• Patients eligible for this study must meet ALL of the following criteria:

• Patient must have signed a written informed consent prior to any trial specific procedures. When the patient is physically unable to give their written consent, a trusted person of their choice, independent from the investigator or the sponsor, can confirm in writing the patient's consent;

• Subject ≥18 years of age on day of signing informed consent form (ICF);

• Histologically proven TNBC defined as follows:

∙ HER2 negativity (ASCO/CAP criteria)

‣ AND less than 10% of cells stained by immunohistochemistry (IHC) for ER and PgR;

• TNBC patients previously treated by standard neoadjuvant chemotherapy with a minimum of 6 cycles of immunochemotherapy containing pembrolizumab, per standard of care (and pembrolizumab label) and anthracyclines and/or taxanes (with/without carboplatin). Other drugs may be acceptable following discussion with the sponsor (with the exclusion of capecitabine);

• Complete resection of the breast tumor(s) (and of any invaded lymph node);

• No complete pathological response, defined as RCB Class I, II or III (per local assessment);

• Available representative formalin-fixed paraffin-embedded (FFPE) tumor block from surgery specimen with its histological report;

• Eastern Cooperative Oncology Group (ECOG) Performance Status \<2;

• Adequate organ and bone marrow function. All screening lab tests should be performed within 28 days before inclusion;

⁃ Resolution to at least grade 1 of all acute toxicities from previous therapies including immune-related toxicity due to pembrolizumab, except alopecia and grade 2 immune-related endocrinopathies controlled by hormone replacement which are allowed;

⁃ Minimal/maximal period for prior treatments (i.e. minimal delay from last dose of prior treatment to C1D1): breast surgery (the wound must have healed prior to C1D1) ≥2 weeks (maximum 10 weeks); last pembrolizumab injection ≥3 weeks;

⁃ Women of child-bearing potential must have a negative serum pregnancy test within 7 days before C1D1;

⁃ Women of child-bearing potential and male patients must agree to use 1 effective form of contraception from the time of the negative pregnancy test up to 6 months after the last dose of study drugs;

⁃ Patient should be able and willing to comply with study visits and procedures as per protocol;

⁃ Patients must be affiliated to a Social Security System (or equivalent).

• Patients eligible for this cohort must meet ALL of the following criteria:

• Patient information prior to study entry and non-opposition to data collection

• Subject ≥18 years of age ;

• Histologically proven TNBC defined as follows:

∙ HER2 negativity (ASCO/CAP criteria)

‣ AND less than 10% of cells stained by immunohistochemistry (IHC) for ER and PgR;

• TNBC patients previously treated by standard neoadjuvant chemotherapy with a minimum of 6 cycles of immunochemotherapy containing pembrolizumab, per standard of care (and pembrolizumab label) and anthracyclines and/or taxanes (with/without carboplatin). Other drugs may be acceptable following discussion with the sponsor (with the exclusion of capecitabine);

• Complete resection of the breast tumor(s) (and of any invaded lymph node);

• No complete pathological response, defined as RCB Class I, II or III (per local assessment);

• Patient should have received at least one injection of pembrolizumab as post-surgery treatment (concomitantly or after radiotherapy).

• Non-exclusion criteria (for standard of care treated external cohort) :

• Patients eligible for this study must not meet ANY of the following criteria:

• Radiological or clinical evidence of metastatic disease documented by imaging or clinical examination after surgery.

• Has received capecitabine or other ICI than pembrolizumab in the NAC regimen;

• Has a known additional malignancy, excepted skin basal cell carcinoma, squamous cell carcinoma of the skin, or in situ cervical cancer or previously treated malignancy with no evidence of disease for ≥2 years;

• Any investigational anticancer therapy (chemotherapy, immunotherapy, biologic for cancer treatment) other than pembrolizumab only as adjuvant treatment.