Sierra Oncology to Present Innovative Clinical Trial Enhancements Leveraging Chk1 Synthetic Lethality in Two Posters at ASCO Annual Meeting

- Company also to present at Jefferies Healthcare Conference at 2pm ET on June 8^th -

VANCOUVER, May 24, 2017 /PRNewswire/ - Sierra Oncology, Inc. (NASDAQ: SRRA), a clinical stage drug development company focused on advancing next generation DNA Damage Response (DDR) therapeutics for the treatment of patients with cancer, today announced that it will be presenting two Trials in Progress posters describing the innovative Phase 1 clinical designs for its Chk1 inhibitor, SRA737, at the 2017 American Society of Clinical Oncology (ASCO) Annual Meeting, being held in Chicago on June 5^th. These ongoing trials were recently amended to include cohort expansions of prospectively selected patients with tumors identified to have genetic aberrations hypothesized to confer sensitivity to Chk1 inhibition via synthetic lethality.

The company also announced that Dr. Nick Glover, President and Chief Executive Officer of Sierra Oncology, will present an overview of the company entitled "Beyond PARP – Next Generation DDR Therapeutics" at the Jefferies Healthcare Conference in New York. The presentation is scheduled for 2:00pm ET on June 8^th. A live audio webcast and archive of the presentation will be accessible through www.sierraoncology.com.

ASCO 2017 Poster Presentations

Title: A phase I study of SRA737 (formerly known as CCT245737) administered orally in patients with advanced cancer.
Trials in Progress Abstract: #TPS2607
Poster: #93b
Poster Session: Developmental Therapeutics — Clinical Pharmacology and Experimental Therapeutics
Date and Time: Monday, June 5, 2017, 8:00 – 11:30am CT
Location: McCormick Place, Event room: Hall A, 2301 S King Dr., Chicago, Illinois

Title: A phase I study of oral SRA737 (formerly CCT245737) given in combination with gemcitabine plus cisplatin or gemcitabine alone in patients with advanced cancer.
Trials in Progress Abstract: #TPS2613
Poster: #96b
Poster Session: Developmental Therapeutics — Clinical Pharmacology and Experimental Therapeutics
Date and Time: Monday, June 5, 2017, 8:00 – 11:30am CT
Location: McCormick Place, Event room: Hall A, 2301 S King Dr., Chicago, Illinois

The posters will be available on June 5, 2017 on the company's website at www.sierraoncology.com

Jefferies Healthcare Conference

Title: Beyond PARP – Next Generation DDR Therapeutics
Date and Time: June 8, 2017, 2:00 pm ET

Live audio webcast and archive of the presentation will be accessible through www.sierraoncology.com.

About Sierra Oncology
Sierra Oncology is a clinical stage drug development company advancing next generation DNA Damage Response (DDR) therapeutics for the treatment of patients with cancer. Our lead drug candidate, SRA737, is a potent, highly selective, orally bioavailable small molecule inhibitor of Checkpoint kinase 1 (Chk1), a key regulator of important cell cycle checkpoints and central mediator of the DDR network. In cancer cells, replication stress induced by oncogenes (e.g., MYC or RAS) or genetic mutations in DNA repair machinery (e.g., BRCA1 or FA) combined with loss of function in tumor suppressors (e.g., TP53 or ATM) results in persistent DNA damage and genomic instability leading to an increased dependency on Chk1 for survival. Targeted inhibition by SRA737 may therefore be synthetically lethal to these cancer cells and have utility as a monotherapy in a range of tumor indications. Chk1 is also believed to facilitate tumor cell resistance to chemotherapy or radiation-induced DNA damage and the combination of SRA737 with these standards-of-care may provide synergistic anti-tumor activity. SRA737 is currently being investigated in two Phase 1 clinical trials in patients with advanced cancer.

Sierra Oncology is also advancing SRA141, a potent, selective, orally bioavailable small molecule inhibitor of Cell division cycle 7 kinase (Cdc7) undergoing preclinical development. Cdc7 is a key regulator of DNA replication and is involved in the DDR network, making it a compelling emerging target for the potential treatment of a broad range of tumor types. For more information, please visit www.sierraoncology.com.

Cautionary Note on Forward-Looking Statements
This press release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements regarding Sierra Oncology's anticipated clinical development, protocol amendments, target indications, trial designs and potential benefits of Sierra Oncology's product candidates. All statements other than statements of historical fact are statements that could be deemed forward-looking statements. These statements are based on management's current expectations and beliefs and are subject to a number of risks, uncertainties and assumptions that could cause actual results to differ materially from those described in the forward-looking statements. Such forward-looking statements are subject to risks and uncertainties, including, among others, the risk that Sierra Oncology may be unable to successfully develop and commercialize product candidates, SRA737 and SRA141 are at early stages of development and may not demonstrate safety and efficacy or otherwise produce positive results, Sierra Oncology may experience delays in the preclinical and anticipated clinical development of SRA737 or SRA141, Sierra Oncology may be unable to acquire additional assets to build a pipeline of additional product candidates, Sierra Oncology's third-party manufacturers may cause its supply of materials to become limited or interrupted or fail to be of satisfactory quantity or quality, Sierra Oncology's cash resources may be insufficient to fund its current operating plans and it may be unable to raise additional capital when needed, Sierra Oncology may be unable to obtain and enforce intellectual property protection for its technologies and product candidates and the other factors described under the heading "Risk Factors" set forth in Sierra Oncology's filings with the Securities and Exchange Commission from time to time. Sierra Oncology undertakes no obligation to update the forward-looking statements contained herein or to reflect events or circumstances occurring after the date hereof, other than as may be required by applicable law.

SOURCE Sierra Oncology