Biological evaluation of novel substituted chloroquinolines targeting mycobacterial ATP synthase
2013
Khan, Shaheb Raj | Singh, Supriya | Roy, Kuldeep K. | Akhtar, Md Sohail | Saxena, Anil K. | Krishnan, Manju Yasoda
The ATP synthase of Mycobacterium tuberculosis is a validated drug target against which a diarylquinoline drug is under clinical trials. The enzyme is crucial for the viability both of actively replicating and non-replicating/dormant M. tuberculosis. Enzyme levels drop drastically as the bacilli enter dormancy and hence an inhibitor would make the dormant bacilli even more vulnerable. In this study, a set of 18 novel substituted chloroquinolines were screened against Mycobacterium smegmatis ATP synthase; 6 compounds with the lowest 50% inhibitory concentration (IC₅₀) values (0.36–1.83μM) were selected for further in vitro studies. All six compounds inhibited the growth of M. tuberculosis H37Rv in vitro, with minimum inhibitory concentrations (MICs) of 3.12μg/mL (two compounds) or 6.25μg/mL (four compounds). All of them were bactericidal to non-replicating M. tuberculosis H37Rv in hypoxic culture; three compounds caused a >2log₁₀ reduction in CFU counts in 4 days at concentrations of 16× or 32× their MICs, compared with a 0.2log₁₀ reduction by isoniazid and a >4log₁₀ reduction by rifampicin at 100× their MICs. The compounds also contributed to a greater reduction in total cellular ATP of the bacilli compared with isoniazid and rifampicin during an exposure time of 18h. The compounds at 100μM caused only 5–35% inhibition of mouse liver mitochondrial ATP synthase, leading to selectivity indices ranging from >55-fold to >278-fold. In vitro cytotoxicity to the Vero cell line measured as the 50% cytotoxic concentration (CC₅₀) of the compounds ranged between 55μg/mL and >300μg/mL.
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