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

Amiodarone (AMI) is a potent antiarrhythmic drug, but its metabolism has no t yet been fully documented. Mono-N-desethylamiodarone (MDEA) is its only k nown metabolite. Our preliminary investigations using rabbit liver microsom es had shown that in vitro AMI was biotransformed to MDEA, and the latter w as rapidly further biodegraded to other unknown products. The aim of the pr esent 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-M S revealed that MDEA had been oxygenated. Hydrogen-deuterium exchange exper iments 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 hydroxyl ation was the n-butyl side chain. NMR analysis (H-1 NMR, one-dimensional-to tal correlation spectroscopy, and heteronuclear multiple-bond correlation s pectroscopy) established the 3-position (omega -1) of the butyl moiety as t he specific carbon atom that is hydroxylated. Rat liver microsomes were als o 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 tissu e of four rats receiving AMI, suggesting that the hydroxylated MDEA was a s econdary metabolite of AMI. Conclusion: in mammals, MDEA is hydroxylated to the secondary metabolite of AMI {2-(3-hydroxybutyl)-3-[4-(3-ethylamino-1-o xapropyl)-3,5-diiodobenzoyl]-benzofuran}.