RegenxBio Reports Positive Results for Rare Disease Drug RGX-121

RegenxBio recently announced promising long-term data from its multiphase study of RGX-121 for treating mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome. According to the 3 September press release, patients who received a pivotal dose level of RGX-121, referred to as dose level 3, exhibited a median reduction of 85% in cerebrospinal fluid (CSF) heparan sulfate D2S6 levels. These levels neared normalcy and were maintained for up to two years. RGX-121 is a gene therapy aimed at delivering a functional copy of the IDS gene to the central nervous system using the NAV AAV9 vector.

The biotech company is preparing to submit a rolling biologics license application (BLA) through the accelerated approval pathway by the third quarter of 2024. RegenxBio plans to use CSF D2S6 as a surrogate marker to predict clinical benefits.

The Phase I/II/III CAMPSIITE study (NCT03566043) is an open-label study divided into two parts. Part 1 served as the Phase I/II single-arm dose escalation study, which evaluated RGX-121 in 16 pediatric participants with neuronopathic MPS II over a span of 24 weeks, primarily to assess safety. Following the initial period, participants continued to be monitored for safety and efficacy for up to 104 weeks.

Part 2 of the study is a single-arm pivotal expansion assessing 30 pediatric patients at different intervals over 24 months. The primary endpoints for this phase included CSF glycosaminoglycan (GAG) biomarker levels, measured by D2S6, and neurodevelopmental outcomes determined by the Bayley Scales of Infant and Toddler Development, 3rd Edition (BSID-III), or Mullen Scales of Early Learning (MSEL).

As of 5 January, the company noted that RGX-121 was well tolerated among the 25 patients who received treatment across all study phases. MPS II is a rare, X-linked recessive disorder characterized by a deficiency in the lysosomal enzyme iduronate-2-sulfatase (IDS). This deficiency leads to an accumulation of large sugar molecules called glycosaminoglycans in body tissues.

Earlier this year, RegenxBio presented data at the 20th Annual WORLDSymposium, revealing that the CAMPSIITE study achieved its primary endpoint. The data were consistent with findings from the dose-finding portion of the study.

“The data continue to support that by restoring the gene missing in boys with Hunter syndrome, RGX-121 changes the course of the disease and has the potential to significantly improve both vital brain function and the systemic manifestations of this devastating disease,” said Dr. Steve Pakola, Chief Medical Officer.

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