Pembrolizumab, Letrozole, and Palbociclib in Treating Postmenopausal Patients With Newly Diagnosed Metastatic Stage IV Estrogen Receptor Positive Breast Cancer

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PRIMARY OBJECTIVES: I. To evaluate the objective response rate (ORR), based on Response Evaluation Criteria in Solid Tumors [RECIST], version 1.1), of pembrolizumab in combination with letrozole and palbociclib in patients with newly diagnosed metastatic estrogen receptor (ER)+human epidermal growth...

PRIMARY OBJECTIVES: I. To evaluate the objective response rate (ORR), based on Response Evaluation Criteria in Solid Tumors [RECIST], version 1.1), of pembrolizumab in combination with letrozole and palbociclib in patients with newly diagnosed metastatic estrogen receptor (ER)+human epidermal growth factor receptor (HER)2- breast cancer, and determine if the addition of pembrolizumab to letrozole and palbociclib combination can achieve an improved response rate (ORR = complete response [CR] + partial response [PR]) measured from the study baseline, based on RECIST version 1.1. SECONDARY OBJECTIVES: I. To determine the safety and tolerability of adding pembrolizumab (200 mg every 3 weeks) to letrozole (2.5 mg) and palbociclib (125 mg 3 weeks on, one week off) in patients with metastatic ER+HER2- breast cancer. II. To evaluate the CR rate. III. To evaluate progression-free survival (PFS). IV. To evaluate overall survival (OS). V. To evaluate duration of response (DOR) using RECIST version 1.1. VI. To evaluate clinical benefit rate (CBR) using RECIST version 1.1. VII. To evaluate toxicities (using the National Cancer Institute [NCI] Common Terminology Criteria for adverse Events [CTCAE], version 4.0) associated with the triple drug combination (pembrolizumab, letrozole, and palbociclib) in patients with metastatic ER+HER2- breast cancer. VIII. To evaluate CR, PR, ORR, PFS, DOR, and CBR using immune-related Response Criteria In Solid Tumors (irRECIST); time to treatment failure will also be assessed. TERTIARY OBJECTIVES: I. To study cellular/humoral immune response by analyzing immune and stromal cell characteristics before and after treatment that correlate with clinical response; this includes programmed cell death 1 ligand 1 (PD-L1) expression levels. II. To study the peripheral serum thymidine kinase (TK) level and its association with treatment response. III. To study circulating tumor deoxyribonucleic acid (DNA) (ctDNA) and the effect of combining pembrolizumab, letrozole, and palbociclib on ctDNA profiles. IV. To evaluate genomic and phenotypic status of breast tumor. OUTLINE: Patients receive letrozole orally (PO) once daily (QD) on days 1-28 and palbociclib PO QD for 3 weeks. Cycless with letrozole and palbociclib repeat every 28 days in the absence of disease progression or unacceptable toxicity. Patients also receive pembrolizumab intravenously (IV) over 30 minutes on day 1. Cycles with pembrolizumab repeat every 21 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 30 days, every 6 months for 3 years, and then every 12 months for 1 year.

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