South San Francisco, California, August 25, 2026
Mahzi Therapeutics announced that the U.S. Food and Drug Administration (FDA) has granted Rare Pediatric Disease Designation (RPDD) to MZ-1866, an investigational gene therapy being developed for Pitt Hopkins syndrome, a rare neurogenetic disorder affecting children. The designation, granted by the FDA’s Office of Orphan Products Development and Office of Pediatric Therapeutics, adds another important regulatory milestone to the MZ-1866 development program, which previously received Orphan Drug Designation and Fast Track Designation from the FDA. At the same time, Mahzi reported that enrollment in the Phase 1/2 UNITE clinical study has surpassed 50%, with 7 of 12 planned participants now enrolled.
MZ-1866 Receives Rare Pediatric Disease Designation
The Rare Pediatric Disease Designation is intended for therapies addressing serious or life-threatening diseases that primarily affect children from birth through 18 years of age. For qualifying products that ultimately receive approval, the designation may provide eligibility for a Priority Review Voucher (PRV), which can be used for a subsequent marketing application or potentially transferred to another sponsor. For Mahzi, the designation further highlights the significant unmet medical need associated with Pitt Hopkins syndrome, for which there are currently no approved disease-modifying treatments. The regulatory milestone comes as the company advances MZ-1866 through clinical development and works toward completing enrollment in its first-in-human study.
Phase 1/2 UNITE Study Reaches 50% Enrollment
Mahzi reported that the Phase 1/2 UNITE study has enrolled 7 of 12 planned participants, with enrollment projected to be completed by the end of 2026. The open-label clinical trial is evaluating a single administration of MZ-1866 in participants with genetically confirmed Pitt Hopkins syndrome across four sites in the United States and Israel. The investigational therapy is administered through an intracerebroventricular route, while the study’s primary objectives focus on safety. Exploratory assessments will examine potential changes in developmental, communication, cognitive and motor functions, providing additional information on the potential clinical effects of the gene therapy. The enrollment milestone represents a key development step for a program targeting the underlying genetic cause of the disorder.
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AAV9-TCF4 Gene Therapy Targets Disease Biology
MZ-1866 is an AAV9-TCF4 gene replacement therapy designed to provide functional copies of the TCF4 gene, which is deficient in Pitt Hopkins syndrome. By targeting the underlying genetic defect rather than focusing only on symptoms, the investigational approach is intended to address the biological basis of the disease. Mahzi developed MZ-1866 in collaboration with the Muotri Lab and licensed the program from the University of California San Diego. The UNITE study has been supported by funding from the California Institute for Regenerative Medicine (CIRM). With RPDD now added to the program’s existing regulatory designations and more than half of planned participants enrolled, Mahzi Therapeutics is advancing MZ-1866 toward completion of Phase 1/2 enrollment and further clinical evaluation in Pitt Hopkins syndrome.
Source: Mahzi Therapeutics press relese
