Surprise! A Food and Drug Administration advisory panel is backing approval for a blood-clotting drug being developed by GlaxoSmithKline and a San Diego firm.

The advisors voted unanimously today to recommend approval for Promacta to treat an immune system disorder. That's a bit of a surprise given that the FDA staff said the data indicated that the drug didn't work any better than a placebo in clinical testing.

After the staff report was released earlier in the week, the stock price of GlaxoSmithKline's development partner, Ligand Pharmaceuticals, plummeted to its lowest level in a decade. Today, Ligard shares popped up by more than 50 percent.

Shares of GlaxoSmithKline, which is a much bigger company, also rose on the action by the Oncologic Drugs Advisory Committee, rising little more than 1 percent.

The full FDA generally follows the recommendations of its advisory panels, but nothing requires it to do so. The key date for FDA action is June 19.