ROCKVILLE, Maryland – August 18, 2005 – Human Genome Sciences, Inc. (Nasdaq: HGSI) announced today that GlaxoSmithKline (NYSE: GSK) has exercised its option to develop and commercialize HGS-ETR1 (mapatumumab) jointly with Human Genome Sciences.

HGS-ETR1 is a human monoclonal antibody that specifically binds to the TRAIL-receptor 1 protein, induces apoptosis in human cancer cell lines expressing TRAIL receptor 1, and has anti-tumor activity in a broad range of tumor types, both as a single agent and in combination with chemotherapy.^1-13 Because HGS-ETR1 mimics the activity of the natural TRAIL-receptor 1 protein, it is considered an agonistic antibody.

As part of its June 1996 agreement with GSK ¹⁴, Human Genome Sciences granted a 50/50 co-development and co-promotion option to GSK for certain Human Genome Sciences human therapeutic products that complete Phase 2a clinical trials. Under the terms of the agreement, GSK and Human Genome Sciences will share equally in Phase 3 and 4 development costs, and will share equally in sales and marketing expenses and profits of any product that is commercialized, under a co-development and co-promotion agreement, the remaining terms of which are being negotiated by the parties. In July 2005, Human Genome Sciences announced that GSK exercised its option to co-develop and co-commercialize LymphoStat-B™ (belimumab), which is currently in Phase 2 clinical trials in systemic lupus erythematosus and rheumatoid arthritis.¹⁵

H. Thomas Watkins, Chief Executive Officer, Human Genome Sciences, said, “We are delighted that GSK has exercised its option to develop and commercialize HGS-ETR1 jointly with Human Genome Sciences. GSK is an important strategic partner and a world leader in the pharmaceutical industry. We look forward to a productive collaboration for the development and commercialization of this novel anti-cancer agent.”

Phase 2 clinical trials of HGS-ETR1 have been initiated in patients with advanced non-small cell lung cancer (NSCLC), advanced non-Hodgkin’s lymphoma (NHL) and advanced colorectal cancer.^16-18 In July 2005, Human Genome Sciences reported that the NSCLC trial results showed that HGS-ETR1 was well tolerated in these patients and could be administered safely and repetitively. Stable disease was observed in 29% (9/32) of the NSCLC patients treated, with 8 patients receiving at least 4 cycles of therapy.^19-20 In June 2005, the Company reported that the interim results of an ongoing Phase 2 clinical trial demonstrated that HGS-ETR1 was well tolerated, showed signs of clinical activity in NHL patients, and could safely be administered intravenously every 21 days at doses up to 10 mg/kg.^{2, 21} It is anticipated that the full data from the Phase 2 NHL study will be presented at an appropriate scientific meeting later in 2005. The Phase 2 trial in colorectal cancer is ongoing, with a presentation of the results anticipated on November 1 at the 2005 European Cancer Conference (ECCO).

Human Genome Sciences, using genomic techniques, originally identified the TRAIL receptor-1 protein. HGS-ETR1, which targets TRAIL receptor 1, was made in a collaboration between Human Genome Sciences and Cambridge Antibody Technology.²²

For more information about HGS-ETR1, see www.hgsi.com/products/ETR1.html. Health professionals interested in more information about trials involving HGSI products are encouraged to inquire via the Contact Us section of the Human Genome Sciences web site, www.hgsi.com/products/request.html, or by calling (240) 314-4400, extension 3550.

Human Genome Sciences is a company with the mission to discover, develop, manufacture and market innovative drugs that serve patients with unmet medical needs, with a primary focus on protein and antibody drugs.

HGS, Human Genome Sciences and LymphoStat-B are trademarks of Human Genome Sciences, Inc.

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 the Company’s unproven business model, its dependence on new technologies, the uncertainty and timing of clinical trials, the Company’s 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 planned facilities, intense competition, the uncertainty of patent and intellectual property protection, the Company’s 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 Securities and Exchange Commission. 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.

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Footnotes:

1. Humphreys, RC. Development and evaluation of cancer therapeutic agents targeting TRAIL receptor 1 and 2. Cancer Drug Discovery and Development: The Oncogenomics Handbook (Ed.: La Rochelle WJ and Shimkets RA, Humana Press, 2005).
2. Younes A, et al. Activity of selective agonistic monoclonal antibodies to TRAIL death receptors R1 and R2 in primary and cultured tumor cells of lymphoid origin. 9th International Conference on Malignant Lymphoma, 2005. Oral presentation.
3. Halpern W, et al. Variable distribution of TRAIL Receptor 1 in primary human tumor and normal tissues. 16 th EORTC-NCI-AACR Symposium on Molecular Targets and Cancer Therapeutics, 2004: Abstract #225.
4. (HGSI Press Release) Human Genome Sciences Reports Results of Preclinical Studies of TRAIL-R1 and TRAIL-R2 Agonistic Human Monoclonal Antibodies at EORTC-NCI-AACR Symposium. October 1, 2004 .
5. Georgakis GV, et al. Selective agonistic monoclonal antibodies to the TRAIL Receptors R1 and R2 induce cell death and potentiate the effect of chemotherapy and bortezomib in primary and cultured lymphoma cells. American Society of Clinical Oncology Annual Meeting, 2004: Abstract #6595.
6. Gillotte D, Zhang Y, Poortman C, et al. Human agonistic anti-TRAIL receptor antibodies, HGS-ETR1 and HGS-ETR2, induce apoptosis in ovarian tumor lines and their activity is enhanced by taxol and carboplatin. Proceedings from the AACR 2004; 73:3579.
7. Humphreys R, et al. Novel, agonistic, human anti-TRAIL receptor monoclonal antibodies, HGS-ETR1 and HGS-ETR2, are capable of potently inducing tumor regression and growth inhibition as single agents and in combination with chemotherapeutic agents in models of human NSCLC. AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics. November 2003. Poster #B72.
8. Georgakis GV, Li Y, Humphreys R, et al. Activity of selective agonistic antibodies to TRAIL death receptors R1 and R2 in primary and cultured tumor cells of hematologic origin. Blood 2003;102:228a (abstract #799).
9. Johnson RL, Huang X, Fiscella M. Human agonistic anti-TRAIL antibodies, HGS-ETR1 and HGS-ETR2, induce apoptosis in diverse hematological tumor lines. Blood 2003;102:981a (abstract #3316).
10. Younes A, Kadin ME. Emerging applications for the tumor necrosis factor family of ligands and receptors in cancer therapy. J Clin Oncol 2003;21:3526-3534.
11. Pukac L, Kanakaraj P, Alderson R, et al. TRAIL-R1 mAb, a human agonistic monoclonal antibody to tumor necrosis factor-related apoptosis-inducing ligand receptor 1, induces apoptosis in human tumor cells in vitro and in vivo. American Association for Cancer Research 94th Annual Meeting. July 2003, Abstract 6429.
12. Salcedo, Alderson R, Basu, et al. TRM-1, a fully human TRAIL-R1 agonistic monoclonal antibody, displays in vitro and in vivo anti-tumor activity. American Association for Cancer Research 93rd Annual Meeting. April 2002, Abstract #4240.
13. Humphreys R, et al. TRAIL-R1 and TRAIL-R2 human agonistic monoclonal antibodies display invitro and invivo activity on human cancer cells. Society for Biological Therapy 2002; oral presentation.
14. (HGSI Press Release) Human Genome Sciences and SmithKline Beecham Sign Revised Genomics Collaboration Agreement. July 2, 1996 .
15. (HGSI Press Release) GlaxoSmithKline Exercises Option to LymphoStat-B. July 7, 2005 .
16. (HGSI Press Release) Human Genome Sciences Completes Enrollment in a Phase 2 Clinical Trial of HGS-ETR1 for the Treatment of Non-Hodgkin’s Lymphoma. March 3, 2005 .
17. (HGSI Press Release) Human Genome Sciences Completes Patient Enrollment in a Phase 2 Clinical Trial of HGS-ETR1 for the Treatment of Colorectal Cancer. February 23, 2005 .
18. (HGSI Press Release) Human Genome Sciences Completes Patient Enrollment in a Phase 2 Clinical Trial of HGS-ETR1 for the Treatment of Non-Small Cell Lung Cancer. November 30, 2004 .
19. Bonomi P, Greco FA, et al. Results of a Phase 2 trial of HGS-ETR1 (agonistic human monoclonal antibody to TRAIL receptor 1) in subjects with relapsed/recurrent non-small cell lung cancer. 11 th World Conference on Lung Cancer. July 4, 2005 . Abstract #1851.
20. (HGSI Press Release) Human Genome Sciences Reports Results of Phase 2 Clinical Trial of HGS-ETR1 in Patients with Non-Small Cell Lung Cancer. July 5, 2005 .
21. (HGSI Press Release) Human Genome Sciences Reports Interim Results of Phase 2 Clinical Trial of HGS-ETR1 in Patients with Advanced Non-Hodgkin’s Lymphoma. June 13, 2005 .
22. (HGSI Press Release) Cambridge Antibody Technology and Human Genome Sciences Announce Second Drug Partnership. January 8, 2002 .

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Kate de Santis
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