Structure–Activity Relationship Analysis of Benzimidazoles As Emerging Anti-Inﬂammatory Agents: an Overview

pharmaceuticals

Review Structure–Activity Relationship Analysis of Benzimidazoles as Emerging Anti-Inﬂammatory Agents: An Overview

Ravichandran Veerasamy 1,2,*, Anitha Roy 3 , Rohini Karunakaran 4 and Harish Rajak 5

1 Pharmaceutical Chemistry Unit, Faculty of Pharmacy, AIMST University, Semeling 08100, Kedah, Malaysia 2 Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai 600077, Tamil Nadu, India 3 Department of Pharmacology, Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai 600077, Tamil Nadu, India; [email protected] 4 Faculty of Medicine, AIMST University, Semeling 08100, Kedah, Malaysia; [email protected] 5 SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur 495009, India; [email protected] * Correspondence: [email protected]

Abstract: A significant number of the anti-inflammatory drugs currently in use are becoming obso- lete. These are exceptionally hazardous for long-term use because of their possible unfavourable impacts. Subsequently, in the ebb-and-flow decade, analysts and researchers are engaged in developing new anti-inflammatory drugs, and many such agents are in the later phases of clinical trials. Molecules with heterocyclic nuclei are similar to various natural antecedents, thus acquiring immense consideration from scientific experts and researchers. The arguably most adaptable heterocyclic cores are benzimidazoles containing nitrogen in a bicyclic scaffold. Numerous benzimidazole drugs are broadly used in the treatment of numerous diseases, showing promising therapeutic poten- Citation: Veerasamy, R.; Roy, A.; tial. Benzimidazole derivatives exert anti-inflammatory effects mainly by interacting with transient Karunakaran, R.; Rajak, H. receptor potential vanilloid-1, cannabinoid receptors, bradykinin receptors, specific cytokines, 5- Structure–Activity Relationship lipoxygenase activating protein and cyclooxygenase. Literature on structure–activity relationship Analysis of Benzimidazoles as (SAR) and investigations of benzimidazoles highlight that the substituent’s tendency and position Emerging Anti-Inflammatory Agents: on the benzimidazole ring significantly contribute to the anti-inflammatory activity. Reported SAR An Overview. Pharmaceuticals 2021, 14, 663. https://doi.org/10.3390/ analyses indicate that substitution at the N1, C2, C5 and C6 positions of the benzimidazole scaf- ph14070663 fold greatly influence the anti-inflammatory activity. For example, benzimidazole substituted with anacardic acid on C2 inhibits COX-2, and 5-carboxamide or sulfamoyl or sulfonyl benzimidazole Academic Editor: Mary J. Meegan antagonises the cannabinoid receptor, whereas the C2 diarylamine and C3 carboxamide substitution of the benzimidazole scaffold result in antagonism of the bradykinin receptor. In this review, we Received: 23 May 2021 examine the insights regarding the SARs of anti-inflammatory benzimidazole compounds, which Accepted: 9 July 2021 will be helpful for researchers in designing and developing potential anti-inflammatory drugs to Published: 11 July 2021 target inflammation-promoting enzymes.

Publisher’s Note: MDPI stays neutral Keywords: benzimidazole; cyclooxygenase; bradykinin; cannabinoid; effect of structural modification with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Inflammation is derived from the Latin word “inflammare”. The body’s immune system initiates an immediate response to harmful stimuli, such as infections or any type Copyright: © 2021 by the authors. of irritation [1]. The inflammatory responses entail several biochemical events (Figure1 ). Licensee MDPI, Basel, Switzerland. They are a defensive attempt by the body to heal infections; however, if inflammation This article is an open access article is not controlled, it can prompt a cluster of acute, chronic and systemic inflammatory distributed under the terms and disorders [2,3]. The major symptoms of inflammation are redness, pain and swelling [4]. conditions of the Creative Commons Some diseases, such as cardiovascular disease, autoimmune diseases, periodontal disease, Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Alzheimer’s disease, asthma, diabetes and COPD, are related to chronic inflammation [1,2]. 4.0/). Steroid drugs have traditionally been used to treat inflammation, but their use has gradually

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ease, Alzheimer’s disease, asthma, diabetes and COPD, are related to chronic inflamma- Pharmaceuticals 2021, 14, 663 tion [1,2]. Steroid drugs have traditionally been used to treat inflammation,2 of 32 but their use

has gradually decreased due to their adverse effects [5]. Non-steroidal anti-inflammatory drugs have been introduced to overcome the adverse effects of steroidal drugs. The role ofease, cyclooxygenase Alzheimer’s disease, and asthma, its coenzyme diabetes and in COPD,the inflammatory are related to chronicprocess inflamma- was an uncreditable discoverytion [1,2]. Steroid [6,7]. drugs In recent have traditionallyyears, elucidatin been usedg the to treatvarious inflammation, complex but mechanisms their use behind the has gradually decreased due to their adverse effects [5]. Non-steroidal anti-inflammatory Pharmaceuticals 2021, 14, 663 2 of 31 inflammatorydrugs have been processintroduced has to indicatedovercome the new adverse methods effects for of steroidalits treatment drugs. [8,9].The role of cyclooxygenaseOver 75% of and the its medications coenzyme in currentlythe inflammatory used haveprocess heterocyclics was an uncreditable containing nitrogen, oxygendiscovery or [6,7]. sulphur, In recent and years, nitrogen elucidatin heterocyclicsg the various complexare present mechanisms in various behind therapeutically the ac- decreasedtiveinflammatory compounds due toprocess their [10,11]. adverse has indicated Pyrazole/pyrazoline, effects [new5]. Non-steroidal methods for its anti-inflammatory benzimidazole, treatment [8,9]. drugs indole have and been pyrimidine are introduced to overcome the adverse effects of steroidal drugs. The role of cyclooxygenase importantOver 75% nitrogen-containing of the medications currently heterocyclic used haves in heterocyclics anti-inflammatory containing research nitrogen, [12]. andoxygen its coenzyme or sulphur, in theand inflammatory nitrogen heterocyclics process was are an present uncreditable in various discovery therapeutically [6,7]. In recent ac- years,tive compounds elucidating [10,11]. the various Pyrazole/pyrazoline, complex mechanisms benzimidazole, behind the indole inflammatory and pyrimidine process has are indicatedimportant new nitrogen-containing methods for its treatment heterocyclic [8,9s]. in anti-inflammatory research [12].

FigureFigure 1.1. TheThe biochemicalbiochemical processprocess ofof inflammation. inflammation. COX—cyclooxygenase;COX—cyclooxygenase; LOX—lipoxygenase;LOX—lipoxygenase; γ PG—prostaglandin;PG—prostaglandin; LT—leukotriene; LT—leukotriene; Tx—thromboxane; Tx—thromboxane; NO—nitric NO—nitric oxide; oxide; IFN-γ—interferon; IFN- —interferon; TNF— Figure 1. The biochemical process of inflammati on. COX—cyclooxygenase; LOX—lipoxygenase; tumourTNF—tumour necrosis necrosis factor; NF- factor;κB—nuclear NF- B—nuclear factor-κB; factor- MAPK—mitogenB; MAPK—mitogen activated proteinactivated kinase; protein JAK— ki- PG—prostaglandin;nase; JAK—Janus kinase; LT—leukotriene;IL—interleukin. Tx—thromboxane; NO—nitric oxide; IFN-γ—interferon; Janus kinase; IL—interleukin. TNF—tumour necrosis factor; NF-B—nuclear factor-B; MAPK—mitogen activated protein kinase;OverBenzimidazole JAK—Janus 75% of the kinase; is medications bicyclic, IL—interleukin. comprising currently useda benzene have heterocyclics fused with an containing imidazole nitrogen, ring, a oxygenheteroaromatic or sulphur, compound and nitrogen with heterocyclicsan amphoteric are property present in(Figure various 2). therapeutically This privileged active scaffold exhibits anti-convulsant, antioxidant, anti-microbial, anticancer, anthelmintic, an- compoundsBenzimidazole [10,11]. Pyrazole/pyrazoline, is bicyclic, comprising benzimidazole, a benzene indole fused and pyrimidine with an imidazole are ring, a importantti-inflammatory, nitrogen-containing anti-fungal, antiviral, heterocyclics antipsychotic in anti-inflammatory and antihistaminic research [effects,12]. among heteroaromaticothersBenzimidazole [13]. Research compound is on bicyclic, the benzimidazole comprisingwith an amphoteric a nucleus benzene has fused property resulted with an in(Figure imidazoledrugs such2). ring,This as al- aprivileged scaf- heteroaromaticfoldbendazole, exhibits mebendazole, compoundanti-convulsant, thia withbendazole, an amphoteric antioxidant, omeprazole, property anti-microbial, lansop (Figurerazole,2). This pantoprazole, anticancer, privileged scaf- aste-anthelmintic, an- foldti-inflammatory,mizole, exhibits enviroxime, anti-convulsant, anti-fungal,candesartan, antioxidant, cilexitil, antiviral, anti-microbial,telmisartan antipsychotic and anticancer, numerous and anthelmintic,antihistaminicother compounds anti- effects, among inflammatory,othersfor treating [13]. other anti-fungal,Research diseases (Figure antiviral,on the 3) benzimidazole antipsychotic[14]. and nucleus antihistaminic has effects,resulted among in oth-drugs such as al- ersbendazole, [13]. Research mebendazole, on the benzimidazole thiabendazole, nucleus has omeprazole, resulted in drugs lansop such asrazole, albendazole, pantoprazole, aste- mebendazole, thiabendazole, omeprazole, lansoprazole, pantoprazole, astemizole, enviroxime,mizole, candesartan, enviroxime, cilexitil, candesartan, telmisartanH and cilexitil, numerous telmisartan other compounds and fornumerous treating other other compounds N N diseasesfor treating (Figure other3)[14 diseases]. (Figure 3) [14]. N H N

H Figure 2. StructureN of benzimidazole. N N H N

Figure 2. Structure of benzimidazole. Figure 2. Structure of benzimidazole.

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Figure 3. Clinically approved drugs with a benzimidazole nucleus. FigureFigure 3.3. ClinicallyClinically approvedapproved drugsdrugs withwith aa benzimidazolebenzimidazole nucleus.nucleus.

TheThe variousvarious targetstargets forfor benzimidazolebenzimidazole areare shownshown inin FigureFigure4 .4. The The NH NH group group of of benzimidazolebenzimidazole isis strongly strongly acidic acidic as as well well as as weakly weakly basic basic in nature.in nature. The The ionisation ionisation constant con- (pKa)stant (pKa) of benzimidazole of benzimidazole is 12.8, is and 12.8, its and conjugate its conjugate acid is acid 5.6 [is15 5.6]. [15].

Figure 4. Various biological targets for benzimidazole. Figure 4. Various biological targets for benzimidazole. Benzimidazoles perform their anti-inflammatory activity mainly by interacting with Benzimidazoles perform their anti-inflammatory activity mainly by interacting with transientBenzimidazoles receptor potential perform vanilloid-1, their anti-inflammatory cannabinoid receptors, activity bradykinin mainly by receptors, interacting specific with transient receptor potential vanilloid-1, cannabinoid receptors, bradykinin receptors, cytokinestransient andreceptor 5-lipoxygenase potential activatingvanilloid-1, protein cannabinoid and cyclooxygenase receptors, (COX)bradykinin (Figure receptors,5). specific cytokines and 5-lipoxygenase activating protein and cyclooxygenase (COX) (Figure(Figure 5).5).

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FigureFigure 5. 5.Benzimidazole’s Benzimidazole’s interactionsinteractions with with clinically clinically approved approved targets. targets.

EvenEven thoughthough benzimidazolebenzimidazole derivativesderivatives areare widelywidely usedused toto treattreat variousvarious diseases,diseases, includingincluding inflammation, inflammation, they they show show some some side side effects, effects, low low potential potential and and physicochemical physicochemical problems.problems. Therefore,Therefore, discovering discovering new, new, safer safer and and more more potent potent anti-inflammatory anti-inflammatory benzimi- ben- dazoleszimidazoles with reducedwith reduced side effects side iseffects urgently is ur warranted.gently warranted. In recent decades,In recent there decades, have beenthere varioushave been reports various on the reports anti-inflammatory on the anti-inflammatory activity of benzimidazoles. activity of benzimidazoles. Hence, the goal ofHence, this reviewthe goal was of tothis collect review the was existing to collect data related the existing to the anti-inflammatorydata related to the activity anti-inflammatory of benzimi- dazolesactivity and of benzimidazoles assess the obtained and results,assess the thus obtained facilitating results, the discussion thus facilitating of structure–activity the discussion relationshipsof structure–activity (SARs). relationships (SARs).

2.2. MethodMethod AA literature literature search search was was carried carried out usingout usin theg Web the of Web Science of citationScience indexing,citation indexing, PubMed andPubMed Google and Scholar Google by using Scholar the termsby using “Heterocyclic”, the terms “Benzimidazole”, “Heterocyclic”, “Structure“Benzimidazole”, activity relationship”“Structure activity and “Anti-inflammatory” relationship” and in“Anti-inflammatory” combination with the in words combination “cyclooxygenase”, with the “cannabinoid”,words “cyclooxygenase”, “cytokine” or “cannabinoid”, “lipoxygenase”, “cytokine” finding thousands or “lipoxygenase”, of publications. finding Addition- thou- ally, the terms “Benzimidazole” and “Anti-inflammation action” were searched alone, and sands of publications. Additionally, the terms “Benzimidazole” and “Anti-inflammation the results were reduced to 75 hits. The publications were reviewed by title, abstract and action” were searched alone, and the results were reduced to 75 hits. The publications text and reduced to those dealing with benzimidazoles with anti-inflammatory action and were reviewed by title, abstract and text and reduced to those dealing with benzimidaz- their structure–activity relationships (SARs). Furthermore, studies without structural and oles with anti-inflammatory action and their structure–activity relationships (SARs). proper pharmacological study information were excluded. The data from the remaining Furthermore, studies without structural and proper pharmacological study information publications were collected and categorised according to the actions of benzimidazoles on were excluded. The data from the remaining publications were collected and categorised different enzymes responsible for inflammation. according to the actions of benzimidazoles on different enzymes responsible for inflammation.

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3. Structure–Activity Relationships of Benzimidazole Derivatives as An- ti-Inflammatory Agents Pharmaceuticals 2021, 14, 663 The reported SAR analysis indicates that substitutions at the N1,5 C2, of 31 C5 and C6 positions of the benzimidazole nucleus greatly influence the anti-inflammatory activity. Various heterocycle substitutions at N1 of benzimidazole account for the effective an- 3.ti-inflammatory Structure–Activity effects Relationships of various of Benzimidazole agents. Derivatives as Anti-Inflammatory Agents 3.1. TheCyclooxygenase reported SAR Inhibitors analysis indicates that substitutions at the N1, C2, C5 and C6 positions of the benzimidazole nucleus greatly influence the anti-inflammatory activity. VariousCyclooxygenase heterocycle substitutions (COX) atis N1a crucial of benzimidazole enzyme in account the biosynthesis for the effective of prostaglandins anti- and inflammatorythromboxanes. effects COX-1 of various is a agents.constitutive enzyme produced in many tissues, while COX-2 is 3.1.inducible Cyclooxygenase and is Inhibitors expressed during inflammation. Hence, drugs that inhibit COX-2 are betterCyclooxygenase anti-inflammatory (COX) is aagents. crucial enzyme in the biosynthesis of prostaglandins and thromboxanes.Paramashivappa COX-1 is a constitutiveet enzymeal., producedin inthe many tissues,year while COX-22003, prepared is2-[[2-alkoxy-6-pentadecyl-phenyl)methyl]thio]-1 inducible and is expressed during inflammation. Hence,H drugs-benzimidazoles that inhibit COX-2 by are incorporating betterbenzimidazole anti-inflammatory with agents.anacardic acid and evaluated its COX-2-inhibitory activity in hu- mansParamashivappa (Figure 6). A et compound al., in the year with 2003, R prepared = H and 2-[[2-alkoxy-6-pentadecyl-phenyl) R1 = methoxy moiety showed potent an- methyl]thio]-1H-benzimidazoles by incorporating benzimidazole with anacardic acid and evaluatedti-inflammatory its COX-2-inhibitory activity, activitywith in384-fold humans (Figureselectivity6). A compound for COX-2 with overR = H COX-1and inhibition. R1Meanwhile, = methoxy moietythe other showed compound potent anti-inflammatory with R = methyl activity, and with R1 384-fold = H moiety selectivity showed 470-fold forselectivity, COX-2 over comparable COX-1 inhibition. to the 375-fold Meanwhile, and the 20 other0-fold compound selectivity with of R celecoxib = methyl and rofecoxib, andrespectively.R1 = H moiety The showed study 470-fold also confirmed selectivity, comparablethe importance to the 375-foldof the and“NH” 200-fold moiety of benzim- selectivity of celecoxib and rofecoxib, respectively. The study also confirmed the importance ofidazole the “NH” in moietyanti-inflammatory of benzimidazole activi in anti-inflammatoryty. However, activity. substitution However, of substitution –CH3 or –NO2 at C5 of 2 ofbenzimidazole –CH3 or –NO2 at showed C5 of benzimidazole moderate showedinhibition, moderate whereas inhibition, –OCHF whereas did –OCHF not show2 favourable didinhibitory not show action favourable [16]. inhibitory action [16].

FigureFigure 6. 6.Structure-Activity Structure-Activity Relationships Relationships (SARs) of anacardic (SARs) acid-conjugated of anacardic benzimidazole acid-conjugated derivatives. benzimidazole derivatives. In 2016, Bukhari et al. studied COX and 5-lipoxygenase inhibition by a few 2-phenyl- substituted benzimidazoles. As shown in Figure7, unsubstituted at R 2,R3 and R4 is preferredIn for2016, COX-1 Bukhari and 2, and et 5-lipoxygenaseal. studied inhibition,COX an whereasd 5-lipoxygenase the amine group inhibition at R1 by a few enhanced2-phenyl-substituted the inhibition of benzimidazoles. all three enzymes. However, As shown the lipophilicin Figure group 7, unsubstituted at R5 favours at R2, R3 and COX-1R4 is preferred inhibition, for a hydrophilic COX-1 and group 2, enhancesand 5-lipox COX-2ygenase inhibition inhibition, and a methoxy whereas substi- the amine group 2 1 tution1 favours 5-lipoxygenase inhibition. One of the compounds with R –CH3,R –NH2 5 at R 3 enhanced4 the inhibition of all three enzymes. However, the lipophilic group at R and R and R –H showed better COX-1 inhibition, with IC50 = 0.72 ± 0.77 µM. However, anotherfavours compound COX-1 inhibition, bearing a nitrile a hydrophilic group at position group R enhances5 was a dual COX- (COX-12 inhibition and -2) in- and a methoxy 1– hibitorsubstitution with IC 50favourss of 8.17 5-lipoxygenase± 2.85 and 6.79 ± inhibition.1.46 µM, respectively. One of the Another compounds compound, with R2–CH3, R 5 havingNH2 and 2-aminopyridin-4-yl R3 and R4–H showed at R , showed better better COX-1 5-lipoxygenase inhibition, inhibition, with IC with50 = an 0.72 IC50 ±of 0.77 µM. How- 8.41ever,± another1.22 µM. Overall,compound the substitution bearing a of nitrile recommended group groupsat position at the above-mentionedR5 was a dual (COX-1 and -2) position favours potent COX-1 and 2, and 5-lipoxygenase inhibition [17]. The SAR of the compoundsinhibitor with is shown IC50 ins of Figure 8.177. ± 2.85 and 6.79 ± 1.46 µM, respectively. Another compound, having 2-aminopyridin-4-yl at R5, showed better 5-lipoxygenase inhibition, with an IC50 of 8.41 ± 1.22 µM. Overall, the substitution of recommended groups at the above-mentioned position favours potent COX-1 and 2, and 5-lipoxygenase inhibition [17]. The SAR of the compounds is shown in Figure 7.

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Figure 7. SARs of 2-phenyl-substituted benzimidazoles. FigureFigure 7.7. SARsSARs ofof 2-phenyl-substituted2-phenyl-substituted benzimidazoles.benzimidazoles. 3.2. Cannabinoid Receptor Agonist 3.2.3.2. CannabinoidCannabinoid ReceptorReceptor receptor AgonistAgonist agonists are effective for the treatment of inflammation. These effectsCannabinoidCannabinoid are mediated receptorreceptor via two agonistsagonists subtypes areare effectiveeffectivof cannabinoide forfor thethe treatmenttreatmentreceptors: of ofCB1, inﬂammation.inflammation. located centrally TheseThese effectsandeffects peripherally, areare mediated mediated and via via CB2, two two on subtypes subtypesimmune of cells cannabinoidof ca ornnabinoid peripheral receptors: receptors: tissues. CB1, CB1, located located centrally centrally and peripherally,and peripherally,Scientists and from CB2,and AstraZeneca CB2, on immune on immune reported cells orcells peripheralcannabinoid or peripheral tissues. receptor tissues. agonist, CB2 agonist and CB1/CB2ScientistsScientists dual fromfromagonist AstraZenecaAstraZeneca activity for reportedreported benzimidazole cannabinoidcannabinoid derivatives receptorreceptor as agonist,agonist, selective CB2CB2 for agonistagonist the man- andand CB1/CB2agementCB1/CB2 of dual pain agonistagonist and inflammation. activityactivity forfor benzimidazole benzimidazole A compound derivatives havingderivatives a carboxamide as as selective selective for substitution for the the manage- man- at mentC5agement of ofbenzimidazole pain of pain and and inflammation. wasinflammation. a highly A compoundselective A compou CB2 havingnd agonist,having a carboxamide awith carboxamide 970-fold substitution selectivity substitution at over C5 at ofCB1C5 benzimidazole of receptors. benzimidazole It also was showedwas a highly a highly Ki selective values selective of CB2 3170 CB2 agonist, and agonist, 3.3 with nM with 970-fold for the970-fold human selectivity selectivity CB1 over and CB1overCB2 receptors. It also showed Ki values of 3170 and 3.3 nM for the human CB1 and CB2 receptors, receptors,CB1 receptors. respectively. It also showed The replacement Ki values of 3170of the and carboxamido 3.3 nM for thegroup human in CB1the previousand CB2 respectively. The replacement of the carboxamido group in the previous compound with the compoundreceptors, respectively.with the sulfamoyl The replacement group produc of edthe another carboxamido comparable group cannabinoid in the previous recep- sulfamoyl group produced another comparable cannabinoid receptor agonist (Figure8). torcompound agonist (Figurewith the 8). sulfamoyl group produced another comparable cannabinoid receptor agonist (Figure 8).

FigureFigure 8.8. SARs of 5-sulfamoyl substitutedsubstituted benzimidazoles.benzimidazoles. Figure 8. SARs of 5-sulfamoyl substituted benzimidazoles. Furthermore,Furthermore, anan AstraZenecaAstraZeneca scientist scientist also also studied studied the the CB1/CB2 CB1/CB2 dual dual agonist agonist activity activ- ofity benzimidazoles ofFurthermore, benzimidazoles withan AstraZeneca with a large a large polar scientist polar 5-N-sulfonamide 5- alsoN-sulfonamide studied substituent the substituentCB1/CB2 for the dual for management agonistthe manage- activ- of painmentity of [ 18ofbenzimidazoles ].pain In 2011,[18]. scientistsIn 2011, with scientists a from large Pﬁzer polar from reported 5-PfizerN-sulfonamide reported CNS-penetrant, CNS-penetrant, substituent selective for selective CB2the manage- agonist CB2 activityagonistment of activity forpain a [18]. few for sulfonylIn a few2011, sulfonyl scientists benzimidazole benzimidazole from Pfizer derivatives reported derivatives as potentialCNS-penetrant, as potential analgesic analgesicselective and anti-CB2and 1 inﬂammatoryanti-inflammatoryagonist activity agents for agents a with few fewersulfonylwith sidefewer benzimidazole effects side [effe19].cts A tertiary[19].derivatives A tertiary butyl as substituent potentialbutyl substituent atanalgesic R resulted at and R1 inresultedanti-inflammatory potent in cannabinoid potent cannabinoidagents receptor with receptorfewer antagonism. side an tagonism.effe However,cts [19]. However, A a tertiary methyl a methylbutyl linker substituent betweenlinker between C2 at ofR1 benzimidazoleresulted in potent and cannabinoid an isobutylgroup, receptor and an benzimidazoletagonism. However, N1 and a amethyl cyclopropyl linkergroup, between is essential for potent agonistic activity (Figure8).

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Pharmaceuticals 2021, 14, 663 7 of 31 C2 of benzimidazole and an isobutyl group, and benzimidazole N1 and a cyclopropyl group, is essential for potent agonistic activity (Figure 8). Gijsen and co-workers, in 2012, evaluated the CB2-receptor agonist activity of 5-sulfonylGijsen benzimidazole and co-workers, derivatives. in 2012, evaluatedOne of the thecompounds CB2-receptor having agonist a 2-ethoxypyridin- activity of 5- sulfonyl4-yl sulfonyl benzimidazole moiety substitution derivatives. at OneC5 of of benzimidazole the compounds showed having good a 2-ethoxypyridin- selectivity, with 4-yl a sulfonyldecent drug-like moiety substitution profile. Further at C5 studies of benzimidazole by the same showed research good group selectivity, led to withthe develop- a decent drug-likement of two proﬁle. potent Further compounds studies with by the methyl-cyclopropyl same research group and led thiazole to the moieties, development respec- of twotively, potent made compounds by replacing with the methyl-cyclopropyl 2-ethoxypyrimidin-4yl and thiazole group moieties,in the compound respectively, shown made in byFigure replacing 9 [20]. the 2-ethoxypyrimidin-4yl group in the compound shown in Figure9[20].

FigureFigure 9.9. SARs of 2-isbutyl-5-sulfonylbenzimidazole derivatives.derivatives.

3.3.3.3. Bradykinin Receptor Antagonists SeveralSeveral acuteacute andand chronicchronic inflammatoryinflammatory pathwayspathways resultingresulting inin pain,pain, oedemaoedema andand vasodilationvasodilation areare aggravated aggravated by kinins, by bradykininkinins, andbradykinin kallidin [21and–23 ]. G-protein-coupledkallidin [21–23]. receptorsG-protein-coupled for bradykinins receptors B1 for and bradykinins B2 mediate B1 these and B2 effects mediate [24]. these In most effects cell [24]. types, In most the bradykinincell types, the B2 bradykinin receptor is expressedB2 receptor under is expressed normal conditions,under normal whereas conditions, the bradykinin whereas the B1 receptorbradykinin is expressed B1 receptor in infections,is expressed inflammatory in infections, diseases inflammatory and traumatic diseases tissue and injury.traumatic tissueGuo injury. et al., in 2008, introduced a benzimidazole nucleus into a benzodiazepine derivative withGuo potentet al., in bradykinin 2008, introduced B1 receptor a benzim antagonisticidazole activity, nucleus by into replacing a benzodiazepine the phenethyl de- benzodiazepinerivative with potent moiety, bradykinin and reported B1 receptor anti-inflammatory antagonistic activity. activity, Their by studyreplacing showed the aphenethyl potent antagonistic benzodiazepine effect moiety, at bradykinin and report B1ed receptors anti-inflammatory with better bioavailability.activity. Their study The β combinationshowed a potent of the antagonistic-alanine linker effect andat bradykinin 2-imidazoline-5-aminopyridine B1 receptors with better group bioavailability. at the C2 of benzimidazole resulted in excellent potency, with IC = 2 nM. They also revealed that the The combination of the β-alanine linker and 2-imidazoline-5-aminopyridine50 group at the 2-carboxamide group on benzimidazole was necessary for the activity. Still, the activity C2 of benzimidazole resulted in excellent potency, with IC50 = 2 nM. They also revealed was reduced when the ethyl linker between the two carboxamide groups was replaced that the 2-carboxamide group on benzimidazole was necessary for the activity. Still, the with a methyl or any other longer alkyl group [25] (Figure 10). They concluded that the activity was reduced when the ethyl linker between the two carboxamide groups was hydrogen bond acceptor in these compounds seemed to enhance the effect. replaced with a methyl or any other longer alkyl group [25] (Figure 10). They concluded As in a previous study, Zischinsky et al. (2010) also studied the bradykinin B1 receptor that the hydrogen bond acceptor in these compounds seemed to enhance the effect. antagonist activity of small benzimidazole derivatives (Figure 11). They found a compound containing an acetamide moiety with better activity, with IC50 = 15 nM, than the parent compound, with an IC50 value of 3500 nM. They performed further optimisation in the substituted moieties and obtained a compound with IC50 = 0.7 nM. A chloroimidazole derivative with IC50 = 0.3 nM was the most active compound in their study [26].

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Figure 10. SARs of 2-substituted N-benzyl benzimidazoles.

As in a previous study, Zischinsky et al. (2010) also studied the bradykinin B1 receptor antagonist activity of small benzimidazole derivatives (Figure 11). They found a compound containing an acetamide moiety with better activity, with IC50 = 15 nM, than the parent compound, with an IC50 value of 3500 nM. They performed further optimisation in the substituted moieties and obtained a compound with IC50 = 0.7 nM. A

chloroimidazole derivative with IC50 = 0.3 nM was the most active compound in their Figurestudy [26]. 10. SARs of 2-substituted N-benzyl benzimidazoles.

Figure 11.11. SARs of 2-aminobenzimidazole asas bradykininbradykinin B1B1 receptorreceptor antagonists.antagonists.

3.4. Anticytokines Anticytokine therapiestherapies are are used used in in the the treatment treatment of chronicof chronic inﬂammatory inflammatory diseases, diseases, par- ticularlyparticularly autoimmune autoimmune diseases diseases such such as rheumatoid as rheumatoid arthritis. arthritis. Based Based on the on ﬁrst the introduced first intro- principleduced principle of cytokine of cytokine blockade blockade in the 1990s, in the tumour 1990s, necrosistumour necrosis factor (TNF)- factorα (TNF)-antagonistsα an- stilltagonists represents still represents the leading the anti-cytokine leading anti-cytokine therapy. therapy.

A numbernumber ofofN N-acridin-9-yl-4-benzimidazo-2-ylbenzamides-acridin-9-yl-4-benzimidazo-2-ylbenzamides (Figure (Figure 12 )12) were were synthe- syn- Figure 11. SARs of 2-aminobenzimidazole as bradykinin B1 receptor antagonists. sisedthesised and and screened screened for anti-inflammatory for anti-inflammator and CDK-inhibitoryy and CDK-inhibitory activities. activities. The study The revealed study thatrevealed the substitution that the substitution at C5 of benzimidazole at C5 of benzimidazole plays a crucial roleplays in anti-inflammatorya crucial role in andan- 3.4. Anticytokines CDK-inhibitoryti-inflammatory activities.and CDK-inhibitory Pronounced activiti activityes. against Pronounced CDK1 andactivity CDK5 against was shownCDK1 byand a compoundCDK5Anticytokine was bearingshown therapiesby a nitro a compound group are used at C5.bearing in By the contrast, atr nitreatmento group an aminoof chronicat C5. or methylBy inflammatory contrast, group an at C5aminodiseases, led toor theparticularly complete autoimmune loss of the anti-inflammatory diseases such as andrheumatoid CDK-inhibitory arthritis. activity Based [on21 ].the first intro- ducedA seriesprinciple of 550 of benzimidazol-2-onecytokine blockade in compounds the 1990s, bearing tumour five necrosis positions factor with (TNF)- substitutedα an- 5-memberedtagonists still heteroaryls represents containingthe leading one anti-cytokine to two heteroatoms therapy. of nitrogen, sulphur or oxygen

(FigureA 13number) was synthesisedof N-acridin-9-yl-4-benzimidazo-2-ylbenzamides by Dombroski et al. (2006) and found to (Figure be potent 12) inhibitors were syn- of p38thesised MAP and kinase screened [22]. In for 2007, anti-inflammator Anderskewitzy et and al. reportedCDK-inhibitory that the compoundactivities. The shown study in Figurerevealed 14 inhibitedthat the thesubstitution CCR3 receptor, at C5 withof benzimidazole a binding constant plays (Ki) a ofcrucial 100 nM. role However, in an- substitutingti-inflammatory the C2-positionand CDK-inhibitory of benzimidazole activities. with Pronounced triﬂuoroethane activity resulted against in better CDK1 CCR3 and receptorCDK5 was inhibition, shown by with a compound Ki = 19 nM bearing [23]. a nitro group at C5. By contrast, an amino or

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methyl group at C5 led to the complete loss of the anti-inflammatory and CDK-inhibitory activity [21].

Pharmaceuticals 2021, 14, 663 9 of 32

Pharmaceuticals 2021, 14, 663 9 of 31 methyl group at C5 led to the complete loss of the anti-inflammatory and CDK-inhibitory activity [21].

Figure 12. SARs of benzimidazole-acridine derivatives. Figure 12. SARs of benzimidazole-acridine derivatives.

A series of 550 benzimidazol-2-one compounds bearing five positions with substituted 5-membered heteroaryls containing one to two heteroatoms of nitrogen, sulphur or oxygen (Figure 13) was synthesised by Dombroski et al. (2006) and found to be potent inhibitors of p38 MAP kinase [22]. In 2007, Anderskewitz et al. reported that the compound shown in Figure 14 inhibited the CCR3 receptor, with a binding constant (Ki) of Figure100 nM. 12.12. However,SARs of benzimidazole-acridine substituting the C2-position derivatives.derivatives. of benzimidazole with trifluoroethane resulted in better CCR3 receptor inhibition, with Ki = 19 nM [23]. A series of 550 benzimidazol-2-one compounds bearing five positions with substituted 5-membered heteroaryls containing one to two heteroatoms of nitrogen, sulphur or oxygen (Figure 13) was synthesised by Dombroski et al. (2006) and found to be potent inhibitors of p38 MAP kinase [22]. In 2007, Anderskewitz et al. reported that the compound shown in Figure 14 inhibited the CCR3 receptor, with a binding constant (Ki) of 100 nM. However, substituting the C2-position of benzimidazole with trifluoroethane resulted in better CCR3 receptor inhibition, with Ki = 19 nM [23].

Figure 13. Benzimidazol-2-one compounds bearing five positions with substituted imidazole. Figure 13.13. Benzimidazol-2-one compounds bearingbearing ﬁvefive positionspositions withwith substitutedsubstituted imidazole.imidazole.

Figure 13. Benzimidazol-2-one compounds bearing five positions with substituted imidazole.

Figure 14. SARs of 1,2,5-trisubstituted benzimidazole as CCR3 receptor inhibitor. Figure 14.14. SARs of 1,2,5-trisubstituted benzimidazole asas CCR3CCR3 receptorreceptor inhibitor.inhibitor.

Mader etet al.al. (2008) (2008) introduced introduced 2-amino-1-isopropylsulfonyl 2-amino-1-isopropylsulfonyl 6-substituted 6-substituted benzimida- benzim- zoleidazole (Figures (Figures 15 and 15 16and) as 16) a potent as a TNF-potentα and TNF- p38αα andMAP p38 kinaseα MAP inhibitor. kinase A inhibitor. 2,6-dichloro A or2,6-dichloro diﬂuoro phenyl or difluoro moiety phenyl at the 2-positionmoiety at the of imidazole 2-position substituted of imidazole at C6 substituted of benzimidazole at C6 of enhancedbenzimidazole the inhibition enhanced of the TNF- inhibitionα and p38 ofα TNF-MAPα kinase and p38 activity.α MAP However, kinase 2-activity. and 4-ﬂuoro How- Pharmaceuticals 2021, 14, 663 phenylever, 2- moieties and 4-fluoro at the phenyl 4-position moieties of imidazole at the substituted4-position of at imidazole C6 of benzimidazole substituted enhanced at10 C6 of of32 ever, 2- and 4-fluoro phenyl moieties at the 4-position of imidazole substituted at C6 of thebenzimidazoleFigure inhibition 14. SARs potential ofenhanced 1,2,5-trisubstituted for the TNF- inhibitionα and benzimidazole p38 potentialα MAP as for kinase CCR3 TNF- receptor activityα and inhibitor.p38 [24].α MAP kinase activity [24]. Mader et al. (2008) introduced 2-amino-1-isopropylsulfonyl 6-substituted benzim- Amino goup shows no effect on idazole (Figures 15 and 16) as a potent TNF-α and p38α MAP kinase inhibitor. A N inhibition potential 2,6-dichloroF H or difluoro phenyl moiety at the 2-position of imidazole substituted at C6 of N NH2 benzimidazole enhancedN the inhibition of TNF-α and p38α MAP kinase activity. How- ever, 2- Nand 4-fluoro SphenylO moieties at the 4-position of imidazole substituted at C6 of F O benzimidazole enhanced theCH3 inhibition potential for TNF-α and p38α MAP kinase activ- H C ity [24]. 3 Replacement or removal of sulfonyl group results in diminished inhibition potential. FigureFigure 15.15. SAR of 2-amino-1-isopropylsulfonyl2-amino-1-isopropylsulfonyl 6-substituted6-substituted benzimidazole.benzimidazole.

Figure 16. SARs of 2-amino-1-isopropylsulfonyl 6-substituted benzimidazole.

In 2005, Dios et al. studied the importance of the N-sulfonyl group incorporated 2-aminobenzimidazole as a p38α MAP kinase inhibitor. The compound with a piprid-4-yl at R3 showed the highest efficacy and selectivity. A 2,4-difluoro substitution at R2 led to better inhibition than mono-fluoro substitution; moreover, optimum steric bulky groups with three carbons at R1 favoured p38α MAP kinase inhibition [27]. How- ever, bulkier groups at R1 led to a reduction in the inhibition of p38α MAP kinase activity. The SAR of the compounds is shown in Figure 17.

Figure 17. SARs of sulfonyl group incorporated 2-aminobenzimidazoles.

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Amino goup shows no effect on N inhibition potential F H N NH2 N N S O F O F CH3 H C 3 Replacement or removal of sulfonyl group Pharmaceuticals 2021, 14, 663 results in diminished inhibition potential. 10 of 31 results in diminished inhibition potential. Figure 15. SAR of 2-amino-1-isopropylsulfonyl 6-substituted benzimidazole.

FigureFigure 16.16. SARs ofof 2-amino-1-isopropylsulfonyl2-amino-1-isopropylsulfonyl 6-substituted6-substituted benzimidazole.benzimidazole.

InIn 2005,2005, DiosDios etet al.al. studiedstudied thethe importanceimportance ofof thethe NN-sulfonyl-sulfonyl groupgroup incorporatedincorporated 2-aminobenzimidazole2-aminobenzimidazole asas a p38a αp38MAPα MAP kinase kinase inhibitor. inhibitor. The compound The compound with a piprid-4-yl with a 3 2 atpiprid-4-yl R showed at R the3 showed highest the efficacy highest and efficacy selectivity. and select A 2,4-difluoroivity. A 2,4-difluoro substitution substitution at R led to at betterR2 led inhibition to better thaninhibition mono-fluoro than mono-fluor substitution;o substitution; moreover, optimum moreover, steric optimum bulky groups steric 1 withbulky three groups carbons with three at R carbonsfavoured at p38 R1 αfavouredMAP kinase p38α inhibitionMAP kinase [27 inhibition]. However, [27]. bulkier How- 1 groupsever, bulkier at R ledgroups to a at reduction R1 led to in a reduction the inhibition in the of inhibition p38α MAP of kinase p38α MAP activity. kinase The activity. SAR of theThe compounds SAR of the compounds is shown in Figureis shown 17 .in Figure 17.

FigureFigure 17.17. SARs ofof sulfonylsulfonyl groupgroup incorporatedincorporated 2-aminobenzimidazoles.2-aminobenzimidazoles.

studied the signiﬁcance of an amide linker at the benzimidazole C2 position by removing the amide carbonyl, which resulted in a substantial reduction in inhibition. Furthermore, their study continued by replacing the amide with urea and sulphonamide groups, which resulted in the loss of IRAK4 inhibition, which indicates the need for an amide bridge between the two aromatic groups for good activity (Figure 18)[28]. As a continuation of the previous research, the Frankel and Powers team obtained a US patent for their study on similar N-acyl 2-aminobenzimidazole derivatives containing various aroyl and hetero aroyl Pharmaceuticals 2021, 14, 663 11 of 32

Amgen Inc. (South San Francisco, CA, USA) carried out high-throughput screening and SAR studies of small molecules, and they found that compound 1 (R = 3-NO2) was a potent inhibitor of interleukin-1 (IL-1) receptor-associated kinase-4 (IRAK4). They also studied the significance of an amide linker at the benzimidazole C2 position by removing the amide carbonyl, which resulted in a substantial reduction in inhibition. Furthermore, their study continued by replacing the amide with urea and sulphonamide groups, which resulted in the loss of IRAK4 inhibition, which indicates the need for an amide

Pharmaceuticals 2021, 14, 663 bridge between the two aromatic groups for good activity (Figure 18) [28]. As a continu-11 of 31 ation of the previous research, the Frankel and Powers team obtained a US patent for their study on similar N-acyl 2-aminobenzimidazole derivatives containing various aroyl and hetero aroyl substituents at the C2 position as potent IRAK4 inhibitors (Figure 18) substituents[29]. Abbott at Corporate the C2 position also asperformed potent IRAK4 high-throughput inhibitors (Figure screening 18)[29]. Abbottthrough Corporate binding alsostudies performed to identify high-throughput potent 1,2-disubstitute screeningdbenzimidazole through binding derivatives studies toas identifyCXCR3 antago- potent 1,2-disubstitutedbenzimidazolenists, inhibiting CXCL10’s binding derivatives to CHO ascell CXCR3 membranes. antagonists, A compound inhibiting having CXCL10’s a me- 3 2 bindingthyl group to CHO at R3 cell, chlorine membranes. at R2 and A compound no substitution having at a methylR1 (Figure group 19) atshowed R , chlorine maximum at R 1 andCXCR3 no substitutionantagonism. at Furthermore, R (Figure 19 )substi showedtution maximum with a CXCR3methoxy antagonism. group at R Furthermore,3 did not re- 3 substitutionduce the activity, with abut methoxy substitutions group at at the R did5- and not 6-positions reduce the were activity, unfavourable but substitutions for activ- at theity (Figure 5- and 6-positions 19) [30]. were unfavourable for activity (Figure 19)[30].

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Figure 18.18. SARs of 2-acetamidopheny-1-(N-substituted) benzimidazoles.benzimidazoles.

Figure 19.19. SARs of 3-methyl-1,2-disubstituted benzimidazoles.benzimidazoles.

InIn 2006,2006, SabatSabat etet al.al. reportedreported thethe anti-inflammatoryanti-inflammatory effectseffects ofof aa fewfew benzimidazolesbenzimidazoles thatthat werewere 1-substituted1-substituted withwith pyrimidin-2-yl.pyrimidin-2-yl. They found that compound 2 (Table1 1),), by by itsits lymphocyte-specificlymphocyte-specific kinase kinase (Lck) (Lck) blocking blocking activity, showedactivity, a potentshowed anti-inflammatory a potent an- effectti-inflammatory [31]. Compound effect [31].2 had Compound an anti-inflammatory 2 had an anti-inflammatory effect and waspotent effect and at 3 was nM inpotent Lck kinaseat 3 nM inhibition in Lck kinase and potent inhibition at 0.054 and mM potent for inhibitingat 0.054 mM IL-2 for cytokine inhibiting production. IL-2 cytokine They alsoproduction. reported They that smallalso reported group substitutions that small group at R3 ofsubstitutions the pyrimidine at R moiety3 of the led pyrimidine to a loss ofmoiety the Lck-inhibitory led to a loss of effect. the Lck-inhibitory In the same yeareffect. (2006), In the Chen same etyear al. (2006), identified Chen compound et al. iden-3 tified compound 3 (Table 1), with a substitution on C6 of benzimidazole, as a potent inhibitor of Janus kinase 3 (JAK3), from their extensive SAR studies [32]. Compound 3 exhibited JAK3-inhibition potency at 45 nM. They found that the substitution of a nitrile group at the 6-position of benzimidazole resulted in excellent JAK3 inhibition. The SARs of the compounds are depicted in Figure 20.

Table 1. N1-substituted benzimidazoles with pyrimidin-2-yl.

Compound 2 Compound 3 R1 H CN

R3 H

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Figure 19. SARs of 3-methyl-1,2-disubstituted benzimidazoles. FigureFigure 19. 19. SARs SARs of of 3-methyl-1,2-disubstituted 3-methyl-1,2-disubstituted benzimidazoles. benzimidazoles.

InInIn 2006, 2006,2006, Sabat SabatSabat et etet al. al.al. reported reportedreported the thethe anti-inflammatory anti-inflammatoryanti-inflammatory effects effectseffects of ofof a aa few fewfew benzimidazoles benzimidazolesbenzimidazoles thatthatthat were werewere 1-substituted 1-substituted1-substituted with withwith pyrimi pyrimipyrimidin-2-yl.din-2-yl. They They found found that that compound compound 2 2 (Table(Table(Table 1), 1),1), by byby itsitsits lymphocyte-specificlymphocyte-specificlymphocyte-specific kinasekinasekinase (Lck)(Lck)(Lck) blblblockingocking activity,activity, showedshowed aa potentpotent an-anti-inflammatory effect [31]. Compound 2 had an anti-inflammatory effect and was potent ti-inflammatoryti-inflammatory effecteffect [31].[31]. CompoundCompound 22 hadhad anan anti-inflammatoryanti-inflammatory effecteffect andand waswas potentpotent at 3 nM in Lck kinase inhibition and potent at 0.054 mM for inhibiting IL-2 cytokine Pharmaceuticals 2021, 14, 663 atat 33 nMnM inin LckLck kinasekinase inhibitioninhibition andand potentpotent atat 0.0540.054 mMmM forfor inhibitinginhibiting IL-2IL-2 cytokinecytokine12 of 31 3 production.production. TheyThey alsoalso reportedreported thatthat smallsmall groupgroup substitutionssubstitutions atat RR3 3 ofofof thethethe pyrimidinepyrimidinepyrimidine moietymoiety led led to to a a loss loss of of the the Lck-inhibitory Lck-inhibitory effect. effect. In In the the same same year year (2006), (2006), Chen Chen et et al. al. iden- iden- tifiedtifiedtified compound compoundcompound 3 3 (Table (Table(Table 1), 1),1), with withwith a aa substitution substitutionsubstitution on onon C6 C6C6 of ofof benzimidazole, benzimidazole,benzimidazole, as asas a aa potent potentpotent inin-in- (Table1), with a substitution on C6 of benzimidazole, as a potent inhibitor of Janus kinase hibitorhibitor of of Janus Janus kinase kinase 3 3 (JAK3), (JAK3), from from their their extensive extensive SAR SAR studies studies [32]. [32]. Compound Compound 3 3 ex- ex-ex- 3 (JAK3), from their extensive SAR studies [32]. Compound 3 exhibited JAK3-inhibition hibitedhibited JAK3-inhibitionJAK3-inhibition potencypotency atat 4545 nM.nM. TheyThey foundfound thatthat thethe substitutionsubstitution ofof aa nitrilenitrile potency at 45 nM. They found that the substitution of a nitrile group at the 6-position groupgroup at at the the 6-position 6-position of of benzimidazole benzimidazole resulted resulted in in excellent excellent JAK3 JAK3 inhibition. inhibition. The The SARs SARs of benzimidazole resulted in excellent JAK3 inhibition. The SARs of the compounds are ofof the the compounds compounds are are depicted depicted in in Figure Figure 20. 20. depicted in Figure 20. Table 1. N1-substituted benzimidazoles with pyrimidin-2-yl. TableTable 1. 1. N1-substituted N1-substituted benzimidazoles benzimidazoles with with pyrimidin-2-yl. pyrimidin-2-yl. Table 1. N1-substituted benzimidazoles with pyrimidin-2-yl.

Compound Compound CompoundCompound 2 22 Compound Compound Compound 33 33 1 RRR11 1 HHH CNCN CN

2 RRR22 2

3 R 3 3 H RRR33 HH

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Figure 20. SARs of 2-benzimidazole substituted with pyrimidine.

In anotheranother study,study, the the activity activity of of pyrimido pyrimido benzimidazoles benzimidazoles in inhibitingin inhibiting lymphocyte- lympho- speciﬁccyte-specific protein protein tyrosine tyrosine kinase kinase (Lck) (Lck) was was explored explored by by Martin Martin et et al. al. (2008) (2008) [ 33[33].]. TheThe SAR studies revealed that 6-(2,6-dimethyl phenyl)-2-((4-(4-methyl-1-piperazinyl)phenyl) amino) pyrimido [5′,4′:5,6]pyrimido-[1,2-a]benzimidazol-5(6H)-one was the most potent Lck inhibitor, with IC50 = 0.007 µM. Replacing the 2,6-dimethyl phenyl group with chlorine at the 6-position of the fused pyrimidobenzimidazole nucleus resulted in a low impact on activity, but shifting the methyl groups to the meta or para position was unfavourable for Lck inhibition (Figure 21). Hunt et al. (2009) reported potent inhibition of Lck kinase and cellular IL-2 release by 4-benzimidazolyl-N-piperazinethyl-pyrimidin-2-amines. The compound shown in Fig- ure 22 inhibited Lck, with IC50 = 0.12 nM, and cellular IL-2 release, with IC50 = 8 nM [34]. The SARs of the compounds are given in Figure 22. Substitution at the 2-position of benzimidazole was unfavourable for Lck inhibition. Furthermore, the author stated that the S, S enantiomers are more potent than the racemic forms of 4-benzimidazolyl-N-piperazinethyl-pyrimidin-2-amines.

Figure 21. SARs of pyrimido-benzimidazoles.

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Figure 20. SARs of 2-benzimidazole substituted with pyrimidine. Pharmaceuticals 2021, 14, 663 13 of 31

In another study, the activity of pyrimido benzimidazoles in inhibiting lymphocyte-specific protein tyrosine kinase (Lck) was explored by Martin et al. (2008) [33]. The SAR studies revealed thatthat 6-(2,6-dimethyl6-(2,6-dimethyl phenyl)-2-((4-(4-methyl-1-piperazinyl)phenyl)phenyl)-2-((4-(4-methyl-1-piperazinyl)phenyl) amino) pyrimido [5[50′,40′:5,6]pyrimido-[1,2-a]benzimidazol-5(6H)-one was was the mostmost potentpotent Lck inhibitor, withwith ICIC5050 = 0.007 0.007 µM.µM. Replacing the 2,6-dimethyl phenyl group with chlorine at the 6-position of the fused pyrimidobenzimidazole nucleus resulted in a low impact on activity, butbut shiftingshifting thethe methyl groupsgroups toto the the meta meta or or para para position position was was unfavourable unfavourable for for Lck Lck inhibition inhibition (Figure (Figure 21). Hunt21). Hunt et al. et (2009) al. (2009) reported reported potent potent inhibition inhibition of Lckof Lck kinase kinase and and cellular cellular IL-2 IL-2 release release by by 4- benzimidazolyl-4-benzimidazolyl-N-piperazinethyl-pyrimidin-2-amines.N-piperazinethyl-pyrimidin-2-amines. The The compound compound shown shown in Figure in Fig- 22 inhibitedure 22 inhibited Lck, with Lck, IC with50 = IC 0.1250 = nM, 0.12 and nM, cellular and cellular IL-2 release, IL-2 release, with ICwith50 = IC 850 nM = 8[ 34nM]. [34]. The SARsThe SARs of the of compoundsthe compounds are givenare given in Figure in Figure 22. Substitution 22. Substitution at the at 2-positionthe 2-position of benzimi- of ben- dazolezimidazole was was unfavourable unfavourable for Lck for inhibition.Lck inhibition. Furthermore, Furthermore, the authorthe author stated stated that that the S,the S enantiomersS, S enantiomers are more potent are than more the racemic potent forms ofthan 4-benzimidazolyl-N-piperazinethyl- the racemic forms of pyrimidin-2-amines.4-benzimidazolyl-N-piperazinethyl-pyrimidin-2-amines.

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Figure 21. SARs of pyrimido-benzimidazoles.

FigureFigure 22.22. SARs ofof 4-benzimidazolyl-4-benzimidazolyl-NN-piperazinethyl-pyrimidin-2-amines.-piperazinethyl-pyrimidin-2-amines.

3.5.3.5. FLAP Inhibitors LeukotrienesLeukotrienes areare responsible responsible for for initiating initiati andng and amplifying amplifying the inﬂammatory the inflammatory response re- bysponse regulating by regulating the recruitment the recruitment and activation and acti ofvation leukocytes of leukocytes in inﬂamed in tissues,inflamed and tissues, they areand lipid they mediators. are lipid mediators. The inhibition The ofinhibition leukotriene of biosynthesisleukotriene biosynthesis may also be may achieved also bybe targetingachieved FLAP.by targeting FLAP. Banoglu et al. (2012) examined benzimidazoles as 5-lipoxygenase (5-LO)-activating protein (FLAP) inhibitors. They performed virtual screening using combined ligand- and structure-based pharmacophore modelling to find compounds targeting FLAP. Their study led to the identification of 1-(2-chlorobenzyl)-2-(1-(4-isobutylphenyl)ethyl)-1H-benzimidazole, which inhibited leukotriene biosynthesis with IC50 = 0.31 mM. In order to optimise the above compound, the authors synthesised a few potent benzimidazole derivatives, with IC50 = 0.12–0.19 mM for intact neutrophils [35]. They also stated that no substitution at C5 of benzimidazole, a methyl linker between the N of benzimidazoleand the aryl or heteroaryl groups favoured the inhibition of leukotriene biosynthesis (Figure 23).

Figure 23. SARs of 2-isobutylphenylethyl benzimidazoles.

AM803, AM643 and AM103 are recently developed FLAP inhibitors that have, pre- clinically, been shown to be efficacious in inflammatory diseases. They have an indole

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Figure 22. SARs of 4-benzimidazolyl-N-piperazinethyl-pyrimidin-2-amines.

3.5. FLAP Inhibitors Leukotrienes are responsible for initiating and amplifying the inflammatory response by regulating the recruitment and activation of leukocytes in inflamed tissues, Pharmaceuticals 2021, 14, 663 14 of 31 and they are lipid mediators. The inhibition of leukotriene biosynthesis may also be achieved by targeting FLAP. Banoglu et al. (2012) examined benzimidazoles as 5-lipoxygenase (5-LO)-activating proteinBanoglu (FLAP) et inhibitors. al. (2012) examinedThey performed benzimidazoles virtual screening as 5-lipoxygenase using combined (5-LO)-activating ligand- and proteinstructure-based (FLAP) inhibitors.pharmacophor Theye performed modelling virtual to find screening compounds using targeting combined FLAP. ligand- Their and structure-basedstudy pharmacophoreled modellingto to ﬁndthe compoundsidentification targeting FLAP. Their studyof led1-(2-chlorobenzyl)-2-(1-(4-isobutylphenyl)ethyl)-1 to the identification of 1-(2-chlorobenzyl)-2-(1-(4-isobutylphenyl)ethyl)-1H-benzimidazole, whichH-benzimidazole inhibited, whichleukotriene inhibited biosynthesis leukotriene with biosynthesis IC50 = 0.31 mM. with In IC order50 = 0.31 to optimise mM. In the order above to optimise compound, the abovethe authors compound, synthesised the authors a few synthesised potent benzimidazole a few potent derivatives, benzimidazole with derivatives, IC50 = 0.12–0.19 with ICmM50 =for 0.12–0.19 intact neutrophils mM for intact [35]. neutrophilsThey also stated [35]. Theythat no also substitution stated that at no C5 substitution of benzimid- at C5azole, of benzimidazole,a methyl linker between a methyl the linker N of between benzimidazoleand the N of benzimidazoleand the aryl or heteroaryl the arylgroups or heteroarylfavoured the groups inhibition favoured of leuk theotriene inhibition biosynthesis of leukotriene (Figure biosynthesis 23). (Figure 23).

Figure 23.23. SARs of 2-isobutylphenylethyl benzimidazoles.

AM803, AM643 and AM103 are recently developeddeveloped FLAP inhibitorsinhibitors that have, pre- clinically, beenbeen shownshown toto bebe efficaciousefficacious inin inflammatoryinflammatory diseases.diseases. TheyThey havehave anan indoleindole scaffold that, structurally, is deficient in one nitrogen atom compared to the benzimidazole nucleus [36–40].

3.6. TRPV-1 Antagonists TRPV-1 is a member of the ion channels that allow the transient influx of Ca2+ ions when activated and predominantly expressed in peripheral sensory neurons involved in nociception and neurogenic inflammation. Ognyanov et al. (2006) evaluated the TRPV-1-antagonising property of 2-(4-pyridin-2- ylpiperazin-1-yl)-1H-benzo-[d]imidazoles, which blocked capsaicin-induced flinch in rats in a dose-dependent manner. One of the compounds, (R,S)-1-(5-chloro-6- ((R)-3-methyl-4-(6- (trifluoromethyl)-4-(3,4,5-trifluorophenyl)-1H-benzo[d]imidazol-2-yl)piperazin-1-yl)pyridin- 3-yl)ethane-1,2-diol, was the most effective orally bioavailable TRPV-1 antagonist [41]. Conclusively, the simultaneous introduction of bulky lipophilic groups at position 4 of 4 6 7 the benzimidazole, a methyl group at R , hydrophilic groups at R and CF3 or Cl at R was favourable for potent TRPV-1-antagonising activity (Figure 24). Additionally, a patent from Amgen has also demarcated the TRPV-1-antagonising action of a piperazine-linked benzimidazole derivative, WO04035549 [42]. PharmaceuticalsPharmaceuticals 20212021,, 1414,, 663663 1515 ofof 3232

scaffoldscaffold that,that, structurally,structurally, isis deficientdeficient inin oneone nitrogennitrogen atomatom comparedcompared toto thethe benzimid-benzimid- azoleazole nucleusnucleus [36–40].[36–40].

3.6.3.6. TRPV-1TRPV-1 AntagonistsAntagonists 2+ TRPV-1TRPV-1 isis aa membermember ofof thethe ionion channelschannels thatthat allowallow thethe transienttransient influxinflux ofof CaCa2+ ionsions whenwhen activatedactivated andand predominantlypredominantly expressedexpressed inin peripheralperipheral sensorysensory neuronsneurons involvedinvolved inin nociceptionnociception andand neurogenicneurogenic inflammation.inflammation. OgnyanovOgnyanov etet al.al. (2006)(2006) evaluatedevaluated thethe TRPV-1-antagonisingTRPV-1-antagonising propertyproperty ofof 2-(4-pyridin-2-ylpiperazin-1-yl)-12-(4-pyridin-2-ylpiperazin-1-yl)-1HH-benzo-[d]imidazoles,-benzo-[d]imidazoles, whichwhich blockedblocked capsai-capsaicin-inducedcin-induced flinchflinch inin ratsrats inin aa dose-dependentdose-dependent manner.manner. OneOne ofof thethe compounds,compounds, ((RR,,SS)-1-(5-chloro-6-)-1-(5-chloro-6- ((((RR)-3-methyl-4-(6-(trifluoromethy)-3-methyl-4-(6-(trifluoromethyl)-4-(3,4,5-trifluorophenyl)-1l)-4-(3,4,5-trifluorophenyl)-1HH-benzo[d]imidazol-2-yl)-benzo[d]imidazol-2-yl) piperazin-1-yl)pyridin-3-yl)ethane-1,2-diol,piperazin-1-yl)pyridin-3-yl)ethane-1,2-diol, waswas thethe mostmost effectiveeffective orallyorally bioavailablebioavailable TRPV-1TRPV-1 antagonistantagonist [41].[41]. Conclusively,Conclusively, thethe simultaneoussimultaneous introductionintroduction ofof bulkybulky lipophiliclipophilic 4 6 groupsgroups atat positionposition 44 ofof thethe benzbenzimidazole,imidazole, aa methylmethyl groupgroup atat RR4,, hydrophilichydrophilic groupsgroups atat RR6 7 and CF3 or Cl at R7 was favourable for potent TRPV-1-antagonising activity (Figure 24). Pharmaceuticals 2021, 14, 663 and CF3 or Cl at R was favourable for potent TRPV-1-antagonising activity (Figure15 of24). 31 Additionally,Additionally, aa patentpatent fromfrom AmgenAmgen hashas alsoalso demarcateddemarcated thethe TRPV-1-antagonisingTRPV-1-antagonising actionaction ofof aa piperazine-linkedpiperazine-linked benzimidazolebenzimidazole derivative,derivative, WO04035549WO04035549 [42].[42].

FigureFigure 24 24.24.. SARsSARs of of 2-(4-pyridin-2--ylpiperazin-1-yl)-1 2-(4-pyridin-2–ylpiperazin-1-yl)-12-(4-pyridin-2--ylpiperazin-1-yl)-1HHH-benzo-[d]imidazoles.-benzo-[d]imidazoles.-benzo-[d]imidazoles.

In 2006,2006, Fletcher Fletcher et al.et studiedal. studied the afﬁnity the af offinity compounds of compounds containing containing a para-substituted a pa- phenylIn at2006, C2 ofFletcher benzimidazole et al. studied for the hTRPV-1the affinity receptors of compounds (Figure 25 ).containing The compound a para-substitutedra-substituted phenylphenyl atat C2C2 ofof benzimidazolebenzimidazole forfor thethe hTRPV-1hTRPV-1 receptorsreceptors (Figure(Figure 25).25). TheThe shown in Figure 25 is potent in antagonising hTRPV-1 receptors, with IC50 = 22 nM, as compound shown in Figure 25 is potent in antagonising hTRPV-1 receptors, with IC50 = measuredcompound in shown a FLIPR-based in Figure assay. 25 is potent Moreover, in antagonising minor changes, hTRPV- such1 as receptors, replacing with the 4-CFIC50 = 22 nM, as measured in a FLIPR-based assay. Moreover, minor changes, such as replacing3 Pharmaceuticals 2021, 14, 663 22 nM, as measured in a FLIPR-based assay. Moreover, minor changes, such as replacing16 of 32 group of the phenyl moiety at C2 of benzimidazole with tert-butyl, –CH3 or F, led to 3 3 thethe 4-CF4-CF3 groupgroup ofof thethe phenylphenyl moietymoiety atat C2C2 ofof benzimidazolebenzimidazole withwith tert-butyl,tert-butyl, –CH–CH3 oror F,F, decreased activity, with IC50 values of 224, 170 and 320 nM, respectively [43]. 50 ledled toto decreaseddecreased activity,activity, withwith ICIC50 valuesvalues ofof 224,224, 170170 andand 320320 nM,nM, respectivelyrespectively [43].[43].

Figure 25.25. SARs of 6-(3-triﬂuoromethylpyridin-2-yl)6-(3-trifluoromethylpyridin-2-yl) benzimidazoles.benzimidazoles.

3.7. Protein Kinase Inhibitors Protein kinaseskinases add add a a phosphate phosphate group group to ato protein a protein in a in process a process called called phosphorylation, phosphory- whichlation, canwhich turn can a protein turn a onprotein or off on and, or therefore, off and, therefore, affect its level affect of its activity level andof activity function. and function.In a study, thiophene substitution at position 1 of benzimidazole resulted in a very potent inhibitor of serine–threonine kinase 3 (IKK3). Moreover, the IKK3-inhibitory ac- In a study, thiophene substitution at position 1 of benzimidazole1 resulted in a very tivity was reduced when one of the hydrogen atoms of R —namely, CH2—was replaced potent inhibitor of serine–threonine kinase 3 (IKK3). Moreover,1 the IKK3-inhibitory ac- with –CH3. This indicates that the methylene bridge at R is essential for potent IKK3 tivity was reduced when one of the hydrogen atoms of R1—namely, CH2—was replaced inhibition. However, electronegative groups substituted at the phenyl ring of R1 resulted with –CH3. This indicates that the methylene bridge at R1 is essential for potent IKK3 inin better IKK3 inhibition than the substitution of electron-donating groups. However, the hibition. However, electronegative groups substituted at the phenyl ring of R1 resulted in inhibitory effect on IKK3 was decreased by replacing the nitrile group with an amide group better IKK3 inhibition than the substitution of electron-donating groups. However, the (compound 4)[44]. The SARs of the compounds are shown in Figure 26. inhibitory effect on IKK3 was decreased by replacing the nitrile group with an amide group (compound 4) [44]. The SARs of the compounds are shown in Figure 26.

Compound 4

Figure 26. SARs of benzimidazole substituted with thiophene.

3.8. Miscellaneous A study reported that benzimidazole linked to oxadiazole via a thioacetamide linker exhibited strong anti-inflammatory activity (Compound 5). A reduction in activity was observed when the phenyl ring was substituted with a hydroxy group at the ortho position. Meanwhile, the thioacetamide linker is essential for anti-inflammatory activity [45]. The SARs of the compounds are shown in Figure 27.

Pharmaceuticals 2021, 14, 663 16 of 32

Figure 25. SARs of 6-(3-trifluoromethylpyridin-2-yl) benzimidazoles.

3.7. Protein Kinase Inhibitors Protein kinases add a phosphate group to a protein in a process called phosphorylation, which can turn a protein on or off and, therefore, affect its level of activity and function. In a study, thiophene substitution at position 1 of benzimidazole resulted in a very potent inhibitor of serine–threonine kinase 3 (IKK3). Moreover, the IKK3-inhibitory activity was reduced when one of the hydrogen atoms of R1—namely, CH2—was replaced with –CH3. This indicates that the methylene bridge at R1 is essential for potent IKK3 inhibition. However, electronegative groups substituted at the phenyl ring of R1 resulted in better IKK3 inhibition than the substitution of electron-donating groups. However, the inhibitory effect on IKK3 was decreased by replacing the nitrile group with an amide group (compound 4) [44]. The SARs of the compounds are shown in Figure 26.

Pharmaceuticals 2021, 14, 663 16 of 32

Pharmaceuticals 2021, 14, 663 16 of 31

Compound 4 Figure 25. SARs of 6-(3-trifluoromethylpyridin-2-yl) benzimidazoles.

3.7. Protein Kinase Inhibitors Protein kinases add a phosphate group to a protein in a process called phosphorylation, which can turn a protein on or off and, therefore, affect its level of activity and function. In a study, thiophene substitution at position 1 of benzimidazole resulted in a very potent inhibitor of serine–threonine kinase 3 (IKK3). Moreover, the IKK3-inhibitory activity was reduced when one of the hydrogen atoms of R1—namely, CH2—was replaced with –CH3. This indicates that the methylene bridge at R1 is essential for potent IKK3 inhibition. However, electronegative groups substituted at the phenyl ring of R1 resulted in better IKK3 inhibition than the substitution of electron-donating groups. However, the inhibitory effect on IKK3 was decreased by replacing the nitrile group with an amide FigureFiguregroup 26. 26.(compound SARs ofof benzimidazolebenzimidazole 4) [44]. The substitutedSARssubstituted of the withwith compounds thiophene.thiophene. are shown in Figure 26.

Compound 4

3.8. Miscellaneous A study reported that benzimidazole linked to oxadiazole via a thioacetamide linker exhibited strong anti-inﬂammatory activity (Compound 5). A reduction in activity was observed when the phenyl ring was substituted with a hydroxy group at the ortho position. Pharmaceuticals 2021, 14, 663 17 of 32 Meanwhile, the thioacetamide linker is essential for anti-inﬂammatory activity [45]. The SARs of the compounds are shown in Figure 27.

Figure 26. SARs of benzimidazole substituted with thiophene.

3.8. Miscellaneous CompoundA study 5 reported that benzimidazole linked to oxadiazole via a thioacetamide linker FigureFigureexhibited 27.27. SARs SARsstrong ofof benzimidazolebenzimidazoleanti-inflammatory substitutedsubstituted activity withwith (Compound oxadiazole.oxadiazole. 5). A reduction in activity was observed when the phenyl ring was substituted with a hydroxy group at the ortho position.Rao RaoMeanwhile, etet al. al. (2013) (2013) the reported thioacetamide reported anti-inﬂammatory anti-inflammatory linker is essential activities activities for for anti-inflammatory 20 forN-Mannich 20 N-Mannich bases activity ofbases substi- [45]. of tutedsubstitutedThe SARs 2-mercapto-1 of 2-mercapto-1 the compoundsH-benzimidazolesH-benzimidazoles are shown using in Figure Swiss using albino27. Swiss rats albino as test rats animals, as test andanimals, the phlo- and gisticthe phlogistic agent was agent 0.1 mL was of carrageenan0.1 mL of carrageenan suspension suspension (1% carrageenan (1% carrageenan in Normal saline). in Normal Var- ioussaline). substituents Various insubstituents the aromatic in ringthe aromatic did not result ring indid much not inhibition,result in much but non-substituted inhibition, but benzimidazolesnon-substituted werebenzimidazoles shown to be were inferior shown to to the be substituted inferior to the compounds. substituted A compounds. compound bearing an electron-releasing methoxy group at position 6 and a two-pyrrolidine substi- A compound bearing an electron-releasing methoxy group at position 6 and a tutiontwo-pyrrolidine at the nitrogen substitution of benzimidazole at the nitrogen (Compound of benzimidazole6) showed strong (Compound anti-inﬂammatory 6) showed activity;strong anti-inflammatory the percentage inhibition activity; forthe thepercentage paw oedema inhibition volume for the was paw 43.5. oedema A compound volume bearingwas 43.5. an A electron-withdrawing compound bearing an nitro electron-w group atithdrawing the 6-position nitro was grou morep at active the 6-position than the remainingwas more compounds,active than the whereas remaining electron-donating compounds, whereas groups led electron-donating to lower potency groups [46]. The led SARsto lower of the potency compounds [46]. The are SARs shown of inthe Figure compounds 28. are shown in Figure 28.

Replacement of R with -H or -OCH3 or -NO2 group results in promising anti-inflammatory activity, but replacement with -CH group 3 NO results in complete loss of activity 2 Dinitro substitution is N essential for activity S NO R N 2 R1 R2 Compound 6:

R=-OCH3 Introduction of alkyl groups at R1,R2 results in a reduction of the activity. R1,R2 = N Introduction of pyrolidino or piperalidino or morpholino group at R1,R2 results in enhanced anti-inflammatory action Figure 28. SARs of N-Mannich bases of substituted 2-mercapto-1H-benzimidazoles.

Another study by Kankala et al. (2012) explained the synthesis, anti-inflammatory activity and SARs of 3,5-disubstituted isoxazole at the 1-position of benzimidazole. In- flammation and, consequently, oedema in the hind paws of mice were induced using carrageenan at 100 mg/kg body weight. Compounds 7 and 8 possess the electron-withdrawing groups 4-fluorophenyl and 4-cyanophenyl at the C3 position of the isoxazole moiety, which resulted in excellent anti-inflammatory activity, inhibiting the hind paw oedema volume by 60.76 and 58.46%, respectively. However, electronegative groups at C5 of the benzimidazole scaffold resulted in more potent anti-inflammatory activity than the substitution of electron-donating groups or no substitution at the same position [47]. The SARs of the compounds are shown in Figure 29.

Pharmaceuticals 2021, 14, 663 17 of 32

Compound 5 Figure 27. SARs of benzimidazole substituted with oxadiazole.

Rao et al. (2013) reported anti-inflammatory activities for 20 N-Mannich bases of substituted 2-mercapto-1H-benzimidazoles using Swiss albino rats as test animals, and the phlogistic agent was 0.1 mL of carrageenan suspension (1% carrageenan in Normal saline). Various substituents in the aromatic ring did not result in much inhibition, but non-substituted benzimidazoles were shown to be inferior to the substituted compounds. A compound bearing an electron-releasing methoxy group at position 6 and a two-pyrrolidine substitution at the nitrogen of benzimidazole (Compound 6) showed strong anti-inflammatory activity; the percentage inhibition for the paw oedema volume

Pharmaceuticals 2021, 14, 663 was 43.5. A compound bearing an electron-withdrawing nitro group at the 6-position17 of 31 was more active than the remaining compounds, whereas electron-donating groups led to lower potency [46]. The SARs of the compounds are shown in Figure 28.

R=-OCH3 Introduction of alkyl groups at R1,R2 results in a reduction of the activity. R1,R2 = N Introduction of pyrolidino or piperalidino or morpholino group at R1,R2 results in enhanced anti-inflammatory action Figure 28.28. SARs of N-Mannich bases of substituted 2-mercapto-12-mercapto-1H-benzimidazoles.H-benzimidazoles.

Another studystudy byby Kankala Kankala et et al. al. (2012) (2012) explained explained the the synthesis, synthesis, anti-inflammatory anti-inflammatory activityactivity and and SARs SARs of 3,5-disubstituted of 3,5-disubstituted isoxazole isoxaz atole the at 1-position the 1-position of benzimidazole. of benzimidazole. Inflamma- In- Pharmaceuticals 2021, 14, 663 18 of 32 tionflammation and, consequently, and, consequently, oedema inoedema the hind in paws the hind of mice paws were of inducedmice were using induced carrageenan using atcarrageenan 100 mg/kg at body 100 weight. mg/kg Compounds body weight.7 and Compounds8 possess the electron-withdrawing7 and 8 possess the groups elec- 4-fluorophenyltron-withdrawing and groups 4-cyanophenyl 4-fluorophenyl at the C3and position 4-cyanophenyl of the isoxazole at the C3 moiety, position which of the re- sultedisoxazole in excellentmoiety, anti-inflammatorywhich resulted in activity,excellent inhibiting anti-inflammatory the hind paw activity, oedema inhibiting volume the by 60.76hind paw and 58.46%,oedema respectively. volume by 60.76 However, and 58.46%, electronegative respectively. groups However, at C5 of electronegative the benzimida- zolegroups scaffold at C5 resultedof the benzimidazole in more potent scaffold anti-inflammatory resulted in activitymore potent than theanti-inflammatory substitution of electron-donatingactivity than the substitution groups or noof electron-don substitutionating at the groups same position or no substitution [47]. The SARsat the of same the compoundsposition [47]. are The shown SARs in of Figure th e compounds 29. are shown in Figure 29.

Pharmaceuticals 2021, 14, 663 18 of 32

Figure 29.29. SARs of 3,5-disubstituted isoxazoleisoxazole atat N1N1 ofof benzimidazole.benzimidazole.

Mariappan and his co-researchers, in 2011, studied the anti-inflammatory activity of a few Mannich bases of 1-(N-substituted amino)methyl-2-ethylbenzimidazoles in mice with carrageenan-induced rat paw oedema. The amount of compound administered was 100 mg/kg p.o. Compounds with 4-fluoro or bromo substituted aniline at R (Figure 30) showed substantial anti-inflammatory effects, with 23 and 25% inhibition of rat paw oedema. Moreover, 2- or 3-halogenated aniline was conducive to anti-inflammatory activity; this may be due to steric hindrance. Overall, electron-withdrawing groups with lip- ophilicity were preferable for activity [48]. The SARs of the compounds are shown in Figure 30.

Figure 30. SARs of Mannich bases of 1-(N-substituted amino)methyl-2-ethylbenz–imidazoles.

In 2010, Gaba et al. studied the anti-inflammatory activity of 5-substituted-1-(phenylsulfonyl)-2-methyl benzimidazole derivatives using carrageenan-induced paw oedema in a rat model at 200 mg/kg body weight. Compounds sub- Figurestituted 29. with SARs ortho of 3,5-disubstituted amino (37% reduction) isoxazole at and N1 ofpara benzimidazole. amino (39.7% reduction) groups at R (Figure 31) exhibited good anti-inflammatory activity with low ulcerogenic potential due to beingMariappan neutral andmolecules his co-researchers, bearing –NH in2 . 2011,The introductionstudied the anti-inflammatory of –CH2 between activitythe amino of aand few aryl Mannich moieties bases of benzimidazole of 1-(N-substituted did not amino)methyl-2-ethylbenzimidazoles influence the anti-inflammatory action in mice[49]. withThe SARs carrageenan-induced of the compounds rat are paw shown oedema. in Figure The amount 31. of compound administered was 100 mg/kg p.o. Compounds with 4-fluoro or bromo substituted aniline at R (Figure 30) showed substantial anti-inflammatory effects, with 23 and 25% inhibition of rat paw oedema. Moreover, 2- or 3-halogenated aniline was conducive to anti-inflammatory activity; this may be due to steric hindrance. Overall, electron-withdrawing groups with lip- ophilicity were preferable for activity [48]. The SARs of the compounds are shown in Figure 30.

Figure 30. SARs of Mannich bases of 1-(N-substituted amino)methyl-2-ethylbenz–imidazoles.

Pharmaceuticals 2021, 14, 663 18 of 32

Pharmaceuticals 2021, 14, 663 18 of 31

Figure 29. SARs of 3,5-disubstituted isoxazole at N1 of benzimidazole.

Mariappan and his co-researchers, inin 2011,2011, studiedstudied thethe anti-inflammatoryanti-inflammatory activityactivity ofof a fewfew MannichMannich basesbases ofof 1-(1-(NN-substituted-substituted amino)methyl-2-ethylbenzimidazolesamino)methyl-2-ethylbenzimidazoles inin micemice withwith carrageenan-inducedcarrageenan-induced ratrat pawpaw oedema.oedema. TheThe amountamount ofof compoundcompound administeredadministered waswas 100100 mg/kgmg/kg p.o. p.o. Compounds Compounds with with 4-fluoro 4-fluoro or or bromo bromo substituted aniline aniline at at R (Figure (Figure 3030)) showed substantial anti-inflammatory anti-inflammatory effects, effects, with with 23 23 and and 25% 25% inhibition inhibition of rat of ratpaw paw oe- oedema.dema. Moreover, Moreover, 2- 2-or or3-halogenated 3-halogenated aniline aniline was was conducive conducive to toanti-inflammatory anti-inflammatory activ- ac- tivity;ity; this this may may be bedue due to steric to steric hindrance. hindrance. Overall, Overall, electron-withdrawing electron-withdrawing groups groups with with lip- lipophilicityophilicity were were preferable preferable for for activity activity [48]. [48 The]. The SARs SARs of ofthe the compounds compounds are are shown shown in inFigure Figure 30. 30 .

Figure 30. SARs of Mannich bases of 1-(N-substituted1-(N-substituted amino)methyl-2-ethylbenz–imidazoles.amino)methyl-2-ethylbenz–imidazoles.

In 2010,2010, Gaba Gaba et al. studiedet al. the anti-inflammatorystudied the anti-inflammatory activity of 5-substituted-1-(pheny- activity of lsulfonyl)-2-methyl5-substituted-1-(phenylsulfonyl)-2-methyl benzimidazole derivatives benzimidazole using carrageenan-induced derivatives using paw carragee- oedema innan-induced a rat model paw at 200oedema mg/kg in bodya rat model weight. at Compounds200 mg/kg body substituted weight. withCompounds ortho amino sub- (37%stituted reduction) with ortho and amino para amino (37% reduction) (39.7% reduction) and para groups amino at (39.7% R (Figure reduction) 31) exhibited groups good at R anti-inflammatory(Figure 31) exhibited activity good withanti-inflammatory low ulcerogenic activity potential with due low to ulcerogenic being neutral potential molecules due bearingto being –NH neutral2. The molecules introduction bearing of –CH –NH2 between2. The introduction the amino andof –CH aryl2 moietiesbetween of the benzimi- amino Pharmaceuticals 2021, 14, 663 19 of 32 dazoleand aryl did moieties not influence of benzimidazole the anti-inflammatory did not influence action [the49]. anti-inflammatory The SARs of the compounds action [49]. areThe shown SARs of in the Figure compounds 31. are shown in Figure 31.

Figure 31.31. SARs of 5-substituted-1-(phenylsulfonyl)-2-methylbenzimidazoles.5-substituted-1-(phenylsulfonyl)-2-methylbenzimidazoles.

Dunwel and his research teamteam triedtried toto evaluateevaluate thethe anti-inflammatoryanti-inflammatory effects ofof aa new seriesseries ofof compounds compounds synthesised synthesised by by the th bio-isosterice bio-isosteric replacement replacement of benzoxazole of benzoxazole with benzimidazolewith benzimidazole and benzothiazole, and benzothiazole, in the in context the context of carrageenan-induced of carrageenan-induced paw oedema paw oe- in rats.dema The in rats. resulting The resulting compounds compounds did not reduce did no ratt reduce paw oedema;rat paw oedema; this may this be due may to be lower due solubilityto lower solubility or altered or drug–receptor altered drug–receptor interactions interactions (Figure 32)[ (Figure50]. Many 32) researchers [50]. Many have research- tried toers develop have tried novel to anti-inflammatorydevelop novel anti-inflamma agents by introducingtory agents substituentsby introducing simultaneously substituents atsimultaneously C2 and C5 of at benzimidazole. C2 and C5 of benzimidazole. Evans et al., Evans in 1996, et al., taking in 1996, benoxaprofen taking benoxaprofen as a lead, synthesisedas a lead, synthesised 72 benzimidazoles 72 benzimidazoles and evaluated and evaluated their anti-inflammatory their anti-inflammatory activities activities in rat adjuvantin rat adjuvant arthritis arthritis [51]. Only [51]. two Only compounds—a two compounds—a compound compound with a diethylaminowith a diethylamino ethoxy moietyethoxy moiety at the 6-position at the 6-position and a compound and a compound bearing bearing a substitution a substitution with 1-methoxy with 1-methoxy ethyl at C5ethyl of benzimidazole—wereat C5 of benzimidazole—were comparable comparable to indomethacin, to indomethacin, with 43 and with 33% 43 improvement, and 33% im- respectivelyprovement, respectively (Figure 33). In(Figure 2004, Terzioglu33). In 2004 et, al.Terzioglu introduced et al. VUF6002 introduced (Figure VUF6002 34), a (Figure potent 34), a potent anti-inflammatory agent with strong antinociceptive effects, using JNJ7777120 (Figure 34) as a lead [52].

Figure 32. SARs of bio-isosteric replacement of benzoxazole.

However, introducing an amide moiety instead of a carbonyl group between the C2 position of benzimidazole (Figure 34) and piperazine resulted in VUF6007, failing to show any such activity. This result conflicts with a few reports indicating the significance of the benzimidazole C2-position amino group [53].

Pharmaceuticals 2021, 14, 663 19 of 32

Figure 31. SARs of 5-substituted-1-(phenylsulfonyl)-2-methylbenzimidazoles.

Dunwel and his research team tried to evaluate the anti-inflammatory effects of a new series of compounds synthesised by the bio-isosteric replacement of benzoxazole with benzimidazole and benzothiazole, in the context of carrageenan-induced paw oedema in rats. The resulting compounds did not reduce rat paw oedema; this may be due to lower solubility or altered drug–receptor interactions (Figure 32) [50]. Many researchers have tried to develop novel anti-inflammatory agents by introducing substituents simultaneously at C2 and C5 of benzimidazole. Evans et al., in 1996, taking benoxaprofen as a lead, synthesised 72 benzimidazoles and evaluated their anti-inflammatory activities Pharmaceuticals 2021, 14, 663 in rat adjuvant arthritis [51]. Only two compounds—a compound with a diethylamino 19 of 31 ethoxy moiety at the 6-position and a compound bearing a substitution with 1-methoxy ethyl at C5 of benzimidazole—were comparable to indomethacin, with 43 and 33% improvement, respectively (Figure 33). In 2004, Terzioglu et al. introduced VUF6002 (Figure anti-inﬂammatory34), a potent anti-inflammatory agent with strong agent antinociceptive with strong antinociceptive effects, using JNJ7777120effects, using (Figure 34) asJNJ7777120 a lead [52 (Figure]. 34) as a lead [52].

Pharmaceuticals 2021, 14, 663 20 of 32 Pharmaceuticals 2021, 14, 663 20 of 32 Figure 32. 32. SARsSARs of ofbio-isosteric bio-isosteric replacement replacement of benzoxazole. of benzoxazole.

Figure 33 33.. SARsSARs of of 4- 4-or or5-position-substituted 5-position-substituted benzimidazoles. benzimidazoles. Figure 33. SARs of 4- or 5-position-substituted benzimidazoles.

Figure 34. SARs of piperazinyl-substituted benzimidazoles. Figure 34 34.. SARsSARs of ofpiperazinyl-substituted piperazinyl-substituted benzimidazoles. benzimidazoles. In another study, Avanir Pharmaceuticals developed a compound with potent anti-IgEInHowever, another study, introducing Avaniractivity, anPharmaceuticals amide moiety developed insteadAVP ofa compounda carbonyl with group potent between13358 an- the C2 positionti-IgE[2-(4-adamantanecarboxamido)phenyl)- of benzimidazoleactivity, (Figure 34)N and-(pyridin-2-yl)-1 piperazineAVPH resulted-benzoimidazole-5-carboxam in VUF6007, failing13358 to show any[2-(4-adamantanecarboxamido)phenyl)-ide], suchbearing activity. an amide This linker; result IgE conflicts plays aN role-(pyridin-2-yl)-1 with in a various few reports inflammatoryH-benzoimidazole-5-carboxam indicating conditions the significance (Figure of the ide], bearing an amide linker; IgE plays a role in various inflammatory conditions (Figure benzimidazole35) [54]. C2-position amino group [53]. 35) [54]. In another study, Avanir Pharmaceuticals developed a compound with potent anti-IgE activity, AVP 13358 [2-(4-adamantanecarboxamido)phenyl)-N-(pyridin-2-yl)-1H-benzoimidazole-5-carboxamide], bearing an amide linker; IgE plays a role in various inflammatory conditions (Figure 35)[54].

Figure 35. SARs of adamantane-substituted benzimidazoles. Figure 35. SARs of adamantane-substituted benzimidazoles. The compound 2-cyclohexylamino-1(4-methoxyphenyl)benzimidazole exhibited potentThe anti-inflammatory compound 2-cyclohexylamino-1(4-me activity according to thoxyphenyl)benzimidazoleTaniguchi et al. (1993) [55]. exhibited The an- potentti-inflammatory anti-inflammatory activity of activitythe compounds according at 100to Taniguchimg/kg p.o. etdoses al. (1993)was evaluated [55]. The using anti-inflammatorythe carrageenan-induced activity of rat the paw compounds oedema moat 100del. mg/kg2-cyclohexylamino-1 p.o. doses was (4-methoxyevaluated using phe- thenyl) carrageenan-induced benzimidazole exhibited rat paw 53.2% oedema inhibiti moondel. of rat2-cyclohexylamino-1 paw oedema. Replacing (4-methoxy the amino phenyl)group benzimidazole with methylene exhibited at C2 53.2%of benzimidazole inhibition of significantly rat paw oedema. reduced Replacing the activity, the amino indi- groupcating thewith significance methylene ofat theC2 guanidineof benzimidazole fraction significantlyin the activity. reduced Moreover, the activity,a bulkier indi- aro- cating the significance of the guanidine fraction in the activity. Moreover, a bulkier aro-

Pharmaceuticals 2021, 14, 663 20 of 32

Figure 33. SARs of 4- or 5-position-substituted benzimidazoles.

Figure 34. SARs of piperazinyl-substituted benzimidazoles.

In another study, Avanir Pharmaceuticals developed a compound with potent anti-IgE activity, AVP 13358 Pharmaceuticals 2021, 14, 663 [2-(4-adamantanecarboxamido)phenyl)-N-(pyridin-2-yl)-1H-benzoimidazole-5-carboxam20 of 31 ide], bearing an amide linker; IgE plays a role in various inflammatory conditions (Figure 35) [54].

Figure 35. SARsSARs of of adamantane-substituted adamantane-substituted benzimidazoles. benzimidazoles.

The compoundcompound 2-cyclohexylamino-1(4-methoxyphenyl)benzimidazole 2-cyclohexylamino-1(4-methoxyphenyl)benzimidazole exhibited exhibited potent Pharmaceuticals 2021, 14, 663 anti-inflammatorypotent anti-inflammatory activity activity according according to Taniguchi to Taniguchi et al. (1993) et al. [55 ].(1993) The anti-inflammatory[55]. The an-21 of 32

activityti-inflammatory of the compounds activity of the at 100compounds mg/kg p.o. at 100 doses mg/kg was p.o. evaluated doses was using evaluated the carrageenan- using the carrageenan-induced rat paw oedema model. 2-cyclohexylamino-1 (4-methoxy phe- Pharmaceuticals 2021, 14, 663 induced rat paw oedema model. 2-cyclohexylamino-1 (4-methoxy phenyl) benzimidazole21 of 32 nyl) benzimidazole exhibited 53.2% inhibition of rat paw oedema. Replacing the amino maticexhibited substitution 53.2% inhibition at the N1 of position rat paw oedema.of benzimidazole Replacing was the aminonot suitable group for with activity methylene (Fig- group with methylene at C2 of benzimidazole significantly reduced the activity, indi- ureat C2 36). of Furthermore, benzimidazole they significantly extended their reduced study the to activity, designing, indicating synthesising the significance and evaluat- of thecating guanidine the significance fraction of in the the guanidine activity. frac Moreover,tion in the a bulkieractivity. aromaticMoreover, substitution a bulkier aro- at the ingmatic the substitution anti-inflammatory at the N1 positionactivity ofof benz 2-iminocycloheptimidazoleimidazole was not suitable derivativesfor activity (Fig-and re- N1 position of benzimidazole was not suitable for activity (Figure 36). Furthermore, they portedure 36). thatFurthermore, these compounds they extended showed their inferiorstudy to anti-inflammatorydesigning, synthesising action and compared evaluat- to extended their study to designing, synthesising and evaluating the anti-inflammatory benzimidazoleing the anti-inflammatory analogues. Theyactivity concluded of 2-iminocycloheptimidazole that the substitution of derivatives hydrophilic and groups re- at theactivityported N1 positionthat of 2-iminocycloheptimidazolethese of compoundscycloheptimidazole showed resultedinferior derivatives anti-inflammatoryin a loss and of reported activity, action that but these4-methoxyphenyl compared compounds to showed inferior anti-inflammatory action compared to benzimidazole analogues. They substitutionbenzimidazole at analogues. the C2 Theyposition concluded of cycloheptimidazole that the substitution ofdid hydrophilic not reduce groups the at an- concluded that the substitution of hydrophilic groups at the N1 position of cycloheptimi- ti-inflammatorythe N1 position of potential. cycloheptimidazole resulted in a loss of activity, but 4-methoxyphenyl dazolesubstitution resulted at inthe a lossC2 ofposition activity, butof 4-methoxyphenylcycloheptimidazole substitution did not reduce at the C2 the position an- of cycloheptimidazoleti-inflammatory potential. did not reduce the anti-inflammatory potential.

Figure 36. SARs of 1,2-disubstituted benzimidazoles. Figure 36 36.. SARsSARs of of 1,2-disubstituted 1,2-disubstituted benzimidazoles. benzimidazoles. Shen et al. (2010) incorporated the imidazole nucleus with the phenyl ring of an active metaboliteShen et et al. al. (2010)isolated (2010) incorporated incorporatedfrom Curvularia the theimidazole verruculo, imidazole nucleus nucleusa synthetic with the with analoguephenyl the phenyl ring of off152A1. ringan ac- of The an syntheticactivetive metabolite metabolite analogue isolated isolated had from the from Curvulariain Curvulariavitro inhibitory verruculo, verruculo, effect a synthetic aof synthetic f152A1 analogue analogueon TNF- of f152A1.α of transcription f152A1. The The andsyntheticsynthetic showed analogue good therapeutic had the inin vitrovitroeffects inhibitoryinhibitory in arthritis effecteffect [56]. ofof f152A1f152A1 onon TNF-TNF-αα transcriptiontranscription and showedand showed good good therapeutic therapeutic effects effects in arthritisin arthritis [56 [56].].

Conjugated benzimidazolesbenzimidazoles of of 2-methyl- 2-methyl-N-sugarN-sugar (Figure (Figure 37) 37)were were synthesised synthesised and and evaluatedevaluated forfor analgesicanalgesic and and anti-inflammatory anti-inflammatory activity activity by byEl-Nezhawy El-Nezhawy et al. et in al. 2009 in 2009 [57]. [57]. TheThe anti-inflammatory activity activity of of the the compou compoundsnds at at100 100 mg/kg mg/kg p.o. p.o. doses doses was was evaluated evaluated usingusing the carrageenan-inducedcarrageenan-induced rat rat paw paw oedema oedema model. model. The The effects effects of theof thesystemic systemic ad- ad- ministrationministration ofof eacheach of of the the test test drugs drugs at at doses doses of of15, 15, 30 30and and 60 60mg/kg mg/kg (0.5 (0.5 mL, mL, i.p., i.p.,n = n = 6/group)6/group) werewere studied;studied; they they found found that that the the methoxy methoxy connector connector between between the theN of N ben- of ben- zimidazolezimidazole andand thethe hydroxyhydroxy groupgroup of of the the sugar sugar was was essential essential for forpotent potent anan- ti-inflammatoryti-inflammatory action.action. A A sugar sugar moiety moiety with with free free hydroxy hydroxy groups groups has has been been reported reported to to resultresult in greatergreater potency potency than than a aprotected protected or or derivatised derivatised moiety. moiety. A sugar A sugar connected connected at its at its positionposition N3N3 withwith benzimidazolebenzimidazole was was found found to toresult result in ingreater greater activity activity (67.8% (67.8% at 60 at 60 mg/kg)mg/kg) than thatthat bridged bridged at at position position 5 (33.3%5 (33.3% at at60 60mg/kg) mg/kg) (Figure (Figure 37). 37).

Pharmaceuticals 2021, 14, 663 21 of 31

Conjugated benzimidazoles of 2-methyl-N-sugar (Figure 37) were synthesised and evaluated for analgesic and anti-inflammatory activity by El-Nezhawy et al. in 2009 [57]. The anti-inflammatory activity of the compounds at 100 mg/kg p.o. doses was evaluated using the carrageenan-induced rat paw oedema model. The effects of the systemic administration of each of the test drugs at doses of 15, 30 and 60 mg/kg (0.5 mL, i.p., n = 6/group) were studied; they found that the methoxy connector between the N of benzimidazole and the hydroxy group of the sugar was essential for potent anti-inflammatory action. A sugar moiety with free hydroxy groups has been reported to result in greater potency than a

Pharmaceuticals 2021, 14, 663 protected or derivatised moiety. A sugar connected at its position N3 with benzimidazole22 of 32 was found to result in greater activity (67.8% at 60 mg/kg) than that bridged at position 5 (33.3% at 60 mg/kg) (Figure 37). Pharmaceuticals 2021, 14, 663 22 of 32

Figure 37 37.37. .SARs SARs of of sugar-linked sugar-linked benzimidazoles. benzimidazoles.benzimidazoles.

AA seriesseries of of of 1,2,6-trisubstituted 1,2,6-trisubstituted 1,2,6-trisubstituted benzimid benzimidazoles benzimidazolesazoles were were synthesiwere synthesised synthesised andsed and evaluated and evaluated evaluated for for anti- for inﬂammatoryanti-inflammatory activity activity activity by by Thakurdesai byThakurdesai Thakurdesai et et al. al. inet in 2007 al.2007 in [ [58].582007]. The [58]. C2 The position position C2 position was was sub- substituted was sub- withstituted various withwith carboxylicvarious various carboxylic carboxylic acids, whereas acids, acids, wh theereas wh C6ereas positionthe C6 the position was C6 substitutedposition was substituted was with substituted electron-rich with with or poorelectron-rich groups. or Theor poor poor activity groups. groups. mainly The The activity depends activity ma oninly ma the dependsinly groups depends on substituted the ongroups the at groupssubstituted C6 of benzimidazole. substituted at at TheC6 ofof activity benzimidazole.benzimidazole. was inversely The The activity relatedactivity was to wasinvers the invers lengthely relatedely of therelated to linker the tolength betweenthe oflength the the linker of carboxyl the be- linker group be- andtween C2 thethe of carboxyl benzimidazole carboxyl group group and (Figure and C2 ofC2 38 benzimid ).of The benzimid substitutionazole azole(Figure of(Figure 38). the benzylThe 38). substitution The group substitution at of the the 1-position of the benzyl group at the 1-position enhanced the anti-inflammatory action. enhancedbenzyl group the anti-inﬂammatoryat the 1-position enhanced action. the anti-inflammatory action.

Figure 38 38.. SARsSARs of of 1,2,6-trisubstituted 1,2,6-trisubstituted benzimidazoles. benzimidazoles.

FigureTheThe 38 .anti-inflammatory anti-inflammatorySARs of 1,2,6-trisubstituted activity activity displayed benzimidazoles. displayed by bybenzimidazole benzimidazole compounds compounds bearing bearing dif- ferentdifferent substituents substituents hashas encouragedencouraged rese researchersarchers to toconcentrate concentrate on onfused fused benzimidaz- benzimidazoles. Tojaoles. etTheToja al. (1984)etanti-inflammatory al. (1984) first reportedfirst reported theactivity anti-inflammatory the anti-inflammatorydisplayed by activitybenzimidazole activity of fused of fused imidazolecompounds imidazole derivatives bearing substitutedderivativesdifferent substituents substituted at the 1- and athas the 2-positions encouraged 1- and 2-positions (Figure researchers 39 (Figure), which to concentrate39), were which non-acidic were on fusednon-acidic agents benzimidaz- [ 59]. The agents [59]. The anti-inflammatory activity of the compounds at 100 mg/kg p.o. doses anti-inflammatoryoles. Toja et al. (1984) activity first of reported the compounds the anti-inflammatory at 100 mg/kg p.o. activity doses was of fused evaluated imidazole using was evaluated using the carrageenan-induced rat paw oedema method. The SARs of the thenaphthimidazolesderivatives carrageenan-induced substituted revealed at that rat the paw4-methoxyphe 1- oedemaand 2-positionsnyl method. substitution (Figure The SARs at 39),the of 2-positionwhich the naphthimidazoles were of the non-acidic imidazoleagents [59]. nucleus The favouredanti-inflammatory anti-inflammatory activity activity of the (45–50% compounds paw oedema at 100 inhibition), mg/kg p.o. doses whilewas evaluated a chloro- or using hydroxy-subs the carrageenan-inducedtituted phenyl moiety rat at paw this oedemaposition reducedmethod. the The activ- SARs of the itynaphthimidazoles (1–20% paw oedema revealed inhibition). that 4-methoxyphenyl substitution at the 2-position of the imidazole nucleus favoured anti-inflammatory activity (45–50% paw oedema inhibition), while a chloro- or hydroxy-substituted phenyl moiety at this position reduced the activity (1–20% paw oedema inhibition).

Pharmaceuticals 2021, 14, 663 22 of 31

Pharmaceuticals 2021, 14, 663 23 of 32

Pharmaceuticals 2021, 14, 663 23 of 32 revealed that 4-methoxyphenyl substitution at the 2-position of the imidazole nucleus favoured anti-inﬂammatory activity (45–50% paw oedema inhibition), while a chloro- Pharmaceuticals 2021, 14, 663 23 of 32 or hydroxy-substituted phenyl moiety at this position reduced the activity (1–20% paw oedema inhibition).

Figure 39. SARs of 1,2-substituted naphthimidazoles. Figure 39. SARs of 1,2-substituted naphthimidazoles. In another study, benzimidazole–NSAID conjugates were screened for an- FigureFigure 39 39.. SARsSARs of of 1,2-substituted 1,2-substituted naphthimidazoles. naphthimidazoles. ti-inflammatoryIn another activity.study, Thebenzimidazole–NSAI synthesised conjugatesD conjugates matched thewere parent screened NSAIDs for in an-po- ti-inflammatorytency,In while anotheranother the study, activity. study,conjugates benzimidazole–NSAID benzimidazole–NSAI The with synthesised better anti conjD conjugatesoxidant ugatesconjugates activitymatched were were screenednotably thescreened parent forreduced anti-inﬂammatory forNSAIDs an- gastric in po- ul- tency,activity.cers.ti-inflammatory whileAmong The the synthesised activity.the conjugates synthesised The conjugates synthesised with betterbenzimidazole–NSAIDmatched conj antiugatesoxidant the matched parent activity the NSAIDs conjugates, parentnotably NSAIDs in reduced potency, benzimidazole– in po- gastric while theul- cers.conjugatesmesalaminetency, Amongwhile withthe showed conjugatesthe better synthesisedthe antioxidant most with potentbetter benzimidazole–NSAID activity antianti-inflammatoryoxidant notably activity reduced notably activity conjugates, gastric reduced [60]. ulcers. gastricbenzimidazole– Amongul- the cers. Among the synthesised benzimidazole–NSAID conjugates, benzimidazole– mesalaminesynthesised benzimidazole–NSAIDshowed the most potent conjugates,anti-inflammatory benzimidazole–mesalamine activity [60]. showed the mesalamine showed the most potent anti-inflammatory activity [60]. most potent anti-inﬂammatory activity [60].

In 2010, Achar et al. incorporated substituted anilines at the 2-position of 6-substitutedIn 2010,2010,2010, Achar Acharbenzimidazoles,Achar et et al. etal. incorporated al.incorporated incorporated and screened substituted substituted substituted their anilines anilinesanti-inflammatory atanilines theat 2-positionthe at2-position the activity of 2-position 6-substituted of in carra- of 6-substituted benzimidazoles, and screened their anti-inflammatory activity in carra- 6-substitutedbenzimidazoles,geenan-induced benzimidazoles, inflammation/oedema and screened theirand screened anti-inflammatory in rat hindtheir paws, anti-inflammatory activity performing in carrageenan-induced a SARactivity study in (Figure carrageenan-induced inflammation/oedema in rat hind paws, performing a SAR study (Figure geenan-inducedinflammation/oedema40). inflammation/oedemaN-(1H-benzimidazol-2-ylmethyl) in rat hind paws, in rat performing hind paws, a performing SAR studyaniline a (Figure SAR study 40). (FigureN-(1andH- 40). N-(1H-benzimidazol-2-ylmethyl)N H aniline and 40).benzimidazol-2-ylmethyl)N-(1 H-benzimidazol-2-ylmethyl)-3-chloroanilineN-(1H-benzimidazol-2-ylmethyl) aniline and -(1 -benzimidazol-2-ylmethyl)-3-chloroaniline showed potentaniline anti-inflammatory and showed(100%N-(1H-benzimidazol-2-ylmethyl)-3-chloroaniline at potent 100 mg/kg) anti-inflammatory activities compared (100% at to 100 nimesulideshowed mg/kg) activitiespotent (100% atanti-inflammatory compared 50 mg/kg) to [61]. nimesulide The in- N(100%-(1H -benzimidazol-2-ylmethyl)-3-chloroanilineat 100 mg/kg) activities compared to nimesulide (100%showed at 50potent mg/kg) [61].anti-inflammatory The in- (100%corporation at 50 mg/kg) of electron-taking [61]. The incorporation groups at th ofe electron-takingbenzimidazole groups 6-position at the reduced benzimidazole the an- (100%corporation at 100 of mg/kg) electron-taking activities groups compared at the tobenzimidazole nimesulide 6-position(100% at 50reduced mg/kg) the [61]. an- The in- 6-position reduced the anti-inflammatory activity. However, a meta chloro (40–50% paw corporationti-inflammatory of electron-takingactivity. activity. However, However, groups a meta a metaat chloro th echloro benzimidazole (40–50% (40–50% paw oedema paw6-position oedema inhibition) reduced inhibition) or the an- or oedema inhibition) or para methoxy group (30–40% paw oedema inhibition) at R resulted ti-inflammatorypara methoxymethoxy group groupactivity. (30–40% (30–40% However, paw paw oedema a metaoedema i nhibition)chloro inhibition) (40–50% at R resultat paw Red resultoedema in potented ininhibition) an-potent an-or in potent anti-inflammatory activity (Figure 40). parati-inflammatory methoxy activitygroup activity (Figure(30–40% (Figure 40). 40). paw oedema inhibition) at R resulted in potent anti-inflammatory activity (Figure 40). ForFor potent potent anti-inflammatory anti-inflammatory activity activity the molecule must have -H at 1st position the molecule must have -H at 1st positionSubstitution of m-Cl group results ofFor benzimidazole potent anti-inflammatory activity Substitution of m-Cl group results of benzimidazole potent activity the molecule must have -H at 1st position potent activity N Substitution of m-Cl group results of benzimidazole N Introductionpotent activity of o-Cl group shows 1 N HN R Introduction of o-Cl group shows R H NN reduction in activity R1 HN R reduction in activity H Introduction of o-Cl group shows N HN R Introduction of electron withdrawingR1 reduction in activity H Introduction of p-OCH3 group shows potent Introductiongroup at C5 of of phenyl electron ring withdrawingresults anti-inflammatoryIntroduction activity of p-OCH3 group shows potent groupin loss of at activity C5 of phenyl ring results anti-inflammatory activity Introduction of electron withdrawing in loss of activity Introduction of p-OCH3 group shows potent Figuregroup 40. at C5SARs of phenyl of substituted ring results anilines at C2 positionanti-inflammatory of 6-substituted benzimidazoles activity . FigureFigurein loss 40. of40. activity SARs ofof substitutedsubstituted anilinesanilines atat C2C2 positionposition ofof 6-substituted6-substituted benzimidazoles.benzimidazoles. The Pharmaceutical Research Centre of Kanebo Ltd. (Japan) developed a few Figure2-(2-pyridinyl)benzimidazoles 40. SARs of substituted anilinesby the isostericat C2 position replacement of 6-substituted of the thiazole benzimidazoles ring in thia-. TheThe PharmaceuticalPharmaceutical ResearchResearch Centre Centre of of Kanebo Kanebo Ltd. Ltd. (Japan) (Japan) developed developed a few a 2-(2-few pyridinyl)benzimidazolesbendazole, since thiabendazole by the has isosteric moderate replacement anti-inflammatory of the thiazole effects ring[62–64]. in thiabendazole, They 2-(2-pyridinyl)benzimidazolesThe Pharmaceutical Research by the Centre isosteric of replacementKanebo Ltd. of (Japan) the thiazole developed ring ina thia-few 2-(2-pyridinyl)benzimidazolesbendazole, since thiabendazole by has the moderateisosteric replacementanti-inflammatory of the thiazoleeffects [62–64]. ring in Theythia- bendazole, since thiabendazole has moderate anti-inflammatory effects [62–64]. They

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sincealso found thiabendazole that one of has the moderate developed anti-inflammatory compounds, 2-(5-ethyl-2-pyridinyl)benzimidazole effects [62–64]. They also found that(KB-1043),(KB-1043), one of showedshowed the developed betterbetter anti-inflammatoryanti-inflammatory compounds, 2-(5-ethyl-2-pyridinyl)benzimidazole activityactivity thanthan phenylbutazonephenylbutazone andand (KB-1043),tiaramide.tiaramide. showedKB-1043 betteralso showed anti-inflammatory slightly less activity gastrointest thaninalinal phenylbutazone irritationirritation andand and aa 2–3-fold-better2–3-fold-better tiaramide. KB-1043 thera-thera- alsopeutic showed index slightlythan the less reference. gastrointestinal A compound irritation with and a 6-ethyl-2-pyridinyl a 2–3-fold-better therapeutic moiety at the index C2 thanposition the reference.of benzimidazole A compound also withshowed a 6-ethyl-2-pyridinyl comparable activity moiety to KB-1043. at the C2 However, position of elec- ben- zimidazoletron-withdrawingtron-withdrawing also showed groupsgroups comparable atat C5C5 ofof benzimidazolebenzimidazole activity to KB-1043. resultedresulted However, inin aa lossloss electron-withdrawing ofof anti-inflammatoryanti-inflammatory groupsactivity at(Figure C5 of benzimidazole41). resulted in a loss of anti-inflammatory activity (Figure 41).

FigureFigure 41.41. SARs ofof 2-(2-pyridinyl)benzimidazoles.2-(2-pyridinyl)benzimidazoles.

RavindernathRavindernath andand Reddy Reddy (2017) (2017) reported reported the the anti-inflammatory anti-inflammatory activity activity of benzo[d]- of ben- imidazolylzo[d]imidazolyl tetrahydropyridine tetrahydropyridine carboxylates carboxylates in carrageenan-induced in carrageenan-induced inflammation/oedema inflamma- intion/oedemation/oedema rat hind paws. inin ratrat They hindhind found paws.paws. that TheyThey all founfoun the synthesisedd that all the compounds synthesised had compounds moderate anti-had inflammatorymoderate anti-inflammatory activity (0.18 toactivity 0.43 at (0.18 4 h, to volume 0.43 at of4 h, oedema volume (mL)) of oedema compared (mL)) to com- the standardpared to the drug standard diclofenac drug sodium diclofenac (0.60 sodium± 0.02 at(0.60 4 h; ± control, 0.02 at 4 3.25 h; control,± 0.03 at3.25 4 h, ± volume0.03 at 4 ofh, oedemavolume of (mL)). oedema An unsubstituted(mL)). An unsubstituted phenyl or ph phenylenyl or substituted phenyl substituted with 2-position with 2-position electron- 1 withdrawingelectron-withdrawing groups or groups 4-position or 4-posi electron-donatingtiontion electron-donatingelectron-donating groups at R groupsgroupsresulted atat R inR11 potentresultedresulted anti- inin 2 inflammatorypotent anti-inflammatory activity. Meanwhile, activity. Meanwhil an orthoe, phenolic an ortho substitution phenolic substitution at R favoured at R anti-22 fa-fa- inflammatoryvoured anti-inflammatory activity (Figure activity 42)[ 65(Figure]. 42) [65].

H N HS N IntroductionIntroduction of of phenyl phenyl moiety moiety NH O with C2 electron withdrawing OCH2CH3 or C4 electron donating group 2 3 shows potent anti-inflammatory 1 N 2 shows potent anti-inflammatory R1 N R2 activity. If the substitutions are opposite in position shows Replacement with phenyl or minor loss in activity oo-phenolic-phenolic group group enhances enhances N anti-inflammatory activity. NH o-chloro substitution HS showsshows moderate moderate action. action.

FigureFigure 42.42. SARs ofof benzo[d]imidazolylbenzo[d]imidazolyl tetrahydropyridinetetrahydropyridine carboxylates.carboxylates.

InIn anotheranother study,study, aaa fusedfusedfused pyrimido[1,2-a]benzimidazolepyrimido[1,2-a]bpyrimido[1,2-a]benzimidazole withwith phenylsulfonylphenylsulfonyl moietymoiety waswas evaluated for for its its effect effect on on anti-inflamma anti-inﬂammatorytorytory activity.activity. activity. AA compou Acompou compoundnd with with a fused a fused py- pyrimidorimido ring ring exhibited exhibited less less anti-inflammatory anti-inﬂammatory activity activity than than indomethacin indomethacin (Figure (Figure 43)43)[ [66].66]. BothBoth electropositiveelectropositive and and negative negative groups groups at theat the phenyl phenyl group, group, which which is directly is directly connected con- tonected the fused to the pyrimido fused pyrimido moiety, moiety, were tested. were tested.

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Figure 43. SARs of pyrimido[1,2-a]benzimidazoles. Figure 43.43. SARs of pyrimido[1,2-a]benzimidazoles. El-Nezhawy et al. (2013) reported the synthesis and anti-inflammatory activity of compoundsEl-Nezhawy with eta few al.al. (2013)pyrid-2-yl reported moieties the synthesis syntsubstitutedhesis andand and anti-inflammatoryanti-inflammatory polyhydroxy sugar activityactivity conju- ofof compoundscompoundsgated to the withwith N-benzimidazole a a few few pyrid-2-yl pyrid-2-yl moiety. moieties moieties substitutedThe substituted compounds and and polyhydroxy 2-methyl- polyhydroxyN sugar-((3,4-dimethoxy sugar conjugated conju- topyridin-2-yl)methyl)-1gated the Nto-benzimidazole the N-benzimidazoleH-benzimidazol-5-amine moiety. moiety. The compounds The compounds 2-methyl- 2-methyl-N-((3,4-dimethoxyN-((3,4-dimethoxy pyridin-and 2-yl)methyl)-11-(1,2,3,5-tetrahydroxy-a-pyridin-2-yl)methyl)-1H-benzimidazol-5-amineH-benzimidazol-5-amineD-mannofuranose)-5-(((3,4-dimethoxypyridin-2yl)methyl) and 1-(1,2,3,5-tetrahydroxy-a- D-mannofuranose)- and 5-(((3,4-dimethoxypyridin-2yl)methyl)1-(1,2,3,5-tetrahydroxy-a-amino)-2-methyl-1H-benz-imidazoleD-mannofuranose)-5-(((3,4-dimethoxypyridin-2yl)methyl) significantly amino)-2-methyl-1 reducedH -benz-imidazolethe inflammation significantly in the paw reducedoedemaamino)-2-methyl-1 themodel inflammation by H62-benz-imidazole and in72%, the pawrespectively, oedemasignificantly modelwhich reduced byis comparable 62 and the 72%, inflammation respectively,to the effect in whichofthe diclo- paw is comparableoedemafenac (73%). modelto These theby effect62 compounds and of 72%, diclofenac respectively,also showed (73%). Thesewhich significant compoundsis comparable antiulcerogenic also to showed the effectactivity. significant of diclo-They antiulcerogenicfenacalso showed (73%). Thesethat activity. electron-donating compounds They also also showed methoxyshowed that significantgroups electron-donating in antiulcerogenicthe pyrid-2-yl methoxy moietyactivity. groups mainly They in thealsocontributed pyrid-2-yl showed to that moietythe electron-donatingcompounds’ mainly contributed potency. methoxy A to benzimidazole the groups compounds’ in the bearing potency.pyrid-2-yl a polyhydroxy A benzimidazolemoiety mainly sugar bearingatcontributed position a polyhydroxy one to thehad compounds’ a potent sugar anti-inflammatory at positionpotency. oneA benzimidazole had effect, a potent with anti-inflammatory remarkablebearing a polyhydroxy anti-ulcer effect, activity sugar with remarkableat(Figure position 44) one[67]. anti-ulcer had a potent activity anti-inflammatory (Figure 44)[67]. effect, with remarkable anti-ulcer activity (Figure 44) [67].

Introduction of -CH3 or -OCH3 group results in reduced activity Introduction of -CH3 or -OCH3 group results in reduced activity

R2 Introduction of -OCH3 groups R3 at R1 and R2 position results in 2 Introduction of -OCH3 groups R 1 R3 R atpotent R1 and anti-inflammatory R2 position results activity in N R1 potent anti-inflammatory activity

1 2 3 N HN No substitution at R ,R and R N results in least anti-inflammatory1 2 3 HN No substitution at R ,R and R N CH3 resultsactivity in least anti-inflammatory N CH3 activity NR4 R4

Substitution of polyhydroxy sugar shows Substitutionremarkable anti-ulcer of polyhydroxy effect in sugar comparable shows to unsubstituted benzimidazole remarkable anti-ulcer effect in comparable to unsubstituted benzimidazole Figure 44.44. SARs of 5-aminopyridinyl substitutedsubstituted benzimidazoles.benzimidazoles. Figure 44. SARs of 5-aminopyridinyl substituted benzimidazoles. Earlier in 2002,2002, SodhiSodhi etet al.al. studiedstudied thethe anti-inflammatoryanti-inflammatory activity of tetrahydropyrimido[1,6-a]benzimidazol-1(2rimido[1,6-a]benzimidazol-1(2Earlier in 2002, Sodhi et al.H )-thione)-thionestudied derivatives the anti-inflammatory with 5050 mg/kgmg/kg activity p.o. of in tetrahydropy- the modelmodel ofof carrageenan-inducedrimido[1,6-a]benzimidazol-1(2 inflammation/oedemainflammation/oedemaH)-thione derivatives in in rat rat hind hind with paws. paws. 50 mg/kg One One of ofp.o. the in reported the model com- of pounds,carrageenan-inducedpounds, 3-methyl-8-nitro-3,4,4a,5-tetrahydropyrimido[1,6-a]benzimidazol-1(23-methyl-8-nitro-3,4,4a,5-tetr inflammation/oedemaahydropyrimido[1,6-a]benzimidazol-1(2 in rat hind paws. One of the reportedHH)–thione,)–thione, com- showedshowedpounds, comparablecomparable3-methyl-8-nitro-3,4,4a,5-tetr anti-inflammatoryanti-inflammatoryahydropyrimido[1,6-a]benzimidazol-1(2 activity (46.0, maximum % reduction ininH oedema))–thione,oedema) 1 toshowedto ibuprofen comparable (51.0, maximum maximum anti-inflammatory % % reduction reduction activity in in oedema). oedema). (46.0, maximum Electron-rich Electron-rich % reduction groups groups at in atR oedema)1 Randand R2, 2 2 Rtoand ,ibuprofen and an electron-withdrawing an electron-withdrawing (51.0, maximum %group reduction group at R at2 inwere R oedema).were unfavourable unfavourable Electron-rich for foranti-inflammatory groups anti-inflammatory at R1 and ac-R2, 1 activity,tivity,and an whereas electron-withdrawing whereas an an –NO –NO2 2groupgroup group at at R Rat1 resulted resultedR2 were in unfavourable moderate anti-inflammatoryanti-inflammatory for anti-inflammatory activityactivity ac- (Figuretivity,(Figure whereas 4545))[ [68].68]. an –NO2 group at R1 resulted in moderate anti-inflammatory activity (Figure 45) [68].

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Figure 45. SARs of tetrahydropyrimido[1,6-a]benzimidazol-1(2H)-thione. Figure 45.45. SARs of tetrahydropyrimido[1,6-a]benzimidazol-1(2H)-thione. Sondhi et al. (2010) reported the microwave-irradiation synthesis and anti-inflammatorySondhi etet al. activity (2010)al. (2010) reported of tricyclic reported the benzimid microwave-irradiation the miazoles.crowave-irradiation The anti-inflammatory synthesis andsynthesis anti-inflammatory activity and of an-the activityti-inflammatorycompounds of tricyclic was activity evaluated benzimidazoles. of tricyclic by using Thebenzimid anti-inflammatorythe meazoles.thod Theof carrageenan-inducedanti-inflammatory activity of the compounds activity inflamma- of was the evaluatedcompoundstion/oedema by was usingin rat evaluated thehind method paws. by ofTwo using carrageenan-induced of thethe symenthesisedthod of inflammation/oedemacompounds,carrageenan-induced one having ininflamma- a rat methyl hind paws.tion/oedemasubstituent Two ofat in theeach rat synthesised hindposition paws. R compounds,2 Twoand Rof3 the(39.4, sy one nthesisedmaximum having acompounds, % methyl reduction substituent one in oedema)having at each a andmethyl posi- an- 2 3 tionsubstituentother R havingand at R a each (39.4,hydroxy position maximum group R2 atand % R reduction 1R (39.2,3 (39.4, maximum maximum in oedema) % %reduction and reduction another in inoedema), having oedema) a displayed hydroxy and an- 1 groupotheranti-inflammatory having at R (39.2, a hydroxy maximum activity group comparable % reductionat R1 (39.2, to in thatmaximum oedema), of ibuprofen displayed % reduction (39.0, anti-inflammatory maximum in oedema), % displayedreduction activity comparableanti-inflammatoryin oedema) (Figure to that activity of46) ibuprofen [69]. comparable (39.0, maximum to that of %ibuprofen reduction (39.0, in oedema) maximum (Figure % reduction 46)[69]. in oedema) (Figure 46) [69].

FigureFigure 46.46. SARs ofof tricyclictricyclic benzimidazoles.benzimidazoles. Figure 46. SARs of tricyclic benzimidazoles. AnotherAnother seriesseries of of fused fused benzimidazole benzimidazole ring ring compounds, compounds, 1,2,4-triazolobenzimidazol 1,2,4-triazolobenzimidazol -3- yl-3-yl acetohydrazide Anotheracetohydrazide series derivatives, of derivatives, fused benzimidazole were were reported repo byringrted Mohammed compounds,by Mohammed et al.1,2,4-triazolobenzimidazol in et 2013. al. in The 2013. synthesised The syn- compounds-3-ylthesised acetohydrazide compounds exhibited derivatives,exhibited significant significant anti-inflammatorywere repo anrtedti-inflammatory by activity,Mohammed which activity, et wasal. whichin evaluated 2013. was The evalu- using syn- carrageenan-inducedthesisedated using compounds carrageenan-induced exhibited inflammation/oedema significant inflamma antion/oedema inti-inflammatory rat hind paws. in ratactivity, Two hind compounds whichpaws. wasTwo showed evalu- com- potentatedpounds using analgesic showed carrageenan-induced andpotent anti-inflammatory analgesic andinflamma anti-inflammatory (84.2 andtion/oedema 89.3, maximum (84.2in rat and %hind reductions89.3, paws. maximum inTwo oedema) %com- re- activity.poundsductions Theyshowed in oedema) also potent showed activity. analgesic GI safetyThey and comparablealso anti-inflammatory showed to GI that safety of (84.2 indomethacin comparable and 89.3, (78.8,maximumto that maximum of %indo- re- %ductionsmethacin reduction in(78.8, inoedema) oedema) maximum activity. [70 %]. reduction They also in showedoedema) GI [70]. safety comparable to that of indo- methacinInIn 2006,2006, (78.8, thethe maximum samesame researchersresearchers % reduction reportedreported in oedema) thethe anti-inflammatoryanti-inflammatory [70]. andand analgesicanalgesic activitiesactivities ofof aa fewfewIn 2006, 1-acyl-2-alkylthio-1,2,4-triazolo1-acyl-2-alkylthio-1,2,4-triazolo the same researchers reported [3,2-a]benzimidazole[3,2-a]benzimidazole the anti-inflammatory derivatives.derivatives. and analgesic AA compoundcompound activities containingofcontaining a few 1-acyl-2-alkylthio-1,2,4-triazoloN N-acetyl-2-isopropylthio-1,2,4-triazolo-acetyl-2-isopropylthio-1,2,4-triazolo [3,2-a]benzimidazole [3,2-a]benzimi-dazole [3,2-a]benzimi-dazole derivatives. exhibited exhibitedA compound the most the potentcontainingmost potent anti-inflammatory N anti-inflammatory-acetyl-2-isopropylthio-1,2,4-triazolo activity. activity. This study This highlightedstudy [3,2-a]benzimi-dazole highlighted the importance the importance exhibited of the aminoof the group at C2 of the benzimidazole nucleus for anti-inflammatory activity (Figure 47)[71]. mostamino potent group anti-inflammatoryat C2 of the benzimidazole activity. Thisnucleus study for highlightedanti-inflammatory the importance activity (Figure of the The compounds showed better GI safety profiles than indomethacin. The SARs of the amino47) [71]. group The compoundsat C2 of the showed benzimidazole better GI nucleus safety profilesfor anti-inflammatory than indomethacin. activity The (Figure SARs 1,2,4-triazolobenzimidazoles are depicted in Figure 47. 47)of the [71]. 1,2,4-triazolobenzimidazoles The compounds showed better are depicted GI safety in profilesFigure 47.than indomethacin. The SARs of the 1,2,4-triazolobenzimidazoles are depicted in Figure 47.

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Figure 47. SARs of 1,2,4-triazolobenzimidazoles.

Soni etet al.al. (2011)(2011) reportedreported the the anti-inflammatory anti-inflammatory activity activity of of N-substituted N-substituted benzimida- benzim- zoleidazole derivatives derivatives at doses at doses of 200 of mg/kg 200 mg/kg p.o. inp.o. the in carrageenan-induced the carrageenan-induced model model of inflamma- of intion/oedemaflammation/oedema in rat hind in paws.rat Onehind of thepaws. synthesised One of compounds, the synthesised 1-[1H-benzimidazol-1- compounds, yl(phenyl)methyl]-5-methyl-2-phenyl-1,2-dihydro-31-[1H-benzimidazol-1-yl(phenyl)methyl]-5-methyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one, showedH-pyrazol-3-o a higher log Pne, value showed and anti-inflammatorya higher log P value activity and anti-inflammatory (75.0, maximum % reductionactivity (75.0, in oedema) maximum than % cele- re- coxibduction (83.3, in oedema) maximum than % reduction celecoxib in(83.3, oedema). maximum As shown % reduction in Figure in 48 oedema)., a 2-aryl As substitution shown in atFigure the 5-methyl-substituted48, a 2-aryl substitution pyrazol-3-one at the 5-methyl-substituted at the benzimidazole-1-yl pyrazol-3-one methyl at the moiety benzim- en- hancedidazole-1-yl the anti-inflammatory methyl moiety enhanced activity, whereasthe anti-inflammatory an amide substitution activity, in whereas the same an position amide resultedsubstitution in moderate in the same activity position [72]. resulted in moderate activity [72].

Figure 48. SARs of benzimidazol-1-yl (phenyl)methyl derivatives.

Arora etet al. al. (2014) (2014) suggested suggested that that electron-deficient electron-deficient groups groups (–Cl or (–Cl –Br) favour,or –Br)whereas favour, electron-richwhereas electron-rich groups (–OCH groups3) reduce, (–OCH the3) anti-inflammatoryreduce, the anti-inflammatory potency of 2-acetamidoben- potency of zimidazoles.2-acetamidobenzimidazoles. They also stated They that also the amide stated linkagethat the in amide the molecule linkage in was the not molecule favourable was fornot anti-inflammatoryfavourable for anti-inflammatory activity (Figure activity49)[73]. (Figure 49) [73].

Introduction of electron-withdrawing groups (-Cl or -Br) results in enhanced anti- inflammatory potency, but electron-donating groups show a decrease in potency.

R O N N H N O H O

Replacement or removal of amide linker has an impact on the anti-inflammatory activity

Figure 49. SARs of 2-acetamidobenzimidazoles.

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In another study, Sharma et al. (2017) evaluated methanesulphonamido-benzimidazolesIn another study, Sharma for thei etr al. gastro-sparing (2017) evaluated anti-inflammatory methanesulphonamido-benzimidazoles effects using a carra- forgeenan-induced their gastro-sparing model anti-inflammatory of inflammation/oedema effects using in rat a carrageenan-inducedhind paws. Compounds model substi- of inflammation/oedematuted with n-hexyl, inn-pentyl rat hind and paws. n-butyl Compounds at N1 substitutedof benzimidazole with n-hexyl, showed n-pentyl better andan- n-butylti-inflammatory at N1 of benzimidazoleactivity (92.73,showed 95.64 and better 97.62, anti-inflammatory maximum % reduction activity (92.73,in oedema) 95.64 than and 97.62,rofecoxib maximum and indomethacin % reduction in(78.95 oedema) and 75.00, than rofecoxib maximum and % indomethacinreduction in oedema). (78.95 and All 75.00, the maximumsynthesised % reductioncompounds in oedema). were non-ulcerogenic All the synthesised at compounds the tested were doses. non-ulcerogenic The me- atthane-sulphonamido the tested doses. The substitution methane-sulphonamido at C5 of benzimidazole substitution is at essential C5 of benzimidazole for potent an- is essentialti-inflammatory for potent activity anti-inflammatory (Figure 50) [74]. activity (Figure 50)[74].

Sulphonamido function is essential for better anti-inflammatory activity. Replacement of sulphonamido group with amine or nitro group results in a decrease of the action potential.

HN N CH H3C 3 O2S N

Introduction of alkyl substitution shows Substitution with -H exhibits execellent anti-inflammatory activity. moderate action potential R Longer alkyl groups shows better activity than smaller groups (n-hexyl > n-pentyl > n-butyl).

FigureFigure 50.50. SARsSARs ofof methanesulphonamido-benzimidazoles.methanesulphonamido-benzimidazoles.

4.4. SummarySummary andand PerspectivesPerspectives TheThe substituentsubstituent groups groups in in benzimidazole benzimidazole favoured favoured for for potent potent anti-inflammatory anti-inflammatory activity ac- aretivity shown are inshown Figure in 51 .Figure For developing 51. For developing benzimidazoles benzimidazoles as anti-inflammatory as anti-inflammatory agents, substi- tutionsagents, atsubstitutions N1, C2, C5 and at C6N1, were C2, preferable. C5 and Specifically,C6 were preferable. an aryl/heteroaryl Specifically, substitution an ar- atyl/heteroaryl C5 or C6 enhances substitution the anti-inflammatory at C5 or C6 enhances activity, the while anti-inflammatory being unsubstituted activity, at C4 while and C7being is betterunsubstituted for anti-inflammatory at C4 and C7 activity.is better However,for anti-inflammatory an amine group activity. at C2 However, consistently an Pharmaceuticals 2021, 14, 663 enhanced the anti-inflammatory activity, while a methyl or ethyl linker29 betweenof 32 the N1 of amine group at C2 consistently enhanced the anti-inflammatory activity, while a methyl benzimidazole and the substituent groups was essential for potent anti-inflammatory activity. or ethyl linker between the N1 of benzimidazole and the substituent groups was essential for potent anti-inflammatory activity. Worldwide, NSAIDs have been successfully used to relieve pain and inflammation and continue to be used every day by thousands of people. There has been tremendous growth in the last two to three decades in the design and development of anti-inflammatory agents. However, safe and effective therapy for inflammatory conditions remains challenging in many ways. The chronic use of NSAIDs is linked to adverse effects on cardiovascular health. Therefore, a benzimidazole scaffold-containing compound may be used since it possesses analgesic and anti-inflammatory action along with angiotensin II-receptor-blocking activity. Benzimidazole derivatives act through different mechanisms, such as reducing cytokines, TRPV-1 antagonism, cannabinoid receptor agonism and FLAP inhibition. Hence, there is a possibility that a single benzimidazole derivative can be optimised to act through multiple pathways involved in pain and in- Figure 51.FigureSummary 51. Summary offlammation. substituents of substituentsat benzimidazole at benzimidazo nucleus favourablele nucleus for potentfavourable anti-inflammatory for potent activity.anti-inflammatory activity. Worldwide, NSAIDs have been successfully used to relieve pain and inflammation and This approachcontinue may to beprovide used every benefits day over by thousands the use of of combinations people. There of has analgesic been tremendous an- growth ti-inflammatoryin the drugs, last twowith to different three decades mechanisms in the design and andfewer development side effects. of A anti-inflammatory benzimid- agents. azole scaffoldHowever, may be the safe best and pharmacophore effective therapy for fordesigning inflammatory active small conditions molecules remains with challenging in good anti-inflammatorymany ways. effects The chronic based useon the of NSAIDs above-mentioned is linked to unique adverse properties. effects on cardiovascular health. Therefore, a benzimidazole scaffold-containing compound may be used since it possesses 5. Conclusionsanalgesic and anti-inflammatory action along with angiotensin II-receptor-blocking activity. Benzimidazole is a vital scaffold in medicinal chemistry because of its diverse biological activities. Benzimidazoles act via different mechanisms in treating numerous diseases, as discussed in the Introduction of this review. Omeprazole, lansoprazole, pantoprazole, albendazole, mebendazole, thiabendazole, astemizole, enviradene, candesartan, cilexitil and telmisartan are clinically approved drugs that contain benzimidazole nuclei. Several researchers have explored the synthesis, structure–activity relationships, QSARs, molecular modelling and other physicochemical and pharmacokinetic profiles of benzimidazoles. In our view, a complete understanding of the structural, physical and chemical properties of the benzimidazoles may help researchers to better determine their potential use in treating inflammation. However, some severe side effects associated with benzimidazoles can be identified and need to be rectified. Further research in this field is need using advanced techniques, such as QSAR analysis, pharmacophore mapping and docking studies, and known SAR information reported in the literature will bring about novel benzimidazoles with considerable scope for use. These methods can provide a much more direct picture of the structural features contributing to the SARs of benzimidazoles.

Author Contributions: All authors (R.V., A.R., R.K. and H.R.) have involved in collecting articles, writing manuscript, read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statements: Data sharing not applicable. Acknowledgments: The authors acknowledge AIMST University, Malaysia, and the Saveetha In- stitute of Medical and Technical Sciences, India, for providing the facilities needed to write the review. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Hansson, G.K.; Libby, P. The Immune Response in Atherosclerosis: A Double-Edged Sword. Nat. Rev. Immunol. 2006, 6, 508– 519. 2. Libby, P. Atherosclerosis: The New View. Sci. Am. 2002, 286, 46–55.

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Benzimidazole derivatives act through different mechanisms, such as reducing cytokines, TRPV-1 antagonism, cannabinoid receptor agonism and FLAP inhibition. Hence, there is a possibility that a single benzimidazole derivative can be optimised to act through multiple pathways involved in pain and inflammation. This approach may provide benefits over the use of combinations of analgesic anti- inflammatory drugs, with different mechanisms and fewer side effects. A benzimidazole scaffold may be the best pharmacophore for designing active small molecules with good anti-inflammatory effects based on the above-mentioned unique properties.

5. Conclusions Benzimidazole is a vital scaffold in medicinal chemistry because of its diverse biological activities. Benzimidazoles act via different mechanisms in treating numerous diseases, as discussed in the Introduction of this review. Omeprazole, lansoprazole, pantoprazole, albendazole, mebendazole, thiabendazole, astemizole, enviradene, candesartan, cilexitil and telmisartan are clinically approved drugs that contain benzimidazole nuclei. Several researchers have explored the synthesis, structure–activity relationships, QSARs, molecular modelling and other physicochemical and pharmacokinetic profiles of benzimidazoles. In our view, a complete understanding of the structural, physical and chemical properties of the benzimidazoles may help researchers to better determine their potential use in treating inflammation. However, some severe side effects associated with benzimidazoles can be identified and need to be rectified. Further research in this field is need using advanced techniques, such as QSAR analysis, pharmacophore mapping and docking studies, and known SAR information reported in the literature will bring about novel benzimidazoles with considerable scope for use. These methods can provide a much more direct picture of the structural features contributing to the SARs of benzimidazoles.

Author Contributions: All authors (R.V., A.R., R.K. and H.R.) have involved in collecting articles, writing manuscript, read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Acknowledgments: The authors acknowledge AIMST University, Malaysia, and the Saveetha Insti- tute of Medical and Technical Sciences, India, for providing the facilities needed to write the review. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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