Updated efficacy of anti-TROP2 ADC ESG401 for first-line metastatic TNBC in phase 1b study.

2 Background: ESG401, a novel ADC comprising a humanized anti-TROP2 IgG1 monoclonal antibody conjugated to SN-38, with a Drug-Antibody Ratio of 8 and a stable cleavable linker, demonstrated preliminary efficacy and tolerability in the Phase Ia trial (Ma, ASCO 2023). This report presents updated results from the first-line mTNBC cohort of the Phase Ib trial and results from patients with brain metastases. Methods: Patients (pts) aged ≥18 years with confirmed local advanced/unresectable or metastatic TNBC, without prior metastatic treatment, received ESG401 (16 mg/kg IV on Day 1,8, and 15, with a 28-day cycle) until unacceptable toxicity, progressive disease, or consent withdrawal. Results: As of the cutoff date (Apr 7 th , 2024), 24 pts in the cohort received ≥1 dose of ESG401. Median age was 53 years (range: 33-73), 33.3% were ECOG 0, 25.0% were de novo stage IV, 70.8% had visceral disease (8.3% brain, 33.3% liver, 50.0% lung). Updated efficacy results are shown in the Table. Thirteen pts (68.4% of 19 efficacy evaluable pts) remained on treatment, with the longest on-treatment duration being 10.3 mons. Median PFS has not been reached. Two pts in this cohort had brain metastases, with one achieving a complete intracranial response and an overall response of PR, and the other demonstrating overall PR with a -13.8% reduction in brain lesion size. Totally 16 pts with brain metastases were enrolled in the entire Phase Ia/1b trials, for these pts, the best overall intracranial response rate was 31%, with an intracranial disease control rate of 75%, while the corresponding overall response rate and disease control rate were 50% and 69%, respectively. The response of intracranial lesions to the treatment is consistent with that of extracranial lesions. The safety profile of ESG401 remained consistent with previous reports, showing no new or unexpected safety signals. Conclusions: ESG401 monotherapy demonstrates promising antitumor activity in 1L mTNBC pts, characterized by a high response rate and durable response. The ORR numerically exceeded that reported in the BEGONIA Study arm 7 in which pts with the same indication were treated with TROP-2 ADC and PD-L1 combination therapy. Additionally, ESG401 shows clear potential for treating HER2-negative breast cancer with brain metastases, including mTNBC and HR+/HER2- BC. These results support further clinical investigation of ESG401. Clinical trial information: NCT04892342 . [Table: see text]