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Regenxbio Plans to File BLA for MPS II Gene Therapy, After Pivotal Trial Met Primary Endpoint

NEW YORK – Regenxbio said on Wednesday that it plans to file a biologics license application with the US Food and Drug Administration this year for RGX-121, its candidate gene therapy for mucopolysaccharidosis type II (MPS II).

Regenxbio's planned application follows the report of favorable results from a pivotal phase trial of RGX-121 in which levels of a biomarker of the disease approached normal after treatment. 

MPS II, also called Hunter syndrome, is a rare, X-linked lysosomal storage disorder caused by a deficiency in the lysosomal enzyme iduronate-2-sulfatase (I2S). This causes glycosaminoglycans (GAGs) to build up within lysosomes, leading to damage that affects physical and neurological development. RGX-121 is an investigational, one-time adeno-associated virus serotype 9-vector gene therapy meant to deliver a functional copy of the I2S gene to the central nervous system.

"We have shared CAMPSIITE results with FDA leadership, and they have confirmed that, based on the totality of the evidence, they are open to accelerated approval, if supported by review of the full data," Kenneth Mills, president and CEO of Regenxbio, said in a statement.

Approximately 48 boys with neuronopathic MPS II between the ages of 4 months and 5 years old are enrolled in the Phase I/II/III open-label CAMPSIITE trial. The trial's primary endpoint is the measurement of heparan sulfate and D2S6, a component of heparan sulfate. Both are GAGs found in cerebrospinal fluid (CSF) and are biomarkers of I2S enzyme activity.

In the trial's pivotal phase, CSF levels of D2S6 fell by a median 86 percent among patients treated with RGX-121 at 16 weeks to approach normal levels. The Rockville, Maryland-based company presented top-line results from CAMPSIITE at the annual WORLDSymposium conference, held in San Diego.

These results were consistent with data from CAMPSIITE's dose-finding phase in which Regenxbio said most patients are exceeding expectations in neurodevelopmental function compared to natural history data up to four years.

Additionally, long-term follow-up of patients treated with RGX-121 in the dose-finding phase showed a high rate of patients for whom trial investigators discontinued standard-of-care intravenous enzyme replacement therapy (ERT) or who remained ERT-naïve. At the pivotal dose level, 80 percent of patients were ERT-free, as of the last time point.

As of Jan. 3 of this year, RGX-121 continued to be well tolerated in 25 patients dosed across all phases of the CAMPSIITE trial.

RGX-121 has received fast-track, orphan drug, regenerative medicine advanced therapy, and rare pediatric disease designations from the FDA, and advanced therapy medicinal products classification from the European Medicines Agency.

Following a meeting with the FDA at the end of last year, Regenxbio said it will continue to use CSF levels of D2S6 as a surrogate endpoint for accelerated approval and intends to file a BLA in the second half of this year. Regenxbio hopes RGX-121 will also be awarded a rare pediatric disease priority review voucher in 2025.

Regenxbio is also testing gene therapies in other diseases, such as Duchenne muscular dystrophy (DMD). Late last year, a DMD patient received the company's investigational gene therapy RGX-202 at the second dose level within a Phase I/II trial.