A semisynthetic ansamycin antibiotic with potent antimycobacterial properties. Rifabutin inhibits bacterial DNA-dependent RNA polymerase, thereby suppressing the initiation of RNA formation and leading to inhibition of RNA synthesis and transcription. Check foractive clinical trials using this agent. or A drug used to prevent the spread of a bacterium called in patients with advanced HIV (the virus that causes AIDS) infection. It blocks an enzyme that the bacteria need to grow. It is a type of antibiotic. Also called Mycobutin.