Tysabri® (natalizumab)

Parent companies: Biogen Idec and Elan Pharmaceuticals, Inc.

Administered via intravenous infusion every four weeks. Dose is 300 mg.

This laboratory-produced monoclonal antibody acts against a molecule that is involved in the activation and function of lymphocytes and their migration into the central nervous system.

It was approved for the relapsing forms of MS based on a 68-percent reduction in relapses compared to placebo (AFFIRM trial) and a 54-percent reduction in relapses for those taking Tysabri plus Avonex, versus Avonex alone (SENTINEL trial). Disability and MRI lesions were also less with Tysabri.

Following a suspension of the drug after two patients developed Progressive Multifocal Leukoencephalopathy (PML), an often-fatal viral infection of the brain, Tysabri was re-released. All patients now receive the drug through safety monitoring programs such as the TOUCH Prescribing Program and registered infusion centers and pharmacies, and the international Tysabri Global Observation Program in Safety (TYGRUS). Nearly 31,800 patients are on the drug worldwide, and there have been no new reports of PML in 18 months.

In early 2008, the FDA mandated a label change to include the possibility of serious liver injury associated with the drug's use.

Three-year data show continued effectiveness, as measured by a reduction in relapses, favorable MRI data, and a reduction in progression of disability. In a small initial trial, it also showed promise as a therapy in patients who had a previously poor response to other disease-management agents.

Recent data also shows that the drug significantly improves patients' perception of health-related quality of life, reduces MS-associated pain, has at least a mild positive effect on fatigue and depression, and may reduce loss of vision associated with relapsing-remitting MS.

A long-term safety study of 2,500 people with relapsing forms of MS is now enrolling participants.

A Phase I study of alternative routes of administration will test the efficacy and safety of subcutaneous and intramuscular routes of administration.