FDA Grants Immusoft RPPD for MPSII Cell Therapy

Rare Daily Staff

The U.S. Food and Drug Administration granted Immusoft Rare Pediatric Disease designation to ISP-002, the company’s experimental cell therapy for the treatment of the rare lysosomal storage disorder mucopolysaccharidosis type II.

The FDA grants Rare Pediatric Disease designation for serious or life-threatening diseases that primarily affect children 18 years old or younger and affect fewer than 200,000 people nationwide. Rare Pediatric Disease designation allows recipient companies, upon approval of their designated treatment, to be eligible for a Priority Review Voucher, which may be used for priority review of a future submission of a New Drug Application or Biologics License Application. Priority review can reduce FDA review time by several months. Priority review vouchers may be sold to a third party, and recent prices paid for such vouchers have been as much as $200 million.

Mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome, is an inherited disorder caused by a deficiency of the enzyme iduronate-2-sulfatase (IDS), which leads to the accumulation of glycosaminoglycans in tissues and organs throughout the body. The disease primarily affects pediatric patients and is associated with progressive, multisystem involvement, including neurocognitive impairment, cardiopulmonary complications, skeletal abnormalities, and reduced life expectancy. The current standard of care is enzyme replacement therapy, which requires frequent lifelong infusions and may not achieve consistent enzyme exposure across all affected tissues.

ISP-002 is designed to deploy a patient’s own B cells as long-lived protein biofactories capable of continuously producing and secreting therapeutic levels of IDS. By leveraging the natural biology of B cells and their ability to engraft in the bone marrow, ISP-002 is intended to enable sustained systemic enzyme exposure following a single treatment. This approach aims to address limitations associated with existing therapies, including treatment burden, fluctuating enzyme levels, and challenges associated with durable tissue penetration.

Immusoft’s proprietary platform modifies autologous B cells ex vivo to produce gene-encoded medicines prior to reinfusion into the patient. Once administered, the engineered B cells are designed to persist for extended periods of time and provide continuous protein expression. The company believes this strategy represents a differentiated therapeutic modality for rare diseases that require long-term or lifelong protein replacement.

The Rare Pediatric Disease designation program is offered by the FDA to encourage the development of new drugs for rare pediatric diseases. Under the program, a sponsor who receives approval for a drug or biologic for a rare pediatric disease for which the designation has been granted may qualify for a Priority Review Voucher at the time of market approval. A Priority Review Voucher can be used to shorten FDA review time from 10 months to six months for any subsequent marketing application.

They are potentially lucrative because they are transferable: Companies can sell or transfer them to other developers. In January 2026, Jazz Pharmaceuticals sold a Priority Review Voucher for $200 million.

“The FDA’s Rare Pediatric Disease designation for ISP-002 recognizes the serious unmet medical need in pediatric patients with MPS II and underscores the potential of Immusoft’s platform as a novel therapeutic approach,” said Sean Ainsworth, CEO of Immusoft. “This designation supports our continued efforts to advance engineered B cell therapies designed to deliver durable protein expression while maintaining a favorable safety and tolerability profile.”

Photo: Sean Ainsworth, CEO of Immusoft