VOLUNTARY ANNOUNCEMENT - PHASE I CLINICAL STUDY DATA OF NOVEL B7-H3 ANTIBODY-DRUG CONJUGATE SYS6043 PRESENTED IN AN ORAL SESSION AT THE 2026 ASCO ANNUAL MEETING

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CSPC PHARMACEUTICAL GROUP LIMITED (Incorporated in Hong Kong with limited liability) (Stock Code: 1093) ʮ ̡ VOLUNTARY ANNOUNCEMENT PHASE I CLINICAL STUDY DATA OF NOVEL B7-H3 ANTIBODY-DRUG CONJUGATE SYS6043 PRESENTED IN AN ORAL SESSION AT THE 2026 ASCO ANNUAL MEETING The Board of Directors (the ‘‘Board ’’) of CSPC Pharmaceutical Group Limited (the ‘‘Company ’’, together with its subsidiaries, the ‘‘Group ’’) is pleased to announce that CSPC Megalith Biopharmaceutical Co., Ltd., a subsidiary of the Group, has presented the Phase I clinical study (the ‘‘Study ’’) data of SYS6043 in an oral session at the 2026 American Socie ty of Clinical Oncology (‘‘ASCO ’’) Annual Meeting. SYS6043 is a novel antibody-drug conjugate (ADC) targeting B7-H3, with a topoisomerase I inhibitor as the payload and a drug-to-antibody ratio (DAR) of approxi mately 6. It utilises an Fc γ receptor effector silencing design and a cleavable linker, aimed at e nhancing anti-tumour activity and reducing off-target toxicity. B7-H3 is widely and highly expressed i n multiple tumour types, but is minimally expressed in normal tissues. The results of the Study indica ted that SYS6043 had the potential to become a treatment option for multiple tumour types. The Study is a multi-centre, open-label, single-arm Phase I /II clinical study (ChiCTR2400094683) conducted in China, designed to evaluate the safety, tolera bility, pharmacokinetic profile, and preliminary efficacy of SYS6043 in patients with advanced s olid tumours. The Study consists of two phases: dose escalation and dose expansion. In the dose esca lation phase, the Study adopted the Bayesian Optimal Interval (BOIN) design to explore a total o f eight treatment regimens, including 1.2 mg/kg, 2.4 mg/kg, 4 mg/kg, 6 mg/kg, 8 mg/kg, and 10 mg/kg once e very 3 weeks (Q3W), as well as 4 mg/kg and 6 mg/kg once every 2 weeks (Q2W). The primary endpoi nts include safety, tolerability, and the recommended Phase II dose (for the Phase I stage), as well as the objective response rate (for the Phase II stage). – 1 – As at 31 March 2026, a total of 627 patients were enrolled in th e Study, including 168 patients with lung cancer, 147 with gynaecological tumours, 137 with brea st cancer, 66 with nasopharyngeal carcinoma, 50 with head and neck squamous cell carcinoma, 22 with oesophageal squamous cell carcinoma, 14 with liver cancer, and 23 with other tumour typ es. The median number of prior lines of therapy was 2 (range: 1 –10), the median age was 57.0 years, and male patients account ed for 58.9%. SYS6043 demonstrated favourable anti-tumour activity in p atients across multiple tumour types. Among the 533 efficacy-evaluable patients, the objective r esponse rate ( ‘‘ORR’’) was 41.8%, the disease control rate (DCR) was 81.4%, and the median duratio n of response ( ‘‘mDoR ’’) was 6.9 months. The efficacy data for patients with various tumour t ypes are as follows: for lung cancer (n=134), the ORR was 46.3% and the mDoR was 6.9 months (95% con fidence interval [CI]: 4.3, not evaluable [NE]); for breast cancer (n=125), the ORR was 56.8 % and the mDoR was 7.0 months (95% CI: 3.9, NE); for gynaecological tumours (n=139), the ORR wa s 42.4% and the mDoR was 6.9 months (95% CI: 5.6, NE); for nasopharyngeal cancer (n=63), the ORR was 28.6% and the mDoR was 5.8 months (95% CI: 4.4, NE); for oesophageal squamous cell carc inoma (n=19), the ORR was 15.8% and the mDoR was 4.5 months (95% CI: 1.3, NE); in addition, for hea d and neck squamous cell carcinoma (n=30), the ORR was 16.7%, and for liver cancer (n=7), the ORR was 42.9%. The preliminary analysis results of the Study showed no sign ificant correlation between B7-H3 expression levels and the efficacy of SYS6043. SYS6043 was g enerally well-tolerated, with only 2 cases of dose-limiting toxicities (DLT) occurring in the 10 mg/kg Q3W dose cohort, which were Grade 3 gastrointestinal disorders and Grade 4 febrile neutropen ia, respectively. Treatment-related adverse events (TRAEs) occurred in 96.7% of the patients. The incide nce of Grade ≥3 TRAEs was 37.2%, which was generally low and predominantly manifested as hae matological toxicities. The most frequent TRAEs included anaemia (16.1%), neutropenia (12.6%), and l eucopenia (9.7%), while the incidence of all other Grade ≥3 TRAEs was ≤3.5%. Based on the positive results of the Study, the Group is curre ntly actively advancing related studies in ovarian cancer, breast cancer, and small cell lung cancer. By Order of the Board CSPC Pharmaceutical Group Limited Cai Dong Chen Chairman Hong Kong, 2 June 2026 As at the date of this announcement, the Board comprises Mr. C AI Dong Chen, Dr. CAI Lei, Mr. WEI Qingjie, Mr. ZHANG Cuilong, Mr. WANG Zhenguo, Mr. WANG Huaiy u, Dr. LI Chunlei, Dr. YAO Bing, Mr. CAI Xin, Mr. CHEN Weiping, Mr. QU Zhiyong and Mr. ZHANG Yiw ei as Executive Directors; and Mr. WANG Bo, Mr. CHEN Chuan, Prof. WANG Hongguang, Mr. AU Chun Kwok Alan, Mr. LAW Cheuk Kin Stephen and Ms. LI Quan as Independent Non-executive Dir ectors. – 2 –

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