RGX-121 is an investigational one-time gene therapy, designed using the adeno-associated virus 9 (AAV9) vector to deliver the gene that encodes the iduronate-2-sulfatase enzyme to the central nervous system through intracisternal administration. Mucopolysaccharidosis type II is a rare, X-linked recessive lysosomal disease that affects 1 in 100,000 children.

The company plans to launch a prospective natural history study that will increase the understanding of the neurocognitive effects and key biomarkers of the disease. Regenxbio plans to expand enrollment of its phase I/II study to include up to six additional patients in a second cohort who will be dosed at a second level (6.5x10^6 genome copies per gram) of brain mass. The company anticipates further updates on the trial by the end of 2020.

In addition, Regenxbio has been cleared for an investigational new drug application by the U.S. Food and Drug Administration and plans to initiate a second phase I/II multicenter, open-label trial of RGX-121 for the treatment of mucopolysaccharidosis type II in older patients ages 5-18 years old. Up to six patients will be enrolled.