Input rate-dependent stereoselective pharmacokinetics: effect of pulsatile oral input.

Computer simulation was used to test the effects of pulsatile oral input on the stereoselectivity in the area under the blood concentration-time curves (AUCs) of the enantiomers of racemic drugs. The effects of input rate determinants, namely, dose, dosage interval, and formulation on the stereoselectivity were investigated under both steady-state and nonsteady-state conditions. Simulations were carried out for drugs undergoing Michaelis-Menten hepatic metabolism with different enantiomeric maximum velocity (Vmax) or constant (Km) values. With pulsatile input, the enantiomeric AUC ratios of both types of drugs were dependent on all the determinants of input rate. However, in most cases, the direction of input rate-dependent changes in the enantiomeric AUC ratios for drugs with different enantiomeric Vmax was opposite of that for drugs with different enantiomeric Km. The direction and magnitude of changes in the enantiomeric AUC ratios were also dependent on the selected dose, dosage interval, and formulation. Further, different conclusions could be reached based on the nonsteady-state and steady-state data. Additional simulations were then performed to test the effects of input rate-dependent stereoselective pharmacokinetics on the bioequivalence of chiral drugs with nonlinear metabolism. These simulations suggested that bioequivalence studies based on the racemic drug measurement may result in erroneous conclusions for the individual enantiomers. The results of this study may be used as a tool for the design of experiments to test the input rate dependence of stereoselective pharmacokinetics and bioequivalence of racemic drugs in animals and humans.