What is BSEP

Why does BSEP inhibition cause liver injury?

Bile salts are produced in hepatocytes and must be actively pumped into bile by BSEP to prevent toxic accumulation. When BSEP is inhibited by a drug, bile salts accumulate inside hepatocytes to concentrations that cause direct membrane damage, mitochondrial dysfunction, and activation of apoptotic pathways. This manifests clinically as cholestatic hepatitis — elevated alkaline phosphatase, conjugated bilirubin, and GGT, with or without elevated transaminases.

How is BSEP inhibition tested?

BSEP inhibition is assessed using the vesicular transport assay with inside-out membrane vesicles prepared from cells overexpressing human BSEP (ABCB11). The probe substrate is taurocholic acid (TCA). The test compound is incubated at multiple concentrations alongside TCA and ATP — the reduction in ATP-dependent TCA accumulation gives the IC50. An IC50 below the estimated unbound hepatic concentration triggers a DILI risk flag requiring further investigation.

Zulassungsstatus

FDA's 2020 DDI guidance does not explicitly mandate BSEP testing. ICH M12 (2022), EMA DDI guideline, and PMDA all recommend BSEP inhibition assessment for drugs with hepatic involvement. Omitting BSEP from an IND package for a hepatically-eliminated drug routinely generates regulatory questions.

Key Facts

• BSEP is encoded by the ABCB11 gene and belongs to the ABC (ATP-binding cassette) transporter superfamily
• Located exclusively on the canalicular (bile-facing) membrane of hepatocytes — transports bile salts from liver cells into bile
• BSEP inhibition causes intracellular bile salt accumulation → oxidative stress → mitochondrial dysfunction → hepatocyte apoptosis and necrosis
• BSEP inhibition is the primary mechanism of cholestatic DILI — examples include troglitazone, bosentan, and cyclosporin A
• ICH M12, EMA and PMDA recommend BSEP inhibition testing for all drugs with hepatic elimination
• Assessed in vitro using inside-out membrane vesicle assay with taurocholic acid as probe substrate
• IC50 below clinical hepatic exposure triggers DILI risk flag in IND/BLA submissions
• Cell4Pharma BSEP Vesicle Kit — available from SeamlessBio for in-house IND-enabling BSEP inhibition testing

Further Reading

• BSEP Inhibition & DILI — Complete Guide
• What is a Vesicular Transport Assay?
• FDA & EMA Mandatory Transporters for IND
• DMPK Transporter Kit Portfolio

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