Characterization of human cytochrome P450 mediated bioactivation of amodiaquine and its major metabolite N-desethylamodiaquine

Aims: Oxidative bioactivation of amodiaquine (AQ) by cytochrome P450s to a reactive quinoneimine is considered as an important mechanism underlying its idiosyncratic hepatotoxicity. However, because internal exposure to its major metabolite N-desethylamodiaquine (DEAQ) is up to 240-fold higher than AQ, bioactivation of DEAQ might significantly contribute to covalent binding. The aim of the present study was to compare the kinetics of bioactivation of AQ and DEAQ by human liver microsomes (HLM) and to characterize the CYPs involved in bioactivation of AQ and DEAQ. Methods: Glutathione was used to trap reactive metabolites formed in incubations of AQ and DEAQ with HLM and recombinant human cytochrome P450s (hCYPs). Kinetics of bioactivation of AQ and DEAQ in HLM and involvement of hCYPs were characterized by measuring corresponding glutathione conjugates (AQ-SG and DEAQ-SG) using a high-performance liquid chromatography method. Results: Bioactivation of AQ and DEAQ in HLM both exhibited Michaelis–Menten kinetics. For AQ bioactivation, enzyme kinetical parameters were K_m, 11.5 ± 2.0 μmol l^–1, V_max, 59.2 ± 3.2 pmol min⁻¹ mg⁻¹ and CL_int, 5.15 μl min⁻¹ mg⁻¹. For DEAQ, parameters for bioactivation were K_m, 6.1 ± 1.3 μmol l^–1, V_max, 5.5 ± 0.4 pmol min⁻¹ mg⁻¹ and CL_int 0.90 μl min⁻¹ mg⁻¹. Recombinant hCYPs and inhibition studies with HLM showed involvement of CYP3A4, CYP2C8, CYP2C9 and CYP2D6 in bioactivation. Conclusions: The major metabolite DEAQ is likely to be quantitatively more important than AQ with respect to hepatic exposure to reactive metabolites in vivo. High expression of CYP3A4, CYP2C8, CYP2C9, and CYP2D6 may be risk factors for hepatotoxicity caused by AQ-therapy.