- The FDA has agreed to reverse its February rejection of Regenxbio’s Navsunli gene therapy for Hunter syndrome, a rare fatal brain disease affecting ~2,000 people worldwide (almost all boys) that typically kills patients in their midteens — with the FDA indicating it could complete its review in as little as two months.
- The reversal is the third recent FDA about-face on rare-disease drugs under new leadership: Replimune’s twice-rejected melanoma therapy got a third chance after White House pressure, and uniQure received a green light for its Huntington’s disease gene therapy after the FDA dropped demands for a new trial.
- The U-turns mark a broad retreat from demands by former FDA biotech division leader Vinay Prasad that rare-disease drugmakers include placebo arms in trials — a requirement companies called impractical and unethical for diseases diagnosed in only dozens of patients per year.
- The FDA now requires only that the original 13 Navsunli trial subjects be followed longer to monitor neurocognitive development, matching the original study design the agency had approved before reversing itself in mid-2025; Regenxbio shares surged more than 10% in premarket trading.
What Happened?
Regenxbio announced that the FDA has agreed to reverse its February 2026 rejection of Navsunli (RGX-121), a one-time gene therapy for Hunter syndrome. The disease causes progressive, irreversible brain damage and cell death, almost exclusively in boys, and is typically fatal by the midteens. About 50 patients are diagnosed annually in the US. Navsunli works by replacing a defective gene so patients produce a protein that prevents toxic substance buildup in the brain and tissues. The FDA had originally approved a trial design without a placebo group before reversing itself in mid-2025 and then formally rejecting the application in February. After CEO Curran Simpson met with then-FDA Commissioner Marty Makary in April — who encouraged a formal appeal with “fresh eyes” — the agency indicated last week it would not require new patients to be dosed, only that the original 13 subjects be followed longer. The FDA indicated a potential review window of as little as two months.
Why It Matters?
The reversal is part of a pattern of FDA policy shifts under new leadership that is reshaping the regulatory landscape for rare-disease biotechs. The prior FDA regime, under Vinay Prasad, insisted that even tiny rare-disease trials include placebo arms — a standard drug-development principle that becomes deeply problematic when a disease affects only dozens of patients per year and untreated subjects will almost certainly suffer irreversible harm. The broader signal to biotech investors is significant: gene therapy developers targeting ultra-rare diseases may no longer face the placebo-requirement barrier that had effectively halted or delayed several programs. Three reversals in rapid succession — Replimune, uniQure, and now Regenxbio — suggest a systematic rethink rather than case-by-case exceptions.
What’s Next?
An FDA approval decision on Navsunli could come as early as August. If approved, it would be the first gene therapy for Hunter syndrome and one of the first one-time curative treatments for any progressive fatal pediatric brain disease. How durably the new FDA posture holds — and whether it extends beyond rare diseases to larger therapeutic areas — will be a major determinant of biotech sector sentiment and investment in the months ahead. For Regenxbio specifically, approval would validate years of development and potentially open a path to expand its gene therapy platform across other lysosomal storage diseases with similar profiles.
Source: The Wall Street Journal
