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The following is an excerpt from a S-1 SEC Filing, filed by ARENA PHARMACEUTICALS INC on 4/28/2000.

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BUSINESS

OUR COMPANY

We have developed a broadly applicable, platform technology that identifies drug candidates more efficiently than traditional drug discovery techniques. Our proprietary CART technology allows us to develop novel biochemical assays to discover drug candidates that target GPCRs, an important class of receptors. Additionally, we believe that CART is applicable to other human receptor classes, such as tyrosine kinase receptors, or TKRs, as well as to non-human receptors for the discovery of animal therapeutics and agricultural products.

In the recent past, the pharmaceutical and biotechnology industries have increasingly focused on receptor-based drug targets due to their potential for increased specificity and reduced side effects. Of the leading 100 pharmaceutical products, based on 1998 revenues, 33 target receptors as their primary mechanism of action. Of these 33 receptor-based drugs, 25 wholly or in part act on GPCRs. In 1998, these GPCR-based pharmaceutical products represented over $23 billion in sales, and included Claritin(RM) for allergies, Zantac(RM) for gastric ulcers, Imitrex(RM) for migraines and Cozaar(RM) for hypertension.

We use our CART technology to discover drug candidates by genetically altering, or CART-activating, receptors to mimic the biological response that occurs when the native ligand binds to the receptor. These CART-activated receptors are then used as a screening tool to identify chemical compounds that alter this biological response and that are the basis for drug candidates. Using CART technology, we have discovered drug candidates that have demonstrated pharmacological activity in pre-clinical, or animal, studies through our own internal research and drug development efforts, as well as through those of our collaborators. We have entered into collaborative relationships with a number of pharmaceutical and biotechnology companies, including Eli Lilly, Fujisawa, Lexicon Genetics and Neurocrine Biosciences.

THE DRUG DISCOVERY PROBLEM

Diseases in humans are caused by the abnormal function of cells. Both normal and abnormal cellular function is principally the result of communication between cells. This cellular communication occurs when a ligand is released from a cell and binds to a receptor on the surface of that cell or another cell. This binding triggers the initiation of various signals within that cell, resulting in changes in cellular function. By interacting with the receptor to mimic or block ligand-receptor binding, drugs affect abnormal cellular function and thereby regulate the disease process.

Receptors are classified into categories based upon similarities in their biochemical and structural properties. They are located in various tissues throughout the body and affect a variety of cellular functions. There are four principal classes of human receptors: GPCRs; TKRs; ligand-gated ion channel receptors; and intracellular receptors. Although we believe CART technology is applicable to all of the principal classes of receptors, we focus on GPCRs because they are the predominant class of receptors involved in cellular function.

The ligand that naturally binds to a receptor and activates or inhibits a biological response is referred to as a receptor's native ligand. A receptor for which the native ligand has been discovered is called a known receptor, while a receptor for which the native ligand has not been identified is called an orphan receptor. There are believed to be approximately 2,000 GPCRs within the human genome which are potential targets for drug development. Approximately 1,900 of these are orphan GPCRs.

Advances in genomics research have enabled researchers, including us, to directly identify the genetic sequence of previously unidentified receptors, including GPCRs, from basic genetic information. As more GPCRs are made available, the opportunity to use this information for drug discovery efforts should increase. However, although hundreds of new, orphan GPCRs are being made publicly available through genomics research, traditional drug discovery techniques to find new drug candidates cannot be applied to orphan GPCRs until the native ligands for these orphan GPCRs are identified.

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The process of identifying native ligands is very uncertain, generally involving many stages of tissue extraction and extensive purification. To our knowledge, only eight definitive examples exist where a novel native ligand has been discovered by intentionally targeting an orphan GPCR. Even when successful, identifying the native ligand typically requires four to five years and costs millions of dollars per GPCR. For example, a GPCR called GPR 14 was discovered in 1995, but its native ligand, urotensin II, was not identified until 1999. The process of identifying native ligands is typically the step that limits the rate at which drugs are discovered at receptor targets.

OUR SOLUTION -- CART TECHNOLOGY

We do not use, and therefore do not need to identify, the receptor's native ligand for our drug discovery efforts. We use our CART technology to discover drug candidates by CART-activating receptors to mimic the biological response that occurs when the native ligand binds to the receptor. Therefore, CART technology avoids a major bottleneck in drug discovery efforts at orphan receptors.

CART technology can be applied broadly to GPCRs because all GPCRs have highly similar structural elements, consisting of:

- three extracellular loops on the outside of the cell

- three intracellular loops on the inside of the cell

- seven regions that cross through the cell surface, or membrane, and connect the extracellular and intracellular loops

When a ligand binds to the extracellular portion of the GPCR, changes occur to the intracellular portion of the GPCR that permit a signaling molecule located within the cell, called a G protein, to bind to the intracellular portion of the GPCR. This leads to further intracellular changes, resulting in a biological response within the cell.

[DIAGRAM DEPICTING GPCR-MEDIATED BIOLOGICAL RESPONSE]

Under normal physiological conditions, a GPCR exists in equilibrium between two different states: an inactive state and an active state. When the GPCR's equilibrium shifts to an active state, the GPCR is able to link to a G protein, thus producing a biological response. When the GPCR's equilibrium shifts to an inactive state, the receptor is typically unable to link to a G protein, and therefore unable to produce a biological response. When a native ligand binds to the GPCR, the GPCR's equilibrium shifts and the GPCR is stabilized in the active state.

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[DIAGRAM DEPICTING LIGAND-DEPENDENT GPCR ACTIVATION]

By altering the genetic structure of a GPCR, our CART technology stabilizes the GPCR in the active state in the absence of the native ligand.

[DIAGRAM DEPICTING CART-ACTIVATED GPCR]

Drug screening and discovery targeting GPCRs using CART technology is comprised of four stages:

- altering the molecular structure of an intracellular loop or intracellular portion of the GPCR to generate a CART-activated form of the GPCR

- introducing the CART-activated form of the receptor into mammalian cells, which, in turn, manufacture the CART-activated form of these receptors at the cell surface

- analyzing the cells containing the CART-activated GPCR to detect biological responses that result from the linking of the CART-activated GPCR to a G protein

- screening chemical libraries of small molecule compounds against the cell membranes containing the CART-activated GPCR to identify compounds that interact with the GPCR

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Screening using CART technology allows us to simultaneously identify compounds that act as receptor inhibitors to decrease the detected biological responses, or act as receptor activators to increase the detected responses. Therefore, our CART technology allows us to discover drugs that either inhibit or enhance biological activity.

CART technology is also useful for identifying drug candidates that reduce cellular responses resulting from ligand-independent activity of receptors. These drugs are termed inverse agonists and are the preferred drugs for treating diseases in which ligand-independent receptor activity may be important, such as schizophrenia. In general, traditional ligand-based drug screening techniques can only be used to identify neutral antagonists, which do not affect the ligand-independent activity of the receptor. We can directly identify inverse agonists using our CART technology by screening for ligand-independent receptor activity. We believe the inverse agonists that we identify will possess improved properties over neutral antagonists because they inhibit both ligand-induced as well as ligand-independent activity.

In addition, because CART does not require the use of the native ligand, we are not limited to finding drug candidates that bind to a receptor at the receptor's ligand binding site. Instead, CART technology exposes the entire receptor surface to drug candidates, allowing for the detection of drug candidates which bind at any point on the receptor surface. We believe that this feature of CART technology is important not only with respect to orphan receptors, but known receptors as well, because it provides us with the ability to discover new drugs with unique mechanisms of action.

In summary, we believe that our platform CART technology offers several key advantages for drug discovery over other screening techniques. Screening CART-activated receptors:

- does not require prior identification of the native ligand for an orphan receptor

- enhances the detection of, and simultaneously identifies, both receptor inhibitor and receptor activator compounds

- provides the ability to discover novel and improved therapeutics at known receptor targets

- allows for the identification of compounds or drug candidates that inhibit both ligand-induced and ligand-independent activity

OUR STRATEGY

Our strategy is to become a leader in the development of novel receptor-based screening assays by using our proprietary CART technology to rapidly discover drug candidates. The major elements of our strategy are to:

Apply our CART technology to orphan GPCR targets to leverage available genomics information. Recent advances in genomics research have provided gene sequence information on an unprecedented number of receptor drug targets, including numerous previously unidentified GPCRs. CART technology can be applied to these orphan GPCR targets to discover drug candidates. This can be done more quickly and efficiently using CART technology than with traditional drug discovery screening techniques because drug discovery using CART does not require the identification and characterization of a receptor's native ligand, a process which typically requires several years and costs millions of dollars per receptor.

Discover new drug candidates that have unique mechanisms of action for known GPCRs. We believe that CART provides us with the ability to discover new drug candidates with unique mechanisms of action at known receptor targets, which may be more effective and may have fewer side effects than existing drugs. Unlike traditional drug discovery methods, we are not limited to finding drug candidates that bind to a receptor at the receptor's ligand binding site. Because CART technology exposes the entire GPCR surface to drug candidates, we can discover drug candidates that act upon any part of the receptor surface.

Develop multiple pharmaceutical product candidates for GPCR targets. CART technology allows us to identify drug candidates that act as receptor inhibitors to reduce biological activity, or receptor activators to increase biological activity. Therefore, CART provides the opportunity to simultaneously

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discover multiple different drug candidates with unique mechanisms of action for each receptor target, any of which may ultimately become successful commercial pharmaceutical products.

Enter into strategic collaborations to discover and develop novel drug candidates. We intend to enter into a number of strategic collaborations to discover and develop novel drug candidates using our CART technology. We believe that the broad applicability of our CART technology will allow us to enter into collaborations that focus on a variety of diseases and target a large number of orphan and known GPCRs. We have recently entered into collaborations with Eli Lilly and Fujisawa under which we will CART-activate a significant number of GPCR targets and may receive revenues in the form of development fees, milestone payments and royalties on products, if any, developed to target these GPCRs.

Apply our CART technology to other human receptors and non-human receptors for human therapeutic, agricultural and other applications. CART technology can also be applied to other types of human receptors, such as TKRs, which are often implicated as important factors in various diseases, such as breast cancer. We are also applying our CART technology to non-human receptors for a variety of applications including plant receptors to discover chemical growth factors, insect receptors to discover insect control agents and viral receptors to discover novel anti-viral drug candidates. We have CART-activated a number of these other types of receptors and intend to pursue opportunities developed from these receptors.

Continue to protect and expand our intellectual property rights. We have filed 96 independent patent applications with the United States Patent and Trademark Office, and are filing some of these patent applications worldwide. Although no patents have been issued to us, we believe that we can obtain patents on our CART technology and that we can obtain patents on our CART-activated receptors because our technology genetically modifies these receptors and changes their function. We intend to continually seek ways to vigorously protect and enforce our rights with respect to our intellectual property.

APPLICATIONS OF OUR CART TECHNOLOGY

Over the past three years, we have obtained the full-length genetic sequences of 235 GPCRs and made them available for CART-activation and screening. Of these, 120 are human orphan GPCRs and 110 are human known GPCRs. The remaining five are non-human receptors, including plant, viral and insect receptors. Through the use of our proprietary CART technology, we have successfully identified drug compounds that inhibit or activate a number of known and orphan receptor targets.

Orphan GPCRs

An important element of our CART technology involves using the genetic sequences of orphan GPCRs to understand and define the tissue and cellular distribution of these GPCRs. The genetic sequences provide us with the necessary tools to locate the orphan receptors in tissues. Once we have identified the location of an orphan receptor in tissues, we can determine the normal function of the orphan receptor and compare that function to the function of the orphan GPCR in diseased tissues. We then use our CART technology to screen the targeted receptor for drug candidates that can be employed to verify the proposed receptor function.

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We have prioritized and applied our CART technology to 15 orphan GPCRs, identified below by our code names, as having high potential value as drug discovery targets against specific diseases or indications, based upon the distribution of the GPCR in specified tissues. These GPCRs and their intended disease targets are:

--------------------------------------------------------------------------------------------- INDICATION ORPHAN GPCRS TISSUE DISTRIBUTION Obesity 18F Hypothalamus ----------------------------------------------------------------- 19U Hypothalamus ----------------------------------------------------------------- 19X Forebrain ----------------------------------------------------------------- 19NY Hypothalamus --------------------------------------------------------------------------------------------- Cancer: Ovarian 18A Adrenal/ovary ----------------------------------------------------------------- Colorectal 18AI Intestine ----------------------------------------------------------------- Osteosarcoma 19AG Bone/pancreas ----------------------------------------------------------------- Leiomyoma 19Y Uterus ----------------------------------------------------------------- Breast 18AD Breast --------------------------------------------------------------------------------------------- Cardiovascular disease 18C Blood vessels ----------------------------------------------------------------- 18D Heart/other --------------------------------------------------------------------------------------------- Diabetes 19AJ Pancreas --------------------------------------------------------------------------------------------- Inflammation 18AF Spleen/lymph nodes/brain ----------------------------------------------------------------- 19W T cells --------------------------------------------------------------------------------------------- Alzheimer's Disease 18L Hippocampus ---------------------------------------------------------------------------------------------

Obesity. National Institutes of Health statistics indicate that approximately 100 million adults in the United States are overweight and that 25% of these are considered clinically obese. The few currently approved drugs for the treatment of obesity in the United States act either as appetite suppressants or blockers of fat absorption. However, cardiovascular or gastrointestinal side effects may limit the long-term effectiveness of these drugs. Consequently, more effective therapeutics are urgently needed for this major public health problem.

We have an ongoing program directed towards the development of novel anti-obesity drugs. We have identified a number of orphan GPCRs on brain cells related to the control of feeding and metabolism, including the 18F, 19U, 19X and 19NY GPCRs. For example, we have discovered an over-abundance of the 18F GPCR in the brain metabolism centers of genetically obese rats. We believe that this discovery indicates that overactivity of this GPCR may be associated with obesity.

We are using our CART technology to identify drug candidates that inhibit the activity of the 18F GPCR. Repeated administration of the drug candidates we have identified has resulted in reduced food intake and sustained weight loss in normal laboratory animals. Similar results were also obtained in a diet-induced animal model of human obesity. In this diet-induced animal model, these drug candidates also acted to increase fat metabolism and resulted specifically in a loss of fat mass. We have found that the 18F GPCR is also located on human fat cells. Therefore, we believe that these drug candidates may provide the basis for a novel approach to the treatment of human obesity by simultaneously reducing food intake and increasing fat metabolism. Additionally, our animal data indicate that our drug candidates may not have the same side effects that are associated with currently available anti-obesity drugs.

Our anti-obesity drug program demonstrates the advantages of CART technology for rapid drug candidate discovery. The process of discovering promising drug candidates took approximately 18 months from our initial discovery of the over-abundance of the 18F GPCR in genetically obese animals to the animal testing of the drug candidates that we discovered using our CART technology. We intend to enter into one or more collaborations to further expand our anti-obesity drug program with the ultimate goal of

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selecting one or more of our novel anti-obesity drug candidates that target orphan GPCRs, such as the 18F GPCR, for future clinical development.

Cancer. We have identified several orphan GPCRs, including the 18A, 18AI, 19AG and 19Y GPCRs, which we believe represent therapeutic targets for the treatment of a variety of cancers. These orphan GPCRs are attractive therapeutic targets because they have been shown to be present at abnormally high levels in ovarian, colorectal, gastrointestinal and uterine cancer cells and cause unwanted proliferation of cells in laboratory experiments. Additionally, we have identified another orphan GPCR, the 18AD GPCR, which potentially suppresses the growth of tumor cells. We determined that there are high levels of the 18AD GPCR present in tissues adjacent to cancerous breast tissue. We tested a CART- activated form of the 18AD GPCR in a non-animal, or in vitro, cancer model and showed that this GPCR suppressed cell proliferation. This suggests to us that drug candidates that activate the function of the 18AD GPCR should inhibit tumor growth, providing a potential new treatment approach for breast and other cancers.

Cardiovascular disease. We have identified several orphan GPCRs, including the 18C and 18D GPCRs, that are located within the cardiovascular system, such as on heart tissues and blood vessel walls. Blood pressure is regulated by constriction or relaxation of blood vessels which is effected by GPCRs located on blood vessel walls. We believe that some of the drug candidates that we have identified using CART technology that target these GPCRs have potential to regulate blood pressure and treat diseases such as hypertension. According to the National Institutes of Health, hypertension affects approximately 50 million people in the United States, and can lead to heart disease and stroke, which are among the leading causes of death in the United States.

Diabetes. One of the orphan GPCRs that we discovered, the 19AJ GPCR, is specifically located on pancreatic, insulin producing, beta cells. Normally, glucose stimulates the beta cell to produce insulin, but in diabetes the beta cell often becomes less sensitive to glucose and the ability of the beta cell to produce insulin is impaired. The 19AJ GPCR appears to make the beta cells more responsive to glucose concentrations, resulting in enhanced insulin release. By applying CART technology to the 19AJ GPCR we will seek to discover drug candidates to treat diabetes, which, according to the National Institutes of Health, affected approximately 15.7 million people in the United States in 1997.

Inflammation. We have identified several orphan GPCRs, including the 18AF and 19W GPCRs, that may mediate inflammatory responses in various locations of the body. Our preliminary data suggest that the 18AF GPCR may regulate brain cells related to inflammation. Based upon its sequence structure, the 18AF GPCR appears to be related to a group of GPCRs called chemokine receptors. Chemokine receptors are known to be involved in the inflammation process, and brain inflammation is involved in a number of neurodegenerative disorders, including stroke. The number of 19W GPCRs is increased in dying cells during inflammation, suggesting that the 19W GPCR may be involved in controlling the process of cell death. We have CART-activated the 19W GPCR and have developed an assay for screening of chemical compounds against this GPCR. Drug candidates that modulate the activity of these GPCRs may provide a unique therapeutic approach to the treatment or mediation of inflammatory responses. According to the National Institutes of Health, diseases involving inflammation afflict over 25 million people in the United States.

Alzheimer's Disease. Several of our orphan GPCR targets are located on cells within the central nervous system, including the 18L GPCR. The 18L GPCR is located on nerve cells in an area of the brain called the hippocampus, which is responsible for controlling memory function. In Alzheimer's Disease, normal memory processes in the hippocampus are severely impaired. We believe drugs that modulate the 18L GPCR could be useful for controlling memory function and for the treatment of symptoms of Alzheimer's Disease, which, according to the National Institutes of Health, affects four million people in the United States.

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Known GPCRs

Although we primarily focus on orphan GPCRs, we also apply our CART technology to known GPCRs. We believe that the application of our CART technology to known GPCRs will identify novel classes of drug candidates that may be more effective and may have fewer side effects than existing drugs that target known GPCRs.

Our principal advantage in applying CART technology to known GPCRs is our ability to directly identify drug candidates that act as inverse agonists, which cannot be directly identified using traditional ligand-based screening techniques. Inverse agonists are particularly relevant in treating diseases in which ligand-independent GPCR activity, or overactivity, is implicated. Diseases in which we believe overactive known GPCRs are implicated include:

DISEASE OVERACTIVE KNOWN GPCRS Schizophrenia 5HT(2A), D(4) -------------------------------------------------------------------- Depression 5HT(2A) -------------------------------------------------------------------- Hyperthyroidism Thyrotropin -------------------------------------------------------------------- Hypertension Angiotensin AT(1A) -------------------------------------------------------------------- Asthma Adenosine A(1) -------------------------------------------------------------------- Melanoma MC-1 -------------------------------------------------------------------- Retinitis Pigmentosa Rhodopsin --------------------------------------------------------------------

We have identified drug candidates that are capable of inhibiting both ligand-independent and ligand-dependent activity at selected known GPCR targets. We are currently developing drug candidates that target these overactive known GPCRs to treat the related diseases. Our most advanced program targets the serotonin 5HT(2A) GPCR for potential treatment of schizophrenia and other psychotic disorders.

Psychosis. According to the National Institutes of Health, approximately 2.7 million people in the United States suffer from schizophrenia. We have tested currently available anti-psychotic drugs and have found that they act as inverse agonists at a known GPCR, referred to as the 5HT(2A) GPCR. Using our CART technology, we have discovered and are developing a number of new drug candidates that act as inverse agonists at the 5HT(2A) GPCR. These drug candidates displayed activities in tests involving laboratory animals indicating that they would be useful in treating psychiatric disorders such as schizophrenia. Moreover, our CART-identified 5HT(2A) inverse agonists possess a higher degree of receptor selectivity than currently marketed anti-psychotics, which suggests our inverse agonists may be more effective. To date, these drug candidates exhibit no evidence of side effects in laboratory animals.

Our anti-psychotic drug program also demonstrates the advantages of CART technology for rapid drug candidate discovery. The process of discovering promising drug candidates took approximately 18 months from the application of our CART technology to the 5HT(2A) GPCR to the animal testing of the drug candidates that we discovered using CART technology. We intend to enter into a collaboration to further expand our anti-psychotic drug program with the goal of selecting one or more of our novel anti-psychotic drug candidates that target the 5HT(2A) GPCR, for future clinical development.

Other areas of CART application

Olfactory and taste GPCRs. A specialized multigene family of GPCRs has been identified in the nasal membrane and is responsible for the sense of smell. Another family of GPCRs has recently been discovered in the tongue and is believed to be responsible for the perception of taste. We are applying our CART technology to a number of olfactory and taste GPCRs to identify novel compounds that we believe will be of potential commercial value in the fragrance and food additive industries.

Plant GPCRs. Plants respond to a variety of environmental and internal signals that regulate aspects of their growth and development. GPCRs have recently been identified in a variety of plants and have

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been implicated in the action of a variety of plant hormones. We are presently applying our CART technology to plant GPCRs in an attempt to identify novel regulators of the life cycle of plants that may affect crop growth and development.

Viral GPCRs. GPCRs are involved in either replication or infection in a number of viruses. Some herpes viruses, including the Kaposi's sarcoma-associated virus, have GPCRs within their genome which are important for replication. Other GPCRs appear necessary for primary infection. For example, HIV infects cells by binding to a GPCR that transports the virus into cells. A number of orphan GPCRs have been identified which appear to act in a similar manner for other viruses. Our goal is to identify novel anti-viral drugs using CART technology.

Insect GPCRs. Insect genomes also include GPCRs, and we have begun the process of applying CART technology to insect GPCRs in an attempt to identify compounds that may offer the potential for improved and environmentally safer insect control agents. Our goal is to use CART-activated insect GPCRs to find compounds that selectively reduce pest reproduction and feeding behavior.

Tyrosine kinase receptors. In addition to applying our CART technology to orphan GPCRs, we are also applying our CART technology to other human receptor classes, including orphan TKRs. A number of orphan TKRs have been located on cancerous tissues and may be involved in excessive cell proliferation and growth. As with GPCRs, our CART technology allows us to activate orphan TKRs in the absence of native ligands and screen the activated TKRs to identify novel inhibitors of TKR activity. We are currently evaluating nine orphan TKRs for drug screening.

In vivo genetic "knock-in" models. We are collaborating with Lexicon Genetics to develop mice that produce CART-activated GPCRs, or GPCR knock-ins, by using state-of-the-art molecular genetic techniques. By producing CART-activated orphan GPCRs in animals, we believe that we will gain valuable insight into the functionality of individual GPCRs, as well as indications of human disease for which drugs that target these GPCRs may be useful. In addition, we expect that these knock-in animals will provide an animal model that can be used to test the in vivo potency of drug candidates discovered using CART-activated GPCRs.

OUR GPCR COLLABORATORS

Our success will depend in large part upon our ability to enter into successful collaborations with other pharmaceutical and biotechnology companies. We are active in the scientific community and within the industry and regularly make presentations regarding our research and development programs and the applications of our CART technology at scientific conferences and industry conventions. We believe that our participation at these events has led, and will continue to lead, to contacts with existing and potential collaborators. We have entered into a number of strategic collaborations in the recent past to discover novel drug candidates using our CART technology, and we expect to enter into additional collaborations and expand our existing collaborations in the future.

Eli Lilly

In April 2000, we entered into a research alliance with Eli Lilly, one of the world's leading pharmaceutical companies. Our collaboration with Eli Lilly will principally focus on the central nervous system and endocrine therapeutic fields. We will also focus on the cardiovascular field and may expand our collaboration to other therapy classes, including cancer.

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During our collaboration, we will pursue an agreed upon research plan with Eli Lilly that has several objectives. During the term of our collaboration, we will mutually review and select GPCRs that will become subject to the collaboration. These GPCRs may be provided either by us or by Eli Lilly. All of our existing CART-activated GPCRs are excluded from the collaboration. We and Eli Lilly will each share our respective knowledge of the GPCRs that become subject to the collaboration to validate and CART-activate selected receptors. We will jointly select a number of proprietary central nervous system, endocrine and cardiovascular GPCRs for CART-activation, and we will then provide Eli Lilly with enabled high-throughput screens for use at their screening facilities. We will receive research funding from Eli Lilly for our internal resources committed to these tasks, which will be augmented by substantial resource commitments by Eli Lilly. Eli Lilly will be responsible for screening of its chemical compound library using selected CART-activated receptors, for identifying drug candidates and for the pre-clinical and clinical testing and development of drug candidates. We will receive a technology access fee, and, if various milestones are achieved, we are entitled to receive development fees per GPCR assay provided to Eli Lilly, development milestone payments per drug discovered and advanced into clinical trials, if any, and royalties depending on the sales levels of drugs, if any.

Once the assay development fee has been paid for a CART-activated GPCR, Eli Lilly will have exclusive rights to screen chemical libraries, discover drug candidates that target that GPCR, and to develop, register and sell any resulting products worldwide. We retain rights to partner or independently develop GPCRs that do not become subject to the collaboration.

The term of our collaboration agreement with Eli Lilly is five years beginning on the effective date of the agreement. Eli Lilly can terminate the agreement with or without cause by giving us written notice during the first nine months after the date of the agreement, and this termination would be effective one year after the date of the agreement. If Eli Lilly terminates the agreement without cause during this time, it must pay us a termination fee. Either Eli Lilly or we can terminate the agreement with or without cause effective three years after the date of the agreement by giving written notice prior to the conclusion of the 33rd month after the effective date of the agreement. In addition, either party can terminate the agreement at any time if the other party commits a material breach, and Eli Lilly can terminate the agreement at any time if, among other reasons, key staff leave our employ. The parties will continue to have various rights and obligations under the agreement after the agreement is terminated. The extent of these continuing rights and obligations depends on many factors, such as when the agreement is terminated, by which party and for what reason. These continuing obligations may include further research and development efforts by us and a variety of payments by Eli Lilly.

Fujisawa

In January 2000, we entered into a collaborative agreement with Fujisawa, a leading Japan-based pharmaceutical company with significant drug discovery research efforts. During the collaboration, we will jointly validate up to 13 orphan GPCRs as drug screening targets. We will be responsible for receptor identification, location and regulation, and will apply our CART technology to GPCRs selected by Fujisawa. We will also seek to validate screening assays based on the selected GPCRs. Fujisawa will be entitled to screen selected assays against its chemical compound library to identify drug candidates. Fujisawa will also be responsible for the pre-clinical and clinical development of any drug candidates that we or Fujisawa discover. We may also screen the selected GPCRs using our in-house chemical library. If we or Fujisawa achieve various milestones, we will receive research and development fees, including milestone payments per drug candidate discovered and advanced into clinical trials, and royalties depending on drug sales, if any.

Our collaborative agreement with Fujisawa will terminate upon the expiration of Fujisawa's obligation to make royalty payments under the agreement, if any. Fujisawa may terminate the agreement at any time by providing us with written notice of their intention to do so and by returning any proprietary rights they have acquired under the agreement. Additionally, either party may terminate the agreement for a material breach of the agreement by the other party. The termination or expiration of the agreement will not affect any rights that have accrued to the benefit of either party prior to the termination or expiration.

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Lexicon Genetics

In April 2000, we signed a binding letter of intent and memorandum of agreement with Lexicon Genetics, a genomics company that uses a proprietary technology to clone mice, enabling the large-scale definition of gene function. The agreement establishes a research collaboration with Lexicon Genetics using their proprietary technology to clone gene-targeted mice whose genomes have been altered using specified CART-activated orphan GPCRs. Our collaboration with Lexicon Genetics consists of a feasibility phase to determine both the utility of this novel approach and the scope of any resulting licensing alliance. If we proceed beyond the feasibility stage, the agreement establishes a licensing alliance in which we and Lexicon Genetics will each contribute up to ten unique GPCRs to clone mice containing CART-activated GPCRs for use as drug discovery tools, and to discover drug candidates using these GPCRs. We will share equally in the fees, milestones and royalties generated from any licensing agreement with a third-party involving GPCRs developed through our licensing alliance.

Neurocrine Biosciences

In September 1999, we entered into an agreement with Neurocrine Biosciences, a biotechnology company focused on the discovery and development of novel therapeutics to treat diseases and disorders of the central nervous system and immune system. Neurocrine Biosciences has the right, prior to September 2000, to provide us with up to three orphan GPCRs obtained by them to which we will apply our CART technology. We will not receive any payment for CART-activating these GPCRs. We may also screen the activated orphan GPCRs using our in-house chemical library, or using Neurocrine Biosciences' in-house chemical library, for which we will receive payments. Neurocrine Biosciences will retain the rights to the CART-activated GPCRs and any drug candidates that are discovered using these GPCRs.

OTHER ELEMENTS OF OUR BUSINESS

T-82

In 1998, we licensed the rights from SSP to develop T-82, a novel drug candidate to treat Alzheimer's Disease. We believe T-82 possesses a unique pharmacological profile that will translate into enhanced therapeutic activity over currently available drugs for Alzheimer's Disease. We have completed three Phase I clinical trials of T-82, involving single and multiple dose studies, as well as a study to evaluate the possible interaction of T-82 with food. We intend to begin Phase II clinical testing of T-82 in 2000 and to enter into a collaborative relationship with a third-party to further develop T-82.

Currently available drugs to treat Alzheimer's Disease inhibit the enzyme acetylcholinesterase. Acetylcholinesterase breaks down acetylcholine, an important chemical for normal memory and cognitive function. By inhibiting the enzyme acetylcholinesterase, currently available drugs prevent the breakdown of acetylcholine. T-82 also acts to prevent the breakdown of acetylcholine by inhibiting acetylcholinesterase, but unlike existing drugs which have limited clinical effectiveness, we believe T-82 may cause the release of additional acetylcholine from nerve terminals. This additional mechanism results from the ability of T-82 to block a serotonin receptor, 5HT(3), which normally acts to inhibit the release of acetylcholine from nerve terminals. Moreover, the side-effects of currently-approved Alzheimer's Disease drugs include drug- induced nausea and vomiting, which may cause some patients to reduce or discontinue their use of the drugs. We believe T-82 possesses a pharmacological advantage over current Alzheimer's drugs because of evidence that inhibition of the 5HT(3) receptor not only enhances acetylcholine release, but also reduces vomiting by inhibiting certain neural pathways.

We have worldwide rights to clinically develop and market T-82, except in Japan. We share rights with SSP to license and market T-82 in ten Asian countries. We will share data related to pre-clinical studies, clinical studies and the manufacturing of T-82 with SSP, and SSP will share similar data developed by SSP with us. SSP has agreed to manufacture and supply T-82 to us for all clinical trials, at no cost to us. Our license requires us to make milestone payments to SSP upon the successful completion of Phase II clinical studies, after successful completion of Phase III clinical studies and, if applicable, after

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receiving marketing approval by the FDA and European regulatory agencies. In addition, we will pay SSP escalating royalties based upon net sales of T-82, if any.

Aressa

In August 1999, we formed Aressa, a wholly-owned subsidiary, to take advantage of opportunities to in-license and develop niche products from other pharmaceutical or biotechnology companies. In November 1999, Aressa entered into a licensing agreement with SSP with respect to a patented anti-fungal compound discovered by SSP, called SS750. Aressa is currently supervising pre-clinical studies of SS750, which are funded by SSP. Aressa may explore other opportunities to in-license niche products.

ChemNavigator.com

In early 1999, we developed an Internet-based search engine that allows scientists to search for chemical compounds based primarily on the similarity of chemical structures. We believe this is important for drug discovery purposes because chemical similarity can be used as an indicator of biological activity. ChemNavigator.com was formed in May 1999 and subsequently obtained independent third-party financing. We licensed the search engine's underlying technology and related intellectual property to ChemNavigator.com in exchange for stock. We currently beneficially own approximately 46% of the outstanding common stock of ChemNavigator.com.

INTELLECTUAL PROPERTY

Our success depends in large part on our ability to protect our proprietary technology and information, and operate without infringing on the proprietary rights of third parties. We rely on a combination of patent, trade secret, copyright and trademark laws, as well as confidentiality agreements, licensing agreements and other agreements, to establish and protect our proprietary rights. Since our inception, we have filed 96 patent applications in the United States regarding our:

- CART technology

- orphan receptors and CART-activated orphan receptors

- CART-activated known receptors

- small molecule chemical compounds

- acetylcholine enhancers

- web-based search engine technologies

The term of all of our patents, if any are issued, will commence on the date of issuance and terminate 20 years from the earliest effective filing date of the patent application. Because the time from filing to issuance of biotechnology patent applications is often more than three years, our patent protection, if any, on our products and technologies may be substantially less than 20 years.

We seek patent protection for all of our key inventions, including our CART technology, new receptors that we discover, genetically-altered receptors, and drug candidates identified by our CART technology. It has been possible to obtain broad, composition-of-matter patents on novel chemical compounds, such as the drug candidates that we identify using our CART technology. It has also been possible to obtain broad method patents for techniques and procedures for screening and drug-identification technologies, such as those embodied by our CART technology. It has generally not been possible to obtain broad composition-of-matter patents for nucleic acid and amino acid sequences. However, it has been possible to obtain patents that protect specific sequences and functional equivalents of those sequences. Furthermore, intellectual property law allows for separate and distinct patents for altered genetic sequences over previously disclosed sequences. We believe that we can obtain patents on our CART-activated receptor sequences because they are not functional equivalents of the receptor that exists in nature. We have filed and will continue to file patent applications on these types of technologies. We believe that our CART technology does not infringe on third-party claims related to any aspect of our proprietary technology.

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As a general matter, obtaining patents in the biotechnology and pharmaceutical fields is highly uncertain and involves complex legal, scientific and factual matters. Obtaining a patent in the United States in the biotechnology and pharmaceutical fields can be expensive and can, and often does, require several years to complete. Our patent filings in the United States may be subject to interference or reexamination proceedings. The defense and prosecution of interference and reexamination proceedings and related legal and administrative proceedings in the United States involve complex legal and factual questions. We also file patent applications outside of the United States. The laws of some foreign countries may not protect our proprietary rights to the same extent as do the laws of the United States. Third parties may attempt to oppose the issuance of our patents in foreign countries by way of opposition proceedings. Additionally, if an opposition proceeding is initiated against any of our patent filings in a foreign country, that proceeding could have an adverse effect on the corresponding patents that are issued or pending in the United States. If we become involved in any interference, reexamination, opposition or litigation proceedings in the United States or foreign countries regarding patent or other proprietary rights, those proceedings may result in substantial cost to us, regardless of the outcome, and may have a material adverse affect on our ability to develop, manufacture, market or license our technologies or products, or to maintain or form strategic alliances.

Although we plan to aggressively prosecute our patent applications and defend our patents against third-party infringement, we cannot assure you that any of our patent applications will result in the issuance of patents or that, if issued, such patents will not be challenged, invalidated or circumvented. Moreover, we cannot assure you that our patents, if any, will provide us protection against competitors with other technologies. Our technologies and potential products may conflict with patents that have been or may be granted to competitors, universities or others. As the biotechnology industry expands and more patents are issued, the risk increases that our technologies and potential products may give rise to claims that they infringe the patents of others. Third parties claiming infringement of their proprietary rights could bring legal actions against us claiming damages and seeking to enjoin our use or commercialization of a product or our use of a technology. In particular, patent applications or patents for innovative and broadly applicable technologies, such as our CART technology, are sometimes challenged by third parties as obvious, or as obvious extensions of technologies previously developed by those third parties. We cannot assure you that such claims will not be brought against us in the future. If any actions based on these claims are successful, in addition to any potential liability for damages, we could be required to obtain a license in order to continue to use a technology or to manufacture or market a product, or could be required to cease using those products or technologies. Any claim, with or without merit, could result in costly litigation and divert the efforts and attention of our scientific and management personnel. We cannot assure you that we would prevail in any action or that any license required under any patent would be made available or would be made available on acceptable terms.

All of our employees are required to enter into and adhere to an employment-confidentiality and invention-assignment agreement, laboratory notebook policy, and invention disclosure protocol, as a condition of employment. Additionally, our employment-confidentiality and invention-assignment agreement requires that our employees do not bring to Arena, or use without proper authorization, any third-party proprietary technology. We also require all of our consultants and collaborators that have access to proprietary property to execute confidentiality and invention rights agreements in our favor before beginning their relationship with us. While such arrangements are intended to enable us to better control the use and disclosure of our proprietary property and provide for our ownership of proprietary technology developed on its behalf, they may not provide us with meaningful protection for such property and technology in the event of unauthorized use or disclosure.

We have obtained a worldwide license from SSP, except for Japan, to issued and pending patents with claims directed to the chemical composition of T-82, and Aressa has obtained a similar license to issued and pending patents with claims directed to the chemical composition of SS750. We have also obtained an exclusive, worldwide license from Albany Medical College to a pending patent application with claims directed to mutations to several known GPCRs. In addition, we have entered into a research agreement with the University of Glasgow to jointly develop screening strategies using our CART-

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activated GPCRs, combined with techniques claimed in a patent application owned by the University. Under this agreement, we have an option to take an exclusive license to this patent application, as well as techniques that are developed during the course of the research agreement. Although neither we nor Aressa are currently in default under any of these agreements, we cannot assure you that we or Aressa will not default under these agreements in the future. Should such a default occur, our licenses could be terminated and we could lose the right to continue to develop T-82, SS750, or our other products or technologies that are subject to these agreements. The loss of our rights to develop T-82, SS750 or our other licensed products or technologies could harm our business.