MRP1 (Multi-drug resistance associated protein 1)
Gene name: ATP Binding Cassette Protein C1 (ABCC1)
ABCC1 or multidrug resistance-associated protein 1 (MRP1), is a uni-directional efflux transporter with a ubiquitous tissue distribution and wide substrate specificity including important therapeutics.
The main roles of this transporter are (i) efflux of xenobiotic and endogenous metabolites, (i) transport of inflammatory mediators (e.g. LTC4) and (iii) defense against oxidative stress. MRP1 plays a role in the development of drug resistance of various types of cancer, and contributes to inflammatory responses [1, 2]. It does not appear to play a significant role in the absorption or eliminations of drugs, but does seem to be an important modulator of drug tissue exposure and metabolite cellular elimination. The FDA and EMA guidances do not mention MRP1, as there are no clinical demonstrations of a notable role in the absorption or elimination of drugs or DDI, although it’s role in chemotherapeutic pharmacology and drug distribution may be important for some NCEs.
MRP1 is ubiquitously expressed in humans. The highest mRNA levels are reported for testis, cardiomyocytes, placenta, prostate, lung, thymus and kidney, with lower expression in small intestine, colon, brain, and mononuclear cells. MRP1 is most highly expressed in cells with a barrier function (epithelial and endothelial), but is located on the basolateral membrane of these polarized cells. It is often more highly expressed in rapidly dividing cells, unlike other ABC transporters such as MRP2 and MDR1 [1,2]. Although apical localization in brain capillary endothelial cells has been suggested , this has not been confirmed by other workers [4,5].
Structure, function, physiology
The 190-kDa MRP1 has a core structure consisting of two trans-membrane domains (TMD), each followed by a nucleotide binding domain (NBD). In common with MRP2, 3, 6 and 7, MRP1 contains a third TMD (TMD0) with five predicted trans membrane segments and an extra cytosolic NH2 terminus connected to the core structure by a linker region (L0) . The TMD0 appears to be important for MRP1 trafficking to the plasma membrane , and the precise roles, mechanisms and dependencies of TMD0 and L0 are the subject of significant research [8,9,10,11].
MRP1 has broad substrate specificity, transporting hydrophobic and anionic molecules, glucuronide and glutathione conjugates, as well as endogenous glutathione. Although many MRP1 substrates are conjugated to glutathione, co-transport of free glutathione is often observed, and appears to stimulate transport of e.g. vincristine and daunorubicin . Glutathione itself is a low affinity substrate of MRP1 (Km = 1-5 mM) . Multiple allosterically cooperative, non-overlapping substrate-binding sites are postulated, which may explain why various substrates both cross-inhibit and cross–stimulate . Whereas the role of glutathione in preventing oxidative stress is well understood, the precise dynamics of MRP1 in regulating cellular glutathione levels require clarification . Cellular exposure to reactive oxygen species (ROS) rapidly depletes GSH, whilst increasing GSSG. GSSG is transported more efficiently by MRP1 than GSH, therefore it may help to maintain a healthy GSSG/GSH ratio . Selective inhibition of MRP1 by MK-571 also promotes 4-Hydroxy-2-nonenal (HNE) induced oxidative stress and cell death. HNE is a chemically reactive aldehyde produced during lipid peroxidation, thus a protective role for MRP1 is postulated, possibly by MRP1-mediated transport of the HNE-SG complex .
The inflammatory cytokine LTC4 and its main metabolite LTD4 are one of the highest affinity MRP1 substrates, suggesting a key role for MRP1 in cytokine release from LTC4 producing cells. In fact, intracellular LTC4 accumulation was observed in mrp1 (-/-) mice . Additionally, although viable, healthy, and fertile with normal phenotype, knockout mrp1 (−/−) mice were hypersensitive to cytotoxic drugs .
Numerous chemotherapeutic agents, including doxorubicin and vinblastine, have been reported to induce MRP1 expression, and a role for nuclear hormone regulation via CAR has been reported .
Clinical significance and polymorphisms
Despite its broad substrate specificity, there are no clear demonstrations of a role for MRP1 in the absorption or elimination of drugs. However, a role in modulating drug tissue distribution and metabolite efflux is evident, implying a role in pharmacology and toxicity. However, tissues levels are somewhat challenging to measure in vivo. Therefore, most information has been derived from preclinical species, in vitro studies and clinical observations:
MRP1 is implicated in the lack of chemotherapeutic response of several clinically important drugs. Cells that highly express MRP1 confer resistance to a variety of natural product anticancer drugs such as vinca alkaloids, anthracyclines and epipodophyllotoxins. The topoisomerase I inhibitor irinotecan (CPT-11), and its major active metabolite, SN-38 and its glucuronide are actively effluxed through MRP1. Peripheral blood mononuclear cells from HIV positive patients with lower MRP1 expression showed a significantly higher accumulation of both ritonavir and saquinavir in lymphocytes compared with those with higher MRP1 expression. Multiple chemical toxicants and their metabolites are also known substrates of MRP1 e.g. aflatoxin and the GSH conjugates of herbicide metolachlor . Using triple knockout mice (mdr1a/1b/mrp1), mrp1 did not significantly influence grepafloxacin systemic exposure or elimination, but did modulate distribution to heart, trachea, kidney and spleen, implying that MRP1 may play a significant role in drug tissue distribution, but not absorption or elimination.
There are at least 15 naturally occurring mutations identified in MRP1, and many of them have been found to affect its in vitro transport activity. Polymorphisms and mutagenesis studies have been reviewed in . Although there many MRP1 SNPs are known, their incidence in populations is reported to be relatively low. In mainland Chinese population the MRP1 polymorphism allelic frequencies of Cys43Ser (128G>C), Thr73Ile (218C>T), Arg723Gln (2168G>A) and Arg1058Gln (3173G>A) were 0.5%, 1.4%, 5.8% and 0.5%, respectively . The clinical relevance of MRP1 polymorphisms has yet to be systematically evaluated, however, an association between doxorubicin cardiotoxicity and the Gly671Val variant of MRP1 has been reported.
As there is no strong association of MRP1 to the absorption and elimination of drugs, and a relatively poor understanding of its role in drug distribution, neither the FDA or EMA guidances make any specific mention of this transporter. Nonetheless, given its broad substrate specificity, and role in modulating cellular exposure to drugs, its evaluation may be appropriate for some NCEs. 
|Location||Endogenous substrates||In vitro substrates used experimentally||Substrate drugs||Inhibitors|
Leukotrienes (C4, D4*, E4) prostaglandins (A2-SG and JA2-SG) 15-deoxy-D12,14 hydroxynonenal-SG, Folic acid, L-leucovorin, Folic acid, L-leucovorin, GSH, GSSG, N-acetyl-Leu-Leu-norleucinal (ALLN)*Bilirubin*, sphingosine 1-phosphate, Glucuronide conjugates of: 17β-Estradiol,Bilirubin , hyodeoxycholate, dehydroepiandrosterone sulfatolithocholate, sulfatolithocholyl taurine
Fluorescent probes, Calcein, Calcein-AM*, Fluo-3, BCECF, SNARF, CFDA*Toxins, Aflatoxin B1, methoxychlor, fenitrothion, chlorpropham, zearalenone* α-zearalenol*
adefovir, indinavir, saquinavir, ritonavir methotrexate, edatrexate, ZD1694, doxorubicin, daunorubicin, epirubicin, idarubicin, etoposide, vincristine, vinblastine, paclitaxel, irinotecan, SN-38 flutamide, hydroxyflutamide FK228*, FR901228, NSC-630176, apicidin*Difloxacin, grepafloxacin, ciprofloxacin* berberine*, pirarubicin*Sodium arsenite/arsenate*, potassium antimonite/antimony tartrate
MK571, indomethacin, benzbromarone, euchrestaflavanone sophoraflavanone, cyclosporine A, quercetin
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- FDA 2012 Draft Guidance for Industry, Drug Interaction Studies