• Must be competent and able to comprehend, sign, and date an IRB approved ICF before the performance of any study specific procedures or tests.

• Participants 18 years or older.

• Pathologically documented breast cancer that:

∙ Is defined as unresectable/metastatic disease.

‣ Is Human Epidermal Growth Factor Receptor 2 (HER2)-negative, defined as HER2- immunohistochemistry (IHC) 0, 1+ or 2+ ISH negative, and estrogen receptor (ER)- and progesterone receptor (PgR)- negative, both defined as IHC \<10%

‣ PD-L1 positive disease as per IHC via CPS \> 10 by IHC (defined by the number of PD-L1 staining cells \[tumor cells, lymphocytes, macrophages\] divided by the total number of viable tumor cells, multiplied by 100) via FDA-approved SP263 assay..

‣ Has been treated with up to one line of systemic cytotoxic chemotherapy with pembrolizumab in the metastatic setting. If recurrence occurred within six months of (neo)adjuvant chemotherapy, (neo)adjuvant chemotherapy would count as one line of therapy.

‣ Prior immune checkpoint inhibition with systemic chemotherapy is required in either neo(adjuvant) or metastatic settings

‣ Prior targeted therapies (e.g., olaparib, or others upon discussion with study sponsor-investigator) do not count as systemic cytotoxic chemotherapy lines and are unlimited prior to enrolment.

• Documented radiologic progression (during or after most recent treatment) or intolerance to prior line of therapy regardless of prior response with subsequent medical need for change of therapy.

• Must have an adequate archival tumor sample \<3 years old available for assessment of PD-L1 status by IHC via CPS. If archival tissue is not available or inadequate for assessment (e.g., decalcified bone, cytology, or other), a fresh biopsy is required on enrolment.

• Presence or absence of measurable lesion based on computed tomography (CT) or MRI per modified Response Evaluation Criteria in Solid Tumors (mRECIST) version 1.1 are both allowed.

• ECOG PS 0 or 1.

• Left ventricular ejection fraction (LVEF) ≥50% within 6 months prior to enrolment.

• Adequate laboratory parameters (table 1) within 14 days from C1D1

⁃ Adequate contraception when indicated (table 2), such as:

• Evidence of post-menopausal status or negative serum pregnancy test for participants of childbearing potential who are sexually active with a non-sterilized male partner.

∙ For participants of childbearing potential, a negative result for serum pregnancy test (test must have a sensitivity of at least 25 mIU/mL) must be available at the screening visit and urine beta-human chorionic gonadotropin (β-HCG) pregnancy test prior to each administration of study drugs.

∙ Participants of childbearing potential are defined as those who are not surgically sterile (i.e., underwent bilateral salpingectomy, bilateral oophorectomy, or complete hysterectomy) or post-menopausal. Participants will be considered post-menopausal if they have been amenorrheic for 12 months without an alternative medical cause.

∙ Participants of childbearing potential who are sexually active with a non-sterilized male partner must use at least one highly effective method of contraception, presented in table 3, from the time of screening and must agree to continue using such precautions for seven months after the last dose of study drugs.

∙ Female participants must refrain from breastfeeding while on study and for seven months after the last dose of study drugs.

∙ Female participants must not donate, or retrieve for their own use, ova from the time of enrolment and throughout the study treatment period, and for at least seven months after the final study drug administration. Preservation of ova may be considered prior to enrolment in this study.

∙ Complete heterosexual abstinence for the duration of the study and drug washout period is an acceptable contraceptive method if it is in line with the participant's usual lifestyle (consideration must be made to the duration of the clinical trial); however, periodic, or occasional abstinence, the rhythm method, and the withdrawal method are not acceptable.

∙ Non-sterilized male participants who are sexually active with a female partner of childbearing potential must use a condom from screening to five months after the final dose of study drugs.

∙ Male participants should refrain from fathering a child or freezing or donating sperm from the time of enrolment, throughout the study, and for five months after the last dose of study drugs. Preservation of sperm should be considered prior to enrolment in this study.