Efficacy of disitamab vedotin (RC48) plus tislelizumab and S-1 as first-line therapy for HER2-overexpressing advanced stomach or gastroesophageal junction adenocarcinoma: A multicenter, single-arm, phase II trial (RCTS).

4009 Background: Integrating immunotherapy with trastuzumab and chemotherapy has shown a significant improvement in overall response rate (ORR) and progression-free survival (PFS) in HER2-overexpression gastric or gastroesophageal junction cancer (GC/GEJC). However, the overall survival (OS) did not achievestatistical significance, and there was no benefit in patients with a PD-L1 combined positive score (CPS) <1. RC48, an antibody-drug conjugate drug, has proven its effect in advanced HER2-positive (HER2 3+ or 2+ by IHC, regardless of FISH positivity) GC/GEJC in later-line treatment setting. This trial was to assess the efficacy and safety of combining RC48, immunotherapy (Tislelizumab), and S-1 as a first-line treatment for HER2-overexpressing GC/GEJC. Methods: This was a single-arm, multi-center, phase II trial conducted in patients with HER2-overexpressing metastatic or unresectable GC/GEJC for first-line treatments. Participants received RC48 (2.5 mg/kg), Tislelizumab (200 mg), and S-1 (40-60 mg BID for 14 days) every 3 weeks until disease progression or intolerable toxicities. The primary endpoint was ORR, while the secondary endpoints included disease control rate (DCR), PFS, OS, and safety. Results: A total of 47 patients were enrolled from 8 centers, median 65 (39 ~ 81) years old, 37 male, 37 GC and 8 GEJC, and all adenocarcinoma. Of them, 30 (63.8%) were HER2 IHC 3+, 11 (23.4%) HER2 IHC 2+/FISH+, and 6 (12.8%) IHC 2+/FISH-. Patients with PD-L1 CPS≥5, ≥1 accounted for 14.9%, 36.2% respectively. With a median follow-up duration of 116.5 days, patients received a median of 5.0 treatment cycles. As of January 20, 2024, 40 patients were included for efficacy analysis, and 44 patients for safety. The ORR reached 95.0% (38/40, 95% CI: 83.1-99.4%), clinical Complete Response rate was 20.0% (5/40, 95% CI: 4.2-26.8%), and the DCR was 100.0% (95% CI: 91.2-100.0%). The median PFS and OS were not reached, while the 6-month and 9-month PFS rates were 100.0% and 80.8%, the 6-month and 9-month OS rates were 100.0% and 83.8%. ORR among the HER2 IHC 3+, IHC 2+/FISH+, and IHC 2+/FISH- groups showed no significant difference (93.3%, 100.0%, 100.0%). ORR for patients with CPS≥1 or < 1 were 100.0%, 92.9%, respectively. One stage IVA GC patient (cT4bN3M0, HER2 3+, CPS 0) and another stage IVB GEJC one (cT3N2M1, HER2 2+/FISH+, CPS≥5) received radical surgery and both achieved pathological complete response. Grade 3/4 treatment-related adverse events occurred in 40.9% of patients, notably neutropenia, fatigue, and diarrhea. Conclusions: The combination of RC48, Tislelizumab, and S-1 showed notable efficacy with manageable safety in the first-line treatment of advanced HER2-overexpressing GC/GEJC patients. These findings support further exploration in this context. Clinical trial information: NCT05586061 .