Design, Synthesis And Antibacterial Activity Of Novel 4″-Benzimidazolyl Clarithromycin Derivatives

Macrolides which are used for the treatment of the infections of respiratory system, digestive system and uropoietic system in clinical application led to serious bacterial resistance because they were abused. This became major threat to successful treatment of infectious diseases. Therefore, it has attracted extensive attention to develop new potent macrolides against resistant bacteria.The mechanism of macrolides' action indicates that the binding sites of macrolides are located inside the nascent peptide exit tunnel of the ribosomal 50S subunit near the peptidyl transferase center. The macrolides can inhibit bacterial protein synthesis by blocking up the peptide tunnel when they bind to the binding sites, bringing forth antibacterial activity. Unfortunately, resistant bacteria have many mechanisms of resistance to macrolides and the commonest mechanism of resistance is mediated by erm-encoded methylation of 23S rRNA. Expression of an erm-resistant determinant in bacteria leads to production of a methyltransferase which modifies the key nucleotide, A2058, in the macrolide-lincosamide-streptogramin B (MLSB) binding site, thereby conferring resistance to macrolides. Therefore, design and synthesis of new macrolide derivatives against resistant bacteria based on new target sites is an important way to overcome bacterial resistance.During the research about the interaction between some kinds antibiotics and binding site, we found that, there are many ribonucleotide binding sites in peptide tunnel from the macrolides binding areas to the center of peptidyl transferase. And macrolides which were introduced side chains at C-4" positions could combine ribonucleotide binding site at P or A site near peptidyl tunnel via hydrogen bond to show antibacterial activity against resistanct strains. Based on the above thoughts,18 novel 14-membered macrolide derivatives were designed and synthesized by introduction of various substituted hydrazinecarboxylate benzimidazole side chains at C-4" positions of clarithromycin selected as the lead compound. Their structures were confirmed by MS, IR and 1H-NMR spectra. In addition, the synthetic routes for the 2 series were successfully established and had such features as high yields, simple operations and mild conditions after optimization of the reaction conditions.In vitro antibacterial activity of the target compounds was determined by using the 96-well microdilution method. The results were as follow:(1) Antibacterial activity against susceptible bacteria:almost all of target compounds showed antibacterial activity against susceptible S. aureus(MIC≤0.25μg/mL), S. pyogenes(MIC≤0.25μg/mL) and S. pneumoniae(MIC≤0.5μg/mL). Among them, compounds N4 - N8 displayed excellent activity against susceptible S. pneumoniae(MIC≤0.03μg/mL). Besides, compounds M6, N4, N5 and N7 had improved activity compared with the control drugs against S. aureus( MIC≤0.125μg/ mL) Except N1, N7 and N10,.all the compounds showed very susceptible activity to S. pyogenes(MIC≤0.03μg/mL). Compounds M4-M6, N1-N5 and N7 showed good activity to all the susceptible S. pneumoniae S. pyogenes and S. aureus. (2) Antibacterial activity against resistant bacteria:compared with the control drugs, nearly all the target compounds exhibited improved activity against the resistant strains S. pneumoniae B5 (ermB),S. pneumoniae A22072 (mefA) and S. pneumoniae A+B14 (ermB+mefA), respectively. All the target compounds were susceptible to the resistant strains S. pneumoniae A22072 (mefA). Most of them exhibited excellent activity (MIC≤0.125μg/mL). Especially, compounds N1-N7, N9 and N11 were most susceptible to this strain (MIC≤0.03μg/mL). What's more, most compounds showed 4-8 fold better activity than control drugs even more to S. pneumoniae B5 (ermB) and S. pneumoniae A+B14 (ermB+mefA) resistant strains. Compounds N1-N4, N6 and N9-N11 displayed excellent activity against all the three tested resistant strains.Structure activity relationships were summarized as follows:(1) Introduction of the bis-hydrazide benzimidazole side chains to the C-4"-OH of the clarithromycin showed improved activity against both susceptible and resistant S. pneumonia. (2) Substitution of the benzimidazoles with aliphatic chains which were prolonged or the polarity of which were changed made little improvement on the antibacteria activity to resistance S. pneumoniae. (3) Substitution of the benzimidazole side chains with aromatic rings displayed much better activity against the resistant S. pneumoniae than that with the aliphatic chains, which implicated the length and rigidity of the side chains had effect on the combination of the compounds and the target site. (4) Introduction of the substituent to the ortho or para position of terminal benzene rings in the benzimidazole side chains showed better activity than that to the meta position. Particularly, the compounds with the electron withdrawing group, e.g.-OCH3,-Cl, CF3, Br or NO2, which were introduced to the ortho or para position of the benzene rings exhibited more potent activity. It is possible to infer the conjugated effect and the polarity of the side chains influence the interaction between the compounds and the target site.Because of increasing bacterial resistance to the existing macrolide antibiotics, the research and development in the areas of macrolides are expected to be intensified urgently. With further investigation on the SARs of the novel derivatives, new structure modifications based on SARs, establishment and application on drug screening method, we can anticipate that even more potent macrolide antibiotics for the treatment of multidrug-resistant bacterial infections will be developed in the near future.