Vol. 29, Issue 10, 1307-1315, October 2001

Mechanism of Intestinal Absorption and Renal Reabsorption of an Orally Active Ace Inhibitor: Uptake and Transport of Fosinopril in Cell Cultures

Cathaleen Shu, Hong Shen, Ulrich Hopfer, and David E. Smith

College of Pharmacy and Upjohn Center for Clinical Pharmacology, University of Michigan, Ann Arbor, Michigan (C.S., H.S., D.E.S.); and Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio (U.H.)

The objective of this study was to delineate the transepithelial transport mechanisms of fosinopril in cultured cell lines expressing the intestinal and renal peptide transporters. Lineweaver-Burk, Dixon, and dose-response analyses revealed that GlySar uptake was competitively inhibited by fosinopril in both Caco-2 (K_i, 35.5 µM) and SKPT cells (K_i, 29.6 µM). Intracellular accumulations of fosinopril were 3 to 4 times higher from apical versus basolateral surfaces of the membrane, as was the apical-to-basal flux of the drug. The apical peptide transporter had a significantly greater affinity for fosinopril than did the basolateral peptide transporter in Caco-2 cells (K_m, 154 versus 458 µM, respectively; p < 0.001) and SKPT cells (K_m, 22 versus 104 µM, respectively; p < 0.001). Moreover, fosinopril uptake by the basolateral peptide transporter was less sensitive to changes in medium pH than the apical peptide transporter in both cell lines. Although Caco-2 cells are known to express PEPT1 protein (and not PEPT2), our immunoblot analyses provide definitive evidence that SKPT cells express PEPT2 protein (and not PEPT1). Taken as a whole, our findings demonstrate that fosinopril is transported intact by PEPT2 and PEPT1, with high-affinity and by a proton-coupled, saturable process. Our results also suggest that distinct peptide transporters exist at the basolateral and apical membranes and that they play an important role in modulating the intestinal absorption and renal reabsorption of peptides and peptide-like drugs.