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103 Background: Novel strategies to improve the efficacy of immune checkpoint inhibitors in microsatellite stable (MSS) mCRC are needed. CV301 is a vector-based vaccine that expresses carcinoembryonic antigen (CEA) and mucin 1 (MUC1), and in a phase II study in resected hepatic limited mCRC significantly improved OS compared with unvaccinated contemporary controls. Methods: In this multi-center randomized phase II study, patients with previously untreated resectable hepatic-limited mCRC were randomized to perioperative nivolumab + mFOLFOX +/- CV301 (Arm B) with a primary endpoint of 3-year OS. Treatment included mFOLFOX-nivo (+/- CV) x 4 cycles followed by resection, then 8 more cycles of mFOLFOX-nivo followed by maintenance nivo monthly for two years in both arms, and CV boosters concurrently with mFOLFOX, and then every 3 months for two years in arm B. Secondary endpoints of ORR (following induction pre-resection), PRR, and safety were determined. Correlative analyses included immune cell quantification using Immunoscore and T-cell clonality. Results: 17 patients were enrolled prior to premature closure for slow accrual (8 arm A, 9 arm B). At the time of data cutoff, 5 patients remained on treatment and no deaths had occurred. One patient was removed from study due to protocol non-compliance. The median age was 61, majority were male (59% vs 41%), and ECOG PS 0-1 (71% 0, 17% 1). All patients had complete surgical resection. Four patients (24%) experienced a SAE related to drug. The TRAE rate was 40.3%,. No AEs delayed/prevented surgical resection. The ORR in arm A was 50% (including 4 CR) and 87.5% in arm B (including 7 CR) (p=0.129, NS). There was no significant difference in pathologic response (p=0.9047). Correlative analyses demonstrated the Immunoscore CD3/CD8 predicted response to mFOLFOX + nivolumab, but did not correlate with response to CV301, though CV301 may induce a shift to predominantly cytotoxic CD8+ T cells. While there was no significant difference in T cell repertoire, clonality, fraction (TCFr) or richness, patients in arm B had significant decreases in blood TCFr and increase in tumor TCFr with treatment; those with CR had higher TCFr and clonality. Conclusions: The addition of CV301 to perioperative nivolumab and mFOLFOX was safe, did not delay or prevent surgical resection, and gave a higher response (p=ns due to sample size). Changes in T cells suggest a vaccine response. Clinical trial information: NCT03547999 . [Table: see text]

TPS257 Background: Refractory microsatellite stable colorectal cancer (MSS CRC) is immunologically cold and single-agent anti-PD-1/PD-L1 drugs are ineffective; novel immune-based approaches are needed. Evorpacept (E, ALX148) is an engineered protein (high-affinity CD47-blocker fused to an inactive IgG Fc region), which blocks the CD47/SIRPα innate immune inhibitory phagocytosis checkpoint expressed on CRC and phagocytes, respectively. The Fc region of E does not bind to Fcγ receptors, thereby limiting hematologic toxicity, and is intended to be given in combination. In CT26 CRC syngeneic models, E ± anti-PD-1 monoclonal antibody decreases tumor growth, reduces myeloid immunosuppression, increases dendritic cell activation, and increases T cell activation (Kauder, 2018); E enhances the antibody-dependent cellular phagocytosis activity of cetuximab (C) in vitro (Kauder, 2018); and E + pembrolizumab (P) was well-tolerated in the first-in-human trial (Lakhani, 2021). Methods: AGICC-ALX148 21CRC01 (NCT05167409) is a phase 2, single-arm, multicenter, investigator-initiated trial of E (15 mg/kg weekly), C (400 mg/m2 then 250 mg/m2 weekly), and P (200 mg every 3 weeks) in 21-day cycles for patients with unresectable MSS/proficient mismatch repair CRC refractory to oxaliplatin, irinotecan, and a fluoropyrimidine, regardless of tumor sidedness and RAS/BRAF status. Additional key eligibility criteria include ECOG performance status 0-1, evaluable disease per RECIST v1.1, adequate hematologic and end organ function, absence of prior checkpoint inhibitor use, and absence of significant autoimmune disease. Six patients will be enrolled in Stage 1 (safety run-in) and treated with ECP. The study will proceed to Stage 2 (dose expansion, N = 42, and all treated with ECP) if less than 33% of patients in Stage 1 experience a dose-limiting toxicity. Otherwise, additional patients will be enrolled in Stage 1 at lower dose level(s). The co-primary objectives are to determine 1) the recommended dose of E with CP, and 2) objective response rate by RECIST v1.1 (by one-sided exact test with α = 0.05, H0 p ≤ 3% [historical controls], HA p ≥ 15%; power is 87%). The study will close for futility if there are no responses (partial or complete) in the first 24 evaluable patients (by MinMax design with α = 0.025 [1-sided]; power is 87%). Secondary and exploratory aims include determination of progression-free survival, overall survival, safety, response assessment by iRECIST, and blood- and tumor-based immune modulation and baseline tumor expression (PD-L1, EGFR, and CD47) for association with tumor response. The study is open through the Academic GI Cancer Consortium and 5 patients have been enrolled at time of submission. Clinical trial information: NCT05167409 .

144 Background: TAS-102 (trifluridine/tipiracil) is a novel oral antimetabolite for late line metastatic colorectal cancer (CRC) approved in 2018. Many patients are treated early in their course with oxaliplatin (OX), particularly adjuvant, and may benefit from re-treatment. In this trial we combine the typical late line use of TAS with OX (BEV [bevacizumab] added at investigator discretion) with goal of improved response. Methods: Eligibility included measurable CRC previously treated with all approved drugs per TAS package insert (irinotecan, oxaliplatin, 5FU, anti-VEGF, anti-EGF) as appropriate, PS = 0-1, labs within usual range, neuropathy < grade 2, ability to take oral meds, appropriate contraception. If no contraindication to BEV, this could be added at patient. TAS was dosed at 35 mg/m2 days 1-5 with OX 85/m2 d1 every 14 days (and BEV 5 mg/kg, if given). All supportive care was allowed including growth factors. Results: 47 patients (pts, median age 55) were enrolled in a Simon mini-max design, including 45% female, 21% black, 11% Asian, 11% Hispanic and 5% mixed. 26 pts received BEV. For the first 40 pts, 385 cycles were given (mean = 7 cycles, median 8) with 18 pts (45%) requiring dose reductions (1 dose reduction = 9 pts, 2 = 6, 3 = 3), and 9 receiving (peg)/filgrastim. Toxicities leading to SAEs included gr 3 heme (2), heart failure, abd pain/n/v (6), sepsis (2), urinary (4); and related gr 3 included one gr 3 vomiting and one gr 3 neutropenia. Independently reviewed RECIST Response (N = 32) included PR 2(6%), SD 23 (72%), PD 7 (22%). Mean TTP was 4.5 m (median 4, range 1 – 18) with 9 (28%) pts more than 6 months. Conclusions: In patients with late-line CRC and candidates for TAS (trifluridine/tipiracil), treatment with TAS plus OX is both well tolerated and active. RR is higher than single agent and 78% (95% CI, 60-91%) of patients had stable disease or response, with 60% receiving 8 or more cycles. Randomized trials comparing to single agent TAS are warranted in this setting. Clinical trial information: NCT04294264 .

178 Background: Anti-EGFR therapy has the potential to increase a localized anti-tumor immune response. We recently published the results of a phase Ib/II trial of the anti-EGFR antibody cetuximab in combination with the anti-PD1 antibody pembrolizumab in pts with advanced, RASwt CRC (PMID: 34645646). Despite its partial local immunologic efficacy, this combination of cetuximab and pembrolizumab was inactive. Here we present the results of comprehensive molecular characterization of pts with tumors amenable to DNA and RNA sequencing. Methods: Forty-two pts with RASwt mCRC were treated with cetuximab plus pembrolizumab. Archival or fresh tumor samples were obtained at baseline and, in select patients, on-treatment. Tumor samples underwent targeted DNA sequencing and whole transcriptome sequencing. Gene set enrichment analysis (GSEA), metabolic dysregulation assessment, and immune deconvolution were performed. Genomic data were linked to clinical outcomes. Results: Eighteen pts had tissue available for dual DNA/RNA extraction and sequencing. Of these, 10 had available matched on-treatment tumor samples. The most common mutations detected in protein coding regions were TP53 (14 pts), ERBB4 (5 pts), CDKN2A and APC (6 pts each). There were no statistically significant differences in progression-free survival (PFS) in pts with and without resistance-associated mutations (i.e., RAS, MET, ERBB4). Further, there was no significant difference associated with the consensus molecular subtype (CMS). However, when we compared patients with stable/increased tumor change percentage to those with decreased tumor change percentage, we found downstream transcriptional differences associated with altered metabolism, and in particular lipid and amino acid metabolism. Conclusions: We identified a significant number of patients with mutations predicting resistance to either or both cetuximab and pembrolizumab; however, none were associated with survival. However, we did identify metabolic pathways of interest, which may be associated with response to therapy. It is important to note, our analysis is limited by the small number of specimens (Trial registration NCT02713373). Clinical trial information: NCT02713373 .

e15587 Background: For resected stage II MSI-high (MSI-H) colorectal cancer (CRC) without high-risk features (HRF), standard of care recommendation per NCCN guidelines is observation. For tumors with HRF, the prognostic significance and impact of adjuvant therapy remains uncertain. The NCDB was queried to assess outcomes. Methods: Adult patients with stage II MSI-H CRC, surgically resected between 2010 and 2017, were identified in the NCDB. Univariate and multivariate Cox proportional hazards models and Kaplan Meier survival curves were generated to assess impact of HRF, clinical and demographic variables, and chemotherapy use on overall survival (OS). Results: 9634 patients with stage II MSI-H CRC who met criteria were identified. In multivariate analysis T4 tumor status, < 12 lymph nodes (LN) examined, perineural invasion (PNI) and positive margins were associated with worse OS, as were older age and increased comorbidity index (CDCC). Poor differentiation and lymphovascular invasion (LVI) were not associated with OS. No OS difference was observed between T4a vs T4b tumors or based upon tumor sidedness, tumor size, or sex. Excluding poor differentiation (33%) and LVI (17%), HRFs were present in 26% of cases: 21% (n = 2033) had 1 HRF and 5% (n = 505) had > 1 HRF. HRFs were associated with decreased 5-year OS. For 1 HRF OS was 66% (HR 1.48, p < 0.0001) and for > 1 HRF OS was 54% (HR 2.31, p < .0001), compared to 77% with 0 HRFs. HRF presence was associated with increased chemotherapy use: 46% for > 1 HRF, 26% for 1 HRF, and 8% with 0 HRFs. pT4 tumors (T4b > T4a) and younger age were also associated with increased chemotherapy use. By age group, chemotherapy was utilized in 31% of those < 50, 27% of those 51-60, 17% of those 61-70 and 6% of those > 70 years old. In the overall population, chemotherapy administration was associated with improved OS on multivariate analysis compared with no treatment (N = 1344; HR 0.76, p < 0.0001). 69% of patients received multi-agent chemotherapy. However, survival was similar between those receiving single-agent vs multi-agent chemotherapy (HR 1.2, p = 0.2016). In patients < 60 years old, chemotherapy use was associated with decreased OS (HR 1.4, p = 0.0156). Conclusions: In this retrospective, uncontrolled large database survey, HRFs are associated with decreased OS in stage II MSI-H CRC. This data indicates that poor differentiation and LVI do not independently worsen outcomes, but that the presence of multiple HRFs bears relevance. Though chemotherapy was associated with improved OS, the association is reversed in the < 60 year old population; this analysis cannot fully control for extent of disease and PS, among other factors. Chemotherapy should be judiciously administered in Stage II MSI-H CRC with HRFs. Immunotherapy trials are justified.[Table: see text]

TPS259 Background: Detection of circulating tumor DNA (ctDNA) shed into the bloodstream represents a highly specific and sensitive approach for identifying microscopic or residual tumor cells after surgical resection. For patients (pts) with colon cancer (CC), the detection of ctDNA is associated with persistent disease after resection and outperforms traditional clinical and pathological features in prognosticating risk for recurrence. However, for pts with stage II CC, there are currently no validated biomarkers predicting benefit in identifying pts whose residual disease cancer be cleared by adjuvant chemotherapy. We hypothesize that for pts whose stage II colon cancer has been resected and who have no traditional high-risk features, a positive ctDNA status may identify those who will benefit from adjuvant chemotherapy. Methods: In this prospective phase II/III clinical trial, pts (N=1,408) with resected stage II CC without traditional high-risk features and whom the evaluating oncologist deems suitable for active surveillance (i.e., not needing adjuvant chemotherapy) will be randomized 1:1 into 2 arms: standard-of-care/observation (Arm A), or prospective testing for ctDNA (Arm B). Postoperative blood will be analyzed for ctDNA with the Guardant Reveal assay, covering CC-relevant mutations and CC-specific methylation profiling. Pts in Arm B with ctDNA detected will be treated with 6 months of adjuvant (FOLFOX) chemotherapy. For all pts in Arm A, ctDNA status will be analyzed retrospectively at the time of endpoint analysis. The primary endpoints are clearance of ctDNA with adjuvant chemotherapy (phase II) and recurrence-free survival (RFS) for “ctDNA-detected” pts treated with or without adjuvant chemotherapy (phase III). Secondary endpoints will include time-to-event outcomes (OS, RFS, TTR) by ctDNA marker status and treatment, prevalence of detectable ctDNA in stage II CC, and rates of compliance with assigned intervention. Archived normal and matched tumor and blood samples will be collected for exploratory correlative research. Enrollment continues across North America to the 540-patient phase II endpoint. Support: U10CA180868, -180822; UG1CA189867; GuardantHealth. Clinical trial information: NCT04068103 .

TPS4121 Background: There are currently no validated predictive biomarkers for stage II resected colon cancer (CC) after adjuvant chemotherapy. However, circulating tumor DNA (ctDNA) that is shed into the bloodstream represents a highly specific and sensitive approach for identifying microscopic or residual tumor cells. For patients (pts) with CC, the detection of ctDNA is associated with persistent disease after resection and may outperform traditional clinical and pathological features as a prognostic factor to assess risk for recurrence. We hypothesize that for pts whose stage II colon cancer has been resected and who have no traditional high-risk features, a positive ctDNA status may identify those who will benefit from adjuvant chemotherapy. Methods: In this prospective phase II/III clinical trial, pts (N=1,408) with resected stage II CC without traditional high-risk features and whom the evaluating oncologist deems suitable for no adjuvant chemotherapy will be randomized 1:1 into 2 arms: standard-of-care/observation (Arm A), or prospective testing for ctDNA (Arm B). Postoperative blood will be analyzed for ctDNA with the GuardantHealth LUNAR panel, covering CC-relevant mutations and CC-specific methylation profiling. Pts in Arm B with ctDNA detected will be treated with 6 months of adjuvant (FOLFOX) chemotherapy. For all pts in Arm A, ctDNA status will be analyzed retrospectively at the time of endpoint analysis. The primary endpoints are clearance of ctDNA with adjuvant chemotherapy (phase II) and recurrence-free survival (RFS) for “ctDNA-detected” pts treated with or without adjuvant chemotherapy (phase III). Secondary endpoints will include time-to-event outcomes (OS, RFS, TTR) by ctDNA marker status and treatment, prevalence of detectable ctDNA in stage II CC, and rates of compliance with assigned intervention. Archived normal and matched tumor and blood samples will be collected for exploratory correlative research. The trial is actively accruing towards the phase II endpoint across all US and Canadian cooperative groups. Support: U10-CA-180868, -180822; UG1CA-189867; GuardantHealth. Clinical trial information: NCT04068103 .

3040 Background: Pembrolizumab was recently FDA approved across solid tumors for TMB scores ≥ 10mut/Mb as assessed by next-generation sequencing (NGS) of tissue (tTMB). A prior study of advanced cancer patients treated with immunotherapy found that higher somatic TMB, as defined by the 80th percentile in each histology, was associated with better overall survival. Previously, bTMB assessed by ctDNA from patients with newly diagnosed advanced NSCLC at a score of 16 mut/MB correlated with a tTMB score of 10 mut/MB. TMB levels vary by cancer type, line of treatment, and therapy received; the distribution of bTMB scores across solid tumor types has not been well characterized. Here we report the distribution of bTMB scores in patients with advanced malignancies. Methods: We queried 5,610 samples from patients with different cancer types undergoing clinical cell-free DNA testing (Guardant360; Redwood City, CA) and assessed bTMB scores from October 2020 - January 2021. bTMB score was derived via a previously described computational algorithm examining the total number of synonymous and non-synonymous SNVs and indels across a 1.0MB genomic footprint. We assessed the success rate of bTMB evaluation, overlap with microsatellite instability (MSI) status, and defined the distribution of bTMB levels across indications in this dataset. Results: bTMB score was successfully assessed in 4,275/5,610 (76.3%) samples (Table). The majority of samples (58%) were tested at disease progression as compared to initial diagnosis (42%). The median turnaround time from sample receipt to clinical reporting was 11 days and decreased to 9 days over the course of the study. For the majority of cancer types the 80th percentile TMB was ≥ 16 mut/MB tissue equivalency. Conclusions: Our analysis demonstrates the feasibility of measuring bTMB using a commercially available liquid biopsy assay. bTMB scores trended higher than tTMB previously reported in these cancer types, reflecting the ability of ctDNA to better capture tumor heterogeneity. cfDNA may allow for exploration of bTMB evolution throughout treatment. TMB should be interpreted in the context of disease, treatment, and method; these data establish a pan-cancer benchmark for bTMB which will serve as a resource for further studies.[Table: see text]

124 Background: Microsatellite stable (MSS) colorectal cancer and CRCLM are highly resistant to current immunotherapeutic approaches. Understanding the TME and the impact of standard chemotherapeutics is of utmost importance in developing optimal therapeutic strategies. Data is limited on the impact of NACT in the TME. In this study, we sought to explore the effects of chemotherapy on the proportions of different immune cell populations [Total leukohematopoetic cells, CD45+, T cell markers, CD3+, CD8+, and macrophage (M) markers, CD163+, CD68+)], as well as to assess their prognostic capacity. Methods: CRCLM pts who underwent liver resection were divided in 2 groups: pts who had NACT within 4 months of liver resection [NACT (+)] and no NACT [NACT (-)]. Whole slide staining of metastatectomy specimen for CD3, CD8, CD68, CD163, and CD45 was done. Cellular density within representative tumor core sections was analyzed. Retrospective chart review was done to obtain clinical data. Disease free (DFS), relapse free (RFS), and overall survival (OS) were analyzed using Kaplan Meir methods. Association between markers and outcomes was evaluated using Cox Regression Models. Results: A total of 43 pts were studied [NACT (+), n= 14; NACT (-), n=29). There were no significant differences in baseline characteristics between the two groups, including age, primary site, T stage, N stage, and grade. Median OS was 48 months. Outcomes were not significantly different with use of NACT. Within the NACT (+) CRCLM significant increases were seen in densities of CD3, CD8, CD45 and CD163 cells. Densities of CD8 cells were strongly correlated to CD163 cell densities ([r] 0.77, p < .0001). The NACT (+) cohort saw significantly increased ratios of T-cells/macrophages as compared to NACT (-) (CD3/CD68, CD3/CD163, CD8/CD163). High CD45 density was associated with improved OS (HR = 0.28, p = 0.006). CD3, CD8, CD68, and CD163 were not independently associated with OS. However, both the ratio of CD3/CD68 and CD3/163 were associated with improved OS (HR 0.44, p=0.03 and HR 0.27, p=0.03 respectively). Conversely, CD68/45 and CD163/CD45 ratios were associated with a worse OS (HR 1.42, p=0.05 and HR 1.43, p=0.03 respectively). No difference was seen in cellular populations based on the type of NACT received: oxaliplatin vs irinotecan. Conclusions: Our study highlights that the administration of NACT is associated with favorable changes in LM TME. While there is a general increase in leukocytes (CD45), the density of CD3+ and CD8+ T cells is particularly increased. Further, this increase is proportionately greater than the concurrent increases on monocytoid populations, reflected by increased CD3/CD163 and CD8/CD163 ratios. Ratios of T-cells/ macrophages are predictive of survival. This study was partly supported by the American Cancer Society Grant, 126771-IRG-14-194-11-IRG.

4032 Background: Pembrolizumab provides effective antitumor immunity and durable responses in patients (pts) with advanced, colorectal cancer (CRC) with microsatellite instability-high (MSI-H) tumors. We present data on antitumor immunity with pembrolizumab in pts from the phase 2, KEYNOTE-164 study who had approximately 3 years of follow-up, and in pts re-treated after disease progression. Methods: KEYNOTE-164 enrolled pts with metastatic MSI-H CRC, MSI-H status confirmed locally by IHC or PCR, and ≥2 (cohort A) or ≥1 (cohort B) prior lines of therapy (fluoropyrimidine, oxaliplatin, irinotecan, or anti VEGF/EGFR). Eligible pts received pembrolizumab 200 mg Q3W for 2y (35 administrations) or until progression, unacceptable toxicity, or withdrawal. Pts who stopped pembro due to a confirmed CR or after completing 2y of treatment and who progressed after stopping were eligible for re-treatment with up to 17 administrations in the second-course phase, at investigator discretion. Tumor response was assessed Q9W per RECIST v1.1 by independent review. The primary endpoint was ORR. Secondary endpoints included DOR, PFS, OS, and safety. The data cutoff date was Sep 9, 2019. Results: At data cutoff, the median follow-up was 31.4 mo (range, 0.2-47.8) for 61 pts in cohort A and 36.1 mo (0.1-39.3) for 63 pts in cohort B. ORR was 32.8% (3CR, 17PR; 95% CI% 21.3-46.0) for cohort A and 34.9% (8CR, 14PR; 95% CI 23.3-48.0) in cohort B. Median DOR was not reached (NR [range, 6.2-41.3+]) and not reached (range, 3.9+ to 37.1+), respectively. Fifteen pts in cohort A and 17 in cohort B had ongoing responses at data cutoff. Median PFS was 2.3 mo (95% CI 2.1-8.1) with 3-yr PFS rate of 31% in cohort A and was 4.1 mo (2.1-18.9) with 3-yr PFS rate of 34% in cohort B. Median OS was 31.4 mo (21.4-NR) with 3-yr OS rate of 49% in cohort A and was not reached (19.2-NR) with 3-yr OS rate of 52% in cohort B. Nine pts (6 in cohort A, 3 in cohort B) had a second course of treatment. The best response in second course was PR in 1 patient each in cohort A and B. Grade 3-4 drug-related adverse events occurred in 10 (16%) pts in cohort A and 8 (13%) pts in cohort B. No grade 5 drug-related events occurred. Conclusions: After approximately 3 y of follow-up, pembrolizumab continues to provide effective long-term antitumor immunity with durable responses, with small numbers of drug-related adverse events and no drug-related deaths in pts with advanced, MSI-H CRC. Clinical trial information: NCT02460198 .

e16102 Background: FL118 is a small molecule anticancer drug with novel mechanisms of action, although its chemical structure is similar to camptothecins. Currently, FL118 is completing its IND process to enter clinical trials for patients with advanced colorectal cancer (CRC) and pancreatic ductal adenocarcinoma (PDAC) cancers. Precision medicine is becoming a trend for modern cancer treatment. Thus, identification of drug efficacy-relevant biomarker(s) for patient selection is critical for enhancing patients’ response rates and avoiding unnecessary treatment in the clinic. Methods: Animal models of CRC and PDAC patient-derived xenograft (PDX) tumors were used to test FL118 efficacy; next generation sequencing (NGS) was used to determine PDX tumor molecular profiles; Western blots (WB) and real time RT-PCR were used to determine gene expression and/or apoptosis; Immunoprecipitation (IP) plus WB was used to determine protein phophorylation. Proteasome inhibitor was used to determine protein expression pathway. FL118 affinity purification was used to identify FL118 binding protein(s). Results: In association with WuXi Apptec, we have completed FL118 CMC and produced a clinical compatible product formulation. In this new product format, FL118 exhibits excess toxicity at a dose of ≥ 7.5 mg/kg via weekly x 4 schedules in mice. However, dosages of ≤ 6 mg/kg via the same schedule are tolerable. FL118 can eliminate the Kras/p53/APC triple-mutated SW620-established CRC tumors in a high percentage of mice at doses of 6 (maximum tolerated dose, MTD), 5, 4, 3, and 2 mg/kg. Tumor elimination under MTD is a typical characteristic of targeted drugs, which do not need MTD but only need the target to be saturated with drug for maximal efficacy. High expression of DDX5 (also called p68), a multifunctional oncogenic transcriptional co-activator and a DEAD-box RNA helicase, is associated with high FL118 efficacy in both CRC and PDAC PDX tumors. Mechanistically, FL118 binds to DDX5 to induce DDX5 tyrosine dephosphorylation and protein degradation without decreasing its mRNA. Tumors with Kras mutation (mKras) and/or TP53/p53 mutation (mp53) possess intrinsic resistance to multiple classic chemotherapeutic agents. However, PDAC PDX tumors with mp53 exhibit higher mKras expression and greater sensitivity to FL118 than tumors with wild type p53 and mKras. Conclusions: DDX5, mKras and mp53 appears to be favorable biomarkers for predicting FL118 high efficacy. However, further studies with a large cohort will be needed for confirmation.

e19047 Background: Colorectal cancer is one of the leading causes of cancer deaths in the US. Area of residence affects survival in many cancers, though largely unknown in colorectal cancer. The purpose of this study is to evaluate disparities between the metropolitan, urban and rural populations and the impact on survival in colorectal cancer. Methods: Data was obtained from the National Cancer Database (NCDB) colon, rectosigmoid, and rectal data sets. Patients (pts) were categorized by area of residence: metropolitan (MA), urban (UA), or rural (RA). Gender, race, insurance status, income, marital status, and tumor related data was collected. Univariate and multivariate analyses were performed to evaluate variables affecting overall survival (OS). Results: A total of 973,139 pts, spanning 2004-2013 were included in the study, 83% MA, 15% UA, 2% RA. RA and UA pts were more likely to be white than MA pts (92.7 v 91.4% v 83%). RA and UA were more likely to have lower income, with slightly lesser amounts of women and pts with no comorbidities. Uninsured rates were similar (3.3%). On univariate analysis, OS was worse for RA (HR 1.10) and UA (1.06) pts, as compared with MA. On multivariate analysis, small differences persisted with worse OS for RA (HR 1.02, p = .043) and UA (HR 1.01, p = .003). Black (HR 1.14) and native American (HR 1.17) pts had worse outcomes, while Asian (HR 0.8) pts had improved OS. Women (HR 0.88) and pts with higher incomes ( > 46K or 63K) had improved OS (HR 0.88). Conclusions: This study identifies socio-demographic disparities in colorectal cancer outcomes. Pts from rural and urban areas had worse OS, even though the proportion of minority pts with poor outcomes was lower. The difference in OS for RA and UA remained statistically significant in multivariate analysis, though it was largely corrected when adjusting for influencing variables. Part of this difference may be due to economic disparities, as lower income was linked to worse OS. This can independently limit access to care, especially for geographically isolated individuals. Additional research efforts are needed in order for us to better understand the key issues facing and the optimal means to improve outcomes in at risk populations.

4062 Background: Cetuximab is an EGFR-targeting IgG1 mAb. Pre-clinical data suggests cetuximab induces CD8+ cytotoxic T-cell (CTL) infiltration of tumors. We hypothesized that augmentation of CTLs in the tumor microenvironment (TME) may provide the proper milieu for effective PD-1 inhibition in metastatic colorectal cancer (mCRC). We conducted a phase Ib/II study of cetuximab with the PD-1 antibody, pembrolizumab, in mCRC. Correlative blood and tissue samples were collected to assess the impact of this treatment on CTLs, as well as potential compensatory alterations in regulatory T-cells (Tregs) and suppressive MDSCs (NCT02713373). Methods: 3 week treatment cycles included pembrolizumab at 200 mg on day 1 and cetuximab 250 mg/m2 following the 400 mg/m2 loading dose. Tumor biopsies were obtained at baseline and at c4d1 (Day 64). Peripheral blood (PB) was drawn at baseline, c2d1 (day 22) and c4d1 (Day 64). Flow cytometry was performed within 24 hours with additional samples stored for future analysis. In the present analysis, we assessed changes in levels of the cellular populations of interest between cycle 4 and cycle 1. Results: Forty-two RAS-wt patients were enrolled through October 2019. Paired tumor tissues were successfully analyzed for 16 patients and PB for 38. Intratumoral CTLs (CD3+CD8+) increased significantly (+47%, p < .05). In PB, there was a slight overall decrease in PB CTLs (-5%, p = NS) and a significant decrease in CD8+CD45RO+PD1+ cells (-42%, p < .05). We saw simultaneous decreases in PD-1+ CTLs in the tumor and PB. There was a trend for increase in Tregs (CD4+ Cd25+ FoxP3+) in PB (+11%, p = NS), but an overall increase in the Teff:Treg ratio (+30%, p = NS). CD4+CTLA4+ cells significantly increased (+37%, p < .05). Granulocytic MDSCs (CD11b+CD14−CD33+HLADR−) in PB decreased significantly on-treatment (-30%, p < 0.5). The sample size and tissue limitations prohibited meaningful evaluation of tissue Tregs and MDSCs via the present methods. Conclusions: Cetuximab and pembrolizumab induced dynamic changes in the TME and PB. The treatment associated increase in intratumoral CTLs was particularly pronounced, consistent with their local expansion and/or influx. This was accompanied by a decrease in PB CTLs. Decreases in multiple PD-1+ lymphoid populations were observed in both tumor and PB, notably PD-1+ CTLs. Of note, we saw a synchronous increase in immunosuppressive CD4+CTLA4+ T-cells in PB. Patient outcomes are pending maturation. Further analyses are planned, coupled with integration of clinical data. Clinical trial information: NCT02713373 .