Abstract

A systematic in vitro study was carried out to elucidate the enzymes responsible for the metabolism of haloperidol (HAL) using human liver microsomes and recombinant human cytochrome P450 isoenzymes. In the first series of experiments, recombinant cytochrome P450 (P450) isoenzymes were used to evaluate their catalytic involvement in the metabolic pathways of HAL. Recombinant CYP3A4, CYP3A5, and CYP1A1 were shown to be able to catalyze the metabolism of HAL to its pyridinium analog (HP⁺) and the oxidation of reduced HAL (RH) back to HAL; Recombinant CYP3A4, CYP3A5, CYP1A1, CYP2C19, CYP2C8, CYP2C9, and CYP2D6 were able to catalyze the dealkylation of HAL to 4-(4-chlorophenyl)-4-hydroxypiperidine (CPHP). CYP3A4 was capable of metabolizing HAL to its tetrahydropyridine analog 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6-tetrahydropyridine and metabolizing to CPHP; CYP3A4 and CYP3A5 were able to metabolize RH to its pyridinium analog (RHP⁺); CYP1A1, CYP1A2, and CYP3A4 were able to catalyze the oxidation of RHP⁺ to HP⁺. In the second series of experiments, the metabolic activities of human liver microsomes from 12 donors were correlated with catalytic activities of selective substrates of different P450 isoenzymes and immuno-reactivities toward different P450 isoenzymes. CYP3A4 activities were found to correlate to all the seven metabolic pathways of HAL mentioned above. This suggests a prominent role for CYP3A4 in the metabolism of HAL. Interestingly, it was found that recombinant CYP1A1 has the highest activity for oxidizing RHP⁺ to HP⁺. The activity of recombinant CYP1A1 was 50 times higher than CYP1A2 and 220 times higher than CYP3A4.