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ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION

Mechanism-Based Inactivation of Human Cytochrome P4502C8 by Drugs in Vitro

Department of Clinical Pharmacology, Flinders University and Flinders Medical Centre, Adelaide, Australia

Studies were conducted to evaluate the potential mechanism-based inactivation of recombinant and human liver microsomal CYP2C8 by clinically used drugs. Several tricyclic antidepressants, calcium channel blockers, monoamine oxidase inhibitors, and various other known CYP3A4 inhibitors exhibited greater inhibition of CYP2C8 (paclitaxel 6-hydroxylation) following preincubation, consistent with mechanism-based inactivation. Inactivation of recombinant CYP2C8 by phenelzine, amiodarone, verapamil, nortriptyline, fluoxetine, and isoniazid was of the pseudo-first order type and was characterized by respective inactivation kinetic constants (K_I and k_inact) of 1.2 µM and 0.243 min^–1, 1.5 µM and 0.079 min^–1, 17.5 µM and 0.065 min^–1, 49.9 µM and 0.036 min^–1, 294 µM and 0.083 min^–1, and 374 µM and 0.042 min^–1. Spectral scanning of recombinant CYP2C8 demonstrated the formation of metabolite-intermediate complexes with verapamil, nortriptyline, fluoxetine, and isoniazid, but not amiodarone. In contrast, inactivation by phenelzine resulted from heme destruction by free radicals. Studies with human liver microsomes (HLMs) revealed that nortriptyline, verapamil, and fluoxetine were not mechanism-based inactivators (MBIs) of CYP2C8. Simultaneous inactivation of CYP2C8 and CYP3A4 (paclitaxel 3'-phenyl-hydroxylation) was observed using amiodarone, isoniazid, and phenelzine with the efficiency of inactivation greater for the CYP3A4 pathway. With the exception of phenelzine, glutathione and superoxide dismutase failed to protect CYP2C8 (recombinant and HLMs) or CYP3A4 from inactivation by MBIs. However, the alternate CYP2C8 substrate, torsemide, prevented CYP2C8 inactivation in all cases. These data are consistent with mechanism-based inactivation of CYP2C8 by a range of commonly prescribed drugs, several of which have been implicated in clinically important drug-drug interactions.

Address correspondence to: Professor John O. Miners, Department of Clinical Pharmacology, Flinders University and Flinders Medical Centre, Bedford Park, Adelaide, South Australia 5042, Australia. E-mail: john.miners{at}flinders.edu.au

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