Comeback Begins for Tysabri

The FDA's clearance was partly based on a risk-management plan designed to inform doctors and patients of the benefits and risks of Tysabri. Doctors who want to prescribe the drug, pharmacies and infusion centers that want to distribute it and patients who want to take Tysabri will have to enroll in a registry as part of a program that will monitor for new cases of PML or other infections.

Bernstein Research analyst Geoffrey Porges said in a report focusing on Biogen that the FDA, in requiring stricter language on Tysabri's label than its advisory committee recommended, appears to be trying to ensure that promotion efforts focus on keeping the product out of first-line usage, at least initially.

"While we were surprised to see the FDA advisory panel vote to reintroduce the product with such confidence, it is clear that the FDA harbors concerns about the potential for substantial numbers of PML cases to ultimately emerge with the reintroduction and promotion of this drug," Porges wrote in his research note. "The label contains a number of elements that are likely to restrict demand."

Because of the past PML cases, Tysabri isn't meant to be used with other immune-system suppressing or immune-modulating drugs, or in patients whose immune systems are compromised or who have undergone organ transplants.

During the companies' conference call, Martin pointed out that the FDA is requiring nothing on the label that would preclude the use of Tysabri as a first-line therapy for MS. He also said he believes there will "undoubtedly" be times when doctors will use Tysabri early on in the course of treating a patient.

As for the drug's price, Martin said the matter was complicated, but that Elan and Biogen will make a decision in the near future.