CSPC PHARMA (01093): Results from the Phase I/II Clinical Study of SYS6043 (B7-H3 ADC) for the Treatment of Advanced Breast Cancer Presented as a Rapid Oral Presentation at the 2026 ESMO Congress

CSPC PHARMA (01093) announced that the results of the Phase I/II clinical study of SYS6043, a novel B7 homolog 3 (B7-H3)-targeted antibody-drug conjugate (ADC) independently developed by the Group, for the treatment of patients with advanced breast cancer (the Study), were presented as a rapid oral presentation (abstract number: 5RO) at the 2026 Annual Congress of the European Society for Medical Oncology (ESMO) on October 26, 2026. The results were presented by Professor Wang Hongxia of Fudan University Shanghai Cancer Center. The Study is a multicenter, open-label Phase I/II clinical study jointly led by Professor Zhang Li of Sun Yat-sen University Cancer Center and Professor Wu Lingying of Cancer Hospital Chinese Academy of Medical Sciences, aiming to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy of SYS6043 in patients with advanced solid tumors. This presentation focused on the breast cancer cohort. As of the data cutoff date (June 15, 2026), a total of 139 patients with advanced breast cancer were enrolled in this cohort, including hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) breast cancer (HR+/HER2- BC, n=84), triple-negative breast cancer (TNBC, n=32), and HER2-positive (HER2+) breast cancer (HER2+ BC, n=23). The median age of patients was 54 years, and most patients had previously received multiple lines of therapy. At this congress, the Group announced the positive results of the Study. The relevant safety and efficacy data are as follows: Safety: Among 139 patients who received at least one dose, SYS6043 was generally well tolerated. The incidence of treatment-related adverse events (TRAEs) was 99.3%, the incidence of Grade 3 TRAEs was 28.8%, and the incidence of serious TRAEs was 5.8%. The most common TRAEs included nausea (61.2%), leukopenia (60.4%), neutropenia (55.4%), anemia (51.1%), and fatigue (51.1%). The proportions of patients with dose interruptions and dose reductions due to TRAEs were 24.5% and 6.5%, respectively, and only 1 patient (0.7%) discontinued treatment due to TRAEs. No treatment-related deaths occurred in the Study. The incidence of interstitial lung disease (ILD) was 1.4%. Efficacy: SYS6043 demonstrated positive antitumor activity across all three breast cancer subtypes in efficacy-evaluable patients: HR+/HER2- BC (n=82): after a median follow-up of 5.1 months, the confirmed objective response rate (ORR) was 53.7%, the disease control rate (DCR) was 92.7%, and both median duration of response (DoR) and median progression-free survival (PFS) were not reached; TNBC (n=32): after a median follow-up of 6.5 months, the confirmed ORR was 75.0%, the DCR was 93.8%, median PFS was 7.2 months, and median DoR was 5.5 months; HER2+ BC (n=23): after a median follow-up of 6.4 months, the confirmed ORR was 69.6%, the DCR was 95.7%, median PFS was not reached, and median DoR was 7.0 months. SYS6043 demonstrated manageable safety and robust durable antitumor activity in pretreated patients with advanced breast cancer, supporting further investigation at the 6 mg/kg every three weeks (Q3W) dose. SYS6043 is an ADC targeting B7-H3 independently developed by the Group, with a topoisomerase I inhibitor payload and a drug-to-antibody ratio (DAR) of approximately 6. SYS6043 employs a crystallizable fragment gamma (Fc) receptor effector-silencing design and a cleavable linker, and has a bystander antitumor effect, aiming to enhance antitumor activity and reduce off-target toxicity. B7-H3 is highly expressed in a variety of solid tumors and has a relatively high expression frequency in breast cancer, making it a therapeutic target with good clinical application prospects.