REGENXBIO said the FDA placed a clinical hold on its investigational gene therapy RGX-121, also known as clemidsogene lanparvovec or Navsunli, after expanded spinal MRI monitoring identified asymptomatic abnormalities in five CAMPSIITE study participants. The patients received the treatment about three to six years ago, and investigators classified the findings as nonserious while radiologists considered them likely benign. No clinical or pathological evidence has established their nature or cause, and no brain nodules or masses were found. All five participants remain clinically well, with neurocognitive and neurobehavioral assessments showing overall stability to improvement. REGENXBIO does not expect to resubmit the RGX-121 Biologics License Application in the near term. The company and partner NS Pharma are reviewing further imaging and longer-term follow-up data and will incorporate FDA feedback, including the full clinical hold letter. Shares fell 23.79% to $8.170 in premarket trading on Monday, according to Benzinga Pro data. RGX-121 is a one-time gene therapy designed to deliver the iduronate-2-sulfatase gene to the central nervous system for MPS II, or Hunter Syndrome.