Sensei Biotherapeutics Reports Third Quarter 2021 Results and Recent Business Highlights - SNS-101 identified as a tumor-selective anti-VISTA antibody product candidate from TMAb platform - - SNS-101 preclinical data acc

Sensei Biotherapeutics Reports Third Quarter 2021 Results and Recent Business Highlights

- SNS-101 identified as a tumor-selective anti-VISTA antibody product candidate from TMAb platform -

- SNS-101 preclinical data accepted for presentation at the Society for Immunotherapy of

Cancer s (SITC) 36th Annual Meeting; abstract available online in the Journal for ImmunoTherapy of Cancer -

- VISTA science symposium to be webcast Tuesday, November 16 at 4:00 p.m. ET -

- Ended third quarter 2021 with cash, cash equivalents and marketable securities of 156.7 million; company reiterated cash runway at

least into first half 2024 -

BOSTON, MA November 9, 2021 Sensei Biotherapeutics, Inc. (NASDAQ: SNSE), an

immunotherapy company focused on the discovery and development of next generation therapeutics for cancer, today reported financial results for the third quarter ended September 30, 2021 and provided recent corporate updates.

In the third quarter, we witnessed rapid preclinical progress with our TMAb platform, which is designed to address the challenge of resistance to

checkpoint blockade. The global PD-1/PD-L1 checkpoint inhibitor market in 2020 was valued at greater than

$30 billion1, yet approximately only 20-30 percent of people respond to treatment2,

said John Celebi, president and chief executive officer of Sensei Biotherapeutics. VISTA is an immune checkpoint that is widely expressed on myeloid cells within the tumor microenvironment, a hub of immunosuppressive activity, and is

implicated in resistance to checkpoint blockade. VISTA has been historically difficult to address therapeutically due to the presence of a pharmacokinetic sink in the blood and unique pH-dependent biology

where the pH of the tumor is more acidic than the rest of the body. We have identified a promising first product candidate, SNS-101, by characterizing a robust set of

pH-selective fully human anti-VISTA antibodies. Later this week at SITC, we plan to present preclinical data demonstrating how we rationally designed SNS-101 to block

VISTA at low pH within the tumor microenvironment and present initial in vivo data with SNS-101.

Mr. Celebi continued, We also continue to advance our ImmunoPhage platform to generate new tumor-specific

T-cells and are looking forward to providing more updates when we finalize the genetic design of the new ImmunoPhage backbone. Importantly, we are well funded with more than $156 million to advance our

Third Quarter Highlights and Pipeline Milestones:

TMAb (Tumor Microenvironment Activated Biologics) Platform

VISTA (V-domain Ig suppressor of T cell activation) is an immune checkpoint that is implicated in resistance to PD-1/PD-L1 and correlates with poor survival across numerous cancers. In the third quarter, Sensei achieved the following milestones for this program:

VSIG4 (V-Set and Immunoglobulin Domain

Containing 4) is a B7-family related protein and a potent inhibitor of T-cell activity, frequently overexpressed on tumor-associated macrophages and a potential

driver of immunosuppressive macrophage polarization. VSIG4 is implicated in resistance to checkpoint blockade. Expression of VSIG4 is also found within normal tissues, presenting potential safety challenges, making VSIG4 an ideal candidate for

Sensei s TMAb platform.

ImmunoPhage Platform

SNS-401-NG is a potential first-in-class, multi-antigenic personalized ImmunoPhage candidate being developed in collaboration with the University of Washington designed to treat a broad range of cancers. The first proof-of concept clinical application is directed to the treatment of Merkel Cell Carcinoma (MCC), an aggressive form of skin cancer commonly driven by the Merkel Cell Polyoma Virus. Once clinical proof of concept

is achieved, Sensei plans to evaluate a broader study in patients with multiple tumor types, potentially including head and neck cancer, lung cancer, melanoma, and triple negative breast cancer based on the prevalence of Phortress antigens.

Third Quarter 2021 Financial

Cash Position Cash, cash equivalents and marketable securities were $156.7 million as of

September 30, 2021, as compared to $16.6 million as of December 31, 2020. Sensei expects the current cash balance to fund operations at least into the first half of 2024.

Research and Development (R&D) Expenses R&D expenses were $6.4 million for the quarter ended

September 30, 2021, compared to $3.6 million for the quarter ended September 30, 2020.

The increase in R&D expenses was primarily attributable to increased headcount to support Sensei s

research, development, and manufacturing activities.

General and Administrative (G&A) Expenses G&A expenses

were $3.9 million for the quarter ended September 30, 2021, compared to $1.8 million for the quarter ended September 30, 2020. The increase in G&A expenses was primarily attributable to higher personnel costs, including

stock-based compensation expense, and costs associated with operating as a public company.

was $9.7 million, for the quarter ended September 30, 2021, compared to $5.4 million for the quarter ended September 30, 2020.

SNS-101 is a potent, pH-dependent fully human monoclonal antibody designed to block the interaction of VISTA, a novel immune checkpoint that is expressed primarily on myeloid

cells, with its receptor, PSGL-1. Selectivity is achieved because SNS-101 targets the active (i.e., protonated) VISTA present in the low pH tumor microenvironment. SNS-101 was selected based on 1) the lack of significant binding to VISTA at physiologic pH (i.e., deprotonated VISTA in the blood), and 2) its high-affinity binding to active VISTA (pH 6.0), which yielded a > 600-fold selectivity. Based on the biochemical properties of SNS-101, we anticipate tumor microenvironment selective activity for this preclinical product candidate. VISTA has

been shown to be expressed in numerous tumor types, including non-small cell lung cancer (NSCLC).

Sensei Biotherapeutics is a biopharmaceutical company engaged in discovery, development, and delivery of next generation

immunotherapies with an initial focus on treatments for cancer. Sensei has developed two unique approaches its TMAb (Tumor Microenvironment Activated biologics) platform, comprising

unique human monoclonal antibodies and alpaca derived nanobodies that are selectively active in the tumor microenvironment, and its ImmunoPhage platform that leverages bacteriophage to drive

the generation of tumor antigen-specific immune responses. Using its TMAb platform, the company has developed SNS-101, an antibody-based therapeutic targeting an immune checkpoint gene that inhibits anti-tumor

immune responses called V-domain Ig suppressor of T cell activation (VISTA). Using the ImmunoPhage platform, Sensei is developing a library of ImmunoPhage, called Phortress , with multiple tumor-associated antigens to create a personalized, yet off-the-shelf cocktail approach for

treating cancer patients. The platform is designed to enable efficient, scalable and cost-effective manufacturing to support all of Sensei s clinical programs.

SNS-401-NG is an ImmunoPhage cocktail in preclinical development for the treatment of Merkel Cell Carcinoma. For more

information, please visit www.senseibio.com, and follow the company on Twitter @SenseiBio and LinkedIn.

Cautionary Note Regarding Forward-Looking Statements

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 such as believe , 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 of Sensei s product candidates and platforms, the availability of data from Sensei s preclinical studies, and its belief that its existing cash and cash equivalents will be

sufficient to fund its operations at least into the first half of 2024. 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 preclinical discovery and development, conduct of clinical trials and related regulatory requirements,

Sensei s reliance on third parties over which it may not always have full control, and other risks and uncertainties that are described in Sensei s Annual Report on Form 10-K filed with the

U.S. Securities and Exchange Commission (SEC) on March 30, 2021 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.

Condensed Statement of Operations

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

Selected Condensed Balance Sheet Data

(Unaudited, in thousands)

VP, Investor Relations and Communications

Sam Brown Inc. Healthcare Communications