Unexpected drug-drug interactions in human immunodeficiency virus (HIV) therapy: induction of UGT1A1 and bile efflux transporters by Efavirenz.

INTRODUCTION Efavirenz is an inducer of drug metabolism enzymes. We studied the effect of efavirenz and ritonavir-boosted darunavir on serum unconjugated and conjugated bilirubin, as probes for UGT1A1 and bile transporters. MATERIALS AND METHODS Healthy volunteers were enrolled in a clinical trial. There were 3 periods: Period 1, 10 days of darunavir 900 mg with ritonavir 100 mg once daily; Period 2, 14 days of efavirenz 600 mg with darunavir/ritonavir once daily; and Period 3, 14 days of efavirenz 600 mg once daily. Serum bilirubin (conjugated and unconjugated) concentrations were obtained at baseline, at the end of each phase and at exit. RESULTS We recruited 7 males and 5 females. One subject developed grade 3 hepatitis on efavirenz and was excluded. Mean serum unconjugated bilirubin concentrations were 6.09 μmol/L (95% confidence interval [CI], 4.99 to 7.19) at baseline, 5.82 (95% CI, 4.88 to 6.76) after darunavir/ritonavir, 4.00 (95% CI, 2.92 to 5.08) after darunavir/ritonavir with efavirenz, 3.55 (95% CI, 2.58 to 4.51) after efavirenz alone and 5.27 (95% CI, 3.10 to 7.44) at exit (P <0.01 for the efavirenz phases). Mean serum conjugated bilirubin concentrations were 3.55 μmol/L (95% CI, 2.73 to 4.36) at baseline, 3.73 (95% CI, 2.77 to 4.68) after darunavir/ritonavir, 2.91 (95% CI, 2.04 to 3.78) after darunavir/ritonavir with efavirenz, 2.64 (95% CI, 1.95 to 3.33) after efavirenz alone and 3.55 (95% CI, 2.19 to 4.90) at exit (P <0.05 for the efavirenz phases). CONCLUSION Efavirenz decreased unconjugated bilirubin by 42%, suggesting UGT1A1 induction. Efavirenz also decreased conjugated bilirubin by 26%, suggesting induction of bile efflux transporters. Ritonavir-boosted darunavir had no effect on bilirubin concentrations. These results indicate that efavirenz may reduce concentrations of drugs or endogenous substances metabolized by UGT1A1 or excreted by bile efflux transporters.