NatalizumabA Pilot Study to Characterize the Biological Effect of a Pre-planned 12 Week Dose Interruption of Natalizumab

Inclusion Criteria:

To be eligible for entry into this study, candidates must meet all of the following eligibility criteria at the time of randomization:

Screening Visit:

1. Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations.
2. At least 18 years old at the time of informed consent.
3. Must be a patient with a relapsing form of Multiple Sclerosis enrolled in the TOUCH Prescribing Program who is not expected to discontinue Tysabri® therapy prior to completion of the requirements of this study.
4. Must weigh between 50 and 110 kg, inclusive.
5. Patients must be considered clinically stable and scheduled for their pre-planned annual dose interruption of 2 consecutive skipped doses.
6. No evidence of disease activity with on standard (28-day interval) dosing of natalizumab.

Exclusion Criteria:

Candidates will be excluded from study entry if any of the following exclusion criteria exist at the time of screening:

Medical History

1. If subject answers 'Yes" to any question on the PML questionnaire that is not resolved prior to infusion as per standard operating procedure for natalizumab infusion.
2. If subject consumes alcohol within 24 hours of blood specimen collection.