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632 Background: Paltusotine is an oral, nonpeptide, selective somatostatin receptor 2 agonist. In a phase 2 study, treatment with once-daily paltusotine reduced the frequency and severity of carcinoid syndrome (CS) symptoms and was well tolerated. The effect of paltusotine on biomarkers of serotonin and 5-hydroxyindoleacetic acid (5-HIAA) was further explored. Methods: This exploratory study, without formal power calculations, included an 8-week randomized treatment phase (completed) and a 102-week open-label extension phase (currently ongoing). The purpose was to evaluate safety, pharmacokinetics, and exploratory efficacy endpoints, including changes in biomarkers of paltusotine treatment. Enrolled patients were adults with a stable, documented grade 1 or 2 neuroendocrine tumor (NET) and CS. These patients were actively symptomatic and either untreated with somatostatin receptor ligand (SRL) therapy (average of ≥4 bowel movements [BMs] per day or >2 flushing episodes per day in ≥2 days over a 2-week period) or washed out of SRL therapy (symptoms previously controlled on SRL), with demonstrated symptom worsening after washout. Patients were randomized to once-daily paltusotine 40 mg or 80 mg; one optional uptitration (from 40 mg to 80 mg or 80 mg to 120 mg) was permitted. Blood samples for assessment of biomarkers (serum serotonin and plasma 5-HIAA) were collected longitudinally from screening to end of treatment. Upper limit of normal was prespecified as 541 ng/mL for serotonin and 22 ng/mL for 5-HIAA. This analysis focuses on changes in biomarker from baseline to the end of 8-week randomized treatment phase and after approximately one year (48 weeks). Results: Thirty-six patients (n=9 untreated; n=27 SRL washout) were randomized. Mean age was 60.8 years (range 35-83), and 52.8% were female. Nineteen patients had grade 1 NETs, and 17 had grade 2 NETs. As reported previously, treatment with once-daily, oral paltusotine was well tolerated and reduced the frequency and severity of CS symptoms, justifying further clinical development of the 80 mg dose. Mean serum serotonin decreased from 1553.0 ng/mL at baseline to 643.9 ng/mL at Week 8, and mean plasma 5-HIAA decreased from 245.1 ng/mL to 159.7 ng/mL. There was a trend of untreated patients achieving a greater reduction in comparison to washout patients. At 48-week follow-up, mean serum serotonin was stabilized at 791.50 ng/mL (n=20), and mean plasma 5-HIAA was stabilized at 190.45 ng/mL (n=20). Conclusions: Paltusotine significantly reduced serotonin and 5-HIAA levels, with effects maintained at 48 weeks. Biomarker reductions paralleled reductions in BM and flushing frequency. These findings support further investigation in the ongoing phase 3 study (CAREFNDR, NCT07087054). Clinical trial information: NCT05361668 .

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.

622 Background: Colorectal small and large cell neuroendocrine carcinoma (SCNEC and LCNEC) comprise <1% of colorectal cancers and have poor prognosis. Detailed survival information is limited due to rarity. We analyzed survival outcomes using Survival, Epidemiology, and End Result (SEER) database (2000-2022) and compared with NETs and adenocarcinoma (AC). Methods: All patients with SCNEC, LCNEC, NETs or AC with colorectal primaries were extracted. AJCC stage was calculated with 8 th ed. TNM classification. Kaplan-Meier estimator, log-rank test, and Cox regression (including AJCC stage, surgery, radiation, and chemotherapy variables) were used to analyze survival outcomes. Results: Comparing colon and rectal primaries, the only significant mOS difference was in stage IV SCNEC (2 vs 7 months, p<0.0001). Comparing LCNEC and SCNEC, the only significant mOS difference was in stage IV colon SCNEC and LCNEC (2 vs 5 months, p<0.0001). Stage III and IV colorectal SCNEC and LCNEC had significantly shorter mOS than stage III and IV AC and NETs. In multivariate Cox regressions, surgery and chemotherapy were associated with significantly improved survival for both SCNEC and LCNEC at both primaries (p<0.001), while radiation was never significant. Conclusions: Most colorectal SCNEC and LCNEC patients present with stage IV disease. Stage IV colon SCNEC had significantly shorter mOS than both stage IV colon LCNEC and stage IV rectal SCNEC. Further research should explore early diagnosis and effective therapies for this rare, aggressive malignancy. Median overall survival (months) Colon SCNEC (n/mOS) Colon LCNEC (n/mOS) Colon NETs (n/mOS) Colon AC (n/mOS) SCNEC vs NETs in colon: P-value LCNEC vs NETs in colon: P-value SCNEC vs AC in colon: P-value LCNEC vs AC in colon: P-value SCNEC vs LCNEC in colon: P-value Rectal SCNEC (n/mOS) Rectal LCNEC (n/mOS) Rectal NETs (n/mOS) Rectal AC (n/mOS) SCNEC vs NETs in rectum: P-value LCNEC vs NETs in rectum: P-value SCNEC vs AC in rectum P-value LCNEC vs AC in rectum SCNEC vs LCNEC in rectum: P-value Colon SCNEC vs rectal SCNEC P-value Colon LCNEC vs rectal LCNEC: P-value Stage I 19/32 17/122 4496/NA 114,213/144 <0.0001 <0.0001 <0.0001 0.34 0.25 27/23 10/28 5185/NA 54,702/160 <0.0001 <0.0001 <0.0001 0.00047 0.88 0.99 0.24 Stage II 17/38 30/49 259/NA 121,757/106 0.0015 <0.0001 0.4 0.0068 0.65 17/16 3/NA 33/NA 38,417/106 <0.0001 0.57 <0.0001 0.87 0.3 0.065 0.55 Stage III 40/14 107/13 582/184 104,793/82 <0.0001 <0.0001 <0.0001 <0.0001 0.48 50/19 37/17 103/102 46,687/107 <0.0001 <0.0001 <0.0001 <0.0001 0.45 0.48 0.47 Stage IV 245/2 217/5 393/49 83,542/13 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 260/7 57/6 158/36 36,652/17 <0.0001 <0.0001 <0.0001 <0.0001 0.25 <0.0001 0.95

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

605 Background: Multiple treatment modalities are available for localized hepatocellular carcinoma (HCC), and current Barcelona Clinic Liver Cancer (BCLC) guidelines recommend ablation for patients with a limited number of small tumors, not amenable to resection or transplantation. However, in recent years, stereotactic body radiation therapy (SBRT) has been increasingly used for localized disease. Patient outcomes based on treatment modality are not well understood. Methods: We conducted a retrospective analysis using the National Cancer Database (NCDB) to identify patients with a history of stage I/II HCC with tumors ≤ 5 cm who had received local therapy with either radiofrequency ablation (RFA) or SBRT between 2006 and 2021. A propensity score matched analysis was conducted accounting for patient demographics, facility region and tumor level characteristics, as well as an inverse probability of treatment weight-adjusted (IPTW) analysis. Overall survival was estimated using the Kaplan-Meier method. Results: We identified 14,516 patients meeting inclusion criteria, including 13,036 who received RFA and 1,480 who received SBRT. Demographic factors associated with receipt of SBRT included age >50 years, female sex, and treatment in the Middle Atlantic and East North Central regions, as well as clinical factors including larger tumor size (>3 to 5 cm), severe fibrosis/cirrhosis, and stage II disease ( p <0.05 for all). Factors associated with receipt of RFA include black race, higher Charlson-Deyo Comorbidity Scores, treatment at a comprehensive cancer program or integrated network cancer program, and treatment in the East South Central, West North Central, West South Central, Mountain, and Pacific Regions ( p <0.05 for all). Median time from diagnosis to treatment initiation was longer for those who received SBRT (66.3 vs 84.6 days, p <0.001). In a propensity matched analysis, mOS was 45.1 months for those receiving RFA vs 30.8 months for those receiving SBRT ( p <0.001). An IPTW analysis showed similar survival benefit with RFA over SBRT (mOS 51.0 vs 30.5 months, p <0.001). This OS benefit was observed in subset of patients with tumors <3cm in size. Conclusions: In this propensity matched analysis controlling for relevant patient and tumor factors, RFA was associated with improved survival. Prospective studies will be important to further analyze differences in patient outcomes.

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.

593 Background: TP53 and CDKN2A are the most frequently mutated tumor suppressors in BTC, each linked to poor outcomes. Their co-alteration remains unexplored due to the current single-gene classification. Methods: This is an international multicenter study involving 35 institutions across 3 continents (North America, Asia, South America). Genomic profiling was performed on tissue or blood samples using NGS. Patients (pts) were categorized as wild-type (wt) TP53/ wt CDKN2A (WT/WT) (42%), mutated (mut) TP53 /wt CDKN2A (45%), wt TP53 /mut CDKN2A (5%), mut TP53 /mut CDKN2A (8%). Associations between TP53 / CDKN2A status , clinical and genomic features were evaluated using Chi-squared/Kruskal-Wallis tests adjusted for multiple comparisons. Overall survival (OS) from start of first-line systemic therapy was analyzed by Kaplan–Meier and multivariate Cox regression (covariates: age, sex, stage, primary site, first-line treatment, TP53/CDKN2A status), p or q<0.05. Results: We included 1079 pts with BTC. Median age 66 years with intrahepatic BTC (46%), gallbladder cancer (25%), extrahepatic BTC (22%), undefined (6%), ampullary cancer (< 1%). Most had de novo metastatic disease (54%), followed by recurrent (38%) and locally advanced disease (8%). Treatment regimens included chemotherapy (87%), chemo-immunotherapy (12%) and immunotherapy alone (<1%). The most frequently actionable genes were HER2 (8%), IDH1 (7%), BRAF (6%), FGFR2 fusions (4%), MSI-High (H) (3%). HER2 alterations were prevalent in mut TP53 /wt CDKN2A vs WT/WT (11 vs 4%, q = 0.002). IDH1 alterations were more frequent in WT/WT vs TP53 mut/wt CDKN2A (10 vs 4%, q < 0.001). FGFR2 fusions were more common in wt TP53 /mut CDKN2A vs WT/WT (17 vs 6%, q < 0.001), mut TP53 /mut CDKN2A (17 vs 3%, q = 0.001) and mut TP53 /wt CDKN2A (17 vs 1%, q < 0.001). MSI-H was enriched in mut TP53 /mut CDKN2A vs wt TP53 /mut CDKN2A (11 vs 2%, q = 0.035), mut TP53 /wt CDKN2A (11 vs 3%, q = 0.002) and WT/WT (11 vs 2%, q < 0.001). TMB-H was more prevalent in mut TP53 /mut CDKN2A vs mut TP53 /wt CDKN2A (16 vs 8%, q < 0.001) and WT/WT (16 vs 3%, q < 0.024). There was no significant difference in ORR, DCR and PFS among the four groups. mut TP53 /mut CDKN2A had the shortest median OS, while WT/WT had the longest (Table). Only 14% of mut TP53 /mut CDKN2A received regimens including immunotherapy. Conclusions: mut TP53 /mut CDKN2A represents a high-risk subset enriched with MSI-H and TMB-H pts, underscoring the need to prioritize the development of immunotherapy strategies. TP53 / CDKN2A status is prognostic, stratifying patients into distinct survival groups and supporting NGS use in the first-line setting. mOS (months) (95% CI) HR (95% CI) P-value WT/WT 21.4 (19.0-24.4) reference wt TP53 /mut CDKN2A 17.3 (12.4-25.9) 1.16 (0.79-1.71) 0.442 mut TP53 /wt CDKN2A 17.4 (16.0-19.2) 1.21 (1.01-1.45) 0.040 mut TP53 /mut CDKN2A 13.8 (11.5-18.1) 1.54 (1.13-2.09) <0.001

567 Background: IDH2 mutation is rare (1%) in patients (pts) with cholangiocarcinoma and there are no data on prognosis in these pts and response to first line (1L) chemotherapy or off-label IDH2 inhibitor enasidenib. Methods: This IRB approved retrospective study included consecutive pts 18 years or older with IDH2 mutated biliary cancer identified between 2010-2024 at 4 large academic centers. Pt characteristics, systemic therapy(ies), next-generation sequencing (NGS) and survival data were obtained via review of electronic medical records. Survival probabilities were estimated using the product-limit method of Kaplan-Meier using SAS (v9.4, Cary, NC). Results: In 40 pts who met criteria, 9 (22.5%) underwent curative intent resection/ ablation, 34 (85%) had recurrent or de novo locally advanced or metastatic stage and of them 33 (82.5%) received any therapy for advanced cancer. Of these 33 pts, 25 (75.7%) were female, 29 (87.9%) non-Hispanic White and 32 (97%) had intrahepatic subtype with a median (range) age of 65 (28-82) years. NGS on tissue (51.5%), plasma (21.2%) or both tissue and plasma (27.3%) identified IDH2 R172* in 28 (84.8%), I98T in 1 (3%), F278fs in 1 (3%) pts and not reported/missing in 1 (3%) pt. Most common co-altered genes included TP53 (27.5%), ARID1A (17.5%), PBRM1 (12.5%), CDKN2A (10%), BAP1 (10%). FGFR2 fusion or N/KRAS mutation was present in 2 (5%) pts each. Of 33 pts with advanced stage, 1L therapy was gemcitabine/platinum +/- devimistat or durvalumab in 84.9%. Median (95% CI) progression-free survival (PFS) on 1L chemotherapy and overall survival were 6.6 (4.1-7.8) and 16.3 (12.1-28.9) months, respectively. Eight of 33 (24.2%) pts with advanced stage received off label enasidenib as Nth line therapy with median PFS (95% CI) 3.9 (1.1-5.1) months. Conclusions: Most pts with IDH2 mutation had advanced ds with median survival similar to those with IDH1 mutated advanced cholangiocarcinoma. Off label enasidenib had modest benefit in this rare subset of pts and should be investigated further.

566 Background: FGFR inhibitors (FGFRi) have an established role in treating FGFR2-altered (alt) cholangiocarcinoma (CCA). Tinengotinib, a novel FGFRi, has shown promising activity overcoming acquired resistance to prior FGFRi. We present biomarker analyses evaluating tinengotinib’s ability to overcome prior FGFRi resistance and to elucidate mechanism of resistance to tinengotinib. Methods: Eligible patients (pts) with advanced/metastatic CCA who had received ≥ 1 prior systemic chemotherapy were treated with tinengotinib 10 mg daily across 4 Cohorts: A1: FGFR2 fusion (fus) with primary progressive disease (PD) on prior FGFRi; A2: FGFR2 fus with PD after prior response to FGFRi; B: FGFR alt without FGFR2 fus; C: FGFR wild-type. Efficacy was assessed by RECIST v1.1. All pts underwent liquid biopsy for circulating tumor DNA (ctDNA) at baseline; additional timepoints include cycle 3 day 1 (C3D1), time of PD, and end of treatment (EOT). Samples were analyzed using FoundationOne Liquid panel. Results: 27 out of 55 enrolled pts had biomarker samples collected at baseline and C3D1. Among them, pts in A1 (n=10, 107 mutations (mut)), B (n=8, 90 mut) and C (n=5, 74 mut) showed a significant decrease in short variant allele frequencies (VAF) from baseline to C3D1 by a median relative VAF reduction of 100% (p<0.0001) for each of the three cohorts. Although a median relative VAF reduction of 33.3% was seen in A2 (n=4, 15 mut), this was not significant (p=0.6093) likely due to low baseline mutation burden. A ≥50% reduction in ctDNA was associated with improved progression free survival (p<0.0014). Additionally, 100% of median relative reductions in FGFR2 kinase domain (KD) mut were seen in A1 and B (p<0.0001, 95% CI 100-100% and p=0.0156, 95% CI 83.6-100%, respectively), with 92% and 57% clearance of initial FGFR2 KD mut by C3D1. In a subgroup of 10 pts with biomarker sampling at baseline and EOT, 3 pts in A1 with acquired resistance showed emergence of novel FGFR2 mut, loss of FGFR-sensitive mut and new mut in alternate pathways (e.g., AKT, EGFR, RAS/MAPK) while retaining the original FGFR2 fus. 1 pt from B developed resistance due to new FGFR2 rearrangements and FGFR2 mut. 4 FGFR wild-type pts from C developed resistance involving mut in TP53, KRAS, and other oncogenic drivers. Conclusions: These findings provide genomic evidence that tinengotinib may overcome resistance to prior FGFRi treatment in FGFR2 fus-positive CCA, as reflected by significantly reductions in ctDNA VAFs and frequent clearance of FGFR2 KD mutations. The emergence of resistance alterations including de novo FGFR2 mutations and bypass pathways activations indicates acquired resistance that may limit long-term benefit. These results support further investigation with expanded biomarker sampling to better characterize resistance evolution and guide therapy optimization in an ongoing Phase 3 trial. Clinical trial information: NCT04919642 .

561 Background: For patients with aHCC, second-line treatment options are not well established. The primary aim of this study was to evaluate the efficacy of Futi, an FGFR 1-4 inhibitor, in combination with Pem as a second line therapy in patients with FGF19-expressing aHCC. Methods: Patients aged ≥ 18 years, with histological/cytological diagnosis or radiological diagnosis of HCC, Child-Pugh (CP) class A-B7, Barcelona Clinic Liver Cancer (BCLC) A, B or C, ECOG performance status of 0-2, and FGF19+, by in-situ hybridization or immunohistochemistry, received Futi 20 mg once daily plus i.v. Pem 200mg every 3 weeks. The primary endpoint was progression-free survival at 6 months (PFS6). Secondary endpoints include overall survival (OS). The study was designed to enroll 22 evaluable patients to detect PFS6 from 25% (historical control) to 47%, assuming a 1-sided alpha of 0.1 with 80% power. All patients provided written consent. Eligible patients with consent who started treatment are considered evaluable. Results: The study was closed to enrollment early at the request of funding agency due to poor accrual. Between 8/2021 and 3/2024, 14 patients enrolled with 1 patient withdrawing consent before beginning protocol treatment. Among 13 evaluable patients, median age was 72 (range: 60-87) years, 7 (53.8%) were male, 12 CP A (92.3%), 7 BCLC C (53.8%), with 7 (53.8%) having received prior liver directed therapy. Median PFS was 15 weeks (95% confidence interval [CI]: 9.0-Not Estimable) and PFS6 was 23.1% (95% CI: 8.6%, 62.3%). Objective response rate was 0% (95% CI: 0%, 24.71%) and stable disease occurred in 7 (53.9%) of patients. Median OS was 55.7 weeks (95% CI: 47.3-NE). Nine (69.2%) patients experienced at least 1 grade 3+ adverse event (AE), and 2 (15.4%) patients experienced at least a grade 4+ AE. One patient had a grade 5 AE unrelated to study treatment (acute kidney injury). Most commonly reported AEs (occurring in more than 31% of patients) were hyperphosphatemia (76.9%), fatigue (69.3%), diarrhea (61.6%), AST (53.9%) and ALT (46.2%), dry eye (38.5%), nail changes (38.5%), rash-maculo-papular (38.5%), and mucositis oral (38.5%). Conclusions: This prospective study evaluated the use of a targeted treatment in combination with immunotherapy following first line immune-based therapy. This combination had modest activity as a second line therapy for patients with aHCC but did not meet the primary endpoint. The safety profile was acceptable and consistent with prior reports. An optimal second-line regimen remains a critical need. Clinical trial information: NCT04828486 .