FDA Grants Rare Pediatric Disease Designation to Mahzi's AAV9 Gene Therapy MZ-1866 for Pitt Hopkins Syndrome

The FDA has granted Rare Pediatric Disease Designation (RPDD) to MZ-1866, Mahzi Therapeutics' investigational gene therapy for Pitt Hopkins syndrome, awarded by the agency's Office of Orphan Products Development and Office of Pediatric Therapeutics.¹ The designation was announced as Mahzi's Phase 1/2 UNITE study surpassed 50% enrollment.¹

What is MZ-1866, and how does it work?

MZ-1866 is an AAV9-TCF4 gene replacement therapy designed to address the underlying biology of Pitt Hopkins syndrome by delivering functional copies of the TCF4 gene. The disorder is caused by haploinsufficiency of TCF4, a transcription factor gene essential to nervous system development; loss of a functional copy leads to severe developmental delay, absent or minimal speech, and characteristic breathing and motor abnormalities.² Prevalence estimates for Pitt Hopkins syndrome vary substantially across sources, ranging from roughly 1 in 11,000 to less than 1 in 200,000, reflecting how recently molecular diagnosis has become available and likely ongoing underdiagnosis.²

MZ-1866's use of an AAV9 vector places it within a broader class of gene therapies that rely on AAV for its safety and durability profile. As Emmanuelle Cameau, strategic technology partnership leader for genomic medicine at Cytiva, explained in earlier BioPharm International© coverage of viral vector platforms generally, "AAV is non-pathogenic to humans, does not cause disease, and has a low immunogenicity, which are crucial factors for the safety of gene therapy patients," adding that "AAV can mediate long-term expression of the therapeutic gene, which is an important factor when treating chronic conditions that require sustained gene expression."³ Cameau's comments describe AAV vector biology broadly rather than MZ-1866 specifically; Mahzi's program was not the subject of that interview. MZ-1866 was developed in collaboration with the Muotri Lab and licensed from the University of California San Diego, with phase 1/2 funding from the California Institute for Regenerative Medicine.¹

What does Rare Pediatric Disease Designation mean for this program?

RPDD applies to therapies for serious or life-threatening diseases primarily affecting patients from birth to 18 years old.¹ Upon approval of a qualifying marketing application, RPDD-designated drugs may become eligible for a Priority Review Voucher (PRV), which sponsors can use to expedite review of a future application or sell to another company; recently disclosed PRV sales have ranged from $150 million to $205 million.¹ This mirrors a pathway BioPharm International has covered before in a comparable rare pediatric gene therapy program — VeonGen's Stargardt disease candidate VG801 received RPDD in 2025 en route to a subsequent RMAT designation, illustrating how these designations often accumulate as programs advance.³ MZ-1866 previously received Orphan Drug and Fast Track designations from the FDA.¹

"Receiving Rare Pediatric Disease Designation is a major milestone for the MZ-1866 program and an important recognition of the critical unmet need for Pitt Hopkins patients," said Yael Weiss, MD, PhD, chief executive officer of Mahzi. "We are grateful for the continuing support of the Pitt Hopkins community and the Pitt Hopkins Research Foundation in advancing the MZ-1866 program and the Phase 1/2 UNITE study."¹

What does the Phase 1/2 UNITE study involve?

UNITE is an open-label study evaluating a single dose of MZ-1866 delivered via intracerebroventricular administration in patients with genetically confirmed Pitt Hopkins syndrome.¹ The trial is designed to enroll 12 participants across four sites in the US and Israel, with primary safety objectives alongside exploratory endpoints assessing developmental, communication, cognitive, and motor function.¹ As of this announcement, 7 of the 12 planned participants have enrolled, with completion projected by the end of 2026.¹

"For families living with Pitt Hopkins syndrome, the absence of any approved treatment has meant managing symptoms with no way to address the underlying cause or to change the course of this disease," said Alex Fay, MD, PhD, principal investigator for the phase 1/2 study at UCSF Benioff Children's Hospitals. "The MZ-1866 Phase 1/2 trial has now enrolled more than 50% of the planned participants. Meeting this important milestone ahead of schedule reflects both the urgency families feel and the strength of the scientific rationale behind gene replacement for TCF4 deficiency."¹

What's next?

Enrollment in the UNITE study is expected to complete by the end of 2026, ahead of any potential efficacy readout on the trial's exploratory developmental and functional endpoints.¹ There is currently no approved treatment that addresses the underlying genetic cause of Pitt Hopkins syndrome.¹

References

1. Mahzi Therapeutics announces FDA Rare Pediatric Disease Designation for MZ-1866 investigational therapy for Pitt Hopkins syndrome. News release. Mahzi Therapeutics; August 25, 2026. Accessed August 25, 2026. https://www.prnewswire.com
2. About Pitt Hopkins. Pitt Hopkins Research Foundation. Accessed August 25, 2026. https://pitthopkins.org/about-pitt-hopkins/
3. VeonGen's Stargardt disease gene therapy gets FDA RMAT status. BioPharm International. Published December 15, 2025. Accessed August 25, 2026. https://www.biopharminternational.com/view/veongen-stargardt-disease-gene-therapy-gets-fda-rmat-status