Biogen Idec Inc. reported 7 more cases of a rare brain infection among multiple sclerosis patients taking Tysabri, sold with Elan Corp. bringing the total number of affected patients to 102 as of March 4.

The Weston, Mass., biotech company reported that one more of those patients has died, bringing the total to 21. The cases of the infection–known as progressive multifocal leukoencephalopathy, or PML–are closely watched as the MS market becomes increasingly competitive.

The drug is generally regarded as highly effective, but mostly used for patients that have stopped responding to other drugs or have aggressive cases of the disease.

Biogen recently halted enrollment in a major clinical trial aimed at testing the earlier use of Tysabri, citing a slower-than-expected pace of finding participants and the desire to devote resources toward “risk stratification” efforts for the drug.

Biogen is developing a blood test that may better determine the chances of patients contracting PML. The test recently received CE Mark approval in Europe and the companies are conducting large studies of its effectiveness.

Regulators have said that they monitor cases of PML as they occur and have concluded that the benefits of the medicine outweigh the risks. Tysabri was withdrawn from the market in 2005 and relaunched in 2006–because of its effectiveness–with a strict access plan that monitors patients.

The overall global PML rate is now at 1.23 per 1,000 patients. A Biogen spokeswoman said the current PML figure is “still generally within what we saw in clinical trials.”

Of the total PML cases, 55 were in the U.S., 42 were in the European Union and five in other areas.

The risk of the infection generally increases with the number of monthly infusions that a patient receives, but that rate appears to drop after 30 months. Biogen is studying this drop and warns there are limited data from patients on the drug longer than 36 months.

The most recent data translate to a rate of 1.87 cases per 1,000 patients on the drug for a year or longer, rising to 2.41 per 1,000 for those on the drug for two years or longer, and dropping to 1.4 for more than three years.

Looked at another way, the rate is about 1.68 cases per 1,000 patients on the drug for between two and three years. The incidence is about 0.44 case per 1,000 patients in those using it for one to two years, and it is essentially nonexistent in patients using it for less than a year.

Source: Wall Street Journal Copyright ©2011 Dow Jones & Company, Inc (19/03/11)