Abstract

Irinotecan (CPT-11) is a water-soluble camptothecin (CPT) derivative that has been recently approved in the United States for patients as a first-line therapy in advanced colorectal cancer. Phase I clinical trials using oral CPT-11 have shown poor and variable oral bioavailability. The present study was designed to investigate the intestinal absorption and efflux mechanisms of CPT-11 using in vitro cell culture models, Caco-2 cells, and engineered Madine-Darby canine kidney (MDCK) II cells overexpressing P-glycoprotein (Pgp), canalicular multispecific organic anion transporter (cMOAT), and multidrug resistance-associated protein (MRP1). The intestinal absorptive and secretory transport of CPT-11 was investigated using Caco-2 cell monolayers. Secretory transport was concentration-dependent and saturable. The secretory efflux permeability (P_eff) of CPT-11 decreased with decreasing temperature, with an estimated activation energy of 19.6 ± 2.9 kcal/mol suggesting the involvement of active transporters. The involvement of potential secretory transporters was further characterized in MDCK II cells. The secretory efflux carrier permeability (P_c) was ∼4- and ∼2-fold greater in MDCK II/Pgp and MDCK II/cMOAT cells than that in MDCK II/wild-type cells. Furthermore, the secretory effluxP_eff of CPT-11 was significantly decreased by Pgp inhibitors, elacridar (GF120918) (IC₅₀ = 0.38 ± 0.06 μM) and verapamil (IC₅₀ = 234 ± 48 μM) in MDCK II/Pgp cells and by cMOAT inhibitor 3-([{3-(2-[7-chloro-2-quinolinyl]ethyl)phenyl}-{(3-dimethylamino-3-oxoprphyl)-thio}-methyl]-thio) propanoic acid (MK571) (IC₅₀ = 469 ± 60 μM) in MDCK II/cMOAT cells. Overall, the current study suggests that Pgp and cMOAT are capable of mediating the efflux of CPT-11 in vitro. Since both Pgp and cMOAT are expressed in the intestine, liver, and kidney, it is likely that these efflux transporters play a significant role limiting the oral absorption and disposition of this important anticancer drug.