Mahzi Therapeutics Announces FDA Rare Pediatric Disease Designation for MZ-1866 Investigational Therapy for Pitt Hopkins Syndrome

Mahzi Therapeutics announced on August 25, 2026, that the U.S. Food and Drug Administration (FDA) granted Rare Pediatric Disease Designation (RPDD) to MZ-1866, the company’s investigational gene therapy for Pitt Hopkins syndrome. The designation, granted by the FDA’s Office of Orphan Products Development and Office of Pediatric Therapeutics, represents a regulatory milestone for the company’s AAV9-TCF4 gene-replacement program.

MZ-1866 is being developed to address the underlying biology of Pitt Hopkins syndrome, a rare neurogenetic disorder associated with functional loss of the TCF4 gene. The investigational therapy is designed to deliver a functional copy of TCF4, with the aim of addressing the disease biology rather than solely managing symptoms. MZ-1866 previously received FDA Orphan Drug Designation and Fast Track Designation.

Phase 1/2 UNITE Study Advances Clinical Development

MZ-1866 is being evaluated in the Phase 1/2 UNITE study (NCT07135050), an open-label clinical trial evaluating a single administration of the investigational therapy in participants with genetically confirmed Pitt Hopkins syndrome. The study is designed to enroll 12 participants across four sites in the United States and Israel, with MZ-1866 administered through intracerebroventricular delivery. Primary objectives focus on safety, while exploratory assessments include developmental, communication, cognitive and motor-function outcomes.

Seven of the 12 planned participants have been enrolled, representing more than 50% of the planned enrollment. Mahzi expects enrollment to be completed by the end of 2026. The progress reflects the significant unmet medical need associated with Pitt Hopkins syndrome and the scientific rationale supporting TCF4 gene replacement.

The study is focused on evaluating the safety of MZ-1866 while also exploring potential effects across developmental, communication, cognitive and motor-function measures. No clinical efficacy results are reported in the available source material.

Regulatory Incentives and Commercial Development Implications

The FDA’s Rare Pediatric Disease Designation program applies to therapies intended for serious or life-threatening diseases primarily affecting children from birth through 18 years of age. If a qualifying product is subsequently approved and applicable statutory and regulatory requirements are met, the designation may provide an opportunity to receive a Priority Review Voucher (PRV).

Recently disclosed PRV sales have ranged from approximately $150 million to $205 million. Any future PRV eligibility for MZ-1866, however, would remain dependent on subsequent regulatory requirements and product approval. The designation itself does not constitute approval of MZ-1866 or guarantee that the therapy will qualify for a voucher.

Development Milestones and Expected Regulatory Pathway

Mahzi Chief Executive Officer Yael Weiss described the designation as a major milestone for MZ-1866 and highlighted the collaborative efforts supporting the program. Principal Investigator Alex Fay, M.D., PhD, noted that the ongoing study addresses a significant unmet need because families currently have no approved treatment capable of changing the course of Pitt Hopkins syndrome.

The immediate development milestone is completion of enrollment in the Phase 1/2 UNITE study, targeted for the end of 2026. No projected date has been provided for pivotal development, submission of a marketing application or potential FDA approval. The regulatory pathway beyond study enrollment therefore remains dependent on clinical development, safety and efficacy findings, and subsequent FDA review.

References

Mahzi Therapeutics Announces FDA Rare Pediatric Disease Designation for MZ-1866 Investigational Therapy for Pitt Hopkins Syndrome – Mahzi Therapeutics