Evaluation of in Vitro Models for Screening Alkaline Phosphatase-Mediated Bioconversion of Phosphate Ester Prodrugs

Generating a phosphate prodrug is one of the common approaches for circumventing poor solubility issues of a parent drug. Alkaline phosphatase (ALP) level was determined in rat intestine mucosa scraps, human colon carcinoma (Caco-2) cells, and Madin-Darby canine kidney (MDCK) cells to characterize in vitro models for ALP-mediated phosphate prodrug conversion. In addition, fosphenytoin and fosfluconazole were used as probe prodrugs to evaluate the models. The highest amount of ALP was detected in rat intestinal mucosa scraps, whereas ALP in 5-day cultured MDCK cells was minimal. As anticipated, ALP levels correlated with the parent drug conversion; the shortest cleavage half-life (t_1/2) was observed in rat mucosa scraps; and MDCK cells showed the slowest conversion. Furthermore, the polarized conversion for the prodrugs was observed in Caco-2 monolayer cells, suggesting the polarized localization of alkaline in differentiated Caco-2 cells. The rate of ALP-mediated conversion was prodrug concentration-dependent with Michaelis-Menten constants of 1160 and 351 μM for fosphenytoin and fosfluconazole, respectively, determined in Caco-2 cells. The results revealed that whereas the intestinal mucosa scraps reserved the highest ALP activities and were shown as a promising in vitro tool for screening the bioconversion of phosphate prodrug, Caco-2 monolayers could provide the predictive information of bioconversion and further offer the capability in characterizing the permeability of prodrug and parent drug.