The Effect of Ritonavir on Human CYP2B6 Catalytic Activity: Heme Modification Contributes to the Mechanism-based Inactivation of CYP2B6 and CYP3A4 by Ritonavir

The mechanism-based inactivation of human CYP2B6 by ritonavir (RTV) in a reconstituted system was investigated. The inactivation is time, concentration, and NADPH dependent and exhibits a K(I) of 0.9 μM, a k(inact) of 0.05 min⁻¹, and a partition ratio of approximately 3. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that the protonated molecular ion of RTV exhibits an m/z at 721 and its two major metabolites are an oxidation product with MH⁺ at m/z 737 and a deacylated product with MH⁺ at m/z 580. Inactivation of CYP2B6 by incubation with 10 μM RTV for 10 min resulted in an approximately 50% loss of catalytic activity and native heme, but no modification of the apoprotein was observed. RTV was found to be a potent mixed-type reversible inhibitor (K(i) = 0.33 μM) and a type II ligand (spectral dissociation constant-K(s) = 0.85 μM) of CYP2B6. Although previous studies have demonstrated that RTV is a potent mechanism-based inactivator of CYP3A4, the molecular mechanism responsible for the inactivation has not been determined. Here, we provide evidence that RTV inactivation of CYP3A4 is due to heme destruction with the formation of a heme-protein adduct. Similar to CYP2B6, there is no significant modification of the apoprotein. Furthermore, LC-MS/MS analysis revealed that both CYP3A4 and human liver microsomes form an RTV-glutathione conjugate having a MH⁺ at m/z 858 during metabolism of RTV, suggesting the formation of an isocyanate intermediate leading to formation of the conjugate.