Inclusion Criteria:
1. Male or female aged 18 years or older.
2. Signed informed consent for study participation.
3. Histologically and/or cytologically confirmed invasive triple-negative breast adenocarcinoma (ER and PR \< 10%, HER2-negative), documented on all tumor samples.
4. Stage II disease: T2-T3N0M0 or T1-T2N1M0.
5. Tumor-infiltrating lymphocytes (TILs) ≥ 30% on the initial biopsy (TILs will be assessed according to the recommendations of the International TILs Working Group 2014).
6. Availability of tumor tissue samples.
7. ECOG Performance Status ≤ 1.
8. No contraindication to neoadjuvant chemotherapy or pembrolizumab.
9. Adequate bone marrow function, defined as follows (laboratory assessments performed within 21 days prior to enrollment):
Absolute neutrophil count ≥ 1,500/μL (1.5 × 10⁹/L) Hemoglobin ≥ 10 g/dL Platelet count ≥ 100,000/μL (100 × 10⁹/L)
10. Adequate hepatic function, defined as follows (laboratory assessments performed within 21 days prior to enrollment):
Total serum bilirubin ≤ 1.5 × ULN; for subjects with known Gilbert's syndrome, ≤ 3 × ULN is permitted AST (SGOT) ≤ 2.0 × ULN ALT (SGPT) ≤ 2.0 × ULN Alkaline phosphatase (ALP) ≤ 2.5 × ULN
11. Adequate renal function, defined as follows (laboratory assessments performed within 21 days prior to enrollment):
Serum creatinine ≤ 1.5 × ULN or estimated glomerular filtration rate (eGFR) ≥ 40 mL/min/1.73 m²
12. Left ventricular ejection fraction (LVEF) ≥ 50%, assessed by echocardiography. A routine LVEF assessment is acceptable if performed within 28 days prior to the start of screening.
13. Adequate serum potassium, magnesium, and calcium levels (\> ULN).
14. Negative serum pregnancy test at screening for subjects of childbearing potential.
15. Women of childbearing potential must agree to use a highly effective method of contraception during the screening period, throughout study treatment, and for at least 7 months after the last administration of study treatment. It is strongly recommended that the male partner of a female participant also use a male condom with spermicide throughout this period.
16. Affiliation with, or beneficiary of, a health insurance system.
17. Ability to communicate and understand French without difficulty.
Exclusion Criteria:
1. Metastatic disease at diagnosis.
2. Relapse of a previous breast cancer.
3. Bilateral breast cancer.
4. Invasive breast adenocarcinoma with ER and PR \> 10% and/or HER2-positive disease.
5. Any other malignancy within the previous 3 years, except for cancers considered adequately treated and at low risk of recurrence.
6. Pregnant or breastfeeding women, or women likely to become pregnant due to lack of effective contraception.
7. Contraindication to MRI.
8. Diagnosis of immunodeficiency.
9. Active autoimmune disease (e.g., inflammatory bowel disease, systemic lupus erythematosus, ankylosing spondylitis, scleroderma, multiple sclerosis, celiac disease, Wegener's granulomatosis) requiring systemic treatment within the previous 3 years (i.e., disease-modifying agents, corticosteroids, or immunosuppressive drugs).
10. Prior treatment with systemic immunosuppressive medications (except physiologic replacement corticosteroids) within 30 days before the planned start of study treatment.
11. Receipt of a live (attenuated) vaccine within 30 days before the planned start of study treatment.
12. Active infection, including:
Tuberculosis (TB); Hepatitis B (known positive hepatitis B surface antigen \[HBsAg\]). Subjects with past or resolved HBV infection (defined by positive anti-HBc antibodies and negative HBsAg) are eligible; Hepatitis C. Subjects positive for anti-HCV antibodies are eligible only if HCV RNA is negative by polymerase chain reaction (PCR); Human immunodeficiency virus (HIV) infection. HIV-positive subjects are eligible if receiving treatment and having an undetectable viral load.
13. Significant cardiovascular disease, including New York Heart Association (NYHA) Class II or higher heart disease, cardiomyopathy, acute inflammatory heart disease, myocardial infarction, transient ischemic attack (TIA), stroke within the previous 3 months, unstable arrhythmias, and/or unstable angina.
14. Participation in another study involving one or more investigational medicinal products within 3 weeks before study entry (or within a period equivalent to 5 half-lives before the first dose of the study intervention). Participation in observational studies or long-term follow-up of other studies is permitted provided that no procedures likely to interfere with the interpretation of the present study results are performed.
15. Prior systemic therapy or curative radiotherapy for the current breast cancer.
16. Ongoing acute urinary tract infection, pre-existing hemorrhagic cystitis, or known urinary tract obstruction.
17. Known or suspected hypersensitivity, or severe allergy, to any active substance or excipient of the study drugs, including lactose.
18. Any uncontrolled medical condition, neuropsychiatric disorder, or significant substance dependence that, in the opinion of the Principal Investigator, could interfere with study completion.
19. Vulnerable persons as defined under Article L.1121-6 of the French Public Health Code, adults under legal protection measures, or individuals unable to provide informed consent in accordance with Article L.1121-8 of the French Public Health Code.
20. Current use of St. John's Wort (Hypericum perforatum).
