Stereoselective metabolism of propafenone by human liver CYP3A4 expressed in transgenic Chinese hamster CHL cells lines

Aim: To investigate the stereoselective metabolism of propafenone (PPF) by human liver CYP3A4. METHODS: A chiral and an achiral HPLC were combined to determine the enantiomer of PPF in S-9 incubates prepared from transgenic C hinese hamster CHL cells lines expressing CYP3A4. The time-dependent study was performed using individual enantiomer or racemate at low or high substr ate concentration. Kinetic parameters were determined employing individual enantiomers as substrates. Enantiomeric inhibition experiments were perform ed by using R(-)-PPF as an inhibitor and S(+)-PPF as a substrate. RESULTS: Stereoselectivity was found in metabolism of racemic PPF at low substrate c oncentration (10 mg/L) (S<R), and lost at high substrate concentration (400 mg/L). When an individual enantiomer of high concentration (200 mg/L) was used as substrate, S( +)-PPF was eliminated faster than its isomer (S>R). H owever, the opposite situation was observed at low concentration (5 mg/L) ( S<R). The V-max of S (+) -PPF was significantly greater than that of R(-) - PPF [(2.66<plus/minus>0.32) vs (1.71 +/-0.19) mu mol.mg(-1).min(-1)] The K- m of R(-)-PPF was significantly lower than that of S (+) -PPF [(73 +/- 11) vs (185 +/- 17) mu mol.L-1]. R (-)-PPF inhibited S (+) -isomer with an IC50 value of 125 mu mol.L-1. CONCLUSION: It is concluded that stereoselectivit y in metabolism of propafenone via CYP3A4 depend on substrate concentration . Enantiomer/enantiomer interaction of PPF occurred at high concentration o f substrate, and resulted in the loss of stereoselectivity. There maybe no enantiomer/enantiomer interaction at low concentration thus keeping the sup eriority of R(-)-PPF in metabolism.