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

This phase II trial studies how well a vaccine, STEMVAC, works in combination with standard endocrine-based therapy (ET) with a CDK4/6 targeted drug therapy, or with the chemotherapy drug capecitabine, in treating patients with hormone receptor (HR)-positive, HER2-negative breast cancer that has spread from where it first started (primary site) to other places in the body (metastatic). STEMVAC is designed to target proteins that cancer cells use when they become more aggressive and start to spread, and it is believed to work by boosting the immune system to recognize and destroy the invader tumor cells that are causing the disease. Standard ET is treatment that adds, blocks, or removes hormones in order to slow or stop the growth of cancer. Standard CDK4/6 inhibitors may stop the growth of tumor cells and may kill them by blocking some of the enzymes needed for cell growth. Capecitabine is in a class of medications called antimetabolites. It is taken up by tumor cells and breaks down into fluorouracil, a substance that kills tumor cells. Giving STEMVAC in combination with standard ET or chemotherapy may be an effective treatment for metastatic HR 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, 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 computed tomography (CT) or ultrasound-guided biopsies for research purposes, as well as collection of blood samples and 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 CT or ultrasound-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.