Regenxbio (NASDAQ: RGNX) Shares Fell Approximately 25% to $8.05 Tuesday August 25, 2026 After the FDA Placed a Clinical Hold on the RGX-121 Gene Therapy Program for the Treatment of Mucopolysaccharidosis Type II (MPS II, Also Known as Hunter Syndrome, a Rare X-Linked Lysosomal Storage Disease Caused by Iduronate-2-Sulfatase (IDS) Deficiency That Results in Accumulation of Glycosaminoglycans in Multiple Tissues Including the Central Nervous System, With Progressive Cognitive Decline, Skeletal Abnormalities, and Cardiac and Respiratory Involvement in the Severe Neuronopathic Form); RGX-121 Is an AAV Gene Therapy Delivering an IDS-Encoding Transgene Via Intracerebroventricular Administration Designed to Address the Neurocognitive Manifestations That the Currently-Approved Enzyme Replacement Therapy Elaprase (Idursulfase) Does Not Reach
Regenxbio (NASDAQ: RGNX) shares fell approximately 25% to $8.05 Tuesday August 25, 2026 after the FDA placed a clinical hold on the RGX-121 gene therapy program for the treatment of Mucopolysaccharidosis type II (MPS II, also known as Hunter Syndrome). Disease context: MPS II is a rare X-linked lysosomal storage disease affecting approximately 1 in 100,000-170,000 male births globally, caused by iduronate-2-sulfatase (IDS) enzyme deficiency that results in accumulation of glycosaminoglycans (GAGs) in multiple tissues including the central nervous system. Clinical manifestations include progressive cognitive decline, skeletal abnormalities, cardiac and respiratory involvement, and reduced life expectancy in the severe neuronopathic form (approximately two-thirds of patients). RGX-121 mechanism: an AAV (adeno-associated virus) gene therapy delivering an IDS-encoding transgene via intracerebroventricular administration designed to establish stable IDS expression in the central nervous system, addressing the neurocognitive manifestations that the currently-approved enzyme replacement therapy Elaprase (idursulfase, Takeda) does not reach because Elaprase administered intravenously does not cross the blood-brain barrier. Specific reasons for the FDA clinical hold have not been publicly disclosed by Regenxbio; details are expected in subsequent regulatory correspondence. The clinical hold delays but does not necessarily preclude the RGX-121 development pathway. Regenxbio's broader pipeline includes RGX-202 for Duchenne muscular dystrophy in Phase 3, RGX-314 for wet AMD in Phase 3, and multiple additional gene therapy candidates.