Phase II Study to Evaluate Safety and Efficacy of Neratinib, an Irreversible Pan-Her Tyrosine Kinase Inhibitor, and Trastuzumab Biosimilar in Patients with HER2 Mutated Advanced Solid Tumors (KCSG AL20-17).

3134 Background: Neratinib, an irreversible pan-HER tyrosine kinase inhibitor, has single agent clinical activity in HER2 mutated cancer. This open-label single arm phase 2 trial investigated the efficacy and safety of the dual anti-HER2 therapy, neratinib plus trastuzumab in heavily pretreated patients with HER2 mutated solid tumors excluding HER2 amplifications. Methods: Neratinib was administered 240mg p.o. daily with trastuzumab biosimilar (Herzuma) every 3 weeks. The primary endpoint was objective response rate (ORR) by RECIST v1.1. Secondary endpoints were duration of response (DoR), progression free survival (PFS), and safety. An exploratory biomarker study was also conducted. Results: Forty patients were enrolled with a median follow-up of 11.08 months (range, 0.39-21.63 months) and a median of 3 (range 1-9) lines of prior systemic therapy. Tumor types included lung (42.5%), colorectal (20%) biliary (12.5%), and breast (7.5%). Sixty percent (60%) of patients (n=24) had HER2 mutation in the kinase domain. Among evaluable patients (n=39), the ORR was 23.1% (CR=0, PR=9) with a median DoR of 11.18 months (95% CI 0-22.63). Median PFS was 3.42 months (95% CI 1.57-5.27) and median overall survival was 9.47 months (95% CI 2.93-16.01). Grade ≥3 adverse events (AEs) occurred in 47.5% of patients (n=19) and diarrhea was the most frequently reported AE (n=10; 25%) followed by bilirubin elevation (n=2; 5%), and anemia (n=2; 5%). Conclusions: In heavily pretreated patients with HER2 mutated solid tumors, neratinib plus trastuzumab showed a moderate response rate, with a durable duration of response. Most AEs were manageable, highlighting the need for prophylactic diarrhea management. Exploratory biomarker results will be presented at the upcoming meeting. Clinical trial information: NCT06083662 .