The study's primary objectives are to evaluate the safety and tolerability of AX 250 administered to subjects with MPS IIIB via an ICV reservoir and catheter and to evaluate the impact of AX 250 on cognitive function in patients with MPS IIIB as assessed by the Development Quotient.

Inclusion Criteria:

Individuals eligible to participate in Part 1 of this study must meet all of the following criteria:

• Has deficient NAGLU enzyme activity at Screening. Blood for NAGLU enzyme activity will be collected and analyzed centrally.
• Is ≥ 1 and < 11 years of age (at least 1 of the 3 subjects in Part 1 must be ≥ 1 and < 6 years of age)
• Has presented with signs/symptoms consistent with MPS IIIB; for individuals who have not presented with signs/symptoms of disease (eg, siblings of known patients), the determination of eligibility will be at the discretion of the BioMarin medical monitor in conjunction with the site investigator.
• 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

Individuals eligible to participate in Part 2 of this study must meet all of the following criteria:

• Participated in and met protocol requirements for transitioning from Study 250-901 or participated in Part 1 of Study 250-201
• Written informed consent from parent or legal guardian and assent from subject, if required

Exclusion Criteria:

Individuals who meet any of the following exclusion criteria are ineligible to participate in Part 1 of the study:

• Has received stem cell, gene therapy or ERT 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 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

Individuals who meet any of the following exclusion criteria are ineligible to participate in Part 2 of this study:

• Has received stem cell, gene therapy or ERT 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)
• Is prone to complications from intraventricular drug administration, including patients with hydrocephalus or ventricular shunts
• Has received any investigational medication 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