In vitro activity of new rifamycins against rifampicin-resistant M. tuberculosis and MAIS-complex mycobacteria.

Comparisons were made of the in vitro activity of rifampicin, and the rifamycin derivatives, rifapentine, rifabutin, CGP 29861, CGP 7040 and CGP 27557, against rifampicin-sensitive and rifampicin-resistant strains of Mycobacterium tuberculosis and against the Mycobacterium avium/intracellulare/scrofulaceum (MAIS) complex. The new rifamycins had MICs four to eight times lower than those of rifampicin against sensitive M. tuberculosis strains. Of the 35 rifampicin-resistant strains of M. tuberculosis, 31% were sensitive to rifabutin but only 3-11% to the other rifamycins. The proportions of the MAIS strains found to be sensitive were 35% for rifampicin, 50-60% for CGP 27557, rifapentine and rifabutin and 85-92% for CGP 29861 and CGP 7040.