The FDA's clinical hold on Navsunli (clemidsogene lanparvovec-sngl [RGX-121]), an investigational adeno-associated virus (AAV) gene therapy for Mucopolysaccharidosis type II (MPS II), has forced Maryland-based RegenxBio Inc. (Nasdaq: RGNX) to delay its planned Biologics License Application (BLA) resubmission indefinitely, the company said on August 24, 2026. The hold was triggered by asymptomatic spine MRI findings in five participants from the CAMPSIITE study who received intracisternal or intraventricular Navsunli approximately three to six years ago.
RegenxBio identified small nodules or cystic masses on spine MRIs in the five participants through its expanded MRI monitoring plan. No equivalent findings appeared on brain MRIs, all five patients remain clinically asymptomatic, and investigators rated the findings as nonserious, with radiologists characterizing them as likely benign. RegenxBio said there is no clinical or pathological evidence to confirm the nature or causation of the findings. Because spine MRI is not standard practice in MPS II clinical care or trials, the background prevalence of such findings in this patient population is unknown.
The FDA had issued a Complete Response Letter (CRL) for the original Navsunli BLA in February 2026, citing concerns about study eligibility criteria, the comparability of the natural history external control, and the suitability of cerebrospinal fluid heparan sulfate D2S6 as a surrogate endpoint. RegenxBio subsequently appealed the CRL and in June 2026 announced alignment with the FDA on a path forward under the accelerated approval pathway, with no additional studies required and a BLA resubmission targeted for Q3 2026. That timeline is now suspended pending longer-term follow-up and additional data analysis, the company said.
The spine MRI monitoring program that identified the findings was itself implemented in response to an earlier clinical hold related to RGX-111, RegenxBio's investigational gene therapy for MPS I. That expanded monitoring protocol — covering both brain and spine MRI — was the mechanism through which the Navsunli findings came to light, the company said.