4215 Background: Pancreatic cancer (PDAC) remains associated with poor prognosis and limited treatment options, particularly in patients with advanced disease. Diffusing Alpha-Emitter Radiation Therapy (Alpha DaRT), a novel alpha-emitting radiation source, has shown initial clinical benefit in superficial epithelial solid tumors. The pooled studies presented here evaluated the safety and efficacy outcomes for endoscopic ultrasound (EUS)-guided alpha radiotherapy using Alpha DaRT in patients with PDAC. Methods: This analysis pooled data from three prospective clinical studies at 3 medical centers in Canada and Israel evaluating Alpha DaRT in patients with locally advanced or metastatic PDAC, regardless of previous lines of therapy. Primary outcome was safety, as assessed by CTCAE V5. Secondary outcomes were best overall response (BOR) of the primary tumor assessed per RECIST V1.1 and overall survival (OS), defined from Alpha DaRT treatment. Results: A total of 58 subjects (31 male, 27 female), median age 72 (range 41-91), were recruited between 2023 and 2025 across the three trials. Treatment-associated AEs of any grade occurred in 21 patients (36%). Grade ≥3 AEs occurred in 5 patients (9%) (biliary obstruction, abdominal pain, fever, liver enzyme imbalance and bacteremia). There were no treatment-related deaths and all grade ≥3 AEs resolved. Across all patients evaluable for response, the BOR showed a disease control rate of 87% and an objective response rate of 27%, with 3 complete and 9 partial responses. Median OS for the cohort was 7.9 months (95% CI: [6.1–11.6]): 10.4mo (95% CI: [7.3–13.4]) when given in 2nd line (N = 24) and 7.5 months (95% CI: [4.2–11.8]) when given in 3 rd line (N = 21). Conclusions: In this pooled analysis of three clinical studies, EUS-guided Alpha DaRT demonstrated a reassuring safety profile and promising early efficacy signals in patients with advanced PDAC. The observed survival outcomes and radiographic responses support further clinical investigation, which is underway. Clinical trial information: NCT04002479 ; NCT05781555 ; NCT05657743 .

Abstract Liquid biopsy using ultra-low-pass whole-genome sequencing (ULP-WGS, ∼0.25x coverage) is a promising tool to detect circulating tumor DNA (ctDNA) for cancer management, and the use of the native Oxford Nanopore (ONT) sequencing platform adds DNA methylation to the set of detectable features. Here, we test the performance of methylation-based cell-type deconvolution in ULP-WGS samples from diverse epithelial malignancies and investigate several new computational strategies using our CelFiE-ISH deconvolution framework. We find that incorporating larger numbers of markers restricted to the epithelial cell lineage can reduce the cancer fraction limit of detection down to 1.7-3.1%, matching or exceeding the 3% floor of established copy-number alteration (CNA) benchmarks. Our study provides a useful strategy for analysis of ULP-WGS ONT data and indicates that marker selection remains a key challenge for analyzing methylation-based cancer datasets.

BACKGROUND:Glecirasib, an inhibitor of Kirsten rat sarcoma viral oncogene homolog glycine-to-cysteine substitution at codon 12 (KRAS G12C), has exhibited clinical activity in non-small-cell lung cancer (NSCLC) and colorectal cancer (CRC). Here, we investigated the efficacy and safety of glecirasib in patients with pancreatic ductal adenocarcinoma (PDAC) and other solid tumors (excluding NSCLC and CRC) that rarely harbor the KRAS G12C mutation but for which effective treatment options remain limited. METHODS:We conducted and analyzed two open-label, phase I/II trials in adult patients with KRAS G12C mutant solid tumors, in which glecirasib was administered orally. The two trials had similar eligibility criteria and endpoints but differed in the regions of patient recruitment. We performed a pooled analysis of all patients, excluding NSCLC and CRC, from both trials. The primary endpoint in the pooled population was objective response rate (ORR). Efficacy and safety were assessed in patients who received at least one dose of glecirasib. RESULTS:As of June 30, 2024, the pooled analysis included 54 patients who were treated with glecirasib: 32 PDACs, 8 biliary tract cancers (BTCs), 4 small intestinal cancers, 3 gastric cancers, 2 appendiceal cancers, and 5 other tumors. At baseline, 24 received ≥ two prior lines of systemic therapy. Of the 53 efficacy-evaluable patients, the confirmed ORR was 50.9% (95% confidence interval [CI], 36.8%-64.9%), with an ORR of 46.9% (95% CI, 29.1%-65.3%) in PDAC patients. Among other solid tumors, ORR was 71.4% (5/7) in BTC, 100% (4/4) in small intestinal cancer, and 66.7% (2/3) in gastric cancer. Median progression-free survival and median overall survival were 6.9 and 10.8 months, respectively, in the overall population, and 5.5 and 10.8 months, respectively, in patients with PDAC. Treatment-related adverse events (TRAEs) of any grade occurred in 94.4% patients, with grade ≥ 3 TRAEs in 27.8%. No fatal TRAEs or TRAEs leading to treatment discontinuation occurred. CONCLUSIONS:Glecirasib showed promising efficacy and was well tolerated in patients with PDAC and other advanced solid tumors (beyond NSCLC and CRC), warranting further expedited clinical development in this patient population. TRIAL REGISTRATION:ClinicalTrials.gov identifier: NCT05009329 and NCT05002270.

Pancreatic cancer surgery frequently results in positive margins and local recurrence despite multimodal treatment. This study evaluated whether combining neoadjuvant stereotactic body radiotherapy (SBRT) with intraoperative electron radiotherapy (IOeRT) during resection could improve local control and surgical outcomes. A retrospective analysis was performed on 15 patients with resectable or borderline resectable pancreatic adenocarcinoma treated between 2021 and 2023. All patients received image-guided, motion-managed SBRT (35–40 Gy/5 fractions to PTV_high; 25 Gy/5 fractions to PTV_low) followed by surgical resection and IOeRT (median 10 Gy; 12 Gy when margins were at risk). Toxicities were graded by CTCAE v5.0 and postoperative complications by Clavien–Dindo criteria. Follow-up included imaging and CA 19–9 every 3 months. Survival was estimated using Kaplan–Meier analysis. Mean patient age was 66 years; 60

741 Background: This study aims to report the initial feasibility, safety, and tumor response of delivering Diffusing Alpha-Emitter Radiation Therapy (Alpha DaRT) to the pancreas. This innovative approach involves the interstitial implantation of a novel alpha-emitting radiation source via endoscopic ultrasound (EUS) for the treatment of pancreatic cancer. Methods: The study enrolled patients treated with Alpha DaRT to the pancreas across two protocols (PANC and ALL). The primary objective was to assess the safety of Alpha DaRT delivery to the target lesion via EUS, while the secondary objective was to evaluate initial tumor response using the Response Evaluation Criteria in Solid Tumors version 1.1 at 1 and 3 months post-Alpha DaRT source insertion. Toxicity was evaluated according to the Common Terminology Criteria for Adverse Events version 4.03. The PANC protocol included patients with biopsy-proven adenocarcinoma, either unresectable, recurrent, or metastatic, who were unfit for standard treatment. The ALL protocol allowed for any malignancy with a targetable lesion and permitted concurrent systemic therapy. For this analysis, only patients from the ALL protocol with pancreatic adenocarcinoma treated to the primary tumor were included. Pre-procedure planning involved CT simulation and EUS, and EUS-guided Alpha DaRT placement was confirmed by CT. The median follow-up time was 91 days. Results: A total of 12 patients (9 male, 3 female) median age 73 (range: 59-85) were enrolled (4 in PANC, 8 in ALL). Four patients had localized, unresectable disease and eight had metastatic disease. Three patients had a history of prior radiotherapy. Successful placement of Alpha DaRT sources into the targetable pancreatic lesions was achieved in all cases (100%), as confirmed by post-treatment imaging. Grade 1-2 acute toxicities included fever in 2 of 12 patients (16.7%), fatigue/weakness in 2 of 12 patients (16.7%) and abdominal or back pain in 3 of 12 patients (25.0%). One patient was hospitalized for obstructive jaundice (Grade 3) possibly secondary to inflammation seven days post-implantation requiring percutaneous transhepatic cholangiography; all hepatic laboratory values returned back to normal post procedure. All adverse events resolved. No long-term toxicities ( > 3 months) have been seen to date. Tumor response data were available for 8 of 12 patients. Local control was 100% for all evaluable patients. Of these, 6 (75.0%) exhibited stable disease, while 2 (25.0%) showed a partial response ( > 30% tumor reduction). One patient had a complete metabolic response on PET-CT at both the primary pancreatic lesion and liver metastasis. Conclusions: Alpha DaRT delivered via endoscopy is feasible with a favorable safety profile and demonstrates promising tumor responses. Ongoing longer follow-up and larger studies are required to validate these findings. Clinical trial information: NCT05781555 and NCT05657743 .

TPS308 Background: Colorectal cancer (CRC) is the third most common cancer. In the metastatic (m) setting, the 5-year relative overall survival is approximately 15%. Conventional treatment comprises fluorouracil (5-FU)–based chemotherapy. Recently, targeted therapies have been studied for specific molecular subtypes. c-Met overexpression frequently occurs in a variety of tumors, including CRC. ABBV-400 is a c-Met–directed antibody-drug conjugate composed of the monoclonal antibody telisotuzumab conjugated to a potent topoisomerase 1 inhibitor payload. Preliminary data from the first-in-human study of ABBV-400 in patients with advanced solid tumors indicate encouraging efficacy of ABBV-400 monotherapy in patients with third-line or later mCRC. This phase 2 randomized study evaluates the safety, efficacy, and optimal dose of ABBV-400 in combination with 5-FU, folinic acid (FA), and bevacizumab (bev) in patients with mCRC with progression after first line (1L) treatment. Methods: Global, open-label, phase 2 randomized controlled study (NCT06107413). Eligible patients (≥18 years) have confirmed unresectable mCRC and measurable disease per RECIST v1.1, are microsatellite stable or mismatch repair proficient, BRAF V600E wild type, and have progression after 1L combination chemotherapy ± an anti-vascular endothelial growth factor or anti-epidermal growth factor receptor antibody. Primary objectives are (a) optimize ABBV-400 dose in combination with 5-FU, FA, and bev; (b) evaluate the efficacy of the combination, using objective response and progression-free survival as dual primary endpoints; (c) evaluate the safety and tolerability of the combination. Approximately 206 patients planned for enrollment in 2 stages: safety lead-in dose escalation (stage 1; n=30) and dose optimization (stage 2; n=176). In stage 1, patients receive escalating doses of ABBV-400 either every 2 weeks (Q2W; 0.8–2.4 mg/kg) or every 4 weeks (Q4W; 1.6–3.0 mg/kg) in combination with Q2W 5-FU (2400 mg/m 2 infusion), FA (200 mg/m 2 ), and bev (5 mg/kg) in 28-day cycles. Dose escalation of ABBV-400 uses a Bayesian optimal interval design, with target toxicity rate of 30%. Dose-limiting toxicities (DLT) are assessed during cycle 1, with ≥6 DLT evaluable patients required to declare a dose safe for the dose-optimization stage. In stage 2, patients are randomized to up to 4 ABBV-400 dose cohorts (2 with Q2W and 2 with Q4W ABBV-400 schedule; all in combination with Q2W 5-FU, FA, and bev) and a comparator cohort (irinotecan [180 mg/m 2 ] + 5-FU [400 mg/m 2 bolus and 2400 mg/m 2 infusion] + FA [200 mg/m 2 ] + bev [5 mg/kg]; all Q2W). Patients are treated until progression, unacceptable toxicity, or other discontinuation criteria are met. Enrollment was initiated in November 2023, with 3 patients enrolled as of January 4, 2024. Clinical trial information: NCT06107413 .

247 Background: The phase 3 KEYNOTE-585 study (NCT03221426) evaluated neoadjuvant/adjuvant pembrolizumab (pembro) or placebo (pbo) + chemotherapy (chemo) followed by adjuvant pembro vs pbo in locally advanced, resectable gastric or gastroesophageal junction (G/GEJ) cancer. A separate cohort evaluated pembro + FLOT (FLOT cohort). We report results from safety and efficacy analyses of the FLOT cohort at third interim analysis. Methods: In the FLOT cohort, patients (pts) with untreated, locally advanced, resectable G/GEJ cancer were randomized 1:1 to neoadjuvant pembro 200 mg IV Q3W (pembro gp) or pbo (pbo gp) Q3W for 3 cycles + FLOT (docetaxel, oxaliplatin, leucovorin, and 5-FU) Q2W for 4 cycles. After surgery, pts received adjuvant pembro or pbo Q3W for 3 cycles + FLOT Q2W for 4 cycles, then adjuvant pembro or pbo Q3W for 11 cycles. Endpoints evaluated included safety, pathCR rate (BICR), EFS (RECIST 1.1, by investigator), and OS in the ITT. Data cut-off at third interim analysis was 09 Feb 2023. Results: A total of 203 pts were randomized (100 pembro + FLOT; 103 pbo + FLOT). Among these, 12 (6%) pts had cT1-T2, 161 (79%) had cT3, and 9 (4%) had cT4. 140 (69%) had N+ disease, and 79 (39%) had GEJ adenocarcinoma at baseline. Median follow-up was 31.6 months (mo) and 31.1 mo, respectively, at IA3. A total of 94 of 99 (95%) pts completed the neoadjuvant phase, 87 of 99 (87%) completed the surgery phase, and 45 of 77 (58%) completed adjuvant treatment. The R0-resection rate was 79% vs 80% in the pembro gp vs pbo gp, respectively. The pathCR rate was 17.0% (95% CI, 10.2-25.8) in the pembro gp and 6.8% (95% CI, 2.8-13.5) in the pbo gp, estimated difference (10.2% [95% CI, 1.3-19.7]). Median EFS was not reached (95% CI, 28.2-NR) in the pembro gp and 30.9 mo (22.8-NR) in the pbo gp (HR 0.79; 95% CI, 0.52-1.22). The 24-mo EFS rates were 66% and 57%, respectively. Median OS was not reached in either group (HR 1.04; 95% CI, 0.66-1.66). The 24-mo OS rates were 72% and 73%, respectively. Grade ≥ 3 drug-related AE rates in all phases combined were 76% and 63% in the pembro vs pbo gp, with serious drug-related AEs in 42% vs 20%, and surgery-related AEs in 20% vs 13%, respectively. A total of 3 (3%) vs 1 (1%) pt in the pembro vs pbo gp died due to a drug-related AE. Conclusions: Neoadjuvant/adjuvant pembro + FLOT was feasible with no new safety concerns. PathCR and EFS favored pembro + FLOT vs pbo + FLOT in pts with untreated, locally advanced resectable G/GEJ cancer. Clinical trial information: NCT03221426 .

604 Background: KRAS G12C mutation is an oncogenic driver and a validated therapeutic target in solid tumors. Glecirasib, a highly selective, covalent oral KRAS G12C inhibitor, has demonstrated promising clinical activity in NSCLC and CRC; however, the efficacy and safety of glecirasib are unknown in PDAC and other solid tumors with KRAS G12C mutation. Methods: Two phase 1/2 trials (NCT05009329 in China; NCT05002270 in US, Europe and Israel) are evaluating the safety and efficacy of glecirasib in patients (pts) with solid tumor harboring KRAS G12C mutations. As both trials have similar inclusion and exclusion criteria, we have pooled the data from them to assess the effect of glecirasib monotherapy in various types of solid tumor with low incidence of KRAS G12C mutation. Here we report the preliminary efficacy and safety results of glecirasib in pts with PDAC and other solid tumors (excluding NSCLC and CRC) from the pooled population of both trials. Results: As of Sep 8, 2023, a total of 48 pts have received glecirasib monotherapy (41 from NCT05009329 and 7 from NCT05002270) and 47 were evaluable for efficacy. This includes 28 PDAC and 19 other solid tumors (7 biliary tract, 3 gastric, 3 small bowel, 1 appendiceal, 1 hepatocellular, 1 peritoneal, 1 posterior bronchial mediastinal, 1 ameloblastic carcinoma and 1 cervical). At baseline, pts had previously received a median of one line of systemic therapy (range: 0 to 4; with 23 pts have received one prior line); 85% Asian, 15% Caucasian; most (45 pts) at 800 mg QD. Among 28 pts with PDAC, 13 achieved confirmed partial response (PR) with the confirmed objective response rate (ORR) of 46.4% (13/28) and disease control rate (DCR) of 96.4%; the median duration of response (DOR) and progression-free survival (PFS) were 4.1 months and 5.5 months (95%CI 1.2, 13.1), respectively. Among 19 pts with other tumor types, the confirmed ORR of 52.6% (10/19) and DCR of 84.2% (16/19) were observed; the median DOR and PFS were 8.3 months and 7.0 months (95% CI 1.1 to 15.2), respectively. Treatment-related AE (TRAE) of any grade occurred in 89.6% (43/48) pts; the most common (≥10%) TRAE were anemia (52.1%), blood bilirubin increased (39.6%), white blood cell count decreased (18.8%), AST increased (18.8%), diarrhea (16.7%), ALT increased (14.6%), asthenia (14.6%), hypertriglyceridemia (10.4%), and nausea (10.4%); ≥Grade 3 TRAE occurred in 25% (12/48) pts; no TRAE were fatal or led to treatment discontinuation. Conclusions: Glecirasib monotherapy is well tolerated and has a manageable safety profile and exhibits promising anti-tumor activity in pts with KRAS G12C mutated PDAC and other solid tumors. Further clinical development of glecirasib in above mentioned population is ongoing (NCT06008288). Clinical trial information: NCT05009329 and NCT05002270 .

634 Background: Prognosis following surgery for pancreatic cancer (PC) remains poor. Local recurrence is common and negatively impacts survival rate. In an effort to improve local control and potentially improve survival, we have adopted a strategy of adding intra-operative electron radiation therapy (IOeRT) to selected patients undergoing pancreatic resection (PR) for PC. Here we report our initial experience with this treatment modality. Methods: 14 patients (10 male, 4 female), age between 44 - 81 years (mean 66.17), who underwent PR (Head 7, Body7) between 1/2021-1/2023 were included in this analysis. Patients selected for IOeRT included patients that according to surgeon assessment were at risk not to achieve satisfactory resection margins by surgery alone. All had resectable or borderline resectable disease according to standard criteria. 8 patients underwent pre-op chemotherapy followed by stereotactic body radiotherapy (SBRT) to a dose of 25-40 Gy in 5 fractions to the tumor and regional lymphatics followed by PR (5 Whipple, 3 distal pancreatectomy) and IOeRT, 3 patients received preop SBRT followed by PR (3 distal pancreatectomy) + IOeRT, 2 patients underwent PR (whipple 1, distal pancreatectomy 1) + IoeRT, and 1 received neoadjuvant chemotherapy followed by SBRT only and did not undergo surgery due to tumor progression. Immediately following resection, mobile IOeRT accelerator using a 40-80mm applicator with a level of 0-15 degrees was directed at the tumor bed deemed at high risk for recurrence. 10-20 Gy was delivered using 6-8 MeV electrons to the 90% isodose line at the discretion of the treating radiation oncologist. Data including serial imaging, genomic analysis, radiation protocol, complications, pathological results and outcome was collected for analysis. Results: Mean follow up was 9.7 months (2-21). 8 patients are alive: 7 pts with no evidence of disease and 1 pt with liver metastases. 6 patients died: 4 with distant metastatic disease, 1 from disease progression prior to surgery and 1 from early post-op sepsis. Negative surgical margins were achieved in 10 patients of which 2 mutated BRCA gene carriers showed complete pathological response (both who received combined neoadjuvant chemotherapy followed by SBRT). Post-operative complications possibly attributed to RT included hepatic artery occlusion (1 pt), portal vein stenosis (1 pt) and prolonged GI tract dysfunction (1pt). Conclusions: Combining RT with resection appears feasible and safe. Our data possibly show a reduction in local recurrence following resection of PC, however, longer follow up is necessary. Refinements in patient selection and treatment protocol may further improve outcomes. Larger scale studies are required to determine the benefit of this modality.

3620 Background: The role of adjuvant chemotherapy (CT) in stage II CC is debated. The validated 12-gene Oncotype DX Colon Recurrence Score test provides a Recurrence Score (RS) result (range, 0-100) which estimates recurrence risk (RR) in stage II/III pts. We studied treatment and clinical outcomes in CC pts in whom treatment decisions incorporated the RS result. Methods: This prospectively designed cohort study included all stage II, MMR-P, CC pts who underwent the 12-gene Oncotype DX testing through Clalit between 1/2011 and 12/2016 and had available data with minimum 3-yr follow-up. Kaplan-Meier (KM) estimates and log-rank tests were used to compare RR and CC specific mortality (CCSM) between RS categories. Multivariable analysis (MVA) identified variables associated with RR/CCSM. Results: The analysis included 938 pts. Median age, 68 (IQR, 60-76) yrs; 96% had T3 tumors, and 89% had ≥12 nodes examined. Median RS was 26 (IQR, 19-33). The 3 RS categories (0-29, 30-40, and 41-100) included 65%, 24%, and 11% of pts, respectively. The overall CT use rate was 24%, with a significant difference between the 3 categories (14%, 36%, and 60%, respectively, P< .0001). Pts with very low RS (0-15) comprised 14% of the cohort (CT use rate, 11%). Younger pts, and those with invasion/perforation/obstruction were more likely to receive CT. Clinical outcomes with a median follow up of 6.9 (IQR, 5.5-8.6) yrs are presented (Table). Among untreated pts, KM estimates for RR and CCSM differed significantly between the 3 RS categories ( P< .0001). Outcome of untreated RS 0-15 pts was excellent. In an MVA model, male sex, presence of invasion/perforation/obstruction and higher RS category (RS 41-100 vs 0-29 and vs 30-40, but not RS 30-40 vs 0-29) were associated with increased RR. For CCSM, the results were similar, but this time age ≥70 yrs replaced sex as a significant prognostic variable. Clinical outcomes within each RS group did not differ significantly between treated and untreated pts, but were numerically better with CT in the RS 41-100 group. Conclusions: This real-world analysis showed that the RS results provide independent prognostic information in stage II CC. Further studies are needed to investigate the potential role of the RS result as a predictor of CT benefit, but the data suggest that this benefit may be limited to pts with high RS results. [Table: see text]

Colorectal cancer (CRC) constitutes a significant portion of cancer-related deaths in the United States, topping cancer-related mortality among males under 50 years of age. Despite an increase in overall survival throughout the years, the prognosis of metastatic CRC remains poor. In addition to the classic molecular targets in CRC that include epidermal growth factor receptor, BRAF, and vascular endothelial growth factor pathways, human epidermal growth factor receptor 2 (HER2) amplification is an emerging target that has been successfully targeted in advanced CRC, particularly with anti-HER2 monoclonal antibodies and tyrosine kinase inhibitors. With the introduction of potent HER2-directed antibody–drug conjugates, the number of patients who might be eligible for anti-HER2 therapy is expected to increase dramatically as tumors with lower expression of HER2 (termed HER2 low) have shown relevant response rates across several histologies. Yet, several challenges remain to be addressed in this field, including the standardized method to define HER2-low disease and the most efficient payload in this setting, among others. Future studies will help to further characterize HER2-low CRCs, identify additional predictive biomarkers, and assist in the development of better treatments.

Background: The liver and lungs are the most common sites of colorectal cancer (CRC) metastases. Their involvement can take five different clinical scenarios: lung metastases only, liver metastases only, lung metastases before liver metastases, liver metastases before lung metastases and simultaneous lung and liver metastases. Using clinical and morphological data we studied the clonal trajectory of these scenarios. Materials and methods: A total of 465 (CRC) patients with 7952 liver and 6406 lung metastases were evaluated. Metastases clinical route was deciphered from metastases number, timing, and linear/parallel ratio (LPR)- a computerized parameter used for deducing clonal trajectories. LPR of +1 suggest pure linear dissemination and −1 pure parallel. Results: Lung-only metastases: high percentage of metachronous disease with a long lead time and a low LPR suggest parallel dissemination. Liver-only metastases: Rare metachronous disease with a short lead time, and a high LPR suggest linear spread. Lung-before-liver metastases: rare solitary metastasis, a median gap of 21 months between the organs, high lung and low liver LPRs suggest linear progression to the lungs and parallel dissemination to the liver. Liver-before-lung metastases: low liver and high lung LPRs and a median gap of 16.5 months between the organs suggest parallel dissemination to the liver and linear spread from the liver to the lungs. Simultaneous liver and lung metastases: rare solitary metastasis and similar and high LPRs suggest simultaneous linear progression to both organs. Conclusions: CRC metastases have different dissemination trajectories in different clinical scenarios. This information can potentially impact on clinical management.

TPS98 Background: Despite standard of care management with surgery ± adjuvant chemotherapy treatment (ACT), >30% of patients (pts) with resectable CRC recur. The presence of ctDNA in plasma after resection and after ACT, has been shown to be a strong prognostic factor for the risk of recurrence, suggesting that the presence of ctDNA in blood is molecular evidence of residual disease. As such, ctDNA analysis may enable post-surgical risk stratification for ACT decision-making, as well as detection of molecular recurrence during surveillance and subsequent early intervention. The CORRECT-I study aims to validate the association of post-definitive therapy and pre-recurrence follow-up ctDNA positivity with recurrence-free interval (RFI) in pts who have undergone complete surgical resection for stage II or III CRC. Methods: This study is a prospective, observational, multicenter study, which is open in Israel, Italy, Japan, Spain, and the UK. Pts must have pathologically confirmed stage II or III CRC, have undergone complete surgical resection, and have tissue available from the primary resection. Pts may not have started ACT prior to baseline blood draw post-surgery. Pts are asked to provide serial whole blood specimens for ctDNA analysis at a) post-surgical baseline, b) pre-recurrence follow-up visits (max 15 blood draws over max 5 years), and c) clinical recurrence. ctDNA will be analyzed with an NGS-based tumor-informed MRD assay that identifies somatic genomic alterations from DNA derived from the patient’s tumor tissue and detects a selected subset of tumor-specific (bespoke) ctDNA in their blood. The primary objective of this study is to validate the association of post-definitive therapy and serial pre-recurrence follow-up ctDNA positivity with RFI, using a Cox proportional hazards regression analysis. Further objectives include the assessment of sensitivity and specificity of the ctDNA assay, the association between ctDNA at individual timepoints and RFI, ctDNA dynamics and RFI, and the time from ctDNA positivity to clinical recurrence. A multivariable model, including ctDNA, clinicopathological risk features, serial serum CEA assessments, and recurrence risk as determined by the Oncotype Colon Recurrence Score will be fit with RFI as endpoint. The primary analysis will be conducted when ≥30 histologic and/or radiographic confirmed cases of clinical recurrence have been observed. As of April 2, 2024, 158 pts of 400 have been enrolled.

TPS425 Background: Initial signal finding studies demonstrated promising antitumor activity with the anti–PD-1 antibody pembrolizumab in combination with the anti-angiogenic multikinase inhibitor lenvatinib in patients with advanced gastric cancer or head and neck squamous cell carcinoma. Because hypoxia is common in many solid tumors and is known to induce an immunosuppressive tumor microenvironment, targeting hypoxia-inducible factor-2 alpha (HIF-2α) may enhance the activity of immune checkpoint inhibitors. The ongoing phase 2 LITESPARK-016 study (NCT04976634) is evaluating the combination of pembrolizumab plus lenvatinib and the HIF-2α inhibitor belzutifan (MK-6482) in multiple solid tumors. LITESPARK-016 includes 2 esophageal squamous cell carcinoma cohorts: one enrolling patients with immunotherapy-naive and the other enrolling patients with immunotherapy-resistant esophageal squamous cell carcinoma, cohorts F and G, respectively. We report the methodology for these cohorts. Methods: In this phase 2, open-label, multicenter study, eligible patients in the esophageal squamous cell carcinoma cohorts have histologically or cytologically confirmed metastatic disease and have tumors that are either immunotherapy-naive with disease progression on 1 prior line of standard systemic therapy (cohort F) or immunotherapy-resistant with disease progression during or after 1 prior line of treatment that contained an anti–PD-1/PD-L1 therapy (cohort G). Patients must be aged at least 18 years, have measurable disease per RECIST v1.1 (verified by BICR) with no radiographic evidence of major blood vessel encasement or intratumoral cavitation, ECOG PS of 0 to 1, and an archival/new tumor sample for biomarker analysis. Patients in cohort F receive triple therapy with pembrolizumab plus lenvatinib plus belzutifan; patients in cohort G are randomized 1:1 to receive either the same triple therapy or doublet therapy with pembrolizumab plus lenvatinib. The planned starting doses are pembrolizumab 400 mg given intravenously every 6 weeks, lenvatinib 20 mg orally once daily, and belzutifan 120 mg orally once daily. Treatment will continue for up to 2 years (for pembrolizumab) or until PD or discontinuation criteria are met. Approximately 30 patients will be enrolled in cohort F and in each arm of cohort G (approximately 90 total patients), with potential expansion of up to 70 additional patients in cohort F and each treatment arm of cohort G after safety/efficacy review with at least 6 months follow-up. The primary endpoints in cohorts F and G are safety (AEs, treatment discontinuation due to AEs) and ORR per RECIST v1.1 by BICR. Secondary endpoints are duration of response, disease control rate, and PFS per RECIST v1.1 by BICR, and OS. Assessment of biomarkers is an exploratory objective. Enrollment for this study began in September 2022 and is ongoing across 48 global sites. Clinical trial information: NCT04976634 .

Objective Pancreatic ductal adenocarcinoma (PDAC) is commonly diagnosed at an advanced stage. Liquid biopsy approaches may facilitate detection of early stage PDAC when curative treatments can be employed. Design To assess circulating marker discrimination in training, testing and validation patient cohorts (total n=426 patients), plasma markers were measured among PDAC cases and patients with chronic pancreatitis, colorectal cancer (CRC), and healthy controls. Using CA19-9 as an anchor marker, measurements were made of two protein markers (TIMP1, LRG1) and cell-free DNA (cfDNA) pancreas-specific methylation at 9 loci encompassing 61 CpG sites. Results Comparative methylome analysis identified nine loci that were differentially methylated in exocrine pancreas DNA. In the training set (n=124 patients), cfDNA methylation markers distinguished PDAC from healthy and CRC controls. In the testing set of 86 early stage PDAC and 86 matched healthy controls, CA19-9 had an area under the receiver operating characteristic curve (AUC) of 0.88 (95% CI 0.83 to 0.94), which was increased by adding TIMP1 (AUC 0.92; 95% CI 0.88 to 0.96; p=0.06), LRG1 (AUC 0.92; 95% CI 0.88 to 0.96; p=0.02) or exocrine pancreas-specific cfDNA methylation markers at nine loci (AUC 0.92; 95% CI 0.88 to 0.96; p=0.02). In the validation set of 40 early stage PDAC and 40 matched healthy controls, a combined panel including CA19-9, TIMP1 and a 9-loci cfDNA methylation panel had greater discrimination (AUC 0.86, 95% CI 0.77 to 0.95) than CA19-9 alone (AUC 0.82; 95% CI 0.72 to 0.92). Conclusion A combined panel of circulating markers including proteins and methylated cfDNA increased discrimination compared with CA19-9 alone for early stage PDAC.

677 Background: Pancreatic cancer, particularly in its locally advanced and oligometastatic forms, poses a formidable therapeutic challenge. Radiotherapy remains an important treatment in an attempt to gain local control, however there is no consensus on whether stereotactic body radiotherapy (SBRT) is appropriate as compared to conventionally fractionated radiotherapy (CFRT). Herein, we report our experience. Methods: We conducted a retrospective analysis of patients with locally advanced and oligometastatic pancreatic cancer who received definitive radiotherapy at our institution between January 2010 and March 2023 with SBRT 30-50 Gy in 3-5 fractions or CFRT 50-60 Gy in 25-30 fractions. We excluded all patients with resectable disease who underwent surgery. Clinicopathological data, treatment regimens, and radiation parameters were collected and analyzed. Results: A total of 51 patients (17 females, 34 males; median age: 67.9 years). 24 patients (47.1%) were treated with CFRT, and 27 patients (52.9%) were treated with SBRT. Median time of follow-up for the entire cohort was 14.4 months. A total of 13 patients (25%) experienced a Local failure (LF) at a median time of 18.5 months, with no difference observed between CFRT and SBRT groups. Cancer-specific mortality (CSM) was statistically significant with a lower mortality rate in SBRT (37%) vs CFRT group (63%) (p<0.004). Several treatment characteristics and dosimetric factors such as sex, age, receipt of chemotherapy, type of chemotherapy, use of elective nodal irradiation, GTV size, biologically equivalent dose (BED) corrected prescribed dose, minimum dose to GTV, or max dose to GTV did not predict local failure. There was no difference in QOL and toxicity between the two groups. Conclusions: In our cohort, local failure rates remain high and were similar between patients treated with SBRT compared to those treated with conventionally fractionated radiotherapy. We were unable to identify any specific predictors for local recurrence in this patient population. Our observation of a lower CSM rate among patients who received SBRT needs further study. The findings underscore the need to further improve radiotherapeutic approaches in an attempt to improve local control in locally advanced disease.

The analysis of cell-free DNA (cfDNA) in plasma provides information on pathological processes in the body. Blood cfDNA is in the form of nucleosomes, which maintain their tissue- and cancer-specific epigenetic state. We developed a single-molecule multiparametric assay to comprehensively profile the epigenetics of plasma-isolated nucleosomes (EPINUC), DNA methylation and cancer-specific protein biomarkers. Our system allows for high-resolution detection of six active and repressive histone modifications and their ratios and combinatorial patterns on millions of individual nucleosomes by single-molecule imaging. In addition, our system provides sensitive and quantitative data on plasma proteins, including detection of non-secreted tumor-specific proteins, such as mutant p53. EPINUC analysis of a cohort of 63 colorectal cancer, 10 pancreatic cancer and 33 healthy plasma samples detected cancer with high accuracy and sensitivity, even at early stages. Finally, combining EPINUC with direct single-molecule DNA sequencing revealed the tissue of origin of colorectal, pancreatic, lung and breast tumors. EPINUC provides multilayered information of potential clinical relevance from limited (<1 ml) liquid biopsy material.

Legend for Supplementary Figures S1-S8.