The pharmacokinetics of a single I.V dose of natalizumab were studied in a phase 1, dose-escalation study in 28 multiple sclerosis (MS) patients. As part of the clinical trial program to establish the pharmacokinetic/pharmacodynamic and safety profiles of natalizumab, four Phase 1, dose-finding studies were conducted. An additional Phase 2 study was conducted to evaluate the safety and efficacy of natalizumab in combination with glatiramer acetate (GA). One of the recommendations of the therapeutics and technology assessment subcommittee of the AAN about the use of natalizumab was to carefully monitor patients receiving natalizumab to establish its long-term safety i.e. the true risk of progressive multifocal leukoencephalopathy (PML). Large Phase 3 trials of natalizumab alone or in combination with interferon (IFN)β-1a show that natalizumab reduces the progression of disability, and dramatically reduces the frequency of relapses and magnetic resonance imaging (MRI) lesion formation in patients with relapsing MS.