Abstract PS5-03-26: A Prospective Observational Study of Atezolizumab plus Nab-Paclitaxel in Patients with Triple-Negative Breast Cancer: First Analysis of JBCRG-C08/ATTRIBUTE

Abstract Background: Atezolizumab plus nab-paclitaxel has been available as a standard treatment for Programmed Death-Ligand 1 (PD-L1)-positive metastatic triple-negative breast cancer (mTNBC) in Japan since 2019, based on the results of the IMpassion130 trial. However, real-world evidence remains insufficient. Moreover, while immune-related adverse events (irAEs) are known to occur not only during treatment but also after discontinuation, detailed characterization of these events is limited. The ATTRIBUTE study was designed to evaluate the safety and effectiveness of atezolizumab plus nab-paclitaxel in a real-world patient population, including those who would have been excluded from the pivotal clinical trial. Methods: This multicenter, prospective observational study was conducted in Japan. Key eligibility criteria included: 1) age ≥20 years, 2) PD-L1-positive mTNBC, 3) planned atezolizumab treatment, 4) no hypersensitivity to atezolizumab, 5) ≤2 prior systemic therapy lines for mTNBC. The primary outcome was the incidence of adverse events as assessed by investigators. Secondary outcomes included progression-free survival (PFS), objective response rate (ORR), clinical benefit rate (CBR), and others. The observation period was set at 2 years from the final patient enrollment, with this report presenting the first analysis data collected 6 months after the final patient enrollment. The protocol was approved by the institutional review board and informed consent was obtained from all patients. Results: A total of 151 patients were enrolled from 79 institutions between February 15, 2021, and April 30, 2024. The safety analysis population comprised 149 patients, and the full analysis set included 148 patients. Baseline characteristics included: median age: 59 years, ECOG PS 0: 75.2%, postoperative recurrence: 73.2%, and history of neoadjuvant/adjuvant therapy: 59.7%. This study included patients who would have been excluded from IMpassion130: ECOG PS ≥2 (4.1%) and history or presence of autoimmune disease (7.4%). The incidence of serious adverse events (SAE) was 16.8%. SAE occurring in ≥1% of patients included pneumonitis (3.4%, n=5), diarrhea (1.3%, n=2), fever (1.3%, n=2), adrenal insufficiency (1.3%, n=2) and heart failure (1.3%, n=2). Two cases of Grade 5 pneumonitis were observed. The incidence of late-onset irAEs (31 days to 6 months after treatment discontinuation) associated with atezolizumab was 2.7%, including reports of adrenal insufficiency, pneumonitis, and palmar-plantar erythrodysesthesia syndrome. The median progression-free survival details will be presented at the meeting. The ORR was 33.6% (95% CI: 25.3-42.7), and the CBR was 37.7% (95% CI: 29.1-46.9). Conclusion: In this first analysis of one of the largest real-world patient populations, no new safety signals were observed for atezolizumab combination therapy compared with previous reports. Further follow-up is ongoing, and findings will be reported at upcoming scientific conferences and publications. Clinical trial identification: UMIN000041747 Funding: Chugai Pharmaceutical Co., Ltd. Citation Format: T. Shimoi, N. Shibata, M. Kitada, M. Tsuneizumi, M. Yamamoto, Y. Ozaki, Y. Fujimoto, S. Akiyoshi, N. Hashimoto, M. Yamaguchi, Y. Yamamoto, N. Masuda, H. Tada, T. Yamanaka, Y. Kikawa, T. Taira, S. Nakagawa, K. Kiyota, M. Oba, N. Niikura. A Prospective Observational Study of Atezolizumab plus Nab-Paclitaxel in Patients with Triple-Negative Breast Cancer: First Analysis of JBCRG-C08/ATTRIBUTE abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-03-26.