Background:The randomized METIMMOX trial evaluated short-course oxaliplatin-based chemotherapy alternating with the immune checkpoint inhibitor nivolumab in patients with previously untreated, unresectable abdominal metastases from microsatellite-stable/mismatch repair-proficient colorectal cancer. A subgroup receiving this experimental treatment showed remarkably improved outcome compared to control arm patients who received chemotherapy alone and had median progression-free survival (PFS) of 9.3 months. We examined whether the kinetics of serum carcinoembryonic antigen (CEA) could provide insights into responsiveness to the METIMMOX regimen. Methods:Patients were randomly assigned to the control arm (oxaliplatin with bolus 5-fluorouracil/folinic acid, Q2W) or the experimental arm (alternating two cycles of chemotherapy Q2W and nivolumab Q2W). In this post hoc biomarker analysis, CEA levels were measured at baseline, at each treatment administration, and every two months during prespecified treatment breaks. Patients were categorized according to CEA kinetics-Flare (transient ≥20% increase above baseline), Non-responding (gradual increase), and Stable/Responding (values around baseline or gradually decreasing). Results:Flare comprised 16 out of 71 patients. According to Log rank test, experimental-arm Flare (n = 6) had significantly improved PFS (median 34.9 months) compared to Stable/Responding (n = 21; median PFS 9.9 months; p = 0.005) and Non-responding (n = 9; median PFS 2.1 months; p = 0.007). Flare was not associated with improved PFS for control-arm subjects. Adjusted Cox regression analysis indicated that experimental-arm Flare exhibited a lower progression risk than Non-responding for up to 6.2 months, creating an observation period for a potential decrease following the initial increase. Conclusion:CEA-Flare may indicate immune checkpoint inhibitor responsiveness following short-course oxaliplatin-based chemotherapy in this patient population; however, this is an exploratory finding that requires prospective validation.

Pseudomyxoma peritonei (PMP) is a rare abdominal cancer where curative treatment involves cytoreductive surgery and hyperthermic intraperitoneal chemotherapy. Postoperative surveillance relies on radiological imaging and blood tumor markers, which lack sensitivity and specificity. Circulating tumor DNA (ctDNA) analysis could offer a non-invasive alternative, but evidence in PMP is limited. Plasma samples from 95 PMP patients carrying KRAS and/or GNAS tumor mutations were analyzed using droplet digital PCR. Clinicopathological parameters and outcome were assessed, and disease-free survival (DFS) was analyzed using Cox regression. ctDNA was detected in 8 of 95 patients (8%), with low (median 0.1) mutated allele frequency with four positive cases at baseline, three at the time of recurrence, and one follow-up sample. Appendix tumor histology, high-grade peritoneal disease, and elevated baseline CA19-9 were independently associated with inferior DFS. Currently, mutation-based ctDNA detection cannot replace conventional postoperative surveillance for PMP patients.

The randomised METIMMOX trial evaluated short-course oxaliplatin-based chemotherapy alternating with nivolumab for metastatic microsatellite-stable/mismatch repair-proficient colorectal cancer. In a post hoc analysis, we investigated whether tumour mutations or patients’ systemic inflammation might provide insights into responsiveness to the METIMMOX regimen. Patients received either oxaliplatin-based chemotherapy (control group) or alternating two cycles each of chemotherapy and nivolumab (experimental group), with progression-free survival (PFS) as the primary endpoint. Tumour biopsies were sequenced with the TruSight Oncology 500 assay. The median tumour mutational burden (TMB; in mutations/megabase) was 8 (range, 1–13). The experimental-arm patients with TMB ≥9 or BRAF-V600E mutation (n = 17) achieved median PFS of 19.8 months (95% confidence interval, 11.3–28.3), longer (p = 0.0090) than experimental-arm patients with TMB < 9 not BRAF-V600E (n = 19) and control-arm patients with either TMB and BRAF status combination (n = 31). With TMB ≥9 or BRAF-V600E and normal, non-inflammatory level of C-reactive protein when starting nivolumab (n = 11), median PFS was 35.0 months (95% confidence interval, 6.8–63.0; p < 0.0001). TMB, somatic BRAF status and systemic inflammation should be prospectively investigated as practical biomarkers for predicting potential responsiveness to immune checkpoint inhibitors in metastatic microsatellite-stable/mismatch repair-proficient colorectal cancer.

OBJECTIVES:Total neoadjuvant therapy (TNT) for locally advanced rectal cancer (LARC) often uses intensive oxaliplatin-based chemotherapy alongside standard radiotherapy to prevent metastatic progression. The optimal intensity of the chemotherapy component, given a contribution of radiotherapy to clonogenic cell elimination in the primary tumour, remains uncertain. We aimed to assess long-term outcomes of a TNT regimen with low-intensity induction chemotherapy before chemoradiotherapy, as evaluated in the LARC-RRP trial. METHODS:Between 2005 and 2010, 87 LARC patients (36% with T4 organ-infiltrating tumour) were enrolled in the single-arm trial. Induction chemotherapy was given over four weeks prior to chemoradiotherapy and surgery. We report 10-year overall survival by tumour regression grade (TRG) and performed descriptive comparisons with contemporary trials whose experimental TNT arms used more intensive chemotherapy. RESULTS:At 10 years, overall survival was 83% (95% confidence interval, 71-90) for patients with complete or near-complete tumour cell elimination in the surgical specimen (TRG 1-2; 70% of the cohort) versus 52% (95% confidence interval, 31-69) for others (log-rank p = 0.0005). LARC-RRP survival outcomes were comparable with those reported in more contemporary TNT trials. CONCLUSIONS:The LARC-RRP results raise the possibility that shorter-duration chemotherapy combined with standard radiotherapy may suffice to protect against systemic recurrence of high-risk LARC. ADVANCES IN KNOWLEDGE:These long-term data support investigation of less intensive chemotherapy within TNT regimens and may guide the design of risk-stratified TNT trials.

BACKGROUND:Colorectal cancer with microsatellite stable (MSS) status is intrinsically resistant to immune checkpoint inhibitor monotherapy targeting PD-1 or PD-L1. However, emerging evidence suggests that chemotherapies may overcome this resistance when combined with immunotherapy. METHODS:We conducted a pooled analysis of individual patient data from two phase II trials evaluating oxaliplatin-based chemotherapy combined with checkpoint inhibitors as first-line treatment for MSS metastatic colorectal cancer. The MEDITREME trial is a single-arm study assessing durvalumab and tremelimumab with oxaliplatin-based chemotherapy, while the METIMMOX trial is a randomized study comparing standard oxaliplatin-based chemotherapy with an alternating regimen of oxaliplatin-based chemotherapy and nivolumab. We compared overall survival (OS), progression-free survival (PFS), and response rates between patients treated with chemotherapy alone versus chemoimmunotherapy, and evaluated the association between these outcomes and transcriptomic and immunoscore data. RESULTS:A total of 130 patients were included: 38 who received chemotherapy alone and 92 chemoimmunotherapy. The median OS was significantly improved in the chemoimmunotherapy group compared with chemotherapy alone (not reached vs. 15.3 months; HR = 0.58; 95% CI, 0.36-0.96; p = 0.03). The complete response rates were higher with chemoimmunotherapy (14% vs. 5%). No significant differences were observed in PFS. High baseline CD8+ T-cell infiltration was associated with improved outcomes in patients receiving chemoimmunotherapy but not in those treated with chemotherapy alone. CONCLUSIONS:This pooled analysis provides compelling evidence supporting the clinical benefit of first-line chemoimmunotherapy in a subset of patients with MSS mCRC. Baseline CD8+ T-cell infiltration may serve as a predictive biomarker for identifying patients most likely to benefit from this treatment approach.

Hyperthermic intraperitoneal chemotherapy (HIPEC) following cytoreductive surgery is a potentially curative treatment for patients with peritoneal metastases. Currently, there is no standardized protocol for performing HIPEC and there is large variation in the key parameters. In vivo models can be a valuable tool to better understand the impact of these parameters and how to improve this treatment strategy. Peritoneal tumors were established in immunodeficient mice by intraperitoneal injection of the high-grade serous human ovarian cancer cell line B76. Thirty-minute perfusion with cisplatin or mitomycin C at 37 or 41 °C was performed 4 days later. Treatment efficacy was assessed by measuring bioluminescence and overall survival. We successfully established a closed peritoneal chemotherapy perfusion model in mice bearing peritoneal metastases. Perfusion with cisplatin and mitomycin C significantly inhibited tumor growth and increased overall survival by 38–48

This article explores the career pathways of women in the field of surgical oncology, specifically their growth and development as surgical oncologists. Nine global female surgical oncologists and experts in Peritoneal Surface Oncology (PSO) share personal reflections of the challenges and facilitators of career success in this challenging and highly specialized field of surgery. Methods: Nine high profile female surgeons with expertise in PSO were recruited by direct contact with the lead author and asked to provide a detailed reflection of their career pathways, with a focus on barriers and facilitators to success. Results: Most participants were highly driven, motivated by the poor prognosis of the disease early in their careers and the new opportunities PSO offered. Many moved between units or countries in search of training, often sacrificing work-life balance. Strong role models-usually male-were cited as mentors, and many participants now lead the units they helped establish. None expressed regrets, and all took pride in the advances achieved during their careers. While some women mentioned gender bias, most felt that within PSO, surgical skills and commitment outweighed such barriers. Conclusions: These reflections should provide encouragement to the next generation of aspiring surgical oncologists to pursue their desire to enter this challenging area of surgery in the future.

BackgroundMetastatic colorectal cancer (mCRC) is the main cause of CRC mortality, with limited treatment options. Although immunotherapy has benefited some cancer patients, mCRC typically lacks the molecular features that respond to this treatment. However, recent studies indicate that the immune microenvironment of mCRC may be modified to enhance the effect of immune checkpoint inhibitors. This study aimed to explore the metastatic tumor microenvironment (TME) by comparing cell populations in colorectal liver (CLM), lung (mLu), and peritoneal (PM) metastases.MethodsRNA isolated from 20 CLM, 15 mLu, and 35 PM samples was subjected to mRNA sequencing and explored through TME deconvolution tools, consensus molecular subtyping (CMS), and differential gene expression and gene set enrichment analysis, with respect to the metastatic sites. Clinical data and KRAS/BRAF hotspot mutation status were also obtained for all the cases.ResultsThe cell type fractions in the TME were relatively similar between the metastatic sites, except for cancer-associated fibroblasts (CAFs), B cells, endothelial cells, and CD4+ T cells. Notably, PM showed enrichment for CAFs and endothelial cells, consistent with distinct pathways associated with metastatic growth and progression in the peritoneal cavity. PM with the mesenchymal subtype, CMS4, had increased CAFs, endothelial cells, and macrophages, along with up-regulated genes related to TNF-alpha signaling via NF-kappa B, EMT, and angiogenesis.ConclusionsTumor samples from different metastatic sites exhibited a broadly similar TME in terms of immune cell composition, with some intriguing differences. Targeting CAF-associated pathways, macrophages, and TNF-alpha signaling through NR4A could represent potential novel therapeutic approaches in CMS4 PM.

Accurate identification of tumor-specific markers is vital for developing chimeric antigen receptor (CAR)-based therapies. While cell surface antigens are seldom cancer-restricted, their post-translational modifications (PTMs), particularly aberrant carbohydrate structures, offer attractive alternatives. Among these, the sialyl-Tn (STn) antigen stands out for its prevalent presence in various epithelial tumors. Although monoclonal antibodies (mAbs) against STn have been developed, their clinical application has been hindered by concerns regarding specificity. Herein, we describe AM52.1, a mAb with unprecedented specificity for STn and lack of reactivity with healthy tissues. The single-chain variable fragment (scFv) of AM52.1 was assembled into a second-generation CAR scaffold. AM52.1CAR T cells efficiently targeted STn-expressing cancer cell lines and patient-derived organoids (PDOs), while sparing STn-negative cells. In further preclinical models, AM52.1CAR T cells robustly controlled gastric and tubo-ovarian tumors, as well as colorectal cancer mucinous peritoneal metastases, highlighting their strong therapeutic potential for targeting and managing complex solid tumors.

Activated RAS is a common driver of cancer that was considered undruggable for decades. Recent advances have enabled the development of RAS inhibitors, but the efficacy of these inhibitors remains limited by resistance. In this study, we developed a pan-RAS inhibitor, ADT-007, (Z)-2-(5-fluoro-1-(4-hydroxy-3,5-dimethoxybenzylidene)-2-methyl-1H-inden-3-yl)-N-(furan-2-ylmethyl)acetamide, that binds nucleotide-free RAS to block GTP activation of effector interactions and MAPK/AKT signaling, resulting in mitotic arrest and apoptosis. ADT-007 potently inhibited the growth of RAS-mutant cancer cells irrespective of the RAS mutation or isozyme. Wild-type RAS (RASWT) cancer cells with GTP-activated RAS from upstream mutations were equally sensitive. Conversely, RASWT cancer cells harboring downstream BRAF mutations and normal cells were essentially insensitive to ADT-007. Sensitivity of cancer cells to ADT-007 required activated RAS and dependence on RAS for proliferation, whereas insensitivity was attributed to metabolic deactivation by UDP-glucuronosyltransferases that were expressed in RASWT and normal cells but repressed in RAS-mutant cancer cells. ADT-007 displayed unique advantages over KRAS mutant-specific, pan-KRAS, and pan-RAS inhibitors that could impact in vivo antitumor efficacy by escaping compensatory mechanisms that lead to resistance. Local administration of ADT-007 showed robust antitumor activity in syngeneic immunocompetent and xenogeneic immune-deficient mouse models of colorectal and pancreatic cancers. The antitumor activity of ADT-007 was associated with the suppression of MAPK signaling and activation of innate and adaptive immunity in the tumor immune microenvironment. Oral administration of ADT-007 prodrug also inhibited tumor growth. Thus, ADT-007 has the potential to address the complex RAS mutational landscape of many human cancers and to improve treatment of RAS-driven tumors.Significance: ADT-007, a first-in-class pan-RAS inhibitor, has unique selectivity for cancer cells with mutant RAS or activated RAS protein and the capability to circumvent resistance to suppress tumor growth, supporting further development of ADT-007 analogs.

Objectives:Pseudomyxoma peritonei (PMP) is a rare, slow-growing cancer with few efficacious treatment options in unresectable cases. Mutations in the KRAS and GNAS oncogenes are common, but the reported frequencies vary greatly, most likely because of low tumor cellularity in peritoneal tumor samples. With treatments targeting these mutations becoming increasingly available, reliable detection of mutations is essential. Methods:The frequency of KRAS and GNAS mutations was analyzed in tumor samples from 167 patients with verified PMP using targeted DNA sequencing and/or droplet digital polymerase chain reaction. When analysis of fresh-frozen peritoneal tumor samples did not reveal mutations, macrodissected formalin-fixed samples were analyzed. Results:Mutations in cancer-related genes were detected in 98 % of the analyzed samples, with KRAS and GNAS mutated in 148 (89 %) and 139 (83 %) cases, respectively. In 48 % of the analyzed cases, the mutational diagnosis was based on primary tumor samples. Conclusions:High frequencies of KRAS and GNAS mutations support the proposed role as driver mutations and as potential therapy targets. The primary tumor may serve as an alternative source of tumor material, increasing the likelihood of detecting targetable mutations. Combined with highly sensitive analytical methods, this approach facilitates selection of patients for novel targeted therapeutic strategies.

BACKGROUND:We evaluated first-line treatment of metastatic microsatellite-stable colorectal cancer with short-course oxaliplatin-based chemotherapy alternating with immune checkpoint blockade. METHODS:Patients were randomly assigned to chemotherapy (the FLOX regimen; control group) or alternating two cycles each of FLOX and nivolumab (experimental group). Radiographic response assessment was done every eight weeks with progression-free survival (PFS) as the primary endpoint. Cox proportional-hazards regression models estimated associations between PFS and relevant variables. A post hoc analysis explored C-reactive protein as signal of responsiveness to immune checkpoint blockade. RESULTS:Eighty patients were randomised and 38 in each group received treatment. PFS was comparable-control group: median 9.2 months (95% confidence interval (CI), 6.3-12.7); experimental group: median 9.2 months (95% CI, 4.5-15.0). The adjusted Cox model revealed that experimental-group subjects aged ≥60 had significantly lowered progression risk (p = 0.021) with hazard ratio 0.17 (95% CI, 0.04-0.76). Experimental-group patients with C-reactive protein <5.0 mg/L when starting nivolumab (n = 17) reached median PFS 15.8 months (95% CI, 7.8-23.7). One-sixth of experimental-group cases (all KRAS/BRAF-mutant) achieved complete response. CONCLUSIONS:The investigational regimen did not improve the primary outcome for the intention-to-treat population but might benefit small subgroups of patients with previously untreated, metastatic microsatellite-stable colorectal cancer. TRIAL REGISTRATION:ClinicalTrials.gov number, NCT03388190 (02/01/2018).

Abstract Introduction: The peritoneum is a common metastatic site for several abdominal cancers, including ovarian and colorectal cancer. Peritoneal metastasis is associated with poor prognosis and poor response to therapy. A potentially curative treatment is cytoreductive surgery (CRS) to remove all visible tumor tissue, followed by perfusion with hyperthermic intraperitoneal chemotherapy (HIPEC) to eliminate small residual tumors and free-floating tumor cells. Hyperthermia can enhance the cytotoxic effect of the drug, inhibit DNA repair and activate the immune system. At the current time, there is no standardized protocol for performing HIPEC, and there is a large variation regarding key parameters, such as choice of drug, drug concentration, treatment duration, carrier solution, volume and temperature. The impact of these parameters is unknown, and in vivo models can be a helpful tool to better understand some of these parameters to improve this treatment strategy. Methods: HIPEC was established in a murine xenograft model of ovarian cancer to evaluate the response to intraperitoneal perfusion of cisplatin and mitomycin C for 30 min at 41.5°C (HIPEC) and 37°C (NIPEC) compared to no treatment or intraperitoneal perfusion with saline at 41.5°C. The luciferase transfected B76 ovarian cancer cell line was injected intraperitoneally and treatment was performed on day 4 when the tumors were < 2 mm. Treatment efficacy was assessed by weekly luminescence measurement, tumor weight at an early time point (day 17), and overall survival. Results: Intraperitoneal perfusion with cisplatin or mitomycin C significantly inhibited tumor growth as assessed by luminescence, tumor weight on day 17, as well as increased overall survival of the mice compared to control treatment from 25-27 days to 36 days for cisplatin and 37 days for mitomycin C. No significant differences in tumor growth or survival were observed by the addition of hyperthermia. However, a slight reduction of tumor weight at day 17 was observed in the HIPEC groups. Conclusion: We have established a functional closed HIPEC model in mice bearing peritoneal metastases from the B76 ovarian cancer cell line which accurately mimics many features of the procedure in patients. Intraperitoneal perfusion of both cisplatin and mitomycin C efficaciously inhibited tumor growth and improved overall survival of the mice. Addition of hyperthermia caused an additional modest, temporary growth inhibition which did not translate into improved long-term outcome. Citation Format: Karianne Giller Fleten, Yvonne Andersson, Emil Løvstakken, Theodor M. Herud, Stein Waagene, Kjersti Flatmark. Experimental hyperthermic intraperitoneal chemotherapy in mice: Establishment of a novel ovarian cancer xenograft model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2825.

The randomized METIMMOX trial (NCT03388190) examined if patients with previously untreated, unresectable abdominal metastases from microsatellite-stable (MSS) colorectal cancer (CRC) might benefit from potentially immunogenic, short-course oxaliplatin-based chemotherapy alternating with immune checkpoint blockade (ICB). Three of 38 patients assigned to this experimental treatment had metastases from BRAF-mutant MSS-CRC, in general a poor-prognostic subgroup explored here. The ≥70-year-old females presented with ascending colon adenocarcinomas with intermediate tumor mutational burden (6.2–11.8 mutations per megabase). All experienced early disappearance of the primary tumor followed by complete response of all overt metastatic disease, resulting in progression-free survival as long as 20–35 months. However, they encountered recurrence at previously unaffected sites and ultimately sanctuary organs, or as intrahepatic tumor evolution reflected in the terminal loss of initially induced T-cell clonality in liver metastases. Yet, the remarkable first-line responses to short-course oxaliplatin-based chemotherapy alternating with ICB may offer a novel therapeutic option to a particularly hard-to-treat MSS-CRC subgroup.

Abstract Purpose: Most CRC patients harbor primarily non-immunogenic MSS tumor. The randomized METIMMOX trial (NCT03388190) examined if patients with previously untreated, unresectable abdominal metastases from MSS-CRC may achieve therapeutic efficacy from short-course oxaliplatin-based chemotherapy (FLOX) and sequential ICB (nivolumab) repeatedly. Half of study patients assigned to this experimental treatment had significantly extended progression-free survival (PFS) while the other half had shortened PFS, compared to the control-group patients given standard FLOX chemotherapy with median PFS 9.3 months. Here we explored if somatic mutations (mut) unveiled by next-generation sequencing might provide insights into responsiveness to the METIMMOX regimen. Procedures: Patients had MSS-CRC with infradiaphragmatic metastases deemed unresectable and were eligible for first-line oxaliplatin-based therapy. They were randomly assigned to the control group of FLOX (oxaliplatin, 5-fluorouracil, folinic acid) Q2W or the experimental group of alternating 2 cycles each of FLOX Q2W and nivolumab Q2W, with prespecified break periods. Radiologic response assessment was done every 8 weeks with PFS as the primary endpoint. Diagnostic tumor biopsies and baseline metastasis biopsies were sequenced with the TruSight Oncology 500 assay. TMB was calculated using only coding, non-synonymous single-nucleotide variants and insertions/deletions with variant allele frequency ≥5% and sequencing depth ≥50 ×, applying the effective panel size as the megabases (Mb) denominator. Results: Sequencing data were acquired from 20 experimental-group patients with median PFS 7.9 months (minimum 2.0, maximum 41.6). Median TMB was 5.1 mut/Mb (minimum 0.8, maximum 11.8), concordant in patient-paired primary tumor and metastasis specimens. The TMB was associated with treatment outcome; the higher TMB, the better PFS with hazard ratio 0.83 (95% confidence interval (CI) 0.70-0.98; p = 0.026, Cox regression) for a PFS event with increasing TMB. TMB cut-off as low as 5.0 mut/Mb distinguished between patient groups (p = 0.007, log-rank test) with median PFS 34.9 months (95% CI 10.6-59.2; TMB>5, n = 10) and median PFS 4.6 months (95% CI 2.7-6.5; TMB≤5, n = 10). In the TMB>5 group, 8/10 were KRAS- or BRAF-mutant cases. In the TMB≤5 group, 9/10 metastasis specimens were RAS/BRAF-wildtype cases. Conclusions: TMB >5 mut/Mb was associated with tumor KRAS or BRAF driver mutation and remarkably long PFS in first-line treatment of patients with abdominal metastases from MSS-CRC with alternating short-course oxaliplatin-based chemotherapy and ICB. Citation Format: Anne Hansen Ree, Paula A. Bousquet, Hilde L. Nilsen, Torben Lüders, Shixiong Wang, Tina Visnovska, Diana L. Bordin, Eirik Høye, Hanne M. Hamre, Christian Kersten, Eva Hofsli, Marianne G. Guren, Halfdan Sorbye, Kjersti Flatmark, Sebastian Meltzer. Predictive value of tumor mutational burden (TMB) in patients with metastatic microsatellite-stable (MSS) colorectal cancer (CRC) given first-line oxaliplatin-based chemotherapy and immune checkpoint blockade (ICB) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2522.

Abstract Background: Pseudomyxoma peritonei (PMP) is a rare cancer originating from appendiceal mucinous tumors, which may spread to encompass the entire peritoneal cavity. Complete surgical removal and hyperthermic intraperitoneal chemotherapy cures approximately half of the patients, but patients with recurrent or non-resectable disease have no efficacious treatment options. These patients have a high unmet need for novel treatment options, and in the Pseudovax trial we will explore a novel therapeutic concept. A main molecular driver of PMP (in up to 90% of cases) is otherwise rare mutations in the GNAS oncogene, coding for the Gsα protein. We hypothesized that mutated Gsα would be a tumor neoantigen, and in T cells isolated from peripheral blood of PMP patients, a strong T cell proliferative response was observed. In the corresponding tumors, T cells with up-regulated immune checkpoint molecules PD-1 and TIGIT were detected. Study hypotheses: Patients with GNAS-mutated PMP has a preexisting, attenuated immune response directed against mutated Gsα protein. Vaccination with mutated Gsα peptides can be combined with immune checkpoint inhibition (ICI) with acceptable toxicity. The vaccine will reactivate the immune response and ICI will restore measurable anticancer immunity in patients. Study design: The Pseudovax trial is a small (10 patients) first-in-human, proof-of-concept, prospective, open label, phase I trial. Patients with recurrent or non-resectable PMP are eligible for inclusion. During a treatment period of 12 months, a mutated Gsα peptide vaccine + adjuvant (GM-CSF) will be administered as intradermal injections. After 12 weeks, treatment with ICI will be initiated. The primary endpoints are toxicity and circulating immune response against the vaccine, measured as increase in interferon-gamma production or proliferation of circulating vaccine-specific T cells compared to pre-vaccination baseline. Secondary endpoints are progression-free survival (abdominopelvic CT scans, blood levels of carcinoembryonic antigen (CEA) and/or detection of GNAS mutation by ctDNA analysis). Citation Format: Kjersti Flatmark, Annette Torgunrud, Fleten G. Karianne, Ben Davidson, Hedvig V. Juel, Nadia Mensali, Christin Lund Andersen, Else Marit Inderberg. The Pseudovax trial: A novel peptide vaccine targeting mutated GNAS for pseudomyxoma peritonei [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT079.

Background Patients with peritoneal metastasis from colorectal cancer (PM-CRC) have inferior prognosis and respond particularly poorly to chemotherapy. This study aims to identify the molecular explanation for the observed clinical behavior and suggest novel treatment strategies in PM-CRC. Methods Tumor samples (230) from a Norwegian national cohort undergoing surgery and hyperthermic intraperitoneal chemotherapy (HIPEC) with mitomycin C (MMC) for PM-CRC were subjected to targeted DNA sequencing, and associations with clinical data were analyzed. mRNA sequencing was conducted on a subset of 30 samples to compare gene expression in tumors harboring BRAF or KRAS mutations and wild-type tumors. Results BRAF mutations were detected in 27% of the patients, and the BRAF-mutated subgroup had inferior overall survival compared to wild-type cases (median 16 vs 36 months, respectively, p < 0.001). BRAF mutations were associated with RNF43/RSPO aberrations and low expression of negative Wnt regulators (ligand-dependent Wnt activation). Furthermore, BRAF mutations were associated with gene expression changes in transport solute carrier proteins (specifically SLC7A6) and drug metabolism enzymes (CES1 and CYP3A4) that could influence the efficacy of MMC and irinotecan, respectively. BRAF-mutated tumors additionally exhibited increased expression of members of the novel butyrophilin subfamily of immune checkpoint molecules (BTN1A1 and BTNL9). Conclusions BRAF mutations were frequently detected and were associated with particularly poor survival in this cohort, possibly related to ligand-dependent Wnt activation and altered drug transport and metabolism that could confer resistance to MMC and irinotecan. Drugs that target ligand-dependent Wnt activation or the BTN immune checkpoints could represent two novel therapy approaches.