R. Pelletier,1 S. Gravel,2 J. Huguet,2 F. Gaudette,3 J. Turgeon,2 V. Michaud2; 1Université de Montréal, Montreal, QC, Canada, 2CRCHUM/Université de Montréal, Montreal, QC, Canada, 3CRCHUM, Montreal, QC, Canada

BACKGROUND: The antipsychotic drug olanzapine (OLA) can prolong the QT interval. OLA undergoes extensive metabolism through glucuronidation, CYP450 and Flavin Monooxygenase (FMO). We have shown that CYP450s expressed in the human heart exhibit drug-metabolism activities which could regulate intracellular drug concentrations and actions. Our objective was to characterize OLA metabolism by CYP450 and FMO isoforms found in the human heart using human heart microsomes (HHM).
METHODS: Human heart tissues were obtained from left ventricles (n=3). HHM were prepared using differential centrifugation techniques. In vitro incubations were performed with OLA (1.5-250µM) and pooled human liver microsomes (HLM), purified recombinant isoforms (CYP1A1-2, 1B1, 2D6, 2J2, 3A-5 and FMO3) and HHM. Inhibition studies with specific and non-specific inhibitors and imunoinhibition assays were conducted in HLM. OLA and its major metabolites (2-OH OLA, 7-OH OLA, 4-N-oxide OLA and 4-N-demethyl-OLA) were quantified by LC-MS/MS.
RESULTS: Our results demonstratE that CYP2J2 had the highest CLint for 7-OH and 2-OH OLA (21.6 and 1.1µL/min) followed by CYP2D6 (4.9 and 0.9µL/min). The rank order for the CLint of N-oxide OLA was: CYP1B1>1A1>FMO3>3A4>2D6>2J2>1A2>3A5. An inhibition of 50% of N-oxide OLA formation was observed with methimazole and furafylline in HLM. Astemizole decreased OLA metabolism in HLM. Incubations with HHM showed that N-oxide OLA formation was >2-OH OLA>7-OH OLA. N-demethyl OLA was not detected in one heart sample.
CONCLUSION: Our data shows that OLA is a substrate of CYP2J2. We showed that the human heart can metabolize OLA. CYP2J2 and FMO are highly expressed in human hearts suggesting that variations in their activities may affect cardiac OLA concentrations.