RGX-121 Gene Therapy in Patients With MPS II (Hunter Syndrome)

Open to males ages 4 months to 5 years

• Must meet any of the following criteria:
• 1. Has a documented diagnosis of MPS II AND has a neurocognitive testing score ≤ 77 (Bayley or Kaufman), OR
• 1. Has a documented diagnosis of MPS II AND has a decline of ≥ 1 standard deviation on serial neurocognitive testing administered between 3 to 36 months apart (Bayley or Kaufman) OR
• 1. Has a relative clinically diagnosed with severe MPS II who has the same IDS mutation as the subject AND in the opinion of a geneticist has inherited a severe form of MPS II OR
• 1. Has documented mutation(s) in IDS that in the opinion of a geneticist is always known to result in a neuronopathic phenotype AND in the opinion of a clinician has a severe form of MPS II
• Patient's legal guardian must be willing and able to provide written, signed informed consent.
• Is ≥4 months to <5 years of age.

• Has contraindications for intracisternal injection, intracerebroventricular injection, or lumbar puncture
• Has contraindications for immunosuppressive therapy
• Has neurocognitive deficit not attributable to MPS II or diagnosis of a neuropsychiatric condition
• Has a (cerebral) ventricular shunt that may impact the proper dosing of the subject
• Received hematopoietic stem cell transplantation
• Has had prior treatment with an AAV-based gene therapy product
• Received ELAPRASE® via intrathecal (IT) administration within 4 months of signing the ICF or experienced a serious hypersensitivity reaction to ELAPRASE®
• Has received any investigational product within 30 days of Day 1 or 5 half-lives before signing the ICF, whichever is longer
• Has a platelet count <100,000 per microliter (µL), absolute neutrophil count <1.3 × 103/µL, or aminotransferase (ALT) or aspartate aminotransferase (AST) >3 × upper limit of normal (ULN) or total bilirubin >1.5 × ULN at screening unless the subject has a previously known history of Gilbert's syndrome