Tysabri-Associated PML in MS: Better Survival Rates Than Other Patient Types

A follow-up study of 35 MS patients with Tysabri-associated PML provides a somewhat encouraging survival rate of about 70%. This percentage is more than twice the quoted survival rate of other patient types with PML (eg, those with HIV infection or following transplantation), and the authors speculate that the improved survival in Tysabri-related cases may be due (partially) to the ability to reconstitute the immune system more easily in these cases.

Factors that appeared to improve the chances of survival among Tysabri-treated patients specifically were generally not surprising: relative youth, less MS-related disability, a shorter time between symptom onset and the diagnosis of PML, and less extensive disease (ie, unilobar PML vs global disease).

The provided overall risk of PML with Tysabri treatment remains within the ranges observed during the clinical development of the drug (1/1000) and the postmarketing experience (0.9/1000). However, the risk of PML is acknowledged to increase substantially as the duration of treatment increases:

• from 0.01/1000 during months 1-12,
  
• to 0.39/1000 during months 13-24,
• to 1.46/1000 during months 25-36. 

The risk of PML reapproaches the overall risk when treatment extends 37-48 months.

The authors, 3 of whom work for Biogen Idec, the maker of Tysabri, admit that one major deficiency of their follow-up report is the lack of long-term data. Only 12 of 25 PML survivors had follow-up data beyond 6 months at the time of the analysis. However, most Tysabri-treated patients with PML who died did so within 2 months of diagnosis; so the authors predict that the ~70% survival rate will be maintained as the data continue to accumulate.

MS = multiple sclerosis; PML = progressive multifocal leukoencephalopathy.