ASTRAEA: ReinvigorAting ReSponse To ImmunotheRApy in MEtAstatic TNBC With Combination Myeloid Inhibition and Radiation

Inclusion Criteria * Age ≥ 18 years at the time of consent. * Patients with metastatic or locally recurrent TNBC who received prior immunotherapy in any setting (curative or advanced/metastatic). * Have ≥2 measurable, non-osseous, non-hepatic recurrent or metastatic lesions. Of these lesions, at least one is indicated for radiation therapy and at least one will not be irradiated to be used to determine response. Notes on lesions: patients are still eligible if they have liver or bone lesions, so long as at least 2 measurable lesions are non-osseous, non-hepatic recurrent or metastatic per above criterion. * Histologically or cytologically-confirmed TNBC, defined as ER \<10%, PR \<10%, HER2-negative by ASCO CAP guidelines. Note: HER2 0-2+ IHC is allowed (i.e., if FISH-negative, this is still considered HER2-negative). * ECOG Performance Status ≤ 1. * Demonstrate adequate organ function * Female subject of childbearing potential should have a negative serum or urine pregnancy test or documentation of absence of pregnancy by a gynecologist within 14 days of initiating first dose of pembrolizumab for eligibility verification. * Female subjects of childbearing potential should be willing to use highly effective contraceptive methods (Appendix 12.6) during the treatment period through 120 days after the last dose of study medication. Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for \> 1 year. * Male particip…