The makers of a recently approved medication for multiple sclerosis have voluntarily removed the product from the market after reports of serious adverse events involving two patients.

The drug, known as natalizumab and marketed as Tysabri, was approved by the Food and Drug Administration (FDA) as recently as this past November. It is indicated as a therapy for the relapses, or exacerbations, that occur in multiple sclerosis. However, the two companies that make the medication, Biogen Idec and Elan Corporation, decided to withdraw it after it was learned one patient taking a combination of Tysabri and the MS drug Avonex (interferon beta-1a) had died. Another developed a serious condition of the central nervous system known as progressive multifocal leukoencephalopathy (PML). PML is a demyelinating disease that is frequently fatal. Both patients had been taking the combination of the two medications for more than 2 years.

"In light of these two cases, we believe it is prudent to take a step back and evaluate the possible risk of PML," said Jim Mullen, CEO of Biogen Idec in a conference call with reporters and investors on February 28.

Mullen said his company is taking a series of steps in its investigation. Those include analyzing MRI information of the two patients in question; working with clinical investigators to evaluate patients in ongoing clinical trials, including taking extensive MRI scans of them; and working with experts on PML to gain a better understanding of the disease and the possible connection that Tysabri may possess.

"We hope that the totality of what we learn from these steps will allow us in collaboration with regulatory agencies to craft a possible path for Tysabri," Mullen said. The companies say they'll make a determination about re-initiating dosing of the drug in clinical trials and future commercial availability, based on the outcome of their investigation.

The companies' decisions were made in consultation with the FDA, Biogen said.

"While we work through this matter, we must place patient safety above all other considerations," said Burt Adelman, MD, executive vice-president of Development at Biogen.

Adelman said about 3,000 patients with MS, Crohn's disease or rheumatoid arthritis have been treated with Tysabri in clinical trials, and approximately 5,000 patients have received it as treatment since it was approved for MS in November 2004. Biogen stressed that it has received no reports of PML in patients taking Tysabri monotherapy or in those taking Avonex alone. Avonex was approved as an MS treatment in 1996.

Adelman said this decision was made "out of an abundance of caution."

"The reason we're taking this cautious approach is because of the rarity of this event, and because of the severity," Mullen added.

Patients currently taking Tysabri who may have questions can contact Biogen Idec at 1-888-489-7227.

John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include coverage of health news for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.