Mechanism-based inactivation of human cytochrome P4502C8 by drugs in vitro

J Pharmacol Exp Ther. 2004 Dec;311(3):996-1007. doi: 10.1124/jpet.104.071803. Epub 2004 Aug 10.


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 6alpha-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 (KI and kinact) of 1.2 microM and 0.243 min(-1), 1.5 microM and 0.079 min(-1), 17.5 microM and 0.065 min(-1), 49.9 microM and 0.036 min(-1), 294 microM and 0.083 min(-1), and 374 microM 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Algorithms
  • Antidepressive Agents, Tricyclic / pharmacology
  • Antineoplastic Agents, Phytogenic / antagonists & inhibitors
  • Antineoplastic Agents, Phytogenic / pharmacokinetics
  • Aryl Hydrocarbon Hydroxylases / antagonists & inhibitors*
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Carbon Monoxide / metabolism
  • Cytochrome P-450 CYP2C8
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme Inhibitors
  • Cytochrome P-450 Enzyme System / genetics
  • Enzyme Inhibitors / pharmacology*
  • Gene Expression Regulation / drug effects
  • Humans
  • Hydroxylation
  • In Vitro Techniques
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology
  • Monoamine Oxidase Inhibitors / pharmacology
  • Orexin Receptors
  • Paclitaxel / antagonists & inhibitors
  • Paclitaxel / pharmacokinetics
  • Plasmids / genetics
  • Receptors, G-Protein-Coupled
  • Receptors, Neuropeptide / drug effects
  • Receptors, Neuropeptide / genetics
  • Recombinant Proteins / metabolism
  • Spectrophotometry, Ultraviolet
  • Sulfonamides / pharmacology
  • Testosterone / pharmacokinetics
  • Torsemide
  • Ultrafiltration


  • Antidepressive Agents, Tricyclic
  • Antineoplastic Agents, Phytogenic
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Monoamine Oxidase Inhibitors
  • Orexin Receptors
  • Receptors, G-Protein-Coupled
  • Receptors, Neuropeptide
  • Recombinant Proteins
  • Sulfonamides
  • Testosterone
  • Carbon Monoxide
  • Cytochrome P-450 Enzyme System
  • Aryl Hydrocarbon Hydroxylases
  • CYP2C8 protein, human
  • CYP3A protein, human
  • Cytochrome P-450 CYP2C8
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human
  • Paclitaxel
  • Torsemide