Background: Increasingly, HER2-positive early breast cancer (EBC) is treated by NACT combined with trastuzumab and pertuzumab followed by surgery. Ontruzant is registered as a biosimilar trastuzumab based on the totality of evidence including a randomized phase III study of NACT+Herceptin versus NACT+Ontruzant demonstrating similar pCR-rates (Pivot et al. J Clin Oncol 2018;36:968). However, no data exist for the efficacy of the combination of NACT with pertuzumab+Ontruzant (p+O). This investigator-initiated study was conducted to assess real world efficacy in HER2-positive EBC patients treated with NACT+p+O based on data from DBCG. DBCG has since 1977 provided guidelines for treatment of breast cancer and collected data from Danish hospital departments of surgery, pathology, and oncology prospectively on NACT, date and type of surgery and patho-anatomic findings. Methods: From the DBCG database, information was extracted for consecutive patients with unilateral early HER2-positive breast cancer registered to have received NACT+p+O from September 1, 2018 to August 31, 2019. pCR was defined as absence of residual invasive tumor in the breast and axillary lymph nodes (ypT0/Tis ypN0(i-)). Results: 215 patients received NACT+p+O. Median age was 54.8 years (range 24-81). NACT used, in combination with concurrent p+O, was cyclophosphamide+epirubicin followed by paclitaxel (62% on 6 cycles and 35% on 8 cycles) or other chemotherapy followed by paclitaxel (3%). Overall, 56% of patients achieved a pCR (Table). 68% of node-positive patients before receiving NACT+p+O had tumor-free axillary nodes after completing NACT+p+O. Conclusions: Real-world data from a nationwide population based study demonstrated a pCR-rate with NACT+p+O comparable to that seen in clinical studies with NACT+p+Herceptin (Chen et al. BMC Cancer 2019;19:973). pCR-rate was highly dependent on estrogen receptor (ER)-status and malignancy grade but not on clinical nodal status and tumor size. 68% of patients with cN+ converted to ypN0(i-).

*χ2-test p<0.05