Abstract OT2-19-02: Clinical evaluation of the efficacy and liquid molecular analysis of abemaciclib rechallenge upon progression to abemaciclib combination therapies for ER-positive HER2-negative metastatic breast cancer patients

Abstract Background: Abemaciclib is a drug approved for ER-positive and HER2-negative metastatic or recurrent breast cancer and those treatable in an early setting. Some patients with ER-positive and HER2-negative breast cancer pretreated with abemaciclib and endocrine therapy may still benefit from abemaciclib rechallenge, especially in case of acquired resistance to endocrine therapy. However, there is no evidence for abemaciclib rechallenge. To address this, a single-arm phase 2 trial is planned to determine whether abemaciclib rechallenge has clinical benefits. Translational research to evaluate the gene alteration and immunohistochemistry will be conducted as an accompanying research to determine the predictive biomarkers of resistance or sensitivity to abemaciclib. Patients and Methods: The eligible patients are histologically confirmed with ER-positive and HER2-negative invasive breast cancer, locally advanced or metastatic, and previously received no more than two lines of endocrine therapy. The patients who previously received abemaciclib in combination with endocrine therapy led to a clinical benefit, including complete response, partial response, or stable disease, during abemaciclib-based treatment (≥6 months). The patients who previously received chemotherapy, immune checkpoint inhibitors (excluding perioperative systemic treatment), everolimus, olaparib, and palbociclib for advanced or metastatic diseases are excluded. The patients will be treated with abemaciclib 150 mg orally q12h on days 1 to 28 of a 28-day cycle. The other endocrine drug prescribed was not used in previous treatment. We will evaluate the gene mutations and/or amplification of ESR1, PIK3CA, CCNE1, FGFR1, RB1, TP53, NF1, MYC, AR, and MET at the time of inclusion and three months after day 1 and ctDNA and immunostaining levels of HER3, PTEN, CCNE1, FGFR1/2, and AR at the time of inclusion. The primary endpoint is progression-free survival (PFS), while the secondary endpoints are overall response rate, clinical benefit rate, chemotherapy-free interval, overall survival (OS), safety, and evaluation of the mechanism of sensitivity and resistance to abemaciclib. The expected median PFS for the study treatment arm is 5.0 months compared to the previous data of 3.0 months. The number of necessary cases is 59, with a significance level of 5% (two-sided) and power of 80% during the two-year recruitment period and one-year follow-up period. In the planned total of 65 cases, 10% is considered inappropriate. The planned duration of enrollment and follow-up is two years and one year, respectively. The planned total study duration is four years. Subgroup analysis will be performed on ESR1 and PIK3CA mutations, disease site (visceral/bone only/other), previous lines of therapy for advanced or metastatic disease (first and second), endocrine therapy for pretreatment (aromatase inhibitors/fuluvestrant), performance status (0/1), organs involved (1/2/≥3), age (<65/≥65 years), and disease-free interval (<36 months/≥36 months) to examine the interaction of treatment effects on PFS and OS. This study was funded by Eli Lilly and Company. Citation Format: Meiko Nishimura, Takahiro Kogawa, Yuko Akaishi, Misato Ogata, Jun Masuda, Mitsuo Terada, Hitomi Sakai, Kazuki Nozawa, Sasagu Kurozumi, Takamichi Yokoe, Yukinori Ozaki Ozaki, Shu Yazaki, Mai Onishi, Tsutomu Iwasa, Takuma Onoe, Yuta Okumura, Sayaka Nakayama, Kanako Hagio, Yuko Takahashi, Hirokazu Tanino, Junji Tsurutani, Koji Matsumoto, Mototsugu Shimokawa, Toshimi Takano. Clinical evaluation of the efficacy and liquid molecular analysis of abemaciclib rechallenge upon progression to abemaciclib combination therapies for ER-positive HER2-negative metastatic breast cancer patients [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 OT2-19-02.