In vitro antimycobacterial activity of 2-(((2-hydroxyphenyl)amino)methylene)-5,5-dimethylcyclohexane-1,3-dione: A new chemical entity against Mycobacterium tuberculosis

Background: In our on-going efforts towards tuberculosis (TB) drug discovery programme, a high-throughput whole cell-based phenotypic screening of thousands of compounds against Mycobacterium tuberculosis H37Rv lead to the identification of a lead compound; 2-(((2-hydroxyphenyl)amino)methylene)-5,5-dimethylcyclohexane-1,3-dione (PAMCHD). In our earlier studies, it was observed that the test compound exhibited antimicrobial activity that was restricted to M. tuberculosis only. The test compound also proved to be non-toxic against a panel of human cell lines. Therefore, the present study was planned with an aim to further explore the antimycobacterial potential of the PAMCHD against M. tuberculosis H37Rv. Methods & Materials: In this study, we determined the minimum inhibitory concentration (MIC) and Minimum bactericidal concentration (MBC) of the PAMCHD. Kill curve and drug interaction studies were performed. Effect of serum/protein binding on PAMCHD activity was explored. Further, post antibiotic effect (PAE), Mutation frequency (MF) and mutant prevention concentration (MPC) were studied. Results: PAMCHD proved to be tuberculostatic (MIC, 2.5 μg/mL) as well as tuberculocidal (MBC, 5.0 μg/mL) agent. This compound was equipotent against drug resistant M. tuberculosis clinical isolates (MIC, 2.5-10 μg/mL). The dynamics of M. tuberculosis killing revealed its time as well as concentration-dependent anti-TB activity with an Emax of 10.0 μg/mL (at this concentration M.tuberculosis culture were completely sterilized). PAMCHD acts synergistically and additively with key first line and second line ATDs respectively. PAE of PAMCHD's was found to be 63.1 and 103.8 h at 4x and 8x MIC and that of INH (used as a control ATD) were 39.8 and 127.9 h respectively. MF for M. tuberculosis against PAMCHD was lower than that of INH at all the tested concentrations. The capacity of preventing emergence of resistant mutants of PAMCHD was found to be comparable to rifampin (RIF) as mutant MPC of both was observed to be 160 μg/mL. MPC/MIC value of PAMCHD (i.e. 64) matched to the best-known value among ATDs (i.e. 68) exhibited by moxifloxacin reflecting its additional significant potential to prevent emergence of resistant mutants. Conclusion: PAMCHD bears significant antituberculosis potential proven at various levels that warrant its further evaluation towards TB drug development.