A Study of ELAPRASE in Treatment-naïve Participants With Hunter Syndrome (Mucopolysaccharidosis [MPS] II)

The main aim of this study is to evaluate the ability of a prophylactic immune tolerizing regimen (ITR) to prevent or reduce the development of high titer anti-idursulfase antibodies in treatment-naïve participants with Hunter syndrome.

In this open label, single arm study, all participants will receive ELAPRASE treatment and a prophylactic ITR.

Participants will be treated with ELAPRASE for up to 104 weeks. The prophylactic ITR will start 1 day prior to the start of ELAPRASE. The prophylactic ITR will consist of a 5-week cycle of: Rituximab (intravenously \[IV\], weekly for 4 weeks); Methotrexate (oral, 3 times per week for 5 weeks) and intravenous immunoglobulin (IVIG) (IV, every 4 weeks of the cycle).

Following the completion of 1 cycle, an assessment will be made at Month 6, 12, and 18 regarding the need for administering another 5-week cycle of the ITR.

Participants will be in the study for approximately 112 weeks (including 6 weeks for screening, up to 104 weeks for treatment, and 2 weeks for follow-up).


Eligibility Criteria:
Inclusion Criteria:

* Participant is male.
* Participant is ELAPRASE-naïve at study entry.
* Participant must have a documented diagnosis of MPS II. The following combination will be accepted as diagnostic of MPS II:

  * Participant has a deficiency in iduronate-2-sulfatase (I2S) enzyme activity of less than or equal to (\<=) 10 percent (%) of the lower limit of the normal range as measured in plasma, fibroblasts, or leukocytes (based on the reference laboratory's normal range). The participant has a normal enzyme activity level of at least 1 other sulfatase as measured in plasma, fibroblasts, or leukocytes (based on the reference laboratory's normal range).
  * Participant has a documented mutation in the IDS gene; additionally, participants must have a severe mutation (example, large deletion or complex gene rearrangement), which is predicted to lead to development of a persistent anti-idursulfase antibody response.
* Participant will be less than (\<) 6 years of age at enrollment.
* Participant has a negative test result for serum anti-idursulfase antibodies.

Exclusion Criteria:

* Participant has received treatment with any investigational drug within the 30 days prior to study entry.
* Participant has received or is receiving treatment with idursulfase-IT.
* Participant has received growth hormones, a cord blood infusion, or a bone marrow transplant at any time.
* Participant has received blood product transfusions within 90 days prior to screening.
* Participant is unable to comply with the protocol as determined by the investigator.
* Participant has known or suspected intolerance or hypersensitivity to the investigational product(s), closely related compounds, or any of the stated ingredients, including the prophylactic ITR.
* Participant has current or recurrent disease that could affect the action, absorption, or disposition of the investigational product, or clinical or laboratory assessments.
* Participant has current or relevant history of physical or psychiatric illness, or any medical disorder that may require treatment or make the participant unlikely to fully complete the study, or any condition that presents undue risk from the investigational product or procedures.
* Participant has current use of any medication (including over-the-counter, herbal, or homeopathic preparations) that could affect (improve or worsen) the condition being studied, or could affect the action, absorption, or disposition of the investigational product(s), or clinical or laboratory assessment (Current use is defined as use within 30 days).
* Within 30 days prior to the first dose of investigational product, the participant has been enrolled in a clinical study (including vaccine studies) that, in the investigator's opinion, may impact this study.