The primary objectives of this study are to evaluate the long-term safety and tolerability of AX 250 administered to subjects with MPS IIIB by an implanted ICV reservoir and catheter and to evaluate the impact of long-term AX 250 treatment on cognitive function in patients with MPS IIIB as assessed by developmental quotient (DQ).

Inclusion Criteria:

• Must have completed 48 weeks in Part 2 of Study 250-201 and enter 250-202 within 8 weeks of study completion
• Written informed consent from parent or legal guardian and assent from subject, if required
• Has the ability to comply with protocol requirements, in the opinion of the investigator
• Males and females who are of reproductive age should practice true abstinence, defined as no sexual activity, during the study and for 6 months after the study has been completed (or withdrawal from the study). If sexually active and not practicing true abstinence, males and females of reproductive age must use a highly effective method of contraception while participating in the study.
• If female with childbearing potential, must have a negative pregnancy test at the Screening visit and be willing to have additional pregnancy tests during the study.

Exclusion Criteria:

• Has both (1) a cognitive AEq score ≤ 18 months, and (2) a DQ score ≤ 20
• Would not benefit from enrolling in the study in the opinion of the investigator
• Has received stem cell, gene therapy or ERT (other than AX 250) for MPS IIIB
• Has contraindications for neurosurgery (eg, congenital heart disease, severe respiratory impairment, or clotting abnormalities)
• Has contraindications for MRI scans (eg, cardiac pacemaker, metal fragment or chip in the eye, or aneurysm clip in the brain)
• Has a history of poorly controlled seizure disorder
• Is prone to complications from intraventricular drug administration, including patients with hydrocephalus or ventricular shunts
• Has received any investigational medication other than AX 250 within 30 days prior to the Baseline visit or is scheduled to receive any investigational drug during the course of the study
• Has a medical condition or extenuating circumstance that, in the opinion of the investigator, might compromise the subject's ability to comply with protocol requirements, the subject's well-being or safety, or the interpretability of the subject's clinical data.
• Is pregnant at any time during the study