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A Study of GDC-9545 Alone or in Combination With Palbociclib and / or Luteinizing Hormone-Releasing Hormone (LHRH) Agonist in Locally Advanced or Metastatic Estrogen Receptor-Positive Breast Cancer

This study will evaluate the safety, pharmacokinetic (PK), pharmacodynamic (PD) activity, and preliminary anti-tumor activity of GDC-9545 as a single agent and in combination with palbociclib and / or luteinizing hormone?releasing hormone (LHRH) agonist in patients with advanced or metastatic estrogen receptor (ER)-positive (human epidermal growth factor receptor 2 [HER2]-negative) breast cancer.
Phase I
Mol. targeted/Immunotherapy/Biologics
GDC-9545, LHRH agonist, Palbociclib
Mayer, Ingrid
Vanderbilt University


18 Years
Inclusion Criteria:

Inclusion Criteria for Dose Escalation: - Histologically or cytologically proven diagnosis of adenocarcinoma of the breast with evidence of either locally recurrent disease not amenable to resection or radiation therapy with curative intent or with metastatic disease - ER-positive tumor - HER2-negative breast cancer as per local laboratory testing - Measurable disease, or evaluable bone disease; that is, bone lesions that are lytic or mixed (lytic + sclerotic) in the absence of measurable lesion - Required paired pre- and on-treatment tumor biopsies for participants with metastases that are safely accessible as determined by the investigator - Advanced or metastatic ER-positive/HER2-negative breast cancer that has recurred or progressed while being treated with adjuvant endocrine therapy for a duration of at least 24 months and/or endocrine therapy in the incurable, locally advanced, or metastatic setting and derived a clinical benefit from therapy (i.e., tumor response or stable disease for at least 6 months) - No more than 2 prior lines of treatment for advanced or metastatic breast cancer - ? 2 weeks must have elapsed from the use of any other endocrine, targeted therapy or chemotherapy - Single-Agent Cohorts (only applies to Dose Escalation): Advanced or metastatic disease that is either refractory to or intolerant of existing standard therapy or for which no effective standard therapy that confers clinical benefit is available - Cohort B0: No prior treatment with Cyclin-Dependent Kinase (CDK) 4/6 inhibitor - For participants undergoing 18F-fluoroestradiol-positron emission tomography (FES-PET) imaging additional restrictions on prior therapy include: ?2 months must have elapsed from the use of tamoxifen; ?6 months must have elapsed from the use of fulvestrant - Postmenopausal status - Eastern Cooperative Oncology Group (ECOG) Performance Status ?1 - Resolution of all acute toxic effects of prior therapy or surgical procedures to baseline or Grade ?1 (except alopecia or other toxicities not considered to be a safety risk for the patient) - Life expectancy of ?12 weeks - Adequate organ function Inclusion Criteria for Dose Expansion: Same as above, except: - In South Korea: Must have received exactly 2 prior lines of treatment for advanced or metastatic breast cancer - In the rest of the world: No more than one prior line of treatment for advanced or metastatic breast cancer And plus: - Cohort B1?2: No prior treatment with CDK4/6 inhibitor - Cohorts A1, A3, and B1 only: Postmenopausal status - Cohorts A2, A4, and B2 only: Participants not defined as post-menopausal - No prior treatment with an oral selective estrogen receptor degrader (SERD) - For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use non-hormonal contraceptive methods with a failure rate of

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