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750 Background: KRAS mutation is the most common molecular alteration (MA) in pancreatic adenocarcinoma (PDAC), and only 8-10% of patients (pts) harbor KRAS wild-type tumors (WT). This study describes clinical and molecular features of KRAS WT PDAC and compares it to mutant KRAS (m KRAS) PDAC. Methods: A retrospective chart review of clinical/molecular data was performed on all pts with PDAC who had contributive molecular profile at Gustave Roussy. KRAS status was determined on tumor molecular profiling as part of routine care using an in-house panel or Foundation Medicine NGS either on tissue (63%) or liquid biopsy (37%). Results: Overall, 41 pts with KRAS WT PDAC were compared to 197 pts with mKRAS PDAC. Median OS (mOS) from initial diagnosis was significantly higher in the KRAS WT group compared to mKRAS group (32.8 months (mts), CI95% [25.9-NR] vs 21.5 mts, CI95% [18.8-24.9] respectively), as mOS from metastatic diagnosis, 26 mts, CI95% [15.7-NR] for KRAS WT pts vs 17.7 mts CI95% [15.5-19.3] for mKRAS (all p

809 Background: Peritoneal carcinomatosis (PC) is a common manifestation in gastro-intestinal (GI) cancers.10-20 % of patients (pts) will present isolated, unresectable peritoneal disease with poor prognosis. In the past years, circulating tumor DNA (ctDNA) has emerged as a novel and less invasive biomarker for molecular diagnosis and disease monitoring. We aimed to evaluate the ability of PC to shed ctDNA into circulation in a cohort of pts with GI cancers and isolated PC treated in our institution. Methods: We reviewed retrospectively the results of liquid biopsies obtained at progression or at diagnosis. Genomic analysis was done using Foundation One liquid CDx panel. Data on demographics, tumor type, clinical outcomes and molecular alterations (MA) have been collected. Results: Out of 400 GI pts tested, 85 with exclusively PC were included in the analysis. The main characteristics were as follows: 45(53%) female, median age of 57 years (IQR: 24-79), majority have received systemic chemotherapy (all were progressive) (n=63, 74%) with median number of previous lines 1 (range: 0-3). Most common tumor types were as follows: gastric (n=24, 28%), colorectal (n=22, 26%) and primary peritoneal (n=20, 24%) cancers. The median index of peritoneal carcinomatosis was 23(IQR: 6-39). The tumor fraction was very low or undetectable in a proportion of 81% (n=69), median Tumor Mutational Burden was 1 mutation per megabase and was not correlated to tumor volume. Gene alterations were found in a proportion of 56% of pts (n=48) but most common alteration were TP 53 (n=22 pts, 26%) and KRAS mutation (n=14 pts, 16%). Targetable MA were identified only in 11 pts (13%) as follows: ATM, ARID1A, BRAFV600E, ERBB2, krasG12C. Three of them were included in a clinical trial. Germline mutations were found in 5 pts: ATM, CDH1, BRCA2, MSH6, BAP1. For colorectal pts known KRAS and BRAF V600E mutations were confirmed in plasma in a proportion of 32% (7 pts out of 22), for pancreatic cancer the KRAS in 17% (3 out of 18) and for gastric cancer, ERBB2 amplification in 5% (1 out of 22 pts). In pts with primary peritoneal disease: pseudomyxoma and mesothelioma, one patient harbored a known germline BAP 1 and kras G12C mutation without other targetable MA. The median survival in MA group was higher compared to non-MA. Conclusions: Isolated Pc and primary peritoneal cancers have distinct clinical and biological characteristics. The clinical utility of ctDNA seems to be limited in isolated PC or primary peritoneal cancers. However, it is not very clear if this to tumor location or to the ability of peritoneum to seed ctDNA.

605 Background: Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of death by cancer worldwide. Mostly diagnosed with locally advanced or metastatic disease, patients lack for treatment options. Gene alterations (GAs) are frequently observed in PDAC, some of them considered as actionable with molecular targeted therapies (MTTs), with potential clinical benefits and improved outcomes. Methods: We conducted a retrospective analysis of all patients, aged ≥18 years, with histologically confirmed PDAC, who underwent tumor molecular profiling between 2010 and 2020 in our institution as part of personalized medicine trials. Overall survival was the primary study endpoint (minimal follow-up after molecular profiling, 6 months). Results: Of 115 eligible patients, molecular profiling was successful in 102 patients (89%). KRAS mutations were the most frequent GAs, mostly G12D. Actionable GAs were found in 29 patients (28%), involving mainly BRCA1/2 (5 [18%]), HER2 (5 [18%]), MTAP (5 [18%]), and FGFR (3 [11%]). Only 12 of these 29 patients (41%, or 10% of the whole population) could receive MTTs accordingly, with a median progression-free survival of 1.6 months. Median OS was 17 months in patients with actionable GAs treated with MTTs (n = 12 [11.8%]), 14 months in patients with actionable GAs not treated with MTTs (n = 17 [16.7%]), and 19 months in patients without actionable GAs treated with standard therapies (n = 73 [71.5%]; p = 0.26). The absence of liver metastases was associated with better OS (HR = 0.471, p = 0.01). The longest duration of response with MTTs was observed in patients with BRCA mutations treated with olaparib. Conclusions: Actionable GAs are found in more than the quarter of patients with advanced PDAC. Overall, targeting actionable GAs with MTTs was not associated with improved OS in this retrospective study. However, selected GA/MTT duets (e.g., BRCA mutations/olaparib) were associated with better outcome.