Efficacy of neoadjuvant TCHP with primary empegfilgrastim (E) prophylaxis in HER2-positive early breast cancer (HER2+ eBC): Final results of the DEFENDOR SPECIAL study.

e12651 Background: Neoadjuvant therapy (NAT) is the current standard of care for treating HER2-positive breast cancer. Achieving a pathological Complete Response (pCR) – is a strong predictor of betteroutcomes. Dose reductions and delays may impair NAT efficacy in early breast cancer. Strategies to enhance pCR are needed. This prospective observational study assessed primary empegfilgrastim (G-CSF) prophylaxis impact on pCR in HER2+ early BC treated with neoadjuvant TCHP regimen. Methods: From April 2020 to September 2023, 105 patients (pts) with HER2+ eBC received NAТ: 6 docetaxel/carboplatin/trastuzumab + pertuzumab + empegfilrastim (TCHP+E). E was administered 24h post each NAT cycle. The primary endpoint was relative dose-intensity (RDI), pCR (ypT0/is ypN0) in the intention-to-treat (ITT) population was one of the main secondary endpoint (NCT04905329). Results: Median age was 52.4 years. Clinical stages II and III were present in 58.1% and 41% of patients, respectively; 59% had nodal involvement at baseline. Mean RDI of TCHP+E regimen was 96%. pCR was achieved in 72.3% of patients. RCB-I and RCB-II were observed in 6.6% and 15.2%, respectively. The highest pCR rate (87.8%) was in pts with HR-/HER2+ (3+) tumors (n = 33). Not a single case of febrile neutropenia was recorded; neutropenia Gr.4 – in 2 (1.9%) pts, Gr.3 – in 1 (1%) pts; thrombocytopenia Gr. 3, 2, 1 – in 1 (1%), 4 (3.8%), 32 (30.5%) pts respectively. Conclusions: The use of empegfilgrastim allows to provide 96% RDI. TCHP with empegfilgrastim shows high efficacy in HER2+ breast cancer across HR status and tumor stage. HER2 expression level is a significant predictor of pCR. Clinical trial information: NCT04905329 . Pathological complete response. HR +3-8 Allred Score (n=69) HR +7-8 Allred Score (n=47) HR + low3-6 Allred Score (n=22) HR -0-2 Allred Score (n=36) HER2 2 + / FISH + (n=10) HER2 3 + (n=95) IIA-IIIA stages (n=72) IIIB-IIIC stages (n=32) pCR, ypT0/is ypN0 46 (66,6%) 27 (57,4%) 19 (86,4%) 29 (80,6%) 4 (40%) 72 (75,8%) 54 (75%) 21 (65,6%)