S. Pilote,1 A. Kamaliza,1 J. Turgeon,2 V. Michaud,2 C. Simard,1 B. Drolet1; 1Institut Universitaire de Cardiologie et de Pneumologie de Quebec, Quebec City, QC, Canada, 2CHUM, Montreal, QC, Canada

BACKGROUND: Several organs express various levels and combinations of drug metabolizing enzymes. This suggests that local concentrations are driven not only by drug plasma levels, but also by the activities of these tissue-specific enzymes. Thus, a given CYP450 can control the residence time of a drug in a tissue and consequently, its action and toxicity. Interestingly, many drugs were shown to prolong the QT interval by inhibiting the rapid component of the cardiac delayed rectifier potassium current (IKr), encoded by hERG. The aim of this study was to demonstrate that block of hERG current (IKr) by methadone, a known QT-prolonging drug, is modulated by the coexpression of functional CYP450 2B6, responsible for in vivo metabolism and clearance of this drug, in HEK293 cells stably expressing hERG.
METHODS: Whole-cell voltage-clamp recordings were made every 5 min at room temperature: 1) on hERG stably transfected cells, and 2) on double hERG-CYP450 2B6 stably transfected cells. The cytosol of each cell was exposed during 60 min to either intrapipette solution containing no drug (baseline) or methadone 1 or 5 µM.
RESULTS: Methadone 5 µM caused a significant reduction (-27%, p<0.01) of hERG tail current amplitude recorded at -40 mV, after a step of +10 mV, in HEK293-hERG stably transfected cells compared to baseline. Surprisingly, a significant increase (+77%, p<0.001) was observed with methadone 5 µM in double hERG-CYP450 2B6 transfected cells, compared to baseline. No significant difference was observed with methadone 1 µM for the two groups of cells when compared to their respective baseline.
CONCLUSION: Coexpression of CYP450 2B6 increases the maximal amplitude of hERG tail current in cells intracellularly exposed to methadone 5 µM. This suggests an ‘as yet to be determined’ hERG-methadone-CYP450 2B6 interaction.