Immusoft’s ISP-002 Receives FDA Rare Pediatric Disease Designation for Hunter Syndrome

SAN FRANCISCO —January 26, 2026— Immusoft, a California-based clinical-stage biotechnology company developing engineered B cell therapies, announced that the U.S. Food and Drug Administration (FDA) has granted Rare Pediatric Disease (RPD) designation to ISP-002, its investigational therapy for mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome.

The FDA’s RPD designation is awarded to therapies targeting serious or life-threatening diseases that primarily affect children under 18 years of age and impact fewer than 200,000 patients in the United States. Upon potential approval, the designation makes Immusoft eligible to receive a Priority Review Voucher (PRV), which can accelerate FDA review timelines for a future drug or biologics application and may be transferred or sold, with recent PRV transactions reaching values of up to $200 million.

MPS II is a rare inherited lysosomal storage disorder caused by a deficiency of the enzyme iduronate-2-sulfatase (IDS), leading to the accumulation of glycosaminoglycans throughout the body. The disease predominantly affects pediatric patients and is associated with progressive multisystem complications, including neurocognitive decline, cardiopulmonary disease, skeletal abnormalities, and reduced life expectancy. Current standard-of-care enzyme replacement therapies require lifelong, frequent infusions and may not provide consistent enzyme exposure across all affected tissues.

ISP-002 is designed to use a patient’s own B cells as long-lived protein-producing biofactories capable of continuously secreting therapeutic levels of IDS following a single treatment. By leveraging the natural engraftment and persistence of B cells in the bone marrow, the therapy aims to deliver sustained systemic enzyme exposure while reducing treatment burden and variability associated with existing approaches.

Immusoft’s proprietary Immune System Programming (ISP™) platform involves ex vivo modification of autologous B cells to express gene-encoded medicines prior to reinfusion. The engineered cells are intended to persist long term, providing continuous protein expression for diseases requiring durable or lifelong replacement therapy.

“The FDA’s Rare Pediatric Disease designation highlights the significant unmet need in children with MPS II and recognizes the potential of our engineered B cell platform,” said Sean Ainsworth, Chief Executive Officer of Immusoft. “This milestone supports our efforts to advance therapies designed to deliver sustained protein expression with a favorable safety and tolerability profile.”

Preclinical development of Immusoft’s engineered B cell platform has been supported in part by funding from the California Institute for Regenerative Medicine (CIRM). Lisa Kadyk, PhD, CIRM Fellow for Clinical Development, noted that the designation could help accelerate development of Immusoft’s programs for both MPS I and II, aligning with CIRM’s mission to support patients with limited treatment options.

ISP-002 expands Immusoft’s growing pipeline of engineered B cell therapies, which also includes ISP-001 for mucopolysaccharidosis type I (MPS I). ISP-001 has previously received FDA RPD designation and has demonstrated a favorable safety and tolerability profile with early pharmacodynamic activity in clinical evaluation, including successful re-dosing.

Collectively, these programs reflect Immusoft’s strategy to establish engineered B cells as a next-generation therapeutic modality for rare genetic and metabolic diseases requiring sustained systemic protein delivery, as the company continues to advance clinical development of its ISP™ platform.