Aliases: ABC30, CMOAT, DJS, cMRP
Gene name: ATP binding cassette subfamily C member 2 (ABCC2)
ABCC2/MRP2 is a unidirectional efflux transporter that primarily transports organic anions, including drug conjugates and conjugated bilirubin. Although expressed in numerous tissues in humans, particularly in the liver, kidney, gastrointestinal tract (GIT) and placenta, MRP2 has a more restricted tissue distribution than MDR1 or BCRP. MRP2 is most highly expressed in the liver, where it facilitates the elimination of bilirubin glucuronides and positively charged drugs and conjugates into the bile. It plays a similar role in renal elimination, while in other organs such as the placenta and the GIT it restricts the distribution of its substrates. Therefore, the primary role of MRP2 is to limit cellular exposure to its substrates. Dubin-Johnson syndrome patients possess an inherited mutation of the ABCC2 gene, giving rise to non-functional MRP2, and suffer from chronic hyperbilirubinemia. Although there are clinical DDIs ascribed to MRP2, and several drug substrates and inhibitors are known, MRP2 receives little attention in the current FDA guidance, and it is not mentioned in the EMA guidance. Nevertheless, investigations for NCEs should be considered on a case-by-case basis, particularly if hyperbilirubinemia is observed.
MRP2 localizes to the apical membrane domain of polarized cells such as hepatocytes, renal proximal tubule cells, and intestinal epithelia, where it mediates unidirectional transport of substrates to the luminal side of the organ, therefore acting as an ATP-dependent efflux pump. MRP2 is also present in the gallbladder, bronchi, and placenta. Of the nine MRPs, only MRP2 localizes exclusively to the apical membrane of polarized cells.
In contrast with other efflux transporters like MDR1 and BCRP, MRP2 is either absent or below detectable limits in the human blood-testis and blood-brain barriers. It is, however, present in brain capillaries of hippocampus specimens from patients with temporal lobe epilepsy, and in malignant tumor cells from renal clear-cell, hepatocellular, ovarian, colorectal, lung, breast, and gastric carcinomas (reviewed in ).
Function, physiology, and clinically significant polymorphisms
The predicted membrane topology of MRP2 transporter includes 12 transmembrane domains with at least two substrate-binding sites. The localization of MRP2 supports a function in the terminal excretion and detoxification of endogenous and xenobiotic organic anions, particularly conjugates of glutathione (GSH), glucuronate or sulfate, as well as a contribution to resistance towards anticancer drugs targeting solid malignant tumors. MRP2 has a broad substrate specificity: it transports organic anions with the highest affinity for glucuronate, GSH conjugates of lipophilic substances (e.g. LTC4), sulfated bile salts , compounds without anionic conjugate residues (e.g. methotrexate and bromosulfophthalein), reduced and oxidized GSH, nucleotide analogues, and anticancer drugs. MRP2 transports conjugated endogenous and xenobiotic substances, including hormones, toxins and carcinogens, into the bile (from hepatocytes), urine (from renal proximal tubular cells), and the intestinal lumen (from enterocytes) . Intriguingly, while some estrogen conjugates are transported highly actively by BCRP as well as MRPs, androgen glucuronides are excreted exclusively by MRPs, with MRP3 displaying higher affinity compared to MRP2 . The hepatic MRP2 pump contributes to the driving forces of bile flow and is the major transporter responsible for the biliary excretion of bilirubin glucuronides.
Known pharmacological inhibitors of MRP2 include probenecid, cyclosporine, vindesine, PSC833 and MK571. With relevance to potential food-drug interactions, flavonoids like quercetin may not only be substrates but also inhibitors of MRP2 [5, 6]; e.g., the flavone baicalein enhanced the oral bioavailability of silybin by inhibiting MRP2- and BCRP-mediated intestinal efflux .
MRP2 appears to have at least two substrate-binding sites, which demonstrate some inter-dependency. For example, in vitro glutathione (GSH) transport by MRP2 is stimulated by sulfinpyrazone at low concentrations, but at much higher concentrations, the latter occupies both transporter binding sites, preventing the co-transport of glutathione, yet allowing sulfinpyrazone to be transported alone .
While the majority of the >200 identified naturally occurring ABCC2 SNPs do not result in clinically relevant changes in function, selected variants have been shown to alter the PK of clinical drugs. The hereditary disorder Dubin-Johnson syndrome is characterized by complete loss of functionally active MRP2, resulting in chronic hyperbilirubinemia. In instances where sequence variants in the ABCC2 gene correlate with loss of MRP2 function, this is often compensated for by the upregulation of other membrane transporters with similar substrate specificities, notably MRP3 and MRP4, in the basolateral membrane of hepatocytes .
MRP2 expression is regulated at the transcriptional and post-transcriptional level by many endogenous and xenobiotic substances, and is altered in different disease states. Transcriptional levels may respond to changes in the intracellular concentrations of bile acids and other lipophilic compounds that are ligands for nuclear hormone receptors such as PXR, CAR, FXR and HNF4. Induction of MRP2 mRNA by PXR/CAR ligand drugs, however, does not necessarily translate into elevated protein abundance: rifampin and carbamazepine both significantly induced ABCC2 transcription in the duodenum without affecting protein levels . MRP2 is upregulated in acute liver injury and may mitigate the effects of ER stress , whereas MRP2 protein abundance declines in chronic liver diseases . Interestingly, Mrp2 in mice was found to be regulated by the circadian clock, and its expression correlated inversely with the diurnal variation in plasma unconjugated bilirubin concentration .
MRP2 transports numerous clinically important compounds across multiple drug classes. Examples of clinical drug substrates include antibiotics such as ampicillin, azithromycin, and ceftriaxone; anticancer agents such as cisplatin, doxorubicin, irinotecan, and methotrexate (MTX); antihypertensives such as olmesartan and temocaprilate; HIV drugs such as adefovir, lopinavir, and saquinavir ; and phytotherapeutic flavonolignan silybin . MRP2 confers resistance to diverse chemoterapeutics such as cisplatin, MTX, mitoxantrone, and etoposide/teniposide. However, DDIs due to MRP2 modulation are difficult to describe for multiple reasons. To name a few, specific and potent clinical inhibitors and substrates are lacking; many MRP2 substrates are conjugated drug metabolites; and compensation by other MRPs (e.g. MRP3 and 4) may mask the effect of MRP2 on drug clearance. Thus, studies that discuss the involvement of MRP2 in DDIs typically refer to multiple transporters, including OATs, P-gp, BCRP, other MRPs, and OATPs. E.g., increased risk of kidney injury upon coadministration of the semisynthetic vinca alkaloid vindesine with high-dose MTX could be assigned to simultaneous inhibition of BCRP and MRP2 by vindesine and the resultant accumulation of MTX in proximal tubule cells . As another example for the involvement of multiple transporters, leflunomide/teriflunomide increased the kidney/plasma ratio of acyclovir by inhibiting MRP2, but also increased plasma concentration by concomitant inhibition of OAT1 and OAT3 .
MRP2-mediated biliary excretion of estradiol metabolites such as the strongly cholestatic estradiol-17β-glucuronide (E217βG) is relevant to the development of cholestasis associated with oral contraceptive use or pregnancy, as E217βG can only exert its inhibitory effect on BSEP from the luminal side .
Roughly half a dozen polymorphisms in the ABCC2 gene have been shown to be of clinical significance. The SNPs -24C/T, 1249G/A, 2366C/T, 3563T/A, 3972C/T, 4544G/A, and 4348G/A are associated with altered PK of irinotecan, pravastatin, methotrexate (MTX), mycophenolic acid, telmisartan, talinolol, tacrolimus, lopinavir, doxorubicin, and deferasirox. In particular, pediatric cancer patients carrying the MRP2 -24T allele show a gender-specific reduced methotrexate clearance, with -24T allele females having approximately 2-fold higher AUC versus non-variant patients, which results in more severe toxicities [20, 21]. Renal allograft recipients carrying the -24T or the 3972T allele showed a 17% increase in the AUC of mycophenolic acid . The -24T allele also predisposes to diclofenac-induced hepatotoxicity , increases the likelihood of resistance to antiepileptic drugs , and accelerates the clearance of the iron chelator deferasirox . The 1249G>A polymorphism, on the other hand, appeared to decrease the risk of therapy resistance in Asian epileptic patients .
In addition to conjugated hyperbilirubinemia, Dubin-Johnson syndrome patients also have elevated urinary coproporphyrin ratios, which incidentally may provide quantitative information of in vivo MRP2 activity .
Although there are several identified substrates and inhibitors of human MRP2, and a number of clinical citations on DDI and the impact of genetic polymorphisms, there is no specific recommendation for this transporter in the current FDA or EMA guidelines. Nonetheless, in vitro evaluation of the interactions of NCEs and particularly their conjugated metabolites should be considered on a case-by-case basis, especially where conjugated hyperbilirubinemia is observed.
|Location||Endogenous substrates||In vitro substrates used experimentally||Substrate drugs||Inhibitors|
hepatocytes (canalicular membrane), renal proximal tubule cells and enterocytes (luminal side), solid tumors
|bilirubin and its conjugates, sulfated bile salts,
estradiol- 17b-glucuronide, CCK-8, sulfinpyrazone
|glutathione and glucuronide conjugates, ampicillin, azithromycin, cefodezime, ceftriaxone, grepafloxacine, irinotecan, cisplatin, doxorubicin, epirubicin, vinblastine, vincristine, methotrexate, etoposide, mitoxantrone, valsartan, olmesartan, temocaprilate,
SN-38-gluc, adefovir, cidofovir, indinavir, lopinavir, nelfinavir, ritonavir, saquinavir, silybin
cyclosporine, delaviridine, efavirenz, emtricitabine, benzbromarone,
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