Short ReviewNew Insights into Drug Absorption: Studies with SirolimusPaine, Mary F.*; Leung, Louis Y.†; Watkins, Paul B.*Author Information From the *General Clinical Research Center and Division of Pharmacotherapy, University of North Carolina, Chapel Hill, North Carolina; and †Drug Safety & Metabolism, Wyeth Research, Collegeville, Pennsylvania. Accepted for publication March 31, 2004. This work was presented at the Pre-Congress meeting of IATDMCT in Basel in 2003. This work was supported in part by a National Research Service Award from the National Institute of General Medical Sciences, GM19034 (to M.F.P.), and by the National Institutes of Health grant GM38149 (to P.B.W.). Reprints: Mary F. Paine, General Clinical Research Center, Room 3005, Bldg APCF, CB# 7600, UNC Hospitals, Chapel Hill, NC 27599-7600 (e-mail: [email protected]). Therapeutic Drug Monitoring: October 2004 - Volume 26 - Issue 5 - p 463-467 Buy Abstract Sirolimus is a recently marketed immunosuppressant that, in common with cyclosporine and tacrolimus, exhibits a low average oral bioavailability (~20%). Likewise, sirolimus is a substrate for the major drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4) and the efflux transporter P-glycoprotein (P-gp), both of which are expressed in close proximity in epithelial cells lining the small intestine. Using CYP3A4-expressing Caco-2 cell monolayers, we examined the interplay between metabolism and transport on the intestinal first-pass extraction of sirolimus. Modified Caco-2 cells metabolized [14C]sirolimus to the same CYP3A4-mediated metabolites as human small intestinal and liver microsomes. [14C]Sirolimus also degraded to the known ring-opened product, seco-sirolimus. A ring-opened dihydro species (M2) was, surprisingly, the major product detected in cells at all sirolimus concentrations examined (2–100 μmol/L) and in incubations with human liver and intestinal homogenates but not in corresponding microsomes. M2 formation was NADPH-dependent but unaffected by prototypical CYP3A4 inhibitors. Although M2 was formed from purified seco-sirolimus (20 μmol/L) in the homogenates, it was not detected in cells when seco-sirolimus was added to the apical compartment because seco-sirolimus was essentially impermeable to the apical membrane. Sirolimus, seco-sirolimus (basolaterally dosed), and M2 were all secreted across the apical membrane, and secretion of each was inhibited by the P-gp inhibitor LY335979 (zosuquidar trihydrochloride). Along with CYP3A4-mediated metabolism and P-gp-mediated efflux, a novel elimination pathway was identified that may also contribute to the first-pass extraction, and hence low oral bioavailability, of sirolimus. This new insight into the intestinal elimination of sirolimus, which was not identified using traditional drug metabolism/transport screening methods, may represent another source for the limited absorption of sirolimus. © 2004 Lippincott Williams & Wilkins, Inc.