A Vaccine (STEMVAC) With Standard Endocrine-Based Therapy or Chemotherapy for the Treatment of Metastatic Hormone Receptor Positive, HER2 Negative Breast Cancer

OUTLINE: Patients with ET-sensitive disease are assigned to Cohort 1, while patients with ET-resistant disease are assigned to Cohort 2. COHORT 1: After completion of 2 cycles of standard of care (SOC) ET + CDK4/6 inhibitor (CDK4/6i) therapy or abemaciclib alone, patients receive STEMVAC intradermally (ID) on the following schedule: 1) Three "priming" doses every 28 days; 2) Two "booster" doses at 6 and 9 months after "priming" dose #1; and 3) Additional "booster" doses every 6 months in the absence of disease progression or unacceptable toxicity. Patients also undergo image-guided biopsies for research purposes, as well as collection of blood samples and computed tomography (CT) or positron emission tomography (PET) scans throughout the trial. COHORT 2: After completion of 1 cycle of SOC capecitabine treatment, patients receive STEMVAC ID on the following schedule: 1) Three "priming" doses every 28 days; 2) Two "booster" doses at 6 and 9 months after "priming" dose #1; and 3) Additional "booster" doses every 6 months in the absence of disease progression or unacceptable toxicity. Patients also undergo image-guided biopsies for research purposes, collection of blood samples, and fluoroestradiol (FES) positron emission tomography (PET) scans, as well as CT or PET scans throughout the trial. After completion of study treatment, patients are followed every 6 months for 3 years.