Teixeira, Maria Fernanda Botelho

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