Abstract

IC₅₀ shift and time dependent inhibition (TDI) experiments were carried out to measure the ability of amiodarone (AMIO) and its circulating human metabolites to reversibly and irreversibly inhibit CYP1A2, CYP2C9, CYP2D6 and CYP3A4 activities in human liver microsomes. [I]_u/K_i,u values were calculated and used to predict in vivo AMIO drug-drug interactions (DDIs) for pharmaceuticals metabolized by these four enzymes. Based on these values, the minor metabolite di-desethylamiodarone (DDEA) is predicted to be the major cause of DDIs with xenobiotics primarily metabolized by CYP1A2, CYP2C9 or CYP3A4, while AMIO and its mono-desethyl derivative (MDEA) are the most likely cause of interactions involving inhibition of CYP2D6 metabolism. AMIO drug interactions predicted from the reversible inhibition of the four P450 activities were found to be in good agreement with the magnitude of reported clinical DDIs with lidocaine, warfarin, metoprolol and simvastatin. TDI experiments showed DDEA to be a potent inactivator of CYP1A2 (K_I = 0.46 μM, k_inact = 0.030 min^-1), while MDEA was a moderate inactivator of both CYP2D6 (K_I = 2.7 μM, k_inact = 0.018 min^-1) and CYP3A4 (K_I = 2.6 μM, k_inact = 0.016 min^-1). For DDEA and MDEA, mechanism-based inactivation appears to occur through formation of a metabolic intermediate (MI) complex. Additional metabolic studies strongly suggest that CYP3A4 is the primary enzyme involved in the metabolism of AMIO to both MDEA and DDEA. In summary, these studies demonstrate both the diversity of likely inhibitory mechanisms with AMIO and the need to consider metabolites as the 'culprit' in inhibitory P450-based DDIs.