Genetically determined stereoselective excretion of encainide in humans and electrophysiologic effects of its enantiomers in canine cardiac Purkinje fibers.

Encainide metabolism is mediated by the polymorphically distributed cytochrome P450IID6, which displays stereoselectivity for some substrates. In this study we found that urinary recovery during steady-state encainide in three poor metabolizers was high (49% to 80%), consisted mainly of unchanged encainide, was nonstereoselective (+/- ratio, 0.985 to 1.049), and was unchanged by quinidine, a potent inhibitor of P450IID6. In contrast, in seven extensive metabolizers the +/- urinary ratios were 1.20 +/- 0.06 for encainide and 0.81 +/- 0.06 (both p less than 0.01) for the cytochrome P450IID6 products O-desmethylencainide plus 3-methoxy-O-desmethylencainide; with quinidine the total percentage recovery rose from 4% +/- 4% to 37% +/- 9% because of increased recovery of unchanged encainide and became non-stereoselective (+/- ratio, 0.84 +/- 0.08 [encainide alone] versus 0.97 +/- 0.05 [encainide plus quinidine]). In vitro, encainide enantiomers depressed the maximum rate of metabolism with similar frequency and concentration dependence. We conclude that (-)-encainide undergoes preferential metabolism by cytochrome P450IID6; however, this genetically determined stereoselective disposition is unlikely to play a major role in mediating the clinical actions of encainide.