The FDA has issued a complete response ... for the treatment of mucopolysaccharidosis type 2 (MPS II), also known as Hunter syndrome.¹ · REGENXBIO reported that it received the CRL on February 7, 2026. In the letter, the FDA outlined multiple reasons for not approving the gene therapy at this time, despite noting general agreement with the study protocol. These included concerns about whether the clinical trial eligibility criteria adequately differentiate neuronopathic from attenuated disease, whether the ... The FDA has issued a complete response letter (CRL) to REGENXBIO’s biologics license application (BLA) for clemidsogene lanparvovec (RGX-121), an investigational adeno-associated virus (AAV) vector–based gene therapy being developed for the treatment of mucopolysaccharidosis type 2 (MPS II), also known as Hunter syndrome.¹ · REGENXBIO reported that it received the CRL on February 7, 2026. In the letter, the FDA outlined multiple reasons for not approving the gene therapy at this time, despite noting general agreement with the study protocol. These included concerns about whether the clin FDA objections centered on study population misclassification risk, external control validity, and surrogate end point evidentiary strength. The CRL also described several potential paths toward approval, including conducting a new clinical trial, dosing additional patients with longer follow-up, and incorporating an on-study untreated control group. REGENXBIO noted, however, that these approaches present challenges given the ultrarare nature of MPS II. "This decision is devastating for the families of boys living with this progressive, life-threatening disease," Curran M. Simpson, the president and chief executive officer of REGENXBIO, said in a statement.1 "We are concerned about FDA's feedback regarding the overall development path and evaluation of the data in the context of the urgent need for this irreversible ultra-rare disease. We remain confident in the quality and volume of evidence demonstrating the long-term potential of RGX-121 to positively change the trajectory of Hunter syndrome. This program has been in development for over 10 years. We are incredibly grateful to all the patients, their families, investigators, and site staff who have supported this program and our continued efforts to bring a much-needed new treatment option to the Hunter syndrome community.

Original title: “FDA Slams REGENXBIO's MPS II Gene Therapy RGX-121 With CRL | CGTlive®”

In the middle of 2026, the company is planning to submit a BLA for RGX-202 via an accelerated approval pathway. Among 4 patients treated with the pivotal dose of Regenxbio’s RGX-202, an investigational adeno-associated virus (AAV) vector–based gene therapy intended to treat Duchenne muscular dystrophy (DMD), in the phase 1/2 portion of the AFFINITY DUCHENNE clinical trial (NCT05693142), all 4 surpassed expected disease trajectory on the North Star Ambulatory Assessment (NSAA) with utilization of the Collaborative Trajectory Analysis Project (cTAP) disease progression model at 18 months post treatment.1 RGX-202 gene therapy for DMD showed significant improvement in patient outcomes, surpassing expected disease progression at 12 and 18 months post treatment. In addition to RGX-202, one of Regenxbio’s other lead programs is clemidsogene lanparvovec (RGX-121), an investigational AAV vector–based gene therapy for mucopolysaccharidosis type 2 (MPS II, also known as Hunter syndrome).2 RGX-121 is currently being evaluated in the pivotal phase 1/2/3 CAMPSIITE clinical trial (NCT03566043). "2026 is set to be a transformative year for Regenxbio, as we enter commercial stage with two near-term catalysts from our three late-stage assets and a clear path to sustained growth," Curran Simpson, president and CEO of Regenxbio, said in a statement.1 "We are starting the year with exciting new long-term data for our Duchenne program, demonstrating how our comprehensive strategy to maximize the potential for therapeutic benefit across all our programs is resulting in positive outcomes for patients. We are continuing to set the bar high for how potentially life-changing gene therapies are discovered, developed, and manufactured; this year we are sharply focused on advancing our commercial readiness to enable successful launches of these medicines for patients in need.”

Original title: “Patients Treated With Regenxbio’s DMD Gene Therapy RGX-202 Exceed Expected Disease Trajectory on NSAA | NeurologyLive - Clinical Neurology News and Neurology Expert Insights”