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 S9 incubates prepared from transgenic Chinese hamster CHL cells lines expressing CYP3A4. The time-dependent study was performed using individual enantiomer or racemate at low or high substrate concentration. Kinetic parameters were determined employing individual enantiomers as substrates. Enantiomeric inhibition experiments were performed 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 concentration (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 a substrate, S(+)-PPF was eliminated faster than its isomer (S < R). However, the opposite situation was observed at low concentration (5 mg/L) (S < R). The Vmax of S(+)-PPF was significantly greater than that of R(-)-PPF [(2.66 +/- 0.32) vs (1.71 +/- 0.19) micromol . mg-1 . min-1]. The Km of R(-)-PPF was significantly lower than that of S(+)-PPF [(73 +/- 11) vs (185 +/- 17) micromol/L]. R(-)-PPF inhibited S(+)-isomer with an IC50 value of 125 micromol . L-1.
Conclusion: It is concluded that stereoselectivity in metabolism of propafenone via CYP3A4 depend on substrate concentration. Enantiomer/enantiomer interaction of PPF occurred at high concentration of substrate, and resulted in the loss of stereoselectivity. There maybe no enantiomer/enantiomer interaction at low concentration thus keeping the superiority of R(-)-PPF in metabolism.