Biomarkers of response in patients with gastric/gastroesophageal junction adenocarcinoma (GEA) treated with telisotuzumab adizutecan.

490 Background: c-Met protein (MET protein) overexpression (OE) and MET amplification (amp) occur in several cancer types, including GEA, and are associated with poor prognosis. The c-Met–directed antibody-drug conjugate telisotuzumab adizutecan (ABBV-400) is being assessed in advanced solid tumors in an ongoing, first-in-human study (NCT05029882). Results showed promising antitumor activity and a tolerable safety profile in GEA (Strickler et al. ESMO 2024, 1439P). Here, we present our analysis on associations between c-Met OE, MET amp, and ctDNA-derived molecular response with clinical outcomes. Methods: c-Met expression was evaluated retrospectively in FFPE tissue (archival/fresh) collected at study screening using IHC (SP44 Ab clone) or whole transcriptomic RNA sequencing (Caris). Plasma ctDNA collected at baseline (BL) and cycle 3 day 1 (C3) was analyzed with the GuardantINFINITY assay. Molecular response (MR) was assessed using circulating tumor fraction (cTF), estimated on the basis of ctDNA variant allele frequencies in a 74-gene panel (Guardant360 CDx) and methylation signals (GuardantINFINITY methylome platform). MR was defined as a 50% decrease in cTF from BL. MET amp status was determined retrospectively by the Guardant Health algorithm. Radiographic response was assessed per RECIST v1.1. Results: Clinical data from 42 patients (pts) with advanced GEA were included in this biomarker analysis. There was a negative correlation between c-Met expression and tumor size change from BL (R=-0.59). Of 42 pts, 12 (29%) had tumors with MET amp per ctDNA analysis/local site reports, which was higher than the bioinformatic analysis of real-world data (Caris-ConcertAI linked RWD360 and PT360; ~3%). Concordance between c-Met expression per IHC and RNA was observed (R=0.75, p<10 -6 ). There were 12 responders; 7 (58%) had MET amp, and 5 (42%) did not. Responders had lower ctDNA percentage change from BL vs nonresponders (1.5-fold and 1.6-fold difference based on 74-gene and methylation panel, respectively). Pts with MR had a greater decrease in tumor size from BL vs those without MR. A positive correlation between change in ctDNA levels (BL to C3) and change in tumor size was observed using the 74-gene panel and methylation panel (R=0.73 and 0.69, respectively). Additional biomarker analyses focused on the association between MET and other key biomarkers relevant in GEA will be discussed. Conclusions: In pts with advanced GEA treated with telisotuzumab adizutecan, c-Met OE/ MET amp and ctDNA-based MR were associated with enhanced antitumor activity. Compared with real-world data, MET amp was enriched in this study population. Phase 2 studies will continue to explore c-Met OE and MET amp prevalence and their potential predictive value with other biomarkers in a clinically representative population. Clinical trial information: NCT05029882 .