The above data suggest that the longer patients are treated with Tysabri, the higher the risk of PML. Does that mean that in year four and year five the risk increases? Few patients have reached four and five years of treatment, so we do not yet know the answer. As to the skewing between the number of cases in the U.S. and Europe, European patients may be more likely to have been treated with chemotherapy before beginning Tysabri. Does that mean that chemotherapy treatment increases the risk of PML, as some MS-specialists speculate? That remains unclear, but we do know that most Tysabri patients who develop PML have no history of other immunosuppressant therapies, which makes that hypothesis unlikely.
Three main possibilities exist, among them that the FDA and the manufacturers may opt to change the label so that Tysabri would be recommended for only 2 years of treatment or less; that research may advance to the point of identifying patients who innately have a higher risk of PML, although to date very few advances have been made in this area; or that Tysabri doses may be changed and/or the therapy used in combination with other treatment agents.