Antimicrobial activities of benzoxazinorifamycin KRM-1648, clarithromycin and levofloxacin against intracellular Mycobacterium avium complex phagocytosed by murine peritoneal macrophages

The in-vitro activities of KRM-1648, a new benzoxazinorifamycin, clarithromycin and levofloxacin against clinical isolates of Mycobacterium avium complex (MAC) were measured using various methods of assay and compared with their in-vivo therapeutic activities against MAC infection in mice. The MICs varied according to drug in the order KRM-1648 << clarithromycin < levofloxacin. However, KRM-1648 and clarithromycin but not levofloxacin had similar therapeutic outcomes in MAC-infected mice. KRM-1648 and clarithromycin given at clinical dosages caused 1 to 2 log unit reductions in bacterial loads in the lungs of host mice. The values of Cmax (lung)/MBC were more closely related to the therapeutic efficacy of these drugs in mice than were MICs and MBCs alone. Potent microbicidal activity was observed with KRM-1648 and clarithromycin but not with levofloxacin against extracellularly growing MAC (EG-MAC) in a liquid medium. These two agents caused more than 3 log unit killing of MAC during a 5 day incubation, when added at concentrations equivalent to Cmax (lung). The anti-EG-MAC bactericidal activity of these drugs was greater than their efficacy in mice in vivo. KRM-1648 and clarithromycin but not levofloxacin caused respectively 2 and 0.5 log unit killing of intracellularly growing MAC (IG-MAC) in murine peritoneal macrophages. The profiles of bacterial killing effects of these agents against IG-MAC accurately reflected their therapeutic effects in mice, although the in-vivo activity of KRM-1648 was still overestimated using even this parameter.