Arena Pharmaceuticals, Inc.
Oct 11, 2005

Arena Pharmaceuticals Issued Patent for 5-HT2C Receptor Modulators by United States Patent Office

APD356, Arena's Phase 2b Small Molecule for Treatment of Obesity, Covered by Patent

Arena Pharmaceuticals Issued Patent for 5-HT2C Receptor Modulators by United States Patent Office<br>

SAN DIEGO, Oct. 11 /PRNewswire-FirstCall/ -- Arena Pharmaceuticals, Inc. (Nasdaq: ARNA) announced today that it was issued US patent number 6,953,787, entitled "5-HT2C Receptor Modulators," by the United States Patent and Trademark Office. The patent relates to novel compounds that modulate the 5-HT2C serotonin receptor, which helps regulate food intake and may also influence metabolic rate. These modulators could be useful in pharmaceutical compositions for the treatment of obesity. APD356, discovered by Arena, is a selective and orally available 5-HT2C receptor modulator covered by the patent. A 12-week, randomized, double-blind, dose-ranging Phase 2b clinical trial studying APD356 in obese patients is currently ongoing.

"This patent, along with a similar patent granted by the European Patent Office earlier this year, provides important protection for our lead drug candidate, APD356," commented Jack Lief, Arena's President and Chief Executive Officer. "We are looking forward to announcing top-line, longer term safety and efficacy data from our Phase 2b clinical trial of APD356 around the end of the year."

APD356 stimulates the 5-HT2C serotonin receptor, a G protein-coupled receptor located in the part of the brain called the hypothalamus, which helps regulate food intake and may also influence metabolic rate. APD356 has approximately 100-fold selectivity in vitro for the 5-HT2C receptor relative to the 5-HT2B receptor. Arena believes the 5-HT2B receptor is primarily implicated in the cardiac valvulopathy observed with non-selective serotonergic agents. In addition, APD356 has approximately 15-fold selectivity in vitro for the 5-HT2C receptor versus the 5-HT2A receptor, a central nervous system (CNS) receptor thought to be responsible for many of the CNS adverse effects of non-selective serotonergic agents. Arena believes that the selectivity of APD356 for the 5-HT2C receptor will allow the compound to be dosed at a well-tolerated level that will induce clinically relevant weight loss without the side effects observed with non-selective serotonergic agents.

About Obesity

Obesity and a related condition known as metabolic syndrome affect tens of millions of adults and children in the U.S. and pose a serious long-term threat to their health and welfare. The number of overweight and obese people has substantially increased over the past few decades. Approximately two-thirds of all adults in the U.S. are obese or overweight and 16% of children and adolescents ages 6 to 19 in the U.S. are overweight. Medical and related costs of obesity in the U.S. were more than $117 billion in 2000. Being obese or overweight is associated with a number of conditions, including heart disease, stroke, diabetes, cancer and osteoarthritis. Medical treatment options for obesity and metabolic syndrome are currently limited.

About Arena Pharmaceuticals

Arena is a clinical-stage biopharmaceutical company focusing on the discovery, development and commercialization of small molecule drugs in four major therapeutic areas: metabolic, cardiovascular, inflammatory and central nervous system diseases. Arena is developing a broad pipeline of compounds that act on an important class of drug targets called G protein-coupled receptors, or GPCRs, using its knowledge of GPCRs and its technologies, including CART (Constitutively Activated Receptor Technology) and Melanophore. Arena has three internally discovered, clinical-stage product candidates. Arena's most advanced clinical compound, APD356, a selective 5-HT2C serotonin receptor agonist for the treatment of obesity, is in Phase 2b. Arena's lead product candidate for the treatment of insomnia, APD125, a compound with a novel mechanism of action (a selective 5-HT2A receptor inverse agonist), is scheduled to begin a Phase 2 clinical trial by the end of 2005. As part of Arena's partnership with Merck & Co., Inc., an Arena product candidate for the treatment of atherosclerosis and related disorders is in a Phase 1 clinical trial. Arena also has an active collaboration with Ortho-McNeil, Inc., a Johnson & Johnson company, for the treatment of type 2 diabetes.

Arena Pharmaceuticals® and Arena® are registered service marks of the company. CART is an unregistered service mark of the company.

Forward-Looking Statements

Certain statements in this press release are forward-looking statements that involve a number of risks and uncertainties. Such forward-looking statements include statements about the possible usefulness of 5-HT2C modulators in pharmaceutical compositions for the treatment of obesity, APD356's safety profile, potency and pharmaceutical behavior, the significance of, and the protection provided by APD356 patents, and the expected timing of Arena's clinical trials and announcement of results and other statements about Arena's strategy, technologies, preclinical and clinical programs, and ability to develop compounds and commercialize drugs. For such statements, Arena claims the protection of the Private Securities Litigation Reform Act of 1995. Actual events or results may differ materially from Arena's expectations. Factors that could cause actual results to differ materially from the forward-looking statements include, but are not limited to, the results of Arena's clinical trials, the results of preclinical studies or clinical trials may not be predictive of future results, Arena's ability to partner APD356, APD125 or other of its compounds or programs, the timing, success and cost of Arena's research, out-licensing endeavors and clinical trials, Arena's ability to obtain additional financing, Arena's ability to obtain and defend its patents, Arena's patent protection, and the timing and receipt of payments and fees, if any, from Arena's collaborators, including Ortho-McNeil and Merck. Additional factors that could cause actual results to differ materially from those stated or implied by Arena's forward-looking statements are disclosed in Arena's filings with the Securities and Exchange Commission. These forward-looking statements represent Arena's judgment as of the time of this release. Arena disclaims any intent or obligation to update these forward- looking statements, other than as may be required under applicable law.

SOURCE Arena Pharmaceuticals, Inc.
10/11/2005
CONTACT: Jack Lief, President and CEO, or David Walsey, Director, Corporate Communications, both of Arena Pharmaceuticals, +1-858-453-7200, ext. 1682; or Media & Investor Relations, Carin Canale of Atkins + Associates, +1-858-527-3498, for Arena Pharmaceuticals, Inc.
Web site: http://www.arenapharm.com