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628 Background: Biliary tract cancers (BTC) are aggressive malignancies with poor survival outcomes and limited treatment options. This study compared the epidemiological and molecular differences of advanced BTC patients through a global collaboration involving the Mayo Clinic and National Cancer Center Hospital East. Methods: We included patients with advanced BTC who underwent comprehensive tumor molecular profiling who received at least 30 days of systemic therapy. Patients were grouped based on specific alterations. Overall survival (OS) was calculated from the start of first-line therapy until death and compared among subgroups using Cox regression models. Results: 332 (36%) out of 932 patients had at least one actionable mutation : somatic BRCA1/2 mutations N=106, FGFR2 fusions N=61, HER2 amplifications N=65, KRAS G12C/D mutations N=46, IDH1 mutations N=38, and TMB >10 mut/Mb N=16. BRCA1/2 -mutated BTC were more commonly found in older Asian males with intrahepatic tumors and often presented with peritoneal and liver metastases upon initial diagnosis. FGFR2 fusions were more prevalent in younger White females with intrahepatic tumors, while HER2 amplifications were mainly seen in Asian females with gallbladder tumors. KRAS mutations were primarily observed in Asian males, and IDH1 mutations were common in White males with intrahepatic tumors, frequently presenting with liver metastasis. Survival outcomes varied significantly based on the alteration type: patients with FGFR2 fusions had the longest median OS (mOS) at 23.3 months (mo) (95%CI 18.4-34.9), followed by those with IDH1 mutations (19.3 mo; 95% CI 17.4-NE) and TMB >10 mut/Mb (17.8 mo; 95% CI 13.6-NE). BRCA mutations had a mOS of 16 mo (95% CI 14.6-19.8), while HER2 amplifications and KRAS mutations had mOS of 15.5 (95% CI 11.9-20.2) and 11.6 mo (95% CI 10.0-20.9), respectively. Conclusions: These findings underscore the heterogeneity in clinical and genomic characteristics among BTC patients, highlighting the importance of tailored therapeutic approaches. Baseline patient characteristics stratified by mutation status. KRAS G12C/D (N=46) BRCA 1/2 (N=106) IDH1 (N=38) HER2 amplification(N=65) FGFR2 fusion(N=61) TMB>10 mut/Mb(N=16) P-value Median Age, year 67 68 64 66 52 73 <0.001 Female, n (%) 19 (41) 34 (32) 18(47) 34 (52) 45(74) 6 (38) <0.001 Race, n (%) <0.001 Asian 31 (67) 97 (92) 13 (34) 51 (79) 10 (16) 6 (38) White 14 (30) 8 (8) 24 (63) 13 (20) 48 (79) 10 (63) Primary tumor location, n (%) <0.001 Intrahepatic 28 (68) 45 (45) 33 (100) 19 (31) 54(95) 10 (67) Extrahepatic 9 (22) 27(27) 0 (0) 10 (16) 2 (4) 4 (27) Gallbladder 4 (10) 28 (28) 0 (0) 32 (53) 1 (2) 1 (7) Sites of Metastasis at diagnosis, n (%) Abdominal Wall / Peritoneal 9 (24) 15 (19) 4(13) 7 (12) 6 (10) 4 (27) 0.309 Bone 33 (92) 70 (90) 26 (87) 2 (3) 9(15) 0 (0) 0.212 Liver 20 (49) 52 (61) 28 (82) 45 (73) 45 (76) 6 (38) 0.001 Lung 26 (70) 65 (82) 22 (69) 36 (58) 27(45) 11 (69) 0.006

62 Background: Both Regorafenib (R) and Trifluridine/Tipiracil (T) ± BEV are widely used treatments for mCRC in later lines. This RW study evaluated optimal sequencing and associated outcomes among patients who initiated R and T in sequential lines of therapy (LOT) between LOT1-LOT6 within community oncology settings in the US. Methods: A retrospective study utilizing electronic medical record data from The US Oncology Network examined adult patients with mCRC who initiated R and T ± BEV sequentially between 09/01/2015 and 11/30/2022 (index date), with follow-up until 05/31/2023. Treatment patterns, overall survival (OS) and time to next treatment (TTNT) after sequence were assessed overall, R first (R-T) and T first (T-R). Multivariable Cox regression adjusting for age, sex, ECOG, tumor stage, and prior medication, assessed the association of factors with OS and TTNT. Results: In total, 308 mCRC patients initiating R and T (R-T, 156 pts [140 R-T and 16 R-T+BEV]; T-R, 152 pts [136 T-R and 16 T+BEV-R]) met study criteria. Baseline demographic (mean age 63 yrs, males 55%) and clinical characteristics (70% colon cancer, 58% ECOG 0-1) were similar between the cohorts. Most patients initiated index therapies at the third line of therapy (LOT3) (49% R-T, 45% T-R). The median (IQR) sequence duration was 7.4 (5.6), 6.7 (4.4), and 7.0 (4.7) months, respectively in patients with R-T, T-R, and overall. The median (95% CI) OS was 12.8 (11.2, 14.1) months among R-T and 10.2 (8.8, 11.9) months among T-R patients. Compared to R-T, the adjusted HR for the T-R sequence was 1.2 (95% CI 0.9–1.6; p=0.2). The median (95% CI) TTNT was 9.3 (8.4, 10.3) and 8.6 (7.8, 9.4) months, for R-T and T-R, respectively; the adjusted HR was 1.1 (95% CI 0.8–1.4; p=0.62). Conclusions: Patients initiating R first appeared to have a longer duration of treatment and survival, although not statistically significant. The one-year median duration of survival in this real-world setting confirms the importance of access to these treatments in patients with mCRC and can inform physicians regarding the choice of medications while sequencing therapies in mCRC. Clinical outcomes among R-T and T-R mCRC patients. Variable Level R-T (± BEV)[N 1 = 156] T (± BEV) -R[N 1 = 152] Adjusted 3,4 HR 5 (95% CI)[R-T Reference] Effect OverallSurvival Months – median (95% CI) 12.8(11.2, 14.1) 10.2(8.8, 11.9) 1.20 (0.91, 1.58) 0.20 TTNT 2 Months – median (95% CI) 9.3(8.4, 10.3) 8.6(7.8, 9.4) 1.07 (0.83, 1.37) 0.62 1 Entire R-T and T-R cohorts are presented since 16 patients in each cohort initiated T+BEV. 2 Time To Next Treatment. 3 Overall Survival adjusting for age, gender, stage at diagnosis, ECOG, prior anti-VEGF treatment. 4 Time To Next Treatment adjusting for age, gender, stage at diagnosis, and line of therapy. 5 Hazard Ratio.

614 Background: Liver transplantation (LT) offers a 5-year survival exceeding 70% for selected patients with hepatocellular carcinoma (HCC). Immune checkpoint inhibitors (ICIs) may be used to downstage or bridge to transplantation. This study aims to evaluate the safety of ICIs prior to LT. Methods: Multicenter retrospective study, involving 9 centers, included adults who received ICIs and, subsequent LT, between 2019 and 2023. The ICI cohort was matched by age, sex, liver diseases and transplant date (1:3) with a similar cohort of HCC transplanted patients that did not receive ICIs. Results: The ICI cohort included 45 patients (Table part 1). The most common underlying liver diseases were viral. 15.5% of patients had macrovascular invasion and, 62.2% multifocal disease. The most commonly used ICI was nivolumab (75.6%), followed by atezolizumab-bevacizumab (17.8%). The median follow up was 32.5 months (17.4 - 47.6). The median ICI duration was 147 days (129.5-276.5), with a median interval of 58 days (22–193) between the last ICI dose and LT. There were 8 (17.8%) cases of graft rejection, 2 steroid-resistant leading to 1 graft failure. There were 4 (8%) HCC recurrences. During follow-up there were 6 deaths, 2 related to HCC recurrence and 4 non liver related. When compared with 135 non-ICI cohort patients, there were no differences within the groups (Table part 2). Importantly, we found no significant differences in crude rejection rates (p=0.5). Conclusions: Our study suggest that ICI treatment prior to LT is safe. ICI rejection primarily occurs in the acute post-transplant period (within 3 months), with a low risk of death due to graft failure. Further clinical trials are needed. Part 1: ICI cohort descriptive analysis. N (%); Median (IQR); Part 2: Comparison between cohorts. N (%); Mean (SD). (part 1) ICI-cohort n=45 General data Male 38 (84.4) Age 61.2 (7.6) Etiology of liver diseases Hepatitis C 17 (37.8)Hepatitis B 6 (13.3)MASLD 6 (13.3)Viral +Alcohol 5 (11.1)Alcohol 3 (6.7)Autoimmune 3 (6.7)Other 3 (6.7)Combined viral 2 (4.4) Largest tumor (mm) 39 (22) Macrovascular invasion 7 (15.5) Multifocal 28 (62.2) Locoregional therapy 36 (80.0) Number of locoregional therapies 2 (2) Immunotherapy Nivolumab 34 (75.6) Atezolizumab/bevacizumab 8 (17.8) Pembrolizumab 1 (2.2) Nivolumab+pembrolizumab 1 (2.2) PD1 inhibitor clinical trial 1 (2.2) Days of ICI treatment 147 (203) Radiological tumor response ICI No 17 (37.8) Transplant Deceased donor 43 (95.6) Washout period (days) 58 (171) Induction therapy (anti-thymoglobulin) 8 (17.8) Outcomes Rejection Biopsy proven 8 (17.8) 8 (100) Time to rejection (days) 43 (102) Graft failure Rejection 2 (4.4)1 (2.2) HCC recurrence 4 (8) Deaths 6 (13.3) Follow up Since transplant (months) 32.5 (30.2) (part 2) ICI-cohort (n=45) Non-ICI cohort (n= 135) P Post transplant rejection rate 8 (17.8) 32 (23.7) 0.53 Time to rejection (days) 180.1 (SD 218) 62.2 (SD 62.8) 0.25 Overall deaths 6 (13.3) 10 (7.4) 0.36

5 Background: Metastatic anal squamous cell carcinoma is rare and associated with a poor prognosis. There is limited evidence on the impact of local interventions on survival outcomes in patients with metastatic disease. We aimed to evaluate the survival of patients with liver-only metastatic anal cancer following definitive local management of liver lesions. Methods: This is a single-institution retrospective cohort study of patients with liver-limited metastatic anal cancer who underwent curative-intent local therapy. Clinical characteristics were collected and analyzed. The Kaplan-Meier method was used to calculate disease-free survival (DFS) and overall survival (OS). Prognostic factors associated with DFS and OS were assessed using Cox-proportional hazards models. Results: Twenty-five patients were included, median age was 59 years, and 92% were female. None of the patients had known HIV infection, 92% had HPV-positive disease and 76% had metachronous liver metastases following locoregional disease. All patients received chemoradiation for the primary lesion Unilobar hepatic involvement was present in 76% of the patients, with 56% having a single metastatic lesion. Pre-intervention systemic therapy was provided to 52% of the patients, including 8 who received carboplatin and paclitaxel, with a median duration of 3 months and ORR of 69%. Definitive treatment of metastatic disease included surgical resection (60%), non-surgical interventions (20%), and multimodal therapy (20%). All patients had viable tumor in the pathological specimen. With a median follow-up of 22 months, median DFS was 7.3 months, and median OS was 51.3 months. An ECOG performance status of 2, poorly differentiated histology, and extrahepatic recurrence were identified as significant prognostic factors in the univariate analysis, but not in the multivariate analysis. Conclusions: Our results demonstrate potential for long-term survival in liver-limited metastatic anal squamous cell carcinoma amenable to local therapeutic intervention combined with systemic therapy. These findings warrant further prospective study of local therapy in oligometastatic anal cancer. Characteristics N=25 (%) Median number of liver metastases (range), lesions- 1- 2-3- >3 1 (1-20)14 (56)8 (32)3 (12) Median maximal diameter of liver metastases (range), cm 3 (0.9-11.9) Best response of liver metastases to chemotherapy (N=13)- Complete response- Partial response- Stable disease- Progressive disease 2 (15)7 (54)3 (23)1 (8) Surgical procedure (N=20)- Right/left hemi-hepatectomy- Segmentectomy- Wedge resection 4 (20)14 (70)2 (10) Non-surgical procedure (N=10)- Local ablation- Radiotherapy- Other 5 (50)3 (30)2 (20) Hepatic surgical margins (N=20)- Positive margins- Close margins (<1mm)- Negative margins 02 (10)18 (90) Recurrent disease- Yeso Intrahepatico Extrahepatic- No 20 (80)14 (70)6 (30)5 (20)

566 Background: Immune checkpoint inhibitors (ICIs) have become the preferred treatment for unresectable hepatocellular carcinoma (HCC). Conflicting data exist regarding the prognostic value of immune-related adverse events (irAEs) in these patients. We aimed to explore potential the association between irAEs and prognosis in HCC patients treated with ICIs. Methods: We searched PubMed, Scopus, Web of Science, and CENTRAL databases for articles published from inception to June 2024 using keywords including ICI, HCC, and irAEs. Two reviewers independently screened studies for eligibility and relevant data. A random effects model was used for statistical analysis, and subgroup analysis was performed by the grade of irAEs. Results: Of 3,028 studies, 25 (n=4,116 patients) met criteria for inclusion. Atezolizumab plus bevacizumab was the most common treatment regimen (n=10 studies). About half (49.5%) of patients experienced irAEs. irAEs were associated with increased objective response rate (ORR) (pooled OR: 1.72; 95% CI: 1.36 – 2.19, I 2 =39%), higher complete response rate (pooled OR: 1.45; 95% CI: 1.21-1.74, I 2 =74%), and longer progression-free survival (PFS) (pooled HR: 0.66; 95% CI: 0.52-0.84, I 2 =71%). Although irAEs were associated with longer overall survival, this difference was not statistically significant (pooled HR: 0.83; 95% CI: 0.62 – 1.11, I 2 =73%). Subgroup analysis showed a survival benefit for patients with grade 1-2 irAEs (pooled HR: 0.50; 95% CI: 0.36-0.67, I 2 =0%) but not for those with grade 3-4 irAEs (pooled HR: 0.95; 95% CI: 0.24-3.72, I 2 =84%). Conclusions: The development of irAEs is associated with a favorable prognosis in HCC, including improved PFS and higher ORR. A survival benefit was noted in patients with mild irAEs, but not in those with severe irAEs. Future studies are needed to determine if a differential survival benefit relates to the continuation vs. discontinuation of ICI therapy.

556 Background: Combined hepatocellular carcinoma-cholangiocarcinoma (cHCC-CCA) is a rare and complex liver cancer with features of both hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), making its management challenging. This study aims to analyze the clinical characteristics, genomic profiles, and treatment outcomes of cHCC-CCA patients in a real-world setting. Methods: We conducted a retrospective analysis of cHCC-CCA patients across three first-line treatment groups: chemotherapy alone(Gemcitabine/Cisplatin, FOLFOX), HCC-directed therapies (atezolizumab/bevacizumab, lenvatinib, nivolumab, durvalumab, sorafenib), and chemoimmunotherapy (Gemcitabine/Cisplatin combined with immunotherapy). This international study, involving Mayo Clinic and the National Cancer Center East (Japan), assessed overall survival (OS) using Kaplan-Meier and Cox regression models. Results: A total of 68 patients were included, with a mean age of 66 years; 74% were male and 66% were White Non-Hispanics. Cirrhosis was present in 74% of patients at diagnosis, and Hepatitis B and C were common etiologies in 57%. Prior primary resection and regional therapies (Transarterial chemoembolization (TACE), Transarterial radioembolization (TARE)) were recorded in 39% and 50% of patients, respectively. Advanced/metastatic disease was present in 52% of the cohort. The distribution of first-line treatments was as follows: 47% received chemotherapy alone, 39% received HCC-directed therapy, and 14% received chemoimmunotherapy. Genomic alterations were detected in 44% patients, with TP53 (28%), ARID1A (25%), and TERT (22%) being the most frequent. The HCC-directed therapy group demonstrated the best overall response rate (52%) and had a median OS of 15.8 months (mo), compared to 11.8 mo for chemotherapy and 4.7 mo for chemoimmunotherapy (p=0.8). Patients who received regional treatments had a longer median OS of 15.8 mo compared to those who did not (10.2 mo) (p=0.4). Conclusions: HCC-directed therapy showed a trend towards better survival outcomes compared to other treatments, though not statistically significant. A considerable proportion of patients had genomic alterations, which may have influenced survival outcomes. Ongoing research with larger cohorts is underway to further elucidate the impact of genomic variations on treatment efficacy and refine therapeutic strategies for this challenging malignancy. Baseline characteristics of patients with combined hepatocellular carcinoma-cholangiocarcinoma (cHCC-CCA). Total(N=68) Mean age at diagnosis, years (SD) 66(6.4) Gender, n (%) Male 50 (74%) Ethnicity, n (%) White, Non-Hispanic 45 (66%) Cirrhosis at diagnosis, n (%) 50 (74%) Etiology of Liver disease, n (%) Viral (Hepatitis B and C) 39 (57%) History of Primary resection, n (%) 27 (39%) Regional treatment, n (%) 34 (50%) Afp, n (%) < 200 23 (64%) ca19_9, n (%) < 50 27 (82%)

496 Background: Incidence of early-onset gastroesophageal adenocarcinoma (eoGEA, ages <50 years) has been increasing in the United States since the 1990s, the etiology of which is still unclear, and limited data exists on molecular drivers. This study evaluates somatic and germline profiles in eoGEA compared to average-onset GEA (aoGEA, ages≥ 50 years). Methods: This is a retrospective, cross-sectional study utilizing data from de-identified records of GEA (esophageal, gastroesophageal junction and gastric adenocarcinomas) patients who underwent somatic NGS testing via the Tempus xT assay (595-648 gene DNA panel) from 12/2017 to 07/2024. This assay assesses somatic tumor mutations (including SNVs, Indels, CNVs, and select SVs), MSI, TMB, and incidentally detected germline SNVs and small indels in matched normal tissue. Clinical characteristics and immunotherapy markers were compared between eoGEA and aoGEA by Wilcoxon Rank Sum or Chi-squared test. Somatic and germline mutations were compared between two age groups with false discovery rate adjustments. Results: We compared the demographic, clinical, and molecular features of a total of 5863 patients with eoGEA (n=785, median age 43) and aoGEA (n=5078, median age 68). Over 80% of patients were diagnosed with stage IV disease. The eoGEA group had a higher proportion of females, non-white, and Hispanic or Latino patients compared to the aoGEA group (p<0.001). Prevalence of MSI-H/ dMMR, and TMB-H was lower in eoGEA vs aoGEA (p<0.001), while PD-L1 expression was similar. Prevalence of somatic CDH1 , CDKN2A , and TP53 SNVs, and FGFR2 and KRAS CNAs were higher in eoGEA vs aoGEA (q<0.001). In a subset of 3286 patients with tumor/normal match testing (eoGEA=464 and aoGEA=2822), eoGEA also had a higher prevalence of germline CDH1 mutations (q<0.001). Conclusions: eoGEA has a unique somatic and germline mutation profile compared to aoGEA; further study evaluating the molecular landscape including epigenetic changes could shed light on underlying mechanisms responsible for the rising incidence of eoGEA. eoGEA (n=785) aoGEA (n=5078) p-value/q-value* Baseline characteristics Median age at diagnosis (IQR) 43 (38, 47) 68 (61,74) <0.001 Gender: male 509 (65%) 3798 (75%) <0.001 Race (white) 280 (69%) 2347 (80%) <0.001 Ethnicity (not hispanic or latino) 206 (60%) 1768 (86% <0.001 Stage IV 495 (85%) 2883 (81%) 0.073 Immunologic markers TMB ≥ 10 25 (3.2%) 519(10%) <0.001 MSI-H 13 (1.7%) 261 (5.1%) <0.001 dMMR 7 (2.2%) 120 (5.9%) 0.007 Somatic mutations profiles * CDH1 126 (16%) 334 (6.6%) <0.001 TP53 515 (66%) 3764 (74%) <0.001 CDKN2A 101 (13%) 1011 (20%) <0.001 KRAS 92 (12%) 931 (18%) <0.001 NOTCH1 12 (1.5%) 228 (4.5%) 0.002 Germline mutation profiles * eoGEA (N=464) aoGEA (N= 2,822) Overall prevalence 46 (9.9%) 207 (7.3%) CDH1 10 (2.2%) 8 (0.3%) 0.001 TP53 3 (0.6%) 0 (0%) 0.039 BRCA2 2 (0.4%) 38 (1.3%) 0.6 BRIP1 4 (0.9%) 10 (0.4%) 0.6

277 Background: While HER2-targeted therapies benefit patients (pts) with advanced HER2-positive mCRC, the development of resistance remains inevitable. This study aims to explore the landscape and frequency of genetic alterations putatively associated with acquired resistance to anti-HER2 therapy in HER2-positive mCRC. Methods: In this retrospective-multicenter international study involving Mayo Clinic, Duke Cancer Center and National Cancer Center Hospital East (Japan), we descriptively compared comprehensive genomic analyses from tissue or blood samples of pts with HER2-positive CRC pre-treatment and post-treatment with HER2-targeting agents. Results: A total of 36 pts were included in this study. 29 (80%) had HER2-positive mCRC as confirmed by immunohistochemistry (IHC) and fluorescent in situ hybridization (ISH), the remaining pts had HER2 amplification detected on blood or tissue . 34/36 (94%) pts were administered their first anti-HER2 agent after progressing on ≥2 prior lines of treatment. The most common anti-HER2 regimen administered was trastuzumab/pertuzumab (50%, 18/36), followed by trastuzumab-tyrosine kinase inhibitors (TKIs) (tucatinib or neratinib) (30%, 11/36) and trastuzumab deruxtecan (14%, 5/36). Putative genomic mechanisms of acquired resistance were detected in 72% (26/36), while the remaining pts lacked new genomic alterations that might explain resistance to anti-HER2 therapy, suggesting alternative escape mechanisms. Acquired alterations in downstream or alternate bypass pathways were identified in 72% (26/36) of pts; most commonly in the RAS/RAF pathway and in EGFR (34.6%, 9/26 each), followed by 27% (7/26) of pts with MET point mutations (mts) and amplification; activating muts in PI3K/AKT and in 27% (7/26), most commonly CDK12 (60%, 4/7) . HER2 receptor alterations were detected in 3 pts (8.3%), all of whom received trastuzumab-tucatinib and included HER2 L755S, HER2 V777L, HER2 D769Y, HER2 G727A and HER2 S310F, which are known to cause resistance to , however, information about tucatinib resistance is not yet available. Other altered pathways included AR, RB, NOTCH and HRD. Two pts lost HER2 expression on blood and tissue NGS post anti-HER2 therapy. Conclusions: The emergence of resistance might be in part related to acquired alterations that constitutively activate signaling pathways parallel or downstream of HER2, bypassing HER2 inhibition. Additional histologic studies, deep mutational scanning, larger populations and correlative analysis are needed to validate these findings.

270 Background: While immune checkpoint blockade (ICB) is used to treat metastatic deficient mismatch repair (dMMR) colorectal cancers (CRCs), such tumors are heterogeneous and resistance to ICB is frequent. To date, biomarkers that can consistently predict patient response to ICB are lacking. We determined whether biomarkers that integrate tumor and immune-related molecular mechanisms could predict ICB response. Methods: Consecutive patients with metastatic dMMR CRCs (N=32) who had been treated with anti-PD-1 therapy were profiled using a validated immune-enhanced exome and transcriptome platform (ImmunoID NeXT, Personalis) using tumor and paired normal tissue. This platform provides antigen-specificity of T or B cells, indicated by the TCR or BCR respectively, neoantigen prediction, tumor mutation burden (TMB), immune profile, HLA allele-specific loss of heterogeneity (LOH) and a Neoantigen Presentation Score (NEOPS) that combines a neoantigen prediction tool with mechanisms of immune evasion. Tumor variables were analyzed in relationship to objective tumor response, and a Cox proportional hazards model was fit to predict progression-free survival (PFS) or overall survival (OS). Results: In CRCs, TCR and BCR repertoire diversity were each significantly increased in tumors from responders versus those with stable or progressive disease after anti-PD-1 treatment (p=0.015 and p=0.048, respectively). Neoantigen burden score, TMB, and HLA allele-specific LOH (reflects loss of antigen presentation machinery) were each unable to distinguish responders from nonresponders. An immune checkpoint gene expression signature was increased in nonresponders relative to responders (P= 0.005). Neoantigen presentation score (NEOPS) was prognostic when adjusted for tumor grade only for OS (p=0.048). Conclusions: The T-cell and B-cell receptor (TCR/BCR) repertoire, as well as NEOPS, were each significantly predictive of favorable response to anti-PD-1 therapy in dMMR tumors. In addition, an immune checkpoint gene expression signature was predictive of resistance to ICB.

269 Background: Sirtuins (SIRT1-7) are class III histone deacetylases with different roles in CRC as tumor suppressive and oncogenic, mainly influencing proliferation, migration, invasion, DNA damage and chemoresistance. In the present study, we explored the association between tumor gene expression of sirtuins and first-line treatment outcomes in mCRC pts. Methods: We evaluated the impact of the tumor gene expression of sirtuin genes ( SIRT1 -7) on treatment outcomes (overall survival: OS; progression free survival: PFS) in 433 pts enrolled in the randomized, phase III CALGB/SWOG 80405 (NCT00265850) trial. RNA extracted from FFPE tumor samples was sequenced using the HiSeq 2500 (Illumina). Subgroup analyses were based on treatment (bevacizumab [bev] (N = 226) vs. cetuximab [cet] (N = 207)). For each gene, expression was divided into tertiles of high, medium and low. The associations between tumor gene expression and outcomes (PFS and OS) were assessed using log-rank test in univariate analysis. Multivariable Cox proportional hazards regression model was used to compute hazard ratios (HR) and 95% confidence intervals while adjusting for baseline characteristics. Treatment-gene interactions were analyzed using the likelihood ratio test. Results: Of the seven genes analyzed, SIRT3 high was associated with both longer PFS (HR: 0.59 [95% CI: 0.46-0.74], P < 0.001) and OS (HR:0.64 [95% CI: 0.50-0.83], P = 0.002) overall. Treatment-gene interaction testing found significant interactions for SIRT1 and SIRT5 with PFS ( P values: 0.021 and 0.016, respectively) and OS ( P values: 0.038 and 0.014, respectively) with targeted therapies (bev vs. cet). Multivariate analysis showed SIRT1 high associated with longer PFS (HR: 0.62 [95% CI: 0.42-0.89], P = 0.014) and OS (HR: 0.59 [95% CI: 0.40-0.89], P = 0.036) in cet-treated pts; in contrast with shorter observed PFS (HR: 1.38 [95% CI: 0.96-1.99], P = 0.22) and OS (HR: 1.31 [95% CI: 0.90-1.93], P = 0.23) in bev-treated pts. SIRT5 high showed similar results and associated with longer PFS (HR: 0.64 [95% CI: 0.45-0.91], P = 0.039) and OS (HR: 0.57 [95% CI: 0.39-0.83], P = 0.013) in cet-treated pts, with no similar associations in bev-treated pts. Conclusions: Our results highlight a potential role for SIRT3 as prognostic and, SIRT1 and SIRT5 as predictive biomarkers for first-line treatment in mCRC, with differential effects based on targeted agents. Significant interaction favored cet treatment in SIRT1 high and SIRT5 high tumors. However, previous reports suggest an opposite role of SIRT5 that has been linked to chemotherapy and/or cet resistance in CRC via the inactivation of succinate dehydrogenase complex subunit A. Therefore, further studies are warranted to dissect the mechanism behind the interaction of SIRT5 with cetuximab activity and establish the potential of targeting SIRT1/5 in CRC.