In vitro study on the involvement of CYP1A2, CYP2D6 and CYP3A4 in the metabolism of haloperidol and reduced haloperidol.

To investigate in vitro which CYP isoforms (CYP1A2, CYP2D6 and CYP3A4) are involved in the biotransformation of haloperidol (HAL) and reduced haloperidol (RHAL). The biotransformation of HAL and RHAL is evaluated by measuring HAL and RHAL remaining after incubation with human liver microsomes and with supersomes from human baculovirus-infected cells expressing human P(450) isoforms. The influence of chemical- and immuno-inhibition of specific isoforms on the disappearance of HAL and RHAL was also studied. After 60-min incubation of 2 microM and 20 microM HAL or RHAL with human liver microsomes, for HAL, 58% and 64%, respectively, remained in the incubation mixture, for RHAL, 53% and 66%, respectively. Ketoconazole had the most pronounced inhibitory effect on the biotransformation of both substrates, while for quinidine and furafylline there was only a weak or no influence. Anti-CYP3A4 antibodies inhibited strongly the biotransformation of HAL and RHAL, while the influence of anti-CYP2D6 antibodies was much less pronounced. After incubation with supersomes of recombinant CYP3A4, HAL and RHAL disappeared rapidly; disappearance was slow after incubation with CYP2D6 supersomes, and negligible with CYP1A2 supersomes. The results show that CYP3A4 is the most important CYP isoenzyme involved in the biotransformation of HAL and RHAL, and that the metabolism by CYP2D6 is only a minor pathway; CYP1A2 has no or only a negligible influence.