US regulators have declared that Biogen Idec and Elan Corp’s multiple sclerosis blockbuster Tysabri does not require new warnings despite a rise in the number of cases of a potentially brain infection.

The US Food and Drug Administration says that since July 2006, when Tysabri (natalizumab) was reintroduced, to September 8 this year, 13 cases of progressive multifocal leukoencephalopathy have been confirmed, four of which were reported in US patients. Tysabri is also approved for Crohn’s disease and no PML cases have been reported in those patients, but that indication accounts for less than 2% of the drug's use in the USA.

Given these figures, the overall rate of developing PML with Tysabri in patients who have received at least one infusion remains below one per 1,000 patients. The FDA added that the current rate of PML in patients who have received at least 24 infusions ranges from 0.4 to 1.3 per 1,000 patients.

Therefore, "at this time, the FDA is not requiring changes regarding PML to the Tysabri prescribing information or to the Tysabri risk management plan," the agency stated. All patients receiving the therapy in the USA are enrolled in the TOUCH prescribing programme and are closely monitored for the occurrence of PML “and other serious opportunistic infections”.

However the agency did claim that “the risk for developing PML appears to increase with the number of Tysabri infusions received”. Specifically, the number of monthly infusions of Tysabri in the 13 patients who developed PML ranged from 12 to 35, and the average number received before the diagnosis of the brain infection was 25.

Avonex patent extended
Meantime, Biogen has announced that it has been issued with a new "method of use" patent for its older MS drug Avonex (interferon beta-1b), which will expire in September 2026.

The patent also covers the use of beta interferon for treating viral conditions and diseases, cancers or tumours, as well as MS. Other patents surrounding the production of Avonex, which had sales of $591.2 million in the second quarter (+12.1%), will expire in 2013.