GSK's Momelotinib Pursues Rare VEXAS Syndrome Indication

GSK has secured Orphan Drug Designations from both the FDA and the EMA for momelotinib in the treatment of Vacuoles, E1 enzyme, X-linked, Autoinflammatory, Somatic (VEXAS) syndrome, a rare and severe hemato-inflammatory condition for which no approved therapies currently exist.¹ The dual designation marks a meaningful step in expanding the therapeutic profile of a drug that is already approved for myelofibrosis across several major markets.

VEXAS syndrome is a clonal myeloid disorder that sits at the intersection of rheumatology and hematology.¹ Clinically, it presents with a broad spectrum of inflammatory manifestations, including prolonged fever, relapsing chondritis, vasculitis, and lung involvement, alongside hematologic complications such as macrocytic anemia, thrombocytopenia, and progressive bone marrow failure that can evolve into hematologic malignancy. Caused by a somatic mutation in the UBA1 gene located on the X chromosome, the condition predominantly affects men over 50. Its prognosis is poor: five-year mortality sits between 30 and 40%, yet the disease remains without any approved treatment options. The designations were supported by retrospective case data suggesting that Janus kinase inhibitors may offer therapeutic benefit in VEXAS, along with a case report indicating clinical improvement with momelotinib specifically, including reductions in disease-related inflammation and hematological manifestations.

What Does This Mean for Momelotinib's Development Program?

Momelotinib's mechanism sets it apart from other Janus kinase inhibitors.¹ The compound inhibits JAK1 and JAK2, pathways implicated in constitutional symptoms and splenomegaly, while also targeting activin A receptor type I, which reduces circulating hepcidin levels and contributes to anemia benefit. That triple-pathway inhibition is central to why the molecule is attracting attention beyond its current indication.

The drug is already approved in the US for intermediate or high-risk myelofibrosis in adults with anemia and holds approvals in the EU, UK, and Japan for myelofibrosis with disease-related splenomegaly, symptoms, or moderate to severe anemia.¹ The VEXAS program represents a deliberate expansion into adjacent hematological territory.

GSK has initiated planning for the ATLAS phase II/III trial, which will evaluate momelotinib's efficacy and safety in VEXAS syndrome and serve as the foundation for planned global regulatory submissions.¹ The study design is scheduled to be presented at the 2026 European Hematology Association Congress in June.

Orphan drug status in both the US and EU confers regulatory and commercial incentives, including extended market exclusivity and reduced filing fees, that influence how companies prioritize and resource development programs.¹ For a molecule already navigating post-approval lifecycle management in myelofibrosis, these designations reinforce the case for continued manufacturing investment and supply chain planning.

The VEXAS indication also introduces formulation and development considerations.¹ As a rare disease with a predominantly older male patient population and complex hematological presentation, any eventual commercial program will require close coordination between clinical, regulatory, and manufacturing teams well ahead of any potential approval. The ATLAS trial design, when presented at the European Hematology Association Congress, will be worth monitoring for signals that could shape those downstream requirements.

How Does This Fit into GSK's Broader Hematology & Oncology Push?

The VEXAS designations arrive as GSK pursues an aggressive expansion across hematology and oncology.² The company recently announced a $10.6 billion agreement to acquire Nuvalent, a clinical-stage biopharmaceutical company with two late-stage non-small cell lung cancer assets, zidesamtinib and neladalkib, both currently under FDA review with potential 2026 approval decisions. The deal also includes a phase I HER2 inhibitor and a preclinical portfolio built around precision medicine. Together with the momelotinib VEXAS program, the moves signal a deliberate strategy of targeting conditions where existing therapies carry meaningful efficacy or tolerability limitations.

References

1. GSK plc. GSK’s momelotinib granted orphan drug designations in the US and EU for VEXAS syndrome. Press Release. June 11, 2026. https://www.gsk.com/en-gb/media/press-releases/gsk-s-momelotinib-granted-orphan-drug-designations-in-the-us-and-eu-for-vexas-syndrome
2. GSK plc. GSK enters agreement to acquire Nuvalent, Inc. Press Release. June 8, 2026. https://www.gsk.com/en-gb/media/press-releases/gsk-enters-agreement-to-acquire-nuvalent-inc/