Background: The therapeutic options for patients (pts) with previously treated metastatic TNBC are limited compared with other breast cancer subtypes. SKB264 is an antibody drug conjugate (ADC) composed of an anti-TROP2 antibody coupled to the cytotoxic belotecan-derivative via a novel linker. Here, we present results from a Phase 2 expansion cohort for pts with mTNBC (NCT04152499). Methods: Pts with previously treated mTNBC were enrolled to receive SKB264 4 mg/kg Q2W or 5 mg/kg Q2W in a non-randomized fashion until disease progression/unacceptable toxicity. The assessment for tumor response was performed every 8 weeks per RECIST v1.1 assessed by investigator. The TROP2 expression was scored using the semi-quantitative H-score method, and cut off point was set to 200. TROP2 expression and its association with anti-tumor activity was retrospectively analyzed. Results: At data cut-off date (May 15, 2022), 59 pts were enrolled (23 in 4 mg/kg, 36 in 5 mg/kg), 88% of them (52 pts) had received ≥3 prior therapies for metastatic disease. Among 53 patients with tissue available for TROP2 testing, 29 patients (55%) had TROP2 high (H-score >200-300) tumors. The median follow up was 9.6 months. Of 55 pts (21 in 4 mg/kg and 34 in 5 mg/kg) evaluable for response assessment (≥1 on-study scan), the confirmed ORR (cORR) was 40% (22/55) and disease control rate (PR+CR+SD) was 80% (44/55). The cORR was 55% (16/29) in the subset of patients with high TROP2 expression. The median duration of response (DoR) was not reached with range from 1.0+ to 11.0+ months and the 6-month DoR rate was 82%. Median PFS was 5.7 months (95% Cl: 3.9, 7.6). Treatment-related adverse events (TRAEs) of ≥ Grade 3 were reported in 55.9% (33/59) of pts. The most common ≥ Grade 3 TRAEs (≥ 10%) were neutrophil count decreased (23.7%), anemia (20.3%) and platelet count decreased (16.9%). TRAEs led to dose reduction in 15.2% (9/59) of pts and to discontinuation in 6.8% (4/59) of pts. No treatment-related AEs leading to death or interstitial lung disease (ILD) were reported. Safety and anti-tumor activities of SKB264 by dose level will be presented. Conclusions: SKB264 demonstrates a manageable safety profile and promising antitumor activity in pts with heavily pretreated mTNBC. SKB264 toxicity was mainly hematologic. A Phase 3 study of SKB264 vs investigator selected chemo alone in pts with locally advanced inoperable or metastatic TNBC was initiated (NCT05347134).

Citation Format: Yongmei Yin, Xinhong Wu, Quchang Ouyang, Min Yan, Lihua Song, YunPeng Liu, Zhongsheng Tong, Cuizhi Geng, Ying Wang, Guohua Yu, Xiang Wang, Ying Cheng, Weihong Zhao, Qun Li, Yina Diao, Gesha Liu, Junyou Ge, Jin Li. Efficacy and safety of SKB264 for previously treated metastatic triple negative breast cancer in Phase 2 study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-03-02.