• Adults ≥19 years old.

• Pathologically documented breast cancer that is unresectable or metastatic

• Tumor with homologous recombination deficiency (HRD) defined by

‣ Germline or Somatic BRCA1/2 mutation

⁃ Homologous recombination repair (HRR) genes mutation

⁃ HRD detected through RAD51 foci formation functional assay

⁃ HRR genes: ATM, BARD1, BRIP1, CDK12, CHEK1, CHEK2, FANCL, PALB2, PPP2R2A, RAD51B, RAD51C, RAD51D, and RAD54L

• Previously treated with a taxane, unless this treatment was contraindicated (whether in recurrent/metastatic setting or in neoadjuvant/adjuvant setting).

• Previous treatment with platinum therapy in the advanced or metastatic setting is permitted, provided the patient did not have a progression during the platinum treatment. If the patient was treated with neoadjuvant or adjuvant platinum therapy, at least 6 months of disease-free interval is required after the last dose.

• Documented radiologic progression (during or after most recent treatment or within 6 months after completing adjuvant therapy).

• \- If the patients had relapsed within 6 months after adjuvant therapy, this will be counted as a systemic chemotherapy for advanced or metastatic disease.

• At least 3 weeks has passed since last chemotherapy treatment

• At least 2 weeks has passed since last hormone therapy or radiation therapy (including palliative radiation).

• Eastern Cooperative Oncology Group (ECOG) performance status of 0, or 1

⁃ At least one measurable lesion that can be accurately assessed at baseline by computed tomography (CT) (magnetic resonance imaging \[MRI\] where CT is not feasible) and is suitable for repeated assessment as per RECIST v.1.1.

⁃ Male and female subjects of reproductive/childbearing potential must agree to use a highly effective form of contraception or avoid intercourse during and upon completion of the study and for at least 7.0 months after the last dose of study treatment.

⁃ \- This study recommend Copper T intrauterine device as a highly effective methods of contraception (\<1% failure rate)

⁃ Adequate normal organ and marrow function measured within 28 days prior to administration of study treatment

∙ Hemoglobin ≥9.0 g/dL

‣ Absolute neutrophil count (ANC) ≥ 1.5 x 109/L

‣ Platelet count ≥ 75 x 109/L

‣ Serum bilirubin ≤ 2.0mg/dL \[This will not apply to patients with confirmed Gilbert's syndrome (persistent or recurrent hyperbilirubinemia that is predominantly unconjugated in the absence of hemolysis or hepatic pathology), who will be allowed only in consultation with their physician.\]

‣ AST (SGOT)/ALT (SGPT) ≤2.5 x institutional upper limit of normal unless liver metastases are present, in which case it must be ≤5x ULN

‣ Adequate renal function: Serum creatinine ≤1.5mg/dL or estimated creatinine clearance \>60 mL/min

⁃ Negative urine pregnancy test within 7 days prior to registration in premenopausal patients.

⁃ Ability to understand and comply with protocol during study period

⁃ Patients should sign a written informed consent before study entry