Metabolism of amiodarone (part I): identification of a new hydroxylated metabolite of amiodarone.

UNLABELLED Amiodarone (AMI) is a potent antiarrhythmic drug, but its metabolism has not yet been fully documented. Mono-N-desethylamiodarone (MDEA) is its only known metabolite. Our preliminary investigations using rabbit liver microsomes had shown that in vitro AMI was biotransformed to MDEA, and the latter was rapidly further biodegraded to other unknown products. The aim of the present study was to investigate the chemical structure of the biotransformed compound of MDEA. Upon incubation of MDEA with rabbit liver microsomes and NADPH as cofactor, MDEA was biotransformed into three unknown products: X1, X2, and X3. The products were purified using chromatography. The chemical structure of the major product, X1, was investigated in detail. HPLC-ESI-MS revealed that MDEA had been oxygenated. Hydrogen-deuterium exchange experiments showed that the X1 molecule contained one exchangeable hydrogen atom more than its precursor MDEA, indicating that MDEA had been hydroxylated. Further results from ESI-MS/MS analysis indicated that the site of hydroxylation was the n-butyl side chain. NMR analysis (1H NMR, one-dimensional-total correlation spectroscopy, and heteronuclear multiple-bond correlation spectroscopy) established the 3-position (omega-1) of the butyl moiety as the specific carbon atom that is hydroxylated. Rat liver microsomes were also able to catalyze MDEA hydroxylation. Compound X1, as analyzed by HPLC-ESI-MS and ESI-MS/MS, was detected in the liver, heart, lung, and kidney tissue of four rats receiving AMI, suggesting that the hydroxylated MDEA was a secondary metabolite of AMI. CONCLUSION in mammals, MDEA is hydroxylated to the secondary metabolite of AMI [2-(3-hydroxybutyl)-3-[4-(3-ethylamino-1-oxapropyl)-3,5-diiodobenzoyl]-benzofuran].