Abstract P2-13-32: Pyrotinib in combination with letrozole for estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer: A multicenter, single-arm, phase II trial

Abstract Background: HER2-targeted agents combined with endocrine therapy (ET) has been recommended as an optional therapeutic strategy for hormone receptor (HR)/human epidermal growth factor receptor 2 (HER2) co-positive metastatic breast cancer (MBC). Pyrotinib is an oral irreversible pan-ErbB receptor tyrosine kinase inhibitor targeting EGFR, HER2 and HER4, with proven efficacy in combination with chemotherapy in HER2-positive MBC. This multicenter, single-arm phase 2 trial aimed to investigate the efficacy and safety of pyrotinib plus letrozole in patients with estrogen receptor (ER)-positive, HER2-positive MBC (NCT04407988). Methods: Pre-/perimenopausal or postmenopausal women with histologically confirmed HER2-positive (immunohistochemistry [IHC] 3+ or 2+ with fluorescence in situ hybridization positive) and ER-positive (the percentage of ER+ cells ≥ 10% by IHC) MBC were enrolled. Prior treatment for metastatic disease was not allowed. Eligible patients received pyrotinib (400 mg, po, qd) plus letrozole (2.5 mg, po, qd) until disease progression or unacceptable toxicity. For pre-/perimenopausal patients, additional treatment with ovarian function suppression (OFS) was required. The primary endpoint was clinical benefit rate (CBR), defined as the rate of patients with complete response (CR), partial response (PR), or stable disease (SD) for at least 24 weeks per RECIST 1.1. Secondary endpoints included objective response rate (ORR), progression-free survival (PFS), overall survival (OS), and safety. Results: Between December 3, 2019 and April 2, 2021, 26 patients were enrolled. As of July 5, 2021, CBR was 76.9% (20 of 26; 95% CI 56.4% to 91.0%) and ORR was 57.7% (15 of 26; 95% CI 36.9% to 76.6%). One of the 26 patients (3.8%) had CR, 14 (53.8%) had PR, 5 (19.2%) had SD, 5 (19.2%) had progressive disease, and 1 (3.8%) had not evaluable disease. The benefits in CBR and ORR were observed across all subgroups. The most common any grade AEs were diarrhea (25 of 26, 96.2%), vomiting (8 of 26, 30.8%), nausea (6 of 26, 23.1%), and oral ulceration (6 of 26, 23.1%). Diarrhea was the only reported grade 3 AE that occurred in 5 patients (19.2%). No grade 4 or 5 AEs occurred during this study. Conclusions: Pyrotinib combined with letrozole showed an encouraging antitumor activity with good tolerance in patients with ER/HER2 co-positive MBC, promising as an alternative treatment option for this disease. The study is ongoing. Citation Format: Quchang Ouyang, Huihua Xiong, Min Yan, Jincai Zhong, Li Ran, Ting Luo, Liping Liu, Jing Li, Xiaohong Yang, Huawu Xiao, Ning Xie, Hui Wu, Jianxiang Gao, Jun Lu, Xuming Hu, Zheyu Hu, Can Tian, Zhengrong Shui, Min Cao. Pyrotinib in combination with letrozole for estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer: A multicenter, single-arm, phase II trial [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-32.