Bioactivation of loxoprofen to a pharmacologically active metabolite and its disposition kinetics in human skin

Loxoprofen (LX) is a prodrug‐type non‐steroidal anti‐inflammatory drug which is used not only as an oral drug but also as a transdermal formulation. As a pharmacologically active metabolite, thetrans‐alcohol form of LX (trans‐OH form) is generated after oral administration to humans. The objectives of this study were to evaluate the generation of thetrans‐OH form in humanin vitroskin and to identify the predominant enzyme for its generation. In the permeation and metabolism study using humanin vitroskin, both the permeation of LX and the formation of thetrans‐OH form increased in a time‐ and dose‐dependent manner after the application of LX gel to the skin. In addition, the characteristics of permeation and metabolism of both LX and thetrans‐OH form were examined by a mathematical pharmacokinetic model. TheKmvalue was calculated to be 10.3 mmin the humanin vitroskin. The predominant enzyme which generates thetrans‐OH form in human whole skin was identified to be carbonyl reductase 1 (CBR1) by immunodepletion using the anti‐human CBR1 antibody. The results of the enzyme kinetic study using the recombinant human CBR1 protein demonstrated that theKmandVmaxvalues were 7.30 mmand 402 nmol/min/mg protein, respectively. In addition, it was found that no unknown metabolites were generated in the humanin vitroskin. This is the first report in which LX is bioactivated to thetrans‐OH form in human skin by CBR1.