Phase Ib/II Study of Zanidatamab in Combination with Tislelizumab and Chemotherapy in First-Line HER2-Positive Gastric/Gastroesophageal Junction Adenocarcinoma

Abstract Purpose: This phase Ib/II trial (NCT04276493) assessed the antitumor activity, safety, and pharmacokinetics (PK) of zanidatamab in combination with tislelizumab and chemotherapy in patients with advanced HER2-positive (HER2+) gastric cancer/gastroesophageal junction cancer (GEJC). Patients and Methods: Adult patients with previously untreated, unresectable, locally advanced/metastatic HER2+ gastric cancer/GEJC received zanidatamab 30 mg/kg i.v. (cohort A) or zanidatamab 1800 mg i.v. (weight <70 kg)/2,400 mg i.v. (weight ≥70 kg; cohort B) once every 3 weeks (Q3W). Both cohorts received tislelizumab 200 mg i.v. once every 3 weeks and standard chemotherapy [capecitabine and oxaliplatin (CAPOX)] once every 3 weeks. Primary endpoints were investigator-assessed confirmed objective response rate (cORR) per RECIST v1.1, in addition to the frequency and severity of adverse events (AE) and serious AEs. Secondary endpoints included investigator-assessed progression-free survival (PFS), duration of response (DoR), overall survival (OS), PK, and immunogenicity of zanidatamab. Results: As of December 7, 2023, 33 patients (cohort A, n = 19; cohort B, n = 14) received treatment. The confirmed objective response rate was 75.8%; the median duration of response, progression-free survival, and overall survival were 23.3, 16.7, and 32.4 months, respectively. The most common treatment-related AEs (TRAEs) were diarrhea (100%), nausea (63.6%), and decreased appetite (48.5%). Treatment-related AEs of grade ≥3 were reported in 22 (66.7%) patients; diarrhea was the most common (27.3%). Conclusions: Zanidatamab, in combination with tislelizumab and CAPOX, demonstrated clinically meaningful antitumor activity with a manageable safety profile as first-line therapy for patients with HER2+ gastric cancer/GEJC. These results support a further development of zanidatamab and tislelizumab with chemotherapy in this patient population in the ongoing phase III HERIZON-GEA-01 trial (NCT05152147).