Oral NTCP Proof-of-Pharmacology: Mechanism Validated, Patients Still Unproven

Assembly Bio Presents Positive Phase 1a Data for ABI-6250

Assembly Biosciences announced positive Phase 1a data for its investigational oral NTCP inhibitor, ABI-6250, presented at The Liver Meeting® 2026. The study in healthy participants showed ABI-6250 was well tolerated with no serious adverse events. Pharmacodynamic analysis demonstrated dose-dependent, reversible increases in total and conjugated bile acids, consistent with sodium taurocholate co-transporting polypeptide (NTCP) target engagement. These findings support the company's plans to advance ABI-6250 into Phase 2 studies for chronic hepatitis delta virus (HDV) infection in Q4 2026 and for cholestatic liver diseases, including primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), in Q1 2027.

• The Phase 1a study of ABI-6250 in healthy participants demonstrated a favorable safety profile, with the drug being well tolerated at all evaluated doses and no serious adverse events reported, providing a strong foundation for further clinical development.
• Pharmacodynamic analysis confirmed potent and dose-dependent target engagement, showing reversible increases in total and conjugated bile acids. This finding is consistent with the intended mechanism of sodium taurocholate co-transporting polypeptide (NTCP) inhibition.
• Based on the positive Phase 1a results, Assembly Bio is preparing to initiate two distinct Phase 2 studies: one for chronic hepatitis delta virus (HDV) infection in the fourth quarter of 2026 and another for cholestatic liver diseases, specifically primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), in the first quarter of 2027.

Addressing Critical Unmet Needs in Chronic HDV and Cholestatic Liver Diseases

Chronic HDV infection remains one of the most severe and underaddressed forms of viral hepatitis, with significant gaps persisting across diagnosis, risk stratification, and treatment access. Despite the approval of bulevirtide by the European Medicines Agency, multiple unmet needs continue to define the clinical and strategic landscape for this disease.

• Underdiagnosis and diagnostic gaps: Only approximately 20% to 50% of HDV-infected individuals have been diagnosed, driven by limited awareness and restricted access to reliable HDV antibody and HDV RNA diagnostic tests. In Spain, a nationwide double reflex testing initiative (antibody testing followed by viral load confirmation) demonstrated that HDV antibody testing uptake among HBsAg-positive individuals increased from 55.1% in 2022 to 85.8% in 2024 across 80 centers — yet marked regional heterogeneity persists, with viral load confirmation rates and seroprevalence varying substantially across autonomous communities. Standardisation of reflex protocols and harmonisation of viral load assays remain unresolved.

• HDV RNA assay variability and lack of standardisation: Reliable quantification of HDV RNA — essential for both diagnosis and treatment response monitoring — is challenged by at least 8 viral genotypes, intra-host diversity, rapid viral evolution, and the distinct secondary structure of HDV RNA. Variability in RNA extraction methodologies, primer/probe design, and the absence of automation across laboratories contribute to inconsistent assay performance, complicating drug development endpoints and patient monitoring.

• Patients with early-stage fibrosis at risk of rapid progression: A multicenter European cohort of 168 HDV-RNA-positive patients with mild-to-moderate fibrosis at baseline showed that over 22% progressed to cirrhosis over a median follow-up of 62 months. Baseline liver stiffness ≥7 kPa (HR 6.02 [1.76–20.6]; p=0.004) was the strongest independent predictor of progression, while ALT levels and HDV RNA — at baseline and during follow-up — did not independently predict fibrosis outcomes. This population, often considered lower priority, represents a critical target for earlier treatment intervention and closer monitoring.

• Patients with advanced cirrhosis and portal hypertension: Interferon alfa, the historical standard of care, carries significant safety limitations in patients with advanced cirrhosis. Bulevirtide has demonstrated promising safety and efficacy in this population in both clinical trials and real-world studies, but EMA label guidance specifies long-term treatment "until clinical benefit," with predictors of response still undefined — leaving clinicians without clear stopping rules or response benchmarks for this high-risk group.

• Optimising treatment response and identifying predictors: Real-world data indicate that ALT normalisation typically precedes virological response (occurring between months 2–6 of bulevirtide treatment), and that patients with high viral load and elevated ALT at baseline respond more slowly. Low body mass index and high alpha-fetoprotein at baseline were identified as possible predictors of delayed response. In the ARISTOTLE pilot study (n=108), virological response was achieved in 54.6% of patients at 6 months, with no significant predictors of response identified — underscoring the need for larger studies to define response-predictive biomarkers.

• Combination therapy strategies and pipeline development: The potential combination of bulevirtide with pegylated interferon alfa (PegIFNα) has suggested increased virological and clinical response rates, though this approach remains under evaluation. Additional compounds targeting HDV assembly (e.g., lonafarnib) and other mechanisms are in development or pipeline stages, reflecting the need for regimens that can achieve durable virological clearance beyond what bulevirtide monotherapy currently offers.

Oral NTCP Inhibitor Advances: A Dual Threat for Liver Diseases

The recent positive Phase 1a data for Assembly Biosciences' ABI-6250, an oral sodium taurocholate co-transporting polypeptide (NTCP) inhibitor, signals a promising step forward in addressing significant unmet needs in liver disease. The study in healthy volunteers confirmed that ABI-6250 was well tolerated and demonstrated clear dose-dependent increases in total and conjugated bile acids, a strong indicator of successful NTCP target engagement. This foundational data supports the company's ambitious plans to advance ABI-6250 into Phase 2 studies for both chronic hepatitis delta virus (HDV) infection and cholestatic liver diseases, including primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC).

For HDV, a severe form of viral hepatitis with limited treatment options, the NTCP inhibition mechanism is already validated. Existing evidence shows that entry inhibitors targeting NTCP can significantly reduce HDV RNA levels and normalize liver enzymes. An oral formulation like ABI-6250 could offer a substantial advantage in patient convenience and adherence compared to injectable therapies, potentially improving real-world outcomes for this challenging infection.

Beyond HDV, the strategic move into cholestatic liver diseases is noteworthy. These conditions are characterized by the harmful accumulation of cytotoxic bile acids within the liver, leading to progressive damage. By inhibiting NTCP, ABI-6250 aims to reduce the liver's uptake of these bile acids, thereby mitigating injury. While current therapies for cholestasis often modulate nuclear receptors to influence bile acid metabolism, a direct NTCP inhibitor offers a distinct approach to managing the bile acid burden.

However, several considerations lie ahead. The competitive landscape for HDV already features an approved NTCP inhibitor in Europe, meaning ABI-6250 will need to demonstrate a compelling differentiation in its efficacy, safety, or convenience profile. Furthermore, while reducing bile acid uptake is a logical strategy for cholestatic diseases, the complex interplay of bile acid transport, synthesis, and detoxification pathways means that clinical success in improving long-term outcomes for PBC and PSC will require robust evidence beyond initial target engagement. The potential for systemic bile acid accumulation from sustained NTCP inhibition also warrants careful monitoring in larger studies. Nevertheless, these early results position ABI-6250 as a versatile candidate with the potential to address critical needs across a spectrum of liver pathologies.