Cellectar Biosciences (CLRB) said Monday that the US Food and Drug Administration has granted its investigational drug iopofosine I 131 rare pediatric drug designation as a potential treatment for inoperable relapsed or refractory pediatric high-grade glioma.

Pediatric high-grade gliomas are a collection of aggressive brain and central nervous system tumors.

A phase 1b study of the radioactive isotope treatment showed meaningful improvements in progression-free and overall survival, the company said.

The study showed that six patients who received at least a 55 millicurie dose of the treatment experienced an average of 5.4 months of progression-free survival and 8.6 months of overall survival, the company said.

Three patients who received additional dosing had an average progression-free survival of 8.1 months and overall survival of 11.5 months, the company added.

The treatment was well-tolerated, and its toxicity profile was consistent with the company's previously reported safety data, Cellectar said.

Shares of the company were down more than 4% in recent premarket activity Monday.