FDA Clinical Hold Derails REGENXBIO's RGX-121 Hunter Syndrome Resubmission

Update August 27, 2026: RGNX closed at $9.37 on August 27 (Polygon), up 16% from the $8.06 August 24 close, as the stock partially recovered from the clinical-hold selloff. The RGX-121 program remains on hold with no resubmission planned.

REGENXBIO (Nasdaq: RGNX) said the FDA placed a clinical hold on RGX-121, its Hunter syndrome gene therapy, after asymptomatic MRI findings showed small nodules or cystic masses in the spines of five of 48 participants in the CAMPSIITE study. The company no longer expects to resubmit the biologics license application in the near term, a filing it had planned for the third quarter. RGNX closed at $8.06 on Monday, August 24, down about 25% from Friday’s close of $10.72.

The hold caps a rough regulatory run for RGX-121. The FDA rejected the therapy in February with a complete response letter recommending a new study with more patients and a placebo arm, then reversed course in June, telling REGENXBIO the existing data were sufficient to support a resubmission under the accelerated approval pathway. Monday’s hold puts that path back in doubt.

REGENXBIO flagged the spine findings through an expanded MRI monitoring plan it adopted after RGX-111, a related Hurler syndrome gene therapy, was placed on hold in January when a tumor revealed AAV vector integration. The company said all five RGX-121 patients remain clinically stable, investigators deemed the findings likely benign, and there is no clinical or pathological evidence confirming their nature or cause. CEO Curran Simpson said he believes the findings are unique to the Hunter program and need longer-term follow-up.

Why It Matters

Hunter syndrome (MPS II) is an ultra-rare, progressive lysosomal storage disorder, and RGX-121 was REGENXBIO’s lead gene therapy candidate. The hold removes a near-term resubmission catalyst and revives the AAV safety questions that have followed the field since the RGX-111 tumor finding. With no resubmission planned, the program’s path to approval is now open-ended.

What Happens Next

REGENXBIO will gather longer-term imaging and clinical data while investors shift attention to suravec, its wet AMD gene therapy in Phase 3. The stock’s drop reflects the loss of a near-term catalyst. The next signal is whether the FDA lays out a path forward for RGX-121 or the program is formally set aside.