Abstract P1-18-32: A nationwide real-world study for evaluation of efficacy and safety of T-DM1 in patients with HER2-positive locally-advanced unresectable or metastatic breast cancer in Korea (KCSG BR19-15)

Abstract Purpose: T-DM1, an antibody-drug conjugate, has significant antitumor activity in patients with HER2-positive metastatic breast cancer (mBC) patients who had progressed after trastuzumab-based chemotherapy. This study was conducted to investigate the clinical practice and factors related with outcomes of T-DM1 use for HER2-positive mBC patients in the nation-wide real-world setting. Method: This complete enumeration study included the patients with HER2-positive mBC who received T-DM1 as palliative therapy from August 2017 to December 2018 under the registry of Health Insurance Review & Assessment Service in Korea. Safety and outcomes of T-DM1 including overall response rate (ORR), progression-free survival (PFS), and overall survival were evaluated. Factors significant in univariate analysis were analyzed in multivariate model. Result: From the sixty institutions, a total of 824 patients were enrolled. Mean age was 58 years-old, 818 patients (99.3%) were female and 516 patients (62.6%) had relapsed after curative treatment. About 40% patients received T-DM1 as first or second line treatment, 21.5% received it as third line and 37.3% as fourth or over line. During a median follow-up of 16.8 months, the ORR was 32.8%, median PFS was 7.2 months and median OS was not reached. In multivariate analysis, clinical factors associated with the lower PFS were age (< 65 year-old, hazard ratio[HR] 1.53, 95% confidence interval[CI]: 1.214-1.919, p < 0.001), poor ECOG performance status (PS ≥ 2, HR 1.98, 95% CI: 1.493-2.626, p < 0.001), previous pertuzumab use (HR 1.40, 95% CI: 1.118-1.742, p = 0.003) and previous lapatinib use (HR 1.29, 95% CI: 1.047-1.586, p =0.017). The common grade 3-4 adverse events were thrombocytopenia (13.0%), neutropenia (2.9%), and elevation of liver enzyme (2.5%). Hypokalemia (≤ 3.0 mmol/L) and any-grade bleeding event such as epistaxis and gum-bleeding occurred in 25 (3.1%) and 94 patients (11.4%), respectively. Conclusion: This is the first, nationwide, real-world data about T-DM1 use of the HER2-positive metastatic breast cancer patients in Korea. The efficacy and toxicity profile of T-DM1 in the real-world practice were comparable with those of randomized trials. Patients’ factors and previous anti-HER2 therapy could predict the outcomes of T-DM1. Further studies to reveal the subtypes of good responders to T-DM1 and the sequence of anti-HER2 therapy are warranted. Citation Format: Sun Kyung Baek, Jae-Ho Jeong, Yeon-Hee Park, Hee Kyung Ahn, Min Hwan Kim, In Hae Park, Young Ju Suh, Dae-Won Lee, Sung Hoon Sim, Jee Hyun Kim, Hyun-Jeong Shim, Yeesoo Chae, Su-Jin Koh, Hyorak Lee, Jieun Lee, Jae-Ho Byun, Youngmi Seol, Eun Mi Lee, Jin Seok Ahn, Kyung-Hae Jung, Seock-Ah Im, Keun Seok Lee, Joohyuk Sohn, Kyoung Eun Lee. A nationwide real-world study for evaluation of efficacy and safety of T-DM1 in patients with HER2-positive locally-advanced unresectable or metastatic breast cancer in Korea (KCSG BR19-15) [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 P1-18-32.