The stereoselective metabolism of halofantrine to desbutylhalofantrine in the rat: Evidence of tissue-specific enantioselectivity in microsomal metabolism

The pharmacokinetics of the antimalarial drug (+/-)-halofantrine are stereoselective in humans and rats. To better understand the stereoselective metabolism of the drug to its primary metabolite, desbutylhalofantrine (DHF), a series of in vitro and in vivo experiments were undertaken in the rat. Formation of (-)-DHF exceeded that of (+)-DHF in liver microsomes [(-):(+) ratio of intrinsic formation clearances - 1.4]. In contrast, in intestinal microsomes no significant stereoselectivity was noted in the formation of the DHF enantiomers. Intestinal microsomes were also less efficient at producing the DHF enantiomers than were liver microsomes. Based on kinetic analysis of the DHF formation, there appeared to be more than one enzyme involved in the biotransformation. (+/-)-Ketoconazole (KTZ) effectively inhibited the formation of both DHF enantiomers by both liver and intestinal microsomes, although the reduction was more marked in liver microsomes. Through a combination of the use of CYP antibodies and recombinant CYP isoenzymes, the involvement of CYP 2B1/2, 3A1, 3A2, 1A1, 2C11, 2C6, 2D1, and 2D2 were implicated in the metabolism of halofantrine to DHF. Of these, CYP3Al/2 and CYP2C11 appeared to be the primary isoenzymes involved, although CYP2C11 showed greater (+)-DHF than (-)-DHF formation, whereas for CYP3A1 it was similar to the isolated rat liver microsomes. In vivo, oral (+/-)-KTZ caused significant increases in plasma halofantrine and decreases in DHF enantiomer plasma concentrations.