Four more patients taking Biogen Idec's multiple sclerosis drug Tysabri have developed the serious brain infection known as PML and one of them died, according to the biotechnology company's monthly update.
The four new confirmed cases of potentially fatal progressive multifocal leukoencephalopathy, or PML, were detected between November 2 and December 2, the company said. That brought the total number of confirmed cases as of December 2 to 79, with 16 deaths.
Biogen, which markets the drug with Ireland-based partner Elan Corp Plc, said the rate of PML cases per 1,000 patients has edged slightly higher to 1.0 from 0.96 and is likely due to more patients being on the drug for more than two years.
The chances of contracting PML tends to increase the longer a patient is on the drug, which is considered the most important growth driver for Biogen and Elan. The incidence of PML cases per 1,000 patients who have received between 1 to 12 infusions of Tysabri is just 0.01, but that rises to 2.05 at more than 24 infusions, according to Biogen's statistics.
Tysabri was briefly pulled from the U.S. market due to concerns over PML. But the drug was so effective compared to other available treatments that MS patients considered the risk worth taking and clamored for its return.
Health regulators agreed to make it available with certain restrictions, including close monitoring and reporting of PML cases.