Abstract P5-16-03: Phase II study of trastuzumab biosimilar (Herzuma®) plus gedatolisib in patients with HER-2 positive metastatic breast cancer who progressed after 2 or more HER-2 directed chemotherapy (BR 18-13, KM-10A): Interim analysis

Abstract Background: Prognosis of patients with HER-2 positive metastatic breast cancer (MBC) has been revolutionized with the development of monoclonal antibodies targeting HER-2 and antibody-drug conjugate, but resistance to anti-HER-2 therapy is inevitable ultimately. PI3K-AKT-mTOR pathway aberration is known to be one of the key resistance mechanisms. BR18-13(KM-10A) study is a phase 2 clinical trial evaluating efficacy and safety of Herzuma® (trastuzumab biosimilar) plus Gedatolisib (dual PI3K/mTORC inhibitor) in patients with HER-2 positive MBC who progressed after multiple lines of therapy. Methods: Patients with HER-2 positive MBC with known PIK3CA pathologic mutation or amplification whose disease progressed after more than 2 HER-2 directed therapy were enrolled in the study. They received Herzuma® (8mg/kg IV for 1st cycle loading dose, and then 6mg/kg IV every 3 weeks) plus Gedatolisib (180mg on D1, 8, 15 of every 21 days). We evaluated efficacy of the combination treatment as interim analysis. The data cutoff of this interim analysis was June 8, 2021. Results: 17 patients were enrolled and followed for a median of 6.2 months. At data cutoff, 17 patients were eligible for response assessment. All patients were confirmed to have pathologic PIK3CA aberrations: 9 kinase mutations (H1047X), 5 helical mutations (E545X), 2 other point mutations, and 1 amplification. Overall, response rate was 64.7% and disease control rate was 82.4%. Eleven patients reached partial response (PR) as their best response, three were stable disease (SD), and three had progressive disease (PD). Two patients who have reached PR remain on investigational treatment until the data cutoff point, and the longest one is on treatment for 12.0 months. The median progression-free survival assessed in data cutoff time was 5.9 months. One patient ended treatment due to CNS disease progression, but her visceral metastatic lesions were decreased with experimental treatment. No fatal adverse events related to trial medication were reported. Conclusion: In this phase 2 study, Trastuzumab biosimilar plus Gedatolisib presented 64.7% of response rate with manageable toxicity in patients with HER-2 positive MBC with PIK3CA mutation. Clinical trial information: NCT03698383 Acknowledgement: this research was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health &Welfare, Republic of Korea (Grant number: HI17C2206). Citation Format: Ju Won Kim, Hee Kyung Ahn, Jong Gwon Choi, Yee Soo Chae, Gyeong-Won Lee, Keon Uk Park, Eunmi Lee, Sung Hoon Sim, Jee Hyun Kim, Yeon Hee Park, Miso Kim, Jin Hyun Park, Jeong Eun Kim, Han Jo Kim, Mi Sun Ahn, So Yeon Oh, Min Hwan Kim, Su-Jin Koh, Kyoung Eun Lee, Myoung Joo Kang, Jae Ho Byun, Joo Young Ha, Jung Hye Kwon, Joo Young Jung, Su Ee Lee, Inhae Park, Kyong Hwa Park. Phase II study of trastuzumab biosimilar (Herzuma®) plus gedatolisib in patients with HER-2 positive metastatic breast cancer who progressed after 2 or more HER-2 directed chemotherapy (BR 18-13, KM-10A): Interim analysis [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 P5-16-03.