FDA Orphan Drug Designation Granted to Zavabresib for Myelofibrosis

The U.S. Food and Drug Administration (FDA) has granted orphan drug designation (ODD) to zavabresib (formerly OPN-2853), an investigational therapy for the treatment of myelofibrosis, a rare and serious blood cancer. The designation, which was announced in a news release by Opna Bio, marks an important regulatory milestone and underscores the ongoing need for new treatment strategies for patients living with this disease.

“Receiving orphan drug designation for zavabresib in myelofibrosis is a significant regulatory milestone for Opna Bio and highlights the urgent need for new and effective treatment options for patients with this disease,” said Reinaldo Diaz, CEO of Opna Bio. “Our investigator-sponsored clinical trial with zavabresib and [Jakafi (ruxolitinib)] has shown impressive results to date, including durable spleen reduction in patients with advanced myelofibrosis. We believe that selective BET inhibition alongside JAK inhibition offers a promising new therapeutic approach for patients with myelofibrosis. We are further encouraged by recent positive meetings with the FDA to continue to test zavabresib in additional clinical studies.”

Designation Highlights Unmet Need in Myelofibrosis

Myelofibrosis is a chronic blood cancer characterized by progressive scarring of the bone marrow, which disrupts normal blood cell production. Patients often experience debilitating symptoms, including severe fatigue, anemia and an enlarged spleen that can cause abdominal pain and early satiety. Approximately 25,000 people in the United States are living with myelofibrosis, classifying it as a rare disease.

Orphan drug designation is reserved for therapies intended to treat conditions affecting fewer than 200,000 individuals nationwide. The designation provides several incentives to support drug development, including tax credits for clinical trial costs, certain FDA fee waivers and seven years of market exclusivity if the drug is ultimately approved. For patients, this recognition often signals increased momentum behind a potential new treatment option.

What Is Zavabresib and How Does It Work?

Zavabresib is an oral small molecule inhibitor targeting bromodomain and extra-terminal motif (BET) proteins. These proteins play a role in regulating gene expression involved in cancer cell growth and inflammation. By selectively inhibiting BET proteins, zavabresib is designed to disrupt disease-driving pathways that contribute to myelofibrosis progression.

Opna Bio is evaluating zavabresib in combination with Jakafi, a Janus kinase (JAK) inhibitor that is commonly used as a first-line therapy for myelofibrosis. Although Jakafi can reduce spleen size and improve symptoms for many patients, some individuals eventually stop responding or experience diminishing benefit. For these patients, treatment options are limited, making combination strategies an area of active research.

Early Clinical Trial Results Show Promising Spleen Reduction

The FDA’s decision follows encouraging results from an ongoing phase 1 investigator-sponsored clinical trial known as the PROMise study. Led by Professor Adam Mead at the University of Oxford in collaboration with Cancer Research UK, the study is evaluating zavabresib as an add-on therapy for patients with myelofibrosis who are no longer responding adequately to Jakafi alone.

Data presented at the American Society of Hematology Annual Meeting in December 2025 showed that 16 of 26 evaluable patients achieved a 50% or greater reduction in spleen length compared with baseline measurements. Spleen enlargement is a major source of discomfort and reduced quality of life in myelofibrosis, so reductions of this magnitude are considered clinically meaningful.

According to Opna Bio, many patients experienced durable benefits, and the combination treatment has been generally well tolerated to date. These findings suggest that adding a BET inhibitor to standard JAK inhibition may help extend disease control for patients with advanced myelofibrosis.

Why This Matters for Patients and Families

For patients whose disease no longer responds to existing therapies, news of orphan drug designation can offer cautious optimism. Although zavabresib remains investigational and is not yet FDA approved, the designation supports continued clinical development and regulatory engagement. Opna Bio has also reported positive discussions with the FDA regarding future studies, which may help accelerate the evaluation process.

Additional Research Supports Ongoing Development

Supplementary data presented by Opna Bio in December 2025 further reinforced the potential of zavabresib in myelofibrosis. In updated analyses from the PROMise study, patients continued to show consistent spleen reductions and improvements in symptom burden, with many completing multiple cycles of combination therapy. These results strengthen confidence in the approach and support further clinical investigation.

As research continues, patients are encouraged to discuss emerging therapies and clinical trial opportunities with their care teams.

References

1. “Opna Bio Announces Orphan Drug Designation Granted to OPN-2853 (Zavabresib) for the Treatment of Myelofibrosis,” by Opna Bio. News release; Jan. 21, 2026.
2. “Opna Bio Showcases Multi-Functional Degraders with Potent Anti-Myeloma Activity and Encouraging Spleen Reductions in Patients with Myelofibrosis Treated with OPN-2853 and Ruxolitinib,” by Opna Bio. News release; Dec. 8, 2025.

Editor's note: This article is for informational purposes only and is not a substitute for professional medical advice, as your own experience will be unique. Use this article to guide discussions with your oncologist. Content was generated with AI and reviewed by a human editor.

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