Independent Data and Safety Monitoring Committee Recommends Continuing Study Without Sample Size Adjustment  

IDSMC Did Not Identify Any Safety Concerns

Phase 2 Study Target Enrollment Achieved; Final Data and Communication Expected in October 2023

Conference Call and Webcast Today at 2:00 p.m. PDT/5:00 p.m. EDT

Atara Biotherapeutics, Inc. (Nasdaq: ATRA), a leader in T-cell immunotherapy, leveraging its novel allogeneic EBV T-cell platform to develop transformative therapies for patients with cancer and autoimmune diseases, today announced completion of the planned Interim Analysis (IA) of the ATA188 Phase 2 randomized, placebo-controlled study (EMBOLD) in June 2022. The IA included review by the Independent Data and Safety Monitoring Committee (IDSMC) of the efficacy, safety, and biomarker data available at the data cut off, based on which the IDSMC recommended the study continue without sample size adjustment.

The IA’s primary focus was on expanded disability status scale (EDSS) improvement at six months and overall safety, and also included evaluation of EDSS beyond six months for patients with longer treatment duration, as well as other available clinical data and magnetization transfer ratio (MTR) biomarker data. The IA was also designed to assess if a sample size increase was needed based on a conditional power modeling approach. The IA was planned to occur before enrolling patient 80 to enable an increased sample size if needed.

The main assessment at the EMBOLD IA was EDSS improvement at six months, which in the 24-patient open label Phase 1 study, appeared to be predictive of confirmed EDSS improvement at 12 months (the primary endpoint for the EMBOLD study). At the time of the EMBOLD IA, there were EDSS data on 34 patients evaluable at six months and 15 patients at 12 months (across both placebo and active groups with 1:1 randomization).

Based on the analysis of the EMBOLD data available at the time of the IA, there was not a sufficient dataset to draw conclusions about the predictive value of six months EDSS improvement for 12 months EDSS improvement. The IDSMC believes the six-month interim endpoint may be an inaccurate measure of the potential of this intervention in this condition. The IDSMC recommended continuing the study without sample size adjustment until the final analysis of the primary endpoint of confirmed EDSS improvement at 12 months.

“Following the EMBOLD IA, we are proceeding with the IDSMC’s recommendation that the study continue to completion without sample size adjustment,” said Jakob Dupont, M.D., Head of Global Research & Development at Atara. ”After reviewing the clinical and safety data available at the data cut-off, the IDSMC did not highlight any safety concerns in the ongoing study. We are pleased to have completed the target enrollment for the EMBOLD study and look forward to sharing the topline results as planned at an appropriate forum in October 2023.”

After completing the IA in June, the target enrollment of 80 patients was achieved. Total enrollment will be higher as patients that were already in screening through the end of June will be allowed to continue the enrollment process. Final read out of the study primary endpoint of confirmed EDSS improvement at 12 months is planned to occur in October 2023.

“We look forward to completing the EMBOLD study as we aspire to demonstrate a potentially transformative profile for ATA188 in patients with progressive MS who have high unmet need and limited options,” said Pascal Touchon, President and Chief Executive Officer of Atara.

Conference Call and Webcast Details

Atara will host a live conference call and webcast today, Tuesday July 12, 2022, at 5:00 p.m. EDT to discuss the EMBOLD IA. Analysts and investors can participate in the conference call by dialing 877-407-8291 for domestic callers and 201-689-8345 for international callers, using the conference ID 13731189. A live audio webcast can be accessed by visiting the Investors & Media – News & Events section of atarabio.com. An archived replay will be available on the Company's website for 30 days following the live webcast.

About Atara Biotherapeutics, Inc.

Atara Biotherapeutics, Inc. (@Atarabio) is a pioneer in T-cell immunotherapy leveraging its novel allogeneic EBV T-cell platform to develop transformative therapies for patients with serious diseases including solid tumors, hematologic cancers and autoimmune disease. With our lead program in Phase 3 clinical development and currently under review to support registration in Europe, Atara is the most advanced allogeneic T-cell immunotherapy company and intends to rapidly deliver off-the-shelf treatments to patients with high unmet medical need. Our platform leverages the unique biology of EBV T cells and has the capability to treat a wide range of EBV-associated diseases, or other serious diseases through incorporation of engineered CARs (chimeric antigen receptors) or TCRs (T-cell receptors). Atara is applying this one platform, which does not require TCR or HLA gene editing, to create a robust pipeline including: tab-cel^® (tabelecleucel) in Phase 3 development for Epstein-Barr virus-driven post-transplant lymphoproliferative disease (EBV+ PTLD); ATA188, a T-cell immunotherapy targeting EBV antigens as a potential treatment for multiple sclerosis; and multiple next-generation chimeric antigen receptor T-cell (CAR-T) immunotherapies for both solid tumors and hematologic malignancies. Improving patients’ lives is our mission and we will never stop working to bring transformative therapies to those in need. Atara is headquartered in South San Francisco. For additional information about the company, please visit atarabio.com and follow us on Twitter and LinkedIn.

Forward-Looking Statements

This press release contains or may imply "forward-looking statements" within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. For example, forward-looking statements include statements regarding the potential benefits, safety and efficacy of ATA188; and the timing and progress of ATA188, including (i) regulatory designations for ATA188 granted by FDA and the impact thereof; (ii) the mechanistic link between EBV and multiple sclerosis and the ability of ATA188 to specifically target such link; (iii) data from ATA188 OLE study; (iv) ATA188 clinical trials, (v) the interim analysis for the EMBOLD study; (vi) the possibility of EDSS improvement at six months to be predictive of EDSS improvement at 12 months; and (vii) Atara’s ability to successfully advance the development of ATA188. Because such statements deal with future events and are based on Atara’s current expectations, they are subject to various risks and uncertainties and actual results, performance or achievements of Atara could differ materially from those described in or implied by the statements in this press release. These forward-looking statements are subject to risks and uncertainties, including, without limitation, risks and uncertainties associated with the costly and time-consuming pharmaceutical product development process and the uncertainty of clinical success; the ongoing COVID-19 pandemic and the current events involving Russia and Ukraine, which may significantly impact (i) our business, research, clinical development plans and operations, including our operations in South San Francisco and Southern California and at our clinical trial sites, as well as the business or operations of our third-party manufacturer, contract research organizations or other third parties with whom we conduct business, (ii) our ability to access capital, and (iii) the value of our common stock; the sufficiency of Atara’s cash resources and need for additional capital; and other risks and uncertainties affecting Atara’s and its development programs, including those discussed in Atara’s filings with the Securities and Exchange Commission (SEC), including in the “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” sections of the Company’s most recently filed periodic reports on Form 10-K and Form 10-Q and subsequent filings and in the documents incorporated by reference therein. Except as otherwise required by law, Atara disclaims any intention or obligation to update or revise any forward-looking statements, which speak only as of the date hereof, whether as a result of new information, future events or circumstances or otherwise.

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