UPDATE 1-Human Genome anthrax drug said to work in animals

* FDA staff: Unknown if animal tests predict human results

* Headaches most common side effect - FDA staff

WASHINGTON, Oct 23 - An experimental Human Genome Sciences Inc drug worked better than a placebo in treating anthrax exposure in animals, U.S. drug reviewers said in a summary released on Friday.

The animal studies of the drug, raxibacumab, had several limitations and "it is still unknown how well these models and results predict efficacy in humans," the Food and Drug Administration staff said.

The FDA will ask an advisory panel that meets Tuesday whether the drug's benefits outweigh risks for treating potentially deadly infections caused from inhaling anthrax, a bacterium feared as a possible biological weapon. FDA rules allow companies to study two animals to show effectiveness in lieu of human tests for some deadly infections.

Headaches were the most common reaction in people who took the drug to test its safety, the FDA staff said.