Biogen Idec is suspending the marketing of its multiple-sclerosis drug Tysabri (natalizumab), following one confirmed and fatal case, and another confirmed case, of progressive multifocal leukoencephalopathy (PML) in patients taking the monoclonal-antibody drug. The relationship between Tysabri and PML has yet to be confirmed. However, given the aggressive nature of PML (death usually occurs one to four months after onset), Biogen has also stopped all clinical trials using Tysabri.
PML is a rare viral disorder of the brain and nervous system that primarily affects people with suppressed immune function such as patients with cancers like leukaemia or lymphoma, or victims of AIDS (acquired immunodeficiency syndrome). The infection is characterised by destruction of the fatty myelin sheath that insulates the nerve fibres of the brain. Symptoms of PML include mental deterioration, vision loss, speech disturbances, ataxia (inability to coordinate physical movements), paralysis and, ultimately, coma. In rare cases, seizures may also occur.
There is currently no effective treatment or possible cure for the disorder (www.fda.gov/cder/drug/advisory/natalizumab.htm).