Elhariri, Ahmed

Importance The prognosis of advanced pancreaticobiliary tumors is poor. Next-generation sequencing (NGS) of tissue samples is utilized to identify actionable alterations, but there are occasional limitations due to inadequate tissue acquisition. Circulating tumor DNA (ctDNA) is an alternative method, but its correlation with tissue-based NGS remains unexplored. Objectives To assess the mutation concordance (mCR) rates between ctDNA and tissue testing for patients with pancreatic ductal adenocarcinoma (PDAC) and cholangiocarcinoma (CC) and to evaluate how ctDNA performs as a treatment response biomarker. Design, Setting, and Participants This retrospective cohort study was conducted among patients with PDAC and CC treated at academic institutions from January 2014 to January 2025. Patients underwent ctDNA testing using 1 platform and tissue NGS testing using 2 platforms. Main Outcomes and Measures mCR, which measures the shared gene alterations observed by both ctDNA and tissue-based NGS testing, was calculated using the Spearman correlation for PDAC and CC. The performance of ctDNA as a treatment response biomarker was assessed by comparing serial ctDNA data with restaging scans and cancer antigen 19-9 levels in patients with PDAC. Results Our cohort included 790 patients: 570 with advanced PDAC (265 [46.5%] female; median [IQR] age, 64 [33-84] years) and 220 with advanced CC (95 [43.2%] female; median [IQR] age, 66 [42-88] years). Overall, 461 patients with PDAC (80.9%) and 192 patients with CC (87.2%) underwent ctDNA testing, while 239 patients with PDAC (41.9%) and 70 patients with CC (31.8%) had tissue NGS testing. Among patients with PDAC, 85 of 130 patients (65.4%) showed shared specific gene alterations between ctDNA and tissue testing; there was a significant mCR, with a Spearman correlation of 0.47 (95% CI, 0.28-0.62; P < .001). For patients with CC, 32 of 48 (66.7%) had shared alterations between ctDNA and tissue testing; there was a significant mCR, with a Spearman correlation of 0.56 (95% CI, 0.35-0.70; P < .001). A subgroup analysis of patients with PDAC who underwent serial ctDNA testing suggested that new TP53 subclones and increased ctDNA variant allele frequency levels of TP53 and KRAS were associated with higher odds of progressive disease (eg, increased TP53 frequency: odds ratio, 7.28; 95% CI, 2.15-24.66; P = .001). Conclusions and Relevance In this cohort study of patients with pancreaticobiliary tumors, there was a significant mCR between ctDNA and tissue NGS testing. Additionally, results suggest that ctDNA might detect resistant clones and enable assessment of treatment response.

163 Background: CRC with PM has a poor prognosis and limited treatment options. Immunotherapy has shown promise in improving outcomes for patients with microsatellite instability-high (MSI-H), mismatch repair-deficient (dMMR), and high tumor mutational burden (TMB-H) tumors. However the peritoneal niche has a unique microenvironment, which can influence the effectiveness of immune check point inhibitors (ICIs). We assessed the impact of ICI on progression-free survival (PFS) and overall survival (OS) in CRC patients (pts) and PM, considering key clinical and molecular characteristics. Methods: A retrospective review aiming to identify CRC pts with MSI-H/dMMR and PM treated with immunotherapy between 2015 and 2023 was performed. Data collected included pt demographics, tumor characteristics, and treatment details. Responses were evaluated using RECIST v1.1 criteria. Results: 37 pts were included, 9 (24%) had Lynch syndrome. The median age of diagnosis was 61 years and most pts were females (65%). Most pts had MSI-H/dMMR tumors, while one pt was treated based on ultra-hypermutated phenotype (TMB 202) and POLE mutation. Among those with available TMB data, the median TMB was 52 mutations/Mb (0-202). Common mutations were PMS2 (51%), MLH1 (38%), MSH6 (16.2%), and MSH2 (16.2%). Right-sided tumors were in 54%, mucinous histology in 62%, and ascites in 54%. Pts received a median of 11 cycles of ICI (2-52). Pembrolizumab was the most frequently used ICI (78%), while Nivolumab was used either alone (11%) or in combination with Ipilimumab (11%). Median PFS in our cohort was 7.8 months and median OS was 29.9 months. The disease control rate (DCR) was 81%, with an objective response rate (ORR) of 51.3% (9 complete responses, 12 partial responses). At the time of data collection, 30% of pts were off systemic therapy and 5-FU-based chemotherapy combined with a biologic agent was the most common subsequent treatment in 32% who had PD. Univariate analysis showed no significant differences in PFS or OS based on tumor sidedness, isolated PM, liver metastases, mucinous vs. non mucinous histology or BRAF / RAS status (WT vs mutant). Malignant ascites was associated with worse OS(HR 0.3; p= 0.03). Conclusions: Treatment with ICIs in pts with MSI-H/dMMR and/or TMB-H CRC and PM improved disease control in CRC pts with PM, however, as compared to historical controls for patients with MSI-H/d-MMR disease treated with ICI, this group showed less favorable outcomes. Malignant ascites was associated with poorer overall survival. Further research of the immune response and tumor microenvironment in the peritoneal cavity is warranted.

e14576 Background: Immune checkpoint inhibitors (ICI) have revolutionized the treatment of many solid malignancies. ICI appears particularly effective when tumors exhibit high microsatellite instability (MSI-H) and/or mismatch repair deficient (dMMR) status. Although the response is typically quite good in this setting, there are scenarios where benefit is less than anticipated. In this study, we aimed to explore the efficacy of ICIs in patients with MSI-H/dMMR cancers with peritoneal metastasis (PM). Methods: We conducted a retrospective chart review to identify patients (pts) with PM from primary MSI-H/dMMR cancers treated with immunotherapy between 2010 and 2022 at Mayo Clinic. Data was collected to include pt demographics, tumor characteristics, pathology, tumor markers, imaging, mutational profile, and previous and subsequent treatments. The primary endpoint was disease control rate (DCR), defined as objective response rate (RECIST v1.1 criteria) or stable disease. Results: Forty-six pts with MSI-H/dMMR and PM treated with ICIs. Most of our cohort were female (61%) with a median age at diagnosis of 61 years. Colorectal cancer was the most common primary site (69.6%), followed by endometrial (6.5%). Most pts received treatment with PD-1 antibodies as second-line therapy following systemic chemotherapy. DCR of the PM was 71.6 % (n= 13 PR, 9 CR, and 9 SD), with a median duration of response of 12.6 months. Thirteen pts (28.4%) had progressive peritoneal disease on ICI with a median PFS of 2.2 months. Ascites was a poor prognosticator and predicted a lack of benefit from ICI (HR 6.6, 95% CI=2.24-19.9, p= 0.006). Mucinous and signet ring histology were common in pts with peritoneal progression (61.5%); however, this was not statistically significant (HR=1.6; 95%CI= 0.4383 to 3.899, p 0.5). Multivariate analysis for the presence or absence of BRAF and/or RAS mutations was not statistically significant in predicting response. Neither was the specific MMR gene mutation(i.e., MLH1 or PMS2). Interestingly, discordant responses between the peritoneum and other sites of metastasis to immunotherapy were seen in 13 pts (28.4%), primarily as primary peritoneal progression compared to other tumor sites. Conclusions: In our cohort, the DCR of metastatic peritoneal disease in MSI-H/dMMR tumors was approximately 71%. The presence of ascites was identified as a poor prognosticator in this setting and was associated with a lack of response/benefit from ICIs. Histologic subtype (i.e., mucinous/signet ring) did not show statistical significance in predicting ICI response. The subpopulation with discordant response to ICI suggests that peritoneal immunobiology may differ from other sites. Further research on immunotherapy response in the peritoneum is needed for this special subset of pts.