Antiviral Activities of Oleanolic Acid and Its Analogues

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Review Antiviral Activities of Oleanolic Acid and Its Analogues

Vuyolwethu Khwaza, Opeoluwa O. Oyedeji and Blessing A. Aderibigbe *

Department of Chemistry, University of Fort Hare, Alice Campus, Alice 5700, Eastern Cape, South Africa; [email protected] (V.K.); [email protected] (O.O.O) * Correspondence: [email protected]; Tel.: +27-406022266; Fax: +08-67301846

Academic Editors: Patrizia Ciminiello, Alfonso Mangoni, Marialuisa Menna and Orazio Taglialatela-Scafati Received: 27 July 2018; Accepted: 5 September 2018; Published: 9 September 2018

Abstract: Viral diseases, such as human immune deficiency virus (HIV), influenza, hepatitis, and herpes, are the leading causes of human death in the world. The shortage of effective vaccines or therapeutics for the prevention and treatment of the numerous viral infections, and the great increase in the number of new drug-resistant viruses, indicate that there is a great need for the development of novel and potent antiviral drugs. Natural products are one of the most valuable sources for drug discovery. Most natural triterpenoids, such as oleanolic acid (OA), possess notable antiviral activity. Therefore, it is important to validate how plant isolates, such as OA and its analogues, can improve and produce potent drugs for the treatment of viral disease. This article reports a review of the analogues of oleanolic acid and their selected pathogenic antiviral activities, which include HIV, the influenza virus, hepatitis B and C viruses, and herpes viruses.

Keywords: HIV; inﬂuenza virus; HBV/HCV; natural product; triterpenoids; medicinal plant

1. Introduction Viral diseases remain a major problem for humankind. It has been reported in some reviews that there is an increase in the number of viral diseases responsible for death and morbidity around the world [1,2]. According to the recent release from the National Health Laboratory Service (NHLS), influenza kills approximately 6000–11,000 South Africans every year [3]. Although other treatments eradicate some pathogens, such as polio, mumps and smallpox, other viral diseases, such as the hepatitis C virus (HCV) and the human immunodeficiency virus (HIV), have proven difficult to combat using the conventional treatment approach. In addition, the increase in viral resistance to drugs, as well as the serious adverse effects of antiviral drugs, results in serious medical problems, particularly when drugs are administered in combination over a prolonged treatment period [4]. Thus, there is a great need to develop novel potential antiviral agents from different sources, such as medicinal plants or natural products, which have been used in many regions as antiviral agents for many years [5–7]. Natural products have been proven as the main source of biologically active compounds, and they are potentially useful for drug development [8]. Pentacyclic triterpenes (PTs) are the most significant group of phytochemicals synthesized from plants through cyclization of squalene, and are known as a large class of secondary plant metabolites that are constructed by isoprene (2-methylbutadiene) (C5H8) units [9]. Structurally, they contain 5- and 6-membered rings (A, B, C, D, and E). The carbon skeleton of PTs is divided into six different subgroups: Oleanane (1), ursane (2), friedelane (3), hopane (4), lupine (5), and gammacerane (6) (Figure1)[ 10]. It has been reported that approximately 20,000 triterpenoids exist in nature [11,12].

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E E E E E E C D C D C D C D C D C D B A A B A B B A A B A B 1 2 1 3 2 3

E E E E C E E D C D C D C C C B D D D A B B B A A A A B A B 4 5 6 4 5 6 FigureFigure 1. 1.Types Types ofof pentacyclicpentacyclic triterpenes structures. structures. Figure 1. Types of pentacyclic triterpenes structures. AntiviralsAntivirals are are compounds compounds which which preventprevent viral viral development. Some Some of of these these antivirals antivirals can can be be isolatedisolatedAntivirals from from sources, sources, are compounds such such as as plants, plants,which animals,preventanimals, viral bacteria development. or or fungi, fungi, whileSome while othersof others these can canantivirals be beobtained obtained can be by by chemicalisolatedchemical synthesis from synthesis sources, [13 [13,14 ,14such].] Antiviral. Antivi as plants,ral mechanisms mechanisms animals, bacteria identifiedidentified or fromfungi, from natural naturalwhile productsothers products can have have be shedobtained shed light light byon on howchemicalhow they they interact synthesis interact with with[13 the,14 the] viral. Antivi viral life rallife cycle, mechanisms cycle, such such as as viralidentified viral entry, entry, from replication, replication, natural products assembly, assembly, have and and shed release, release, light as on as well as targetinghowwell they as targeting ofinteract virus–host-specific with of virus the viral–host‑specific life interactions cycle, interactionssuch [5 ,as6]. viral The [5,6] entry, antiviral. The replication, properties antiviral assembly, properties of PTs haveand of r attractedelease, PTs have as the attentionwellattracted as of targeting the many attention researchers. of virusof many–host‑specific Pompei researchers. and interactions colleaguesPompei and [5,6] extracted colleagues. The glycyrrhizic a extractedntiviral properties glycyrrhizic acid (Figure of acid PTs2)( (figure have7) from attracted the attention of many researchers. Pompei and colleagues extracted glycyrrhizic acid (figure the2) crude (7) from extract the crude of Glycyrrha extract of glabra Glycyrrharoots, glabra and roots it was, and found it was to found be active to be against active against the herpes the herpes simplex 2) (7) from the crude extract of Glycyrrha glabra roots, and it was found to be active against the herpes virussimplex [15]. Chen virus et[15] al.,. alsoChen isolated et al., analso oleanane-type isolated an oleanane triterpene-type derivative triterpene known derivative as salaspermic known as acid simplexsalaspermic virus acid [15] (F.igure Chen 2) et(8 ) al.from, also Tripterygium isolated anwilfordii oleanane, which-type blocked triterpene the replication derivative of knownHIV in H9 as (Figure2)( 8) from Tripterygium wilfordii, which blocked the replication of HIV in H9 lymphocytes (IC50: salaspermiclymphocytes acid (IC (50F:igure 10 μM) 2) ([16]8) from. Toshihiro Tripterygium et al. wilfordiialso isolated, which betulinic blocked acid the replication(BA) (Figure of 2)HIV (9 )in from H9 10 µM) [16]. Toshihiro et al. also isolated betulinic acid (BA) (Figure2)( 9) from the bark and leaves of 50 lymphocytesthe bark and (ICleaves: 10 of μ SyzigiumM) [16]. Toshihiroclaviflorum et, which al. also showed isolated a significantbetulinic acid effect (BA against) (Figure HIV 2) (EC(9) 50from: 1.4 Syzigium claviflorum, which showed a significant effect against HIV (EC : 1.4 µM) [17]. Since then, theμM) bark [17] and. Since leaves then, of many Syzigium natural claviflorum PTs have, which been reportedshowed a to significant have antiviral effect50 activities against .HIV (EC50: 1.4 manyμM) natural [17]. Since PTs then, have many been natural reported PTs to ha haveve been antiviral reported activities. to have antiviral activities. X X

O O H HOOC H HOHOOC O HO O HOHOC O HO OO HOOC O OO HHOO 7 HO OH 7 OH

X X

H H H H H H X H H H O H X H HO O HO H HO 8 H 8 HO 9 H 9

X-COOH X-COOH Figure 2. Isolated antiviral compounds from plants. FigureFigure 2. 2.Isolated Isolated antiviral compounds compounds from from plants. plants.

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One natural natural PT PT is Oleanolic is Oleanolic acid acid(OA) (OA)(Figure (Figure 3), which3), whichpossess possesseses many interesting many interesting biological biologicalactivities, such activities, as antiviral such as [18] antiviral, anti-inflammatory, [18], anti-inﬂammatory, analgesic [18,19] analgesic, antibacterial [18,19], antibacterial [20,21], anti- [cancer20,21], anti-cancer[22–26], anti [-22oxidation–26], anti-oxidation [27,28], antimicrobial [27,28], antimicrobial [29–31], and [ 29cardioprotective–31], and cardioprotective activities [32] activities. Chen et [32 al].. Chenreported et al.that reported OA provides that OAextraordinary provides extraordinaryprotection against protection acute and against chronic acute liver and injury, chronic and liver can injury,be used and as an can oral be usedmedication as an oralfor the medication treatment for of the human treatment liver disorders of human [33] liver. OA disorders is isolated [33]. from OA ismore isolated than 1600 from different more than plant 1600 species different [33– plant37], and species is non [33-toxic–37], and and moderately is non-toxic water and-soluble moderately [19]. water-solubleTable 1 below [19 shows]. Table some1 below medicinal shows some plants medicinal contain plantsing OA containing as an active OA as constituent an active constituent,, and their andbiological their biological activities. activities. In this study In this we study review we reviewmodifications modiﬁcations of the basic of the oleanolic basic oleanolic acid structure acid structure that thathave have been been made. made.

30 29

19 20 21

12 18 22 17 11 OH 25 26 13 28 1 9 14 16 2 O 10 8 15

3 7 5 HO 27 4 6 10 24 23 Figure 3. Structure of oleanolic acid (OA). Table 1. Several plants where OA was reported, the plant parts used, and their biological activities. Table 1. Several plants where OA was reported, the plant parts used, and their biological activities. Plant Species (Family) Biological Activity Plant Parts Used References Plant Species (Family) Biological Activity Plant Parts Used References Oleaeuropaea L. (Oleaceae) Anticancer, antimicrobial, anti-diabetic Fruits and leaves [22,28,37,38] Oleaeuropaea L. (Oleaceae) Anticancer, antimicrobial, anti-diabetic Fruits and leaves [22,28,37,38] Fabiana imbricata R. et P. Fabiana imbricata R. et P. Antiviral, antitumor, and antihyperlipidemic Leaves and flowers [39] (Solanaceae) Antiviral, antitumor, and antihyperlipidemic Leaves and flowers [39] (Solanaceae) Syzygium aromaticum Antinociceptive, Anti-inflammatory, Syzygium aromaticum Antinociceptive, Anti-inflammatory, Flower budsFlower and leaves buds and [18,39,40] (Myrtaceae) antihypertensive, and antioxidant [18,39,40] (Myrtaceae) antihypertensive, and antioxidant leaves Ligustrum lucidum Ait Anti-inflammatory, antioxidative, Ligustrum lucidum Ait Anti-inflammatory, antioxidative, antiprotozoal, Fruits and leaves [41] (Oleaceae) antiprotozoal, antimutagenic, and anticancer Fruits and leaves [41] (Oleaceae) antimutagenic, and anticancer Viscum album (Santalaceae) Anti-tumor, analgesic, and anti-inflammatory Leaves and stems [19,41,42] Viscum album (Santalaceae) Anti-tumor, analgesic, and anti-inflammatory Leaves and stems [19,41,42] Phyllanthus amarus Phyllanthus amarus Anti-diabetes Leaves or aerial [43] (Phyllanthaceae) Anti-diabetes Leaves or aerial [43] (Phyllanthaceae) PunicaPunica granatum granatumL. (Punicaceae) L. Antioxidant activity Fruit [27] [27] Antioxidant activity Fruit Rosmarinus officinalis L. Anti-inflammatory, hepatoprotective, Leaves, flowers, (Punicaceae) [28] Rosmarinus(Lamiaceae) officinalis L. Antigastroprotective,-inflammatory, antiulcer hepatoprotective, stems, branches.Leaves, flowers, [28] Gentiana(Lamiaceae) lutea (Gentianaceae) gastroprotective, Antimicrobial antiulcer Dried root andstems, rhizome branches. [30] Gentiana lutea Anti-inflammatory, antioxidative, Dried root and L. camara (Verbenaceae) Antimicrobial Leaves and flowers [41] [30] (Gentianaceae) antiprotozoal rhizome L.Viburnum camara chingii(Verbenaceae(Asteraceae)) Anti-inflammatory, Antimicrobial antioxidative, antiprotozoal LeavesLeaves and flowers [44] [41] SiphonodonViburnum celastrineuschingii Anti-inflammatoryAntimicrobial Root bark, stemLeaves [45,46[44]] (Asteraceae)(Celastraceae) Siphonodon celastrineus Rosa laevigata (Rosaceae) Anti-inflammatoryAnti-inflammatory LeavesRoot bark, stem [47[]45,46] (Celastraceae) Fructus Ligustri Lucidi (FLL) Anti-hepatitis Leaves [18] Rosa laevigata (Rosaceae) Anti-inflammatory Leaves [47] Fructus Ligustri Lucidi Anti-hepatitis Leaves [18] Analogues(FLL) of Oleanolic Acid OA (10) has three active sites (i.e., the hydroxyl C-3 in ring A, the alkene C12-C13 in ring C and 1.1. Analogues of Oleanolic Acid carboxylic acid C-28), which can be modified in order to change its physical structure and improve its biologicalOA (10) effectshas three [47 active–49]. Many sites ( analoguesi.e., the hydroxyl of OA haveC-3 in been ring synthesized A, the alkene and C12 tested-C13 forin ring numerous C and biologicalcarboxylic activitiesacid C-28 [)50, which]. It has can been be modified reported in literatureorder to change that OA its is physical a good precursorstructure and molecule improve for semi-syntheticits biological effects modifications [47–49]. Many due to analogues its multiple of biological OA have properties,been synthesized availability, and tested and low for productionnumerous costbiological [51]. Nkeh-Chungagactivities [50]. It ethas al. been reported reported the methylationin literature andthat acetylationOA is a good of precursor OA originally molecule isolated for semi-synthetic modifications due to its multiple biological properties, availability, and low

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Molecules 2018, 2323, 2300x 44 ofof 1414 production cost [51]. Nkeh-Chungag et al. reported the methylation and acetylation of OA originally productionisolated from cost Syzygium [51]. Nkeh aromaticum-Chungag (clove) et al., reportedwhich resulted the methylation in two compounds and acetylation (Figure of 4 )OA (11 )originally and (12). fromisolatedBoth Syzygium compounds from Syzygium aromaticum exhibited aromaticum(clove), better (clove) which in vivo/in, resultedwhich vitro resulted in twoanti -in compoundsinflammatory two compounds (Figure and (F4 membraneigure)( 11) 4 and) (11-stabilizing ()12 and). Both(12). compoundsBothproperties compounds, respectively exhibited exhibited better, when inbetter vivocompared / inin vitro vivo/in to OAanti-inﬂammatory vitro[52]. anti-inflammatory and membrane-stabilizing and membrane- properties,stabilizing respectively,properties, respectively when compared, when to compared OA [52]. to OA [52].

OR H OR O H O H O O O H H O H 11 R = -CH3

11 R = -CH3 R = -H 12 12 R = -H Figure 4. 3-acetoxy, 28-methyloleanolic acid (11); 3-acetoxyoleanolic acid (12). Figure 4. 33-acetoxy,-acetoxy, 28-methyloleanolic28-methyloleanolic acid (11);); 3-acetoxyoleanolic3-acetoxyoleanolic acid (12(12).). Modification of OA has resulted in compounds with biological activity, such as antidiabetic activity.ModiﬁcationModification Yolanda et of of al. OA OA synthesized has has resulted resulted analogues in compounds in compounds of OA with (three with biological ethers biologicalactivity, and four activity such esters, as such antidiabetic on hydroxyl as antidiabetic activity. C3 in Yolandaactivity.ring A, three etYolanda al. esters synthesized et fromal. synthesized the analogues carboxyl analogues of group OA (three C -of28 ethers,OA and ( threecorresponding and fourethers esters and primary onfour hydroxyl esters alcohol on C3 hydroxyl), in derived ring A, C3 threefrom in estersringthe reduction A from, three the esters of carboxyl carboxylic from groupthe carboxylacid C-28, with and group LiAlH corresponding C4-. 28Cinnamoyl, and corresponding primary ester alcohol),(13) primaryand derivedethyl alcohol ether from ),(14 thederived) ( reductionFigure from 5) of carboxylic acid with LiAlH . Cinnamoyl ester (13) and ethyl ether (14) (Figure5) were found to thewere reduction found to of be carboxylic the most acidPTP4 -with1B inhibitors LiAlH4. .Cinnamoyl The in vitro ester inhibitory (13) and effect ethyl of ether compound (14) (Figure 13 was 5) beweresignificant the found most, and PTP-1B to beit substantially the inhibitors. most PTP lower The-1B in inhibitorsed vitro bloodinhibitory glucose. The in effectlevels vitro ofin inhibitory compoundvivo experiments effect13 wasof compoundwhen signiﬁcant, compared 13 and was to it substantiallysignificantOA. Compound, and lowered it substantially14 bloodexhibited glucose lower better levelsed blood inhibitoryin vivo glucoseexperiments activity levels in and when vivo selectivity experiments compared over to when OA. protein Compound compared-tyrosine 14to exhibitedOA.phosphatase Compound better 1B inhibitory( PTP14 -1B)exhibited, activitywith advanced better and selectivity inhibitory interaction over activity with protein-tyrosine site and B, in selectivity accordance phosphatase over with 1B proteindocking (PTP-1B),-tyrosine studies with advancedphosphatase[53]. interaction 1B (PTP- with1B), with site B,advanced in accordance interaction with dockingwith site studies B, in accordance [53]. with docking studies [53].

H H OH OH O H O 13 R = O RO H H 13 R = O RO 14 R = C H H 2 5 14 R = C H Figure 5. (3b) 5. -3-{[(2E)-3-phenylprop(3b)-3-{[(2E)-3-phenylprop-2-enoyl]oxy}olean-12-en-28-oic-2-enoyl]oxy}olean-12-en-28-oic acid2 (135 ), (3b)- 3-ethoxyolean acid (-1312),- (3b)-3-ethoxyolean-12-en-28-oic acid (14). Figureen-28-oic 5. (3b)acid- 3(14-{[(2). E)-3-phenylprop-2-enoyl]oxy}olean-12-en-28-oic acid (13), (3b)-3-ethoxyolean-12- Theen-28 modification-oic acid (14). of oleanolic acid also resulted in potent antibacterial agents. Hichri et al. The modification of oleanolic acid also resulted in potent antibacterial agents. Hichri et al. explored the effect of introducing an acyl substituent at the hydroxyl C-3 in ring A of OA. A sequence exploredThe the modification effect of introducing of oleanolic an acidacyl substituent also resulted at the in potenthydroxyl antibacterial C-3 in ring agents.A of OA. Hichri A sequence et al. of diverse triterpenic acid esters were prepared from oleanolic acid using suitable cyclic anhydrides, exploredof diverse the triterpenic effect of introducingacid esters were an acyl prepared substituent from atoleanolic the hydroxyl acid using C-3 in suitable ring A ofcyclic OA. anhydrides, A sequence acid chlorides, and N,N-dimethyl-4-aminopyridine (DMAP) as a catalyst (Figure6 and Table2)[ 29]. ofacid diverse chlorides triterpenic, and N ,acidN-dimethyl esters were-4-aminopyridine prepared from (DMAP) oleanolic as acid a catalyst using (suitableFigure 6 cyclic and Table anhydrides, 2) [29]. OA and its acylated analogues were screened for their antimicrobial activity against five fungal plant acidOA and chlorides its acylated, and N analogues,N-dimethyl were-4- aminopyridinescreened for their (DMAP) antimicrobial as a catalyst activity (Figure against 6 and five Table fungal 2 )plant [29]. pathogens, and two Gram-positive and two Gram-negative bacteria. Compound 15 with sulfur and OApathogens, and its acylatedand two analoguesGram-positive were andscreened two Gram for their-negative antimicrobial bacteria. activity Compound against 15 five with fungal sulfur plant and chlorine atom(s), ((3b)-3-((thiophene-2-carbonyl)oxy)-olean-12-en-28-oic acid, was found to be an pathogens,chlorine atom(s) and two, ((3b) Gram-3-((thiophene-positive and-2- carbonyl)oxy)two Gram-negative-olean -bacteria.12-en-28 -Compoundoic acid, was 15 foundwith sulfur to be and an effective antibacterial agent, and the most active antifungal compound. It exhibited good activity chlorineeffective aantibacterialtom(s), ((3b) agent-3-((thiophene, and the- 2most-carbonyl)oxy) active antifungal-olean-12 compound.-en-28-oic acid,It exhibited was found good to activity be an against A. niger, P. italicum, P. digitatum, A. flavus, and T. harzianum. effectiveagainst A. antibacterial niger, P. italicum, agent P., and digitatum the most, A. flavus,active andantifungal T. harzianum. compound. It exhibited good activity against A. niger, P. italicum, P. digitatum, A. flavus, and T. harzianum.

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H H H H H HH H H H OH OH HH H OH OH OH OH OOHH OOHH OH O O H OO O O O H H O X //DMX /ADPMAP H H OO O HH H XX///DDMMX A//ADPPMAP H H HO HO HH H HHOO HO H H RO RO H H RO RO H H HH RROO H H HH FigureFigure 6.. Modification 6 6.. ModificationModiﬁcation of OA of .. OA OA.. FigureFigureFigure 66... ModificationModification 6.. Modification ofof OAOA of... OA.. Table 2. Synthesis of oleanolic acid. TableT able2.. Synthesis 2. Synthesis of oleanolicleanolic of oleanolic acid.acid. acid. TTableableT 2able2... SynthesisSynthesis 2.. Synthesis ofof ooleanolicleanolicleanolic of oleanolicleanolic acid.acid.acid. acid.acid. 1 Compound R X 11 Yield1 (%) CompoundCompound R R X X YieldYield (%)(%) 1(%) 1 CompoundCompoundCompound RR R XX X YieldYieldYield (%)(%)(%) 111(%)(%) 11 O O OO O X 15 15 15 X X RCl RClRCl 98 98 98 1515 15 XX X RClRCl RCl 9898 98 X-S X-S X--S XX---SS X--S Cll Cl O O CClll Cll OO O 16 16 16 RCl RClRCl 94 94 94 1616 16 RClRCl RCl 9494 94 Ph Ph Ph Ph PPPhhh Ph Cll Cl O O CClll Cll OO O 17 17 17 RCl RClRCl 91 91 91 1717 17 RClRCl RCl 9191 91

O O Cll Cl OO O Cll Cll 18 18 18 CClll RCl RClRCl 95 95 95 1818 18 RClRCl RCl 9595 95 Cll Cll CClll Cll O O Cll OO O Cll Cll 19 19 CClll Cl RCl RCl 94 94 19 1919 19 RClRCl RClRCl 9494 94 94 Cll Cl CClll Cll O O O O O OO O O OOO OO O 20 20 OO OO OO O 92 92 20 2020 20 9292 92 92 HOOHCOOC HHOOOOHCCOOC HOOC O O OO O O O OO O 21 21 82 82 21 2121 21 O O 8282 82 82 OO O COOCHOOH COOCHOO H CCOOOOHH O O OO O O O O O OO O OO O 22 22 O O 91 91 22 2222 22 OO O 9191 91 91 COOCHOOH O O CCOOOOCHHO O H O O O OO O O O O OO O O OOO OO O 23 23 OO OO OO O 85 85 23 2323 23 8585 85 85 HOOHCOOC HHOOOOHCCOOC O O O O O OO OO O OO O OO OOO OO O 24 24 81 81 24 2424 24 8181 81 81 HOOHCOOC HHOOOOHCCOOC C ll Cll C Clll Cll 11 1 1 Obtained1 Obtained percentage percentage yield yield of product. of product. 111 Obtained Obtained11 Obtained1 Obtained percentagepercentage percentage percentage yieldyield yieldyield ofof product.product. of of product. product.

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2. Phytochemical S Studiestudies and Anti-ViralAnti-Viral ActivitiesActivities ofof OAOA

2.1. Anti Anti-HIV-HIV Activity Due to the inc increaserease in the the occurrence occurrence of of drug drug-resistant-resistant virus s strains,trains, the improvement improvement of effective treatments fo forr the HIV infection is dependent on the identificationidentification of novelnovel biomedicalbiomedical agents with low toxicity. Synthesis of oleanolic acid,acid, as well as other closely-relatedclosely-related triterpenes,triterpenes, such as betulinic acid and dihydrobetulinic acid,acid, has led to anti-HIVanti-HIV agents [[5353–55].]. Zhu et al.al. synthesized derivatives of OA.OA. These authors modifiedmodified the C12-C13C12-C13 double bond of OA yielding compound 25, which was 3-fold3-fold more active than OA. Esterification Esterification of 25 with anhydrides resulted in compounds 26–28, which were 5-fold5-fold moremore activeactive thanthan OAOA withwith 2828 showingshowing remarkableremarkable activityactivity [[56]56] (Figure(Figure7 ).7).

25 H H 26: HOOC H COOH COOH O 27 HOOC X O O

25-28 28 25-28 28 HOOC

O Figure 7 7.. Oleanolic derivatives with anti anti-HIV-HIV activity activity..

CompoundCompound 25 was further modiﬁedmodified by converting the C28-carboxylC28-carboxyl group to an aminomethyl group,group, resultingresulting in in compounds compound29s and29 and30, which30, which were greater were greater than 10-fold than more 10-fold active more when active compared when tocompared OA (Figure to OA8). (Figure 8).

Z Y X H

29 X Y O 29

Z Y X H

X- COOH X Y O X- COOH 30 Y- C=O Z- NH Z- NH Figure 8.8. Previously derived OA analogues. Yu et al. in their structure-activity relationship study of effective anti-HIV agents, synthesized and Yu et al. in their structure-activity relationship study of effective anti-HIV agents, synthesized evaluated new triterpene derivatives in vitro for antiviral activity. OA analogue compound 31 was and evaluated new triterpene derivatives in vitro for antiviral activity. OA analogue compound 31 inactive, while OA derivative 32 exhibited an EC50 value of 0.32 µM, indicating that OA is a promising was inactive, while OA derivative 32 exhibited an EC50 value of 0.32 μM, indicating that OA is a anti-HIV inhibitor [56,57]. These compounds are potential therapeutics that would beneﬁt from further promising anti-HIV inhibitor [56,57]. These compounds are potential therapeutics that would benefit studies in vivo (Figure9). from further studies in vivo (Figure 9).

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X O H O RO O 31 R= OH H O O O 32 R= OH

X- COOH Figure 9.9. PPreviouslyreviously modified modiﬁed anti-HIVanti-HIV triterpene derivatives.

Kashiwada et al. prepared several 3 3--O--acyl-ursolicacyl-ursolic acids acids and and evaluated evaluated their their anti anti-HIV-HIV activity. activity. The most potent compound indicated an EC value of 0.31 µM and a TI of 155.5 [58]. In another The most potent compound indicated an EC50 value of 0.31 µM and a TI of 155.5 [58]. In another report by Kashiwada et al. al.,, OA derivatives inhibited HIV HIV-1-1 replication in acutely infected H9 H9 cells cells.. OA-triterpenesOA-triterpenes isolated from the leaves of S. claviflorumclaviflorum exhibitedexhibited potent anti-HIVanti-HIV activity,activity, which further revealed thethe potentialpotential ofof OAOA derivativesderivatives forfor thethe treatmenttreatment ofof HIVHIV [[59,6059,60].]. 2.2. Anti-Influenza 2.2. Anti-Influenza The influenza virus is a lethal respiratory virus capable of triggering significant damage to the The influenza virus is a lethal respiratory virus capable of triggering significant damage to the population. Vaccines and antiviral agents are important for controlling the outbreak of influenza. The population. Vaccines and antiviral agents are important for controlling the outbreak of influenza. The research into plant-based drugs against the influenza virus is promising, as some plants have been research into plant-based drugs against the influenza virus is promising, as some plants have been proven to have anti-influenza properties, some of these include: Aster spathulifolius, Pinus thunbergia, proven to have anti-influenza properties, some of these include: Aster spathulifolius, Pinus thunbergia, Thuja orientalis [61], Allium fistulosum [62], Sambucus nigra [63], and Psidium guajava [64]. OA and other Thuja orientalis [61], Allium fistulosum [62], Sambucus nigra [63], and Psidium guajava [64]. OA and other molecules, such as chlorogenic acid baicalein and quercetin, are regarded as active constituents in molecules, such as chlorogenic acid baicalein and quercetin, are regarded as active constituents in traditional Chinese folk medicine, which have been proven to be effective antiviral agents according to traditional Chinese folk medicine, which have been proven to be effective antiviral agents according clinical data [65]. These molecules have been found to be potential neuraminidase inhibitors, which to clinical data [65]. These molecules have been found to be potential neuraminidase inhibitors, which could be helpful for anti-influenza medicine development [66]. Yang et al. also reported that OA works could be helpful for anti-influenza medicine development [66]. Yang et al. also reported that OA as a broad-spectrum entry inhibitor of influenza viruses [67]. Han et al. conjugated sialic acid with works as a broad-spectrum entry inhibitor of influenza viruses [67]. Han et al. conjugated sialic acid OA and its analogues (Figure9). In vitro evaluation of the compounds on the influenza A/WSN/33 with OA and its analogues (Figure 9). In vitro evaluation of the compounds on the influenza A/WSN/33 (H1N1) virus in MDCK cell culture revealed compound 33 as the most potent compound, with an IC50 (H1N1) virus in MDCK cell culture revealed compound 33 as the most potent compound, with an IC50 of of 41.2 µM. It acted as an influenza virus entry inhibitor by inhibiting the binding of the influenza 41.2 μM. It acted as an influenza virus entry inhibitor by inhibiting the binding of the influenza virus virus hemagglutinin protein to host cells [68]. In a similar report by Han et al., compounds 34 and hemagglutinin protein to host cells [68]. In a similar report by Han et al., compounds 34 and 35 displayed 35 displayed anti-influenza activity against the A/WSN/33 (H1N1) virus. C-5 acetylamide and C-9 anti-influenza activity against the A/WSN/33 (H1N1) virus. C-5 acetylamide and C-9 hydroxy of hydroxy of sialic acid were useful for their binding with hemagglutinin during viral entry into host sialic acid were useful for their binding with hemagglutinin during viral entry into host cells [69] cells [69] (Figure 10). (FigureMolecules 10). 2018, 23, x 8 of 14 X X X H N O OH O O O

O X-OH

HO 33

Figure 10. Cont. X X H X OMe N O COOMe N O O N N NH X HO X-OAc 34

X X H X OMe N O COOMe N O O N N NH X HO X-OAc 35 Figure 10. OA derivatives with anti-influenza activity.

2.3. Anti-Hepatitis Previous reports estimated that approximately 170 million people across the world are infected with the hepatitis C virus (HCV), which causes deaths globally [70]. Since the formation of Ribavirin (RBV) and PEGylated interferon alpha (IFN-α) as the standard drugs for the medical treatment of HCV in 1990, large efforts have been made to find new and more effective drugs. New effective HCV inhibitors belonging to direct-acting antivirals (DAAs), which include Boceprevir, Ledipasvir, Sofosbuvir, Telaprevir, Simeprevir, and Daclatasvir, have been developed in recent years [71]. Although great progress in anti-HCV remedies has been surely attained, many barriers nevertheless exist. Monotherapy with DAAs is linked with the rapid occurrence of drug-resistant viral mutations, thus the medical utility of DAAs is usually restricted to mixture regimens, which is associated with extra side effects, drug-drug interactions, high costs, and availability. Therefore, new therapeutic techniques consisting of various HCV inhibitors focused on particular stages of the HCV life cycle, with more effectiveness and wider availability, are still in demand to overcome those obstacles. Historically, many active modern drugs have been established from compounds initially isolated from plants, and now there is still more interest in finding new drugs from herbal sources for the treatment of various human diseases. Many natural compounds have been identified with antiviral effects globally, which includes anti-HCV activity. OA was an important compound in many traditional Chinese hepatic protective drugs, and was considered to play a therapeutic role in many diseases. Indeed, Plantago major L., a well-known traditional Chinese remedy, has been used for the treatment of many diseases including the cold and viral hepatitis [72]. OA and its isomer, ursolic acid, were identified as active compounds of Fructus Ligustri Lucidi (FLL), and were indicated as two

Molecules 2018, 23, x 8 of 14

X X X H N O OH O O O

O X-OH

Molecules 2018, 23H,O 2300 33 8 of 14

X X H X OMe N O COOMe N O O N N NH X HO X-OAc 34

X X H X OMe N O COOMe N O O N N NH X HO X-OAc 35 FigureFigure 10 10.. OAOA derivatives derivatives with with anti anti-influenza-influenza activity activity.. 2.3. Anti-Hepatitis 2.3. Anti-Hepatitis Previous reports estimated that approximately 170 million people across the world are infected Previous reports estimated that approximately 170 million people across the world are infected with the hepatitis C virus (HCV), which causes deaths globally [70]. Since the formation of Ribavirin with the hepatitis C virus (HCV), which causes deaths globally [70]. Since the formation of Ribavirin (RBV) and PEGylated interferon alpha (IFN-α) as the standard drugs for the medical treatment of (RBV) and PEGylated interferon alpha (IFN-α) as the standard drugs for the medical treatment of HCV in 1990, large efforts have been made to find new and more effective drugs. New effective HCV in 1990, large efforts have been made to find new and more effective drugs. New effective HCV HCV inhibitors belonging to direct-acting antivirals (DAAs), which include Boceprevir, Ledipasvir, inhibitors belonging to direct-acting antivirals (DAAs), which include Boceprevir, Ledipasvir, Sofosbuvir, Telaprevir, Simeprevir, and Daclatasvir, have been developed in recent years [71]. Although Sofosbuvir, Telaprevir, Simeprevir, and Daclatasvir, have been developed in recent years [71]. great progress in anti-HCV remedies has been surely attained, many barriers nevertheless exist. Although great progress in anti-HCV remedies has been surely attained, many barriers nevertheless Monotherapy with DAAs is linked with the rapid occurrence of drug-resistant viral mutations, thus exist. Monotherapy with DAAs is linked with the rapid occurrence of drug-resistant viral mutations, the medical utility of DAAs is usually restricted to mixture regimens, which is associated with extra thus the medical utility of DAAs is usually restricted to mixture regimens, which is associated with side effects, drug-drug interactions, high costs, and availability. Therefore, new therapeutic techniques extra side effects, drug-drug interactions, high costs, and availability. Therefore, new therapeutic consisting of various HCV inhibitors focused on particular stages of the HCV life cycle, with more techniques consisting of various HCV inhibitors focused on particular stages of the HCV life cycle, effectiveness and wider availability, are still in demand to overcome those obstacles. Historically, many with more effectiveness and wider availability, are still in demand to overcome those obstacles. active modern drugs have been established from compounds initially isolated from plants, and now Historically, many active modern drugs have been established from compounds initially isolated there is still more interest in finding new drugs from herbal sources for the treatment of various human from plants, and now there is still more interest in finding new drugs from herbal sources for the diseases. Many natural compounds have been identified with antiviral effects globally, which includes treatment of various human diseases. Many natural compounds have been identified with antiviral anti-HCV activity. OA was an important compound in many traditional Chinese hepatic protective effects globally, which includes anti-HCV activity. OA was an important compound in many drugs, and was considered to play a therapeutic role in many diseases. Indeed, Plantago major L., traditional Chinese hepatic protective drugs, and was considered to play a therapeutic role in many a well-known traditional Chinese remedy, has been used for the treatment of many diseases including diseases. Indeed, Plantago major L., a well-known traditional Chinese remedy, has been used for the the cold and viral hepatitis [72]. OA and its isomer, ursolic acid, were identified as active compounds of treatment of many diseases including the cold and viral hepatitis [72]. OA and its isomer, ursolic acid, Fructus Ligustri Lucidi (FLL), and were indicated as two antiviral compounds that seriously suppressed were identified as active compounds of Fructus Ligustri Lucidi (FLL), and were indicated as two the duplication of the Hepatitis C Virus (HCV) genotype 1b replicon, and the HCV genotype 2a JFH1

virus. Furthermore, these compounds exhibited anti-HCV properties, partly by suppressing HCV NS5B RdRp properties, as noncompetitive inhibitors. Therefore, their results conﬁrmed that natural products of OA and ursolic acid are potential HCV antivirals [18]. Yan et al. (2006) reported interesting work on the synthesis of OA structures (Figure 11). These researchers proposed a series of OA analogues which were synthesized by various reactions. All tested analogues (38–41) inhibited the secretion of HBsAg, and also decreased the secretion of HBeAg [73]. Compound 39 showed major inhibition of HBV DNA duplication, which was signiﬁcant when compared to the reference drug. Due to its great performance in vivo and in vitro, compound 39 Molecules 2018, 23, x 9 of 14 antiviral compounds that seriously suppressed the duplication of the Hepatitis C Virus (HCV) genotype 1b replicon, and the HCV genotype 2a JFH1 virus. Furthermore, these compounds exhibited anti-HCV properties, partly by suppressing HCV NS5B RdRp properties, as noncompetitive inhibitors. Therefore, their results confirmed that natural products of OA and ursolic acid are potential HCV antivirals [18]. Yan et al. (2006) reported interesting work on the synthesis of OA structures (Figure 11). These researchers proposed a series of OA analogues which were synthesized by various reactions. All Moleculestested analogues2018, 23, 2300 (38–41) inhibited the secretion of HBsAg, and also decreased the secretion of HBeAg9 of 14 [73]. Compound 39 showed major inhibition of HBV DNA duplication, which was significant when compared to the reference drug. Due to its great performance in vivo and in vitro, compound 39 is a ispotential a potential novel novel anti anti-Hepatitis-Hepatitis B virus B virus drug drug candidate candidate,, with with a adifferent different mechanism mechanism of of action action,, which needs further investigation.investigation.

O 39 O H

(iv)

O R R R (i) O O (iii) O H R R R 36 R 37 H OA H H

(iii) (ii) (v)

R R O O R O H O O R O R H 41 H 38 O H 40

R= -OH Figure 11.11. Synthesis route to derivatives. Reagents Reagents and conditions:conditions: (i)(i) N-Bromosuccinimide-Bromosuccinimide ( (NBS),NBS), ◦ CCl4,, light, reﬂux,reflux, 44 h; h; (ii) (ii chromic) chromic acid acid solution, solution, acetone, acetone, 0 C,0 °C, 1 h; 1 (iii) h; ( Eosiniii) Eosin Y, dichloromethane, Y, dichloromethane, light, ◦ ◦ 10light, h; (iv)10 h; chromic (iv) chromic acid solution, acid solution, acetone, acetone, 0 C, 10 h;°C, (v) 1 chromich; (v) chromic acid solution, acid solution, acetone, acetone, 0 C, 10 h.°C, 1 h. 2.4. Anti-Herpes 2.4. Anti-Herpes Herpes simplex virus (HSV) has two serotypes (HSV-1 and HSV-2), which target the oral and Herpes simplex virus (HSV) has two serotypes (HSV-1 and HSV-2), which target the oral and genital mucous membranes of humans, create latent infections in the sensory neurons, and may genital mucous membranes of humans, create latent infections in the sensory neurons, and may reactivate to cause recurring infections at the primary site [74]. The effects of genital herpes as a human reactivate to cause recurring infections at the primary site [74]. The effects of genital herpes as a health threat are increasing because of its interrelationship with HIV. HSV has been well treated with human health threat are increasing because of its interrelationship with HIV. HSV has been well acyclovir since 1970 [75]. Some licensed anti-herpes virus drugs, such as ganciclovir, cidofovir, and treated with acyclovir since 1970 [75]. Some licensed anti-herpes virus drugs, such as ganciclovir, foscarnet, that destroy herpes virus DNA polymerases, also have toxicity in long-term usage [76]. cidofovir, and foscarnet, that destroy herpes virus DNA polymerases, also have toxicity in long-term Therefore, novel antiviral drugs from natural sources that have different mechanisms of action are in great demand. Mukherjee et al. isolated OA from methanol (MeOH) extract taken from Achyranthes aspera roots. The MeOH extract exhibited a weak anti-herpes virus effect (EC50 64.4 g/mL for Herpes simplex virus 1 (HSV-1) and 72.8 g/mL for Herpes simplex virus 2 (HSV-2)), whereas OA showed potent anti-herpes virus activity against both HSV-1 (EC50 6.8 g/mL) and HSV-2 (EC50 7.8 g/mL) [74]. Keda et al., also examined 15 oleanane-type triterpenoids, and their derivatives, for anti-herpes simplex virus type 1 (HSV-1) activities. OA and its derivative Hederagenin (Figure 12)(42) were found to exhibit moderate anti-HSV-1 activity [77]. Molecules 2018, 23, x 10 of 14 usage [76]. Therefore, novel antiviral drugs from natural sources that have different mechanisms of action are in great demand. Mukherjee et al. isolated OA from methanol (MeOH) extract taken from Achyranthes aspera roots. The MeOH extract exhibited a weak anti-herpes virus effect (EC50 64.4 g/mL for Herpes simplex virus 1 (HSV-1) and 72.8 g/mL for Herpes simplex virus 2 (HSV-2)), whereas OA showed potent anti-herpes virus activity against both HSV-1 (EC50 6.8 g/mL) and HSV-2 (EC50 7.8 g/mL) [74]. Keda et al., also examined 15 oleanane-type triterpenoids, and their derivatives, for anti- Moleculesherpes simplex2018, 23, 2300 virus type 1 (HSV-1) activities. OA and its derivative Hederagenin (Figure 1210) of(42 14) were found to exhibit moderate anti-HSV-1 activity [77].

OH H O H HO H 42 OH Figure 12.12. Hederagenin.

3. Conclusio Conclusionsns Triterpenoids areare anan important important group group of of compounds compounds widely widely found found in in nature. nature. Previous Previous reports reports on triterpenoidson triterpenoids showed showed that that OA OA and and its its analogues analogues have have countless countless beneficial beneficial effects,effects, suchsuch as antiviral, anti-inflammatory,anti-inflammatory, antitumorantitumor promotion,promotion, anticancer,anticancer, andand soso forth.forth. OA is relatively non-toxicnon-toxic and has been sold in in China China as as a a therapeutic therapeutic for for the the treatment treatment of ofhuman human hepatitis. hepatitis. There There are arefew few reports reports on the on theantiviral antiviral activities activities of OA of OA and and its its analogues. analogues. The The treatment treatment of of viral viral di diseasesseases continues continues to to present problems inin currentcurrent medicine, medicine with, with various various studies studies showing showing a great a great increase increase in the in occurrencethe occurrence of drug of resistantdrug resistant viruses viruses [78]. The [78] modification. The modification of the of hydroxyl the hydroxyl and carboxylic and carboxylic acid functional acid functional groups groups on OA hason OA resulted has resulted in potent in compounds potent compounds with antiviral with activity. antiviral The activity. anti-HIV The activity anti-HIV of the activity synthesized of the compoundssynthesized iscompounds via the inhibition is via the of HIV-1inhibition replication. of HIV- The1 replication. compounds The synthesized compounds also synthesized hindered virus also entryhindered byinhibiting virus entry the by bindinginhibiting of the influenza binding virus of influenza hemagglutinin virus hemagglutinin protein to the protein host cells.to the However,host cells. aHowever, thorough a thorough antiviral antiviral mode of mode action of of action these of compounds these compounds needs further needs further investigation. investigation. The initial The informationinitial information obtained obtained from in from vivo and in vivoin vitro and studies in vitro of studies OA derivatives of OA derivatives is promising. is promising Therefore,. furtherTherefore studies, further are needed studies intoare OA, needed and itsinto analogues OA, and from its analo naturalgues products, from natural as an antiviral products agent, as foran variousantiviral human agent for diseases. various human diseases.

Author Contributions:Contributions: V, Khwaza,Khwaza, BlessingBlessing AderibigbeAderibigbe and O.O. OyedejiOyedeji contributedcontributed substantially to the work reported. The authors approved the ﬁnalfinal versionversion ofof thethe manuscript.manuscript.

Funding:Funding: This This research research was was funded funded by South by South African African Medical Medical Research Research Council Council (Self-Initiated (Self-Initiated Research), Research), National Research Foundation South African and Govan Mbeki Research and Development Centre (GMRDC), University National Research Foundation South African and Govan Mbeki Research and Development Centre (GMRDC), of Fort Hare. University of Fort Hare. Acknowledgments: The authors acknowledge Govan Mbeki Research and Development Centre (GMRDC), UniversityAcknowledgments: of Fort Hare, The and authors NRF–Sasol acknowledge Inzalo for Govan financial Mbeki support. Research Medical and Development Research Council Centre (Self-Initiated (GMRDC), Research)University and of Fort National Hare, Research and NRF Foundation,–Sasol Inzalo South for financial Africa are support. also acknowledged Medical Research for their Council financial (Self support.-Initiated ConflictsResearch) ofand Interest: NationalThe Research authors Foundation, declare no conflict South Af of interestrica are also regarding acknowledged the publication for their of thisfinancial paper. support. Conflicts of Interest: The authors declare no conflict of interest regarding the publication of this paper. References References 1. Rumlová, M.; Ruml, T. In vitro methods for testing antiviral drugs. Biotechnol. Adv. 2018, 36, 557–576. 1. Rumlová,[CrossRef][ MPubMed.; Ruml, ]T. In vitro methods for testing antiviral drugs. Biotechnol. Adv. 2018, 36, 557–576. 2. Nováková,Nováková, L.; PavlPavlík,ík, J.; Chrenková, Chrenková, L.; Martinec, O.; Červený,Cervenˇ ý, L. Current antiviral drugs and their analysis in biologicalbiological materials—Partmaterials—Part I: Antivirals against respiratory and herpes viruses.viruses. J.J. Pharm.Pharm. Biomed. Anal. 20120188,, 147, 400–416.400–416. [CrossRef][PubMed] 3. National Health Laboratory Service. Available online: http://www.nhls.ac.za/?page=alerts&id=5&archive= 2016&rows=5&pager=5 (accessed on 5 November 2017). 4. Kitazato, K.; Wang, Y.; Kobayashi, N. Viral infectious disease and natural products with antiviral activity. Drug Discov. Ther. 2007, 1, 14–22. [PubMed] 5. Ganjhu, R.K.; Mudgal, P.P.; Maity, H.; Dowarha, D.; Devadiga, S.; Nag, S.; Arunkumar, G. Herbal plants and plant preparations as remedial approach for viral diseases. VirusDisease 2015, 26, 225–236. [CrossRef] [PubMed] Molecules 2018, 23, 2300 11 of 14

6. Ahmed-Belkacem, A.; Ahnou, N.; Barbotte, L.; Wychowski, C.; Pallier, C.; Brillet, R.; Pohl, R.; Pawlotsky, J. Silibinin and Related Compounds Are Direct Inhibitors of Hepatitis C. Gastroenterology 2010, 138, 1112–1122. [CrossRef][PubMed] 7. Morishima, C.; Shuhart, M.C.; Wang, C.C.; Paschal, D.M.; Apodaca, M.C. Silymarin Inhibits In Vitro T-Cell Proliferation and Cytokine Production in Hepatitis C Virus Infection. Gastroenterology 2010, 138, 671–681. [CrossRef][PubMed] 8. Dinh, T.; Moons, N.; Kim, Y.; Borggraeve, W.D.; Mashentseva, A.; Andrei, G.; Snoeck, R.; Balzarini, J.; Dehaen, W. Synthesis of triterpenoid triazine derivatives from allobetulone and betulonic acid with biological activities. Bioorg. Med. Chem. 2014, 22, 3292–3300. [CrossRef][PubMed] 9. Ruzicka, L. The isoprene rule and the biogenesis of terpenic compounds. Experientia 1953, 9, 357–367. [CrossRef][PubMed] 10. Xiao, S.; Tian, Z.; Wang, Y.; Si, L.; Zhang, L.; Zhou, D. Recent progress in the antiviral activity and mechanism study of pentacyclic triterpenoids and their derivatives. Med. Res. Rev. 2018, 38, 1–26. [CrossRef] [PubMed] 11. Bishayee, A.; Ahmed, S.; Brankov, N.; Perloff, M. Triterpenoids as potential agents for the chemoprevention and therapy of breast cancer. Front. Biosci. 2011, 16, 980–996. [CrossRef] 12. Xu, R.; Fazio, G.C.; Matsuda, S.P.T. On the origins of triterpenoid skeletal diversity. Phytochemistry 2004, 65, 261–291. [CrossRef][PubMed] 13. Villa, T.G.; Feijoo-Siota, L.; Rama, J.L.R.; Ageitos, J.M. Antivirals against animal viruses. Biochem. Pharmacol. 2017, 133, 97–116. [CrossRef][PubMed] 14. Lin, L.T.; Hsu, W.C.; Lin, C.C. Antiviral Natural Products and Herbal Medicines. J. Tradit. Complement. Med. 2014, 4, 24–35. [CrossRef][PubMed] 15. Pompei, R.; Flore, O.; Marccialis, A.M.; Pani, A.; Loddo, B. Glycyrrhizic acid inhibits virus growth and inactivates virus particles. Nature 1979, 281, 689–690. [CrossRef][PubMed] 16. Chen, K.; Qian, S.; Yoshiki, K.; Zhang, D.-C.; Hu, C.-Q.; Jin, J.-Q.; Nozaki, H.; Kilkuskie, R.E.; Tramontano, E.; Cheng, Y.-C.; et al. Anti-aids agents, 6. salaspermic acid, an anti-HIV principle from tripterygium wilfordii, and the structure-activity correlation with its related compounds. Prod. J. Nat. 1992, 55, 340–346. [CrossRef] 17. Fujioka, T.; Kashiwada, Y.; Robert, E.K.; Cosentino, L.M.; Ballas, L.M.; Jiang, J.B.; Lanzen, W.P.; Cheen, I.; Lee, K. Anti- aids agents, 11 betulinic acid and plantanic acid as anti-HIV principles from syzygium claviflorum, and the anti-HIV activity of structurally relate triterpenoids. J. Nat. Prod. 1994, 57, 243–247. [CrossRef][PubMed] 18. Kong, L.; Liao, Q.; Zhang, Y.; Sun, R.; Zhu, X.; Zhang, Q.; Wang, J.; Wu, X.; Fang, X.; Zhu, Y. Oleanolic acid and ursolic acid: Novel hepatitis C virus antivirals that inhibit NS5B activity. Antiviral Res. 2013, 98, 44–53. [CrossRef][PubMed] 19. Rali, S.; Oyedeji, O.O.; Aremu, O.O.; Oyedeji, A.O.; Nkeh-Chungag, B.N. Semisynthesis of derivatives of oleanolic acid from Syzygium aromaticum and their antinociceptive and anti-inflammatory properties. Mediators Inflamm. 2016, 2016, 1–9. [CrossRef][PubMed] 20. Kim, S.; Lee, H.; Lee, S.; Yoon, Y.; Choi, K.H. Antimicrobial action of oleanolic acid on Listeria monocytogenes, Enterococcus faecium, and Enterococcus faecalis. PLoS ONE 2015, 10, e0118800. [CrossRef][PubMed] 21. Hichri, F.; Ben, H.; Cheriaa, J.; Jegham, S.; Mighri, Z. Antibacterial activities of a few prepared derivatives of oleanolic acid and of other natural triterpenic compounds. C. R. Chim. 2003, 6, 473–483. [CrossRef] 22. Li, X.; Song, Y.; Zhang, P.; Zhu, H.; Chen, L.; Xiao, Y.; Xing, Y. Oleanolic acid inhibits cell survival and proliferation of prostate cancer cells in vitro and in vivo through the PI3K/Akt pathway. Tumor Biol. 2016, 37, 7599–7613. [CrossRef][PubMed] 23. Chouaïb, K.; Romdhane, A.; Dlelmasure, S.; Dutartre, P.; Elie, N.; Toutboul, D.; Ben, H.; Ali, M. Regiospecific synthesis, anti-inflammatory and anticancer evaluation of novel 3,5-disubstituted isoxazoles from the natural maslinic and oleanolic acids. Ind. Crops Prod. 2016, 85, 287–299. [CrossRef] 24. Guo, Y.; Han, B.; Luo, K.; Ren, Z.; Cai, L.; Sun, L. NOX2-ROS-HIF-1α signaling is critical for the inhibitory effect of oleanolic acid on rectal cancer cell proliferation. Biomed. Pharmacother. 2017, 85, 733–773. [CrossRef] [PubMed] 25. Shanmugam, M.K.; Dai, X.; Kumar, A.P.; Tan, B. K.H.; Sethi, G.; Bishayee, A. Oleanolic acid and its synthetic derivatives for the prevention and therapy of cancer: Preclinical and clinical evidence. Cancer Lett. 2014, 346, 206–216. [CrossRef][PubMed] Molecules 2018, 23, 2300 12 of 14

26. Oprean, C.; Mioc, M.; Csányi, E.; Ambrus, R.; Bojin, F.; Tatu, C.; Critea, M.; Ivan, A.; Dancui, C.; Dehelean, C.; et al. Improvement of ursolic and oleanolic acids0 antitumor activity by complexation with hydrophilic cyclodextrins. Biomed. Pharmacother. 2015, 83, 1095–1104. [CrossRef][PubMed] 27. Pattnaik, B.; Nayak, L.V.; Sistla, R.; Mallavadhani, V.U. Bioorganic Chemistry Synthesis of ring-C modified oleanolic acid derivatives and their cytotoxic evaluation. Bioorg. Chem. 2016, 68, 152–158. [CrossRef] [PubMed] 28. Fu, Q.; Zhang, L.; Cheng, N.; Jia, M.; Zhang, Y. Extraction optimization of oleanolic and ursolic acids from pomegranate (Punica granatum L.) flowers. Food Bioprod. Process. 2014, 92, 321–327. [CrossRef] 29. Bernatoniene, J.; Cizauskaite, U.; Ivanauskas, L.; Jakstas, V.; Kalveniene, Z.; Kopustinskiene, D.M. Novel approaches to optimize extraction processes of ursolic, oleanolic and rosmarinic acids from Rosmarinus officinalis leaves. Ind. Crops Prod. 2016, 84, 72–79. [CrossRef] 30. Chouab, K.; Hichri, F.; Nguir, A.; Daami-Remadi, M.; Elie, N.; Touboul, D.; Ben, J.H.; Hamza, M. Semi-synthesis of new antimicrobial esters from the natural oleanolic and maslinic acids. Food Chem. 2015, 183, 8–17. [CrossRef][PubMed] 31. Weckesser, S.; Engel, K.; Simon-haarhaus, B.; Wittmer, A.; Pelz, K.; Schempp, C.M. Screening of plant extracts for antimicrobial activity against bacteria and yeasts with dermatological relevance. Phytomedicine 2007, 14, 508–516. [CrossRef][PubMed] 32. Aisha, A.F.A.; Abu-salah, K.M.; Salman, A. Syzygium aromaticum extracts as good source of betulinic acid and potential anti-breast cancer. Rev. Bras. Farmacogn. 2012, 22, 335–343. [CrossRef] 33. Chen, P.; Zeng, H.; Wang, Y.; Fan, X.; Xu, C.; Deng, R.; Zhou, X.; Bi, H.; Huang, M. Low Dose of Oleanolic Acid Protects against Lithocholic Acid–Induced Cholestasis in Mice: Potential Involvement of Nuclear Factor-E2-Related Factor 2-Mediated Upregulation of Multidrug Resistance-Associated Proteins. Am. Soc. Pharmacol. Exp. Ther. 2014, 42, 844–852. [CrossRef][PubMed] 34. Sheng, H.; Sun, H. Synthesis, biology and clinical significance of pentacyclic triterpenes: A multi-target approach to prevention and treatment of metabolic and vascular diseases. Nat. Prod. Rep. 2011, 28, 543–593. [CrossRef][PubMed] 35. Liu, J. Oleanolic acid and ursolic acid: Research perspectives. J. Ethnopharmacol. 2005, 100, 92–94. [CrossRef] [PubMed] 36. Fukushima, E.O.; Seki, H.; Ohyama, K.; Ono, E.; Umemoto, N.; Mizutani, M.; Saito, K.; Muranaka, T. CYP716A Subfamily Members are Multifunctional Oxidases in Triterpenoid Biosynthesis. Plant Cell Physiol. 2011, 52, 2050–2061. [CrossRef][PubMed] 37. Jesus, J.A.; Lago, J.H.G.; Laurenti, M.D.; Yamamoto, E.S.; Passero, L.F.D. Antimicrobial activity of oleanolic and ursolic acids: An update. Evid.-Based Complement. Altern. Med. 2015, 2015.[CrossRef][PubMed] 38. Strehle, A.; Thomas, C.; Sato, H.; Lobstein, A.; Wagner, A.; Mioskowski, C.; Auwerx, J. Anti-hyperglycemic activity of a TGR5 agonist isolated from Olea europaea. Biochem. Biophys. Res. Commun. 2007, 362, 793–798. 39. Sánchez, M.; Theoduloz, C.; Schmeda-hirschmann, G.; Razmilic, I.; Yáñez, T.; Rodríguez, J.A. Gastroprotective and ulcer-healing activity of oleanolic acid derivatives: In vitro–in vivo relationships. Life Sci. 2006, 79, 1349–1356. [CrossRef][PubMed] 40. Somova, L.O.; Nadar, A.; Rammanan, P.; Shode, F.O. Cardiovascular, antihyperlipidemic and antioxidant effects of oleanolic and ursolic acids in experimental. Phytomedicine 2003, 10, 115–121. [CrossRef][PubMed] 41. Banik, R.M.; Pandey, D.K. Optimizing conditions for oleanolic acid extraction from Lantana camara roots using response surface methodology. Ind. Crops Prod. 2008, 27, 241–248. [CrossRef] 42. Jäger, S.; Winkler, K. Solubility Studies of Oleanolic Acid and Betulinic Acid in Aqueous Solutions and Plant Extracts of Viscum album L. Planta Med. 2007, 73, 157–162. [CrossRef][PubMed] 43. Ali, H.; Houghton, P.J.; Soumyanath, A. Amylase inhibitory activity of some Malaysian plants used to treat diabetes with particular reference to Phyllanthus amarus. J. Ethnopharmacol. 2006, 107, 449–455. [CrossRef] [PubMed] 44. Chen, X.Q.; Li, Y.; He, J.; Cheng, X.; Wang, K.; Li, M.M.; Pan, Z.H.; Peng, L.Y.; Zhao, Q.S. Triterpenoids and diterpenoids from Viburnum chingii. Chem. Pharm. Bull. 2011, 59, 496–498. [CrossRef][PubMed] 45. Niampoka, C.; Suttisri, R.; Bavovada, R.; Takayama, H.; Aimi, N. Potentially cytotoxic triterpenoids from the root bark of Siphonodon celastrineus Griff. Arch. Pharm. Res. 2005, 28, 546–549. [CrossRef][PubMed] Molecules 2018, 23, 2300 13 of 14

46. Kaweetripob, W.; Mahidol, C.; Prawat, H.; Ruchirawat, S. Lupane, friedelane, oleanane, and ursane triterpenes from the stem of Siphonodon celastrineus Griff. Phytochemistry 2013, 96, 404–417. [CrossRef] [PubMed] 47. Yan, M.; Zhu, Y.; Zhang, H.J.; Jiao, W.H.; Han, B.N.; Liu, Z.X.; Qiu, F.; Chen, W.S.; Lin, H.W. Anti-inflammatory secondary metabolites from the leaves of Rosa laevigata. Bioorg. Med. Chem. 2013, 21, 3290–3297. [CrossRef] [PubMed] 48. Liby, K.T.; Sporn, M.B. Synthetic Oleanane Triterpenoids: Multifunctional Drugs with a Broad Range of Applications for Prevention and Treatment of Chronic Disease. Pharmacol. Rev. 2012, 64, 972–1003. [CrossRef] [PubMed] 49. Suh, N.; Wng, Y.; Honda, T.; Gribble, G.W. Advances in Brief A Novel Synthetic Oleanane Triterpenoid, 2-Cyano-3,12-dioxoolean-1,9-dien-28-oic Acid, with Potent Differentiating, Antiproliferative, and Anti-Inflammatory Activity. Cancer Res. 1999, 59, 336–341. [PubMed] 50. Chen, J.; Liu, J.; Zhang, L.; Wu, G.; Hua, W. Pentacyclic triterpenes. Part 3: Synthesis and biological evaluation of oleanolic acid derivatives as novel inhibitors of glycogen phosphorylase. Bioorg. Med. Chem. Lett. 2006, 16, 2915–2919. [CrossRef][PubMed] 51. Pollier, J.; Goossens, A. Phytochemistry Oleanolic acid. Phytochemistry 2012, 77, 10–15. [CrossRef][PubMed] 52. Nkeh-chungag, B.N.; Oyedeji, O.O.; Oyedeji, A.O.; Ndebia, E.J. Anti-Inflammatory and Membrane- Stabilizing Properties of Two Semisynthetic Derivatives of Oleanolic Acid. Inflammation 2015, 38, 61–69. [CrossRef][PubMed] 53. Zhao, H.; Holmes, S.S.; Baker, G.A.; Challa, S.; Bose, H.S.; Song, Z. Ionic derivatives of betulinic acid as novel HIV-1 protease inhibitors. J. Enzyme Inhib. Med. Chem. 2012, 27, 715–721. [CrossRef][PubMed] 54. Aiken, C.; Chen, C.H. Betulinic acid derivatives as HIV-1 antivirals. Trends Mol. Med. 2005, 11, 32–36. [CrossRef][PubMed] 55. Yogeeswari, P.; Sriram, D. Betulinic Acid and Its Derivatives: A Review on their Biological Properties. Curr. Med. Chem. 2005, 12, 657–666. [CrossRef][PubMed] 56. Zhu, Y.; Shen, J.; Wang, H.; Mark, L.; Lee, K. Synthesis and Anti-HIV Activity of Oleanolic Acid Derivatives. Bioorg. Med. Chem. Lett. 2001, 11, 3115–3118. [CrossRef] 57. Yu, D.; Sakurai, Y.; Chen, C.; Chang, F.; Huang, L.; Kashiwad, Y.; Kuo-Hsiung, L. Anti-AIDS Agents 69. Moronic Acid and Other Triterpene Derivatives as Novel Potent Anti-HIV Agents. J. Med. Chem. 2006, 49, 5462–5469. [CrossRef][PubMed] 58. Kashiwada, Y.; Nagao, T.; Hashimoto, A.; Ikeshiro, Y.; Okabe, H.; Cosentino, L.M.; Lee, K.H. Anti-AIDS agents 38. Anti-HIV activity of 3-O-acyl ursolic acid derivatives. J. Nat. Prod. 2000, 63, 1619–1622. [CrossRef] [PubMed] 59. Kashiwada, Y.; Wang, H.K.; Nagao, T.; Kitanaka, S.; Yasuda, I.; Fujioka, T.; Yamagishi, T.; Cosentino, L.M.; Kozuka, M.; Okabe, H.; et al. Anti-HIV activity of oleanolic acid, pomolic acid, and structurally related triterpenoids. J. Nat. Prod. 1998, 61, 1090–1095. [CrossRef][PubMed] 60. Sultana, N.; Ata, A. Oleanolic acid and related derivatives as medicinally important compounds. J. Enzyme Inhib. Med. Chem. 2008, 23, 739–756. [CrossRef][PubMed] 61. Lee, S.; Song, D.; Poo, H. Antiviral Activity of the Plant Extracts from Thuja orientalis, Aster spathulifolius, and Pinus thunbergii against Influenza Virus A/PR/8/34. J. Microbiol. Biotechnol. 2013, 23, 125–130. 62. Lee, J.; Miyake, S.; Umetsu, R.; Hayashi, K.; Chijimatsu, T. Anti-influenza A virus effects of fructan from Welsh onion (Allium fistulosum L.). Food Chem. 2012, 134, 2164–2168. [CrossRef][PubMed] 63. Kinoshita, E.; Hayashi, K.; Katayama, H.; Hayashi, T.; Obata, A. Anti-Influenza Virus Effects of Elderberry Juice and Its Fractions. Biosci. Biotechnol. Biochem. 2012, 76, 1633–1638. [CrossRef][PubMed] 64. Sriwilaijaroen, N.; Fukumoto, S.; Kumagai, K.; Hiramatsu, H.; Odagiri, T. Antiviral effects of Psidium guajava Linn. (guava) tea on the growth of clinical isolated H1N1 viruses: Its role in viral hemagglutination and neuraminidase inhibition. Antiviral Res. 2012, 94, 139–146. [CrossRef][PubMed] 65. Wang, X.; Jia, W.; Zhao, A.; Wang, X. Anti-influenza agents from plants and traditional Chinese medicine. Phyther. Res. 2006, 20, 335–341. [CrossRef][PubMed] 66. Liu, Z.; Zhao, J.; Li, W.; Wang, X.; Xu, J.; Xie, J.; Tao, K.; Shen, L.; Zhang, R. Molecular docking of potential inhibitors for influenza H7N9. Comput. Math. Methods Med. 2015, 2015, 1–8. [CrossRef][PubMed] Molecules 2018, 23, 2300 14 of 14

67. Yang, Y.; He, H.; Chang, H.; Yu, Y.; Yang, M.; He, Y. Multivalent oleanolic acid human serum albumin conjugate as nonglycosylated neomucin for influenza virus capture and entry inhibition. Eur. J. Med. Chem. 2018, 143, 1723–1731. [CrossRef][PubMed] 68. Han, X.; Shi, Y.; Si, L.; Fan, Z.; Wang, H.; Xu, R.; Jiao, P.; Meng, K.; Tian, Z.; Zhou, X.; et al. Design, synthesis and biological activity evaluation of novel conjugated sialic acid and pentacyclic triterpene derivatives as anti-influenza entry inhibitors. MedChemComm 2016, 7, 1932–1945. [CrossRef] 69. Han, X.; Si, L.L.; Shi, Y.Y.; Fan, Z.B.; Wang, S.X.; Tian, Z.Y.; Li, M.; Sun, J.Q.; Jiao, P.X.; Ran, F.X.; et al. Synthesis and in vitro anti-influenza virus evaluation of novel sialic acid (C-5 and C-9)-pentacyclic triterpene derivatives. Molecules 2017, 22, 1018. [CrossRef][PubMed] 70. Hattori, M.; Ma, C.; Wei, Y.; Salah, R.; Dine, E.; Sato, N. Survey of Anti-HIV and Anti-HCV Compounds from Natural Sources. Can. Chem. Trans. 2013, 1, 116–140. 71. Pastuch-Gawolek, G.; Chaubey, B.; Szewczyk, B.; Krol, E. Novel thioglycosyl analogs of glycosyltransferase substrates as antiviral compounds against classical swine fever virus and hepatitis C virus. Eur. J. Med. Chem. 2017, 137, 247–262. [CrossRef][PubMed] 72. Chiang, L.C.; Chiang, W.; Chang, M.Y.; Ng, L.T.; Lin, C.C. Antiviral activity of Plantago major extracts and related compounds in vitro. Antiviral Res. 2002, 55, 53–62. [CrossRef] 73. Yan, W.; Zhang, C.; Li, B.; Xu, X.; Liang, M.; Gu, S.; Chu, S.; Xu, B.; Ren, J.; Wang, P.; et al. A Series of Oleanolic Acid Derivatives as Anti-Hepatitis B Virus Agents: Design, Synthesis, and in Vitro and in Vivo Biological Evaluation. Molecules 2016, 21, 402. [CrossRef][PubMed] 74. Mukherjee, H.; Ojha, D.; Bag, P.; Chandel, H.S.; Bhattacharyya, S.; Chatterjee, T.K.; Mukherjee, P.K.; Chakraborti, S.; Chattopadhyay, D. Anti-herpes virus activities of Achyranthes aspera: An Indian ethnomedicine, and its triterpene acid. Microbiol. Res. 2013, 168, 238–244. [CrossRef][PubMed] 75. Whitley, R.J.; Roizman, B. Herpes simplex virus infections. Lancet 2001, 357, 1513–1518. [CrossRef] 76. De Clercq, E. Antiviral drugs in current clinical use. J. Clin. Virol. 2004, 30, 115–133. [CrossRef][PubMed] 77. Keda, T.I.; Okomizo, K.Y.; Suchihashi, M.O.; Injo, J.K. Anti-herpes Virus Type 1 Activity of Oleanane-Type Triterpenoids. Biol. Pharm. Bull. 2005, 28, 1779–1781. 78. Sarrazin, C. The importance of resistance to direct antiviral drugs in HCV infection in clinical practice. J. Hepatol. 2016, 64, 486–504. [CrossRef][PubMed]

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