In vitro metabolism of acitretin by human liver microsomes: Evidence of anacitretinoyl-coenzyme A thioester conjugate in the transesterification to etretinate

The aromatic retinoid acitretin is the primary active metabolite of etretin ate, and in this study we investigated the ethyl esterification of acitreti n to etretinate using [C-14]acitretin and human liver microsomes. Samples w ere analysed by TLC, HPLC, and LC-MS. Essential requirements for the transe sterification reaction were identified and included viable microsomal prote in, ATP, CoASH, and ethanol. Human liver microsomes catalysed formation of acitretinoyl-CoA at. the rate of 0.08 +/- 0.02 nmol/min/mg (mean +/- SD, N = 10). Acitretinoyl-CoA was pivotal for the transesterification to etretina te and in the presence of methanol, ethanol, n-propanol, n-butanol, and hex anol, the corresponding esters, namely methyl-, ethyl (etretinate)-, propyl -, butyl-, and hexyl-acitretinate, were formed. On average, 1.7% of the aci tretin present in the incubation was converted to etretinate in the presenc e of ethanol. In the absence of ethanol, transesterification did not procee d. Inhibition of the ester hydrolysis of etretinate by bis-p-nitrophenylpho sphate (BNPP, 1 mM) prevented futile cycling of etretinate via acitretinoyl -CoA. An additional finding was that acitretin (15-30 mu M) activated signi ficantly human liver microsomal long-chain fatty acid-CoA ligase (E.C.6.2.1 .3, LCL), resulting in enhanced formation of palmitoyl-CoA. This study demo nstrated that in the presence of ethanol the ethyl esterification of acitre tin to etretinate proceeds via formation of acitretinoyl-CoA. Predicting cl earance of acitretin in vivo via this unique metabolic pathway will be a ch allenge, as the intracellular concentration of ethanol could never be predi cted with any degree of accuracy in humans. BIOCHEM PHARMACOL 60;4:507-516, 2000. (C) 2000 Elsevier Science Inc.