Why are agency reviewers hesitant to approve Biogen and Elan’s expensive treatment for the gastrointestinal illness? Potentially fatal side effects and a lack of safety studies the FDA asked the drugmakers to provide, according to documents posted on the FDA web site in advance of an advisory committee meeting this Tuesday in Maryland.

The FDA staffers wrote they’re concerned patients taking Tysabri, which is already approved to treat multiple sclerosis, could develop a rare and incurable nervous system disease that is usually fatal after six months, the Associated Press reports.

Biogen, which codeveloped the drug with Elan, is asking the FDA to approve Tysabri to treat Crohn’s disease, which causes inflammation of the intestines and affects about 1 million people worldwide. But the med has been restricted since three patients developed a rare nervous system disorder known as multifocal leukoencephalopathy, or PML.

Tysabri was temporarily pulled from the market in February 2005, but the FDA agreed to alllowed the drug back on the market last year under the condition that patients use it alone, and not in combo with meds used to suppress the immune system. The agency figured that combining Tysabri with these treatments could increase risk of PML.

In its review of Tysabri for the new use, agency staffers says it’d be more difficult to assure the safety of the drug in Crohn’s patients, since they’re more likely to take immune-system suppressing drugs as additional therapy. They also noted Biogen didn’t conduct several requested safety analyses, and those submitted only tracked patients over a short time. Despite safety concerns, the FDA says Tysabri appeared effective in preventing inflammation of the intestine wall in Crohn’s patients.

In a note to investors, Rodman & Renshaw biotech analyst Michael King wrote: “We believe that the FDA will likely require additional studies for Tysabri to establish its safety profile in Crohn’s Disease. Whether the FDA will require complete trials or allow the drug to come to market with a significantly limited label and post-market requirements is up for question…The FDA panel consists of three voting members of GI Advisory Committee, five members of the Drug Safety Committee, and 11 temporary voting members. We believe that the make-up of the committee is potentially skewed toward the viewpoint of safety.”

To see the meeting’s draft agenda, look here;
To look at the committee rosters, look here, here and here;
To look at Biogen and FDA briefing documents, please look here.