A staff report from the

Food and Drug Administration

reviewing

Biogen's

(BGEN)

experimental psoriasis drug, Amevive, was released Wednesday, raising questions about the drug's long-term safety.

These concerns appeared to be weighing on Biogen's shares, which were down $1.77, or 4.2%, to $39.71 in pre-market trading.

The FDA documents, which are posted on the agency's Web site in advance of Thursday's advisory panel meeting, do not take a position on Amevive's approval. And the safety questions raised by the agency's drug reviewers appear to be

in line with expectations.

Amevive is generally regarded as a new, effective treatment for the painful, scaly skin lesions that make psoriasis such a debilitating disease. But the drug works by suppressing a certain type of T lymphocyte, or immunity-boosting T cell, responsible for the outbreak of skin lesions. The fear is that long-term T-cell suppression could put psoriasis patients at risk for increased rates of infection, even cancer.

Thursday's meeting of the Dermatologic and Opthalmic Drug Advisory Committee is likely to spend a great deal of time discussing Amevive's safety profile before voting on whether to recommend the drug's approval. That recommendation will then be passed onto the full FDA. The FDA usually follows the panel's recommendation but is not obligated to do so.

Regarding Amevive's efficacy, the FDA staff report found that psoriasis patients do respond to the drug, compared to patients taking placebo. Reviewers, however, questioned whether the response was enough to improve patients' quality of life.

On the safety front, the FDA staff is concerned about the suppression of T cells. "Studies consistently showed that lymphocyte counts decrease and do not recover to baseline in a substantial proportion of patients even after a prolonged observation period," the report states.

Biogen has conducted studies that appear to show that this effect does not lead to increased infections in patients, but FDA reviewers appear skeptical. The FDA staff report, however, says, "There was no evidence that depressions in lymphocyte counts were associated with serious adverse events, particularly infections."

There was also "no evidence of opportunistic infections or reactivation of latent/chronic infections," the report states.

However, FDA reviewers left the biggest question for last.

"Long-term studies in larger patient populations are needed to assess the risk of infection and malignancy and the recovery of lymphocyte counts to baseline/normal," the report states.

Does this mean that the FDA wants Biogen to conduct additional safety studies before the agency decides on approval? Or, as it can do, will the FDA approve Amevive on the condition that Biogen conducts the studies while the drug is on the market?

Answers to at least some of these questions are forthcoming Thursday.

Biogen closed Tuesday at $41.48 per share.