The non-nucleoside reverse transcriptase inhibitor efavirenz (EFV) is directly conjugated by the UDP-glucuronosyltransferase (UGT) pathway to form EFV-N-glucuronide (EFV-G), but the enzyme(s) involved has not yet been identified. The glucuronidation of EFV was screened with UGT1A and UGT2B enzymes expressed in a heterologous system, and UGT2B7 was shown to be the only reactive enzyme. The apparent K(m) value of UGT2B7 (21 microM) is similar to the value observed for human liver microsomes (24 microM), whereas the variant allozyme UGT2B7*2 (Tyr(268)) displayed similar kinetic parameters. Because 3'-azido-3'-deoxythymidine (AZT), one of the most current nucleotide reverse transcriptase inhibitors prescribed in combination with EFV, is also conjugated by UGT2B7, the potential metabolic interaction between EFV and AZT has been studied using human liver microsomes. Glucuronidation of both drugs was inhibited by one another, in a concentration-dependent manner. At K(m) values (25 and 1000 microM for EFV and AZT, respectively), EFV inhibited AZT glucuronidation by 47%, whereas AZT inhibited EFV glucuronidation by 23%. With a K(i) value of 17 microM for AZT-glucuronide formation, EFV appears to be one of the most selective and potent competitive inhibitor of AZT glucuronidation in vitro. Moreover, assuming that concentrations of EFV achieved in plasma (C(max) = 12.9 microM) are in a range similar to its K(i) value, it was estimated that EFV could produce a theoretical 43% inhibition of AZT glucuronidation in vivo. We conclude that UGT2B7 has a major role in EFV glucuronidation and that EFV could potentially interfere with the hepatic glucuronidation of AZT.