In vitro susceptibility of Mycobacterium avium complex and Mycobacterium tuberculosis strains to a spiro-piperidyl rifamycin.

The spiro-piperidyl rifamycins are newly synthesized rifamycin S compounds. One of these compounds, LM 427, was tested in vitro against strains of the Mycobacterium avium complex and strains of M. tuberculosis; LM 427 inhibited 81.3% of 155 strains of the M. avium complex tested at a concentration of 1.0 microgram/ml compared with 5.8% inhibited by the same concentration of rifampin. Twenty-nine strains were resistant to both LM 427 and rifampin at 1.0 microgram/ml. Further testing of these 29 strains showed LM 427 inhibitory for all but 5 strains at 2.0 micrograms/ml and inhibitory for all but 1 at 5.0 micrograms/ml. Rifampin, on the other hand, inhibited none at 2.0 micrograms/ml and 11 strains at 5.0 micrograms/ml. The in vitro activity of LM 427 was also compared with rifampin by testing both compounds against M. tuberculosis at 1.0 microgram/ml. This comparison showed that all strains susceptible to rifampin were also susceptible to LM 427. However, 16 strains were susceptible to LM 427 and resistant to rifampin. The inhibition of drug-resistant mycobacterial species that cause pulmonary disease makes this compound an important consideration for future clinical studies.