This is a global, open-label, Phase I/II, interventional trial in participants younger than 6 years of age with Mucopolysaccharidosis Type IIIB (MPS IIIB), designed to assess the safety and tolerability of JR-446, determine its pharmacodynamic effects, and explore its potential to demonstrate early clinical effects on disease-relevant outcomes.

Inclusion Criteria:

1. Patients with confirmed MPS IIIB with a phenotype of MPS IIIB that is not classified as slowly progressive (i.e., not attenuated).
2. A chronological age <6 years.
3. Ability to complete and achieve a Standard score ≥55, using either Bailey Scale of Infant Development-4 (BSID-4) Cognitive domain or Kaufman Assessment Battery for Children-II (KABC-II) Non-Verbal Index, whichever tool is most appropriate for the participant's chronological age.

Exclusion Criteria:

1. Contraindication for lumbar puncture or MRI.
2. A participant who has a medical condition or extenuating circumstance that, in the opinion of the principal investigator or sub-investigator, might compromise the participant's ability to comply with protocol requirements, the participant's well-being or safety, or the interpretability of the participant's clinical data.
3. A participant who has received any other investigational drug product (including but not limited to, tralesinidase alfa enzyme replacement therapy [TA-ERT], Genistein, KINERET [anakinra], ambroxol, miglustat) within 4 months (or 5 half-lives, whichever is longer) before the time of providing informed consent.
4. A participant who has received gene therapy treatment or hematopoietic stem cell transplantation (HSCT) with successful engraftment.
5. Serious drug allergy or hypersensitivity to any components of JR-446 or medications likely prescribed during the trial.
6. A participant has a history of bleeding disorder or current use of medications that, in the opinion of the investigator, place them at risk of bleeding following lumbar puncture.
7. A patient with recurrent epileptic seizures not adequately controlled with anti-seizure medication, and which, in the clinical judgment of the principal investigator, would preclude safe participation in the trial.
8. Serology consistent with human immunodeficiency virus (HIV) exposure or consistent with active hepatitis B (HepB) or hepatitis C (HepC) infection.
9. A participant/family, who, in the opinion of the investigator, may not be able to comply with protocol requirements and cooperate fully with the trial assessments, procedures, and scheduling for JR-446 IMP dose administrations.
10. Study participants for whom informed consent is unable to be provided by a parent or legal guardian; or when applicable for a study participant who is unable to provide assent with respect to study participation in conjunction with parental or legal guardian consent for participation on study.

The above information is not intended to contain all considerations relevant to a patient's potential participation in this clinical trial.