title: In vitro activity of the benzoxazinorifamycin KRM-1648 against drug-susceptible and multidrug-resistant tubercle bacilli.
abstract: We investigated the activity of benzoxazinorifamycin (KRM-1648) against several drug-susceptible and multidrug-resistant strains of tubercle bacilli. Since KRM-1648 is a rifamycin derivative, we included some strains 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, < or = 0.015 and < or = 0.25 micrograms/ml, respectively). The MBC of KRM-1648 (range, 0.007 to 0.03 microgram/ml) was also much lower than that of RIF (range, 0.5 to 1.0 microgram/ml). Postantibiotic effect studies with KRM-1648 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 days thereafter. Parallel postantibiotic effect studies with RIF showed a less significant effect with a faster recovery of growth, and RIF failed to sterilize the organisms even after 72 h of exposure. KRM-1648 at 0.125 and 0.25 microgram/ml caused complete inhibition of intracellular growth of M. tuberculosis in J774 A.1 macrophages after 48 h of exposure. After a similar exposure time RIF at a concentration of 0.25 microgram/ml caused complete inhibition of growth, but a concentration of 0.125 microgram/ml caused only a 50% reduction in growth compared with that of controls at day 7. With 24 h of pulsed exposure of the intracellular organisms to 0.25 micrograms of the drugs per ml, KRM-1648 caused complete inhibition of intracellular growth, while RIF caused only moderate inhibition of intracellular growth. These findings suggest that KRM-1648 is a potentially useful drug for the treatment of tuberculosis.
authors.affiliation: Department of Medicine, University of Illinois at Chicago 60612, USA.
authors.forename: J
authors.initials: J
authors.lastname: Luna-Herrera
authors.forename: M V
authors.initials: MV
authors.lastname: Reddy
authors.forename: P R
authors.initials: PR
authors.lastname: Gangadharam
mesh_terms: D000904:Antibiotics, Antitubercular / Q000494:pharmacology*; D018432:Drug Resistance, Multiple; D008826:Microbial Sensitivity Tests; D009169:Mycobacterium tuberculosis / Q000187:drug effects*; D012293:Rifampin / Q000494:pharmacology; D012294:Rifamycins / Q000494:pharmacology*
chemical_list: D000904:Antibiotics, Antitubercular; D012294:Rifamycins; C068980:KRM 1648; D012293:Rifampin