Biol. Pharm. Bull. 28(4) 682—687 (2005)

flammation drugs (NSAIDs) and it is clinically the first choice among its own class, 2-arylpropionic acids, due to its low incidence of unwanted effects. It contains a chiral center adjacent to the carboxyl group and thus existed as ( ) R and ( ) S enantiomers. Ibuprofen was originally marketed as racemate and later in 1994 dexibuprofen was brought into the market in Austria and Switzerland based on the findings that the pharmacological activity exclusively resided in the ( ) S enantiomer. Despite this, racemate is still extensively used worldwide and there is no indication of replacing the racemate with the single S enantiomer. One important characteristics of ibuprofen pharmacokinetics is that the R enantiomer undergoes metabolic chiral inversion to its antipode but not vice versa. Since its first observance in 1967 by Adams, the pathway and involved biochemical mechanisms of ibuprofen inversion had been fundamentally elucidated in the past forty years. But reports concerning the site of inversion in the current literature still seemed to be contradictory. Mehvar and Jamali claimed that this unidirectional inversion occurred predominantly and presystemically in the gut based from a pharmacokinetics model study. Later observations that formulations with prolonged absorption exhibited greater extent of inversion supported the presystemic inversion of ibuprofen. To the contrary, no significant differences of the area under curve values (AUC) for S-ibuprofen was observed between oral and intravenous administration of racemic ibuprofen to humans, which precluded the existence of presystemic inversion. Similar research conducted in dogs led to the same conclusion. Furthermore, no evidence of intestinal inversion of ibuprofen was observed in an in situ isolated perfused rat intestine/liver system. To date, only one report gave direct evidence to the presystimic inversion of ibuprofen in humans. When ( ) R ibuprofen was incubated with ileum or colon segments excised from the cancer patients, substantial inversion of up to 30% was observed within 200 min. Thus, the site of inversion remained controversial. Although ( ) R ibuprofen was demonstrated to be inverted in all species tested, the inversion extent and site may vary from species to species, thus the results obtained from other species can not be extrapolated to humans. Unfortunately, human intestinal segments could only be obtained from patients who required intestinal removal, which limited the extension of research and might lead to high inter-assay variability. Therefore, development of a human-resourced model for research of the inversion of ibuprofen and other 2-arylpropionic acid drugs is of special importance. Caco-2 cells, a cell line derived from human colon carcinoma, were extensively applied and proved a suitable model in the research of absorption characteristics and mechanisms of drugs. When Caco-2 cells reach confluence, the cells differentiate morphologically into polarized, columnar cells with organized microvillus membranes, and also acquire many biochemical characteristics of enterocytes. Most importantly, long chain acyl-CoA synthetase (EC 6. 2. 1. 3), which was previously demonstrated to be stereoselective and the rate controlling enzyme in the ibuprofen inversion catalyzing ibuprofen-CoA formation, was reported to be active on catalyzing fatty acid metabolism in Caco-2 cells. Taking all information into account, a preliminary experiment had been conducted to test whether ibuprofen could undergo chiral inversion in Caco-2 cells. As a result, substantial inversion of ( ) R ibuprofen (a 20% inversion was observed within 24 h coincubation) was observed when incubated in Caco-2 cells. The present investigation was then designed to: 682 Biol. Pharm. Bull. 28(4) 682—687 (2005) Vol. 28, No. 4