CGP 4832, a new semisynthetic rifamycin derivative highly active against some gram-negative bacteria.

CGP 4832 (5) is a new derivative of rifamycin S, showing a very high degree of activity against certain Gram-negative bacteria, with MICs as much as 400 times lower than those of rifampicin. CGP 4832 and rifampicin inhibit DNA-dependent transcription in vitro to a similar extent, which excludes any difference in their effect on the target enzyme. The most plausible explanation for the potent activity of CGP 4832 is that it penetrates into bacterial cells by way of a specific mechanism. This hypothesis is corroborated by the high rate of mutations leading to bacterial strains resistant against CGP 4832.