Sensei Biotherapeutics Reports Third Quarter 2023 Financial Results and Recent Business Highlights - SNS-101 initial Phase 1 clinical data presented at SITC demonstrate potential best-in-class safety and pharmacokinetic

Sensei Biotherapeutics Reports Third Quarter 2023 Financial Results and Recent Business Highlights

- SNS-101 initial Phase 1 clinical data presented at SITC demonstrate potential best-in-class safety and pharmacokinetic profile, with no cytokine release syndrome or dose-limiting toxicities reported -

- Phase 1/2 trial of SNS-101 continues to enroll ahead of schedule, with new clinical data

milestones anticipated throughout 2024 -

- Strong balance sheet with cash runway into the second half of 2025 -

BOSTON, MA November 7, 2023 Sensei Biotherapeutics, Inc. (Nasdaq: SNSE), a clinical stage immuno-oncology

company focused on the discovery and development of next-generation therapeutics for cancer patients, today reported financial results for the third quarter ended September 30, 2023, and provided recent business updates.

The third quarter of 2023 was marked by significant progress, culminating in the presentation of favorable clinical data supporting our belief that our

lead investigational candidate SNS-101 is a potentially transformative treatment in the oncology space with a best-in-class

safety and pharmacokinetic profile, said John Celebi, President and Chief Executive Officer of Sensei Biotherapeutics. Clinical data to-date support our capability to execute on the promise and

development of conditionally active antibodies, and we believe we are well-capitalized to advance SNS-101 clinically toward our near-term business and scientific milestones.

Highlights and Milestones

SNS-101 is a conditionally active antibody targeting the immune

checkpoint VISTA (V-domain Ig suppressor of T cell activation), which is implicated in numerous cancer indications and whose expression correlates with low survival rates. Sensei is conducting a multi-center

Phase 1/2 clinical trial to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, and efficacy of SNS-101 as both a monotherapy and in combination with Regeneron s PD-1 inhibitor Libtayo (cemiplimab) in patients with advanced solid tumors. Recent updates include:

Anticipated milestones for the SNS-101

Phase 1/2 clinical trial include:

Additional TMAb Pipeline and Platform Updates

Through its Tumor Microenvironment Activated biologics (TMAb) platform, Sensei is advancing several conditionally active antibody programs, including SNS-102 targeting VSIG4 (V-Set and Immunoglobulin Domain Containing 4), SNS-103 targeting ENTPDase1 (ecto-nucleoside triphosphate diphosphohydrolase-1, also known as CD39) and SNS-201, a VISTAxCD28 bispecific antibody.

Quarter 2023 Financial Results

Cash Position: Cash, cash equivalents and marketable securities were $72.0 million as of September 30,

2023, as compared to $107.1 million as of December 31, 2022. The decrease is due to cash used to fund operations and $9.8 million relating to shares repurchased during the nine months ended September 30, 2023. Sensei expects its

current cash balance to fund operations into the second half of 2025.

Research and Development (R&D) Expenses: R&D expenses were

$3.8 million for the quarter ended September 30, 2023, compared to $9.2 million for the quarter ended September 30, 2022. The decrease in R&D expenses was primarily attributable to lower manufacturing-related expenses, lower

personnel costs due to the Company s December 2022 restructuring and lower expense relating to lab supply purchases, partially offset by higher expense associated with clinical trials.

General and Administrative (G&A) Expenses: G&A expenses were $3.9 million for the quarter ended September 30, 2023, compared to

$4.8 million for the quarter ended September 30, 2022. The decrease in G&A expense was primarily attributable to decreased personnel costs due to the Company s December 2022 restructuring and lower expense related to directors and

Net Loss: Net loss was $7.1 million for the quarter ended September 30, 2023, compared to $13.4 million for the

quarter ended September 30, 2022.

About Sensei Biotherapeutics

Sensei Biotherapeutics (Nasdaq: SNSE) is a clinical stage immuno-oncology company focused on the discovery and development of next-generation therapeutics for

cancer patients. Through its TMAb (Tumor Microenvironment Activated biologics) platform, Sensei develops conditionally active therapeutics designed to disable immunosuppressive signals or

activate immunostimulatory signals selectively in the tumor microenvironment to unleash T cells against tumors. Sensei s lead investigational candidate is SNS-101, a conditionally active antibody designed

to block the V-domain Ig suppressor of T cell activation (VISTA) checkpoint selectively within the low pH tumor microenvironment, where VISTA acts as a suppressor of T cells by binding the receptor PSGL-1. The company is also developing SNS-102, a conditionally active monoclonal antibody targeting V-Set and Immunoglobulin Domain

Containing 4 (VSIG-4); SNS-103, a conditionally active monoclonal antibody targeting ecto-nucleoside triphosphate

diphosphohydrolase-1 (ENTPDase1), also known as CD39; and SNS-201, a conditionally active VISTAxCD28 bispecific antibody consisting of a CD28 agonist arm and a pH-sensitive anti-VISTA arm. For more information, please visit www.senseibio.com, and follow the company on Twitter @SenseiBio and LinkedIn.

Condensed Statements of Operations

(Unaudited, in thousands except share and per share data)

Selected Condensed Balance Sheet Data

(Unaudited, in thousands)

Cautionary Note Regarding Forward-Looking Statements

Any statements contained in this press release that do not describe historical facts may constitute forward-looking statements as that term is defined in the

Private Securities Litigation Reform Act of 1995. These statements may be identified by words and phrases such as believe , designed to, expect , may , plan , potential ,

will , and similar expressions, and are based on Sensei s current beliefs and expectations. These forward-looking statements include expectations regarding the development and potential therapeutic benefits of Sensei s product

candidates, the timing of Sensei s Phase 1/2 clinical trial of SNS-101, including reporting of data therefrom, the timing of selection of product candidates, and its belief that its existing cash and cash

equivalents will be sufficient to fund its operations at least into the second half of 2025. These statements involve risks and uncertainties that could cause actual results to differ materially from those reflected in such statements. Risks and

uncertainties that may cause actual results to differ materially include uncertainties inherent in the development of therapeutic product candidates, such as the risk that any one or more of Sensei s product candidates will not be successfully

developed or commercialized; the risk of delay or cessation of any planned clinical trials of Sensei s product candidates; the risk that prior results, such as signals of safety, activity or durability of effect, observed from preclinical

trials, will not be replicated or will not continue in ongoing or future studies or clinical trials involving Sensei s product candidates; the risk that Sensei s product candidates or procedures in connection with the administration

thereof will not have the safety or efficacy profile that Sensei anticipates; risks associated with Sensei s dependence on third-party suppliers and manufacturers, including sole source suppliers, over which Sensei may not always have full

control; risks regarding the accuracy of Sensei s estimates of expenses, capital requirements and needs for additional financing; and other risks and uncertainties that are described in Sensei s Quarterly Report on Form 10-Q filed with the U.S. Securities and Exchange Commission (SEC) on or about November 7, 2023 and Sensei s other Periodic Reports filed with the SEC. Any forward-looking statements

speak only as of the date of this press release and are based on information available to Sensei as of the date of this release, and Sensei assumes no obligation to, and does not intend to, update any forward-looking statements, whether as a result

of new information, future events or otherwise.

Senior Director, Investor Relations

Sensei Biotherapeutics

Ten Bridge Communications