BSEP Transporter Assay (ABCB11)
The Bile Salt Export Pump, or BSEP, is the major transporter responsible for the secretion of bile salts from liver hepatocytes into the bile. BSEP is located on the canalicular (apical) side of hepatocytes and is a member of the ATP-binding cassette transporter family (ABCB11). Unlike uptake transporters which import substrates into cells, BSEP transports its substrates out of the cell (“efflux”), and is capable of doing so against even a steep concentration gradient. In contrast to other transporters that often have multiple family members with overlapping specificity, BSEP receives little to no assistance from other apical transporters in the transport of monovalent conjugated bile salts.
In the liver, primary bile acids are synthesized from cholesterol, and are then conjugated with amino acids (glycine or taurine) to form conjugated bile acids, which are often referred to as bile salts. Bile salts are amphipathic molecules displaying detergent-like properties. When released with bile into the intestine, bile salts play a critical role in the digestion and absorption of fats and lipid-soluble vitamins. However, due to their detergent-like properties, bile salts are cytotoxic, even toward hepatocytes.
Mutations in BSEP can increase bile salt concentration in hepatocytes and lead to diseases ranging from the relatively mild Benign Recurrent Intrahepatic Cholestasis type 2 (BRIC-2) to the much more severe Progressive Familial Intrahepatic Cholestasis type 2 (PFIC-2), which can result in end-stage liver disease in childhood.
BSEP activity can also be adversely affected by drugs. Drugs or metabolic products of drugs can inhibit BSEP, potentially causing increased bile salt levels inside hepatocytes and leading to drug-induced liver injury (DILI). Numerous therapeutics have been restricted or withdrawn from the market due to DILI, and drug-induced disruption of BSEP is believed to be one potential cause. Preclinical models have been relatively poor predictors of potential liver injury associated with BSEP, potentially because the bile salt pool in rodents is simply less cytotoxic than that of humans.
While BSEP has been reported to transport some xenobiotics (such as pravastatin), the main risk is not drug-drug interactions (DDI), but rather toxicity due to xenobiotic-mediated BSEP inhibition. It is therefore recommended that investigational drugs be tested as potential inhibitors of BSEP.
Optivia’s BSEP Assay
Optivia offers BSEP assays in different formats. In the single transporter format, inverted membrane vesicles prepared from Sf9 cells expressing human BSEP are used. In addition, we offer BSEP in multiple-transporter formats in which uptake transporters (such as NTCP and OATP1B1) are expressed along with the BSEP efflux transporter, allowing the assessment of BSEP activity in the biologically relevant efflux mode.
Optivia offers a wide range of BSEP studies, from single concentration screens to full kinetic studies that allow the determination of IC50 values for inhibitors, as well as Km and Vmax values for substrates.
BSEP Transporter Assay Details
|Relevance – FDA & EMA guidance, biliary secretion of bile salts, drug induced liver injury||Single or Multi-Transporter Model – Single (Multi-Transporter models are also available)|
|Transporter Type – Efflux||Model – Vesicle (Multi-Transporter MDCK-II model also available)|
|Assay Type – Vesicle accumulation (Transcellular assay for Multi-Transporter models)||Exactly Matched Control Cells – Yes|
|Species – Human, rat, dog||Probe Substrate – Taurocholate|
|Subcellular Localization in Assay Model – N/A for vesicles, apical for multi-transporter models.||Inhibition Positive Control – Rifampicin|
Clinical & Regulatory Relevance
The BSEP transporter is significant in the evaluation of in vitro safety, as several cases of hepatotoxicity have been linked to BSEP inhibition. BSEP inhibition can cause accumulation of cytotoxic bile salts and ultimately lead to serious liver injury, such as intrahepatic cholestasis. The study of the BSEP transporter is recommended in EMA guideline documents.
Applications & Therapeutic Areas
DDI/regulatory, toxicity/safety, liver failure