IN-VITRO ACTIVITY OF THE BENZOXAZINORIFAMYCIN KRM-1648 AGAINST DRUG-SUSCEPTIBLE AND MULTIDRUG-RESISTANT TUBERCLE-BACILLI

We investigated the activity of benzoxazinorifamycin (KRM-1648) agains t several drug-susceptible and multidrug-resistant strains of tubercle bacilli. Since KRM-1648 is a rifamycin derivative, we included some s trains of Mycobacterium tuberculosis resistant to rifampin (RIF) among the multidrug-resistant strains, For RIF-susceptible strains, the MIC of KRM-1648 was much lower than that of RIF (MICs of KRM-1648 and RIF at which 90% of strains are inhibited, less than or equal to 0.015 an d less than or equal to 0.25 mu g/ml, respectively), The MBC of KRM-16 48 (range, 0.007 to 0.03 mu g/ml) was also much lower than that of RIF (range, 0.5 to 1.0 mu g/ml). Postantibiotic effect studies with KRM-1 648 showed a rapid reduction in the CFU counts with an exposure of 24 h or more, and its sterilizing effect was maintained even up to 21 day s thereafter, Parallel postantibiotic effect studies with RIF showed a less significant effect with a faster recovery of growth, and RIF fai led to sterilize the organisms even after 72 h of exposure, KRM-1648 a t 0.125 and 0.25 mu g/ml caused complete inhibition of intracellular g rowth of M. tuberculosis in J774 A.1 macrophages after 48 h of exposur e, After a similar exposure time RIF at a concentration of 0.25 mu g/m l caused complete inhibition of growth, but a concentration of 0.125 m u g/ml caused only a 50% reduction in growth compared with that of con trols at day 7, With 24 h of pulsed exposure of the intracellular orga nisms to 0.25 mu g of the drugs per ml, KRM-1648 caused complete inhib ition of intracellular growth, while RIF caused only moderate inhibiti on of intracellular growth. These findings suggest that KRM-1648 is a potentially useful drug for the treatment of tuberculosis.