Sierra Oncology Reports Preclinical Data for its Chk1 Inhibitor SRA737 Supporting its Ongoing Clinical Development Strategy

- SRA737 demonstrates synergy with replication stress-inducing agents -
- SRA737 plus gemcitabine efficacious in gemcitabine-resistant tumor models -
- SRA737 potentiated by novel sub-therapeutic gemcitabine dosing -

VANCOUVER, Oct. 30, 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, reported preclinical data supporting the ongoing clinical development strategy for its Chk1 inhibitor, SRA737. The results were presented in a poster on October 29th at the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics held in Philadelphia, Pennsylvania.

"The data generated from these experiments are consistent with recent findings from our research and demonstrate that a potent and selective Chk1 inhibitor such as SRA737 can effectively synergize with sub-therapeutic doses of gemcitabine to induce replication catastrophe and tumor cell death," said Dr. Alan R. Eastman, Professor at the Geisel School of Medicine at Dartmouth and the founding Director of the Molecular Therapeutics Research Program of the Norris Cotton Cancer Center at Dartmouth-Hitchcock. "I look forward to results from the clinical study Sierra is conducting which translates this novel strategy for the treatment of patients with advanced cancers."

"Chk1 is essential for managing replication stress (RS), which is intrinsically elevated in certain oncogene-transformed tumors, and can also be further enhanced by chemotherapeutic drugs like gemcitabine. While gemcitabine likely causes RS by depleting deoxynucleotide (dNTP) and damaging DNA, Chk1 protects against RS through a variety of molecular mechanisms. Consequently, tumor cells become highly reliant on Chk1 to manage replication stress and its downstream consequences in order to survive and continue to proliferate," added Dr. Christian Hassig, Senior Vice President of Research at Sierra Oncology. "Through our research, we have demonstrated that SRA737 has the potential to synergize with several clinically important chemotherapeutic inducers of RS to kill tumor cells in vitro at low concentrations. We also demonstrated that the combination of SRA737 and gemcitabine may prove efficacious in gemcitabine-resistant clinical settings and that SRA737 can be potentiated by sub-therapeutic doses of gemcitabine in animal models of cancer."

"Replication stress has been recognized as a potent driver of genomic instability, a fundamental hallmark of cancer, and is rapidly emerging as an area of dynamic scientific research," stated Dr. Nick Glover, President and CEO of Sierra Oncology.  "Tumors harboring high levels of intrinsic or exogenous forms of replication stress are potential candidates for therapeutic intervention using SRA737.  We are actively leveraging these concepts in our ongoing monotherapy and low-dose gemcitabine combination clinical trials."

About the Poster
Title: The Chk1 inhibitor, SRA737, demonstrates chemical synthetic lethality with replication stress-inducing agents, including novel low-dose gemcitabine, in preclinical models of cancer.
Poster #181; Abstract #B181:
The Poster is available on the company's website at www.sierraoncology.com.

Data reported in the Poster demonstrated that:

• The combination of SRA737 with a range of RS-inducing agents was highly synergistic in a panel of 15 cell lines of diverse tissue lineages, with the strongest synergy observed with gemcitabine.
• Profound synergy between SRA737 and gemcitabine was observed in several bladder cancer cell lines as well as in human patient-derived bladder cancer 3D cultures, further supporting the clinical development of this RS-inducing combination.
• Significant anti-tumor activity and increased survival (vs. control) were observed when SRA737 and gemcitabine were dosed in combination in a highly aggressive gemcitabine resistant bladder carcinoma PDX model. These findings suggest that the combination of SRA737 and gemcitabine may be efficacious in gemcitabine-resistant clinical settings.
• Strikingly, anti-tumor activity was observed when SRA737 was combined with a sub-therapeutic dose of gemcitabine in xenograft models of colorectal adenocarcinoma and osteosarcoma. This combination was shown to increase RS markers by three to five-fold over the change noted with gemcitabine alone. These findings support i) the development of SRA737 in combination with low, sub-therapeutic doses of gemcitabine and, ii) the broader application of this unique combination in tumor indications where gemcitabine is not standard of care.

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. SRA737 is currently being investigated in two Phase 1 clinical trials in patients with advanced cancer: a monotherapy study evaluating SRA737 in patients with tumors identified to have genetic aberrations hypothesized to confer sensitivity to Chk1 inhibition via synthetic lethality, and a study evaluating the combination of SRA737 potentiated by low-dose gemcitabine. Sierra is also preparing for potential clinical studies of SRA737 in combination with other agents where there is a strong biological rationale for synergy with Chk1 inhibition, such as immune oncology therapeutics, and other DDR inhibitors including PARP inhibitors.

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 market and industry position, expectations from current data, anticipated clinical development 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