BackgroundThe cytochrome P450 (CYP) oxidative enzyme system, located primarily in the liver and small intestine, is responsible for metabolism and detoxification of numerous endogenous and exogenous substances. The most abundant CYP enzyme, CYP3A, is known to be involved in the metabolism of more than 200 commonly used medications. In experimental models of renal failure, both hepatic function and CYP enzyme content are reduced; however, direct evidence in humans is lacking. Evaluation of drug metabolism in patients with end‐stage renal disease is important because these patients use a large number of medications and are at risk of adverse reactions and drug‐drug interactions.MethodsWe measured hepatic CYP3A activity at baseline and after rifampin (INN, rifampicin) enzyme induction in 12 patients with end‐stage renal disease and 12 healthy, age‐matched controls. Hepatic CYP3A phenotype was characterized with the erythromycin breath test, and enzyme induction capacity was evaluated with a short course of rifampin (600 mg/d for 6 days).ResultsThe end‐stage renal disease group had 28% lower baseline erythromycin breath test values than controls (P < .05); however, enzyme induction capacity after rifampin administration was similar between groups (P = .70).ConclusionThe findings suggested that one mechanism by which patients with end‐stage renal disease are at increased risk of drug toxicity is reduced activity of the CYP3A enzyme pathway.Clinical Pharmacology & Therapeutics (2003) 73, 427–434; doi: 10.1016/S0009‐9236(03)00056‐0