Disposition kinetics of ML-1035 sulfoxide enantiomers and the prochiral sulfide in rats

ML‐1035, 4‐amino‐5‐chloro‐2‐[2‐(methylsulfinyl)ethoxy]‐N‐[2‐(diethylamino)ethyl]benzamide, is a sulfoxide compound and a racemic gastroprokinetic agent with a chiral center at the sulfur atom. We have investigated the disposition kinetics of (R)‐ML‐1035 sulfoxide (R) and (S)‐ML‐1035 sulfoxide (S) after the single enantiomers and the racemic mixture were administered to rats in separate experiments. There was no noticeable chiral inversion after either enantiomer dose. Both enantiomers were rapidly absorbed. After dosing with enantiomers or with the racemate, the resulting plasma concentration‐time curve of R was closely parallel to that of S in both intravenous and oral experiments, suggesting that the two enantiomers have approximately the same disposition kinetics. After intravenous enantiomer doses, only S underwent conversion to sulfide, suggesting that sulfidation in the liver is enantioselective. However, the enantioselective sulfidation after intravenous dosing did not introduce a difference in the global plasma disposition profiles between R and S, since the reduction reaction is a minor metabolic process. Other metabolic reactions such as sulfonation and mono‐N‐desethylations were not enantioselective. After oral administration, conversion to sulfide was observed for both enantioners, implicating the existence of a nonhepatic pathway in sulfidation. Administration of a prochiral sulfide dose was associated with an enantioselective sulfoxidation, in which the R/S concentration ratios increased as a function of time. In addition, enantiomeric interaction causing changes in pharmacokinetic parameters was observed after the oral racemate dose, while the interaction is negligible after an intravenous racemate dose, indicating a route dependency in enantiomeric interaction. © 1993 Wiley‐Liss, Inc.