Abstract LBO1-02: Pathologic complete response (pCR) of neoadjuvant therapy with or without atezolizumab in HER2-positive, early high-risk and locally advanced breast cancer: APTneo Michelangelo randomized trial

Abstract Background: Neoadjuvant dual targeting of HER2 with trastuzumab (H) and pertuzumab (P) plus chemotherapy is the standard of care for high-risk HER2-positive (HER2+) breast cancer (BC). Compelling data show the contribution of the immune system in prognosis and response/resistance to HER2 directed therapies, supporting combination of immune checkpoint inhibitors with anti HER2 antibodies. We designed APTneo to test the role of adding atezolizumab to neoadjuvant dual targeting of HER2 with chemotherapy, and the value of also using anthracyclines in this setting. Methods: In this multicenter open label phase III trial (NCT03595592), 661 patients (pts) with high-risk early and locally advanced (LA) centrally confirmed HER2+ BC were randomized to neoadjuvant HPCT (H and P d1, carboplatin and paclitaxel iv d1 and 8) given q3 wks for 6 cycles w/o (n=223, ARM A) or with atezolizumab 1200 mg iv d1 (n=438, ARM B). In Arm B pts were randomized to Arm B1 (n=218) to receive anthracycline and cyclophosphamide (AC) + atezolizumab iv d1 q3 wks for 3 cycles followed by HPCT + atezolizumab for 3 cycles, or to arm B2 (n=220) to receive HPCT + atezolizumab for 6 cycles. After surgery pts continued adjuvant HER2 directed therapies w/wo atezolizumab until completion of 1 year. Among intent-to-treat (ITT) pts, 44.8% were LABC, 35% hormonal receptor (HR) negative and 30.4% PD-L1 positive. Primary endpoint is event-free survival (EFS) of Arm B vs A. A key secondary endpoint is the rate of pCR (ypT0/Tis, ypN0) w/wo atezolizumab. Primary population for all efficacy endpoints is ITT. We also assessed baseline PD-L1 status (Ventana SP142) and stromal Tumor Infiltrating Lymphocytes (sTILs). Results: pCR rate in Arm B (57.8%) vs Arm A (52.0%) was not significantly increased (adjHR 1.33, 95% CI 0.95-1.86; p=0.091). Also, the difference in pCR rate in Arm B1 (61.9%) vs Arm B2 (53.6%) was not significant (adjHR 1.402, 95% CI 0.95-2.07; p=0.089). Compared to Arm A, Arm B1 had a 9.9% significantly higher pCR rate (multivariate analysis in Table). The different pCR rate in arms B1 and A was similar regardless of HR and PD-L1 status. High sTILs (≥30%) and PD-L1positive tumors had a higher likelihood of pCR in all arms. Serious adverse events (SAE) after start of therapy occurred in 6.8% pts in Arm A and 14.1% in Arm B (p=0.0064). SAE were numerically more frequent in Arm B1 than Arm B2 (16.7% and 11.6%, respectively), due to hematological toxicity with AC. Immune-related SAE were quite infrequent and similar in B1 (4.7%) and B2 (7.8%), respectively. No grade 5 AE did occur. Conclusions: Addition of atezolizumab to chemotherapy and HP did not significantly increase the rate of pCR in women with HER2+ BC. An exploratory analysis showed that adding atezolizumab to neoadjuvant AC followed by HPCT led to higher pCR rate compared to HPCT and atezolizumab. This could be because of anthracyclines themselves or to drug-drug enhancement of anthracyclines and immune modulation. Atezolizumab did not cause major tolerability issues. Molecular studies of collected biospecimens are ongoing. Patients will continue to be followed up for EFS and overall survival analyses. Table. Supported in part by unrestricted grant from Hoffman-La Roche, Ltd, Switzerland Citation Format: Luca Gianni, Elisabetta Munzone, Mauro Mansutti, Giampaolo Bianchini, Yann Izarzuzaga, Elena Rota Caremoli, Luc Dirix, Lucia Del Mastro, Santiago González-Santiago, Andreas Schneeweiss, Jose Ponce, Chiun-Shen Huang, Pauline Wimberger, Sevilay Altintas, Miguel Martín, Nicoleta Antone, Christian F. Singer, Ugo De Giorgi, Ana Godoy Ortiz, Hans-Joachim Lueck, Riccardo Spezia, Angelica Fasolo, Giuseppe Viale, Pinuccia Valagussa. Pathologic complete response (pCR) of neoadjuvant therapy with or without atezolizumab in HER2-positive, early high-risk and locally advanced breast cancer: APTneo Michelangelo randomized trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr LBO1-02.