A study to evaluate the efficacy and safety of treatment with JR-141

You may be able to join if

• A patient who voluntarily signs an IRB or Independent Ethics Committee (IEC)-approved written ICF. If the patient is aged under 18 years (aged under 16 years in the UK) at the time of enrollment or willingness to participate in the study cannot be confirmed due to MPS II-related intellectual disability, the patient's legally acceptable representative (e.g., his or her parents or guardians) may sign the informed consent on behalf of the patient. Written informed assent should be obtained from the patient, wherever possible.
• Patients with confirmed diagnosis of MPS II, based on all of the following criteria: 1) Deficient activity of IDS in leucocytes, plasma or fibroblasts defined by 10% or less of the lower limit of the measuring laboratory normal range unless the hospital or laboratory has established different criteria 2) Documented mutation identified in the IDS gene 3) Increased levels of urinary glycosaminoglycans (GAGs) (or uronic acid) or clinical symptoms and signs consistent with MPS II (such as dysostosis multiplex, coarse facies, cardiac valve disease, developmental delay, chronic pulmonary disease, hernias, kyphosis, joint contractures, carpal tunnel syndrome, etc)
• Naïve patients or patients who are receiving stable enzyme replacement therapy (ERT) with idursulfase for more than 12 weeks before starting administration of JR-141 or idursulfase for this study.
• Cohort A - Patients aged 36-42 months of age at the time of ICF signing: patients must have a standard score on the cognitive domain measured by the BSID-III of 85 or less at screening - Patients aged 43-71 months old at the time of ICF signing: patients must EITHER have 1. A development quotient (DQ) on the cognitive domain measured by the BSID-III between 20 and 85 at screening 2. A composite standard score on Nonverbal Index (NVI) measured by the KABC-II of 85 or less at screening for only who are able to perform the KABC-II - Patients aged 30-35 months of age at the time of randomization and who are judged as having the severe phenotype by the Expert Board based on presence of one of the following mutations in the IDS gene and other information such as high CSF HS concentrations: 1) Large deletion or rearrangement 2) Small insertions or deletions that are out of frame 3) Missense mutations, nonsense mutations, in frame inserts or deletions that involve neuronopathic disease in other documented neuronopathic or other cases within the patient's family.
• Cohort B - Patients 6 years of age or older at the time of ICF signing. - Intelligence quotient (IQ) measured by the Wechsler test (WISC-V, or WAIS-IV) is 70 or higher at screening. - Enrollment of subjects in Cohort B is contingent on the availability in that country of a validated country-specific version of the test (either WISC-V, WIAS-IV, or T.O.V.A.). - Patients with 1SD deficiency in the omission errors or variability domains of the T.O.V.A. test or Processing Speed or Working memory on the Wechsler tests at screening
• Patients or patients whose female partners are of child-bearing potential i.e., fertile, following menarche and until becoming post-menopausal unless permanently sterile, agree to use a medically accepted, highly effective method of contraception as described in Section 10.4 of protocol, from the time of informed consent. The method of contraception must be used during the study until 90 days for male subjects, and 30 days for female subjects after the final study intervention administration.