Tysabri

Tysabri is the brand name for natalizumab, a humanized monoclonal antibody used in the treatment of multiple sclerosis and Crohn's disease. Developed by Biogen Idec and Elan, natalizumab targets alpha4-integrin (CD49d) on the surface of leukocytes and blocks its interaction with vascular cell adhesion molecule-1 (VCAM-1). This inhibits the migration of immune cells across the blood-brain barrier and into the gut mucosa, reducing inflammatory activity in the central nervous system and intestines.

Indications and administration: In the United States, Tysabri is approved for relapsing forms of multiple sclerosis and for moderately to severely active Crohn's disease in patients who have failed or cannot tolerate conventional therapy. It is given as an intravenous infusion, typically 300 mg every four weeks, though dosing may be adjusted by clinicians.

Safety and history: Tysabri was approved in 2004 for MS but was temporarily withdrawn in 2005 after reports of progressive multifocal leukoencephalopathy (PML), a rare brain infection caused by the JC virus. It was reintroduced in 2006 under a risk management program known as the TOUCH Prescribing Program, designed to monitor and limit PML risk through JC virus antibody testing, exposure history, and restricted prescribing. The risk of PML is higher in patients who are JC virus positive, have had prior immunosuppressive therapy, or have prolonged treatment, particularly after two or more years.

Common adverse effects include infusion reactions, headache, fatigue, and upper respiratory infections. Serious risks include PML, hepatic injury, and hypersensitivity reactions. Patients are monitored for signs of infection or liver damage and may undergo regular MRI scans to detect early PML; pregnancy should be discussed with a clinician, as data on fetal safety are limited.