Guess the Therapy Answer January 14, 2026

Which investigational gene therapy candidate for MPS II (Hunter syndrome) has an upcoming PDUFA date for February 10, 2026?

Answer: clemidsogene lanparvovec (RGX-121)

See below for further reading on RGX-121:

Patients Treated With REGENXBIO's MPS II Gene Therapy RGX-121 Sustain 82% Median Reduction in CSF HS D2S6 Levels Through 1 Year

According to a new data update from REGENXBIO's pivotal phase 1/2/3 CAMPSIITE clinical trial (NCT03566043) evaluating clemidsogene lanparvovec (RGX-121), an investigational adeno-associated virus (AAV) vector-based gene therapy for mucopolysaccharidosis type 2 (MPSII, also known as Hunter syndrome), 13 patients treated in the pivotal phase have shown an 82% reduction in heparan sulfate (HS) D2S6 levels in the cerebrospinal fluid (CSF) through 1 year posttreatment. The company originally reported the updated data at the International Congress of Inborn Errors of Metabolism (ICIEM), held in Kyoto, Japan, from September 2 to 6, 2025, and subsequently summarized key points in a press release.

According to the press release, HS D2S6 is “a key biomarker of MPS II brain disease that is reasonably likely to predict clinical benefit.” The company also noted that the updated data is in line with earlier findings: the study previously met its primary end point for the proportion of treated patients showing CSF HS D2S6 under maximum attenuated levels at 16 weeks posttreatment (P < .0001). In addition, REGENXBIO pointed out that in the dose-finding part of the trial, an 85% decrease in CSF HS D2S6 was maintained through 2 years posttreatment for patients who received the pivotal dose. Furthermore, in the pivotal and dose-finding parts of CAMPSIITE, positive neurodevelopmental outcomes were seen, with neurodevelopmental skill acquisition or stability on all sub-scales of the Bayley Scales of Infant and Toddler Development, 3rd Edition (BSID-III) being seen in patients treated in the pivotal portion at 1 year posttreatment. REGENXBIO states that strong correlation is seen between levels of CSF HS D2S6 at 16 weeks posttreatment and neurocognitive outcomes at 1 year posttreatment, according to new data from both the dose-finding and pivotal parts of the trial, thus supporting the utilization of CSF HS D2S6 as a surrogate end point for the accelerated approval pathway that the company is pursuing for the gene therapy product.

FDA Bumps Back PDUFA for REGENXBIO's MPS II Gene Therapy RGX-121

The FDA has extended the review timeline for REGENXBIO's biologics license application (BLA) for clemidsogene lanparvovec (RGX-121), an investigational adeno-associated virus (AAV) vector-based gene therapy intended to treat mucopolysaccharidosis type 2 (MPSII, also known as Hunter syndrome), setting a new Prescription Drug User Fee Act (PDUFA) goal date of February 8, 2026.

The new PDUFA date is several months later than the original PDUFA date of November 9, 2025. REGENXBIO noted that the extended timeline came after the company responded to an FDA information request by submitting longer-term clinical data for all 13 patients treated in the pivotal phase 1/2/3 CAMPSIITE clinical trial (NCT03566043). REGENXBIO pointed out that the updated data are consistent with previously submitted biomarker and neurodevelopmental findings. The new data will be presented at the International Congress of Inborn Errors of Metabolism (ICIEM), which will be held in Kyoto, Japan, from September 2 to 6, 2025.