Multidrug resistance protein 2 (MRP2) belongs to the ATP binding cassette family of transporters. Its substrates include organic anions and anticancer drugs. We have used transport assays with vesicles derived from Sf9 insect cells overproducing MRP2 to study the interactions of drugs, organic anions, and bile acids with three MRP2 substrates: estradiol-17-␤-D-glucuronide (E 2 17␤G), methotrexate, and glutathione-S-dinitrophenol. Complex inhibition and stimulation patterns were obtained, different from those observed with the related transporters MRP1 and MRP3. In contrast to a previous report, we found that the rate of E 2 17␤G transport by MRP2 increases sigmoidally with substrate concentration indicative of homotropic cooperativity. Halfmaximal transport was obtained at 120 M E 2 17␤G, in contrast to values < 20 M for MRP1 and 3. MRP2 stimulators, such as indomethacin and sulfanitran, strongly increased the affinity of MRP2 for E 2 17␤G (halfmaximal transport rates at 65 and 16 M E 2 17␤G, respectively) and shifted the sigmoidal dependence of transport rate on substrate concentration to a more hyperbolic one, without substantially affecting the maximal transport rate. Sulfanitran also stimulated MRP2 activity in cells, i.e. the transport of saquinavir through monolayers of Madin-Darby canine kidney II cells. Some compounds that stimulate E 2 17␤G transport, such as penicillin G or pantoprazole, are not detectably transported by MRP2, suggesting that they allosterically stimulate transport without being cotransported with E 2 17␤G. We propose that MRP2 contains two similar but nonidentical ligand binding sites: one site from which substrate is transported and a second site that regulates the affinity of the transport site for the substrate.

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Human MRP2 actively transports several HPI and could, based on its known and assumed tissue distribution, therefore reduce HPI oral bioavailability. It may also limit brain and fetal penetration of these drugs and increase their hepatobiliary, intestinal and renal clearance. MRP2 function and enhancement of its activity could adversely affect the therapeutic efficacy, including the pharmacological sanctuary penetration, of HPI. In vivo inhibition of MRP2 function might, therefore, improve HIV/AIDS therapy.

ATP binding cassette (ABC) multidrug transporters such as P-glycoprotein (P-gp, ABCB1) and BCRP (ABCG2) confer resistance against anticancer drugs and can limit their oral availability, thus contributing to failure of chemotherapy. Like P-gp and BCRP, another ABC transporter, MRP2 (ABCC2), is found in apical membranes of pharmacologically important epithelial barriers and in a variety of tumors. MRP2 transports several anticancer drugs and might thus have a similar impact on chemotherapy as P-gp and BCRP. We here show that human MRP2 transduced into epithelial MDCKII cells efficiently transported the taxane anticancer drugs paclitaxel and docetaxel and that this transport could be substantially stimulated with the drug probenecid, a representative of a range of MRP2-stimulating drugs. Transport of 2 previously identified MRP2 substrates, etoposide and vinblastine, was likewise stimulated by probenecid. MRP2 further conferred substantial resistance against paclitaxel toxicity, and this resistance was 2.7-fold stimulated by probenecid. Our data indicate that MRP2 function might affect chemotherapy with taxanes, potentially influencing both tumor resistance and taxane pharmacokinetics. Moreover, coadministration of probenecid and other MRP2-stimulating drugs might lead to unforeseen drug-drug interactions by stimulating MRP2 function, potentially leading to suboptimal levels of taxanes and other anticancer drugs in plasma and tumor. ' 2005 Wiley-Liss, Inc.Key words: ABCC2; multidrug resistance; paclitaxel; probenecid; stimulation ABC multidrug transporters such as P-glycoprotein (P-gp) and BCRP can make tumor cells resistant to many anticancer drugs. In addition, they can limit the oral availability and penetration of these drugs in certain tissues, thus possibly interfering with the clinical efficacy of cancer chemotherapy at several levels. 1 Oral administration of anticancer drugs is rarely feasible because of low and variable oral availability, and this is at least partly caused by ABC transporters. Oral administration has many advantages over i.v. administration as it is less invasive, easier to use for the patient in a chronic regimen and more cost-effective because of decreased hospitalization. We previously demonstrated that the oral availability of the taxane paclitaxel is severely limited by the paclitaxel-transporting P-gp, due to its presence and activity in the apical membranes of hepatocytes and intestinal epithelial cells. 2 P-gp also limits the brain and fetal penetration of taxanes. 3,4 In view of their clinical importance, we wanted to know whether taxanes are also transported by other pharmacokinetically important ATP binding cassette (ABC) transporters. While BCRP/Bcrp1 has similar pharmacokinetic functions as P-gp, paclitaxel is not substantially transported by BCRP or murine Bcrp1. 5 Recent publications suggest that another ABC transporter, multidrug resistance protein 2 (MRP2, ABCC2), might have broader pharmacokinetic and clinical relevance than previously thought. Like Pgp, MRP2 occurs in t...

The low oral bioavailability of the HIV protease inhibitor (HPI) saquinavir is dramatically increased by coadministration of the HPI ritonavir. Because saquinavir and ritonavir are substrates and inhibitors of both the drug transporter P-glycoprotein (P-gp) and of the metabolizing enzyme CYP3A4, we wanted to sort out whether the ritonavir effect is primarily mediated by inhibition of CYP3A4 or P-gp or both. P-gp is known to limit the bioavailability, brain, testis, and fetal penetration of its substrates, so effective inhibition of P-gp by ritonavir in vivo might open up pharmacological sanctuary sites for saquinavir, with the potential of beneficial effects on therapy, but also of increased toxicity. In vitro, P-gp-mediated transport of saquinavir and ritonavir was only moderately inhibited by both HPIs compared with the potent P-gp inhibitor PSC833. When [(14)C]saquinavir was orally coadministered with a maximum tolerated dose of ritonavir to wild-type and P-gp-deficient mice, saquinavir bioavailability was dramatically increased in both strains, but P-gp still limited the oral bioavailability of saquinavir, and its penetration into brain and fetus. These data indicate that in vivo, ritonavir is a relatively poor P-gp inhibitor. The highly increased bioavailability of saquinavir because of ritonavir coadministration most likely results from reduced saquinavir metabolism. Importantly, our data indicate that it is unlikely that ritonavir coadministration will substantially affect the contribution of P-gp to pharmacological sanctuary sites such as brain, testis, and fetus. Thus, if one wanted to effectively open these sites for therapeutic purposes, more efficient P-gp inhibitors should be applied.