Research paperAnti-methicillin resistant Staphylococcus aureus activity, synergism with oxacillin and molecular docking studies of metronidazole-triazole hybrids
Graphical abstract
Introduction
Staphylococcus aureus is the leading cause of infections in the hospitals worldwide. S. aureus strains resistant to methicillin were first found in England, in 1961 [1]. Since, then the incidences of hospital acquired (HA) and community acquired (CA) methicillin-resistant S. aureus (MRSA) is ever increasing [2], [3], [4], [5], [6], [7]. The number of deaths due to MRSA is near to the total number of deaths due to AIDS, tuberculosis and viral hepatitis all together. Over the years, the semi-synthetic β-lactams such as oxacillin, cloxacillin, flucloxacillin were antibiotics of choice for the treatment of S. aureus infections. Besides β-lactam antibiotics, presently other classes of compounds have also been approved by FDA as anti-MRSA agents such as, peptide antibiotics (vancomycin and daptomycin), glycylcyclines (tigecycline), and oxazolidinone class of drugs (linezolid) [8], [9]. Bouley et al., have recently reported quinazolinones as a promising antibiotic for the treatment of highly resistant MRSA infections [10]. Unfortunately, MRSA has developed resistance to β-lactam antibiotics (penicillin, methicillin, oxacillin, dicloxacillin and ceftaroline), vancomycin and glycopeptides [11], [12], [13]. With the emergence of multidrug resistant MRSA, new anti-MRSA therapeutic strategies are needed, especially agents that are orally bioavailable. The rapid evolution of antibiotic resistant bacterial pathogens and the slower pace for the discovery of novel antibiotics, has a devastating effect on the potential for treatment of bacterial infections [14], [15], [16].
When methicillin-susceptible S. aureus (MSSA) acquires the methicillin-resistance gene mecA by horizontal gene transfer mediated by staphylococcal cassette chromosome (SCC), MRSA is born [17], [18]. The β-lactam antibiotics act by associating with the penicillin binding proteins (PBPs), resulting in the inhibition of the cell wall synthesis, leading to growth inhibition or cell lysis. Resistance to β-lactams emerge if penicillin binding protein 2a (PBP2a) is over expressed by MRSA. Because of the low binding affinity between PBP2a (encoded by the gene mecA) and β-lactams, the cell wall synthesis can be renewed. This protein thus acts as an effective equipment for the MRSA to prevent the attack from β-lactams. The reason for the resistance is credited to the induction of specific genes coding for a monofunctional DD-transpeptidase enzyme penicillin binding protein 2a (PBP2a) by β-lactam exposure [19]. These β-lactam antibiotics irreversibly inhibit PBP2 by acylation of active site serine residue (Ser403) [20], in contrast, the PBP2a of MRSA is resistant to β-lactam acylation and successfully catalyzes the DD-transpeptidation reaction necessary to complete the cell wall. Recent report highlights the presence of an allosteric site located 60 Å distance from the PBP2a active site and it is capable of binding three ligands including one molecule of ceftaroline, muramic acid and peptidoglycan [21], [22]. Such allosteric binding triggers conformational changes in PBP2a active site, allowing another inhibitor molecule (ceftaroline) to bind there [20]. Although PBP2a have also developed resistance against ceftaroline in certain strains with mutations (N146K and E150K) observed in allosteric domain and H351N mutation in region away from allosteric and active sites.
Metronidazole (MTZ) and other imidazole derivatives are known for their antibacterial activity [23], [24], [25], [26], [27]. The triazole [28] nucleus is a well known pharmacophore for anti-MRSA activity. In continuation with our earlier work on antibacterial agents, we screened the MTZ-triazole hybrids against MRSA [29], [30], [31], [32], [33], [34], [35]. These MTZ-triazole hybrids were found to be effective against MRSA strains upto 4 μg/mL concentration. We determined that the MTZ-triazoles can also decrease the minimal inhibitory concentration of oxacillin, many-fold against MRSA. The synergistic activity shows that MTZ-trizoles can be used in combination with oxacillin.
Section snippets
Chemistry
Click chemistry has been widely used for the construction of various biologically active molecules due to its ease of synthesis and the possibility of the generation of a library of compounds through various modifications. The copper catalyzed azide-alkyne cycloaddition reaction results in the formation of 1,4-disubstituted 1,2,3-triazoles, which are bioisosteres of the amide bond. In the present study, the synthesis of MTZ-triazole hybrids were accomplished in the sequence of reactions as
Results and discussion
The agar plate dilution method was conducted using 35 metronidazole-triazole hybrids against 30 MRSA strains and the MIC of each compound against each MRSA strain was obtained. Table 1 shows the number of MRSA strains against which the compounds are effective at a particular concentration. All the compounds were active against one or another strain of MRSA at 64 μg/mL concentration. Ten compounds (4, 6, 10, 11, 12, 17, 22, 25, 28 and 31) were active at 8 μg/mL. Compound 4, 11, 22, 25 and 28
Conclusions
All the synthesized compounds were found to be effective against MRSA. The best active compound 22 showed good synergism with the reference drug oxacillin. All the compounds were found to be nontoxic up to 50 μM concentration against THP-1 cell line. Also, the compounds exhibited good pharmacokinetic properties and follow the Lipinski's rule of 5. Thus we believe that these compounds can be considered as a possible lead for the development of new anti-MRSA agents.
In vitro anti-MRSA activity
MRSA strains were isolated using axilla and nasal swabs obtained from Medical Students and Patients at Teaching Hospital Anuradhapura, Sri Lanka. After swabbing axilla and nasal areas, the swabs were dipped in NaCl enriched nutrient broth. Following overnight incubation at 37 °C, those swabs were streaked on Manitol Salt Agar Plates. The colonies which turned adjacent media into yellow color were selected and the Gram stained smears of those were observed through light microscopy. The cultures
Acknowledgments
D.S.R. thanks SERB-Department of Science and Technology [EMR/2014/001127] New Delhi, India for financial support. D.K. and Beena are thankful to CSIR for the award of junior and senior research fellowship. Authors are also thankful to CIF-USIC, University of Delhi, Delhi for spectral data.
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2021, European Journal of Medicinal ChemistryCitation Excerpt :The SAR study indicated that polar groups/substitutions such as –COOH, –CH2NH2, and pyrimidine group diminished the activity, whereas phenyl and benzyl substitution enhanced or were well tolerated for inhibitory activity. Rawat [42] and Jarrad et al. [41] further evaluated anti-MRSA (antibacterial) and antiparasitic activities of similar metronidazole-triazole conjugates (16 and 17) synthesized by the same methods (Scheme 3), which are summarized in Fig. 3 and Fig. 8, respectively. Zhu et al., in 2010 explored the synthesis of metronidazole–deoxybenzoin derivatives 19 by the reaction of 9 with hydroxyl-substituted deoxybenzoin 18 under basic conditions in DMF as solvent as depicted in Scheme 4[43].