Human Genome Sciences Receives Complete Response Letter from FDA for Raxibacumab Biologics License Application

Human Genome Sciences, Inc. (Nasdaq: HGSI) today announced that it has received a Complete Response Letter from the U.S. Food and Drug Administration (FDA) relating to the Company’s Biologics License Application (BLA) requesting the approval of raxibacumab for use in the treatment of inhalational anthrax. The FDA issues Complete Response Letters to request additional information needed to complete the review of a BLA.

"We have responded to all of FDA’s previous questions. We plan to address the current questions as well,” said Sally D. Bolmer, Ph.D., R.A.C, Senior Vice President, Development and Regulatory Affairs, HGS. "In certain respects, the Complete Response Letter appears to be inconsistent with the FDA’s published final rule governing the development of new drugs when human efficacy studies are not ethical or feasible.”

James H. Davis, Ph.D., J.D., Executive Vice President and General Counsel, HGS, and leader of the Company’s raxibacumab program with the U.S. Government, said, "It is unfortunate that it was not possible to resolve these questions before the PDUFA date passed. However, HGS has delivered 20,000 doses of raxibacumab to the U.S. Strategic National Stockpile under our contract with BARDA, so it is currently available in the Stockpile for use in the event of an emergency while we complete our discussions with the FDA.”

Raxibacumab represents a new way to address the anthrax threat. While antibiotics can kill the anthrax bacteria, they are not effective against the deadly toxins that the bacteria produce. Raxibacumab targets anthrax toxins after they are released by the bacteria into the blood and tissues. In an inhalation anthrax attack, people may not know they are infected with anthrax until the toxins already are circulating in their blood, and it may be too late for antibiotics alone to be effective.

The July 9 edition of The New England Journal of Medicine published the results of two pivotal randomized placebo-controlled studies conducted in rabbits and monkeys to evaluate the efficacy of raxibacumab, as well as the results of human safety studies, which supported the use of raxibacumab in the event of life-threatening inhalation anthrax disease.

Raxibacumab is a first-in-class treatment for anthrax, and the first procurement under Project BioShield of a product discovered and developed after the September 11, 2001 terrorist attacks. Raxibacumab is being developed under a contract entered into in 2006 with the Biomedical Advanced Research and Development Authority (BARDA) of the Office of the Assistant Secretary for Preparedness and Response (ASPR), U.S. Department of Health and Human Services (HHS).

About the Raxibacumab Contract with the U.S. Government

Raxibacumab is being developed under a contract entered into with BARDA in 2006 (Contract Number HHS010020050006C). In April 2009, HGS fulfilled its commitment under this contract to deliver 20,000 doses of raxibacumab to the Strategic National Stockpile for emergency use in the treatment of inhalational anthrax. In July 2009, the Company announced that the U.S. Government exercised its option to purchase an additional 45,000 doses of raxibacumab for the Stockpile, to be delivered over a three-year period, beginning near the end of 2009. Both purchase awards were made under the Project BioShield Act of 2004, which is designed to accelerate the development, purchase and availability of medical countermeasures for the Strategic National Stockpile. In May 2009, HGS submitted the BLA to the U.S. Food and Drug Administration (FDA) for raxibacumab for the treatment of inhalational anthrax.

About Anthrax

Anthrax infection is caused by a spore-forming bacterium, Bacillus anthracis, which multiplies in the body and produces lethal toxins. Most anthrax fatalities are caused by the irreversible effects of the anthrax toxins. Research has shown that the bacteria produce protective antigen, the key facilitator in the progression of anthrax toxicity at the cellular level. After protective antigen and the other anthrax toxin components are produced by the bacteria, protective antigen binds to the anthrax toxin receptor on cell surfaces and forms a protein-receptor complex that makes it possible for the other anthrax toxin components to enter the cells where they become active. Raxibacumab blocks the binding of protective antigen to cell surfaces and prevents the anthrax toxins from entering and killing the cells.

About Human Genome Sciences

The mission of HGS is to apply great science and great medicine to bring innovative drugs to patients with unmet medical needs. The HGS clinical development pipeline includes novel drugs to treat lupus, hepatitis C, inhalation anthrax and cancer.

The Company’s primary focus is rapid progress toward the commercialization of its two lead drugs, BENLYSTA™ (belimumab) for lupus and ZALBIN™ (albinterferon alfa-2b) for hepatitis C. BENLYSTA has successfully met its primary endpoint in two pivotal Phase 3 trials in systemic lupus erythematosus, and the submission of marketing applications in the U.S., Europe and other regions is planned in the first half of 2010. ZALBIN has completed Phase 3 development, and the submission of global marketing applications is planned in fourth quarter 2009.

In May 2009, HGS submitted a Biologics License Application to the FDA for raxibacumab for the treatment of inhalation anthrax. In July 2009, the Company secured a new purchase order for 45,000 doses of raxibacumab to be delivered to the U.S. Strategic National Stockpile over a three-year period, beginning near the end of 2009. The Company also has several drugs in earlier stages of clinical development for the treatment of cancer, led by the TRAIL receptor antibody mapatumumab and a small-molecule antagonist of IAP (inhibitor of apoptosis) proteins. In addition, HGS has substantial financial rights to certain products in the GSK clinical pipeline including darapladib, currently in Phase 3 development in patients with coronary heart disease, and Syncria® (albiglutide), currently in Phase 3 development in patients with type 2 diabetes.

For more information about HGS, please visit the Company’s web site at www.hgsi.com. Health professionals and patients interested in clinical trials of HGS products may inquire via e-mail to medinfo@hgsi.com or by calling HGS at (877) 822-8472.

Safe Harbor Statement

This announcement contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. The forward-looking statements are based on Human Genome Sciences’ current intent, belief and expectations. These statements are not guarantees of future performance and are subject to certain risks and uncertainties that are difficult to predict. Actual results may differ materially from these forward-looking statements because of Human Genome Sciences’ unproven business model, its dependence on new technologies, the uncertainty and timing of clinical trials, Human Genome Sciences’ ability to develop and commercialize products, its dependence on collaborators for services and revenue, its substantial indebtedness and lease obligations, its changing requirements and costs associated with facilities, intense competition, the uncertainty of patent and intellectual property protection, Human Genome Sciences’ dependence on key management and key suppliers, the uncertainty of regulation of products, the impact of future alliances or transactions and other risks described in the Company’s filings with the SEC. Existing and prospective investors are cautioned not to place undue reliance on these forward-looking statements, which speak only as of today’s date. Human Genome Sciences undertakes no obligation to update or revise the information contained in this announcement whether as a result of new information, future events or circumstances or otherwise.