Ambrx Biopharma Inc., or Ambrx, (NYSE: AMAM), a clinical stage biopharmaceutical company using an expanded genetic code technology platform to create Engineered Precision Biologics, today announced the first patient has been dosed in its global ACE-Breast-03 Phase 2 clinical study of ARX788 in patients with HER2-positive metastatic breast cancer.

ARX788 is an anti-HER2 antibody drug conjugate (ADC) that is being studied broadly in breast cancer, gastric/GEJ cancer and other solid tumors. The FDA has granted ARX788 Fast-Track Designation for the treatment of HER2-positive metastatic breast cancer in December 2020.

“Dosing the first patient in this Phase 2 study of ARX788 in patients with HER2-positive metastatic breast cancer marks an important milestone for Ambrx,” said Feng Tian, Ph.D., Chairman of the Board, President, and CEO of Ambrx. “We have made excellent progress with our clinical development pipeline over the last few months, highlighted by our positive data of ARX788 for HER2-positive gastric cancer, as well as the dosing of the first patient in a Phase 1 trial of ARX517 for PSMA expressing tumors. Our growing clinical programs, coupled with an influx of capital from our IPO in June 2021, leaves Ambrx well-positioned to potentially attain several near-term clinical and corporate milestones.”

The global ACE-Breast-03 Phase 2 clinical study is a multicenter study to evaluate the efficacy and safety of ARX788 in HER2-positive, metastatic breast cancer patients whose disease is resistant or refractory to T-DM1, and/or T-DXd, and/or tucatinib-containing regimens. The primary outcome measure of the study will be the objective response rate.

About Ambrx Biopharma Inc. (Ambrx)

Ambrx is a clinical stage biopharmaceutical company using an expanded genetic code technology platform to discover and develop Engineered Precision Biologics. These include next generation antibody drug conjugates (ADCs), bispecifics, targeted immuno-oncology therapies, novel cytokines to modulate the immune system, and long-acting therapeutic peptides for metabolic and cardiovascular disease. Ambrx is advancing a robust portfolio of clinical and preclinical programs designed to optimize efficacy, safety and ease of use, in multiple therapeutic areas, including its lead product candidate ARX788. In addition, Ambrx has clinical collaborations with multiple partners, for drug candidates generated using Ambrx technology. For additional information, please visit www.ambrx.com.

Forward-Looking Statements

This press release includes certain “forward-looking statements” intended to qualify for the “safe harbor” from liability established by the Private Securities Litigation Reform Act of 1995, as amended. Forward-looking statements may be identified by the words “anticipate,” believe,” “estimate,” “expect,” “intend,” “plan,” “project,” “may,” “will,” “could,” “should,” “seek,” “potential” and similar expressions, and include, without limitation, express or implied statements regarding Ambrx’s beliefs and expectations regarding the advancement and potential benefits of its product candidates, clinical development and strategic plans, the timing of future events, and anticipated upcoming milestones. Forward-looking statements are based on Ambrx’s current expectations and are subject to inherent uncertainties, risks and assumptions that are difficult to predict. Factors that could cause actual results to differ include, but are not limited to, those risks and uncertainties associated with: the impact of the COVID-19 pandemic on Ambrx’s business, operations, strategy, goals and anticipated milestones; Ambrx’s ability to execute on its strategy including with respect to the timing of its R&D efforts, initiation of clinical studies and other anticipated milestones; risks associated with development of novel therapeutics, including potential delays in clinical trials and regulatory submissions and the fact that future clinical trial results may not be consistent with preliminary results or results from prior preclinical studies or clinical trials; Ambrx’s ability to fund operations as anticipated; and the additional risks and uncertainties set forth more fully under the caption “Risk Factors” in Ambrx’s registration statement on Form F-1 filed with the United States Securities and Exchange Commission (SEC) on June 14, 2021, and elsewhere in Ambrx’s filings and reports with the SEC. Forward-looking statements contained in this announcement are made as of this date, and Ambrx undertakes no duty to publicly update or revise any forward looking statements, whether as a result of new information, future events or otherwise, except as may be required under applicable law.

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