Just ran across this article in the April 13, 2012 Wall Street Journal (a little late): Novartis Gilenya Patient Gets Rare Brain Disease.
Turns out that someone who was using Gilenya got diagnosed with progressive multifocal leukoencephalopathy (PML), the brain disease most associated with use of Tysabri in MS patients (or in people with AIDS). The person had previously been treated with Tysabri.
I have always been under the impression that the PML risk associated with Tysabri went away after stopping the drug, as the immune system returned to normal levels. However, maybe it is possible that it retains some sort of "memory" and certain cells are easily affected again on another drug. I just don't know.
I have not yet been able to find out any details, such as how long the person had taken Tysabri and how long it had been since they had stopped. Nor do I know how many doses of Gilenya this person has taken or what their status is today. This is definitely something to keep an eye on. I would appreciate it if anybody with more details could write into the comments section and share what you know.
UPDATED: I did receive communication from Novartis today that the person who was diagnosed with PML had been taking Tysabri for 3.5 years prior to beginning Gilenya and was JC virus antibody-positive. The risk for PML goes up drastically after a person has been on Tysabri for 2 years (24 infusions) and is JC virus antibody-positive.
I do feel like i need to add a comment of my own about Novartis' conduct in this matter. I do not feel like they are being at all reluctant to share information. From what I can tell, there is simply not that much information to share at this point. I think pretty much anyone would say that the main contributing factor for PML here is Tysabri, but if and how subsequent Gilenya doses somehow interacted with a Tysabri-exposed immune system is the question. It could only benefit Novartis to figure this out as soon as possible and communicate their findings.
Read more about PML: