Prime Medicine's PM359 Gains FDA RMAT Designation for p47phox-Deficient Chronic Granulomatous Disease
Key Insights
The U.S. FDA granted Regenerative Medicine Advanced Therapy (RMAT) designation to PM359, Prime Medicine's investigational Prime Edited hematopoietic stem cell therapy for p47phox-deficient CGD.
The designation was based on Phase 1/2 clinical data published in The New England Journal of Medicine showing rapid engraftment and durable restoration of immune cell function with a single dose.
PM359 now holds four FDA designations: RMAT, Fast Track, Orphan Drug, and Rare Pediatric Disease, underscoring the serious unmet need in CGD.
Prime Medicine, Inc. announced on June 22, 2026, that the U.S. Food and Drug Administration (FDA) has granted Regenerative Medicine Advanced Therapy (RMAT) designation to PM359, an investigational autologous Prime Edited hematopoietic stem cell therapy for the treatment of p47phox-deficient chronic granulomatous disease (search) (CGD). The designation was supported by Phase 1/2 clinical data, including results previously published in The New England Journal of Medicine in December 2025.
"FDA's decision to grant RMAT designation to PM359 reinforces the potential for this program to deliver a meaningful, disease-modifying impact in CGD, where patients face significant morbidity, lifelong complications, and limited treatment options," said Allan Reine, M.D., Chief Executive Officer of Prime Medicine. "The combination of RMAT, Fast Track, Orphan Drug, and Rare Pediatric Disease Designations underscores the seriousness of CGD and the need for transformative therapies that deliver durable benefit, while positioning us to engage with the FDA on the most efficient path to a Biologics License Application."
Clinical Evidence Supporting the Designation
The RMAT designation was granted based on initial clinical trial results reported in the December 2025 New England Journal of Medicine publication. The data demonstrated that a single dose of PM359 drove rapid engraftment and a durable, clinically meaningful restoration of immune cell function in both treated patients. Neutrophil activity was well above the level associated with clinical benefit, and the safety profile was consistent with busulfan-based conditioning alone. These findings support the potential of PM359 to deliver a one-time, disease-modifying treatment for patients with p47phox-deficient CGD.
Regulatory Implications of RMAT Designation
Established under the 21st Century Cures Act, RMAT designation is designed to expedite the development and review of regenerative medicine therapies intended to treat serious or life-threatening diseases when preliminary clinical evidence indicates the therapy has the potential to address unmet medical need. The designation provides all the benefits of Fast Track and Breakthrough Therapy Designations, including intensive FDA interaction, discussions on surrogate or intermediate endpoints that may support accelerated approval, and eligibility for rolling and priority review of a future Biologics License Application (BLA).
With the addition of RMAT, PM359 now holds four distinct FDA designations: RMAT, Fast Track, Orphan Drug, and Rare Pediatric Disease Designations.
PM359: Mechanism and Trial Design
PM359 is an investigational autologous hematopoietic stem cell therapy designed to correct the delGT mutation in NCF1 (search), the most prevalent disease-causing mutation in p47phox-deficient CGD. The therapy is currently being evaluated in an ongoing Phase 1/2, multinational, first-in-human trial.
Disease Background and Unmet Need
Chronic granulomatous disease is a rare inherited hematologic disorder characterized by susceptibility to severe, difficult-to-treat infections and inflammatory and autoimmune complications. CGD is caused by mutations in any one of the subunits comprising the NADPH oxidase (search) complex, which is required for phagocytic cells, particularly neutrophils, to destroy invasive microorganisms. CGD causative mutations are estimated to occur in between one in 100,000 and one in 200,000 births in the United States, with the p47phox form accounting for approximately 25% of cases.
Current standard of care relies on lifelong antimicrobial prophylaxis and, in select cases, allogeneic hematopoietic stem cell transplantation, which carries significant morbidity and is not accessible to all patients. There are no approved gene editing therapies for CGD.
Broader Pipeline Context
Prime Medicine is deploying its proprietary Prime Editing platform to develop a new class of differentiated one-time curative genetic therapies. Beyond PM359, the company is advancing programs in Wilson Disease and Alpha-1 Antitrypsin Deficiency, with a diversified portfolio spanning liver, lung, and immunology and oncology therapeutic areas. Reine noted that the company is "working with urgency to advance PM359 toward potential approval, while building on this foundation across our broader pipeline."