Animal data suggests a proposed Human Genome Sciences Inc drug may help people exposed to anthrax but more study is needed to show the treatment adds benefit beyond antibiotics alone, a U.S. advisory panel said on Tuesday.

The Food and Drug Administration did not ask the panel for a recommendation on whether it should approve the company's bid for approval for treating anthrax infection. The FDA said recent inspections raised questions about tests used to measure the drug's absorption in people, making it impossible to judge if the product was fit for approval.

Anthrax is a bacterium that can cause a potentially fatal lung infection if inhaled. Letters carrying powdered anthrax killed five people in 2001, and experts fear it could be used again in a biological attack.

The U.S. government already has ordered 65,000 doses of Human Genome's drug, known as ABthrax or raxibacumab, for a national stockpile of emergency medicines ahead of the FDA review.

Officials with Human Genome Sciences said they felt they could address the FDA's concerns and hoped the agency would clear ABthrax with a commitment for further studies after approval.

Human Genome's application relies largely on studies in rabbits and monkeys. FDA rules allow animal tests in lieu of human studies to show the effectiveness of new drugs to fight certain deadly infections where testing in humans would be unethical.

The panel voted 16-7, with one abstention, that evidence from the animal studies predicted the response in humans. In those tests, a single intravenous dose of ABthrax worked better than a placebo but similar to antibiotics for treating anthrax infection, FDA reviewers said.

It is clearly protective in these animal models, and I think there is every reason to expect it would be beneficial in humans, said panel member Stephen Leppla, acting chief of the bacterial diseases laboratory at the National Institute of Allergy and Infections Diseases.

The majority, however, voted that the FDA should request evidence from animal studies to show that adding ABthrax to antibiotics worked better than antibiotics alone. The question did not specify if the agency should seek that data before or after the drug's approval.

We definitely want to be sure it has some benefit in addition to the antibiotics, said Dr. Archana Chatterjee, a panel member and chief of the pediatrics infectious diseases division at Creighton University School of Medicine in Nebraska.

The FDA said it will consider if new animal or human studies are needed once an investigation of the testing issues is complete and it weighs the panel's advice.

The panel narrowly voted that evidence showed the medicine would not reduce the effectiveness of antibiotics.

Human Genome said new options to fight anthrax were needed because antibiotics do not always work, and the U.S. government has called for alternatives to protect the public.

Current established therapies are not sufficient, Sally Bolmer, a senior vice president at Human Genome, told the panel.

ABthrax takes a new approach to fighting anthrax by targeting the toxin made by the bacteria rather than the microbe itself. It is designed to attack and neutralize the toxin that makes anthrax so deadly.