BSEP

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BSEP (bile salt export pump)

Aliases: ABC16, BRIC2, PFIC-2, PFIC2, PGY4, SPGP
Gene name: ATP binding cassette subfamily B, member 11 (ABCB11)

Summary

ABCB11, more commonly referred to as BSEP (Bile Salt Export Pump) is a uni-directional, ATP-dependent efflux transporter that plays an important role in the elimination of bile salts from the hepatocyte into the bile canaliculi for export into the gastrointestinal tract (GIT). It is almost exclusively expressed in the liver, with much lower levels reported in the kidney. It is mainly of relevance to hepatotoxicity, as BSEP inhibition by a drug and/or its metabolites can result in the buildup of bile salts in the liver, which can lead to cholestasis and drug-induced liver injury (DILI). Compared to other drug transporters there are only few identified drug substrates and inhibitors of BSEP; thus, its involvement in drug-drug interactions (DDI) is very limited. The relevance of in vitro BSEP inhibition as a predictor of clinical outcomes is not clearly established, but whenever cholestatic liver injury is observed in clinical or preclinical trials, characterization of BSEP interactions should be considered. In contrast with the FDA guidance, the EMA guidance recommends consideration of in vitro BSEP inhibition testing for NCEs.

Localization

BSEP is predominantly expressed in the cholesterol-rich apical (canalicular) membrane of hepatocytes, where it functions in the secretion of bile salts from the liver into the bile canaliculi [1]. Low levels of mRNA expression have also been reported in the kidney, testis, and choroid plexus [2].

Function, physiology, and clinically significant polymorphisms

The BSEP transporter is a ~160 kDa protein with 12 putative membrane-spanning domains. BSEP mediates the hepatic excretion of monovalent conjugated bile acids. It shows high affinity for conjugated bile acids and a relatively poor affinity for unconjugated bile acids, in the order of: taurochenodeoxycholate ~ glycodeoxycholate > taurocholate ~ glycocholate [3]. BSEP also has a low affinity for a limited number of drugs that are substrates for MDR1 e.g. pravastatin [4], although a role for BSEP in clinical drug transport has not been established.
More than 300 SNPs of the ABCB11 gene have been reported, and half of these show possible associations with cholestatic disease, but only few have been investigated at the molecular level. Many ABCB11 polymorphisms are ethnicity-related [5, 6]. Mutations affecting the transmembrane helices tend to have severe functional consequences, suggesting a role for these motifs in substrate recognition, binding, and translocation. Possible outcomes of non-synonymous mutations include aberrant splicing, reduced plasma membrane expression, intracellular retention, and reduced or absent bile salt transport function. Low BSEP protein expression correlates with the C-allele at position 1457 in the ABCB11 gene [7]. In vitro evaluations revealed the association of 616A>G, 1674G>C, 1772A>G, and 3556G>A with significantly impaired taurocholate transport activity; the 890A>G variant had mildly impaired function and 3556G>A was associated with reduced cell surface total protein expression compared with wild-type BSEP [5]. In addition to the severe Progressive Familial Intrahepatic Cholestasis type 2 (PFIC-2) and a milder, transient disorder known as Benign Recurrent Intrahepatic Cholestasis type 2 (BRIC2), intrahepatic cholestasis of pregnancy (ICP) has also been linked to BSEP mutations [6]. Recently, surgical outcomes of PFIC2 patients with different BSEP mutations was investigated and found to depend on the type of the mutation (common mutations with likely residual function vs. rare mutations with complete loss of function) [8].
BSEP expression is regulated by the farnesoid X receptor (FXR), and increased hepatic bile acids and certain herbal substances like licorice root extract or tanshinone IIA can induce transcription through Nrf2 pathway [9]. Hepatitis C infection reduces BSEP expression to 47% of control levels [10].

Clinical significance

PFIC2 is characterized by severe jaundice, hepatomegaly, and high plasma levels of bile acids and aminotransferases. BRIC2 is associated with recurrent episodes of cholestasis and gallstone formation. Polymorphisms in BSEP transporter are associated with ICP and drug-induced cholestasis (reviewed in [11]). In particular, the polymorphism p.V444A increases susceptibility to both ICP and contraceptive-induced cholestasis, the latter condition probably being attributable to the inhibition of BSEP by estrogen/progesterone metabolites [12]. Primary intrahepatic stone is also linked to mutations in BSEP [13].
Because humans, unlike rats, have no compensatory mechanism for the loss of this transporter, mutations or chemical inhibitors can result in decreased biliary bile salt secretion, leading to decreased bile flow and accumulation of bile salts inside the hepatocyte, resulting in hepatotoxicity. Drugs such as bosentan, troglitazone and CI-1034 cause clinical hepatotoxicity that is related to inhibition of BSEP [14].
DILI is a rare but very serious clinical issue that in severe cases necessitates liver transplantation. Although there is a distinct association between modulation of BSEP function and DILI and/or cholestasis, the relationship is complex and multifactorial. This was also shown in a Bsep knockdown rat model where different BSEP/Bsep inhibitors exerted diverse effects on bile acid homeostasis and hepatotoxicity [15]. 
The propensity of drugs to inhibit BSEP can be predicted in vitro with remarkable accuracy. In a screen conducted with 85 pharmaceuticals, BSEP inhibition in vitro neatly correlated with cholestasis [16]. Such information may be helpful in the selection of candidates with reduced DILI liability [3] and/or the design of clinical strategies to manage/monitor DILI. However, it is still not possible to reliably predict DILI based on BSEP inhibition observed in vitro [17, 18]. For individualized patient management it may also be useful to consider that some polymorphisms of the ABCB11 gene, reviewed by Kubitz et al. [19], have been shown to predispose to DILI [20, 21]. 
Evidence shows that the enhancement of BSEP expression through either FXR or Nrf2 can offer protection against cholestasis [22, 23], which opens novel therapeutic avenues. In 2016, the FXR agonist obeticholic acid (OCA) was approved by FDA for use in cholestatic liver diseases [24].

Regulatory requirements

Due to the association between BSEP and DILI, the EMA recommends that hepatically cleared new drugs and their metabolites should be investigated for their potential to inhibit BSEP. The FDA guidance also recommends the consideration of BSEP inhibition studies when appropriate. A positive in vitro result mandates careful monitoring of hepatic function in clinical trials.

References

1.    Cheng, X., D. Buckley, and C.D. Klaassen, Regulation of hepatic bile acid transporters Ntcp and Bsep expression. Biochem Pharmacol, 2007. 74(11): p. 1665-76.
2.    Choudhuri, S., et al., Constitutive expression of various xenobiotic and endobiotic transporter mRNAs in the choroid plexus of rats. Drug Metab Dispos, 2003. 31(11): p. 1337-45.
3.    Kis, E., et al., BSEP inhibition - In vitro screens to assess cholestatic potential of drugs. Toxicol in Vitro, 2011.
4.    Kivisto, K.T. and M. Niemi, Influence of drug transporter polymorphisms on pravastatin pharmacokinetics in humans. Pharm Res, 2007. 24(2): p. 239-47.
5.    Ho, R.H., et al., Polymorphic variants in the human bile salt export pump (BSEP; ABCB11): functional characterization and interindividual variability. Pharmacogenet Genomics, 2010. 20(1): p. 45-57.
6.    Dixon, P.H., et al., Contribution of variant alleles of ABCB11 to susceptibility to intrahepatic cholestasis of pregnancy. Gut, 2009. 58(4): p. 537-44.
7.    Meier, Y., et al., Interindividual variability of canalicular ATP-binding-cassette (ABC)-transporter expression in human liver. Hepatology, 2006. 44(1): p. 62-74.
8.    Bull, L.N., et al., Outcomes of surgical management of familial intrahepatic cholestasis 1 and bile salt export protein deficiencies. Hepatol Commun, 2018. 2(5): p. 515-528.
9.    Weerachayaphorn, J., et al., Nuclear factor erythroid 2-related factor 2 is a positive regulator of human bile salt export pump expression. Hepatology, 2009. 50(5): p. 1588-96.
10.    Drozdzik, M., et al., Protein Abundance of Hepatic Drug Transporters in Patients With Different Forms of Liver Damage. Clin Pharmacol Ther, 2020. 107(5): p. 1138-1148.
11.    Kosters, A. and S.J. Karpen, Bile acid transporters in health and disease. Xenobiotica, 2008. 38(7-8): p. 1043-71.
12.    Meier, Y., et al., Increased susceptibility for intrahepatic cholestasis of pregnancy and contraceptive-induced cholestasis in carriers of the 1331T>C polymorphism in the bile salt export pump. World J Gastroenterol, 2008. 14(1): p. 38-45.
13.    Gan, L., et al., Functional analysis of the correlation between ABCB11 gene mutation and primary intrahepatic stone. Mol Med Rep, 2019. 19(1): p. 195-204.
14.    Sahi, J., et al., Metabolism and transporter-mediated drug-drug interactions of the endothelin-A receptor antagonist CI-1034. Chem Biol Interact, 2006. 159(2): p. 156-68.
15.    Li, Y., et al., Use of a Bile Salt Export Pump Knockdown Rat Susceptibility Model to Interrogate Mechanism of Drug-Induced Liver Toxicity. Toxicol Sci, 2019. 170(1): p. 180-198.
16.    Dawson, S., et al., In vitro inhibition of the bile salt export pump correlates with risk of cholestatic drug-induced liver injury in humans. Drug Metab Dispos, 2012. 40(1): p. 130-8.
17.    Kenna, J.G., et al., Can Bile Salt Export Pump Inhibition Testing in Drug Discovery and Development Reduce Liver Injury Risk? An International Transporter Consortium Perspective. Clin Pharmacol Ther, 2018. 104(5): p. 916-932.
18.    Chan, R. and L.Z. Benet, Measures of BSEP Inhibition In Vitro Are Not Useful Predictors of DILI. Toxicol Sci, 2018. 162(2): p. 499-508.
19.    Kubitz, R., et al., Genetic variations of bile salt transporters. Drug Discov Today Technol, 2014. 12: p. e55-67.
20.    Bao, Y., et al., Genetic Variations Associated with Anti-Tuberculosis Drug-Induced Liver Injury. Curr Pharmacol Rep, 2018. 4(3): p. 171-181.
21.    Sissung, T.M., et al., Severe Hepatotoxicity of Mithramycin Therapy Caused by Altered Expression of Hepatocellular Bile Transporters. Mol Pharmacol, 2019. 96(2): p. 158-167.
22.    Festa, C., et al., Targeting Bile Acid Receptors: Discovery of a Potent and Selective Farnesoid X Receptor Agonist as a New Lead in the Pharmacological Approach to Liver Diseases. Front Pharmacol, 2017. 8: p. 162.
23.    Dong, R., et al., Yangonin protects against estrogen-induced cholestasis in a farnesoid X receptor-dependent manner. Eur J Pharmacol, 2019. 857: p. 172461.
24.    FDA approves Ocaliva for rare, chronic liver disease. Available from: https://www.fda.gov/news-events/press-announcements/fda-approves-ocaliva-rare-chronic-liver-disease.