By Micah Jonah
January 29, 2026
The U.S. Food and Drug Administration (FDA) has placed a clinical hold on two of Regenxbio’s experimental gene therapy programs for rare childhood diseases, sending shockwaves through the biotech market. The company’s shares fell sharply by 30% in premarket trading on Wednesday.
According to Regenxbio, the FDA action followed the discovery of a brain tumor in a five-year-old child who received the experimental therapy RGX-111 four years ago. RGX-111 is being developed to treat Hurler syndrome, a rare genetic disorder affecting one in 100,000 children.
Another therapy, RGX-121, designed as a one-time treatment for Hunter Syndrome (MPS II), was also placed on hold by the FDA. Authorities cited similarities between the two programs and a possible shared risk.
“We are surprised by FDA’s decision to put RGX-121 on hold while investigating this single, inconclusive incident in RGX-111,” said Curran Simpson, CEO of Regenxbio.
The company stressed that no other patients-nine in RGX-111 trials and 32 in RGX-121-have reported tumors. Regenxbio is yet to receive the full FDA clinical hold letter and is awaiting further guidance.
The development is a setback for Regenxbio, which has been pioneering gene therapies for rare childhood diseases. Analysts say the FDA move underscores the high stakes in the race to develop life-saving treatments while maintaining patient safety.
Market Impact: Shares of Regenxbio plunged immediately after the announcement, reflecting investor concern over regulatory hurdles and future program timelines.
The FDA hold serves as a reminder of the delicate balance between innovation and safety in cutting-edge medical research.
