196 Background: HLA Class I (HLA-I) proteins present antigens to CD8+ T-cells to facilitate antitumor responses. The impact of HLA-I diversity on immune checkpoint inhibition (ICI) efficacy is unclear in colorectal cancer (CRC). We explored HLA-I genotypes and loss of heterozygosity (LoH) in relation to T-cell dynamics and ICI efficacy in CRC. Methods: Tumor whole-exome sequencing datasets were from 3 CRC datasets: 580 TCGA patients (pts), 82 pts from a BC Cancer resectable CRC study (VICTORI) and 109 pts from the Canadian Cancer Trials Group CO.26 trial (NCT02870920) comparing durvalumab and tremelimumab (D+T) to supportive care in refractory CRC. POLYSOLVER and LOHHLA were used to determine HLA-I genotypes and LoH status. Samples were labeled somatic LoH or germline homozygosity (Hom) if ≥1 HLA-I gene (HLA-A, B, or C) showed LoH or homozygosity. T-cell populations were inferred using CIBERSORT in TCGA and VICTORI RNA-seq data. In CO.26, T-cell receptor (TCR) Shannon diversity was determined using MiXCR on TCR-seq data. Results: The average rate of LoH and Hom was 24.7% and 25.0%, respectively (dataset and HLA-I-gene rates in table). Rates did not differ by age or sex; however, LoH was more common in white and Asian pts (OR=3.1, 95% CI 1.2-10.6, p=0.015) whereas Hom had no race association. HLA-I supertypes had varied loss prevalence compared to germline: B62 had the highest at 12.5% (95% CI 7.6-19.9) while B08 was lowest at 4.1% (95% CI 1.8-9.2). We next assessed common (>5%) HLA alleles and ICI efficacy. B44:02 and C05:01 were in 13% of pts and associated with reduced overall survival (OS) in pts receiving D+T after correcting for plasma TMB and liver metastases (adjusted hazard ratio (aHR)=3.5, p=0.001, p-interaction=0.005; aHR=3.5, p=0.004, p-int=0.004). These alleles often co-occurred (p<0.001) and B44 was most frequent in white pts (OR=10.7, 95% CI 1.7-440.3, p=0.002). We then examined the role of LoH on immune microenvironment and patient outcomes. LoH in HLA-B and HLA-C correlated with higher expression of each gene (p=0.004; p=0.009). Further, LoH CRCs had higher CD8+ T-cell and active/resting memory CD4+ T-cell scores in VICTORI (p=0.024, p=0.013 respectively) and TCGA (p=0.046, p=0.029). Neither LoH (p=0.13) nor Hom (p=0.86) affected OS in ICI treated CRCs. However, the combo of LoH and low TCR diversity reduced ICI efficacy (HR-int=7.1, p-int=0.01). Conclusions: HLA alleles B44:02 and C05:01 reduced ICI efficacy. HLA-I LoH correlated with higher HLA-I gene expression and active T-cell levels, indicating its selection in T-cell infiltrated tumors. In these tumors, the combo of low TCR diversity and LoH may cause immune evasion and reduced ICI efficacy. CO.26 VICTORI TCGA HLA-I LoH (%) 21.2 28.8 24.9 HLA-I Hom (&) 22.3 25.3 25.5 HLA-A,B,C LoH (%) 16.9, 11,8, 13.1 15.7, 15,8, 22.1 19.1, 17.6, 17.2 HLA-A,B,C Hom (%) 16.8, 4.7, 7.5 14.6, 7.3, 13.9 14.1, 9.1, 12.4

41 Background: Surrogate endpoints of overall survival (OS) are increasingly used as the basis of cancer drug approvals, but a strong association between their treatment effects is essential for validation as appropriate surrogates. We summarize the current evidence landscape for meta-analytic validation of surrogate endpoints in colorectal cancer (CRC) and assess surrogacy evaluation quality through a novel framework to inform the selection of primary endpoints in future trials. Methods: We developed a framework to grade the quality of surrogate endpoints in oncology. We then searched MEDLINE to identify meta-analytic validation studies of endpoints in CRC at the trial level. We consulted a research librarian to develop a search strategy in PubMed using keywords and MeSH terms. Articles published before 07/06/2024 were included and two reviewers independently evaluated each. Grades were assessed according to seven evidence domains: data source (A-B), disease population (0-3), surrogate and true endpoint definitions (0-3), number of trials evaluated (0-4), quality and strength of individual (0-2) and trial-level associations (0-8). Discordance in blinded assessments was resolved by consensus decision. Results: Eighteen articles were identified containing 39 evaluations of 12 unique endpoints. Results are presented for disease-free survival (DFS), Progression-free survival (PFS), overall response rate (ORR), time to progression (TTP) and disease control rate (DCR). PFS was most frequently assessed (n=11). Surrogates were primarily evaluated in stage IV disease (n=34) and analyses of biologic agents (n=29). Individual patient data (IPD) was used in 13 evaluations. A23 was the highest endpoint quality score assigned using IPD and B6 was the lowest using summary trial data (SD). Only DFS consistently demonstrated high-quality surrogacy for OS at the trial level using IPD (≥A18, R 2 ≥ 0.70). Conclusions: DFS consistently shows high quality of surrogacy, whereas other endpoints show inconsistent surrogacy evaluation ranging from low to moderate quality in contemporary CRC trials. Standardizing the methodology of endpoint validation using IPD may improve surrogacy estimates. Assessment of new, or revision of prior surrogate endpoints may be warranted, particularly in the context of metastatic disease. Endpoint Data source (frequency) Treatment setting (frequency) Treatment era (frequency) Trial-level association, R 2 (range) Surrogate quality score (range) DFS (n=5) IPD: 5 Adjuvant Pre-biologics: 4Post-biologics: 1 0.70-0.93 A18-23 PFS (n=11) IPD: 3SD: 8 Metastatic Pre-biologics: 2Post-biologics: 9 0.34-0.99 B6-A22 ORR (n=9) IPD: 1SD: 8 Metastatic Pre-biologics: 1Post-biologics: 8 0.03-0.74 B6-A13 TTP (n=5) SD: 5 Metastatic Post-biologics: 5 0.27-0.65 B7-11 DCR (n=2) SD: 2 Metastatic Post-biologics: 2 0.82-0.89 B7-8

Pancreatic ductal adenocarcinoma (PDAC) is a deadly disease characterized by late-stage manifestation and relative resistance to standard therapies. Challenges with early detection and a paucity of effective therapies lead to one of the lowest 5-year survival rates among all cancers. Individuals around 50 years and over presenting with new onset diabetes (NOD) have a higher risk for PDAC diagnosis within 3 years of diabetes onset compared to the rest of the population. In this review, we contextualize NOD within other types of diabetes presentations such as type 1 diabetes (T1D), type 2 diabetes (T2D), and type 3 diabetes (T3cD), unravel the bidirectional relationship between diabetes and PDAC, and highlight potential biomarkers that may distinguish PDAC-associated diabetes from other predominant types of diabetes. Although practical applications of NOD currently fall short from being clinically actionable, clinical trials are underway to stratify NOD patients with PDAC-associated diabetes. Ultimately, these efforts could offer the rationale to implement early detection screening strategies to this subgroup of PDAC patients.

BACKGROUND:Given inconsistent findings from previous epidemiologic studies on the association between antidepressant exposure and colorectal cancer (CRC), our study provides a rigorous investigation to clarify the temporality of this association, including early-age onset (EAO) and average-age onset (AAO) CRC. METHODS:We conducted a population-based case-control study using administrative health databases from British Columbia, Canada. We included CRC cases and controls, matched (1:10) on age, sex, and index date (i.e., CRC diagnosis date/matched date). Antidepressant exposures were ascertained by duration (i.e., varying windows from 15 to 1 year before CRC diagnosis), drug classes (tricyclic antidepressants (TCAs), selective serotonin reuptake inhibiters (SSRIs), other), cumulative dose and treatment intensity. We used multivariable conditional logistic regression models and interpreted odds ratios as relative risks. RESULTS:Among 10,171 CRC cases (688 EAO-CRC; 9483 AAO-CRC) and 90 928 controls, antidepressants exposure in the 15-year window was associated with a lower risk of CRC overall (adjusted relative risk [aRR] 0.84; 95% CI 0.80, 0.89), EAO-CRC (aRR 0.54; 95% CI 0.44, 0.66), and AAO-CRC (aRR 0.87; 95% CI 0.83, 0.92). Across narrowing exposure windows, associations persisted up to 7 years before CRC diagnosis, then weakened. Inverse associations were also observed for TCAs (aRR 0.83; 95% CI 0.77, 0.89) and SSRIs (aRR 0.86; 95% CI 0.81, 0.91) and CRC. Cumulative dose and treatment intensity showed no associations. CONCLUSIONS:Across all age groups, antidepressant exposure in the earlier exposure windows (15-7 years) was associated with a lower CRC risk, with the strongest effect at the 15-year window.

3547 Background: Benefits of immune checkpoint inhibitors (ICIs) in unselected proficient mismatch repair (pMMR) mCRC remain unestablished. This exploratory analysis of CCTG CO.26 evaluated whether serum VEGF-A concentrations may influence ICI efficacy. Methods: CCTG CO.26 was a phase II trial that randomized refractory mCRC patients (pts) (2:1) to Durvalumab + Tremelimumab + Best supportive care (DT) vs Best supportive care (BSC) alone and was positive for overall survival (OS). VEGF-A concentrations were determined in baseline serum samples using an ELISA method. OS and progression-free survival (PFS) were compared between high and low groups based on median VEGF-A concentration in each treatment arm. An optimal cutoff was also identified using a minimal p-value approach. VEGFA mRNA expression from tissue-based RNA-seq was analyzed in the INSPIRE study, a separate, single arm, multicohort phase II trial comprising multiple tumor types treated with pembrolizumab for external validation. Results: Baseline VEGF-A concentrations were available for 161 pts (47 BSC, 114 DT). Median VEGF-A concentration was 1166 pg/ml (range: 226 – 4898; IQR: 784-1755). VEGF-A concentrations were significantly higher in ECOG 1 vs 0 pts (p = 0.03), and in pts who had previously received regorafenib (p = 0.03). Grouped by median VEGF-A, median OS was longer in DT-treated patients with low VEGF-A (7.39 months for DT-Low vs. 6.08 DT-high; hazard ratio (HR) 0.62, 95% confidence interval (CI): [0.42–0.90], p = 0.01), but not in BSC treated patients (4.44 months BSC-Low vs. 4.11 BSC-High; HR 1.05 [0.57–1.92], p = 0.88; interaction HR 0.56 [0.28–1.14], p = 0.11). An optimal VEGF-A threshold of 1540 pg/mL was identified using a minimal p-value approach, with 33.5% pts having high VEGF-A concentrations. Median OS was 7.29 vs 4.37 months for DT-Low and DT-High pts (HR 0.53 [0.35–0.80], p < 0.005), and 3.55 vs 7.46 months for BSC-Low and BSC-High pts (HR 1.50 [0.80–2.80], p = 0.21; interaction HR 0.33, [0.16–0.70], p < 0.005). In multivariable analyses incorporating ECOG, presence of liver metastases, plasma tumor mutation burden and arginine levels, low VEGF-A concentrations remained statistically associated with improved OS with DT (HR 0.46 [0.30–0.70], p < 0.001; interaction HR 0.18 [0.083–0.41], p < 0.0001). There was no association between VEGF-A concentrations and PFS in any arm. In the INSPIRE study (n = 66), VEGFA mRNA expression was lower in responders (p < 0.001) and significantly associated with improved PFS (HR 0.45 [0.25–0.79], p = 0.006) and OS (HR 0.47 [0.26–0.85], p = 0.013) across tumor types. Conclusions: Refractory mCRC patients with low VEGF-A concentrations may derive benefit from ICIs. This is the first report to suggest that serum VEGF-A concentration is a potential predictive biomarker for benefit from ICIs. These findings should be prospectively validated.

TPS3634 Background: Currently, there are no biomarkers validated prospectively in randomized studies for resected colon cancer (CC) to determine need for adjuvant chemotherapy (AC). However, circulating tumor DNA (ctDNA) represents a highly specific and sensitive approach (especially with serial monitoring) for identifying minimal/molecular residual disease (MRD) post-surgery in CC patients (pts), and may outperform traditional clinical and pathological features in prognosticating risk for recurrence. CC pts who do not have detectable ctDNA (ctDNA-) are at a much lower risk of recurrence and may be spared the toxicities associated with AC. Furthermore, for CC pts with detectable ctDNA (ctDNA+) who are at a very high risk of recurrence, the optimal AC regimen has not been established. We hypothesize that for pts whose CC has been resected, ctDNA status may be used to risk-stratify for making decisions about AC. Methods: In this prospective phase II/III trial, up to 1,912 pts with resected stage III A, B (all pts) and stage II, IIIC (ctDNA+ only) CC will be enrolled. Based on the post-operative ctDNA status using personalized and tumor-informed assay (SignateraTM, bespoke assay), those who are ctDNA- (Cohort A) will be randomized to immediate AC with fluoropyrimidine (FP) + oxaliplatin (Ox) for 3-6 mos per established guidelines vs. serial ctDNA monitoring. Patients who are ctDNA+ post-operatively or with serial monitoring (Cohort B) will be randomized to FP+Ox vs. more intensive AC with addition of irinotecan (I) for 6 mos. The primary endpoints for Cohort A are time to ctDNA+ status (phase II) and disease-free survival (DFS) (phase III) in the immediate vs. delayed AC arms. The primary endpoint for Cohort B is DFS in the FP+Ox vs FP+Ox+I arms for both phase II and phase III portions of the trial. Secondary endpoints include prevalence of detectable ctDNA post-operatively, time-to-event outcomes (overall survival and time to recurrence) by ctDNA status, and the assessment of compliance to adjuvant therapy. Biospecimens including archival tumor tissue, as well as post-operative plus serial matched/normal blood samples, will be collected for exploratory correlative research. Active enrollment across the NCTN started in June 2022. NCT#: NCT05174169. Support: U10-CA-180868, -180822; UG1CA-189867; Natera, Inc. Clinical trial information: NCT05174169 .

Pathology is foundational to biomarker-driven and translational oncology research, yet systemic barriers limit full pathology engagement in Canadian cancer clinical trials, compromising the tissue-based questions such trials are designed to answer. This commentary and white paper synthesizes the perspectives of a national pathology working group convened at the 2025 Canadian Cancer Trials Group (CCTG) Annual General Meeting with a descriptive internal audit of the CCTG Tumour Tissue Data Repository (TTDR), in which biospecimen attrition was tabulated at the patient level by disease site; no inferential testing was performed. The TTDR data revealed substantial attrition across disease sites, with no tissue submitted for 32-44% of patients and slides submitted in place of formalin-fixed paraffin-embedded blocks for up to 51% of cases, reflecting persistent misalignment between protocol expectations and laboratory capacity. From these observations, five interrelated gaps were identified-in trial design, funding and resourcing, digital pathology infrastructure, academic recognition, and knowledge translation around consent and ethics governance. Five corresponding strategies are proposed to align research demands with pathology capacity, reduce attrition, and strengthen biomarker-driven trials. As a consensus- and experience-driven analysis rather than a systematic review, these recommendations are intended to frame a national conversation and a starting point for prospective evaluation.

TPS3676 Background: Despite significant progress and newly approved agents, metastatic colorectal cancer (mCRC) has limited prognosis after progressing on standard cytotoxic chemotherapies. Many patients remain well enough for treatment at the time when their cancer is refractory to all available therapy, creating a significant unmet need. Once refractory, patient management with best supportive care (BSC) results in a median overall survival (OS) of 4–6 months. To date, immune checkpoint inhibitors (ICIs) have shown limited activity in mCRC that is not mismatch repair deficient (dMMR) or microsatellite instability-high (MSI-H). Prior data from the CCTG CO.26 randomized phase 2 trial demonstrated doublet ICI with durvalumab + tremelimumab improved OS in refractory mCRC; however, a phase 3 trial was not undertaken. Botensilimab (BOT) is an Fc-enhanced multifunctional anti–CTLA-4 antibody designed to increase efficacy in poorly immunogenic cancers. The combination of BOT with the anti–PD-1 antibody balstilimab (BAL) has shown impressive objective response rates (ORRs), durations of response (DOR), and survival in heavily treated non–MSI-H/dMMR mCRC across phase 1 and 2 studies. The CO.33/BATTMAN trial will evaluate BOT + BAL + BSC versus BSC alone in patients with refractory mCRC that is not MSI-H/dMMR. Methods: BATTMAN (NCT07152821) is an international multi-centre, open-label randomized phase 3 trial that will be conducted in Canada (CCTG), Australia/New Zealand (AGITG) and France (Unicancer). Eligible patients will be ≥18 years old, have received and progressed on or been intolerant of all available therapies, have an Eastern Cooperative Oncology Group performance status (ECOG PS) of 0/1, have RECIST 1.1 measurable/evaluable disease, have a life expectancy of ≥12 weeks, and have adequate end organ function without concurrent illness contraindicating ICI. Exclusion criteria include: prior organ transplant or primary immunodeficiency, ongoing use of non-physiologic corticosteroid dosing, autoimmune disorders requiring ongoing management, and prior ICI exposure. The trial’s statistical plan will ascertain efficacy of BOT and BAL in all comers, as well as key subgroups based on the presence or absence of liver metastases. A planned 834 patients will be randomized 1:1 to BOT (75 mg IV every 6 weeks up to 4 doses) + BAL (450 mg IV every 3 weeks until progression) + BSC or BSC alone, with stratification for region of recruitment, ECOG PS, and presence of liver metastases. The primary endpoint is OS, with secondary endpoints including progression-free survival, ORR, clinical benefit rate, quality of life, safety, and toxicity, with correlative studies and economic evaluations planned. Support for this study is provided by: The Canadian Cancer Society and Agenus Inc. Clinical trial information: NCT07152821 .

Background: Peptide receptor radionuclide therapy (PRRT) is an established treatment for somatostatin receptor (SSTR)-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs), with eligibility determined by the Krenning score. While SSTR PET provides greater sensitivity than octreotide scintigraphy, its impact on PRRT eligibility remains unclear. We compared Krenning scores between [111In]octreotide scintigraphy and [68Ga]DOTATOC PET and assessed eligibility under NETTER-1 and NETTER-2 criteria. Methods: We retrospectively assessed tumor Krenning scores in 115 patients with well-differentiated GEP-NETs who underwent both [111In]octreotide scintigraphy and [68Ga]DOTATOC PET. PRRT eligibility was defined according to NETTER-1 (Krenning ≥2) and NETTER-2 (Krenning ≥3) criteria. Logistic regression identified predictors of Krenning ≥3 on SSTR PET. Quantitative PET analysis was performed for patients with positive baseline [68Ga]DOTATOC PET scans. Results: Median Krenning scores were higher with SSTR PET than planar imaging (3 [IQR 3–4] vs. 2 [IQR 0–3], p < 10−8). NETTER-1 eligibility increased from 59.1% with planar imaging to 81.7% with PET (p < 10−3), and NETTER-2 from 48.7% to 76.5% (p < 10−4). Among patients eligible on planar imaging, 95.6% (NETTER-1) and 98.2% (NETTER-2) had concordant PET findings. Conversely, 61.7% (NETTER-1) and 55.9% (NETTER-2) of patients ineligible on planar imaging were reclassified as eligible with PET. Metastatic disease was associated with higher SSTR PET uptake (odds ratio 9.5 [95% confidence interval: 2.7–32.8], p < 10−3). SUVmax discriminated patients with Krenning ≥3 using an optimal threshold of 9.2 (AUC 0.93). Conclusions: [68Ga]DOTATOC PET yields higher Krenning scores and increases PRRT eligibility compared with [111In]octreotide scintigraphy. PET may be unnecessary for clearly positive planar studies but identifies additional candidates with borderline or negative planar results.

3629 Background: Circulating tumor DNA (ctDNA) detection of molecular residual disease (MRD) is prognostic in colorectal cancer (CRC) but missed recurrences highlight constraints in assay performance. VICTORI is a prospective observational study investigating an ultrasensitive MRD assay (NeXT Personal) in CRC patients undergoing curative treatment. Methods: Whole-genome informed panels of ~1,800 variants interrogated plasma collected pre-curative loco-regional treatment, q2 weeks post-surgery (weeks 2–8, landmark window), and subsequently every 3 months (3-36 months, surveillance window). Results: 795 plasma samples from 109 patients were analyzed, with median follow-up of 21.1 months and 31 recurrences observed. Cohort characteristics: 61 (56.0%) rectal cancer, 48 (44.0%) colon cancer; 81 (74.3%) stage I-III; 56 (50.5%) had neoadjuvant treatment; 106 (97.2%) had surgery as curative procedure. Pre-surgical sensitivity (treatment-naïve) was 96.0% (n=48/50) and specificity was 100% (no detection in pathologic complete responders; 0/10). Landmark window overall sensitivity for recurrence was 83.9% (n=26/31) with 69.2% (18/26) of detections being <100 ppm. Sensitivity was similar weeks 4–8 (81.5% week 4 [n=22/27]; 82.1% week 6 [23/28]; 85.7% week 8 [24/28]), and lower at week 2 (52.4% [11/21]) when post-surgical cell-free DNA peaked (median 2.80 ng/mL pre-surgery to 9.40 at week 2; p=8.7×10⁻¹²). Sampling between weeks 4-8 yielded a sensitivity of 80.0% (n=24/30) at the first available timepoint and 89.7% (26/29) with incorporation of an additional subsequent sample. Detection of ctDNA at any landmark window timepoint was associated with inferior outcomes (adjusted HR [aHR]: 10.28 [95% CI: 4.03-26.23], p =1.06x10 -06 ). Week-4 detection <100 ppm was prognostic and conferred a 6.5 month longer lead time than detections ≥100ppm (HR 4.09, p=0.018). All patients who recurred with surveillance samples had ctDNA detected prior to recurrence (sensitivity: 100% (n=28/28). ctDNA identified 100% of lung- and liver-only metastases; most initial detections were <100 ppm (75% and 71.4%, respectively). In evaluable patients, lead time from ctDNA detection to recurrence was 176 days (including patients with expedited imaging upon receipt of a ctDNA detection result). If a higher threshold for ctDNA was used (≥100 ppm) in the same population, the lead time would be >two months shorter (105 days, p<0.001). All patients without ctDNA detected on surveillance remain disease free (NPV 100% [n=63/63]). Conclusions: NeXT Personal demonstrated high accuracy, prognostic value, and early detection at ultrasensitive levels with no loss in specificity. The ideal postoperative landmark time point for MRD testing is at 4-8 weeks. Our results show strong clinical potential for an ultrasensitive MRD assay for landmark MRD and longitudinal surveillance in CRC.

Background Neuroendocrine tumours (NETs) are uncommon cancers that secrete vasoactive hormones. Around half of those with metastatic small bowel NETs develop carcinoid syndrome, and historically, up to 50% develop carcinoid heart disease (CHD). We evaluated clinical characteristics, echocardiographic features, and outcomes of patients with NETs and CHD in a contemporary cohort to identify factors associated with CHD development and mortality. Methods We identified patients with NETs who underwent an echocardiogram at a tertiary cancer centre–affiliated laboratory (2013-2023). Patients were classified based on whether they developed CHD. Clinical characteristics, echocardiographic data within 6 months of NET diagnosis, treatments, and outcomes were compared between CHD and non-CHD patients. Results Over a 10-year period, 87 patients with NETs were identified (48% female; median age, 61 years), with 18 (21%) developing CHD. Primary small bowel NETs were more common with CHD (83% vs 55%, P = 0.032). Patients with CHD exclusively had right-sided valve involvement: 100% tricuspid, 61% pulmonary, and 0% mitral/aortic; 17% had a patent foramen ovale. Compared to non-CHD patients, CHD patients demonstrated higher right atrial volume index and right ventricular basal diameter, with more frequent right ventricular dysfunction. Patients with CHD had a lower median survival from the time of NET diagnosis (6.6 vs 13.4 years; P = 0.007). Among CHD patients, primary NET resection was associated with improved survival (P = 0.006). Conclusions Within a contemporary cohort, approximately 1 in 5 patients with NETs develop CHD and exhibit only right-sided valve involvement. CHD was associated with increased mortality whereas primary NET resection was associated with improved survival.

3585 Background: Intratumor heterogeneity (ITH) drives CRC resistance, yet tissue-based assessments remain limited. ctDNA captures heterogeneity, though variable tumor fractions confound measurements. We assessed raw vs fraction-adjusted ITH in ctDNA for overall survival (OS) in metastatic CRC (mCRC). Methods: We analyzed two real-world mCRC cohorts from MD Anderson: a 73-gene panel (1,137 patients, 2014-2023; limit of detection (LoD) 0.1%) and a 701-gene panel (150 patients, 2024-2025; LoD 0.5%). Quality filters included ≥ 3 detected variants, MaxVAF > 0.01, and bioinformatic filtering of putative germline variants. ITH was quantified via raw Mutant-Allele Tumor Heterogeneity (rMATH; VAF distribution width) and the Shannon entropy-based, MaxVAF-normalized (VAF/Max_VAF) blood ITH (bITH; multiplies bin entropy by clonality weights). OS was assessed using Cox models by continuous variables and optimal cut-points determined by maximally selected log-rank statistics. A stress test subset (top 50% MaxVAF) was analyzed to decouple findings from analytical noise inherent at low tumor fractions. Results: In the 73-gene cohort (median age 53.2; 91% MSS, 14% BRAF mutated, 64% RAS mutated), MaxVAF was the strongest predictor of worse OS in both univariable (HR 1.32, p < 0.001) and multivariable (MV) models (HR 1.31, p < 0.001). Paradoxically, high rMATH was associated with improved OS (MV HR 0.89, p = 0.014). This protective signal of high heterogeneity was more pronounced in the dichotomized model (MV HR 0.71, p < 0.001) and persisted in the subset of high-burden patients (top 50% MaxVAF) (MV HR 0.69, p < 0.001) and the MSS subset (MV HR 0.68, p = 0.003). In the 701-gene panel cohort, MaxVAF remained the dominant independent predictor (MV HR 2.37, p < 0.001). bITH was not independently prognostic (MV HR 0.98, NS). Conclusions: MaxVAF serves as the primary ctDNA-derived driver of clinical outcomes in mCRC, confirming that ctDNA-inferred ITH metrics are heavily confounded by total tumor shedding. The unexpected association between high rMATH and improved OS suggests that increased clonal diversity, at a fixed tumor burden, may indicate a less evolutionarily pruned or consolidated tumor compared to those that have undergone a selective clonal sweep. These results highlight that while ctDNA can characterize clonal architecture, the clinical interpretation of heterogeneity must be contextualized within the tumor’s total systemic volume and evolutionary state. Cohort Variable (High vs Low) Multivariable HR (95% CI)* p-value 73-gene panel MaxVAF 1.31 (1.20–1.43) <0.001 Raw MATH 0.89 (0.81–0.98) 0.014 bITH 0.98 (0.90–1.08) 0.716 701-gene panel MaxVAF 2.37 (1.52–3.71) <0.001 Raw MATH 0.85 (0.50–1.44) 0.536 bITH 1.23 (0.82–1.87) 0.320 *HR per 1 SD increase. Raw MATH was adjusted for MaxVAF, while bITH was tested without redundant MaxVAF adjustment. All models adjusted for age, sex, MSI, BRAF, and RAS status.

The CCTG PA.7 study was a randomized phase II trial comparing chemotherapy with and without dual immune checkpoint inhibition in patients with metastatic pancreatic ductal adenocarcinoma (mPDAC). In follow-up to the published primary results of the trial, the analysis herein focused on long-term survival and exploratory analysis.Plasma sequencing analysis identified concurrent mutations in DNA damage repair genes BRCA1, POLE, ATM and FANCA in 18/173 (10.40%) of patients, and presence of two or more mutations in these genes was associated with overall survival benefit for patients receiving immunotherapy (median overall survival 26.2 vs. 9.7 months; hazard ratio, 0.34; 95% CI, 0.16-0.68; p = 0.001; interaction p = 0.003). Partial response was observed in 7/11 (63.64%) patients with concurrent DNA damage repair gene mutations in the immunotherapy arm. As a prospective study in PDAC identifying a potential biomarker beyond mismatch repair deficiency for benefit from immunotherapy, these data highlight an actionable subgroup of mPDAC.

BACKGROUND:Surrogate endpoints are increasingly used as the basis of cancer drug approvals. A strong association between treatment effects on the surrogate and true endpoint is essential for their validation as reliable endpoints. We assess the current evidence of trial-level surrogacy for colorectal cancer (CRC) endpoints through a novel framework. METHODS:We developed a framework to grade the quality of oncology surrogate endpoints in 7 domains of the surrogate relationship: data source quality (A-B); disease setting homogeneity (0-3); uniformness of surrogate and true endpoint definitions (0-3); number of trials evaluated (0-4); and individual- (0-2) and trial-level associations (0-8). We screened PubMed using keywords and medical subject headings terms for meta-analyses published before July 6, 2024, validating surrogate endpoint associations across CRC trials. Two reviewers blindly assessed each article before reaching a consensus score. The main outcomes were framework-derived evidence scores and reported trial-level associations (R2) with overall survival. RESULTS:A total of 18 articles were identified, containing 39 evaluations of 12 unique surrogate endpoints for overall survival. Evidence scores ranged from B6 to A23. Only disease-free survival consistently demonstrated high-quality evidence of trial-level surrogacy (≥A18, median R2 = 0.92). Progression-free survival and objective response rate were low-quality surrogates in metastatic CRC (progression-free survival: B6-A15, median R2 = 0.55; objective response rate: B6-A13, median R2 = 0.33). Other evaluated endpoints were demonstrated to be unreliable surrogates. CONCLUSION:Disease-free survival shows high-quality surrogacy in stage II-III colon cancer. Endpoints identified in the metastatic setting demonstrate inconsistent surrogacy evidence. Reassessment and identification of new endpoints are warranted, particularly with immunotherapy.

101 Background: Mismatch repair deficiency (dMMR) or microsatellite instability-high (MSI-H) status predicts response to immune checkpoint inhibitors (ICIs) in mCRC. In the landmark KEYNOTE177 study, a mPFS of 16.5 months was observed with pembrolizumab, with a 29% progressive disease (PD) rate and 14% treatment discontinuation rate. In this study, we evaluated real-world efficacy and safety of single-agent pembrolizumab in dMMR/MSI-H mCRC and explored patient and tumor factors associated with outcomes. Methods: We conducted a retrospective study of patients (pts) with dMMR/MSI-H mCRC treated with pembrolizumab between 02/2022-11/2024. BC Cancer is the sole funder of oncology drugs across the province, treated pts were identified through the pharmacy database. Demographics, tumor characteristics, and treatment patterns were collected. Descriptive statistics were used for baseline characteristics. Progression-free (PFS) and overall survival (OS) were estimated using Kaplan–Meier methods and compared by log-rank tests. Cox proportional hazards models were used to identify prognostic features. Results: Among the 129 pts included, median age at diagnosis was 71 years (24-88) and 57% were female. BRAF and RAS mutations were identified in 53% and 29% of patients, respectively. The primary tumor was more frequently right-sided (53%). Common metastatic sites included non-regional lymph nodes (37%), liver (33%), and peritoneum (30%). Median PFS was 19.3 months (95% CI, 11.9–NE). Estimated PFS at 3 and 24 months was 80% (95% CI, 73–87) and 49% (95% CI, 40–60), respectively. 14% of patients experienced progression at first restaging. Median OS was not reached (95% CI, 22.4 months–NE), with estimated OS at 3 and 24 months of 87% (95% CI, 82–93) and 57% (95% CI, 47–68). Treatment-related toxicity occurred in 65% of pts; 16% required hospitalization and 16% discontinued treatment. The most frequent toxicities were dermatitis (13%), fatigue (12%), colitis (10%), and hypothyroidism (5%). On multivariable analysis, older age (HR 1.05, 95%CI: 1.02-1.08, p<0.001, per year) and the presence of liver metastasis (HR 2.03, 95%CI: 1.02 – 4.03, p = 0.043) were associated with inferior PFS, whereas the occurrence of any grade of treatment-related toxicity was associated with improved outcomes (HR 0.19, 95%CI – 0.09 – 0.40), p<0.001). Conclusions: In this population-based cohort, pembrolizumab demonstrated durable efficacy and manageable safety, comparable or better than that seen in KEYNOTE 177 with a mPFS of 19.3 mo and 16% discontinuation rate. A primary PD rate of 14% was lower than expected. Age, ECOG performance status, and liver metastases were significant prognostic factors, while development of treatment-related toxicity was associated with improved survival. Now with the availability of dual IO, these factors may help inform IO treatment strategy in the 1L for dMMR/MSI-H mCRC.

IntroductionThe long-acting somatostatin analogues (LA-SSAs) octreotide LAR (OCT) and lanreotide (LAN) improve progression-free survival (PFS) in gastrointestinal neuroendocrine tumors (NETs), however, no head-to-head comparison exists. We compared treatment patterns and efficacy in a small bowel and pancreatic NET population-based cohort from British Columbia, Canada.MethodsWe identified 321 patients receiving either LAN or OCT for retrospective chart review. These somatostatin analogs were evaluated for impact on progression-free and overall survival.ResultsAge, sex, ECOG, and primary site did not differ by treatment, however, LAN was more commonly used in higher grade tumors (P = 0.019). PFS was longer for patients receiving LAN than OCT (Hazard Ratio (HR) 0.60, 95% CI 0.40-0.89, P = 0.011). Similarly, overall survival (OS) was longer for patients receiving LAN than OCT (HR 0.45, 95% CI 0.28-0.73, P = 0.016). Sensitivity analysis among patients diagnosed after both agents were reimbursed showed similar results for PFS (HR 0.50, 95% CI 0.28-0.90, P = 0.018). There was similar dose escalation with LAN vs OCT (OR: 0.80, CI 0.38-1.77, P = 0.70), with 29.4% of patients in the LAN group requiring LA-SSA dose escalation compared to 34.3% in the OCT group. There was numerically less short acting octreotide use in the LAN group (P = 0.087), with none of these patients requiring short acting octreotide, compared to 8.7% of the OCT group.ConclusionLAN was associated with longer time to cancer progression, as well as less use of short acting rescue octreotide in our population-based cohort. However, given the retrospective design and reimbursement-era differences, these findings should be interpreted cautiously and warrant confirmation in prospective or head-to-head studies.

BACKGROUND:Peptide receptor radionuclide therapy (PRRT) is an established treatment for somatostatin receptor (SSTR)-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs), with eligibility determined by the Krenning score. While SSTR PET provides greater sensitivity than octreotide scintigraphy, its impact on PRRT eligibility remains unclear. We compared Krenning scores between [111In]octreotide scintigraphy and [68Ga]DOTATOC PET and assessed eligibility under NETTER-1 and NETTER-2 criteria. METHODS:We retrospectively assessed tumor Krenning scores in 115 patients with well-differentiated GEP-NETs who underwent both [111In]octreotide scintigraphy and [68Ga]DOTATOC PET. PRRT eligibility was defined according to NETTER-1 (Krenning ≥2) and NETTER-2 (Krenning ≥3) criteria. Logistic regression identified predictors of Krenning ≥3 on SSTR PET. Quantitative PET analysis was performed for patients with positive baseline [68Ga]DOTATOC PET scans. RESULTS:Median Krenning scores were higher with SSTR PET than planar imaging (3 [IQR 3-4] vs. 2 [IQR 0-3], p < 10-8). NETTER-1 eligibility increased from 59.1% with planar imaging to 81.7% with PET (p < 10-3), and NETTER-2 from 48.7% to 76.5% (p < 10-4). Among patients eligible on planar imaging, 95.6% (NETTER-1) and 98.2% (NETTER-2) had concordant PET findings. Conversely, 61.7% (NETTER-1) and 55.9% (NETTER-2) of patients ineligible on planar imaging were reclassified as eligible with PET. Metastatic disease was associated with higher SSTR PET uptake (odds ratio 9.5 [95% confidence interval: 2.7-32.8], p < 10-3). SUVmax discriminated patients with Krenning ≥3 using an optimal threshold of 9.2 (AUC 0.93). CONCLUSIONS:[68Ga]DOTATOC PET yields higher Krenning scores and increases PRRT eligibility compared with [111In]octreotide scintigraphy. PET may be unnecessary for clearly positive planar studies but identifies additional candidates with borderline or negative planar results.

PURPOSE:Pre-existing and emerging molecular targeted therapies have been identified for the treatment of pancreatic ductal adenocarcinoma (PDAC) and exemplify the need for incorporating tumor sequencing into routine clinical practice. Tumor profiling through circulating tumor DNA (ctDNA) represents an important opportunity in PDAC because of the aggressive nature of the disease. Circulating tumor fraction (CTF) levels are highly variable within and between cancer types and influence the accuracy of plasma-based tumor profiling, and the variability and factors related to CTF levels in PDAC are not well understood. EXPERIMENTAL DESIGN:ctDNA sequencing (PredicineATLAS) and clinical metadata from a cohort of 166 patients with metastatic PDAC (mPDAC) were generated as part of the PA.7 trial (NCT02879318). Patients were stratified into high CTF (>30% CTF; 35/166, 21.08%) and low CTF (131/166, 78.92%) groups for comparative analysis. Matched RNA sequencing data were available for 20 patients. RESULTS:High CTF was associated with lower overall survival (hazard ratio = 1.87; 95% confidence interval, 1.27-2.75; P = 0.0014) as well as clinical presentation that was indicative of higher disease burden, with CTF highest in patients that had liver metastases and distant metastases including bulky lymph nodes versus patients with no liver metastases (P < 0.001). Exploratory gene expression analysis revealed a positive association between CTF and upregulation of cell cycle-related pathways, which included those involving CDK4 (P = 0.0093) and MT2A (P = 0.012), as well as glycolytic (P = 0.0028) and basal-like (P = 0.029) subtyping genes. CONCLUSIONS:These data demonstrate the heterogeneity of CTF and its associated factors in mPDAC, which converge toward an aggressive phenotype from both clinical and molecular standpoints.