Sierra Oncology Announces Oral Presentation of Momelotinib Pivotal Phase 3 Data at European Hematology Association Annual Meeting

– Oral presentation of MOMENTUM pivotal phase 3 data in myelofibrosis patients who are symptomatic and anemic –

– Poster presentation to highlight improved transfusion independence, symptoms and spleen volume of these myelofibrosis patients who also present with thrombocytopenia –

Sierra Oncology, Inc. (NASDAQ: SRRA), a late-stage biopharmaceutical company on a mission to deliver transformative therapies for rare cancers, today announced two abstracts have been accepted into the program for the 2022 Annual Meeting of the European Hematology Association (EHA). An abstract presenting the full data from the pivotal phase 3 MOMENTUM study in myelofibrosis patients who are symptomatic and anemic has been selected for oral presentation. Additionally, a subset analysis from the trial evaluating safety and efficacy for patients with low platelet counts has been selected for poster presentation.

“We are excited and honored to have the MOMENTUM data selected for an oral presentation at EHA which demonstrates that momelotinib achieved statistically significant and clinically important efficacy across all prespecified primary and key secondary endpoints,” said Barbara Klencke, MD, Chief Medical Officer of Sierra Oncology. “In addition, we are pleased to present the subset analysis in a poster presentation which indicates momelotinib may be safely administered and provide an improvement in symptoms, spleen and anemia in anemic myelofibrosis patients who present with low platelet counts. The data to be presented further expand upon the potential of momelotinib as a treatment option for myelofibrosis patients who are symptomatic and anemic.”

Abstract: S195: MOMENTUM: Phase 3 Randomized Study of Momelotinib (MMB) versus Danazol (DAN) in Symptomatic and Anemic Myelofibrosis (MF) Patients Previously Treated with a JAK Inhibitor

The primary and all key secondary results, as well as safety data, from the MOMENTUM pivotal Phase 3 trial of momelotinib will be presented in an oral presentation by Srdan Verstovsek, MD, PhD, co-Principal Investigator of the study. Topline results from the study were announced in January 2022. Key data to be presented include:

• Primary Endpoint of Total Symptom Score (TSS) of >50%: 25% in the MMB arm vs. 9% in the control arm (p=0.0095)
• Secondary Endpoint of Transfusion Independence (TI): 31% in the MMB arm vs. 20% in the control arm (one-sided p=0.0064; non-inferiority)
• Secondary Endpoint of Splenic Response Rate (SRR) >35%: 23% in the MMB arm vs. 3% in the control arm (p=0.0006)
• The rate of Grade 3 or worse adverse events in the randomized treatment period was 54% in the MMB arm and 65% in the control arm. Serious treatment emergent adverse events were 35% in the MMB arm and 40% in the control arm.
• Mean baseline characteristics for all patients were TSS of 27, Hemoglobin (Hgb) of 8 g/dL and platelet count of 145 x 10⁹/L

Presentation Details

Abstract: S195

Title: MOMENTUM: Phase 3 Randomized Study of Momelotinib (MMB) versus Danazol (DAN) in Symptomatic and Anemic Myelofibrosis (MF) Patients Previously Treated with a JAK Inhibitor

Presenter: Srdan Verstovsek, MD, PhD, The University of Texas MD Anderson Cancer Center

Session Title: Treatments and Complications in MPN

Location: Hall Lehar 1-2

Date and Time: Saturday, June 11, 2022, 11:30 am – 12:45 pm CEST

Abstract: P1050: Thrombocytopenic Myelofibrosis (MF) Patients Previously Treated with a JAK Inhibitor in a Phase 3 Randomized Study of Momelotinib (MMB) versus Danazol (DAN) [MOMENTUM]

Abstract P1050 will highlight an analysis of MOMENTUM patients with baseline platelet counts of <150 x 10⁹/L on key symptom, anemia and spleen volume endpoints at 24 weeks. Results to be presented are consistent with the overall intent-to-treat analysis set. Momelotinib was superior to danazol for symptom responses, transfusion requirements and spleen responses, and showed a trend toward improved overall survival up to Week 24 was seen with momelotinib versus danazol [95% CI = 0.490 (0.195, 1.235)].

Presentation Details

Abstract: P1050

Title: Thrombocytopenic Myelofibrosis (MF) Patients Previously Treated with a JAK Inhibitor in a Phase 3 Randomized Study of Momelotinib (MMB) versus Danazol (DAN) [MOMENTUM]

Presenter: Alessandro Vannucchi, MD, Center Research and Innovation for Myeloproliferative Neoplasms, AOU Careggi, University of Florence, Italy

Session Title: Poster Session

Date and Time: Friday, June 10, 2022, 4:30 – 5:45 pm CEST

About Sierra Oncology

Sierra Oncology is a late-stage biopharmaceutical company on a mission to deliver targeted therapies that treat rare forms of cancer. We harness our deep scientific expertise to identify compounds that target the root cause of disease to advance targeted therapies with assets on the leading edge of cancer biology. Our team takes an evidence-based approach to understand the limitations of current treatments and explore new ways to change the cancer treatment paradigm. Together we are transforming promise into patient impact.

For more information, 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 expectations regarding the potential and future success of momelotinib. 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 commercialize momelotinib, 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 may be unable to obtain and enforce intellectual property protection for its technologies and momelotinib 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.

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