The human multidrug resistance-associated protein (MRP) family currently has seven members. The ability of several of these membrane proteins to transport a wide range of anticancer drugs out of cells and their presence in many tumors make them prime suspects in unexplained cases of drug resistance, although proof that they contribute to clinical drug resistance is still lacking. Recent studies have begun to clarify the function of the MRP family members. MRPs are organic anion transporters; i.e., they transport anionic drugs, exemplified by methotrexate, and neutral drugs conjugated to acidic ligands, such as glutathione (GSH), glucuronate, or sulfate. However, MRP1, MRP2, and MRP3 can also cause resistance to neutral organic drugs that are not known to be conjugated to acidic ligands by transporting these drugs together with free GSH. MRP1 can even confer resistance to arsenite and MRP2 to cisplatin, again probably by transporting these compounds in complexes with GSH. MRP4 overexpression is associated with high-level resistance to the nucleoside analogues 9-(2-phosphonylmethoxyethyl) adenine and azidothymidine, both of which are used as anti-human immunodeficiency virus drugs. MRPs may, therefore, also have a role in resistance against nucleoside analogues used in cancer chemotherapy. Mice without Mrp1, a high-affinity leukotriene C(4) transporter, have an altered response to inflammatory stimuli but are otherwise healthy and fertile. MRP2 is the major transporter responsible for the secretion of bilirubin glucuronides into bile, and humans without MRP2 develop a mild liver disease known as the Dubin-Johnson syndrome. The physiologic functions of the other MRPs are not known. Whether long-term inhibition of MRPs in humans can be tolerated (assuming that suitable inhibitors will be found) remains to be determined.

Membrane transporters can be major determinants of the pharmacokinetic, safety and efficacy profiles of drugs. This presents several key questions for drug development, including which transporters are clinically important in drug absorption and disposition, and which in vitro methods are suitable for studying drug interactions with these transporters. In addition, what criteria should trigger follow-up clinical studies, and which clinical studies should be conducted if needed. In this article, we provide the recommendations of the International Transporter Consortium on these issues, and present decision trees that are intended to help guide clinical studies on the currently recognized most important drug transporter interactions. The recommendations are generally intended to support clinical development and filing of a new drug application. Overall, it is advised that the timing of transporter investigations should be driven by efficacy, safety and clinical trial enrolment questions (for example, exclusion and inclusion criteria), as well as a need for further understanding of the absorption, distribution, metabolism and excretion properties of the drug molecule, and information required for drug labeling.

The human multidrug resistance protein (MRP) family contains at least six members: MRP1, the godfather of the family and well known as the multidrug resistance protein, and five homologs, called MRP2-6. In this review, we summarize what is known about the protein structure, the expression in tissues, the routing in cells, the physiological functions, the substrate specificity, and the role in multidrug resistance of the individual members of the MRP family.

The human multidrug resistance protein (MRP1) causes drug resistance by extruding drugs from tumor cells. In addition to an MDR-like core, MRP1 contains an N-terminal membrane-bound region (TMD 0 ) connected to the core by a cytoplasmic linker (L 0 ). We have studied truncated MRP1 versions containing either the MDRlike core alone or the core plus linker L 0 , produced in the baculovirus-insect (Sf9) cell system. Their function was examined in isolated membrane vesicles. Fulllength MRP1 showed ATP-dependent, vanadate-sensitive accumulation of leukotriene C 4 and N-ethylmaleimide glutathione. In addition, leukotriene C 4 -stimulated, vanadate-dependent nucleotide occlusion was detected. The MDR-like core was virtually inactive. Co-expression of the core with the N-terminal region including L 0 fully restored MRP1 function. Unexpectedly, a truncated MRP1 mutant lacking the entire TMD 0 region but still containing L 0 behaved like wild-type MRP1 in vesicle uptake and nucleotide trapping experiments. We also expressed the MRP1 constructs in polarized canine kidney derived MDCKII cells. Like wild-type MRP1, the MRP1 protein without the TMD 0 region was routed to the lateral plasma membrane and transported dinitrophenyl glutathione and daunorubicin. The TMD 0 L 0 and the MRP1 minus TMD 0 L 0 remained in an intracellular compartment. Taken together, these experiments strongly suggest that the TMD 0 region is neither required for the transport function of MRP1 nor for its proper routing to the plasma membrane. MDR1 P-glycoprotein (MDR1 Pgp)1 and MRP1 (multidrug resistance protein 1) are members of the ATP binding cassette (ABC) transporter family that can cause multiple drug resistance in tumor cells. MDR1 Pgp is an ATP-dependent drug extrusion pump and confers resistance to a wide variety of hydrophobic toxic agents (1). MRP1 has been shown to be a high affinity primary active transporter for the glutathioneconjugated eicosanoid, leukotriene C 4 (LTC 4 ) (2, 3) and to transport various other compounds that are conjugated to glutathione, sulfate, or glucuronide (2, 4 -6). The physiological functions of MRP1 range from the mediation of an inflammatory response to the elimination of certain xenobiotics (7-11), and this protein may play a role in the chemotherapy resistance of several types of cancer (12).Vanadate inhibits ATP-dependent drug transport both by MDR1 and MRP1 (1, 7), and in the presence of vanadate, the trapping of an adenine nucleotide in these proteins has been demonstrated (13,14). Transported compounds specifically increase the rate of vanadate-dependent nucleotide occlusion in MRP1 (14), similarly to what has been shown for hydrophobic drugs in the case of MDR1 Pgp (15). Vanadate-dependent, drug-stimulated nucleotide trapping reflects a partial reaction of the multidrug transporters and thus can be used to examine their functional characteristics.MDR1 Pgp and MRP1 share a similar core structure, consisting of a tandem repeat of transmembrane domains (TMDs) and cytoplasmic ABC-containing regions. However,...

The multidrug resistance-associated protein (MRP) mediates the cellular excretion of many drugs, glutathione S-conjugates (GS-X) of lipophilic xenobiotics and endogenous cysteinyl leukotrienes. Increased MRP levels in tumor cells can cause multidrug resistance (MDR) by decreasing the intracellular drug concentration. The physiological role or roles of MRP remain ill-defined, however. We have generated MRP-deficient mice by using embryonic stem cell technology. Mice homozygous for the mrp mutant allele, mrp-/-, are viable and fertile, but their response to an inflammatory stimulus is impaired. We attribute this defect to a decreased secretion of leukotriene C4 (LTC4) from leukotriene-synthesizing cells. Moreover, the mrp-/- mice are hypersensitive to the anticancer drug etoposide. The phenotype of mrp-/- mice is consistent with a role for MRP as the main LTC4-exporter in leukotriene-synthesizing cells, and as an important drug exporter in drug-sensitive cells. Our results suggest that this ubiquitous GS-X pump is dispensable in mice, making treatment of MDR with MRP-specific reversal agents potentially feasible.