When might rifabutin be used instead of rifampin in HIV-infected patients?

• A. In all HIV-infected patients
• B. In patients on certain ART regimens to reduce drug interactions
• C. Never used
• D. Only with isoniazid

Answer: (B)

The key idea is drug interactions between rifamycins and antiretroviral therapy. Rifampin is a strong inducer of liver enzymes (like CYP3A4), which can dramatically lower the blood levels of many antiretroviral drugs. That can undermine HIV control. Rifabutin, while still a rifamycin, induces these enzymes less strongly, so it causes fewer reductions in antiretroviral drug levels. Therefore, in HIV-infected patients who are on ART regimens that would be adversely affected by rifampin, rifabutin is chosen to allow TB (or MAC) treatment with fewer interactions. Dose adjustments may still be needed and some ART regimens require careful management, but this approach avoids the subtherapeutic ART levels seen with rifampin in those patients.