Citations

9523 Background: Primary Central Nervous System (CNS) Melanoma is a rare and aggressive malignancy, comprising 1% of melanoma cases and 0.07% of brain tumors. Due to its rarity, its natural history, molecular features and optimal management remain poorly defined. Methods: We retrospectively reviewed patients at our institution between January 1998 and October 2025 with primary CNS melanoma or melanocytoma with melanoma transformation. Clinical characteristics, molecular profiling, treatments and survival outcomes were collected. Overall survival (OS) and progression free survival (PFS) were estimated using Kaplan-Meier curves. Results: Nineteen patients were identified, including eight (42%) with melanocytoma to melanoma transformation. Median time of transformation was 34.4 months. Thirteen (68%) patients were female with a median age at diagnosis of 59.3 years. Common presenting symptoms were sensory disturbances (58%), pain (53%), motor deficits (37%) and nausea or vomiting (32%). At diagnosis, location of disease was spinal (n=12), intracranial (n=6) and multifocal (n=1). Thirteen patients had leptomeningeal involvement. Molecular profiling was performed in 15 patients, revealing mutations in GNAQ (n=7), GNA11 (n=3), CKIT (n=2), BAP1 (n=1), EIF1AX (n=1), and BRAF v600 (n=1). Thirteen (68%) patients underwent subtotal resection (STR), five (26%) gross total resection (GTR) and one radiation only. Median resected tumor size was 1.7 cm. Seventeen (94%) patients had recurrence at a median of 9.1 months, and four patients underwent repeat surgery. Fourteen (74%) patients received adjuvant radiation and 14 (74%) received immunotherapy, most commonly ipilimumab with nivolumab (71%). Nine patients received chemotherapy: temozolomide (n=5), bevacizumab (n=2) and one each of intrathecal IL-2, tebentafusp, imatinib and encorafenib with binimetininb. Four patients received chemotherapy before immunotherapy. Median OS from diagnosis of CNS melanocytoma or melanoma was 50.9 months (13.4 – 80). Median OS from time of transformation to melanoma or CNS melanoma diagnosis was 24.0 months (11.9 – NA). One-, three- and five-year OS rates were 73%, 53% and 40% respectively. GTR was associated with increased median OS compared to STR (80.0 vs 19.7 months, p= 0.02). Age at diagnosis, immunotherapy, chemotherapy, adjuvant radiation, STR with or without radiation, disease location, leptomeningeal involvement, tumor size >3cm and presence of GNAQ mutation were not associated with survival. Median PFS was 3 months with chemotherapy and 5.7 months with immunotherapy. Conclusions: To our knowledge, this is the largest single center study of primary CNS melanoma. Outcomes remain poor, with survival primarily associated with GTR. The predominance of GNAQ and GNA11 alterations with rare BRAF v600 mutations highlight the distinct biology of primary CNS melanoma and need for novel targeted therapies.

9030 Background: AI use is ubiquitous among HOT and healthcare workers, but with variable base knowledge and comfort level. Its indiscriminate use may be problematic. To address this educational gap, we developed a WS for HOT to assess their use of AI and educate them on how to employ it in MedEd and patient-centered CP based on LSP. Methods: A faculty member developed a WS on how to use AI in MedEd and CP based on LSP of analogy, contrasting cases, elaboration, generation, and question driven learning. The WS was for HOT, but faculty could join. It had 3 sections: introduction to basic concepts of AI; principles of the Health Insurance Portability and Accountability Act and protected health information; AI in MedEd; and AI in CP. Sections had real use case examples of AI, for participants to interact, ask questions, and share their personal use cases. HOT were invited to compare and generate examples during the WS, based on their educational and clinical experience and needs. Participants’ AI use and self-perceived and objective knowledge about it were assessed in a pre activity survey, a post WS survey, and a post 8 week survey to evaluate long term learning. Results: A 2 hour WS with 9 HOT and 2 faculty was completed successfully. 10 participants completed the pre WS survey, and 11 completed the post WS and the post 8 week surveys. 100% of them used generative AI tools before the WS, while 10% had prior training in AI. 90% used AI in personal life and MedEd, 70% used it in CP, and 30% in academic research. Table 1 shows participants’ self-assessed knowledge of AI, comfort level in using AI for MedEd and CP, self-assessed preparedness to implement AI in MedEd and CP, and objective knowledge. 8 weeks post WS, 91% of participants reported increased AI use in CP, leading anywhere from slight to significant decreases in time spent on documentation tasks for 91% of them. In MedEd, 73% reported increased AI use, and 82% described slight to significant improvement in the quality of time dedicated to it. 82% described the WS as extremely relevant to their daily responsibilities, and 91% reported confidence navigating ethical and privacy risks of AI in HemOnc. All participants were interested in having a similar WS in the future. Conclusions: An LSP based WS about AI increased the self-perceived and objective knowledge of HOT about using this tool in MedEd and CP. It also increased HOT comfort level, preparedness, and efficiency using AI. With the ubiquitous use of AI by HOT, ongoing education about it focused on supporting high-quality patient care should be a part of fellowship curriculum. Pre WS % Post WS % Post 8 week % Moderately/Very knowledgeable about AI 50 91 100 Comfortable using AI in MedEd 60 82 82 Prepared to implement AI in MedEd 30 91 73 Comfortable using AI in CP 40 73 82 Prepared to implement AI in CP 0 91 73 Correct answers on objective assessment 53 68 71

6028 Background: RET alterations are targetable oncogenic drivers in multiple solid tumors and TCs. Pralsetinib is an oral, potent, selective RET inhibitor granted accelerated approval by the FDA for adults and children aged ≥12 y with advanced/metastatic RET fusion-positive TC who require systemic therapy and are radioactive iodine-refractory, based on ARROW trial (NCT03037385) results. Accelerated approval for medullary TC (MTC) was withdrawn by the sponsor in 2023 due to recruitment challenges in the confirmatory trial. We report final efficacy and safety in ARROW patients with RET -altered TC and MTC. Methods: Phase 2 ARROW patients were enrolled from 84 sites in 13 countries. Primary end points were overall response rate (ORR; per RECIST v1.1) and safety. Progression-free survival (PFS), overall survival (OS), and safety were assessed in patients with advanced/metastatic TC and MTC who received ≥1 dose of pralsetinib (Efficacy Population [EP]). ORR and duration of response (DOR) were assessed in the Measurable Disease Population (MDP). Results: At the May 20, 2024, data lock, 28 patients with RET -altered TC and prior systemic treatment and 145 patients with RET -altered MTC received pralsetinib 400 mg/d (median treatment duration: 24.7 mo [TC]; and 35.4 mo [MTC]). In the TC and MTC groups, median age was 58 and 57 y; 39% and 64% were male; median (range) prior lines of treatment was 2 (1, 9) and 1 (1, 6). 53.8% of MTC patients had prior systemic treatment. In TC and MTC patients, ORRs (95% CI) were 91.7% (73.0, 99.0) and 68.2% (59.5, 76.0) (Table). Treatment-related adverse events (TRAEs) occurred in 27 (96%) TC and 142 (98%) MTC patients; 68% and 67% had grade ≥3. TRAEs in ≥35% of TC and MTC patients were increased AST (50% and 38%) and ALT (43% and 30%), decreased white blood cell count (39% and 30%), and anemia (39% each). Death due to a TRAE occurred in 1 TC patient (liver injury) and 1 MTC patient (pneumonia). Safety was consistent with prior reports. Conclusions: The final analysis of ARROW confirms that pralsetinib yields clinically meaningful and durable responses in patients with RET -altered TC and MTC with a manageable safety profile consistent with prior reports. Clinical trial information: NCT03037385 . Efficacy summary. All MTC MTCPrior Cabozantinib/Vandetanib MTC Treatment-Naïve TCPrior Systemic Treatment MDP, n 132 60 62 24 ORR, % (95% CI) 68.2(59.5, 76.0) 56.7(43.2, 69.4) 79.0(66.8, 88.3) 91.7(73.0, 99.0) Complete response, n (%) 12 (9.1) 2 (3.3) 7 (11.3) 4 (16.7) Partial response, n (%) 78 (59.1) 32 (53.3) 42 (67.7) 18 (75.0) DOR, median, mo (95% CI) 39.6(29.4, NE) 21.7(15.1, 34.8) NR(36.8, NE) NR(16.0, NE) DOR follow-up, median, mo (95% CI) 45.7(41.0, 48.4) 48.9(36.9, 58.7) 45.1(40.4, 47.8) 35.4(26.9, 40.0) EP, n 145 67 67 28 OS, median, mo (95% CI) NR(54.6, NE) 42.2(31.2, NE) NR(NE, NE) NR(25.4, NE) PFS, median, mo (95% CI) 37.2(27.5, 55.5) 24.9(19.9, 35.0) 55.3(36.8, NE) NR(14.7, NE) NE, not evaluable; NR, not reached.

560 Background: Patients (pts) with early stage hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer (BC) remain at both short and long term risk of recurrence. While clinico-pathological and gene-based biomarkers provide insight into early and late recurrence risk, additional prognostic tools are needed. We previously developed (Saha et al. Clin Cancer Res , 2025) a new breast MRI image-based artificial intelligence prognostic tool (TumorSight Risk) composed of both radiographic and clinical information. Here we sought to validate this tool in an independent cohort. Methods: In this multi-center, blinded, prospective/retrospective validation study, we sought to validate the 5-year recurrence-free survival (RFS) according to pre-defined risk scores (0-100) and categories (“high” or “low”) via pre-specified cutoff (≥47) for pts with HR+/HER2- BC. Kaplan-Meier and Cox regression analyses were then used to assess the unadjusted and adjusted association between risk category/score and the 5-year RFS. Results: 2,129 pts with HR+/HER2- BC diagnosed between January 1, 2010, and December 31, 2024, who underwent breast MRI prior to surgery (no neoadjuvant therapy) were included from four institutions. Median age was 57 years, 16% were Black, 15% had grade 3 disease, 93% were clinical T1 or T2, and 85%/15% were clinical N0/N1. At a median follow up of 6.24 years, the unadjusted hazard ratio (HR) for RFS comparing low (n=1808) and high risk (n=321) pts was 2.71 (95% CI = 1.95 – 3.76, p = 3x10 -9 ). The estimated 5 year RFS rate according to risk score (low and high) was 95.5% (95% CI: 94.63 – 96.3%) and 89.6% (95% CI = 86.4 – 92.6%) while the 10 year RFS estimates were 88.9% (95% CI = 86.5 - 90.9%) and 69.6% (95% CI = 59.1 – 77.9%). Univariate subgroup analysis demonstrated that the risk score was prognostic by age (< 50 vs. ≥50 years), pathologic nodal status, grade, and surgery type, regardless of chemo-, endocrine, or radiation therapy administration. The observed continuous risk score was strongly associated with risk of recurrence, with a 22% increase in relative risk per 10-unit change in risk score (0-100), translating to a 2.4% (95% CI = 1.2 – 4.5%) and 25.5% (95% CI = 15.4 – 40.4%) risk of recurrence at 5-years for pts at the lowest (0-9) versus highest (90-100) risk score. The prognostic association between risk score and RFS remained after adjusting for pathologic T- and N-stage, grade, age, race, and treatment (p = 0.002). Conclusions: TumorSight Risk is a new image-based biomarker for HR+/HER2- BC that provides prognostic information independent of standard clinico-pathological parameters. Given the ready availability of breast MRI in most pts with BC, TumorSight Risk may be a convenient tool for estimating risk for recurrence.

5068 Background: 177 Lu-PSMA-617 (LuPSMA) is a groundbreaking therapy for mPC, but biomarkers are urgently needed to improve patient selection. Plasma-derived EVs are emerging blood-based markers of tumor burden. Here, we analyzed levels of EVs of 123 patients (pts) from 4 cohorts: a pre- and during LuPSMA treatment for a contemporary prospective Mayo Clinic cohort, pre-treatment plasma levels of EVs for a historical Tulane cohort, localized prostate cancer (LPC) after radical prostatectomy (RP), and male healthy donors (HD). Methods: Blood samples were collected from 51 pts pre-cycles (pre-C) 1-3 of LuPSMA at Mayo Clinic from 12/2024-12/2025 and 26 pts pre-C1 at Tulane Cancer Center between 12/2018-1/2025. Small and large EVs were incubated with fluorescently conjugated antibodies (PSMA, STEAP1, CEACAM5, and TROP2) and enumerated using nanoscale flow cytometry. EVs from blood samples of 10 HD and 36 LPC post-RP from 6/2020-8/2024 at Mayo Clinic as controls. AUC-ROC and logistic regression analyses were performed to identify marker signatures associated with PSA50 response. Results: Baseline demographics are shown in the table. Median levels of PSMA, STEAP1, CEACAM5, and TROP2 EV/milliliter (mL) were all significantly higher for pts with mPC (pooled Tulane and Mayo cohorts) compared to LPC post-RP (p<0.001) and HD (p<0.001). In the Mayo cohort, 10 of 51 pts (20%) had undetectable PSA pre-C1. There were no significant differences in median pre-C1 plasma EV levels for these 4 different types of EVs comparing pts with undetectable vs detectable PSA. For 41 Mayo pts with detectable PSA pre-C1, 20 achieved PSA50 response at time of data cutoff (49%), while 7 pts (17%) had persistent PSA rise on LuPSMA. For pts who achieved PSA50, there was a significant decline in both CEACAM5 and TROP2 EVs/mL pre-C1 compared to pre-C3 (19 x 10 6 vs 5.8 x 10 6 , p=0.003 and 16 x 10 6 vs 5.5 x 10 6 , p<0.001, respectively), while no significant changes occurred for the 7 pts with no PSA response. A composite score of pre-C1 CEACAM5 and TROP2 EVs (large- and small-sized EVs) demonstrated the strongest association with PSA50 response (OR=2.96, 95% CI: 1.27-6.44, p=0.038), achieving AUC of 0.73. PSMA and STEAP1 EVs did not show any significant changes from pre-C1 to pre-C3. Conclusions: Plasma-derived EVs are a promising biomarker to identify patients who will benefit from LuPSMA. Longer follow-up and enrolling a larger cohort are underway. Clinical trial information: NCT06200103 . Median values for pre-LuPSMA (Mayo and Tulane), post-RP for LPC, and HD. Mayo mPC (n=51) Tulane mPC (n=26) LPC (n=36) HD (n=10) Age (range) 73 (57-87) 71 (54-83) 66 (56-77) 55 (50-57) Prior lines (range) 1 (1-3) 4 (1-11) - - Gleason score 9 8 7 - PSA (range) 1.0 (0-182) 75 (0.8-503) - - PSMA EV/mL 3.7 x 10 6 14 x 10 6 2.0 x 10 6 1.4 x 10 6 STEAP1 EV/mL 30 x 10 6 45 x 10 6 12 x 10 6 12 x 10 6 CEACAM5 EV/mL 17 x 10 6 5.1 x 10 6 3.7 x 10 6 3.9 x 10 6 TROP2 EV/mL 13 x 10 6 5.2 x 10 6 4.5 x 10 6 3.6 x 10 6

5005 Background: Androgen receptor pathway inhibitors (ARPIs) may have distinct central nervous system (CNS) penetration and differential effects on cognition or quality of life (QOL). AFT-47, “ARACOG” is a prospective, randomized open-label phase 2 trial comparing objective and patient-reported cognitive and QOL outcomes between patients with metastatic hormone sensitive prostate cancer (mHSPC), metastatic castration-resistant prostate cancer (mCRPC) or non-metastatic castration-resistant prostate cancer (nmCRPC) treated with darolutamide (DAR) or enzalutamide (ENZ). Methods: Between 8/17/2021 and 3/11/2025, 111 pts were enrolled and randomized 1:1 to treatment with ENZ or DAR. CANTAB, a validated computer-based platform evaluating 5 cognitive domains (PALFAM-memory recall, SWM- spatial working memory, SSP-short-term visual memory, working memory, RVP-rapid visual processing, and OTSMCC- executive function), was performed at baseline, 12, 24, and 48 weeks. Crossover (Xover) was allowed at 12 or 24 weeks for significant pre-specified decline in cognitive testing or patient reported outcome measures (PROMs), or for CNS-associated adverse events, including falls. The primary endpoint was the percent change in the maximally changed cognitive domain (MCCD) from baseline to 24 weeks and was compared between groups with the Wilcoxon rank sum statistic. Patients crossing over prior to 24 weeks were evaluated at the time of Xover and analyzed with their randomized arm. Secondary endpoints included comparison of Xover rates. Results: Among 111 pts, 55 received DAR and 56 received ENZ; median age was 71, 83% White. 95 pts (48 DAR, 47 ENZ) were evaluable for the primary endpoint. Baseline CANTAB was similar between groups. Median cognitive change in MCCD between baseline and 24 weeks was -15.8% and -36.1% for the DAR and ENZ pts, respectively (p = 0.009) (Table). Individual cognitive domains suggest possible learning effect (increased scores) for DAR and are stable or show mild decline for ENZ treated patients at 24 weeks. 32 DAR (58%) and 33 ENZ (59%) participants were eligible for Xover by 24 weeks, and 23 pts crossed over by 24 weeks. All Xover was pts randomized to ENZ (crossed to DAR), with the most common causes being decline in objective (N = 14) or subjective (N = 11) cognitive testing. Conclusions: In this US randomized trial comparing DAR to ENZ, the primary endpoint of differential cognitive effects was met. ENZ showed significantly greater cognitive score decline than DAR at 24 weeks in the MCCD after treatment initiation. Learning effect was seen with DAR but not ENZ. Longer term follow-up of cognitive change and PROM analyses are ongoing. Clinical trial information: NCT04335682 . Percent change in maximally changed cognitive domain by treatment. DAR (N=48) ENZ (N=47) Wilcoxon Rank Sum Test Result CANTAB domain PALFAM SWM Degree of change -15.8 (-38.8, -2.6) -36.1 (-59.3, -16.7) P=0.009

4500 Background: BM occur in ~30% of RCC pts and are associated with poor survival and symptomatic skeletal events (SSEs). Radium-223, an alpha-emitting bone-seeking radioisotope, prolongs survival in advanced prostate cancer. A pilot study of radium-223 with VEGF tyrosine kinase inhibitors (TKI) in RCC with BM showed safety and early efficacy (McKay, CCR, 2018). Cabozantinib is a TKI that showed efficacy in BM. RADICAL (NCT04071223) was designed to investigate cabozantinib ± radium-223 in RCC with BM. Methods: This open label multicenter study enrolled RCC pts with ≥1 symptomatic BM not previously irradiated and Karnofsky performance status ≥60%. Prior therapies were allowed; non-clear cell population was limited to 20%. Randomization was 1:1 to cabozantinib with (Arm A) or without (Arm B) radium-223, stratified by osteoclast-targeted therapy (OTT), prior therapy, baseline opioid use, and IMDC risk. In Arm A, cabozantinib was 40 mg daily cycle 1, 60 mg thereafter if tolerated; radium-223 was 1.49 microcurie/kg IV every 28 days for 6 doses. In Arm B, cabozantinib was 60 mg daily. The primary endpoint was SSE-free survival (SSE-FS). Secondary endpoints were safety, objective response rate (ORR), progression-free survival (PFS) and overall survival (OS). The study was designed with 85% power to detect improvement in 6-month SSE-FS from 65% to 78% (one-sided α=0.05), with planned interim futility analysis at 50% of expected events. Results: From 12/2019 to 9/2025, 90 pts were enrolled and evaluable for the interim analysis. Median age was 63 years; 74.4% male, 90.0% White, 83.3% clear cell histology. 11.1% were treatment naïve, 80.0% received OTT, 51.1% had baseline opioid use. IMDC risk was 16.7% favorable, 68.9% intermediate, 14.4% poor. Median cabozantinib dose was 31.4 mg (Arm A) and 40.0 mg (Arm B); median radium-223 cycles was 6. Median follow-up was 13.1 (range 0.1-49.4) months with 50 events, 17 SSEs (Arm A: 10; Arm B: 7) and 33 deaths (Arm A: 13; Arm B: 20). Outcomes are in Table. 71 pts were evaluable for response. ORR was 22.2% (Arm A), 22.9% (Arm B). Any grade treatment-related adverse events were 97.7% (Arm A), 93.2% (Arm B); grade ≥3 were 65.9% (Arm A), 56.8% (Arm B). The trial closed after interim analysis as SSE-FS did not meet the prespecified futility boundary (HR≤1). Conclusions: Radium-223 did not improve the primary endpoint of SSE-FS when added to cabozantinib but a numerical prolongation in OS was observed. Combination treatment was safe. Alternative radiotherapeutics warrant investigation in RCC. Clinical trial information: NCT04071223 . Interim efficacy analysis. Arm Median(95% CI) HR (95% CI) Stratified SSE-FS A 17.9 (15.0-NE) 1.24 (0.62-2.48) B 17.6 (10.9-NE) Unstratified SSE-FS A 17.9 (15.0-NE) 0.90 (0.51-1.59) B 17.6 (10.9-NE) OS A 32.2 (17.9-NE) 0.77 (0.42-1.41) B 21.3 (12.3-NE) Stratified PFS A 11.0 (5.8-17.4) 1.39 (0.72-2.66) B 11.2 (9.0-18.5)

4559 Background: Baseline QOL has been linked to survival in mRCC therapy clinical trials (Cella, 2024); however, there is a paucity of prospectively collected real-world QOL data across front-line management strategies. We used data from ODYSSEY to examine the relationship between baseline QOL and early death. Methods: ODYSSEY is a multi-site, prospective observational study of 468 US pts with mRCC. Pts must have mRCC (any histology); have no prior systemic therapy (ST) or be within 3 months of starting ST; and follow up at a PCORnet study site. Exclusion criteria include treatment for cancer except mRCC. The primary objective is to determine patterns of change in QOL and symptom burden of pts with mRCC. Early death was defined as death within 12 months post-enrollment. All pts with non-missing FKSI-19 or FKSI-DRS at enrollment were included. Cumulative event probabilities as a function of time were stratified by either < or ≥ median QoL value and plotted as a step function vs time since enrollment to create cumulative event curves. To determine association between QoL at enrollment and early death, separate Cox proportional hazards models (unadjusted and adjusted) for time-to-first-event as a function of baseline QoL were fit. The adjusted model controlled for the following baseline covariates: IMDC risk (favorable vs intermediate or poor), histology (clear cell vs non). Results: As of 1/5/26, 468 pts were enrolled; 425 had non-missing FKSI-19 or FKSI-DRS at enrollment (329 immediate ST, 96 deferred ST [DST]). ST included 129 IO-IO, 108 IO-TKI, 92 Other. DST included 57 Active Surveillance, 22 Local Therapy Planned, 17 Other/Missing. 209 and 190 pts have < median FKSI-19 (score = 59) and FKSI-DRS (score = 29), respectively; 216 and 235 pts have ≥ median FKSI-19 and FKSI-DRS, respectively. Those below the median have lower rates of nephrectomy, worse IMDC risk and KPS, shorter time since RCC diagnosis, and higher rates of bone metastasis (all p<0.05). Cumulative event probabilities for death at 12 months are 21% and 7% in those < and ≥ median baseline FKSI-19, respectively; and 21% and 7% in those < and ≥ median baseline FKSI-DRS, respectively. For FKSI-19, the unadjusted hazard ratio (HR) for early death is 0.55 (95% CI: 0.43 – 0.70; p<0.001) and adjusted HR is 0.65 (0.49 – 0.85; p = 0.002) per 1 SD higher score. For FKSI-DRS, the unadjusted HR for early death is 0.57 (95% CI: 0.46 – 0.71; p<0.001) and adjusted HR is 0.66 (0.51 – 0.86; p =0.002) per 1 SD higher score (higher score indicates better QOL). Conclusions: Across management strategies in our ODYSSEY cohort of systemic therapy naïve mRCC patients, better baseline QOL was associated with better survival. This is to our knowledge the largest reported real world mRCC cohort with prospectively collected PROs. Future analyses will focus on association of outcomes with longitudinal PROs. Clinical trial information: NCT04919122 .

4013 Background: Pancreatic ductal adenocarcinoma (PDAC) is driven predominantly by oncogenic KRAS signaling, yet prior attempts to target the MAPK pathway have been limited by toxicity and emergence of resistance. Atebimetinib is a next-generation, deep cyclic inhibitor (DCI) of MEK designed to achieve pulsatile pathway inhibition with improved tolerability. We report a Phase 2a cohort evaluating atebimetinib in combination with modified gemcitabine/nab-paclitaxel (mGnP) in the first-line treatment of advanced or metastatic PDAC (NCT05585320). Methods: In this multicenter, open-label, nonrandomized Phase 2a study, patients with previously untreated advanced or metastatic PDAC were eligible for participation and were treated with orally administered atebimetinib daily in combination with mGnP (modification: every other week dosing). The primary endpoint was objective response rate (ORR) per investigator assessment using RECIST 1.1 criteria in response-evaluable patients. The cohort was powered (~80%, one sided alpha 0.05) using an optimal Simon’s 2-stage design. Results: Fifty-five patients were enrolled and treated with 320 mg atebimetinib daily + mGnP. ECOG performance status was 0-1 (100%); median age was 68 years (range 42-85 years), 62% were ≥65 years, and 55% were male. No grade 5 treatment-related adverse events (AE) were observed. No AE related to atebimetinib were higher than grade 3. Grade 3 AE related to atebimetinib occurred in 29% of participants, with the most frequent being rash (5%), ALT increased (5%), and AST increased (5%). Among the 50 response-evaluable patients, best overall response (BOR) achieved were 21 PR, 21 SD, 7 PD, 1 NE. The ORR was 42%, and DCR was 84%. With a median follow-up of 10.4 months (data cutoff Jan 7, 2026), 6- and 9-month OS rates were 88% and 79%, respectively; median PFS was 8.3 months and median OS was not reached. Conclusions: Atebimetinib in combination with mGnP demonstrated favorable safety and promising efficacy compared to historic standard-of-care (SoC) GnP. 1 A phase 3 registrational trial is planned to start in mid-2026 for evaluation of atebimetinib + mGnP versus SoC GnP. 1. Von Hoff DD, Ervin T, Arena FP, et al. Increased survival in pancreatic cancer with nab-paclitaxel plus gemcitabine. N Engl J Med . 2013;369(18):1691-1703. doi:10.1056/NEJMoa1304369 (PMID: 24131140). Clinical trial information: NCT05585320 .

3516 Background: ICIs produce durable benefit in MSI-H CRC, yet optimal treatment duration and monitoring strategies remain unclear. ctDNA enables minimally invasive assessment of molecular response during and after ICI. We evaluated real-world treatment patterns, outcomes, and longitudinal ctDNA dynamics in pts with MSI-H CRC treated with ICI. Methods: Pts with MSI-H mCRC treated with first-line ICI were identified from Natera’s Real-World Database with linked longitudinal ctDNA results (Signatera, Natera, Inc.) and commercial claims data (Forian’s Hybrid data ecosystem, CHRONOS). Overall survival (OS) was estimated using available mortality data (Veritas Data Research, Fact of Death Mortality Data Index). ctDNA status was assessed pre-ICI when available and longitudinally following ICI initiation. Outcomes included OS and time to next trmt (TTNT); ctDNA dynamics following ICI discontinuation were descriptively evaluated. Results: Among 559 pts treated with first line ICI (pembrolizumab [70.1%], ipilimumab+nivolumab [15.7%], and nivolumab [11.8%]), the median duration of ICI was 266 days. Baseline (pre-ICI) ctDNA results were available for 180 pts, including 38% who were treated with chemotherapy, and most pts (71.7%, 129/180) were ctDNA-positive (ctDNA(+)) prior to ICI initiation. Of these, 89.9% (116/129) had ctDNA-timepoints after the start of ICI, and 81.0% (94/116) achieved anytime ctDNA clearance (from ctDNA(+) to negative), while 19.0% (22/116) remained ctDNA(+). ctDNA(+) at anytime after the start of ICI (HR: 8.0, p < 0.0001) or on first result after the start of ICI (HR: 3.4, p < 0.0001) was associated with inferior OS compared ctDNA-negativity (ctDNA(-)) at these time points. Among pts whose first post-ICI test was ctDNA(+), 32% switched treatments, with a median TTNT of 302 days, compared to only 6.6% of pts (median TTNT 683 days) whose first post-ICI test was ctDNA(-). First post-ICI ctDNA status was associated with longer ICI duration (median 192 days for ctDNA(+) vs 303 days for ctDNA(-)), and ctDNA(+) was predictive of inferior OS (HR: 4.9; p < 0.001). ctDNA after the end of ICI was available for 392 patients; it was persistently negative in 77.2% (303/392), persistently positive in 12.8% (50/392), cleared after the end of ICI (positive to persistently negative) in 4.1%, and the remaining 5.9% had variable patterns (e.g. positive to negative to positive). Conclusions: In this large real-world cohort of MSI-H mCRC treated with ICI, longitudinal ctDNA dynamics strongly correlated with treatment durability, time to subsequent therapy, and OS. These findings support further evaluation of ctDNA to inform treatment duration and surveillance strategies in MSI-H mCRC.