Incyte  announced that the U.S. Food and Drug Administration (FDA) has granted Breakthrough Therapy designation to INCA033989, a first-in-class mutant calreticulin (mutCALR)-targeted monoclonal antibody, for the treatment of patients with essential thrombocythemia (ET) harboring a Type 1 CALR mutation who are resistant or intolerant to at least one cytoreductive therapy.

ET is a chronic myeloproliferative neoplasm (MPN) characterized by persistently elevated platelet counts due to abnormal blood cell production in the bone marrow. CALR mutations are the second most common oncogenic driver mutation and are observed in 25% of patients with ET. A 52-bp deletion, also known as a Type 1 mutation, occurs in 55% of patients with a CALR mutation, and is associated with the highest risk of transformation to myelofibrosis (MF) among all ET patients.

“Incyte has long been committed to improving outcomes for patients with MPNs, and this Breakthrough Therapy designation underscores the potential of INCA033989 to be a novel therapy that could significantly transform the treatment of ET patients, who today have limited treatment options,” said Pablo J. Cagnoni, M.D., President and Head of Research and Development, Incyte. “The designation allows us to expedite the development pathway for INCA033989 in patients with Type 1 mutations. Looking ahead, we plan to initiate a Phase 3 program evaluating INCA033989 in ET patients with all types of CALR mutations in mid-2026, following alignment with regulators in the first half of next year.”

The FDA Breakthrough Therapy Designation was supported by the early Phase 1 data evaluating INCA033989 in ET patients with a Type 1 CALR mutation available at the time of submission.

The preliminary Phase 1 data were presented earlier this year at the 2025 European Hematology Association (EHA) Congress. In the study, INCA033989 was well-tolerated and demonstrated rapid and durable normalization of platelet counts across evaluated doses, with greater responses seen at higher doses across both mutation types. Updated results from the Phase 1 dose escalation and expansion trial are planned for presentation at the 2025 ASH Annual Meeting in Orlando (Session 634; Publication #1024; December 8, 4:30-6:00 p.m. ET).

Incyte plans to develop INCA033989 for patients with Type 1 and non-Type 1 CALR mutations and, following discussions with regulatory agencies, plans to initiate a registrational program evaluating patients with ET with a Type 1 or non-Type 1 CALR mutation who are resistant or intolerant to at least one cytoreductive therapy in the first half of next year.