Real-world biomarker testing patterns, first line treatment and drivers of treatment choice in patients with advanced/metastatic gastric/gastroesophageal junction adenocarcinoma in the United States and Europe.

353 Background: For first line (1L) treatment (tx) in HER2- gastric/gastroesophageal junction (G/GEJ) adenocarcinoma (AC), clinical guidelines recommended platinum-fluoropyrimidine (PF) chemotherapy (CT) in patients (pts) with a PD-L1 combined positive score (CPS) <5 and PF CT in combination with immunotherapy (IO) for CPS ≥5. There is little contemporary evidence into IO tx uptake in the real world. This real world study examined biomarker testing, 1L tx characteristics, and reasons for 1L tx choice in pts with HER2- G/GEJ AC to identify unmet needs and opportunities for optimal clinical practice. Methods: Data were derived from the Adelphi G/GEJ Cancer Disease Specific Programme: a systematic approach involving multi-national, multi-faceted data sources including cross-sectional surveys capturing data from physicians and their consulting pts. Physicians reported pt demographics, clinical characteristics, tx received and reasons for tx decisions, via chart review. Data were collected across US and Europe (EU: France, Germany, Spain, the UK) between Oct 2022 to Apr 2023. All pts were alive at data collection and analyses were descriptive. Results: Overall, 243 treating physicians (US n=60/EU n=183) provided data on 642 HER2- pts (US n=83/EU n=559) receiving 1L tx at data collection. For pts from US/EU, median age was 66/67 years, 84%/79% had an ECOG of 0-1 and 52%/75% had metastatic disease at data collection. In the US/EU, 87%/80% of pts were tested for PD-L1 status respectively, of which 63%/69% had a known CPS. For US/EU, 67%/45% of pts (n=30/n=139) had a CPS ≥5. Differences in biomarker testing rates were observed in US v EU pts respectively for MSI/dMMR (66% v 58%), NTRK-gene fusion (31% v 4%) and FGFR2 (27% v 3%). Of US/EU physicians, 70%/85% (n=60/n=183), reported the main reason for conducting biomarker testing was ‘to inform tx decisions’. Cost related concern (50%/39%) and time constraints (23%/15%) were the main perceived barriers to testing. Among pts with PD-L1 CPS <5, 27% of US pts and 2% of EU pts received IO + CT; in pts with CPS ≥5, 63% of US pts and 76% of EU pts received IO + CT. Whilst progression free survival/overall survival benefit were the most common reasons selected by physicians for 1L tx choice across the US/EU (75%/74% and 60%/61%), differences across regions were reported for tx choice based on maintaining and improving pt’s QoL (4%/28%) and pt’s performance status (2%/24%). Conclusions: PD-L1 testing was not performed by 19% of physicians and approximately one third of pts with CPS ≥5 across the US/EU were not receiving IO + CT in 1L. As informing 1L tx choice was a key driver for biomarker testing, there may be scope for improvement in 1L tx by increasing testing rates. Further studies are needed to address barriers to biomarker testing and uptake of IO in pts who can benefit from 1L IO tx.