ESMO 2026: CSPC and Alphamab’s HER2 Bispecific ADC JSKN003 Achieves 100% ORR in Phase II First-Line Gastric Cancer Study

BOSTON/SHANGHAI — A novel bispecific antibody-drug conjugate from CSPC Pharmaceutical Group and Alphamab Oncology delivered a 100% objective response rate in one of three treatment cohorts in a Phase II trial for first-line HER2-positive gastric cancer, according to data presented at the ESMO 2026 Congress.

The study evaluated JSKN003, a HER2 bispecific ADC also known as pukang'anni tuodankang, in 25 treatment-naive patients with unresectable locally advanced or metastatic gastric or gastroesophageal junction adenocarcinoma whose tumors were HER2-positive with a PD-L1 combined positive score of at least 1. The data cutoff was May 9, 2026.

Patients were assigned to one of three combination regimens. Cohort A received JSKN003 at 6.3 mg per kilogram plus sintilimab at 360 mg every three weeks and capecitabine. Cohort B received JSKN003 plus sintilimab, capecitabine, and anitumab — a HER2 bispecific antibody — at 10 mg per kilogram every three weeks. Cohort C received JSKN003 plus SG001, capecitabine, and oxaliplatin.

The confirmed objective response rates were 71.4% (5 of 7 patients) in Cohort A, 100% (14 of 14 patients) in Cohort B, and 50.0% (2 of 4 patients) in Cohort C. Duration of response, progression-free survival, and overall survival had not yet matured at the time of analysis.

The complete response in Cohort B, which added the HER2 bispecific antibody anitumab to the backbone regimen, is likely to draw attention from oncologists and investors alike. The cohort enrolled 14 patients — the largest of the three — and every one of them showed tumor shrinkage meeting response criteria.

On safety, the most frequently reported treatment-related adverse events across all cohorts included decreased appetite and anemia, each occurring at notable rates, alongside hypoalbuminemia, thrombocytopenia, nausea, and diarrhea. Grade 3 or higher treatment-related adverse events were observed in 57.1% (4 of 7) of patients in Cohort A, 28.6% (4 of 14) in Cohort B, and 100% (4 of 4) in Cohort C. No treatment-related deaths were reported.

Cohort C's 100% rate of severe adverse events, though based on just four patients, stands in sharp contrast to the more manageable safety profile in Cohort B. The addition of oxaliplatin in that regimen is a well-known driver of hematologic toxicity, and the small sample size warrants caution in interpretation.

The trial marks a significant step for Alphamab Oncology, a Suzhou-based biotech listed in Hong Kong, and CSPC Pharmaceutical Group, one of China's largest pharmaceutical conglomerates. The companies are betting that a bispecific ADC approach — targeting two HER2 epitopes simultaneously — can improve both efficacy and tolerability compared with conventional HER2-directed therapies such as trastuzumab-based regimens that have long been the standard of care in HER2-positive gastric cancer.

Gastric cancer remains one of the most lethal malignancies worldwide, with HER2-positive tumors accounting for roughly 15% to 20% of cases in Western populations and an even higher proportion in Asia. First-line treatment options have expanded in recent years with the addition of immunotherapy, but response rates remain suboptimal, creating a clear unmet need for more effective combinations.

The Phase II data, while preliminary and based on a small sample, position JSKN003 as a candidate worth watching as it moves through clinical development. Longer follow-up will be critical to determine whether the early responses translate into durable progression-free and overall survival benefits.