BSEP Inhibition

BSEP is a liver-specific and ATP-dependent transport protein that mediates the excretion of bile salts into bile and is expressed on the apical plasma membrane domain (canalicular surface) of hepatocytes (Lam et al., 2010). Bile formation and excretion is an essential biological process in higher vertebrates and is an important route of xenobiotic elimination, which also plays a key role in intestinal dissolution and absorption of lipids, vitamins, and fat-soluble food components. Bile salts are synthesized within hepatocytes by cytochrome P450-mediated metabolism of cholesterol and are key components of bile (Hofmann and Hagey, 2008). Bile formation and bile flow are regulated physiologically by complex mechanisms, which in hepatocytes include nuclear hormone receptor-mediated transcriptional pathways and a variety of posttranscriptional processes (Kullak-Ublick et al., 2004 Gonzalez, 2012). 

A variety of drugs that cause cholestatic or mixed chole-static/hepatocellular DILI have been found to inhibit the 

An additional in vitro approach that can provide valuable insight into the possible influence on BSEP inhibition of cooperative interactions with other transporters is by studying vectorial transport of probe substrates using transfected ceUs cultured as confluent monolayers in transwell devices. This approach enables quantification of both basal-to-apical transport through the cells and apical efflux. In these studies it is important to use polarized epithelial cells that express distinct apical and basolateral sinusoidal plasma membrane domains and that also exhibit a low level of endogenous expression of transporters other than BSEP that may mediate apical probe substrate efflux. Appropriate cells include dog kidney-derived MDCK cell lines and porcine kidney-derived LLC-PKl cells. It is also necessary to ensure that basolateral solute carriers are present in the cells that mediate probe substrate uptake. This may require coexpression of BSEP and relevant uptake transporter(s). MDCK cell lines 

Studies with SCH require substantial resources and expertise and are dependent upon access to plateable primary hepatocytes of high quality. In addition, a minimum of several days in cnlture is needed to enable SCH to repolarize. Therefore this model is best suited for low- to medium-throughput characterization of small numbers of test compounds and not high-volume compound screening. 

Profiling of elevated plasma coneentrations of bile acids provides evidence of functional inhibition of bile salt clearance in vivo. This has been observed in rats exposed to a variety of drugs that inhibit BSEP activity in vitro, and in some instances also in humans (Fattinger et al., 2001), which snggests that evalnation of plasma (or serum) bile acids could provide a useful noninvasive biomarker of in vivo BSEP inhibition. However, since uptake carriers on the basolateral plasma membrane domain mediate hepatic nptake of bile acids and therefore also play important roles in bile acid clearance (Kullak-Ublick et al., 2004), potential interactions involving these transporters need to be considered when interpreting effects of compounds on plasma bile acid concentrations. In addition, elevated bile acid concentrations have been observed in plasma and urine from rats treated in vivo with a variety of hepatotoxic drugs, some of which do not inhibit BSEP (e.g., acetaminophen, carbam-azepine Yamazaki et al., 2013). Therefore while elevated total plasma bile acid levels can provide a nseful indirect index of impaired in vivo BSEP-mediated bile salt clearance, they may also arise due to other mechanisms and so cannot be considered a specific in vivo BSEP inhibition biomarker. 

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More recently, a much more complex mechanism has been suggested for BSEP inhibition. By comparing the inhibitory effect of Mrp2 substrates on BSEP func-... 

In Vitro and In Vivo Assays to Measure BSEP Inhibition... 

Critical Review Is There a Link between BSEP Inhibition, Drug-Induced Cholestasis and Idiosyncratic Liver Injury ... 

The best clinical evidence that BSEP is involved in hepatotoxicity is provided by human genetic studies which found four highly conserved non-synonymous mutations in two hepatobiliary transporters (BSEP and MDR3) that were specific for drug-induced liver injury [118]. Recently, a consortium of investigators identified a remarkable 82 different ABCBll mutations in 109 families that caused severe BSEP deficiency [119]. It is therefore expected that at least some of these genetic mutations and polymorphisms will put patients at an increased risk of drug-induced cholestasis. Does this justify the implementation of a simple BSEP inhibition screen for all new chemical entities The answer is not quite that simple. 

Although BSEP is not an important transporter of drugs, drug-inhibition of this transporter is a recognized mechanism for hepatotoxicity. Intracellular accumulation of cytotoxic bUe salts subsequent to BSEP inhibition can produce cholestatic liver injury. The antidiabetic... 

The emerging data linking BSEP inhibition with risk of human cholestatic DILI has been acknowledged recently by the International Transporter Consortium (TTC), which also has highlighted that there ranain important gaps in our... 

Overall, in view of the deleterious consequences of dual inhibition of mitochondrial function and BSEP inhibition, it is highly desirable to avoid these two synergistic safety liabilities as mnch as possible during the drug development process. This is especially important for drugs intended for patient populations in which BSEP expression, mitochondrial reserve capacity and function, and antioxidant status are compromised. 

HA, Howell BA (2014) Exploring BSEP inhibition-mediated toxicity with a mechanistic model of drug-induced liver injury. Front Pharmacol 5, 240. 

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