Acelyrin : Corporate Presentation

Accelerating Medicines to Transform Patients' Lives

Corporate Overview

March 20, 2024

Forward Looking Statements & Disclaimer

This presentation contains statements that are not of historical facts, considered 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. Forward-looking statements include, but are not limited to, statements about our expectations regarding the potential benefits, effectiveness, and safety of our product candidates; our expectations with regard to our research, development and regulatory plans, including the design, timing and results of preclinical studies and clinical trials, the timing and availability of data from such studies and trials, and the timing or likelihood of regulatory filings and approvals for our product candidates; our expectations with regard to our ability to license, acquire, discover, and develop additional product candidates and advance such product candidates into, and successfully complete, preclinical studies and clinical trials; the potential market size and size of the potential patient populations for our product candidates and any future product candidates and those indications we target; our expectations about our ability to maintain existing, and establish new, strategic collaborations, licensing, or other arrangements; the scope of protection we are able to establish and maintain for intellectual property ("IP") rights covering our product candidates and any future product candidates; our business strategy; and our future results of operations and financial position.

Such statements reflect the current views of ACELYRIN with respect to future events, and are subject to known and unknown risks (including, without limitation, business, regulatory, economic and competitive risks), uncertainties, assumptions and contingencies about ACELYRIN (including, without limitation, those associated with our successful completion of development and regulatory activities for our product candidates, maintaining and defending IP protection, ability to timely secure adequate supply of our product candidates, legal proceedings, government investigations, macroeconomic conditions, market volatility) and other risks and uncertainties described under the heading "Risk Factors" in the Company's most recent Quarterly Report on Form 10-Q for the three months ended September 30, 2023 filed with the U.S. Securities and Exchange Commission ("SEC") and in subsequent filings made by us with the SEC, which are available on the SEC's website at www.sec.gov.

In light of these risks and uncertainties, many of which are beyond our control, the plans, intentions, expectations or other events described in or implied by our forward-looking statements may not occur. Actual results or events could differ materially and adversely from those expressed in any forward-looking statements. New risks may occur at any time, and we anticipate that subsequent events and developments could cause our views to change. Any reader of this presentation is cautioned not to place undue reliance on these forward-looking statements, which speak to our current beliefs and expectations only as of the date of this presentation. Except as required by law, we disclaim any intention or responsibility for the accuracy and completeness of the forward-looking statements in this presentation, or to update

or revise any forward-looking statements in the event of new information, future developments or otherwise.

No representation is made as to the safety or effectiveness of our product candidates. Additionally, this presentation also contains certain information related to or based on studies, publications, surveys and other data obtained from third-party sources, and our own internal estimates and research, including without limitation relating to market size and growth. While we believe these third-party sources to be reliable as of the date of this presentation, we have not independently verified, and make no representation as to the adequacy, fairness, accuracy or completeness of any third-party information. In addition, market data involves a number

of projections, assumptions, and estimates of our future performance and the future performance of the markets in which we operate, and there can be no guarantee as to the accuracy or reliability thereof as they are necessarily subject to a high degree of uncertainty and risk.

The information in this presentation is as of the date of this presentation, and is subject to change without notice.

TRADEMARKS

This presentation contains trademarks, service marks, trade names and copyrights of ACELYRIN and other companies which are the property of their respective owners.

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"ACELYRIN Is A Leading Clinical-Stage Biopharma Company

Focused On Identifying, Acquiring, And Accelerating

The Development And Commercialization Of

Transformative Medicines In Immunology"

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Creating An Industry Leading Immunology Company

Team of veteran biopharma executives who together bring exceptional track records of developing some of the most successful medicines within immunology and beyond

Building a portfolio of potential new medicines that we think have the opportunity to provide clinically meaningfully differentiated benefit to patients

• We seek "diamonds in the rough" where, based on molecule characteristics, our collective experience and expertise, and the evolving scientific and medical understanding, we can test hypotheses around clinical differentiation for patients

Robust pipeline of clinical programs across several indications representing multi-billion-dollar opportunities in the aggregate

• Izokibep is a "pipeline-in-a-program" in late-stage development for multiple immunological indications including psoriatic arthritis (PsA), hidradenitis suppurativa (HS), axial spondyloarthritis (AxSpA) and uveitis
• Lonigutamab has demonstrated proof-of-concept as a subcutaneously delivered therapy for thyroid eye disease (TED) with the goal to improve upon efficacy, safety as well as convenience for patients
• SLRN-517is an early program targeting mast cell-driven diseases

Well-capitalized having secured more than $1 billion in private and public capital since founding in 2020

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Experienced Leadership Team

Successful Track Record of Delivering Some Of The Most Transformative Medicines For Patients

Shao-Lee Lin | MD, PhD	Melanie Gloria	Mina Kim
Founder and CEO	COO	CL&AO

Leaders In Immunology

Gil Labrucherie	Agnes Lee	Ken Lock
CFO	SVP, IR & Communications	CCO

Shep Mpofu| MD, MRCP, FRCP	Ron Oyston	Patricia Turney
SVP, Development	CPO	CTO

Board of Directors

Shao-Lee Lin

Bruce C. Cozadd

Dan Becker

Alan Colowick

Henry Gosebruch

Patrick Machado

Beth Seidenberg

Dawn Svoronos

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Executing With A Sense Of Urgency For Patients

Q1 2024

October 2020

Series A Completed

July 2020

ACELYRIN Founded

December 2021

Positive Phase 2 Top-line Data For izokibep in PsA

October 2021

$250 Million

Series B

August 2021

First Program, izokibep, Licensed

November 2022

Add'l Phase 2 RCT PsA

Endpoint & QoL Data

Presented at ACR

September 2022

$300 Million Committed In

Series C; Accelerated PsA

Phase 2b/3 Into 2022

June 2022

Phase 2 RCT PsA Data Presented At EULAR

September 2023

Announced HS Part B Data

May 2023

$621 Million IPO,

April 2023

Announced 46-Week

Phase 2 PsA Data

March 2023

Positive Phase 2b/3 Part A Data For izokibep In Hidradenitis Suppurativa

January 2023

Expanded Portfolio With An

All-stock Company

Acquisition, Adding

Lonigutamab And SLRN-517

2023

Izokibep PsA Phase 2b/3 Data

Izokibep HS 32-week Data

Lonigutamab Phase 1/2 Data

2024

2020

20212022

RCT: Randomized Controlled Trial; QoL: Quality of Life	6

Total Addressable Markets Are Significant And Growing

Pursuing multiple indications with significant unmet need for izokibep and announced positive Phase 2b/3 topline data in PsA and long-term data in HS in 1Q 2024

Proof-of-concept for lonigutamab in Thyroid Eye Disease achieved in 1Q 2024

Strong financial position of $788 million in cash on September 30, 2023 expected to fund operations through key value- driving milestones across our portfolio.

					Mast Cell Disease
					e.g., Urticaria
				TED	$5.8B+
			Uveitis	$4.8B	Market by
			2030
			$790M	Market	~$39B
		AxSpA	by 2030
		Market
	HS	$6.8B	by 2030
PsA	$2.9B	Market
by 2030
$17.8B	Market
by 2030
Market
by 2030

~$22B

2023	2030

1 GlobalData Epidemiology Estimates; Helmick C. 2014; Perros P, 2017; Chin Y, 2020; Bartley GB, 1994 See Note 2 on Slide 15 regarding the Phase 2b/3 trial in PsA.

2 Active PsA Market - Emergen Research, 2022; Global AxSpA Market - iHealthcare Analyst, Nov 2022; HS Market Outlook - DelveInsight, Jan 2023; HS	7
Opportunity Analysis - GlobalData 2019; CSU Market - DelveInsight, Dec 2022, Uveitis Global Drug Forecast - GlobalData, June 2021; Tepezza

Consensus Sales Forecast compiled by GlobalData

Robust Portfolio Of Clinical Programs

With Multiple Indications In Late Stage

Preclinical	Phase 1	Phase 2	Phase 3

	Hidradenitis Suppurativa
Izokibep	Psoriatic Arthritis2
(IL-17Ai)3
	Axial Spondyloarthritis4
	Uveitis1
Lonigutamab	Thyroid Eye Disease
(anti-IGF-1R)5
SLRN-517	Mast Cell Diseases	Mast Cell
(anti-c-KIT)6	7	Diseases
e.g., Urticaria

1. Phase 2b/3 trial in uveitis. Planned inclusion into registrational package for non-infectious uveitis (as applicable) if granted orphan drug designation and following consultation with relevant health authorities. We have not previously completed any clinical trials for uveitis and are currently conducting our first Phase 2b/3 trial.
2. Phase 2b/3 trial in PsA.
3. IL-17AInhibitor; Excludes (i) development, commercialization and manufacturing rights in mainland China, Hong Kong, Macau, South Korea and Taiwan, and (ii) development rights in certain other Asia Pacific countries including, without limitation, Australia, India, New Zealand and Singapore. We retain decision making authority for izokibep global development. Potential opportunity to extend certain IP protection into early 2040's.

1. Based on data from our Phase 2 and ongoing Phase 2b/3 trials in PsA, we intend to discuss with the FDA initiation of the Phase 3 program in AxSpA without completing earlier clinical trials in AxSpA. The FDA may require us to complete a Phase 2 trial in AxSpA prior to initiating our planned Phase 3 program.
2. Worldwide rights to non-oncology indications. Potential opportunity to extend certain IP protection into 2043.
3. Potential opportunity to extend certain IP protection to 2039.
4. Based on preclinical studies demonstrating highly potent inhibition of the c-KIT pathway targeting mast cell proliferation and degranulation across mast-cell driven diseases such as Chronic Urticaria, an inflammatory disease that is driven by the release of histamine and other vasoactive molecules produced by mast cells

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Izokibep's High Potency & Small Size Enables Potential To Improve Clinical Response With SC Exposures Others Require IV To Achieve

Validated Target

IL-17A is associated with autoimmune inflammation. Marketed monoclonal antibodies have demonstrated targeting IL-17A results in dose-responsive increases in efficacy without dose-limiting toxicity.

Targeting more broadly than IL-17A as a means to more effectively inhibit the IL-17 axis has demonstrated risk for increased fungal infection, suicidal ideation & behavior, and liver toxicity with a requirement for routine monitoring - all raising the potential of association specifically with inhibition of IL-17F.

Hitting IL-17A the hardest may be the sweet spot of achieving increased exposure/efficacy without introducing additional or new safety liability. The high potency and small size of izokibep has the potential to impact clinical response.

High Potency

Blocks the homodimeric IL-17A target protein by binding to both sub-units simultaneously with the high affinity (KD: 0.3 pM) versus other IL-17A inhibitors.

Small Size

~1/10th the size of a mAb (~18.6 kD) enabling potential to reach difficult to treat tissues.

IL-17A homodimer

IL-17A binding domain

IL-17A binding domain

Albumin binding domain

Extends half-life to 12 days

IZOKIBEP (~18.6 kDa)

pM, picomolar; kD, kilodalton	9

PsA

PsA Is A Disease With Multiple Manifestations

Addressing Totality Of Manifestations Is Necessary To Achieve Disease Control & Restore Quality of Life

Arthritis	Psoriasis

Enthesitis

Dactylitis

Spondylitis

Psoriatic arthritis (PsA) is a chronic, inflammatory disease with multiple clinical manifestations including

arthritis, psoriasis, enthesitis (inflammation of dense, non- vascular tissues that connect ligaments and tendons to bone), spondylitis, and dactylitis

~1.6M PsA patients in the U.S.

Among moderate-to-severe PsA patients, over a third fail non-biologictherapy

More complete and faster resolution of disease symptoms manifesting in dense tissues (e.g., enthesitis) remain an unmet need

Addressing totality of manifestations is the goal

for patients

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