Loxoprofen is a 2-arylpropionic acid nonsteroidal anti-inflammatory drug (profen NSAID) with a cyclopentanone moiety. After absorption as unchanged form from gastrointestinal tract, the drug is transformed to the active metabolite, (2S, 1'R, 2'S)-trans-alcohol, which exerts pharmacological effects based on prostaglandin synthesis inhibition. The stereoselective determination method of the active metabolite was developed by combination of antibody-mediated extraction and diastereomeric amide derivatization for HPLC. Using the method, (2S, 1'R, 2'S)-trans-alcohol was found to be preferentially present in plasma among the possible eight monoalcohol enantiomers. The three stereoselective metabolic reactions, reduction of the cyclopentanone ring, conversion of cis-alcohol to trans-alcohol and chiral inversion of the propionic acid side chain, were favorable for the formation of the active metabolite. These stereoselective metabolism studies contributed to the development of this drug as a prodrug.
The mechanism of chiral inversion was studied by analysis of MS and NMR spectra of the rat urinary metabolite, tert-alcohol, after administration of 2-(2-isopropylindan-5-yl)propionic acid labeled with deuterium. Upon chiral inversion, deuterium atoms at the methyl position were retained but the methine deuterium was lost, contrary to the Wechetr's 2, 3-dehydro intermediate theory⁹⁾. Recently, many investigators have shown the results in conflict with 2, 3-dehydro intermediate mechanism. Nakamura et al¹¹⁾. proposed that the essential reaction in chiral inversion is the stereospecific CoA thioester formation of the R-enantiomers.
Since the activation of carboxylic acid to CoA thioester is also the fir st step for the amino acid conjugation, only the R-enantiomers was thought to yield amino acid conjugate in profen NSAID. This hypothesis was examined in dogs using 2-phenylpropionic acid (2-PPA) as a model compound. Unexpectedly, the 2-PPA glycine conjugate was produced from both enantiomers, and further the reverse chiral inversion (S to R) was observed in a simillar extent as the chiral inversion (R to S). The reverse chiral inversion was dependent on species and substrates employed, suggesting the presence of acyl CoA synthetases exhibting different stereoselectivity. The stereospecificity of glycine N-acyl transferase for the S-2-PPA CoA was presumed.