Citations

373 Background: Recent studies have demonstrated encouraging outcomes with neoadjuvant ICIs in pts with localized deficient mismatch repair (dMMR) and/or microsatellite instability-high (MSI-H) GEC, prompting discussions around the feasibility of nonoperative management (NOM). However, current evidence is largely from phase 1 and 2 clinical trials with limited sample sizes. Thus, the efficacy and safety of neoadjuvant ICI in this pt population remain subject to ongoing debate. Methods: We conducted a systematic review and meta-analysis of prospective and retrospective studies evaluating neoadjuvant ICIs in pts with localized dMMR/MSI-H GEC according to the PRISMA guidelines. The primary outcomes were pathological complete response (pCR) and complete clinical response (cCR) rates. cCR was defined as no evidence of residual disease on tumor-specific imaging and on endoscopy with biopsy. Secondary outcomes included major pathologic response (MPR) rates and the incidence of grade 3–4 immune-related adverse events (irAEs). Proportions with 95% confidence intervals (CI) were used to pool event rates for occurrence of cCR, pCR, MPR and grade 3-4 irAEs. The DerSimonian and Laird random-effects model was used. NOM was assessed as part of the qualitative analysis. Results: 20 studies comprising 396 pts treated with neoadjuvant ICIs were included. Three studies utilized double ICI blockade (PD-1/ PD-L1 inhibitor plus CTLA-4 inhibitor), while the remaining studies primarily utilized monotherapy with a PD-1 inhibitor, with or without chemotherapy. Overall, 320 (81.0%) underwent surgical resection. The pooled pCR rate was 41.6% (95% CI, 33.1%–50.8%), cCR 63.8% (95% CI, 45.6%–78.8%), and MPR 64.2% (95% CI, 49.1%–76.9%). The pooled incidence of grade 3–4 irAEs was 5.1% (95% CI, 0.6%–12.2%). Subgroup analysis showed higher pCR with ≥3 months versus <3 months of ICI (50.2% [95% CI 42.2–58.3] vs 28.4% [18.9–40.2]; χ²=8.69, p=0.003). Forty-four pts were managed with NOM, two experienced early local regrowth over median follow-up ranging from 11.5 to 28 months. Conclusions: Neoadjuvant ICI therapy appears to be a safe and effective therapeutic strategy for pts with localized dMMR/MSI-H GEC. Neoadjuvant ICI exposure for ≥3 months seems to yield higher pCR rates than <3 months. The high and prolonged cCR rates support the nonoperative strategy in selected patients with dMMR/MSI-H GEC.

334 Background: Sarcopenia, or decreased skeletal muscle mass, has been associated with worse surgical and oncologic outcomes across multiple malignancies. This study investigates the impact of sarcopenia on outcomes after multimodal therapy for esophageal cancer. Methods: Retrospective observational cohort study including adult patients who underwent neoadjuvant therapy followed by esophagectomy for primary esophageal cancer from January 2000 - December 2021. A fully automated abdominal segmentation using deep convolutional neural network was utilized to calculate skeletal muscle index (SMI) from preoperative CT scans performed at our institution. Males with SMI <52.4 cm 2 /m 2 and females with SMI <38.5 cm 2 /m 2 at any point before surgery were classified as sarcopenic. The relationship between sarcopenia and patient demographics, operative details, and complications were explored using chi-square tests. The relationship between sarcopenia and disease-free survival and overall survival were investigated using parsimonious multivariable analysis retaining variables with p-value <0.05 using backward selection method. Results: We identified 200 patients meeting the study criteria with 142 (71%) having sarcopenia at any time prior to surgery. Patients with sarcopenia were significantly older [65.2 (59.6, 71.2) vs 63 (53.6, 67.5), p = .023] and more likely to have had open surgery [129 (90.8%) vs 46 (79.3%), p = .034]. There were no significant differences in sex, history of diabetes, coronary artery disease, renal disease, smoking, cancer type, cancer stage, tumor size, tumor location, or margin status between sarcopenic groups ( p > .05). Sarcopenic patients were significantly more likely to require blood transfusion ≥3 units after esophagectomy [79 (55.6%) vs 8 (13.8%), p < .001] or to experience complications requiring surgical intervention [36 (24.6%) vs 7 (12.1%), p = .048]. Multivariate analysis revealed that sarcopenia was associated with disease-free and overall survival (Table). Conclusions: Sarcopenia is a prognostic factor in patients undergoing multimodal therapy for esophageal cancer. Interventions that aim to prevent decreases in SMI after initiating neoadjuvant therapy should be investigated. Multivariate analysis with sarcopenia predicting outcomes. Disease free survivalHR (95% CI) p-value Overall survivalHR (95% CI) p-value Age 1.04 (1.02, 1.06) < .001 1.04 (1.02, 1.07) < .001 Epicenter Middle Upper 1.88 (1.21, 2.86)3.16 (1.67, 5.98) .001.005< .001 1.96 (1.26, 3.04)4.00 (1.90, 8.44) < .001.003< .001 Stage Stage II Stage III Stage IV 1.88 (0.95, 3.71)2.90 (1.46, 5.75)16.02 (6.12, 41.93) < .001.069.002< .001 1.65 (0.83, 3.25)2.76 (1.37, 5.55)10.14 (4.03, 25.53) < .001.151.005< .001 Cancer type - - 1.76 (1.03, 3.02) .039 Transfusion > 3 pRBC - - 1.54 (0.99, 2.40) .057 BMI prior to cancer diagnosis - - 0.98 (0.94, 1.01 .208 Sarcopenic 1.76 (1.16, 2.67) .007 1.64 (1.04, 2.60) .033

331 Background: The relationship between plant-based dietary patterns and upper gastrointestinal (GI) cancers remains unclear. The Healthful Plant-Based Diet Index (hPDI) is a tool that measures adherence to plant-based diet and generates “scores” for clinical comparison. We conducted a systematic review and meta-analysis to examine the association between hPDI and the risk of esophageal and gastric cancers. Methods: A PubMed search identified relevant studies reporting on the association between hPDI and upper GI cancers. Reference lists were manually searched for additional studies. Study selection followed PRISMA guidelines, and quality was assessed using the Newcastle–Ottawa Scale (NOS). Pooled risk estimates were calculated using inverse-variance fixed-effects models, with subgroup analyses by cancer type. Sensitivity analyses were conducted using leave-one-out approach and by switching between fixed- and random-effects models. Forest plots and funnel plots were generated with RevMan 5.4, while Begg’s and Egger’s tests for publication bias were performed using Stata 15. Results: A total of four studies were included, all with high NOS scores. Overall, higher hPDI scores were associated with a significantly lower risk of upper GI cancers (RR = 0.64, 95% CI: 0.56–0.75). In subgroup analyses, the inverse association was consistent for both esophageal cancer (RR = 0.66, 95% CI: 0.54–0.79) and gastric cancer (RR = 0.63, 95% CI: 0.50–0.79). Between-study heterogeneity was low (I² = 35%, P = 0.16). Publication bias assessment indicated no evidence of small-study effects, as Begg’s test was non-significant (P = 0.88) and Egger’s test showed no significant bias (P = 0.76). Sensitivity analyses confirmed the robustness of the findings, with consistent results after sequential exclusion of individual studies and when using random-effects models. Conclusions: Higher hPDI scores were associated with a significantly reduced risk of esophageal and gastric cancers, indicating that adherence to a healthy plant-based diet may reduce upper GI cancer risk. These findings highlight the potential role of dietary modification in upper GI cancer prevention.

292 Background: Fibroblast activation protein (FAP) has high expression in cancer associated fibroblasts (CAFs) in the tumor microenvironment of many solid tumor malignancies. [ 18 F]FAPI-74 is a fluorine-18 labeled FAP-targeting PET imaging tracer with promising characteristics for detection of malignant disease in a variety of cancers. In this multicenter prospective Phase 2 trial, we hypothesized that [ 18 F]FAPI-74 PET would have a high positive predictive value (PPV) for detecting FAP-expressing tumor tissue in a variety of gastrointestinal (GI) malignancies. Methods: This Phase 2 multicenter trial was conducted in the United States at MGH, MSKCC, BAMF, Northwell, UCLA and Mayo (NCT05641896). Eligible GI cancers in the study included hepatocellular carcinoma, cholangiocarcinoma, gastric, esophageal, pancreatic, and colorectal cancer. Each subject underwent one [ 18 F]FAPI-74 PET scan, and a tissue sample was obtained by biopsy and/or surgical resection. The imaging reads were performed centrally by experienced PET readers. The primary objective was to evaluate the PPV of [ 18 F]FAPI-74 PET in detecting FAP-expressing cells from patient collected tissues using centrally evaluated immunohistochemistry (IHC) with positivity defined by the sponsor as an H score higher than 50. The secondary objective was to evaluate the safety profile of [ 18 F]FAPI-74 and its PPV in detecting the presence of GI malignancies using histopathology as the standard of truth. Results: The trial was activated in May 2023, with the last patient imaged in Jan 2025. Of 103 subjects with GI malignancies, the primary site was gastroesophageal in 46 (45%), pancreatic in 22 (21%), colorectal in 17 (17%), hepatobiliary in 10 (10%) and of other GI origin in 8 (8%) cancers. 60 participants were male and 43 female. In total, 60 patients had viable and representative tissue data for efficacy analysis of primary and secondary endpoints. No serious adverse events were reported because of the investigational product. The PPV of [ 18 F]FAPI-74 PET to detect FAP-expressing cells using IHC was 100% (52/52). The sensitivity and specificity for the detection of FAP were 90% (52/58) and 100% (2/2) respectively. The PPVs of [ 18 F]FAPI-74 PET to detect cancer using histopathology as truth standard was 94% (49/52). The sensitivity for the detection of disease was 91% (49/54). Conclusions: The Phase 2 study in GI cancers demonstrates that [ 18 F]FAPI-74 PET detects FAP expression and presence of disease in cancers of the GI tract. PPV of 100% for primary objective and 94% for secondary objective were determined. These results support initiation of a Phase 3 study of [ 18 F]FAPI-74 PET in patients with GI cancers. In addition, [ 18 F]FAPI-74 PET’s reliability in visualizing FAP expression cells in the tumor microenvironment supports a further role in selection of patients for FAP-directed therapies. Clinical trial information: NCT05641896 .

210 Background: ERBB2/HER2 amplification defines a rare but clinically distinct subtype of colorectal cancer (CRC), associated with favorable responses to HER2-targeted therapies. In contrast, ERBB2/HER2 -mutant CRCs show limited therapeutic benefit, and their clinical and molecular features remain poorly defined. This study investigated the molecular and clinical features of ERBB2/HER2 -mutant CRCs to provide insight into the molecular basis of their differential therapeutic vulnerabilities compared to amplified CRC. Methods: CRC cases were identified from cBioPortal studies with available clinical and ERBB2/HER2 genomic data. After quality control and deduplication, the cohort included 4407 unique cases: 97 with driver mutation(s) only (2.2%), 86 with amplification only (2%), and 4224 wild-type (95.9%). Demographic, clinical, and molecular features, including microsatellite instability (MSI), tumor mutational burden (TMB), and alterations in 94 oncogenic pathway genes, were compared across groups. Statistical analyses used chi-square/Fisher’s exact tests for categorical data, Kruskal-Wallis tests for continuous variables, and the Benjamini-Hochberg correction for multiple comparisons. Odds ratios and FDR-adjusted p-values were calculated to assess associations. Results: Across groups, the distributions of gender, race, and primary cancer site (colon or rectal cancer) were comparable. However, significant differences were observed in age (mean 58.7 ± 15.0 years for ERBB2/HER2 -mutant, 55 ± 13.9 for amplified, and 59 ± 14.8 for wild-type; p = 0.035) and tumor sidedness (among ERBB2/HER2 -mutant cases: right-colon 30.8%, left-colon 15.2%; among ERBB2/HER2 -amplified: right-colon 12.1%, left-colon 37.9%, p = 0.011). KRAS alterations were significantly more common in ERBB2/HER2 -mutant CRCs (50.5%) compared with both amplified (19.8%) and wild-type (37.6%). MSI-H was detected in 34.5% of ERBB2/HER2 -mutant cases, whereas no MSI-H was observed among amplified tumors (p < 0.001). Expectedly, ERBB2/HER2 -mutant tumors exhibited a hypermutated phenotype, with significantly higher TMB (30.3 ± 45.5) and total mutation count (232.8 ± 926.0), compared to amplified (6.1 ± 9.7 and 15.3 ± 28.4) and wild-type (10.5 ± 20.3 and 126.3 ± 424.5; both p < 0.001). Conversely, ERBB2/HER2 -amplified tumors had the highest fraction of overall genomic alterations including chromosomal changes (0.24 ± 0.17 in amplified vs 0.14 ± 0.17 in mutants and 0.20 ± 0.16 in wild-type; p < 0.001). Hierarchical clustering demonstrated that ERBB2/HER2 -mutant tumors form a distinct molecular cluster, whereas amplified tumors cluster closely with wild-type CRC. Conclusions: ERBB2/HER2 - mutant and amplified CRCs show highly distinct genomic and clinical features, supporting the need for tailored treatment approaches.

198 Background: The Hedgehog (Hh) signaling pathway regulates tissue patterning, growth and differentiation, including within gastrointestinal tract. Mammalian homologs include Sonic (SHH), Indian (IHH) and Desert (DHH) Hedgehog, which signal through PTCH and SMO and the GLI transcription factors. In the digestive tract, both SHH and IHH are expressed, but the functional role of Hh signaling in mCRC remains incompletely defined. Methods: We analyzed 433 mCRC patients treated with either bevacizumab (bev = 226) or cetuximab (cet = 207) plus first line chemotherapy in CALGB/SWOG 80405 (NCT00265850). RNA expression (Illumina HiSeq 2500) of 9 Hh pathway genes (SHH, IHH, DHH, PTCH1, PTCH2, SMO, GLI1, GLI2, GLI3) was evaluated. Median overall survival (mOS) and progression free survival (mPFS) in months (mo) were compared across expression tertiles (low [L], medium [M], high [H]). Likelihood ratio tests, hazard ratios and 95% confidence intervals were computed from multivariable Cox proportional hazards models, adjusting for age, sex, ethnicity, ECOG PS, tumor location, number of metastatic sites, KRAS , microsatellites instability status and treatment. Prognostic effects were modeled as marginal terms; predictive effects were modeled as multiplicative interactions. Results: Overall, IHH and GLI2 demonstrated prognostic value for OS after adjusting for multiple testing (Benjamini-Hochberg False Discovery Rate (FDR) < 0.05). High IHH expression (H group, n = 143) correlated with improved mOS (H: 36.0 mo vs M: 32.8 mo vs L: 23.5 mo; p= 0.031), whereas low GLI2 expression (L group, n = 145) was associated with longer mOS (H: 25.6 mo vs M: 33.9 mo vs L: 33.7 mo; p= 0.0016). Moreover, IHH showed predictive value for cet benefit in both OS (mOS H: 38.1 mo vs M: 34.0 mo vs L: 18.4 mo; p= 0.0032) and PFS (mPFS H: 12.6 mo vs M: 12.9 mo vs L: 8.8 mo; p= 0.0025) with significant treatment interaction for OS and PFS (p= 0.0046 and p= 0.0049, respectively; FDR< 0.05). No significant associations were found in bev treated pts. No other Hh genes showed significant prognostic or predictive effects after FDR correction. Conclusions: These results highlight a dual role of Hh pathway in mCRC. Consistent with prior preclinical evidence, reduced IHH expression appears to mark poor prognosis, likely reflecting the loss of IHH-mediated restraint on Wnt signaling, which drives proliferation and dedifferentiation. Conversely, low GLI2, a downstream transcription factor, was linked with favorable outcomes, underscoring context-specific effects of Hh signaling. The strong predictive association of IHH with cet efficacy supports its potential as a biomarker in RAS wild type mCRC. These results suggest that Hh pathway modulation, particularly IHH signaling, may influence disease biology and treatment response. Validation in independent cohorts and mechanistic studies is warranted to clarify implications.

179 Background: Neuroligins are a family of postsynaptic cell adhesion molecules that play a crucial role in synapse formation and function. Recent studies suggest Neuroligin family genes (NLGN1,2,3,4X) may promote cancer cell growth and metastasis in various cancers, including colorectal cancer. Here, we investigated the association between neuroligin family gene expression and first-line treatment outcomes in mCRC. Methods: We evaluated the expression of 4 genes in the neuroligin family (NLGN1, NLGN2, NLGN3 and NLGN4X) on treatment outcomes of 433 pts with mCRC treated with either bevacizumab (Bev, n = 226) or cetuximab (Cet, n = 207) in combination with first-line chemotherapy within the CALGB/SWOG 80405 trial. mRNA was isolated from FFPE tumor samples and sequenced on the HiSeq 2500 (Illumina). Overall survival (OS) and progression-free survival (PFS) were compared between groups of pts categorized by low expression (Q1) (≤25 percentile), medium expression (Q2-3) (25th-75th percentile) and high expression (Q4)(>75th percentile) of NLGN family genes. The associations between gene expression and clinical outcomes were assessed via the multivariable Cox model and plotted with Kaplan-Meier curve. Treatment-gene interactions were evaluated using the likelihood ratio test. False discovery rate (FDR) was controlled through the Benjamini-Hochberg procedure. Results: High NLGN2 expression was associated with shorter PFS (median 10.0 mos, 95% CI (8.3, 11.3)) and OS (median 25.2mos, 95%CI (20.9, 30.0)) compared to low NLGN2 expression (PFS: median 11.2mos, 95% CI (9.8,14.3))(HR 1.41, 95% CI (1.06, 1.89)), P = 0.029). (OS: (median 31.8mos, 95%CI (26.8, 37.5)( HR 1.49, 95% CI (1.10, 2.01) P =0.0095) in multivariate analysis. Patients treated in the cetuximab arm(n=207), high HLGN2 expression significantly associated with shorter PFS (median 8.0 vs 13.4, HR 2.02, 95% CI (1.35, 3.03), P = 0.0021) and OS (median19.6 vs 36.3, HR 2.30, 95% CI (1.50, 3.53), P =8.3e-05). High NLGN3 expression was associated with shorter OS (23.9 vs 36.0 mos, P =0.0083) in cetuximab treatment arms, no significance found in PFS. We found significant treatment interaction for OS with NLGN2 and NLGN3 favoring cetuximab. In pts treated with cetuximab, High NLGN2 expression was significantly associated with shorter OS (median 29.1vs 36.3) (p=0.024) compared to pts treated with bevacizumab. For NLGN3, pts treated with cetuximab, high NLGN3 expression was significantly associated with shorter OS (median 25.0 vs 36.0) (p=0.018) compared to pts treated with bevacizumab. No significant association was found in NLGN1 and NLGN4X. Conclusions: Our results suggest NLGN2/3 gene expression may predict mCRC treatment outcomes. Potential therapeutic drug target NLGN in combination with anti-EGFR treatment in mCRC is warranted.

175 Background: BRAF V600E mutations occur in ~8–12% of metastatic colorectal cancer (mCRC) and are linked to poor prognosis, therapy resistance, and limited response to anti-EGFR monotherapy. While the BEACON trial established encorafenib plus cetuximab as standard, the added value of triplet regimens remains unclear. Clarifying the efficacy and toxicity of doublet vs triplet therapies is essential to optimize care in this high-risk group. Methods: Studies published across PubMed, Embase, and Google Scholar from inception till May 2025 with reported outcomes of the desired targeted therapies for BRAF V600E-mutated mCRC were screened and finally 12 studies included in this review. Data on efficacy parameters such as overall survival (OS), progression-free survival (PFS), objective response rate (ORR), disease control rate (DCR) as well as safety indicators such as adverse events (AEs), and discontinuation rates were extracted and analyzed. Results: In the ≥2nd-line setting, doublet and triplet regimens (doublet plus binimetinib) demonstrated improved outcomes compared with cetuximab plus irinotecan/FOLFIRI, with higher OS (7.2–9.2 vs 5.6 months), PFS (4.2–4.6 vs 1.5 months), DCR (65–75% vs 31%), and a comparable incidence of severe AEs (3–5% vs 4%). Panitumumab plus vemurafenib, also in the ≥2nd-line setting, reported an OS of 7.6 months, PFS of 3.2 months, and a DCR of 27%. In the 1st-line setting, the doublet regimen plus mFOLFOX6 with longer treatment duration (~50 weeks) showed improved outcomes compared with the doublet regimen plus higher-dose cetuximab, with higher OS (30.3 vs 19.5 months), PFS (12.8 vs 6.8 months), ORR (65.7% vs 45.6%), but also a higher incidence of severe AEs (46% vs 30%). Similarly, the cetuximab + vemurafenib + irinotecan regimen outperformed conventional chemotherapy (mFOLFOX6, FOLFOXIRI, or CAPOX) ± bevacizumab, with improved OS (25.3 vs 10–15 months), PFS (11.9 vs 7.5 months), ORR (63.2% vs 37–40%), DCR (94.7% vs 75%), and a lower rate of grade ≥3 AEs (34.2% vs 65%). Conclusions: The survival benefit observed with the doublet regimen was comparable to that of the triplet, but with lower rates of adverse events and treatment discontinuation, supporting current evidence that the doublet remains the preferred option for patients with BRAF V600E–mutated mCRC who have received one or more prior lines of therapy. Although adding conventional agents such as mFOLFOX6 to the doublet in the first-line setting improved OS, PFS, ORR, and DCR, this approach was associated with higher toxicity and discontinuation rates, raising questions about its overall benefit. Emerging data on vemurafenib in combination with cetuximab and conventional agents show promise, but further studies directly comparing encorafenib- versus vemurafenib-based regimens are warranted to optimize outcomes in this patient population.

163 Background: Being physically inactive is an independent predictor of poor outcomes in patients with stage III colon cancer. Whether physical activity can mitigate the oncologic risk of postoperative MRD is unknown. Methods: We concluded a post hoc analysis of a phase III trial evaluating celecoxib vs. placebo and 3 vs. 6 months of adjuvant chemotherapy in patients with resected stage III colon cancer. Self-reported physical activity was assessed in standardized measures of metabolic equivalent hours per week (MET-h/wk) at 2 timepoints: 1) midway through and 2) 6 months after completing adjuvant chemotherapy, modeled as a time-weighted average. MRD was determined via a clinically validated, tumor-informed circulating tumor DNA (ctDNA) assay (Signatera, Natera, Inc.). Disease-free survival (DFS) and overall survival (OS) were estimated using flexible parametric survival models. Results: Among 763 patients (median follow-up 6.0 years), 24% were highly active (≥18 MET-h/wk) and 32% were inactive (<3 MET-h/wk); 18% were ctDNA positive. Adjusting for covariates, DFS improved in patients who were ctDNA negative and moderately active (Ref: inactive; 3-17.9 MET-h/wk; HR=0.62, 95% CI [0.40-0.97]) or highly active (HR = 0.53 [0.30-0.94]). No benefit was seen among patients who were ctDNA positive (Ref: inactive; moderately active: HR = 1.20 [0.75-1.92]; highly active: HR=0.83 [0.45-1.54]). The interaction was not significant (Pint=0.269). Physical activity did not impact OS regardless of ctDNA status. The interaction of physical activity and adjuvant celecoxib use is shown in Table. Conclusions: Among patients who were ctDNA negative, physical activity was associated with improved DFS, and may demonstrate increased survival benefit alongside adjuvant celecoxib. The additive effects of both interventions were inconclusive among those who were ctDNA positive. Support: U10CA180821, U10CA180882, U24CA196171; https://acknowledgments.alliancefound.org; Natera, Pfizer; NCT01150045. Disease-free survival by physical activity level and celecoxib use, stratified by ctDNA status. 3-yr event # at risk Adjusted HR (95% CI) P-value Pint ctDNA Negative 0.258 Inactive + placebo 23 182 Ref Active + placebo 17 132 0.74 (0.43-1.28) 0.283 Inactive + celecoxib 23 167 0.92 (0.57-1.48) 0.720 Active + celecoxib 10 146 0.40 (0.22-0.79) 0.007 ctDNA Positive 0.221 Inactive + placebo 26 33 Ref Active + placebo 15 20 0.96 (0.50-1.84) 0.903 Inactive + celecoxib 34 54 0.62 (0.37-1.05) 0.076 Active + celecoxib 14 29 0.34 (0.18-0.64) 0.001 ctDNA, circulating tumor DNA; HR, hazard ratio; CI, confidence interval. Active = ≥9 MET-h/week, inactive = <9 MET-h/week. Celecoxib use defined by assignment in CALGB/SWOG 80702. Models adjusted by T and N stage, age, sex, race, assigned cycles of fluorouracil, leucovorin, and oxaliplatin, KRAS and BRAF status, BMI, and Western diet.

162 Background: In preclinical studies, the tryptophan-metabolizing enzyme, Indoleamine 2, 3-dioxygenase 1 (IDO1), promotes resistance to radiation in colorectal cancer, and IDO1 inhibition with epacadostat improves tumor radiosensitivity. In a phase 1 trial, patients (pts) with locally advanced rectal cancer (LARC, n=17) were treated with epacadostat in combination with short course radiotherapy (SCRT) and CAPOX which was well-tolerated, and the recommended phase 2 dose (RP2D) of epacadostat was 400mg BID. Here we report efficacy and safety from the NCI-supported phase 2 (NCT03516708) multicenter, open-label trial (treatment cohort). Methods: Pts were treated with standard of care (SOC) SCRT, then SOC neoadjuvant chemotherapy (NAC) followed by SOC surgical resection or non-operative management (NOM). Epacadostat (400 mg BID) started with SCRT and continued until starting NAC (a minimum of 21 days). Eight pts were enrolled prior to discontinuation of epacadostat by Incyte in February/March 2025. The primary endpoint is Neoadjuvant Rectal (NAR) score. In patients who underwent NOM, we calculated MRI-based tumor regression grade (MR-TRG). Secondary endpoints are pathologic complete response (pCR) rate, complete clinical response (cCR) rate, and progression free survival (PFS). Results: Table 1 summarizes baseline characteristics and endpoints (primary and secondary). NOM was pursued in 7/8 (87.5%) pts, surgical resection was performed on 1/8 (12.5%). At median follow-up of 12 months, local tumor regrowth occurred in 1 pt initially treated with NOM (after tumor regrowth, he underwent salvage surgery and is currently on surveillance). Treatment with epacadostat was interrupted in 2/8 (25%) patients due to grade 3 maculopapular skin rash. Most frequent treatment-emergent adverse events (TEAEs) regardless of causality (all grades (G)/G3-4) were bloating (75%/0%), increased ALT (62.5%/0%), increased AST (50%/0%), nausea (50%/0%), and constipation (50%/0%). Grade 3 diarrhea and grade 3 non-cardiac chest pain occurred in 1 patient each (both unrelated to epacadostat). Conclusions: Epacadostat demonstrates promising efficacy and manageable safety in patients with LARC. The biomarker cohort and correlative studies are ongoing. Clinical trial information: NCT03516708 . Baseline characteristics and endpoints. Patient Age Sex Ethnicity Stage Surgery/NOM NAR MR-TRG cCR/pCR Recurrence 1 52 M Caucasian T3N1 NOM NA 2 Y/NA No 2 45 M Caucasian T3N1 NOM NA 3 Y/NA Local tumor regrowth 3 48 M Caucasian T3N2 NOM NA 3 Y/NA No 4 61 M Caucasian T4N2 NOM NA 3 Y/NA No 5 57 M Caucasian T3N0 NOM NA 1 Y/NA No 6 60 F Caucasian T3N1 Surgery 15 3 N/ypT3N0 No 7 67 M Caucasian T4N0 NOM NA 3 Y/NA No 8 51 M Caucasian T4N2 NOM NA P NA No