A phase II study evaluating KN026 monotherapy in patients (pts) with previously treated, advanced HER2-expressing gastric or gastroesophageal junction cancers (GC/GEJC)

4040 Background: Outcomes of second or later-line treatment for pts with advanced GC/GEJC remain inferior. KN026 is a novel HER2-targeted bispecific antibody composed of VH regions of trastuzumab and pertuzumab, targeting the HER2 juxtamembrane domain (IV) and the dimerization domain (II) simultaneously. KN026 has shown promising antitumor efficacy in preclinical and phase I studies. Here we present the results of KN026 monotherapy in previously treated, advanced HER2-expressing GC/GEJC. Methods: In this multi-center, single-arm, open-label, 2-cohort phase II study, adult pts with previously treated, advanced GC/GEJC were assigned into a HER2 high-level cohort (Cohort 1: IHC3+ or IHC 2+ ISH+) or a HER2 low-level cohort (Cohort 2: IHC 1+/2+ ISH- or IHC 0/1+ISH+). KN026 was given intravenously in 10 mg/kg QW, 20 mg/kg Q2W, or 30 mg/kg Q3W. Primary endpoints were objective response rate (ORR) and duration of response (DoR) assessed by investigators per RECIST 1.1. Secondary endpoints included progression-free survival (PFS), overall survival (OS), and safety outcomes. This trial is registered at ClinicalTrials.gov (NCT03925974). Results: At data cut-off (Oct. 29, 2021), 45 pts were enrolled and treated with KN026; 39 were eligible for response evaluation (25 pts in Cohort 1 and 14 pts in Cohort 2). In Cohort 1, ORR was 56% (95%CI 35%-76%) with 14 PR; the median DoR was 9.7 months (mo) (95%CI 4.2-not evaluable (NE)). At a median follow-up of 14.7 mo (95%CI 9.4-16.5), the median PFS was 8.3 mo (95%CI 4.2-11.4), and median OS was 16.3 mo (95% CI 11.0- NE). In Cohort 2, ORR was 14% (95%CI 2%-43%), with median DoR being 6.2 mo (95%CI 3.2-NE). At a median follow-up of 27.5 mo (95%CI 4.1-NE), the median PFS was 1.4 mo (95%CI 1.4-4.1), and median OS was 9.6 mo (95% CI 3.5-14.9). The table shows efficacy in Cohort 1 pts who have progressed on the prior trastuzumab treatment. KN026-related adverse events (TRAEs) were observed in 37 (82%) pts; the most common TRAEs (any grade) were increased aspartate aminotransferase (12, 27%), increased alanine aminotransferase (9, 20%), rash (7,16%), anemia (7,16%), and infusion-related reaction (7,16%). Four pts reported 5 grade 3 TRAEs, including infusion-related reaction, renal hydrocele, ureteral stenosis, increased blood pressure, and abnormal hepatic function (1 each). No grade 4 or 5 TRAEs occurred. Conclusions: KN026 monotherapy yielded promising efficacy with mild to moderate toxicities in pts with previously treated, advanced GC/GEJC. Further investigation is warranted. Clinical trial information: NCT03925974. [Table: see text]