This open-label, prospective, single-arm clinical study aims to evaluate the efficacy and safety of pegfilgrastim in preventing febrile neutropenia (FN) among patients with triple-negative breast cancer (TNBC) receiving anthracycline/cyclophosphamide (AC) chemotherapy following neoadjuvant paclitaxel/carboplatin therapy. In the Keynote-522 regimen, paclitaxel and carboplatin are administered prior to the AC phase, which may increase cumulative myelosuppression and subsequently elevate the risk of FN during AC. Despite this clinical concern, real-world evidence supporting the prophylactic use of pegfilgrastim in Korean patients undergoing this regimen remains insufficient.

A total of 40 adult TNBC patients will be enrolled. Pegfilgrastim 6 mg will be administered subcutaneously once per cycle on Day 2 of each AC cycle (Cycles 1-4), approximately 24 hours after chemotherapy completion. The primary objective is to assess the incidence of FN during the four AC cycles. Secondary objectives include hospitalization due to FN, incidence of Grade 4 neutropenia, delays or dose reductions in chemotherapy due to neutropenia, and evaluation of hematologic and non-hematologic toxicities.

This study is descriptive in nature and does not involve hypothesis-testing sample size calculations. The sample size of 40 was determined based on feasible drug supply and is expected to provide clinically meaningful insight when compared with existing real-world data, in which the FN risk during AC without prophylactic G-CSF is historically reported at approximately 20-25%. The findings from this study may offer essential clinical evidence supporting the preventive use of pegfilgrastim during the AC phase in TNBC patients treated with the paclitaxel/carboplatin-leading neoadjuvant regimen.

Inclusion Criteria:

• 1. Women diagnosed with triple-negative breast cancer by pathology or cytology. 2. Age ≥19 and ≤70 years. 3. ECOG performance status 0-1. 4. Stage II-III breast cancer confirmed pathologically or clinically. 5. Planned to receive AC chemotherapy following paclitaxel/carboplatin as neoadjuvant therapy.

  6. Adequate organ function:

  1. Absolute neutrophil count (ANC) ≥1,000 cells/mm³

  2. Platelet count ≥75,000 cells/mm³

  3. Creatinine clearance ≥50 mL/min or serum creatinine <1.5× ULN

  4. Total bilirubin ≤1.5× ULN

  5. AST ≤2.5× ULN

  6. ALT ≤2.5× ULN 7. Ability to provide written informed consent and willingness to comply with study procedures.

     Exclusion Criteria:

• 1. Prior treatment with cytotoxic chemotherapy. 2. Prior treatment with G-CSF or pegylated G-CSF. 3. History of conditions that may affect bone marrow function (e.g., severe liver or kidney disease).

  4. Receipt of radiotherapy within 4 weeks prior to study initiation. 5. Active uncontrolled infection or inflammatory disease that may affect study outcomes.

  6. Receipt of blood transfusion within 60 days prior to study initiation. 7. Pregnant, breastfeeding, or unwilling to use appropriate contraception. 8. Any condition judged by the investigator to make the patient unsuitable for participation.