Regenxbio shares plunged Monday following a regulatory hold on its investigational gene therapy-the biotechnology company's second program paused by regulators in recent months.

An expanded MRI program initiated a few months ago led to the discovery of an asymptomatic nodule or a small cystic mass on the spine of each of five patients enrolled in the study, Regenxbio said. All five patients had received an injection of RGX-121 roughly three to six years prior.

According to the company, researchers categorized these findings as non-serious and radiologists assessed them as likely benign, with no clinical or pathological data proving a direct cause.

Shares nonetheless plunged 22% to $8.34 in Monday's premarket session. Trading was temporarily halted ahead of the announcement.

The biotech was evaluating RGX-121 in patients with Hunter syndrome, a rare genetic disorder in which the body lacks an enzyme to break down complex sugar molecules. The resulting buildup causes widespread cellular and organ damage. The condition mostly occurs in boys and is life-limiting, with patients typically surviving into their 20s.

The Food and Drug Administration's clinical hold on RGX-121 follows a similar hold placed on another Regenxbio candidate, RGX-111, earlier this year, occurring just weeks before a key approval decision. The hold extended to RGX-121 at the time due to similarities in the therapies.

Following the latest setback, the company said it does not expect to resubmit its Biologics License Application (BLA) for RGX-121 in the near future. A BLA is a formal request to market and distribute a medical product derived from living organisms across state lines.

CEO Curran Simpson emphasized that while the findings appear "unique and limited" to the company's Hunter syndrome program, they still "require longer-term follow-up and additional data analysis."

The company's candidates for Duchenne musuclar dystrophy and an eye disease called we age-related macular degeneration "utilize a different capsid and routes of administration," Simpson said, adding that Regenxbio plans to submit a BLA for its Duchenne candidate in the current quarter.

Other drugmakers targeting these disorders, namely Sarepta Therapeutics and EyePoint, have seen their shares plummet this year in light of recent setbacks.