Once and again: Withdrawn in February 2005, Tysabri is likely to return to the market.

A multiple sclerosis drug yanked off the market last year is on track to return, prompting some experts to say it should perhaps not have been withdrawn in the first place.

A US Food and Drug Administration (FDA) advisory panel in March voted unanimously to allow the drug, Tysabri, back on the market with some restrictions. Their decision followed a two-day meeting at which people who had benefited from the drug tearfully testified that the treatment was their only hope.

Tysabri launched in November 2004 after a fast-track approval from the FDA. Three months later, the drug's makers, Elan and Biogen Idec, withdrew it after discovering that a rare brain infection called progressive multifocal leukoencephalopathy (PML) had struck two people and killed one.

Experts say the abrupt halt and return to market demonstrates a need for less drastic reactions when the FDA first learns of serious side effects.

The FDA has really got to have more alternatives than just an on-off switch. Raymond Woosley, The Critical Path Institute

“The FDA has really got to have more alternatives than just an on-off switch,” says Raymond Woosley, president of The Critical Path Institute, an independent institute that evaluates the drug approval process. “[The system] ignores the realities of science.”

Once a treatment is on the market, the FDA cannot pull it off the shelves unless the information on its label is fraudulent. But the agency can use its power to persuade a company to withdraw it. About three percent of approved drugs are withdrawn after release, but only one drug—Lotronex for irritable bowel syndrome—has previously been returned to the market.

Following Tysabri's withdrawal, the FDA also halted clinical tests of at least one other drug in its class—GlaxoSmithKline's 683699, which was in phase 2 trials. Officials at the FDA would not comment on the status of the other drugs in development.

Scientists had warned more than ten years earlier that those who take Tysabri or others in its class may be at risk for infection (Nat. Med. 11, 359; 2005). Tysabri works by blocking immune cells called lymphocytes from entering the central nervous system—a mechanism that may also weaken the immune system.

A study conducted after the companies withdrew the treatment showed that the risk of developing PML is 1 in 1,000 (N. Engl. J. Med. 354, 924; 2006). Other studies on the drug's efficacy were also incomplete at the time of the withdrawal.

Still, the FDA and the companies had to take some sort of action to show the public that they are committed to safety, says David Feigal, former director of the FDA's Center for Devices and Radiological Health.

“Nobody knew the scope of the problem,” adds Karl Kieburtz, chairman of the advisory panel and a professor of neurology at the University of Rochester. “In the absence of information you have to act on the principle of 'do no harm.'”

As a condition for reapproval, the advisory committee recommended that Tysabri not be used along with other multiple sclerosis drugs. The panel also said that everyone who takes Tysabri must be registered and that they can only receive the treatment at authorized centers. As of 21 March, FDA officials had not yet announced their decision to allow the drug back on the market, but are expected to follow the committee's advice.