Opna Bio's Zavabresib Receives FDA Orphan Drug Designation for Myelofibrosis Treatment
Key Insights
The FDA has granted Orphan Drug Designation to zavabresib (search) (OPN-2853), a BET (search) inhibitor developed by Opna Bio (search), for treating myelofibrosis (search), a rare blood cancer (search) affecting approximately 25,000 people in the U.S.
Clinical trial data from the Phase 1 PROMise study showed that 16 of 26 evaluable patients experienced a 50% or greater reduction in spleen length when zavabresib (search) was combined with ruxolitinib.
The designation provides significant regulatory benefits including tax credits for clinical trials, FDA fee waivers, and seven years of market exclusivity upon approval.
Opna Bio (search) announced that the U.S. Food and Drug Administration has granted Orphan Drug Designation to zavabresib (search) (OPN-2853), a bromodomain and extra-terminal motif (BET (search)) small molecule inhibitor, for the treatment of myelofibrosis (search). The International Nonproprietary Names for Pharmaceutical Substances has also approved the generic name zavabresib for OPN-2853.
Clinical Significance for Rare Blood Cancer
Myelofibrosis (search) is a rare and serious type of blood cancer (search) characterized by bone marrow scarring, which leads to ineffective blood cell production and symptoms such as severe fatigue, enlarged spleen, and anemia. The disease affects approximately 25,000 people in the United States.
"Receiving Orphan Drug Designation for zavabresib (search) in myelofibrosis (search) is a significant regulatory milestone for Opna Bio (search) and highlights the urgent need for new and effective treatment options for patients with this disease," said Reinaldo Diaz, chief executive officer of Opna Bio.
Promising Clinical Trial Results
The ongoing Phase 1 PROMise study, led by Professor Adam Mead at the University of Oxford through a collaboration with Cancer Research UK, is evaluating zavabresib (search) as an add-on to ruxolitinib in patients with myelofibrosis (search) who are no longer responding to ruxolitinib alone. Data presented at the American Society of Hematology conference in December 2025 demonstrated significant clinical activity.
The combination treatment achieved a 50% or greater reduction of spleen length in 16 of 26 evaluable patients when compared to baseline. Diaz noted that the investigator-sponsored clinical trial with zavabresib (search) and ruxolitinib "has shown impressive results to date, including durable spleen reduction in patients with advanced myelofibrosis (search)."
Regulatory Benefits and Strategic Advantages
The FDA grants Orphan Drug Designation to investigational therapies intended for the treatment, diagnosis or prevention of rare diseases that affect fewer than 200,000 people in the United States. The designation provides several benefits, including tax credits for clinical trial costs, a waiver of certain FDA fees, and eligibility for seven years of market exclusivity upon approval.
Opna Bio (search) believes that selective BET (search) inhibition alongside JAK (search) inhibition offers a promising new therapeutic approach for patients with myelofibrosis (search). The company has had recent positive meetings with the FDA to continue testing zavabresib (search) in additional clinical studies.
Advancing Multi-Functional Protein Degrader Program
Beyond zavabresib (search), Opna Bio (search) is developing novel protein degraders designed to block multiple oncogenic targets – EP300, CBP, IKZF1 (search) and IKZF3 (search) – concurrently in the same cancer cell. These targets are known to promote the progression of multiple myeloma (search), a type of blood cancer (search) derived from malignant plasma cells in the bone marrow.
In a proof-of-concept OPM-2 multiple myeloma (search) model, OPN-5667 (search) potently reduced the levels of key oncoproteins in vitro and caused tumor regression in all treated animals in vivo. The company's medicinal chemistry campaign has produced compounds with improved potency and pharmacological properties, advancing the program towards clinical candidate selection.
"These promising data support our goal of developing a single agent 'super drug' for hematological malignancies, such as multiple myeloma (search) and lymphoma," said Gideon Bollag, PhD, chief scientific officer of Opna Bio (search). The company anticipates identifying a lead candidate in mid-2026 and submitting an IND in 2027.
The degrader program builds on foundational studies presented at ASH in 2024 with OPN-6602, an oral EP300/CBP inhibitor, in combination with immunomodulatory drugs. The combination resulted in strong synergy in vivo including complete regressions and improved response durability. A Phase 1 study of OPN-6602 is currently enrolling patients with relapsed or refractory multiple myeloma (search) at multiple sites in the U.S.