195 Background: Peritoneal metastasis (PM) from gastrointestinal (GI) malignancies respond poorly to traditional systemic therapies. Intraperitoneal (IP) paclitaxel (PTX) is safe and effective in gastric and ovarian PM in Phase III trials and is now being tested for appendiceal-adenocarcinoma PM (AA-PM) (NCT06207305). The peritoneal cavity is immune-cell–rich, yet their diversity across GI-PM subtypes and the impact of IP PTX remain unevaluated. We asked how the peritoneal immune landscape varies across GI-PM subtypes and how, particularly in myeloid cells and T cells, it is altered by IP PTX. Methods: We profiled peritoneal immune components by scRNA-seq and scTCR-seq in patients without PM and in those with PM from AA or colorectal cancer (CRC-PM). We also analyzed AA-PM before and after IP PTX in a clinical trial. In parallel, a mouse PM model with spectrum flow cytometry examined how peritoneal immune cells shape responses to IP PTX. Results: Across all groups, peritoneal fluid contained monocytes, macrophages, T cells, and DC2s, with very few B cells and DC1s. Cell-type frequencies were similar in no-PM and CRC-PM, whereas AA-PM showed marked monocyte enrichment with reduced T cells, consistent with an immune-inhibitory milieu. Within the myeloid compartment, most macrophage subsets—including LYVE1⁺SPP1⁺ tumor-associated macrophages—were comparably prevalent with and without GI PM, indicating baseline anti-inflammatory tone. In contrast, CD8⁺ T cells in GI-PM peritoneal fluid displayed robust activation signatures (TBX21, GZMB, IFNG). In one MSI-high CRC-PM patient on anti-PD-1, the top four TCR clonotypes comprised ~30% of all T cells, indicating strong clonal expansion and potential peritoneal antitumor activity. IP PTX reshaped the peritoneal immune system in AA-PM: TIMD4⁺ resident macrophages decreased post-treatment and macrophages acquired pro-inflammatory programs, including IFN-α/β signaling. In DC2s, GSEA showed upregulated antigen processing/presentation, co-stimulation, cytokine/chemokine, and migratory pathways. Post-treatment peritoneal T cells increased activation and tissue-residency features (CD69, ITGAE, CXCR6, TBX21), suggesting that IP PTX may activate innate cells and augment T-cell responses. In mice, intraperitoneal MC38-luciferase cells established PM, and IP PTX (25 mg/kg twice weekly) markedly reduced tumor bioluminescence. Flow cytometry showed decreased ICAM2⁺ large peritoneal macrophages and increased neutrophils and MHCII⁺ monocytes, recapitulating the patient phenotype. Effector-memory CD4⁺ and CD8⁺ T cells (CD44⁺CD62L⁻) expanded. Anti-tumor activities of these peritoneal macrophages and T cells are under study. Conclusions: Overall, these data demonstrate changes in the immune milieu in GI PM and after IP PTX and suggest cooperative roles for mononuclear phagocytes and T cells in IP PTX efficacy.

Natural killer (NK) cells are a principal component of the body's innate immune response to both primary and metastatic cancer.1 In recent years, NK cells have been heavily investigated as both therapeutic targets and as adoptive cell therapies for the treatment of cancer, with the majority of prior in-human clinical investigation of NK cell therapies focusing on soluble blood cancers.2 Both human and animal studies have demonstrated that robust NK cell function facilitates improved control of tumors in the gastrointestinal tract, including gastrointestinal stromal tumor, gastric, and colorectal cancer.1 Previous work has not specifically examined the role of NK cells in either localized or metastatic appendiceal adenocarcinoma (AA), a molecularly distinct gastrointestinal tumor that has recently been increasing in incidence.3 Here, we assess the role of NK cells in AA by analyzing single-cell RNA sequencing (scRNA-seq) data from NK cells recovered from tumors of peritoneal metastasis of appendiceal adenocarcinoma (PMAA). We demonstrate that PMAA-infiltrating NK cells express a transcriptional program consistent with cell exhaustion, with only a small minority of NK cells expressing a classical cytotoxic phenotype. These findings suggest NK cell functional rescue as a possible strategy for the treatment of AA.

155 Background: Modern therapy can achieve a clinical complete response (cCR) of the primary tumor in the patients with rectal cancer (RC). However, use of a rectal organ preservation (OP) strategy for patients achieving cCR among patients who present with synchronous liver metastases (SLM) remains an area of active investigation. This study aimed to evaluate the outcomes of OP in patients with a cCR after liver first treatment sequencing for metastatic RC. Methods: Patients with RC and SLM who underwent liver-first approach at an NCI-designated Comprehensive Cancer Center from 1998-2024 were identified. Patients who achieved a cCR of the primary tumor were stratified with respect to whether the primary tumor was resected or managed nonoperatively via an OP strategy. A time benchmark of 12 weeks from the date of cCR was used to control for immortal time bias. Rates of local regrowth (LReg) in the rectum, local recurrence in the pelvis after rectal resection (LRec), and extrapelvic recurrence (DR) were modeled with cumulative incidence functions, and 3-year overall survival (OS) was compared between groups. Results: Among 217 patients identified, 48 (22.1%) patients achieved a cCR of the primary tumor. Among these patients, 8 patients (16.7%) demonstrated progression prior to the benchmark and were excluded from analyses. Of these, 5 patients demonstrated extrahepatic DR, 1 intrahepatic DR, and 2 LReg. An additional 3 patients (6.2%) underwent rectal resection following the benchmark date without documented evidence of LReg and were also excluded. Of the remaining 37 patients, 12 underwent rectal resection prior to the 12-week benchmark (32.4%) while 25 did not (67.6%). At a median follow-up of 36.9 months, 14 patients managed with an OP strategy experienced LReg (56.0%). Of these patients, 9 patients (64.3%) underwent salvage resection, with an R0 resection rate of 88.9%. None of these patients experienced post-salvage LRec, and LRec rate was also 0% among patients who underwent initial resection. The remainder of those who did not undergo salvage resection had developed prior DR at the time of LReg (4/5, 80%), with one patient electing to continue OP. Rates of DR as the initial site of disease progression were similar between groups (52.0% resection vs. 58.3% OP, p=0.892). 3-year OS rates were not significantly different between groups (100.0% resection vs. 82.5% OP, p=0.18). Conclusions: In patients with RC who have SLM, undergo liver-first approach, and achieve cCR, an OP strategy is associated with high rates of LReg but has high rate of successful salvage and is not associated with significant differences in DR or 3-year OS rate compared to primary tumor resection. An OP strategy for metastatic RC may be a feasible management strategy for select patients, with future work aimed at identifying patients with favorable tumor biology in whom this strategy would be most oncologically appropriate.

840 Background: There are no prospective studies evaluating the benefit of adjuvant chemotherapy (AC) for patients diagnosed with early-stage Appendiceal adenocarcinoma (AA). Current NCCN guidelines suggest that AA be treated the same as colon cancer, and that 5FU-based AC should be considered if CRC-derived “high-risk” features are present. Here we evaluate the benefit of AC and identify features associated with risk of relapse in the largest study of early-stage AA to date. Methods: Foundry software (Palantir, Denver, CO) was used to query the MD Anderson (MDA) EHR to identify and extract data from all patients all AA diagnosed between 2000 and 2024. Results: Among a total of 3,662 patients with AA, 439 (12.0%) were identified as early-stage. Interestingly goblet cell histology (GCA) was most common, 176 (40%), followed by mucinous 131 (30%), enteric 117 (27%), and signet ring cell (SRC) 15 (3%); this is a notable difference from metastatic patients where GCA account for only 12.8%. In 202 patients where surgery was performed at MDA only 19 (9.4%) relapsed. As expected, 5-yr relapse incidence was higher in stage 3 vs. 2 (30% vs 9.0%, HR 3.1). GCA had a particularly low 5-yr relapse (2%), relative to mucinous (14.5%), SRC (16.7%) and enteric (20%, p=0.006). In multivariate analysis of stage 2 tumors histologic subtype (HR 5.6 mucinous, 6.6 enteric, p<0.001) and pT4 (HR 3.5, p<0.001) predicted relapse, while CRC-derived risk factors (grade, <12 LN examined, perforation, LVI, PNI) were not significant. Of 428 evaluable patients, 196 (46%) received AC. Relapse occurred in 29% with AC vs 16% without (OR 2.2, p=0.001). AC was associated with worse RFS in univariate (HR 2.1, p=0.001) but not multivariate analysis (HR 1.0, p=0.94), consistent with higher risk patients more likely to receive AC. Importantly no OS benefit was observed in the overall cohort, all stage II patients, or any subset of ‘high-risk’ stage II patients. The majority of relapse was confined to the peritoneal space (88%), and 56% of relapsed patients were candidates for complete cytoreductive surgery. This likely explains why even the majority of patients with relapse (77%) remained alive at 5 years (vs. 96% non-relapsed; HR 5.5). Conclusions: The risk of relapse following surgical resection of early-stage AA is low, particularly in GCA, and survival outcomes are favorable. We find no RFS or OS survival advantage to AC in all stage 2 patients nor in any high-risk subset. Given the lack of prospective data supporting the use of AC for patients with appendix cancer, these data suggest patients with stage 2 AA should not be offered AC.

The efficacy of immune checkpoint inhibitors (ICIs) in colon cancer (CC) is limited by the tolerogenic immune landscape of both the tumor and the intestine. Here, we investigated the impact of microbial metabolic pathways associated with ICI responsiveness on anti-tumor immunity in CC. Sulfur amino acid (Saa) metabolic pathways were enriched in ICI responders and in individuals without CC. In murine models, high-Saa diets restricted CC progression, increased colonic mucus thickness, and promoted the expansion of mucus-dwelling Mucispirillum schaedleri (M. schaedleri). Dietary Saa supplementation increased the frequency of activated intratumoral cytotoxic CD8+ T cells. The protective effect of high-Saa diets required type 1 conventional dendritic cells (cDC1s) in tumor-draining lymph nodes, where M. schaedleri was sufficient to drive cDC1 expansion and activation. Mechanistically, M. schaedleri stimulated natural killer T (NKT) cell expansion and secretion of the chemokine XCL1, which was essential for cDC1 recruitment and anti-tumor immunity. Dietary modulation of this NKT-XCL1-cDC1 axis in combination with ICIs may open avenues for improved treatments of CC.

847 Background: The biological behavior of appendiceal epithelial neoplasms (AENs) depends on histological subtype and grade. Due to their rarity, previous molecular analyses have grouped diverse biological types together. We aimed to examine genomic and outcomes differences among distinct biological subgroups of appendiceal cancer. Methods: Tempus Lens was used to analyze de-identified clinical genomic, and transcriptomic information for patients (pts) diagnosed with AENs (N=807): mucinous adenocarcinomas (MA) (N=282), non-mucinous adenocarcinomas (NMA) (N = 311), signet-ring cell carcinoma (SRC) (N=46), and goblet cell adenocarcinoma (GCA) (N = 168). Pts were stratified by GNAS/KRAS co-mutations (mut) (N=151) or wild-type (wt) (N=231). RNA-seq data were normalized and quantified as transcripts per million (TPM) and reported as log2(TPM+1). Immune infiltration was estimated via quanTIseq and enrichment via single sample gene set enrichment analysis (ssGSEA). Real-world overall survival (rwOS) was defined as the time from sample collection to death or loss to follow up. Median OS (mOS) was estimated using Kaplan-Meier curves and compared using log-rank tests. Results: Across the cohort, median age was 63 years; 87% were Stage IV with grade distribution of 18% G1, 34% G2, 48% G3; 54% were female; 79% were White, 10% Black, 2% Asian, 8% other and 9% Hispanic ethnicity. Most common mutations across histologies are shown in the table. In MA and NMA, the most common KRAS alterations were 37% and 25% G12D, 20% and 14% G12V, 7% and 6% G13D, and 5% and 4% G12C. GNAS / KRAS -mut was present in 38% MA, 14% NMA and 7% SRC. Grade 2 MA was genomically more like G1 than G3 with KRAS and GNAS rates for G1/2/3 of 90%/83%/46% and 64%/41%/17%, respectively. In MA pts, mOS was shorter for G3 compared to G2 and G1, respectively (13.2m vs 28.8m vs 25.9m, p=0.002). Although, immune cell infiltration was similar between MA and NMA pts; pts with GNAS/KRAS-mut showed higher levels of M1 macrophages (p<0.05), higher enrichment in the tertiary lymphoid structure signature (p<0.001) and lower levels of M2 macrophages (p<0.0001) and NK cells (p<0.0001) compared to wt pts. mOS was improved in GNAS/KRAS-mut (N=307) pts compared to wt pts (20.5m vs 16m, p=0.008). Conclusions: AENs show unique DNA alterations by histological subtype. Grade 2 MA closely resembles G1, not G3, in survival and genomics, supporting a three-tier grading system over a high-grade (G2/G3) grouping. KRAS G12D represented 37% of KRAS mutations, and pts with KRAS/GNAS co-mutations had better survival and a favorable immune profile, supporting further immunotherapy research in low-grade MA. Most common mutations across histological subtypes. MA NMA GCA SRC KRAS *** 75% 55% 14% 20% TP53 *** 44% 59% 34% 17% SMAD4 24% 26% 15% 30% GNAS *** 41% 15% 8% 7% APC *** 8% 23% 2% 4% RHOA *** 0.5% 3% 8% 4% KDM6A * 2% 3% 4% 15% PIK3CA 7% 14% 4% 11% ***p<0.001; *p<0.05.

In colorectal cancer (CRC)-related carcinomatosis, the risk of ovarian metastases (OM) is not well described, and practices during cytoreductive surgery (CRS) vary widely. Defining this risk is critical for counseling patients on the necessity of oophorectomy during CRS. This retrospective study analyzed female patients with CRC-related carcinomatosis undergoing curative intent CRS with or without hyperthermic intraperitoneal chemotherapy (HIPEC) at a single center (2008–2024). The study identified 74 patients, 66

Importance Relapse risk and benefit of adjuvant chemotherapy after resection of appendiceal adenocarcinoma (AA) are uncertain. Objective To identify clinicopathologic and genomic factors associated with relapse and assess efficacy of adjuvant chemotherapy in localized AA. Design, Setting, and Participants This retrospective cohort study (January 2000 through February 2024; median follow-up, 62.6 months) used Kaplan-Meier and Cox proportional hazards modeling. It took place at the University of Texas MD (UT MD) Anderson Cancer Center with validation from Memorial Sloan Kettering Cancer Center (MSKCC). Participants included a complete localized cohort of 439 patients with stage I to III AA from UT MD Anderson, of whom 202 underwent surgery at UT MD Anderson and also included a validation cohort of 128 patients with stage II AA from MSKCC. Exposures Surgical resection with or without adjuvant chemotherapy. Main Outcomes and Measures Rate of recurrence, recurrence-free survival (RFS), and overall survival (OS). Results There were 439 patients with localized AA (median age, 56.5 [IQR, 22.2-83.7] years; 50% female and 50% male) managed at MD Anderson between January 2000 and February 2024. Of 202 MDA surgical patients, 19 (9.4%) had a relapse including 9 (6%) patients with stage II and 8 (19.5%) of patients stage III disease. Five-year OS was 95.7% without vs 77.2% with relapse (hazard ratio [HR], 5.50; 95% CI, 3.07-9.83; P < .001). Relative to goblet cell tumors, mucinous (HR, 5.60; 95% CI, 2.1-15; P < .001) and enteric-type (HR, 6.60; 95% CI, 2.9-15; P < .001) histologies were independently associated with relapse, as was pathologic T4 (HR, 3.30; 95% CI, 1.9-5.7; P < .001). Importantly, poor differentiation, perforation, lymphovascular invasion, and perineural invasion, known risk factors in colorectal cancer, were not significantly associated with relapse. For the complete localized cohort, adjuvant chemotherapy was not associated with improved RFS (univariate HR, 2.06; 95% CI, 1.36-3.13; P = .001 and multivariable HR, 0.98; 95% CI, 0.43-2.28; P = .90) or OS (univariate HR, 1.80; 95% CI, 1.0-3.2; P = .04 and multivariable HR, 0.71; 95% CI, 0.24-2.1; P = .53). TP53 mutation in goblet cell tumors (HR, 6.93; 95% CI, 1.50-31.00; P = .01) and GNAS mutation in nongoblet tumors (HR, 17.0; 95% CI, 3.09-93.3; P = .001) were associated with greater risk of relapse. Conclusions and Relevance These results demonstrate that relapse after resection of localized AA is uncommon. Molecular profiling and histopathologic subtype refine risk. Adjuvant chemotherapy were not associated with benefit.

The role of immunotherapy in the care of surgical oncology patients promises to expand as investigators and clinicians evaluate new targets and approaches. Currently active clinical trials evaluate new immune checkpoints, including lymphocyte activation gene 3, T cell immunoreceptor with Ig and ITIM domains, and killer Ig-like receptor 2DL1/2L3. Vaccines delivered through mRNA have demonstrated exciting results in early clinical trials and hold promise for expanded application. Investigational approaches include dendritic cell vaccines, peptide vaccines, cytokines therapies, and cellular therapies. These studies have the potential to revolutionize the management of surgical oncology patients and promote durable cures following surgical resection.

The role of immunotherapy in the care of surgical oncology patients promises to expand as investigators and clinicians evaluate new targets and approaches. Currently active clinical trials evaluate new immune checkpoints, including lymphocyte activation gene 3, T cell immunoreceptor with Ig and ITIM domains, and killer Ig-like receptor 2DL1/2L3. Vaccines delivered through mRNA have demonstrated exciting results in early clinical trials and hold promise for expanded application. Investigational approaches include dendritic cell vaccines, peptide vaccines, cytokines therapies, and cellular therapies. These studies have the potential to revolutionize the management of surgical oncology patients and promote durable cures following surgical resection.

Peritoneal carcinomatosis from lower gastrointestinal malignancies causes debilitating intra-abdominal complications, including bowel obstruction, ascites, and biliary/ureteral obstruction. Guidelines addressing palliative cytoreductive surgery (CRS) in patient’s ineligible for curative resection remain limited. Patients who underwent CRS for appendiceal cancer (AC)- or colorectal cancer (CRC)-related carcinomatosis not amenable to complete cytoreduction at one referral center (2004–2025) were retrospectively reviewed. Tumor grade was dichotomized into low- (well- and well-to-moderately differentiated ACs) versus high-grade (moderately or poorly differentiated/signet ring ACs, all CRCs). Descriptive statistics summarize patient demographics. Carcinomatosis-related complication-free (CRCFS), obstruction-free (OFS), ascites-free (AFS), and overall survival (OS) were estimated by using the Kaplan-Meier method. A total of 131 patients underwent palliative CRS. The most common reasons for incomplete cytoreduction were high tumor burden/multiple unresectable areas 62 (47

Abstract Background: Appendiceal Adenocarcinoma (AA) is a rare cancer, though recent data indicate a rising incidence, now exceeding 1 per 100,000 individuals annually. Goblet cell adenocarcinoma (GCA) of the appendix, which accounts for ∼15% of all AA, is associated with worse survival than non-goblet AA. To elucidate the unique tumor biology of GCA, we systematically compared the genomic and transcriptomic landscapes of GCA versus non-GCA AA, identifying GCA-specific driver mutations and dysregulated pathways that distinguish GCA from other AA histologic subtypes. Methods: Whole-exome sequencing (WES) data from 1,202 AA tumors were generated as part of the standard workflow for personalized, tumor informed circulating tumor DNA testing (SignateraTM), including 385 patients with GCA, 450 with mucinous, 206 with enteric-type, and 161 with signet ring cell histology. Transcriptomic profiling was available for 63 tumors (n=25 GCA, 38 non-GCA) using AlteraTM tumor genomic profiling test (Natera, Inc.). Isogenic organoids with gene knock-out (KO) were generated using CRISPR/Cas9 in a KRAS/GNAS co-mutant murine tumor. Results: There were striking differences in the somatic mutation landscapes between GCA and non-GCA tumors, with frequent mutations in KRAS (73.4%), TP53 (39.5%), and GNAS (36%) in non-GCA tumors. In contrast, GCA did not harbor a single gene mutation in more than 20% of the cohort. In GCA, the most commonly mutated gene was ZFP36L2 (18%), a zinc finger protein that binds and stabilizes mRNA transcripts, and has previously been reported to inhibit cell proliferation. Other commonly mutated genes in GCA included TP53 (17%) and SMAD4 (10%), with only 10% of tumors harboring KRAS mutations. Transcriptional regulators such as WTAP (9%), SOX9 (8%), ARID1A (7%), MLLT4 (7%), KMT2D (7%), and KMT2B (4%) were also frequently altered. Performing Gene Set Enrichment Analysis (GSEA) on the differentially expressed genes between GCA and non-GCA tumors revealed that the bile acid and adipogenesis pathways were upregulated in GCA. Survival analysis showed worse OS in GCA vs non-GCA AA (94.7 months vs not reached; HR 3.51, 95% CI 2.67-4.63; p < 0.0001). To explore the potential significance of ZFP36L2, organoids derived from ZFP36L2 KO showed decreased doubling time and allowed propagation of the organoid culture without doxycycline-induced expression of GNASR201C, a known oncogenic driver in AA. Transcriptomic analysis comparing ZFP36L2 KO to parental lines showed enrichment for androgen response, EMT, TNFα, Wnt, Hedgehog, KRAS, and TGFβ signaling. Conclusion: Clear molecular differences between GCA and the more common mucinous AA were observed. It is clear that strategies for developing new drugs for AA must treat GCA as a distinct entity and not merge these patients with other histological subtypes, as has frequently been done previously. Citation Format: Saikat Chowdhury, Ichiaki Ito, Samuel Rivero-Hinojosa, Chia-Mei Young, Eleanor A. Fallon, Beth A. Helmink, Taylor Neilson, Vasily Aushev, Sacha El Khoury, Abdelrahman MG Yousef, Mahmoud Mohamad Gamal Yousef, Shruti Sharma, Robert Lentz, Adham Jurdi, Paul F. Mansfield, Keith Fournier, Yuan-Hung Lo, Michael G. White, John Paul Shen. Goblet cell appendiceal adenocarcinoma is molecularly distinct from other histologic subtypes: Insights from large-scale genomic, transcriptomic, and organoid modeling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4125.

Background: Appendiceal adenocarcinoma (AA) is a rare gastrointestinal cancer that frequently presents with peritoneal metastases; the standard of care for resectable peritoneal metastases from AA limited to the peritoneum is cytoreductive surgery (CRS) followed by hyperthermic intraperitoneal chemotherapy (HIPEC). Three serum tumor markers (TMs)-carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA19-9), and cancer antigen 125 (CA125)-may play a role in preoperative and postoperative risk estimation and decision-making in patients undergoing CRS-HIPEC. Objective: To evaluate the association of preoperative and postoperative serum TM levels with outcomes in patients with AA or goblet cell adenocarcinoma (GCA) undergoing CRS-HIPEC. Design, Setting, and Participants: This retrospective cohort study was conducted in a single quaternary referral cancer center. Software was used to query the MD Anderson internal patient database of patients with AA or GCA to identify a cohort receiving CRS with or without HIPEC between March 2016 and August 2024, with median postoperative follow-up of 35.6 months (range, 1.4-100.5 months). Data were analyzed from June 2024 to March 2025. Exposure: Preoperative and postoperative levels of CEA, CA19-9, and CA125. Main Outcomes and Measures: Association of preoperative and postoperative serum TM levels with tumor burden and likelihood of complete CRS, disease-free survival (DFS), and overall survival (OS). Results: A total of 421 CRSs were included in the study, 376 (89.3%) of which were performed in patients with AA (229 [60.9%] in females; median age at surgery, 56 [IQR, 47-64] years; 290 [77.1%] were complete CRS). In AA, greater vs lesser preoperative tumor burden was associated with increased median serum TM levels (CEA: 9.5 [IQR, 3.7-42.3] ng/mL vs 3.0 [IQR, 2.1-5.9] ng/mL; CA19-9: 31.1 [IQR, 10.0-102.3] U/mL vs 15.0 [IQR, 6.1-25.5] U/mL; CA125: 28.7 [IQR, 13.9-58.5] U/mL vs 11.5 [IQR, 8.0-17.3] U/mL; all P < .001), and after CRS vs before, percentages of patients with TM elevation were significantly lower (CEA: 77 of 223 [34.5%] vs 222 of 340 [65.3%]; CA19-9: 45 of 194 [23.2%] vs 117 of 324 [36.1%]; CA125: 13 of 203 [6.4%] vs 98 of 325 [30.2%]; all P < .001). However, complete vs incomplete CRS was associated with normalized TM levels in a greater percentage of patients (55 of 90 [61.1%] vs 7 of 52 [13.5%]; P < .001). Elevated vs normal preoperative TM levels were associated with increased rate of incomplete CRS (30 of 193 [15.5%] vs 1 of 95 [1.1%]; P < .001) and shorter DFS (hazard ratio [HR], 2.30; 95% CI, 1.46-3.64; P < .001) but not shorter OS (HR, 1.36; 95% CI, 0.71-2.60; P = .36). Postoperative TM elevation was associated with shorter DFS (HR, 3.73; 95% CI, 2.33-5.95; P < .001) and OS (HR, 4.10; 95% CI, 2.02-8.31; P < .001) in univariate analyses and also in multivariate analyses (HR, 3.34; 95% CI, 1.44-7.75; P = .005), whereas postoperative normalization of all TMs was associated with improved DFS (HR, 3.54; 95% CI, 2.01-6.23; P < .001) and OS (HR, 6.00; 95% CI, 2.06-17.46; P = .001) in univariate analyses and also in multivariate analyses (HR, 4.21; 95% CI, 1.33-13.35; P = .02). Conclusions and Relevance: In this cohort study of CRS in patients with AA, elevated serum TM levels were associated with worse outcomes, suggesting that patients with postoperative TM elevation after complete CRS may represent a cohort with residual tumor that requires more cautious surveillance and that both preoperative and postoperative levels of CEA, CA19-9, and CA125 should be assessed in clinical practice.

Young-onset rectal cancer (YORC, <50 years) incidence is rising, representing 20% of colorectal cancers, yet underlying mechanisms driving this epidemic remain unclear. The tumoral microbiome has emerged as a critical modulator of colorectal cancer pathogenesis, affecting tumor growth, inflammation, metastasis, and chemoresistance through complex host-microbe interactions. Emerging evidence demonstrates that specific bacterial species, including Fusobacterium nucleatum, promote tumorigenesis and therapeutic resistance in colorectal cancer models. We previously reported distinct microbial signatures between YORC and later-onset rectal cancer (LORC, ≥50 years), with tumor-associated oral bacteria correlating with treatment failure. Building on these findings, we expanded our analysis to quantify oral bacterial burden across multiple sample types and determine its clinical impact on therapeutic response. We conducted metagenomic analysis on oral (61), fecal (82), tumor (110), and 111 tumor-adjacent normal (TAN) samples from 227 treatment-naïve patients with locally advanced rectal cancer receiving standardized neoadjuvant chemoradiotherapy. Oral bacterial burden was quantified using reference bacterial taxonomies from the Human Oral Microbiome Database. Major pathological response (MPR) was defined as ≤10% residual viable tumor cells following neoadjuvant therapy. Metagenomes are being evaluated to detect and quantify known microbial genomic markers associated with colorectal cancer, including Bacteroides fragilis toxin and polyketide synthase genes found in colibactin producing E. coli. In this expanded cohort, both YORC and LORC tumors demonstrated significantly higher burden of oral bacteria compared to paired TAN tissues (p<0.001), confirming tumor-specific bacterial enrichment. Predominant oral species colonizing tumors included Parvimonas micra, Gemella morbillorum, Streptococcus sanguinis, Streptococcus salivarius, Prevotella intermedia, and multiple Fusobacterium species. Remarkably, tumoral oral bacterial burden negatively correlated with achieving MPR (p=0.014), with the strongest association observed in LORC patients. TAN tissues showed no correlation with pathological response (p>0.05), while fecal samples demonstrated significantly lower oral bacterial burden than tumors (p<0.05) with no correlation to treatment response, emphasizing the unique and clinically relevant tumoral microenvironment. Tumoral oral bacterial burden represents a potential biomarker for predicting neoadjuvant therapy response in rectal cancer patients. This discovery suggests that precision medicine approaches through targeted antimicrobial interventions to deplete tumor-associated oral bacteria may improve therapeutic outcomes. Our findings support the scientific rationale for ongoing clinical trials testing anaerobe-targeting antibiotics as adjuvant therapy (NCT06569368). Further validation in expanded cohorts and additional clinical evidence is needed to establish the clinical utility of microbiome-guided treatment approaches in colorectal cancer care. Nadim J. Ajami, Ashish V. Damania, Abderrahman Day, Matthew C. Wong, Pranoti V. Sahasrabhojane, Yasmine M. Hoballah, Vivian R. Orellana, Jillian S. Losh, Brenda D. Melendez, Mona M. Ahmed, Lon W. Fong, Bharat B. Singh, Melissa W. Taggart, Khalida Wani, Davis R. Ingram, Diana D. Shamsutdinova, Alexander Lazar, Jumanah Y. Alshenafi, Zuzana Lutter-Berka, Ryan B. Morgan, Taylor M. Neilson, Laurence Diggs, Ramy S. Behman, Paula M. Smith, George J. Chang, David Menter, Christopher D. Johnston, Susan Bullman, Yi-Qian Nancy. You, Scott Kopetz, Michael G. White, Jennifer A. Wargo. Distinct Oral Bacterial Signatures in Rectal Cancer Tumors Associated with Age of Onset and Treatment Response [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: The Rise in Early-Onset Cancers—Knowledge Gaps and Research Opportunities; 2025 Dec 10-13; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(23_Suppl):Abstract nr PR013.

Colorectal peritoneal metastases (CPM) are the third most common site of metastatic spread of colorectal cancer and are associated with worse survival than other sites of metastatic disease. In recent years tumoral circulating tumoral DNA (ctDNA) mutational status has been increasingly utilized in clinical decision making for metastatic colorectal cancer patients despite its utility in CPM being poorly understood. Here we describe standard of care performed mutational profiles and associated outcomes for unresectable CPM patients, with a contextual comparison to 160 unresected colorectal liver metastases (CLM) patients. Of 508 patients, 288 (57 %) had CPM alone and 220 (43 %) had CPM with extraperitoneal metastases. Patients with synchronous CPM and CLM had worse overall survival (HR 1.67 [95 %CI 1.26-2.22]). Mutations in ctDNA were noted in 110/145 (75.9 %) of CPM patients, with mutations in KRAS or PIK3CA ctDNA being associated with worse survival. Importantly, the association between tumoral mutational profile and survival differed by site of metastatic disease. The prognostic significance of specific mutations, particularly BRAF and KRAS, differs between patients with CPM and CLM, and supports the distinct biology of these metastatic sites and the importance of tissue and circulating genomic profiling to risk-stratify these patients according to site of metastasis.

BACKGROUND:Patients with peritoneal metastases from colorectal adenocarcinoma (CRC) benefit from cytoreductive surgery (CRS). However, the role of heated intraperitoneal chemotherapy (HIPEC) remains unclear. We studied associations between HIPEC, peritoneal disease-free survival (PDFS), and overall survival (OS) in patients with CRC undergoing CRS. STUDY DESIGN:All patients with CRC undergoing CRS between 2008 and 2023 were retrospectively identified at our center. Patient clinicodemographic factors were determined by chart review. OS and PDFS were evaluated using the Kaplan-Meier method. Associations between clinical factors and survival were determined using Cox proportional hazard models. RESULTS:A total of 147 patients were identified with 24.5-month median follow-up. Most patients (111; 75.5%) had a low peritoneal cancer index (PCI; less than 11), whereas 36 patients (24.5%) had high PCI (11 or more). Eighty-three patients (55.8%) had CRS alone. Sixty-four patients underwent CRS with HIPEC (42 [66%] mitomycin C [MMC], 19 [30%] oxaliplatin [OX], 3 [4.7%] cisplatin). The median OS was 59.8 (95% CI 41.7 to 83.0) months. The median PDFS was 13.3 (95% CI 10.4 to 18.5) months. OS was equivalent comparing HIPEC with MMC, HIPEC with OX (hazard ratio [HR] 0.49, 95% CI 0.22 to 1.09, p = 0.08), and CRS alone (HR 0.74, 95% CI 0.41 to 1.32, p = 0.31). Multivariable analysis showed only PCI was associated with OS (HR 1.08, 95% CI 1.03 to 1.14, p = 0.003). Low PCI patients had similar PDFS after CRS-HIPEC (HR 1.07, 95% CI 0.61 to 1.87, p = 0.814) compared with CRS alone. High PCI patients had improved OS comparing CRS-HIPEC with CRS (MMC: HR 0.17, 95% CI 0.04 to 0.76, p = 0.02; OX: HR 0.048, 95% CI 0.006 to 0.37, p = 0.004). CONCLUSIONS:HIPEC was not associated with OS or PDFS in patients with PCI 10 or less but was associated with improved OS and PDFS in patients with PCI 11 or more. The use of HIPEC for high PCI may be warranted, although randomized prospective data are needed to confirm this.