Kate Johnson –
June 7, 2011 (Montreal, Quebec) — Numbers are higher than previously reported for the postmarketing incidence of progressive multifocal leukoencephalopathy (PML) associated with the use of natalizumab (Tysabri, Biogen Idec), according to an update presented here at the Consortium of Multiple Sclerosis Centers 25th Annual Meeting.
As of May 2011, there have been 124 reported cases of natalizumab-associated PML worldwide among 83,300 patients treated for multiple sclerosis (MS), said John Foley, MD, from Rocky Mountain Multiple Sclerosis Clinic, in Salt Lake City, Utah, and a consultant for the company.
That is an overall incidence of 1.4 cases per 1000 patients — an increase from the 1.2 per 1000 rate announced by the US Food and Drug Administration (FDA) and reported by Medscape Medical News in April.
Rare Opportunistic Infection
Natalizumab was first approved by the FDA in 2004 to treat relapsing forms of MS but was temporarily withdrawn from the market the following year because of its association with PML — a rare opportunistic infection of the central nervous system infection caused by the John Cunningham virus (JCV).
The drug was reintroduced in 2006 with stricter safety warnings and monitoring recommendations, and this spring, labeling was updated to quantify postmarketing PML incidence rates according to duration of exposure.
Now, a further update presented at this meeting shows the highest PML incidence continues to be in patients who have received 2 to 3 years of monthly natalizumab infusions — in whom there is a current PML rate of 1.89 cases per 1000 patients, up from the previously reported 1.5 cases, reported Dr. Foley.
In contrast, patients who have received between 3 and 4 years of infusions continue to have a lower incidence, at 1.10 cases per 1000 — up from 0.9 cases.
“We’re, of course, very interested in what’s happening here,” commented Dr. Foley. “Is this real or a statistical artefact? We still have small numbers of patients out in this range.”
Finally, the lowest PML rates continue to be in patients who have received 2 years or less of monthly infusions, in whom the most recent incidence is 0.49 cases per 1000 patients — up from 0.3 cases, he said.
“Clearly, the statistics are suggesting that the longer you’re on the drug, the higher the risk. It also looks like there’s a more definitive association being established with prior immunosuppressant therapy. And, we now have, probably what you might call a secondary risk stratification factor — that being the JCV antibody — [for which a screening test] is currently completing its experimental phase and will likely move into a commercial phase most likely this year.”
JCV Antibody
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