Fountzilas, C.

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 .

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 .