To evaluate the safety and tolerability of escalating doses GX-I7 in combination with standard dose pembrolizumab, and to evaluate objective response rate (ORR) in subjects with refractory or relapsed TNBC

Key Inclusion Criteria:

1. Triple negative must be defined as guidelines of American Society of Clinical Oncology(ASCO)/ College of American Pathologist(CAP): Estrogen Receptor (ER) < 1% positive tumor nuclei, Progesterone Receptor (PR) < 1% positive tumor nuclei, and negative for HER2 by IHC 1+, 0 or negativity status confirmed by in situ hybridization (ISH).
2. Subject must have received anthracycline and taxane based chemotherapy for TNBC
3. Has measurable disease as defined by RECIST 1.1 as assessed by the the local site Investigator/radiology.
4. Female subjects, age ≥ 19 years at the time of consent.

Key Exclusion Criteria:

1. Known severe hypersensitivity (≥ Grade 3) to pembrolizumab, pembrolizumab formulation excipients or GX-I7 formulation excipients or cyclophosphamide formulation.
2. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or with an agent directed to another co-inhibitory T-cell receptor (ie, CTLA-4, OX-40, CD137) or has previously participated in a Merck pembrolizumab (MK-3475) clinical trial.