molecules Article Sulfadiazine Salicylaldehyde-Based Schiff Bases: Synthesis, Antimicrobial Activity and Cytotoxicity Martin Krátký 1,*, Magdaléna Dzurková 2, Jiˇrí Janoušek 3, Klára Koneˇcná 4, František Trejtnar 3, JiˇrinaStolaˇríková 5 and Jarmila Vinšová 1 1 Department of Organic and Bioorganic Chemistry, Faculty of Pharmacy in Hradec Králové, Charles University, Akademika Heyrovského 1203, 500 05 Hradec Králové, Czech Republic; [email protected] 2 Department of Chemistry, Faculty of Science, J. E. Purkinje University, Ceskˇ é mládeže 8, 400 96 Ústí nad Labem, Czech Republic; [email protected] 3 Department of Pharmacology and Toxicology, Faculty of Pharmacy, Charles University, Akademika Heyrovského 1203, 500 05 Hradec Králové, Czech Republic; [email protected] (J.J.); [email protected] (F.T.) 4 Department of Biological and Medical Sciences, Faculty of Pharmacy, Charles University, Akademika Heyrovského 1203, 500 05 Hradec Králové, Czech Republic; [email protected] 5 Laboratory for Mycobacterial Diagnostics and Tuberculosis, Regional Institute of Public Health in Ostrava, Partyzánské námest˘ í 7, 702 00 Ostrava, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-495067343; Fax: +420-495067166 Received: 25 August 2017; Accepted: 13 September 2017; Published: 19 September 2017 Abstract: The resistance among microbes has brought an urgent need for new drugs. Thus, we synthesized a series of Schiff bases derived from the sulfa drug sulfadiazine and various salicylaldehydes. The resulting 4-[(2-hydroxybenzylidene)amino]-N -(pyrimidin-2-yl)benzene-sulfonamides were characterized and evaluated against Gram-positive and Gram-negative bacteria, yeasts, moulds, Mycobacterium tuberculosis, nontuberculous mycobacteria (M. kansasii, M. avium) and their cytotoxicity was determined. Among bacteria, the genus Staphylococcus, including methicillin-resistant S. aureus, showed the highest susceptibility, with minimum inhibitory concentration values from 7.81 µM. The growth of Candida sp. and Trichophyton interdigitale was inhibited at concentrations starting from 1.95 µM. 4-[(2,5-Dihydroxybenzylidene)amino]-N-(pyrimidin-2-yl)-benzenesulfonamide was identified as the most selective Schiff base for these strains with no apparent cytotoxicity and a selectivity index higher than 16. With respect to M. tuberculosis and M. kansasii that were inhibited within the range of 8 to 250 µM, unsubstituted 4-[(2-hydroxy-benzylidene)amino]-N-(pyrimidin-2-yl)benzenesulfonamide meets the selectivity requirement. In general, dihalogenation of the salicylic moiety improved the antibacterial and antifungal activity but also increased the cytotoxicity, especially with an increasing atomic mass. Some derivatives offer more advantageous properties than the parent sulfadiazine, thus constituting promising hits for further antimicrobial drug development. Keywords: antibacterial activity; antifungal activity; antimycobacterial activity; cytotoxicity; Schiff bases; sulfadiazine; sulfonamides 1. Introduction Schiff bases are condensation products of primary (aromatic) amines with aldehydes or ketones carrying the azomethine (imino) moiety (-CR=N-). They are considered versatile pharmacophores for various pharmacological activities where the azomethine group has been demonstrated to be critical to the bioactivity. For example, Schiff bases, whether of natural or non-natural origin, have exhibited promising antibacterial, antitubercular, antifungal, antiparasitic, antiviral, antioxidant, anticancer, Molecules 2017, 22, 1573; doi:10.3390/molecules22091573 www.mdpi.com/journal/molecules Molecules 2017, 22, 1573 2 of 15 analgesic, anti-inflammatory properties etc. [1–3]. Altogether, they represent very frequently used and useful scaffold in medicinal chemistry. SchiffMolecules 2017 bases, 22, 1573 of substituted salicylaldehydes (2-hydroxybenzaldehydes) are well-known2 of 15 antimicrobialMolecules 2017 agents, 22, 1573in “free” form or as ligands in metallic complexes [4–7]. Similar biological2 of 15 Schiff bases of substituted salicylaldehydes (2-hydroxybenzaldehydes) are well-known action have been reported for Schiff bases of various sulfonamides [4,8,9]. In addition, antimicrobialSchiff bases agents of in “free”substituted form orsalicylaldehyde as ligands in metallics (2-hydroxybenzaldehydes) complexes [4–7]. Similar arebiological well-known action cotrimoxazole, sulfamethoxazole and sulfadiazine (1, SDZ, Figure1) have exhibited activity against haveantimicrobial been reported agents infor “free” Schiff form bases or asof ligands various in su metalliclfonamides complexes [4,8,9]. [4–7]. In addition, Similar biological cotrimoxazole, action Mycobacterium tuberculosis [10] as well as nontuberculous (atypical) mycobacteria (NTM) [11] at sulfamethoxazolehave been reported and for sulfadiazine Schiff bases (1 , ofSDZ, various Figure su 1)lfonamides have exhibited [4,8,9]. activity In addition, against cotrimoxazole,Mycobacterium clinicallytuberculosissulfamethoxazole achievable [10] as concentrations welland sulfadiazineas nontuberculous after (1, SDZ, administration (atypical) Figure 1)mycobacteria haveper exhibited os. To (NTM) the activity best [11] of atagainst ourclinically knowledge, Mycobacterium achievable there is no reportconcentrationstuberculosis about [10] an after identicalas well administration as propertynontuberculous ofper sulfathiazole,os. (atypical)To the best mycobacteria usedof our now knowledge,only (NTM) topically, there[11] at is clinically no or otherreport achievable sulfonamides.about an El-Baradieidenticalconcentrations reported property after [ 12of administration] sulfathiazole, an antibacterial per used os. activityTonow the only best of oftopically, a our SDZ-based knowledge, or other Schiff theresulfonamides. base is no with report El-Baradie unsubstituted about an salicylaldehyde.reportedidentical [12]property Itan exhibited antibacterial of sulfathiazole, minimum activity ofused inhibitory a SDZ-ba now onlysed concentrations Schiff topically, base withor (MICs)other unsubs sulfonamides. oftituted 100–250 salicylaldehyde.µ g/mLEl-Baradie for both It exhibited minimum inhibitory concentrations (MICs) of 100–250 µg/mL for both Gram-positive Gram-positivereported [12] and an antibacterial Gram-negative activity bacteria. of a SDZ-ba Thesed Schiff Schiff base base derived with unsubs fromtituted 5-bromosalicylaldehyde salicylaldehyde. and Gram-negative bacteria. The Schiff base derived from 5-bromosalicylaldehyde exhibited a exhibitedIt exhibited a broad minimum spectrum inhibitory of concentrations antibacterial (MICs) and of antifungal 100–250 µg/mL properties for both Gram-positive [13]. Isosteric broad spectrum of antibacterial and antifungal properties [13]. Isosteric 4-(5-chloro-2- 4-(5-chloro-2-hydroxybenzylideneamino)-and Gram-negative bacteria. The Schiff Nbase-(pyrimidin-2-yl)benzenesulfonamide derived from 5-bromosalicylaldehyde (exhibited2c, Figure a 2) hydroxybenzylideneamino)-broad spectrum of antibacterialN-(pyrimidin-2-yl)benzenesulfonamide and antifungal properties ([13].2c, Figure Isosteric 2) showed 4-(5-chloro-2- activity showed activity against Staphylococcus aureus, including a methicillin-resistant strain (MRSA), againsthydroxybenzylideneamino)- Staphylococcus aureusN, -(pyrimidin-2-yl)benzenesulfonamideincluding a methicillin-resistant strain (2c (MRSA),, Figure M.2) showedtuberculosis activity and M. tuberculosis Mycobacterium avium µ Mycobacteriumagainst Staphylococcusand avium (MIC aureus values, including within thea(MIC methicillin-resistant range values of 125–250 within µM) strain the whereas range (MRSA), it of caused 125–250M. tuberculosis a significantlyM) whereasand it causedhigherMycobacterium a significantlyinhibition avium of Mycobacterium higher(MIC values inhibition withinkansasii of the strainsMycobacterium range (8–32 of 125–250 µM) kansasii [4]. µM) whereasstrains (8–32it causedµM) a significantly [4]. higher inhibition of Mycobacterium kansasii strains (8–32 µM) [4]. FigureFigure 1.1. Sulfadiazine 1.1 . Figure 1. Sulfadiazine 1. Keeping in mind these facts, we decided to combine two well-established pharmacophores for Keeping in mind these facts, we decided to combine two well-established pharmacophores for antimicrobialKeeping activity,in mind sulfadiazine these facts, andwe decidedsalicylaldehyd to combinee, into two one well-establishedmolecular Schiff basepharmacophores entity (Figure for 2) antimicrobial activity, sulfadiazine and salicylaldehyde, into one molecular Schiff base entity (Figure2) andantimicrobial to screen activity, systematically sulfadiazine their and antimicrobial salicylaldehyd ande, into cytotoxic one molecular properties. Schiff In base our entity study (Figure [4], the 2) and to screen systematically their antimicrobial and cytotoxic properties. In our study [4], the activities activitiesand to screen of sulfadiazine systematically amides their and antimicrobialimines based onand the cytotoxic salicylic properties.scaffold were In comparableour study [4],against the of sulfadiazinetheactivities majority of amides sulfadiazineof mycobacterial and iminesamides strains basedand andimines on drug-susceptible the based salicylic on the scaffold salicylic Staphylococcus were scaffold comparable aureus were, butcomparable further against research theagainst majority of mycobacterialdiscoveredthe majority that of strains mycobacterial the amides and drug-susceptible are strains significantly and drug-susceptible