The drug Tysabri which was approved by the Food and Drug Administration (FDA) for the treatment of multiple sclerosis in November 2004, was taken off the market in late February, after two cases of progressive multifocal leukoencephalopathy, or PML, appeared in patients given the drug during clinical trials.
Tysabri was considered a major breakthrough in the treatment of multiple sclerosis, and drug companies Biogen and Elan had also been testing the drug for the treatment of Crohn's disease, which like MS, is believed to be caused by the immune system attacking the body.
Tysabri appears to be able alleviate those attacks.
According to Dr. Jeffrey Drazen, the editor-in-chief of NEJM, while he sees a link between Tysabri and the onset of PML, further study is needed to determine whether the drug's benefits justify its risks. Tysabri is also known as natalizumab.
He says natalizumab does present a dilemma, as though it appears to be a promising therapy for multiple sclerosis and has raised the hopes of patients, the complication of PML can be fatal.
Drazen made his comments in an editorial about the formidable risks that clinical trial patients face. The editorial accompanied the analyses of data collected from three clinical trial patients who used Tysabri and subsequently contracted PM, two died from the disease. He says that though the association between natalizumab and the occurrence of PML seems clear, the level of that risk of PML per year of exposure is unknown.
PML is believed to be caused when a virus called the JC virus is reactivated and attacks the brain. and medical experts estimate that up to about 80% of the public carry the JC virus, which almost always remains dormant. PML, which is extremely rare, is most often seen in patients who are severely immuno-suppressed, such as those suffering from advanced AIDS, and is often fatal.
Other experts speculate that PML victims might stand a better chance of recovering if Tysabri was discontinued in the very early stages of the disease.
Biogen researchers hope that Tysabri could return to the marketplace if patients are strictly monitored for the onset of PML.
They say it is possible that testing for the appearance of JC virus in blood and discontinuing the natalizumab therapy would allow patients to recover.