Opna Bio, a clinical-stage biopharmaceutical company focused on developing novel cancer therapies, has received a key regulatory boost for its myelofibrosis program. The U.S. Food and Drug Administration has granted Orphan Drug Designation to OPN-2853, a bromodomain and extra-terminal motif (BET) small-molecule inhibitor, for the treatment of myelofibrosis, a rare and life-threatening blood cancer. In parallel, the International Nonproprietary Names (INN) organization has approved the generic name zavabresib for the investigational therapy.
Myelofibrosis is characterized by progressive scarring of the bone marrow, which disrupts normal blood cell production and leads to debilitating symptoms including severe fatigue, anemia, and enlargement of the spleen. The disease affects an estimated 25,000 people in the United States and remains an area of high unmet medical need, particularly for patients who stop responding to standard therapies.
The Orphan Drug Designation underscores the FDA’s recognition of both the rarity of myelofibrosis and the need for new treatment options. The designation provides several incentives to developers, including tax credits for qualified clinical trial expenses, exemptions from certain regulatory fees, and the potential for seven years of market exclusivity in the U.S. if the drug is approved.
Opna Bio’s chief executive officer highlighted the milestone as a validation of the company’s scientific strategy. Zavabresib is being developed as a selective BET inhibitor and is being studied in combination with the JAK inhibitor ruxolitinib, which is a standard therapy for myelofibrosis. According to the company, combining BET inhibition with JAK inhibition may offer a novel therapeutic approach, particularly for patients with advanced disease who have limited options.
Clinical evidence supporting the program has emerged from the ongoing Phase 1 PROMise study, led by Professor Adam Mead at the University of Oxford in collaboration with Cancer Research UK. In this investigator-sponsored trial, zavabresib is being evaluated as an add-on therapy in patients whose disease is no longer adequately controlled by ruxolitinib alone. Data presented at the American Society of Hematology meeting in December 2025 showed that 16 of 26 evaluable patients achieved a 50% or greater reduction in spleen length compared with baseline, suggesting meaningful clinical activity.
Encouraged by these results and recent interactions with the FDA, Opna Bio plans to continue advancing zavabresib into additional clinical studies as it seeks to address the unmet needs of patients living with myelofibrosis.