Pembrolizumab in Combination With Paclitaxel in the Hormone Receptor-positive Metastatic Breast Cancer With High Tumor Mutational Burden Selected by Whole Exome Sequencing: Korean Cancer Study Group Trial (KCSG BR20-16)

Inclusion Criteria: * Pre or postmenopausal women with stage IV hormone receptor-positive breast cancer by histological or cytological confirmation * Be willing and able to provide written informed consent/assent for the trial * Progression after 1 or more lines of any systemic therapy (endocrine, HER2-targeted or chemotherapy) in the metastatic setting * Be over 19 years of age on day of signing informed consent * 70 or more nonsynonymous mutations per tumor by WES * Subject who has biopsy-accessible tumor for WES. Biopsy on breast tumor or axillary nodes is acceptable if locoregional recurrence after primary surgery occurs or de novo stage IV breast cancer is diagnosed * Have measurable disease based on RECIST 1.1. Biopsied tumor may be counted a measurable lesion if it is not excised * Documented disease progression on the most recent therapy * Life expectancy of \> 12 weeks * Have a performance status of 0 or 1 on the ECOG Performance Scale. * Demonstrate adequate organ function as defined in Table 3, all screening labs should be performed within 10 days of treatment initiation. * Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. * Female subjects of childbearing potential (Section 5.5.2) must be willing to use an adequate method of contraception as outlined…