LINKAccording 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.

The Biogen team say they are committed to a thorough evaluation of the safety profile of natalizumab.

Drazen has praised Biogen specifically for its handling of the clinical trial patients.

Biogen and Elan suspended all sales and testing of Tysabri in late February after two patients using the drug in a clinical trial were reported to have contracted PML. A third PML victim, also from a clinical trial, was identified in March. A fourth suspected case of the disease was reported to the FDA recently.

According to Biogen and the FDA, of the three confirmed PML cases, two of the patients have died, while a third survived, the fourth suspected victim is also alive.

Three of the patients, who suffered from multiple sclerosis, used Tysabri along with another Biogen drug, Avonex. A fourth patient, who suffered from Crohn's, had been taking Tysabri, along with a variety of other medications for his condition.

Since the discovery of the first two cases in February, Biogen and Elan have carried out a comprehensive medical review of the 5,000 patients who took Tysabri, to identify the trigger for PML. They hope to have the review completed by late summer, and will then discuss with the FDA whether Tysabri should be put back on the market and in what capacity.