Human Transporters

BCRP

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BCRP (breast cancer resistance protein)

Aliases: ABC15, ABCP, BCRP1, BMDP, CD338, CDw338, EST157481, GOUT1, MRX, MXR, MXR-1, MXR1, UAQTL1
Gene name: ATP binding cassette sub-family G member 2 (ABCG2)

Summary

ABCG2, more commonly referred to as BCRP (Breast Cancer Resistance Protein), is an efflux transporter that serves two major drug transport functions. Firstly, it restricts the distribution of its substrates into organs such as the brain, testes, placenta, and across the gastrointestinal tract (GIT). Secondly, it eliminates its substrates from excretory organs, mediating both biliary and renal excretion, and occasionally direct gut secretion. Although less well studied than e.g. MDR1, BCRP is generally co-expressed with MDR1, and shares many of its substrates, inhibitors and inducers. Of its known substrates, rosuvastatin has been implicated in DDI, especially with perpetrator drugs that also inhibit OATPs (e.g. cyclosporine). It is probable that a synergy exists between the action of BCRP, MDR1, and the drug-metabolizing enzyme CYP3A4, particularly in the GIT.
BCRP is included in the list of important drug transporters that both the FDA and EMA consider necessary to investigate regarding liabilities for NCEs. Drugs whose ADME, and bioavailability in particular, is influenced by BCRP may require clinical investigation to reveal a potential DDI with potent clinical BCRP inhibitors. For instance, since the GIT absorption of rosuvastatin is modulated by BCRP, it may be necessary to study the impact of BCRP inhibitors on the oral absorption of rosuvastatin. Because of the potential synergy between BCRP, CYP3A4, and MDR1, a clinical investigation examining the contribution of both drug transporters and enzymes to drug ADME may be necessary.


Localization

BCRP is a 72 kDa „half transporter” encoded by the ABCG2 gene; it consists of six transmembrane domains and functions as a homodimer or homotetramer [1]. It is highly expressed in barrier tissues such as the colon, small intestine, blood-brain barrier (BBB), placenta, and liver canalicular membrane. BCRP localizes to the apical membrane of polarized cells, where it mediates unidirectional transport of substrates to the luminal side of the organ, therefore acting as an efflux pump [2], and playing a key role in maintaining the barrier function of the organ. BCRP is strongly induced in the lactating breast [3], and it is also expressed at high levels on the apical surface of many tumor cell lines, and transformed cell lines such as Caco-2.


Function, physiology, and clinically significant polymorphisms

In common with MDR1, BCRP restricts the distribution of its substrates from the extracellular space into cells e.g. in the GIT and BBB, or eliminates drugs/metabolites entering excretory cells from the blood side of the organ by transporting them into the bile, urine, or GIT. BCRP contributes to the observed multidrug resistance to many chemotherapeutic agents in patients, and is expressed in hematological malignancies and solid tumors.
Substrates and inhibitors of BCRP include a wide range of clinically important and structurally diverse drugs (e.g. rosuvastatin, glyburide, nitrofurantoin, dipyridamole, cimetidine, chlorothiazide, sulfasalazine, and leflunomide), dietary components (e.g. porphyrins) and endogenous molecules (e.g. estrones and bile acids) [4-6]. BCRP inhibitors may be highly potent and relatively specific (e.g. fumitremorgin C and its analog Ko143), highly potent but relatively non-specific (e.g. GF120819, which is also a MDR1 inhibitor) or more general inhibitors of ADME mechanisms (e.g. cyclosporine A and some of the anti-HIV protease inhibitors).
There is a significant overlap in substrate specificity between BCRP and MDR1 (e.g. glyburide, imatinib, methotrexate, mitoxantrone, and prazosin). Because of the general co-expression of these two transporters, a synergistic effect is anticipated, which results in enhancement of the barrier function of these efflux transporters [7, 8]. While MDR1 and BCRP often share hydrophobic substrate compounds, BCRP additionally shows efficient transport of conjugated organic anions, particularly sulfate conjugates.
Given that MDR1 and CYP3A4 share many substrates, and BCRP shares many substrates with MDR1, a synergistic effect of these three proteins on their joint substrates is postulated, in particular because of their co-expression in the GIT.
In common with many drug-metabolizing enzymes, BCRP expression is known to be regulated by nuclear hormone receptors, including the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), which regulate expression in response to chronic treatment with xenobiotics such as rifampicin, phenobarbital, and TCDD [9].
There are over 80 reported single-nucleotide polymorphisms for the BCRP transporter gene, although few modify transport activity. The non-synonymous SNP e5/C421A is associated with lower BCRP expression, as the protein is less stable and has reduced plasma membrane localization [10]. This variant causes elevated plasma exposure to of atorvastatin and rosuvastatin; therefore, dose adjustment of statins is recommended in C421A carriers [11-13]. Q141K is found at high allele frequencies (30 –60%) in Japanese and Chinese populations and at relatively low allele frequencies (5 – 10%) in Caucasians and African-Americans. Clinically, this variant is also associated with higher plasma levels of BCRP substrate drugs such as topotecan, rosuvastatin, sulfasalazine, diflomotecan, imatinib, atorvastatin, and methotrexate [12, 14] but not of irinotecan, pitavastatin, or lamivudine [15]. Porphyrin transport is affected by the variants Q126stop, F208S, S248P, E334stop, and S441N, and reduced urate transport in Q141K carriers has been implicated in gout [16].


Clinical significance

Thus far, clinical Inhibition of BCRP appears to be of greatest relevance in the absorption of drugs from the GIT. The oral bioavailability of topotecan, a BCRP substrate, more than doubles after co-administration with GF120918 [17]. Bcrp1-knockout mice have >100 times higher systemic exposure to sulfasalazine following oral administration, versus wild-type mice [18]. Inhibition of both GIT BCRP and hepatic OATPs by cyclosporine-A leads to the much greater increase in rosuvastatin exposure than is predicted from inhibition of hepatic OATPs alone.
BCRP actively transports xenobiotics, including anticancer drugs, and thus restricts the uptake of its substrates from the gut lumen and through the BBB. On the other hand, due to its high expression in the lactating mammary gland, BCRP may secrete toxic xenobiotics such as dietary carcinogens, anticancer agents, and other drugs including the over-the-counter antiulcerative cimetidine, into breast milk [3]. BCRP expression in cancer cells confers drug resistance in leukemia patients, and higher levels are reported in solid tumors from the digestive tract, endometrium, lung and melanoma [19]; as a contrast, expression is generally low in breast cancer [20]. There is significant association between BCRP expression and tumor response to chemotherapy and progression-free survival [21]. BCRP is implicated in mitoxantrone efflux in 70% of adult acute lymphoblastic leukemia patients studied, despite very low mRNA levels [22]. High BCRP expression in the placenta appears to protect the fetus; e.g. the antidiabetic drug glyburide has limited fetal penetration due to efflux by BCRP. In the combined antiretroviral treatment of HIV-positive pregnant women, when tenofovir disoproxil fumarate (TDF) is given together with etravirine, a non-nucleoside reverse transcriptase inhibitor shown to inhibit BCRP, fetal protection against TDF is weakened and the transplacental passage of the victim drug is increased [23]. In Abcg2-deficient pregnant mice, when with topotecan as a substrate was co-administered with GF120918, the fetal plasma contained twice the levels of topotecan as the mother’s [24]. BCRP plays a major role in the biliary excretion but only a minor role in the intestinal transport of troglitazone sulfate [25]. Although BCRP transports bile salts, this is believed to be mainly from the intestine and kidney, with a relatively minor role in the liver [26].


Regulatory requirements

Because of its role in the PK of major drugs such as rosuvastatin, and consequent drug-drug interactions (DDI), regulatory agencies consider BCRP a clinically important drug transporter. Agencies expect an in vitro assessment of both substrate and inhibitory potency for registration. A positive outcome in either assessment may require further clinical investigation.
Direct clinical evidence of the contribution of BCRP inhibition or induction to drug-drug interactions is limited because of cross-specificity with P-gp and CYP3A substrates and inhibitors, and the activity of other transporters such as OATPs. Due to its widespread distribution, BCRP may modulate absorption, distribution, metabolism, and elimination (ADME) of its substrates, which complicates, and may even confound, clinical investigation of DDI potential.
Although BCRP is involved in a number of clinically relevant DDIs, none of the cited clinical probe substrates or inhibitors is truly specific for this transporter. The current recommendation for investigating BCRP-related DDI involves oral sulfasalazine for intestinal BCRP, oral rosuvastatin for both intestinal and hepatic BCRP, and intravenous rosuvastatin for hepatic BCRP only, with curcumin and lapatinib as reference inhibitors [27].

Location
Endogenous substrates
Substrates used experimentally
Substrate drugs
Inhibitors
Intestinal enterocytes, hepatocytes: canalicular membran, kidney proximal tubule, brain endothelia, placenta, stem cells, mammary glands
dietary flavonoids, porphyrins, estrone 3-sulfate
Fluorophores such as rhodamine 123 and Hoechst 33342, Conjugates such as estrone-3-sulfate and E217ßG
Anthracyclines, daunorubicin, doxorubicin, topotecan, SN-38, irinotecan, methotrexate, imatinib, irinotecan, Mitoxantrone, nucleoside analogs, prazosin, pantoprazole, statins, topotecan, rosuvastatin, teriflunomide, chlorothiazide
Fumitremorgin C, Ko132, Ko134, Ko143, gefitinib, Iressa, Imatinib mesylate, novobiocin, estrone, 17ß-estradiol, ritonavir, omeprazole, ivermectin [5], cyclosporine

Rerefences


1.    Kage, K., et al., Dominant-negative inhibition of breast cancer resistance protein as drug efflux pump through the inhibition of S-S dependent homodimerization. Int J Cancer, 2002. 97(5): p. 626-30.
2.    Maliepaard, M., et al., Subcellular localization and distribution of the breast cancer resistance protein transporter in normal human tissues. Cancer Res, 2001. 61(8): p. 3458-64.
3.    Jonker, J.W., et al., The breast cancer resistance protein BCRP (ABCG2) concentrates drugs and carcinogenic xenotoxins into milk. Nat Med, 2005. 11(2): p. 127-9.
4.    Beery, E., et al., ABCG2 modulates chlorothiazide permeability in vitro - characterization of the interaction. Drug Metab Pharmacokinet, 2011.
5.    Jani, M., et al., Kinetic characterization of sulfasalazine transport by human ATP-binding cassette G2. Biol Pharm Bull, 2009. 32(3): p. 497-9.
6.    Kis, E., et al., Leflunomide and its metabolite A771726 are high affinity substrates of BCRP: implications for drug resistance. Ann Rheum Dis, 2009. 68(7): p. 1201-7.
7.    Kodaira, H., et al., Kinetic analysis of the cooperation of P-glycoprotein (P-gp/Abcb1) and breast cancer resistance protein (Bcrp/Abcg2) in limiting the brain and testis penetration of erlotinib, flavopiridol, and mitoxantrone. J Pharmacol Exp Ther, 2010. 333(3): p. 788-96.
8.    Polli, J.W., et al., An unexpected synergist role of P-glycoprotein and breast cancer resistance protein on the central nervous system penetration of the tyrosine kinase inhibitor lapatinib (N-{3-chloro-4-[(3-fluorobenzyl)oxy]phenyl}-6-[5-({[2-(methylsulfonyl)ethy l]amino}methyl)-2-furyl]-4-quinazolinamine; GW572016). Drug Metab Dispos, 2009. 37(2): p. 439-42.
9.    Jigorel, E., et al., Differential regulation of sinusoidal and canalicular hepatic drug transporter expression by xenobiotics activating drug-sensing receptors in primary human hepatocytes. Drug Metab Dispos, 2006. 34(10): p. 1756-63.
10.    Furukawa, T., et al., Major SNP (Q141K) Variant of Human ABC Transporter ABCG2 Undergoes Lysosomal and Proteasomal Degradations. Pharm Res, 2008.
11.    DeGorter, M.K., et al., Clinical and pharmacogenetic predictors of circulating atorvastatin and rosuvastatin concentrations in routine clinical care. Circ Cardiovasc Genet, 2013. 6(4): p. 400-8.
12.    Keskitalo, J.E., et al., ABCG2 polymorphism markedly affects the pharmacokinetics of atorvastatin and rosuvastatin. Clin Pharmacol Ther, 2009. 86(2): p. 197-203.
13.    Elsby, R., et al., Solitary Inhibition of the Breast Cancer Resistance Protein Efflux Transporter Results in a Clinically Significant Drug-Drug Interaction with Rosuvastatin by Causing up to a 2-Fold Increase in Statin Exposure. Drug Metab Dispos, 2016. 44(3): p. 398-408.
14.    Warren, R.B., et al., Genetic variation in efflux transporters influences outcome to methotrexate therapy in patients with psoriasis. J Invest Dermatol, 2008. 128(8): p. 1925-9.
15.    Ieiri, I., et al., SLCO1B1 (OATP1B1, an uptake transporter) and ABCG2 (BCRP, an efflux transporter) variant alleles and pharmacokinetics of pitavastatin in healthy volunteers. Clin Pharmacol Ther, 2007. 82(5): p. 541-7.
16.    Woodward, O.M., et al., Identification of a urate transporter, ABCG2, with a common functional polymorphism causing gout. Proc Natl Acad Sci U S A, 2009. 106(25): p. 10338-42.
17.    Kruijtzer, C.M., et al., Increased oral bioavailability of topotecan in combination with the breast cancer resistance protein and P-glycoprotein inhibitor GF120918. J Clin Oncol, 2002. 20(13): p. 2943-50.
18.    Zaher, H., et al., Breast cancer resistance protein (Bcrp/abcg2) is a major determinant of sulfasalazine absorption and elimination in the mouse. Mol Pharm, 2006. 3(1): p. 55-61.
19.    Diestra, J.E., et al., Frequent expression of the multi-drug resistance-associated protein BCRP/MXR/ABCP/ABCG2 in human tumours detected by the BXP-21 monoclonal antibody in paraffin-embedded material. J Pathol, 2002. 198(2): p. 213-9.
20.    Robey, R.W., et al., ABCG2: determining its relevance in clinical drug resistance. Cancer Metastasis Rev, 2007. 26(1): p. 39-57.
21.    Kim, Y.H., et al., Expression of breast cancer resistance protein is associated with a poor clinical outcome in patients with small-cell lung cancer. Lung Cancer, 2008.
22.    Suvannasankha, A., et al., Breast cancer resistance protein (BCRP/MXR/ABCG2) in adult acute lymphoblastic leukaemia: frequent expression and possible correlation with shorter disease-free survival. Br J Haematol, 2004. 127(4): p. 392-8.
23.    Reznicek, J., et al., Etravirine inhibits ABCG2 drug transporter and affects transplacental passage of tenofovir disoproxil fumarate. Placenta, 2016. 47: p. 124-129.
24.    Jonker, J.W., et al., Role of breast cancer resistance protein in the bioavailability and fetal penetration of topotecan. J Natl Cancer Inst, 2000. 92(20): p. 1651-6.
25.    Enokizono, J., H. Kusuhara, and Y. Sugiyama, Involvement of breast cancer resistance protein (BCRP/ABCG2) in the biliary excretion and intestinal efflux of troglitazone sulfate, the major metabolite of troglitazone with a cholestatic effect. Drug Metab Dispos, 2007. 35(2): p. 209-14.
26.    Mennone, A., et al., Role of breast cancer resistance protein in the adaptive response to cholestasis. Drug Metab Dispos, 2010. 38(10): p. 1673-8.
27.    Lee, C.A., et al., Breast cancer resistance protein (ABCG2) in clinical pharmacokinetics and drug interactions: practical recommendations for clinical victim and perpetrator drug-drug interaction study design. Drug Metab Dispos, 2015. 43(4): p. 490-509.
28.    Jani, M., et al., Ivermectin interacts with human ABCG2. J Pharm Sci, 2011. 100(1): p. 94-7.

 

 

 

 

 

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