Jefferies 2015 Health Care ConferenceNASDAQ: ARNACraig M. Audet, PhD | Sr. VP, Operations & Head of Global Regulatory Affairs

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PROPRIETARY | 2

Forward-Looking Statements

This presentation includes forward-looking statements that involve a number of risks and uncertainties, including statements about our and others’ focus, mission, goals, strategy, plans, timelines, milestones and expectations; the potential, opportunity, significance, value, safety, efficacy, indication, research and development (R&D), advancement, differentiation, status and related timing, results, regulatory activities and commercialization with respect to BELVIQ or lorcaserin, R&D programs and GPCR drugs in general; need for new treatments, addressing such need and impacting lives; commercializing BELVIQ, including with respect to focus, strategy, pricing, reimbursement, sales/prescriptions, supply, marketing, sales force, and outreach and awareness; collaborations, including related strategy, goals, progress, potential, activities, commitment, revenues, payments and expectations; financial guidance; intellectual property; our ability to discover and develop compounds and supply and commercialize drugs; and other statements that are nothistorical facts, including statements that may include words such as “may,” “will,” “intend,” “plan,” “expect,” “potential” or other similar words. For such statements, we claim the protection of the Private Securities Litigation Reform Act of 1995. Actual events or results may differ materially from expectations, and you are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the time they were made. Factors that could cause actual results to differ materially from such statements include, without limitation, risks related to commercializing drugs, including regulatory, manufacturing, supply and marketing issues and BELVIQ’s or lorcaserin’s availability and use; cash and revenues generated from BELVIQ; our revenues are based in part on estimates, judgment and accounting policies, and incorrect estimates or disagreement regarding estimates or accounting policies may result in changes to guidance or previously reported results; the timing and outcome of regulatory review is uncertain, and lorcaserin may not be approved for marketing when expected or ever in combination, for another indication or using a different formulation or in any other territory for any indication; regulatory decisions in one territory may impact other regulatory decisions and business prospects; reimbursement and pricing decisions; risks related to relying on collaborative arrangements; payments, if any, from collaborators; the entry into or modification or termination of collaborative arrangements; unexpected or unfavorable new data; nonclinical and clinical data is voluminous and detailed, and regulatory agencies may interpret or weigh data differently and reach different conclusions than we or others, request additional information, have additional recommendations or change their guidance or requirements; information related to R&D may not meet regulatory requirements or otherwise be sufficient for (or we or a collaborator may not pursue) further R&D, regulatory review or approval or continued marketing; our and third parties’ intellectual property rights; the timing, success and cost of R&D; study and trial results are subject to different interpretations and may not be predictive of future results; trials and studies may not proceed at the time or in the manner expected or at all; having adequate funds; and satisfactory resolution of litigation or other disagreements. Additional factors that could cause actual results to differ materially from those stated or implied by our forward-looking statements are disclosed in our SEC filings. We disclaim any intent or obligation to update these forward-looking statements, other than as may be required under applicable law.

PROPRIETARY | 3

Embracing the Challenge of Improving Health by Bringing Innovative Medicines Targeting

G Protein-Coupled Receptors to Patients

Arena’s Mission

PROPRIETARY | 4

Arena: Embracing Innovation; Impacting Lives

BELVIQ®Lorcaserin HCI for

Chronic Weight Management

LORCASERINLifecycle

Management

PIPELINEInternally Discovered,Novel Drug Candidates

Targeting G Protein-Coupled ReceptorsTechnologies and Expertise

OurFOCUS

OurSTRATEGY

Discover, Develop and Commercialize Novel Oral Compounds

OurPORTFOLIO

CommercialFirst-in-Class

Clinical StageDevelopment Programs

PROPRIETARY | 5

Preclinical GPCR ResearchTargets Include CV, CNS and Metabolic

GPCR Development-Stage Programs

IND Phase 1 Phase 2 Phase 3 NDA Phase 4PROGRAM/INDICATION

Once-Daily Formulation

Smoking Cessation

Co-Admin w/Phentermine

CVOT/Conversion to T2D/MACE+

Ralinepag - Vascular (PAH)

APD334 - Autoimmune (IBD)

APD371 - Pain & Fibrotic Disease

Temanogrel - Thrombotic Diseases

Nelotanserin -Neurological Diseases

LORCASERIN LIFECYCLE

MANAGEMENT*

CLINICAL STAGE

PIPELINE*

Development Expense Shared

or Funded by Collaborators

Collaboration with Ildong for

South Korea

Collaborationwith Roivant

*See full presentation for program status

PROPRIETARY | 6

• First-in-class molecule discovered by Arena

• MOA: Selective activation of 5-HT2C receptors in hypothalamus are believed to decrease food consumption and promote satiety

• Large addressable patient population– Overweight (BMI ≥27) with at least one weight-

related medical condition– Obese (BMI ≥30)

• Launched in US by Eisai in June 2013

• Launched in S. Korea by Ildong in February 2015

BELVIQ® for Chronic Weight Management

PROPRIETARY | 7

Eisai Collaboration

• BELVIQ sales force

– 90 internal and 230 contract sales representatives promoting BELVIQ focusing on high-prescribers

• Eisai marketing program

– Pay no more than $75 savings card

– Drive patient demand with focus on increasing prescription refill rates

– Increase access by improving insurance coverage

• Eisai’s goal– Make BELVIQ the weight loss agent of choice of physicians and obese and

overweight patients

PROPRIETARY | 8

BELVIQ Launched in South Korea

• Arena received $3.0 million milestone payment in 1Q15

• Arena net sales of $2.2 million in 1Q15

• ~12,770 one-month prescriptions through February 24 - March 31

• Ildong estimates 617,044 tablets were prescribed in April; ~10,284 one-month prescriptions

PROPRIETARY | 9

44K 59K77K

110K

143K 148K

182K

Q3 Q4 Q1 Q2 Q3 Q4 Q1

US South Korea

Steady Prescription Growth

~865K TRxs Filled as of 5/22/15

2013 2014

BELVIQ QUARTERLY TRx

2015

PROPRIETARY | 10

BELVIQ ROW Update

• Review ongoing

Mexico & Brazil

• Bridging study (6 months) needed; study began enrolling in April

• Study is intended to enable filing for marketing approval once completed

Taiwan

• Review ongoing

Israel

• Meeting requests filed with MHRA & MPA

• Meetings expected in 3Q15

• Resubmission planned for end of March 2016

European Union

PROPRIETARY | 11

BELVIQ® Lifecycle Management Update

• Meeting request filed with FDA

• Meeting expected in 3Q15

Coadministration with Phentermine

• May provide additional market differentiation

– Increased patient convenience and compliance

• NDA submission planned for later this year

BELVIQ XR

• Multiple commercial scenarios being assessed

Smoking Cessation

Ralinepag for Vascular Disease

PROPRIETARY | 13

• Stable, orally bioavailable, highly selective, non-prostanoid drug candidate

• Unique pharmacokinetic profile expected to provide low peak-to-trough ratio more closely approximating continuous i.v. infusion, thereby providing potential for optimal clinical outcome

• Differentiated pharmacologic profile based upon robust IP receptor potency and in vitro target cell effects (smooth muscle relaxation and proliferation, platelet inhibition)

Ralinepag: Novel, Oral Prostacyclin Receptor Agonist to Treat Pulmonary Arterial Hypertension (PAH)

t ½ (hr)

Treprostinil 4.5hrs 1

Selexipag 8 hrs 2

Ralinepag ~20-26 hrs 3

Improved receptor coveragegiven long half-life

1 Tapson, et al. Am J Respir Crit Care Med (2009)2 Lang, et al Expert Opin Pharmacother (2014)

3 Arena Pharmaceuticals, Inc

Lower peak-to-trough ratio may improve tolerability

Note: modeling based on published and internal data

0 12 24 36 48 60 721

10

Time (hr)

Pla

sm

a C

on

c.

(ng

/mL

)TherapeuticRange

Adverse Events

Long T½ low peak to trough

Short T½ high peak to trough

Long T½ modified release

PROPRIETARY | 14

Ralinepag: Phase 2 Trial for PAH

• Randomized, double-blind, placebo-controlled

• ~60 patients with WHO group 1 PAH; WHO/NYHA functional class II-IV symptomology

• 22 week-study including dose titration period followed by an open-label extension

• Primary outcomes: change in pulmonary vascular resistance, change in 6-min walk distance

OBJECTIVES AND STUDY DESIGN

PROPRIETARY | 15

Ralinepag: IP Agonist Market Opportunity

Oral Monotherapy Oral Combos

EARLY STAGE DISEASE LATE STAGE DISEASE

Inhaled/SC/IV

Endothelin Receptor Antagonists: +$2 Billion

PDE5 Inhibitors: +$500 Million

Launched Prostacyclins: +$1 Billion

Potential advancement to earlier lines of therapy with oral formulations

An oral, IP receptor agonist with a long half-life could move treatment earlier in

the treatment paradigm

Soluble Guanylate Cyclase (sGC) Stimulator: ~$100 Million

Oral Therapies Dominate First Line Treatment

PROPRIETARY | 16

Ralinepag: IP Agonist Market Opportunity

Convenience of Administration

(Oral Drug)

Symptomatic Disease Management (Vasodilation)

Earlier Interventionin PAH

Once-Daily, Oral Drug that Approximates Continuous Therapeutic Exposure of I.V. Infusion

Potential Disease Modification

(Vascular Remodeling)

PROPRIETARY | 17

Ralinepag: Activity in Rat Monocrotaline Model of PAH

Treatment with ralinepag lowered pulmonary artery pressure

(9)

Sham

MCT +

Veh

icle

MCT +

APD81

1 3

0mpk

0

5

10

15

20

25

30

35

(5)(9)

p<0.01

(8)

mP

AP

(mm

Hg

)

Pulmonary Artery Pressure

• Monocrotaline (MCT) induced PAH in rats is a common model used to investigate potential therapies for PAH.

• MCT induces a significant increase in pulmonary artery pressure, medial thickness of small pulmonary arteries, and right ventricular hypertrophy.

PROPRIETARY | 18

Ralinepag: Activity in Rat Monocrotaline Model

Treatment with ralinepag blocked development of PAH & reduced pulmonary vascular and cardiac remodeling

(9)

Sham

MCT +

Vehic

le

MCT +

APD811 3

0mpk

0

10

20

30

40

50

(5)(5)

p<0.01

(5)

% w

all

thic

kne

ss (

%)

Sham MCT MCT + APD811

Wall Thickness

(9)

0.00

0.25

0.50

0.75

1.00

*

*p<0.001 Vs. MCT +Vehicle

RV

/LV

+S

Right Ventricle Weight

APD334 for Autoimmune Diseases

PROPRIETARY | 20

APD334: Novel, Oral S1P1 Modulator to Treat Autoimmune Diseases (Phase 1 MAD Results)

Attributes of A Potential Best-in-ClassOnce-Daily, Oral Therapy Inhibiting Lymphocyte Trafficking

No clinically significant elevations in liver enzyme tests in any dose group

No clinically significant safety findings with respect to heart rate or rhythm

• Dose-dependent effect on lymphocyte count lowering in blood, with mean decreases from baseline up to 69%

• Recovery of lymphocyte counts to baseline within one week (half-life ~35 hours)

Lymphocyte Reduction with Rapid Recovery

Favorable Cardiovascular Safety Profile

Favorable Hepatotoxicity Safety Profile

Lymphocyte Reduction with Rapid Recovery

Favorable Cardiovascular Safety Profile

Favorable Hepatotoxicity Safety Profile

PROPRIETARY | 21

Inflammatory Bowel Disease (IBD): Market Overview

Disease Burden

• Chronic disease with sizeable economic burden:

− Over 1.6 million people (907,000 with ulcerative colitis and 780,000 with Crohn’s disease) in the US

− Annual US economic costs estimated at +$31 billion

• Growing disease of the developed world, particularly urban areas and northern climates

• ~$9.0 billion global IBD market in 2014

Unmet Need

• Novel, oral, efficacious maintenance therapy with fewer side effects

− ~48% of ulcerative colitis patients are in remission;70% of patients with disease will have a reoccurrence in the next year

− ~50% of people with Crohn’s disease will be in remission, or have mild disease, over the next five years; more than 50% of patients in remission will have a relapse over a year

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Ulcerative Colitis Crohn's Disease

In Remission 48%

Mild Disease 30%

Moderate Disease

20%

Severe Disease

1-2%

Ulcerative Colitis: Disease Activity

• Preparing for Phase 2 study start in ulcerative colitis later this year.

PROPRIETARY | 22

Mild

Aminosalicylates (5-ASAs)

S1P1 Receptor Modulator

Corticosteroids

Immunosuppressants and Anti-Integrins

Mild Moderate Severe

TNF-α Inhibitors

Primary Market Research Suggests that S1P1 Receptor Modulator May be Prescribed to a Similar Patient Population as TNF-α Blockers

IBD Treatment Algorithm

PROPRIETARY | 23

S1P1 Receptor Modulators

$? Billion

$2.5 Billion (2014)2

TNF-α Blockers

~$32 Billion (2014)1

~$2 Billion (2002)2

16X

Rheumatoid Arthritis

Psoriatic Arthritis

Ankylosing Spondylitis

Crohn’s Disease

Psoriasis

Ulcerative Colitis

Multiple Sclerosis

Rheumatoid Arthritis

Psoriatic Arthritis

Ankylosing Spondylitis

Crohn’s Disease

Psoriasis

Ulcerative Colitis

S1P1 Receptor Modulator Class Potential

Multiple Products and Autoimmune Indications Could Expand Commercial Potential Over Time Similar to the TNF-α Blocker Class

APD371 for Pain

PROPRIETARY | 25

Pain: Market Overview

Disease Burden

• 100 million people in US are burdened with persistent pain each year

• Pain is a leading cause of medically related work absenteeism

− ~>50 million workdays lost each year

− ~$560-635 billion estimated annual US economic costs

• ~$35 billion in global pain management market in 2012

Unmet Need

• Pain is often under treated

• Current treatment options do not provide adequate balance of safety and efficacy

− NSAIDs: modest efficacy with potential for increased risk of GI bleeding

− Opioids: stronger efficacy with potential for dependence/abuse and GI side effects

− COXIBs: narrow therapeutic benefit over NSAIDs with enhanced risk of CV adverse events

• With the prevalence of pain on the rise, the need for safe, well-tolerated and more-efficacious pain relievers is substantial

At least 1 in 3 Americans suffer from chronic pain

According to NHANES data, prevalence of chronic pain in the US has been increasing, particularly in patients aged 45-64

% r

ep

ort

ing

pai

n in

pri

or

mo

nth

10

20

30

40

NHANES 1999-2000 NHANES 2001-2002 NHANES 2003-2004

Ages 20 to 44 Ages 45 to 64 Ages 65 and over

Trends in Pain Prevalence: United States, 1999-20041

PROPRIETARY | 26

APD371: Novel, Oral Agonist of the Cannabinoid 2 (CB2) Receptor

Need for non-opioid, non-NSAID approaches to pain

• Opioids: strong efficacy with potential for dependence/abuse andGI side effects

• NSAIDs (including COXIBs): modest efficacy with increased risk ofGI bleeding, renal toxicity and CV events

Full agonist

Selectively targeting CB2 receptor may provide pain relief without the liabilities of opiates and NSAIDs

Phase 1 single-ascending trial dosing completed

• No adverse events that limited dose escalation up to 400mg

• Blood drug levels achieved greatly exceed the in vitro CB2 receptor activation constant [EC50]

• Plan to initiate Phase 1 multiple-ascending dose study later this year

Initial Market Opportunity

Attributes

Status

Additional Potential

Fibrotic diseases

PROPRIETARY | 27

Nelotanserin: Novel 5-HT2A Inverse Agonist for Behavioral and Neuropsychiatric Disturbances

• Nelotanserin

– Internally discovered inverse agonist of the serotonin 5-HT2A receptor

– Previous trials support potential to treat a variety of neuropsychiatric conditions

• Development, Marketing and Supply Agreement

– Roivant Sciences:

• Granted exclusive worldwide rights to develop and commercialize nelotanserin

• Responsible for all development expenses

• Intend to initiate Phase 2 clinical trials in certain neurological diseases

– Arena Pharmaceuticals GmbH:

• Will manufacture clinical supply and commercial product to sell to Roivant

• Receive $4 million upfront payment; eligible for $41.5 million in milestone payments

• 15% of net sales and up to $60 million in purchase price adjustments

PROPRIETARY | 28

ARNA Investment Highlights

• Revenue stream from BELVIQ® (lorcaserin HCl), indicated for weight management

• Strategy to create additional value with lorcaserin by pursuing lifecycle management opportunities

• Clinical-stage pipeline of novel, internally discovered compounds targeting large market opportunities

• Cash and cash equivalents of $241M at 3/31/15

PROPRIETARY | 28

PROPRIETARY | 29

Embracing the Challenge of Improving Health by Bringing Innovative Medicines Targeting

G Protein-Coupled Receptors to Patients

Arena’s Mission

Jefferies 2015 Health Care ConferenceNASDAQ: ARNACraig M. Audet, PhD | Sr. VP, Operations & Head of Global Regulatory Affairs

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