In vitro activities against mycobacteria of two long-acting rifamycins, FCE22807 and CGP40/469A (SPA-S-565).

The in vitro activities of two new long-acting rifamycins, FCE22807 a derivative of FCE22250, and CGP40/469A (SPA-S-565) were studied. When compared with rifampicin, the minimal inhibitory concentrations (MIC) against Mycobacterium tuberculosis of both were 4 times lower but neither was particularly active against rifampicin-resistant strains of M. tuberculosis nor against M. avium-intracellulare-scrofulaceum complex strains. A drug is likely to be particularly effective in widely spaced intermittent dosage if it has a long half-life and high bactericidal activity. When tested against M. tuberculosis in the logarithmic and in the stationary phase of growth, FCE22807 was amongst the most bactericidal of the rifamycins while CGP40/469A had little bactericidal activity.