Quantitative prediction of oral absorption of PEPT1 substrates based on in vitro uptake into Caco-2 cells

The method for predicting the fraction absorbed (Fa) of the PEPT1 substrates was established based on the in vitro uptake into Caco-2 cells. Uptake of a drug into Caco-2 cells was measured, and the carrier-mediated initial uptake clearance (ΔCLuptake) was calculated as the difference between the uptake clearance in the absence of glycyl-sarcosine (Gly-Sar) and that in the presence of 30 mM Gly-Sar. The ΔCLuptake of each drug was then divided by that of cephradine to obtain ΔCLuptake∗, which was a normalized parameter to correct for inter-day and/or inter-cell variability. Then, cephradine (CED), cefixime (CFIX), and cefotiam (CTM) were selected as marker compounds having excellent, medium and poor absorption, respectively. The ΔCLuptake∗ and Fa values for CED, CFIX and CTM were fitted to the equation derived from the complete radial mixing (CRM) model, and the scaling factor (A′) was obtained. Using the A′ value, Fa was predicted from the ΔCLuptake∗ value of each drug. Good correlation was observed between the predicted and reported Fa values, which demonstrated that Fa of PEPT1 substrates can be predicted based on the in vitro uptake in Caco-2 cells.