Regenxbio (RGNX) said Wednesday that the US Food and Drug Administration has placed a clinical hold on its investigational gene therapies RGX-111 and RGX-121 for potential treatment of Hurler syndrome and Hunter syndrome, respectively.

The company said the FDA's decision regarding RGX-111 was made after a preliminary analysis of a case of neoplasm in an asymptomatic participant treated with RGX-111 four years earlier in a phase 1/2 study. An investigation to determine whether the case is drug-related is being conducted, Regenxbio said, adding that causality has not been established.

The FDA's hold on RGX-121 is due to similarities in products, study populations, and shared risk between its clinical studies with RGX-111, the company said.

Regenxbio Chief Executive Curran Simpson said the clinical hold on RGX-121 came as a surprise because RGX-111 and RGX-121 are separate therapies, and RGX-121 has a positive safety profile.

Shares of Regenxbio were down more than 30% in recent premarket activity Wednesday.