BIOTRANSFORMATION OF LIFIBROL (U-83860) TO MIXED GLYCERIDE METABOLITES BY RAT AND HUMAN HEPATOCYTES IN PRIMARY CULTURE

Lifibrol (U-83860), K12.148) is a lipid-lowering drug that has the pot ential to accumulate in the liver and induce hepatic peroxisome prolif eration. To investigate the identity and potential human relevance of persistent lifibrol-related residues in rat liver, rat and human hepat ocyte primary cultures were treated with 30 mu M of [C-14]lifibrol. Af ter a steady uptake for 24 hr, cellular levels of radioactivity became stable for the next 24-48 hr. A nonradioactive lifibrol chase caused an efflux of intracellular radioactivity. Cellular autoradiography rev ealed the association of radioactivity with small lipid drops at 6 hr exposure and with large lipid drops at 24 hr exposure, HPLC analysis o f media revealed that lifibrol acyl glucuronide and a t-butylcarboxyli c acid metabolite (U-94613) were the major metabolites of rat and huma n hepatocytes, respectively, Using an HPLC method suitable for nonpola r metabolites, the analysis of rat and human cell extracts revealed a broad band of multiple, radioactive peaks that had a similar retention and UV spectrum to a synthetic standard of lifibrol cholesterol ester , Fetch extracts of liver from rats treated with [C-14]lifibrol or unl abeled lifibrol and [C-14]acetate had a unique radioactive TLC band th at had similar HPLC retention to hepatocyte residues, The group of non polar peaks from the hepatocytes was purified by HPLC, Conversion of t he lifibrol sec-hydroxy group to a nicotinate ester afforded particle beam-electron impact mass spectra of the cholesterol ester standard an d hepatocyte residues. The derivatized rat hepatocyte residue did not contain detectable lifibrol cholesterol ester (M(+).816), but did cont ain molecular ion clusters corresponding to a mixed triglyceride of li fibrol and two fatty acids, Lifibrol-specific product ions of molecula r ion clusters centered at M(+).1021, 1047, and 1073 were observed at m/z 448, 430, and 310. The major lifibrol-containing triglycerides had a fatty acid composition of C-16-C-20 with 0-6 unsaturations.