Sonbol, Mohamad Bassam

830 Background: Immune checkpoint inhibitors (ICIs) are standard therapy for MSI-H GI cancers. Lynch syndrome, caused by germline mutations in mismatch repair (MMR) genes, leads to MSI-H tumors and robust ICI responses, but its effect on irAEs remains poorly defined. Clarifying this risk may inform treatment selection and toxicity monitoring. Methods: We retrospectively reviewed MSI-H or MMR-deficient (dMMR) GI cancer patients (pts) treated with ICIs across Mayo Clinic (AZ, MN, FL) from January 2017 to June 2025. Eligible pts were identified using Mayo Data Explorer. Demographics, treatment, and irAEs (graded per CTCAE v5.0) were collected. Lynch status was determined by germline testing when available. Logistic regression estimated odds ratios (ORs) for any grade and grade ≥3 irAEs, adjusting for age, cancer type, prior chemotherapy, and treatment regimen. Results: A total of 209 pts with dMMR/MSI-H GI cancer were included: 61 with Lynch syndrome and 148 without. Lynch pts were younger (median 58 vs 76 years, p < 0.001) and more likely to have received prior chemotherapy (62% vs 43%, p = 0.009). Colorectal cancer was the predominant cancer type (74%), but Lynch pts had proportionally more non-colorectal GI cancers (15% vs 2.7%, p = 0.007). Overall, 29% of pts experienced an irAE, with rates significantly higher in Lynch vs non-Lynch pts (43% vs 24%, p = 0.005). The most frequent irAEs were gastrointestinal, endocrine, and dermatologic, followed by hepatic and pulmonary events. Grade ≥3 irAEs occurred in 16% overall, with no significant difference between groups (p = 0.124). In adjusted models, Lynch syndrome was independently associated with a higher risk of any grade irAE (OR 2.14, 95% CI 1.09 - 4.19, p = 0.026), while risk of severe irAEs was not increased (OR 1.55, 95% CI 0.64 - 3.59, p = 0.316). Lynch syndrome pts were more likely to have combination therapy (17% vs 6.8%, p = 0.029). However, there were no differences observed in any grade irAE between monotherapy and combination immunotherapy (OR 1.36, 95% CI 0.49 - 3.54, p = 0.535). Conclusions: Lynch syndrome is associated with a higher prevalence of any-grade irAEs among dMMR/MSI-H GI cancer pts treated with ICIs, though severe events were not increased. Incorporating Lynch status into immunotherapy planning may improve risk stratification and guide regimen selection.

776 Background: Surgery is the only potentially curative option for PC. However, only a minority of patients (pts) undergo resection with current perioperative (periop)-chemotherapy (CT). In the absence of phase III trials, selection between mFOLFIRINOX and gemcitabine/nab-paclitaxel (gem-nab) is based on limited evidence. We assessed whether NGS–based tumor profiling can guide tailoring of CT. Methods: pts with resected PC (resectable, borderline resectable, and downstaged locally advanced) that underwent Tempus xT, xF (DNA), and/or xR (RNA) testing were selected (89% tissue, 11% blood). Most samples (73%) were collected post-CT. Samples were stratified by KRAS status into KRAS -mutated ( KRAS mut) and KRAS wild-type ( KRAS wt). PD-L1 IHC was reported as tumor proportion score (TPS). Primary endpoint was median overall survival (mOS) defined from CT initiation to death with censoring of pts alive at the last follow-up or at a study cutoff of 5 years. Secondary endpoint was molecular profiling by KRAS status. Pearson chi-square and Wilcoxon rank-sum tests were used for descriptive comparisons; univariate Cox regression and log-rank tests (p<0.05) for survival. Results: We included 1,325 pts (median age 66 years [IQR 59–72]) with resected PC [stage II (15%), stage III (12%) and stage I (10%), unknown (62%)]. 95% had adenocarcinoma. Pts received neoadjuvant (50%), neoadjuvant plus adjuvant (26%), neoadjuvant with indeterminate adjuvant allocation (13%) or adjuvant CT (11%). First-line periop-CT was mFOLFIRINOX in 73% and gem-nab in 27% (median duration 4.3 months (mo) [range, 2.6–6.7]). KRAS mutations were present in 75% (G12X 91%: G12D 44%, G12V 34%, G12R 21%, G12C <1%). mOS was 26.7 mo (95%CI, 22.4-35.7) in the total cohort; KRAS mut pts showed a clinically meaningful worse mOS compared to KRAS wt [24.1 vs 35.7 mo; HR 1.61 (95%CI, 0.98 - 2.63); p=0.06]. In the KRAS G12D pts, mFOLFIRINOX showed a trend towards longer mOS compared to gem-nab [17.79 vs 14.53 mo; HR 0.75, (95% CI, 0.56 - 1); p=0.050]; no significant differences were observed in other KRAS subgroups. Compared to KRAS wt, KRAS mut pts were enriched for TP53 (76% vs 22%; p<0.001), SMAD4 (28% vs 5.8%; p<0.001), CDKN2A (33% vs 8.5%; p<0.001), MTAP deletions (7.7% vs 1.2%; p<0.001), and chromatin-regulator alterations ( ARID1A 7.8% vs 1.2%; p<0.001; KMT2C 2.5% vs 0.6%; p=0.034). KRAS wt pts were enriched for BRAF V600E (1.5% vs 0%; p<0.001). PD-L1 TPS ≥1% was more frequent in KRAS mut compared to KRAS wt (11.0% vs 8.1%; p=0.033). Conclusions: KRAS status does not predict benefit from mFOLFIRINOX vs gem-nab in resected PC. Interestingly, resected PC showed a higher prevalence of KRAS wt compared to the metastatic setting. KRAS status is associated with distinct profiles of potentially targetable co-alterations. These findings may suggest the integration of genomic profiling in clinical trials to develop biomarker-driven tailored strategies in the early stage.

679 Background: DNA-damaging agents improve outcomes in metastatic pancreatic cancer (mPC) with homologous recombination repair (HRR) deficiency, albeit use is limited to 6-9% of patients (pts) with BRCA1/2 or PALB2 mutations. We hypothesized that BRCA/PALB2 wildtype mPC may exhibit platinum sensitivity driven by altered HRR gene expression. We correlated HRR gene RNA profiles with real-world overall survival (rwOS) following first-line (1L) platinum therapy (PT). Methods: Tempus Lens, a platform used to query multimodal data from millions of de-identified patient records in the Tempus Database was used to identify mPC pts with wildtype somatic BRCA1/2 and PALB2 who had Tempus xT DNA and xR RNA testing. RNA-seq data were normalized to correct for assay/batch effects, quantified as transcripts per million (TPM) and reported as log2(TPM+1) for 17 HRR genes and GATA6 . Pts were classified as high and low expressors based on the top and bottom quartile gene expression. Pts were grouped as PT-treated (received 1L PT) or PT-naïve (never received PT). Real world overall survival (rwOS) was calculated from 1L start to death or loss to follow up. Risk set adjustment was applied to mitigate immortal time bias. Median rwOS was estimated using Kaplan-Meier and univariate Cox models. A multivariate Cox Proportional (CoxPh) model included PT use, RAS status, and expression of genes (as continuous variables). Results: Among 1,068 pts, the median age was 66 years and 609 (57%) received 1L PT. In low RAD51 expressors, PT-treated pts (n=136) had significantly longer rwOS than PT-naïve (n=112): 11.4 months (95% CI, 7.8–13.8) vs 8 months (95% CI, 5.5–9.0); p=0.028. In contrast, in high RAD51 expressors, rwOS was similar between PT-treated (n=150) and PT-naïve (n=97): 7.9 vs 6.9 months (p=0.236). High GATA6 expression was also associated with improved rwOS in PT-treated pts vs PT-naïve: 13.5 vs. 9.3 months; p=0.011. In multivariate analysis, increased GATA6 expression remained a positive predictor of rwOS (HR 0.82; 95% CI, 0.76–0.88; p<0.001), and high RAD51 expression was associated with shorter rwOS with PT (HR 1.18; 95% CI, 1.01–1.39; p=0.043). Conclusions: In BRCA/PALB2 wt mPC, transcriptomic profiling identified low RAD51 and high GATA6 expression as predictors for improved rwOS when treated with PT. Integrating these biomarkers may improve development of DNA-damaging therapies beyond canonically defined HRD. Hazard ratios of mPC mortality by selected HRR genes estimated by multivariate CoxPh models. HR 95% CI p value 1L PT (vs PT-naïve) 0.87 0.75, 1.00 0.054 BRCA1 1.15 0.98, 1.35 0.079 BRCA2 0.89 0.77, 1.04 0.14 GATA6 0.82 0.76, 0.88 <0.001 NBN 0.97 0.81, 1.15 0.7 RAD51 1.18 1.01, 1.39 0.043 RECQL4 0.94 0.84, 1.04 0.2 Any RAS mutation 1.43 1.14, 1.81 0.002

629 Background: NENs are a heterogeneous group of tumors with variable clinical behavior. While most NENs occur sporadically, a subset arises in the context of germline cancer predisposition syndromes. The prevalence of GPVs in NENs is increasingly recognized, but the reported frequency and spectrum of genes involved remain inconsistent across studies. Therefore, systematic review and meta-analysis is helpful to clarify the prevalence and clinical significance of PGVs in pts with NENs. Methods: A systematic review with meta-analysis of prospective and retrospective studies reporting germline testing in pts with NENs was conducted according to the PRISMA guidelines. The primary outcome was the prevalence of GPVs. Random-effects models (GLMM) with logit transformation were applied to estimate pooled prevalence with 95% confidence intervals (CIs). Results: Nine studies comprising 1304 patients with NENs were included. The median age was 53.8 (10-88.5) years, 52% were female (n=678/1304), and 56% (n=736/1304) were unselected cohorts (all-comers) and 44% (568/1304) were selected cohorts (testing based on clinical suspicion and high-risk features defined by each study). Across all cohorts, the pooled prevalence of PGVs was 19.6% (95% CI: 10.68–33.15%), with high heterogeneity (I² ≈ 96%). Subgroup analyses demonstrated comparable GPV prevalence between selected (13.2%, 95% CI 6.3–25.8) and unselected cohorts (23.0%, 95% CI 12.0–39.6) (p=0.25). GPV prevalence stratified by pancreatic NENs (17.8%, 95% CI 12.6–24.5) and other NENs (10.6%, 95% CI 6.5–17.0) was also not statistically different (p=0.08). The most altered genes involved DNA repair and cellular signaling pathways, including APC, ATM, BRCA1/2, BAP1, CHEK2, MEN1, MUTYH, VHL, RET, and TSC1/2. Germline variants in mismatch repair (MMR) genes were reported in approximately 2.5% of cases. Conclusions: This study represents the first systematic review and meta-analysis evaluating the prevalence of germline mutations in NENs. Approximately 20% of pts with NENs harbor a germline pathogenic variant, underscoring the importance of genetic counseling and germline testing. The wide variability across studies highlights the need for standardized genetic testing approaches. These findings support routine referral for genetic counseling and panel testing in patients with NENs to guide management and familial risk assessment.

619 Background: Well-differentiated G3 GEP-NETs are rare and heterogeneous diseases, limiting phase III trials and definition of clear standards of care. Building upon our preliminary cohort of 76 patients (pts), we present an updated and expanded retrospective analysis across three Mayo Clinic tertiary centers. Methods: We included pts with histologically confirmed well-differentiated G3 GEP-NETs diagnosed since 2017 (new WHO classification) from the three Mayo Clinic sites (Rochester, Phoenix, Jacksonville). Endpoints were ORR, DCR, PFS, and OS across treatment lines. PFS/OS were estimated by Kaplan-Meier. OS was defined from treatment start. Multivariate Cox regression was used to evaluate the association of clinic-pathologic features (age, sex, tumor functionality, Ki-67, primary tumor site, stage at diagnosis, presence of liver metastasis, number of metastatic sites at diagnosis) with OS (p<0.05). Results: 111 pts (median age 62 y, range 22–83). Primary sites were pancreas (58%), small bowel (19%), unknown (9%), and rectum (5%). Most tumors were G3 at diagnosis (88%); the remainder evolved from prior G1/G2. Ki-67 was 20–55% in 83% of cases. Most were nonfunctional (70%); functional subtypes included carcinoid (13%) and gastrinoma (8%). Median plasma 5-HIAA was 38.5 ng/ml (5–1394). Advanced disease was present in 85%. Common metastatic sites included liver (85%), lymph nodes (32%), bone (26%), peritoneum (10%), and lung (9%). Median number of metastatic sites was 1 (range 0–5). Median number of systemic treatment lines was 2 (range 0–8). The most frequent regimens and outcomes are shown in the Table. Insulin-secreting tumors (HR 2.16, 95% CI 1.33–25.73, p=0.019) and ≥2 metastatic sites (HR 2.13, 95% CI 1.05–4.31, p=0.035) were associated with inferior OS. Conclusions: This represents one of the largest real-world datasets of well-differentiated G3 GEP-NETs and provides a deeper evaluation of treatment strategies and prognostic factors. CAPTEM, SSAs, and PRRT with Lu-177 emerged as the most commonly used therapies, with PRRT showing the most favorable outcomes. Insulin secretion and higher metastatic burden were confirmed as adverse prognostic factors. CAPTEM SSAs PRRT Lu-177 Carboplatin plus etoposide FOLFOX Everolimus Sunitinib N of pts (%) 64 (58) 44 (40) 37 (33) 24 (19) 18 (13) 11 (10) 8 (7) Line of therapy (%) First 36 (56.3) 27 (61.4) 5 (13.5) 13 (59.2) 5 (7.2) 1 (9.0) 1 (12.5) Second 15 (23.4) 13 (29.5) 13 (35.2) 3 (13.6) 5 (35.7) 3 (27.3) 2 (37.5) Third or higher 13 (20.3) 4 (9.1) 19 (51.3) 6 (27.2) 8 (57.1) 7 (63.7) 5 (62.5) DCR (%) 60.9 56.8 75.0 52.2 71.4 45.5 37.5 ORR (%) 34.4 13.6 44.4 30.4 35.7 18.2 12.5 mPFS (95% CI) 8.0 (5.3-10.6) 6.2 (2.8-9.5) 11.2 (9.1-13.3) 7.3 (2.8-11.8) 4.4 (2.4-6.4) 3.9 (2.2-5.6) 3.9 (1.54-6.26) mOS (95% CI) 40.1 (23.0-57.2) 50 (22.6-77.4) 76.1 (16.3-135.8) 33.0 (21.0-45.0) 21.1 (15.0-27.2) NE NE

599 Background: Biliary tract cancers (BTC) remain aggressive malignancies with limited treatment options. Alterations in PIK3CA are implicated in tumor progression across solid tumors, but their prognostic impact in BTC remains unclear. We investigated the association of PIK3CA alterations(alt), alone and in combination with TP53 and ARID1A , with survival outcomes in advanced BTC. Methods: We conducted a retrospective analysis of 1,079 patients with advanced BTC treated with first-line chemotherapy (CT) or chemoimmunotherapy (CIO) across three sites in the USA, Japan, and Brazil. Genomic profiling was performed using next-generation sequencing on tissue and/or liquid biopsy samples. Survival outcomes were estimated by Kaplan-Meier analysis, with comparisons by log-rank testing and hazard ratios (HR) derived from Cox regression models. Results: PIK3CA alterations were identified in 11% (123/1,079) of patients. The median age at diagnosis was 68 yrs (range, 20–79), with 50% (62/123) female. The most common primary site was intrahepatic in 43.1% (53/123) while majority of patients received CT only 92.5% (111/123). When analyzed with TP53 , 68 pts had PIK3CA/TP53 co-alterations(co-alt), 501 had TP53 alt, 55 had PIK3CA alt alone, and 455 had neither PIK3CA nor TP53 alt (WT/WT). Overall survival (OS) was significantly worse in patients with PIK3CA alt, particularly in the setting of TP53 co-alt. Median OS (mOS) was 22.6 months(m) (95% CI, 20.4–24.8) for PIK3CA/TP53 WT, 15.9 m (95% CI, 11.6–18.5) for PIK3CA alt /TP53 WT, 14.6 m (95% CI, 13.1–19.5) for PIK3CA alt/ TP53 alt, and 17.1 m (95% CI, 15.5–18.8) for TP53alt/PIK3CA WT (p<0.001). In analyses with ARID1A , 32 pts had PIK3CA/ARID1A co-alt, 91 had ARID1A alt only, 120 had PIK3CA alt only, and 836 were WT/WT. PIK3CA/ARID1A co-alt tumors had a mOS of 12.8 m (95% CI, 9.1–NE) compared with 19.3 m (95% CI, 18.1–20.9) in PIK3CA WT /ARID1A WT (p=0.009). Multivariate analysis identified PIK3CA alt (hazard ratio [HR] 1.37, 95%CI 1.09–1.72; p=0.008), PIK3CA alt/ TP53 alt (HR 1.52 95% CI, 1.11–2.07; p=0.008), ARID1A alt/ PIK3CA WT (HR 1.40, 95% CI 1.08-1.82, p=0.011), and metastatic status (HR 1.82, 95% CI 1.52-2.19, P<0.001) as independent prognostic factors for shorter OS. Conclusions: In advanced BTC, PIK3CA alt were associated with significantly worse overall survival, particularly when co-occurring with TP53 or ARID1A alt. These findings establish PIK3CA as a prognostic biomarker of poor outcome and highlight the need to explore PI3K-targeted therapeutic strategies in BTC.

554 Background: Atezo/Bev is the 1 st -line standard for advanced hepatocellular carcinoma (aHCC). Optimal 2 nd -line therapy post-Atezo/Bev remains undefined. This study evaluates continuation of atezolizumab with a MKI versus MKI alone in the 2 nd -line setting. Methods: This multicenter, randomized phase II trial enrolled patients with unresectable aHCC who progressed on Atezo/Bev. Patients were randomized 2:1 to atezolizumab plus MKI (cabozantinib or lenvatinib; Arm A) or MKI alone (Arm B). Stratification factors included MKI choice, viral etiology, and alpha-fetoprotein levels. Dual primary endpoints were overall survival (OS) and progression-free survival (PFS). Secondary endpoints included objective response rate (ORR), duration of response (DOR), and safety. With 84 death and 89 PFS events, from 122 patients, this trial had 80% power to detect a hazard ratio (HR) of 0.556 in OS and HR of 0.571 in PFS with a one-sided type I error (alpha) of 0.05 for each endpoint. Results: Twenty patients were accrued across 5 U.S. centers, between 01/2022 and 03/2025, before early closure due to slow accrual. Eighteen received treatment (Arm A: n=13; Arm B: n=5). Baseline characteristics: median age 68 yr, extrahepatic 56%, Hep B+ 0%, Hep C+ 33%, NASH 33% and CP A (5 to 6) 100%. MKIs used were cabozantinib (Arm A: 6/13; Arm B: 4/5) and lenvatinib (Arm A: 7/13; Arm B: 1/5). Median follow-up was 28.0 months (mo), with 14 deaths reported. Median (95% CI) OS were 13.9 (7.1-Not Estimable [NE]) and 6.2 (5.1-NE) mo (Arm A and B respectively); and that of PFS 4.3 (2.3-NE) and 4.1 (3.7-NE) mo. ORR was 7.7% (1/13) in Arm A (responder received lenvatinib) and 0% (0/5) in Arm B. Grade ≥3 adverse events (at least possibly related) occurred in 46.2% of Arm A and 60.0% of Arm B patients. G3+ events occurring in >10% of patients included hypertension, fatigue, anorexia, and ALT elevation in Arm A; and hypertension, hallucination, nausea, hypomagnesemia, hypokalemia, mucositis, AST increased and colitis in Arm B. Conclusions: In this limited cohort, continuation of atezolizumab with MKI showed relative activity and manageable safety in aHCC post-Atezo/Bev. The AE profile for atezo/MKI is consistent with the known profile of respective agents. These findings support further investigation of immunotherapy-based combinations in the 2nd-line setting. Clinical trial information: NCT05168163 .

399 Background: The standard of care for resectable GEA adenocarcinomas is perioperative chemotherapy with the FLOT (fluorouracil, leucovorin, oxaliplatin, docetaxel) regimen. Recent studies investigating the addition of checkpoint inhibitors (CPIs) to perioperative chemotherapy have shown mixed results. An NMA can leverage both direct and indirect comparisons to evaluate the relative efficacies of available regimens in this setting. Methods: MEDLINE, EMBASE, Scopus, Web of Science and CENTRAL were searched until August 27th, 2025. Eligible studies included phase 3 randomized trials evaluating perioperative or neoadjuvant chemotherapy +/- CPIs ± radiation in resectable GEA. The primary outcomes were disease-free survival (DFS) and overall survival (OS). Hazard ratios and 95% confidence intervals (CIs) were estimated using a frequentist NMA framework. Results: Fifteen trials including 8072 patients were analyzed. For DFS, perioperative durvalumab + FLOT (dFLOT) ranked highest (P-score = 0.99), followed by FLOT (P-score = 0.84) (HR for dFLOT vs. FLOT was 0.71, 95% CI 0.58-0.86). dFLOT also ranked higher than perioperative pembrolizumab + cisplatin and fluorouracil/capecitabine (KN-585), though this comparison was not statistically significant (HR 0.65, 95% CI 0.40-1.04). Compared with CROSS, dFLOT showed superior DFS (HR of 0.48, 95% CI 0.36-0.63). For OS, dFLOT ranked the highest (P-score=0.95) and was superior to CROSS (HR 0.57, 95% CI 0.42-0.77). While dFLOT appeared favorable versus FLOT (HR 0.78, 95% CI 0.62-0.98), this finding was primarily driven by MATTERHORN, which did not meet its prespecified OS significance boundary (P = 0.03 vs threshold P < 0.0001), and the proportional hazards assumption was violated. Similarly, no significant OS difference was observed between dFLOT and the KN-585 arms (HR 0.75, 95% CI 0.45-1.23). Conclusions: In this NMA, perioperative FLOT remained the benchmark regimen for resectable GEA. dFLOT has shown promising early results, ranking the highest for DFS and showing favorable, but not statistically definitive OS trends in comparison to FLOT. Importantly, the network enabled indirect comparisons not tested in head-to-head trials, demonstrating that dFLOT ranked above pembrolizumab+CF/CX (KN-585) without reaching statistical significance, and also outperformed CROSS. However, these findings should be interpreted with caution, as they are primarily driven MATTERHORN data in which OS did not cross the prespecified threshold and proportional hazards assumptions were violated. Perioperative immunotherapy combinations appear promising, but final MATTERHORN results and further confirmatory studies are needed before dFLOT can be considered as a new standard of care.

392 Background: PAC+RAM is the standard second-line therapy for advanced GEA. However, PAC-related neuropathy is a major challenge, particularly since many pts have pre-existing neuropathy from prior platinum exposure. An effective “neuropathy-free” regimen remains an unmet need. We evaluated FTD-TPI+RAM as an alternative to PAC+RAM. Methods: This multi-center, randomized, phase II study enrolled GEA pts who progressed on or were intolerant to fluoropyrimidine+ platinum therapy. Pts were randomized to FTD-TPI+RAM (Arm A) or PAC+RAM (arm B) at a 1:1 ratio. The primary endpoint was progression-free survival (PFS), defined as time interval between randomization date and the date of disease progression or death; Secondary endpoints included overall survival (OS) and adverse events (AEs). The design hypothesized non-inferiority (NI) of Arm A to Arm B with a NI margin of hazard ratio 1.19. 90 PFS events from 116 pts provide 80% power with a one-sided alpha of 0.15. The interim futility analysis, conducted in December 2024 with 20 events, did not cross the futility boundary (Z statistic ≥ 1.172 ) and continued enrolling pts. The trial closed enrollment in March 2025 due to slow accrual. At the time of interim analysis, 20 events had occurred and the Z statistic was 0.4532. Final analysis included the 29 pts that were eligible, consented, randomized, and received any protocol treatment. Results: Among 29 pts (15 Arm A; 14 Arm B), baseline characteristics were balanced (in Arms A vs B, median age 64 vs 63; 80% vs 85.7% male; 86.7%% vs 78.6% White; 80% vs 78.6% ECOG 1). At data freeze of 8/4/2025, 26 PFS events were reported. Median PFS was 3.2 vs. 3.9 months in Arm A vs. Arm B (HR 1.37, 95% CI 0.61-3.02). NI was not demonstrated (Z =0.34, threshold for NI<-1.036). Median OS was 8.4 vs. 10.7 months, in Arms A and B, respectively. Grade ≥3 AEs occurred in 93% (Arm A) vs 71% (Arm B) with hematologic AEs occurring more frequently in Arm A than Arm B (53% vs 29%). Peripheral sensory neuropathy (any grade) occurred more frequently in Arm B (86%) vs. Arm A (47%). Conclusions: Although NI was not achieved due to early closure and small sample size, FTD-TPI+RAM was relatively safe and associated with less neuropathy compared with PAC+RAM. This regimen warrants further evaluation for pts in whom neuropathy limits taxane use. Clinical trial information: NCT04660760 .

378 Background: RATIONALE-305 (NCT03777657) enrolled the highest number of pts with PM among similar trials and stratified pts with advanced GC/GEJC by presence of PM. We used statistical modeling to evaluate the impact of varying PM levels on outcomes with TIS+CT. Methods: We randomized adults with GC/GEJC (1:1) to TIS or placebo (PBO) + CT. Primary endpoints: OS in intent-to-treat (ITT) population and pts with programmed death-ligand 1 (PD-L1) Tumor Area Positivity (TAP) score ≥5%. We weighted pts using the method of moments to match a hypothetical population to ITT, with the same distribution of sex, primary tumor location, PD-L1 (TAP score <5% vs ≥5%), number of metastatic sites (0-2 vs 3+), and liver metastasis. PM representation was set from 20% to 45%. Weighted OS, PFS, and objective response rate (ORR) were estimated. Results: Of 997 pts enrolled, 434 (43.5%) had PM. Among pts with PD-L1 TAP scores ≥5% (54.8%), 39.7% had PM, and among pts with scores ≥1% (88.9%), 43.6% had PM (data cutoff Feb 28, 2024). Fewer pts with (+) PM had PD-L1 high tumors (TAP score ≥5%: 50.0%) and more had primary stomach tumors (86.6%) compared to those without (-) PM (58.4% and 75.5%). TIS+CT improved OS, PFS, and ORR in ITT and in pts +/- PM vs PBO+CT (Table). Across 20%–45% PM, TIS+CT showed consistent benefit, with stable HRs for OS and PFS (20% and 45% shown). TIS+CT showed survival benefits in pts with PM and PD-L1 TAP score ≥5% (OS: HR=0.71; PFS: HR=0.65) and ≥1% (OS: HR=0.79; PFS: HR=0.79) vs PBO+CT. ORR was higher with TIS+CT vs PBO+CT in pts + PM and PD-L1 TAP scores ≥5% (47.3% vs 30.8%) and ≥1% (44.2% vs 33.2%). Conclusions: Our analysis showed that lowering PM representation marginally improved OS and ORR, underscoring the poor prognosis of peritoneal disease. In RATIONALE-305, TIS+CT provides clinically meaningful response and survival benefits irrespective of PM, with higher responses in PD-L1+ GC/GEJC. Clinical trial information: NCT03777657 . Arm(95% CI) ITT + PM - PM 20%+ PM 45%+ PM OS, months TIS+CT 15.0 (13.6, 16.5) 12.3 (10.6, 14.3) 17.3 (15.0, 20.3) 15.4 (13.9, 18.0) 15.0 (13.5, 16.5) PBO+CT 12.9 (12.1, 14.1) 11.8 (10.5, 13.0) 14.0 (12.6, 16.0) 13.1 (12.3, 14.5) 12.8 (12.1, 14.1) HR 0.79 (0.69, 0.91) 0.78 (0.64, 0.96) 0.79 (0.65, 0.95) 0.81 (0.69, 0.95) 0.79 (0.69, 0.90) PFS, months TIS+CT 6.9 (5.7, 7.2) 5.8 (5.6, 7.3) 7.0 (5.7, 8.4) 6.9 (5.7, 7.3) 6.9 (5.7, 7.2) PBO+CT 6.2 (5.6, 6.9) 5.7 (5.3, 6.9) 6.9 (5.7, 7.1) 6.5 (5.6, 7.0) 6.2 (5.6, 6.9) HR 0.79 (0.68, 0.91) 0.80 (0.64, 0.98) 0.77 (0.64, 0.94) 0.79 (0.67, 0.94) 0.79 (0.69, 0.91) ORR, % TIS+CT 47.3 (42.9, 51.8) 43.6 (37.0, 50.5) 50.2 (44.2, 56.2) 48.0(42.9, 53.1) 47.3 (42.8, 51.8) PBO+CT 40.5 (36.2, 45.0) 32.2 (26.0, 39.0) 46.8 (40.9, 52.8) 43.1(38.1, 48.3) 40.3(36.0, 44.8) ORR difference, % 6.8 (0.6, 12.9) 11.4 (2.3, 20.3) 3.4 (-4.9, 11.6) 4.9 (-2.2, 11.9) 6.9 (0.8, 13.0)