The cytochrome P450 2B6 (CYP2B6) is the main catalyst of efavirenz primary and secondary metabolism: implication for HIV/AIDS therapy and utility of efavirenz as a substrate marker of CYP2B6 catalytic activity.

We used human liver microsomes (HLMs) and recombinant cytochromes P450 (P450s) to identify the routes of efavirenz metabolism and the P450s involved. In HLMs, efavirenz undergoes primary oxidative hydroxylation to 8-hydroxyefavirenz (major) and 7-hydroxyefavirenz (minor) and secondary metabolism to 8,14-dihydroxyefavirenz. The formation of 8-hydroxyefavirenz in two HLMs showed sigmoidal kinetics (average apparent Km, 20.2 micro M; Vmax, 140 pmol/min/mg protein; and Hill coefficient, 1.5), whereas that of 7-hydroxyefavirenz formation was characterized by hyperbolic kinetics (Km, 40.1 micro M and Vmax, 20.5 pmol/min/mg protein). In a panel of 10 P450s, CYP2B6 formed 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz from efavirenz (10 micro M) at the highest rate. The Km value for the formation of 8-hydroxyefavirenz in CYP2B6 derived from hyperbolic Eq. 12.4 micro M) was close to that obtained in HLMs (Km, 20.2 micro M). None of the P450s tested showed activity toward 7-hydroxylation of efavirenz. When 8-hydroxyefavirenz (2.5 micro M) was used as a substrate, 8,14-dihydroxyefavirenz was formed by CYP2B6 at the highest rate, and its kinetics showed substrate inhibition (Ksi, approximately 94 micro M in HLMs and approximately 234 micro M in CYP2B6). In a panel of 11 HLMs, 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz formation rates from efavirenz (10 micro M) correlated significantly with the activity of CYP2B6 and CYP3A. N,N',N"-Triethylenethiophosphoramide (thioTEPA; 50 micro M) inhibited the formation rates of 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz from efavirenz (10 micro M) by > or = 60% in HLMs) and CYP2B6, with Ki values < 4 micro M. In conclusion, CYP2B6 is the principal catalyst of efavirenz sequential hydroxylation. Efavirenz systemic exposure is likely to be subject to interindividual variability in CYP2B6 activity and to drug interactions involving this isoform. Efavirenz may be a valuable phenotyping tool to study the role of CYP2B6 in human drug metabolism.