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Tysabri was originally approved by the FDA in 2004 to treat Crohn’s disease and Multiple Sclerosis (MS), but was removed from the U.S. market a few months later after several patients were diagnosed with a rare and often deadly brain infection called progressive multifocal leukoencephalopathy (PML). In 2006, Tysabri was allowed back onto the market to treat Multiple Sclerosis only, after Biogen Idec Inc. and Elan Corp., the makers of the drug agreed with the FDA to a add new warning to the drugs label informing patients of the associated risk.

On June 17, 2010, Biogen Idec Inc’s announced that six new patients taking the multiple sclerosis drug Tysabri developed a potentially deadly brain infection known as PML, according to a recent Reuters news report.

Biogen announced in its latest monthly update that as of June 7th, there were 55 confirmed cases of progressive multifocal leukoencephalopathy, or PML, up from 49 as of May 6th. Biogen, which markets the drug with partner Elan Corp Plc, said the number of deaths from PML remained at 11.

Sales of Tysabri, the most important growth driver for both Elan and Biogen, have been crimped by concerns over PML as investors and physicians try to gauge how great the risk the risk of contracting the disease might be over time.

The rate of PML increases with the length of time a patient remains on the drug. The incidence of PML in patients taking Tysabri for roughly a year is 0.01 per 1,000 patients. That rate rises to 0.35 cases per 1,000 in patients taking the drug for between one and two years, and to 1.47 cases for patients taking it between two and three years.

The company says the rate of PML is still within the 1 in 1,000 level seen in its clinical trials and noted on the drug’s label.

The May 6th figure of 49 represented an increase of three new cases compared with the 46 cases reported as of April 6th. That in turn represented an increase of four from March 10th, when there were 42 cases.

Tysabri was initially approved by the FDA in 2004 to treat Crohn’s disease and Multiple Sclerosis (MS), but was taken off the U.S. market a few months later after numerous patients were diagnosed with a rare and often deadly brain infection called progressive multifocal leukoencephalopathy (PML). Tysabri was allowed back onto the market in 2006 to treat only Multiple Sclerosis, after Biogen Idec Inc. and Elan Corp., the makers of the drug agreed with the FDA to add a new warning to the label informing patients of the associated risk.

On February 5, 2010, FDA officials warned doctors that the risk of a deadly brain inflammation linked to the multiple sclerosis treatment Tysabri increases with use of the drug, according to an Associated Press report.

Tysabri has been plagued for years by rare cases of progressive multifocal leukoencephalopathy (PML), which can cause swelling of the brain that is usually fatal.

The FDA, in a statement revealed that it has confirmed a total of 31 cases of PML since mid-2006. Eight of those patients have died, as of last month, according to the FDA. The agency posted updated labeling information for the drug to its Web site.

In the new labeling, the FDA notes there have been no reports of PML in patients taking Tysabri for less than 12 months, but in patients taking the drug for two to three years the rate of PML is one case per 1,000 patients.

The new labeling also warns of a complication of PML, known as Immune Reconstitution Inflammatory Syndrome that can dramatically damage patients’ health even after they have stopped taking Tysabri. The agency urges doctors to monitor patients for signs of inflammation.

According to a Wall Street Journal report, Biogen will converse with doctors once a month on the occurrence of new cases of a rare brain infection in patients using its multiple sclerosis treatment Tysabri. The situation is closely watched because Tysabri, a highly effective treatment sold with Elan Corp. was previously pulled from the market because of its association with progressive multifocal leukoencephalopathy (PML), a debilitating and often fatal condition.

Since, Tysabri reentered the U.S. market in 2008, Biogen and regulators have struggled with how to provide information on PML because case-by-case updates of a specific side effect are unprecedented. Last summer, the company stopped providing updates altogether.  As of mid-January, the number of cases stands at 31, which puts the overall incidence rate at about 1-in-1,000 patients, as implied by the drug’s label.

Under the new plan, Biogen will proactively update doctors midmonth and provide information through a password-protected Web site. It will include the number of PML cases, with an incidence rate broken down by duration of use, as well as a cumulative patient exposure figure, which is different than the quarterly patient count provided to investors.

Biogen withdrew the drug from the market for 18 months beginning in 2005 after three patients developed PML. Infections re-emerged in 2008, and Biogen provided regular weekly updates to the public until last July, when it officially stopped providing any information.

In October, European and U.S. regulators said the number of PML cases had risen to 24, well above Biogen’s July disclosure of 11, surprising Wall Street and raising questions about Biogen’s disclosure policies and refusal to comment on PML case numbers. The company began to re-think its approach at about the same time.

On January 21, 2010, the European Medicines Agency’s Committee for Medicinal Products for Human Use, known as CHMP, recommended increased risk-mitigation measures for Tysabri after reviewing its safety. The panel concluded that Tysabri’s benefit for MS patients outweighs its risks and the drug should stay on the market.

Tysabri had more than $1 billion in 2009 sales.