Patients on HAART
Rifamycins induce hepatic CYP3A4 enzymes and can accelerate metabolism of protease inhibitors and some non-nucleoside reverse transcriptase inhibitors (NNRTIs), decreasing their serum concentrations, possibly to ineffective levels. The degree to which each drug activates CYP3A4 differs: rifampin is the most potent and rifabutin the least. In addition, rifabutin is a substrate for CYP3A4; protease inhibitors slow its metabolism, increasing serum concentrations and possibly toxicity.