Design, Synthesis And Antibacterial Activity Of 15-Membered Azalide Derivatives Against Resistant Bacteria

Macrolides are a class of weak base antibiotics produced by streptomycetes. Erythromycin A and its analogs such as clarithromycin and azithromycin, have been widely used for treating upper and lower respiratory tract, mycoplasma, chlamydia, pneumonia and soft tissue infections in clinical application because of their superior antibacterial activity. However, marolide-resistant infections have been observed with increasing frequency in recent years, which presents an urgent need for a new generation of macrolid antibiotics with effective activity against resistant bacteria. Therefore, it has attracted extensive attention to develop new potent macrolides against resistant bacteria.The molecular biological studying indicated that there are three binding sites from peptidyl transferase center (PTC) to the contraction of peptidyl tunnel in bacterial ribosome. The first site is A2058 or A2059 in domain V,which situates in the entrance of peptide releasing tunnel; The second site is U2609 in domain V or A752 in domain II, which locates in another side of entrance of peptide releasing tunnel; The third site is the binding site of amphemycin or clindamycin, which locates A and P sites of PTC. In order to overcome the drawbacks of macrolides and ketolides in their activity against resistant bacteria, new macrolide derivatives could be designed and synthesized based on the known binding sites in the upper part of the nascent peptide exit tunnel in ribosome. Based on the thoughts above, three series of 15-membered azalides were designed and synthesized with azithromycin as the lead compound. These derivatives were prepared through introduction of side chains at C-11, C-12 and C-4" positions of macrocyclic skeleton. The synthetic routes for these series were successfully established, and their structures were confirmed by MS, IR and 1H NMR. In addition, the synthetic routes had advantages of high yields, simple operations and mild conditions.In vitro antibacterial activity of the target compounds was determined by using the doubling dilution method. The results were as follows:(1) Antibacterial activity against susceptible bacteria:target compounds all possessed moderate antibacterial activity against sensitive bacteria, and demonstrated better activity against S. aureus than against S. pneumoniae. In addition, the activity of series L was superior to J and K. Among these compounds, compounds L4, L12 and L17 showed potent antibacterial activity, especially compounds L4 and L17 exhibited excellent activity against S. pneumoniae (MIC<0.03μg/mL). (2) Antibacterial activity against resistant bacteria: All compounds showed improved activity against resistant bacteria, and the activity of compounds against M-resistant S. pneumoniae A022072 was superior to against other bacteria tested. Series L possessed excellent activity against resistant bacteria, and series J and K displayed similar anti-resistant bacteria activity. Otherwise, compounds L1 and L17 showed improved activity against MLSB-resistant bacteria, compounds L12, L15, L17, L19 and L22 also exhibited potent activity against M-resistant bacteria, and compounds L12 and L23 possessed excellent activity against mixed-type resistant bacteria, which were 32-fold potent activity than erythromycin, clarithromycin and azithromycin.The results of structure-activity relationship study of these derivatives were summarized as follows:(1) The basic skeletons of macrolides are essential for activity against susceptible bacteria. (2) The introduction of substituted benzoylpiperazinyl side chains to C-4" carbon position of the skeleton had improved activity against resistant bacteria. (3) The introduction of 11,12-cyclic carbonate moiety to C-4" benzoylpiperazinyl derivatives couldn't influence antibacterial activity. but the substituent on benzene ring in it's C-4" side chains contributed to different antibacterial activity. For example:the electron-donating group would increase antibacterial activity against sensitive strains and the electron-withdrawing group would lead to an increase in antibacterial activity against resistant strains. (4) The introduction of C-4" substituted benzoylpiperazinyl and C-11 alkyl carbamate side chains to the skeleton simultaneously would improve antibacterial activity significantly. In particularly, the electron-withdrawing group on benzene ring in it's C-4" side chains would show more potent antibacterial activity than the electron-withdrawing group.The above studies indicated that azithromycin is an ideal lead compound which can help us develop new anti-resistant bacteria drugs, for many sites of the structure can be modified, such as C-11, C-12 and C-4". There is no doubt that introduction of carbon chains having different length and terminal group to azithromycin will help us discover efficacious agents for the treatment of infections by resistant bacteria.