Abstract 4648: Circulating tumor DNA analysis demonstrates newly emerging alterations in previously treated IDH1 mutated cholangiocarcinoma patients enrolled in the ClarIDHy study

Abstract Background: Ivosidenib (IVO) is a first-in-class oral inhibitor of the IDH1 mutant (mIDH) enzyme. In the ClarIDHy phase 3 study of previously treated patients with mIDH cholangiocarcinoma (CCA), IVO treatment led to improved progression-free survival (PFS) and overall survival (OS) vs placebo (median PFS: 2.7 vs. 1.4 months and OS 10.3 vs. 5.1 months, when adjusted for the 70% crossover rate; HR, 0.49 [95% CI, 0.34-0.70]; 1-sided P<0.001. Here, we studied the dynamics of mutations emerging after 6 months of IVO treatment using ctDNA analyses. Methods: Prior to enrollment, all subjects in ClarIDHy had IDH1 R132 mutations confirmed by a central NGS panel utilizing tumor biopsy tissue. Since post-progression tissue samples were unavailable, discovery of emerging mutations was limited to plasma ctDNA assessments. To explore molecular changes associated with acquired resistance, an analysis was performed on 18 subjects treated with IVO for ≥6 months (4 partial response; 12 stable disease, 2 NA) using matched baseline and end of treatment (EOT) samples (PGDx elioTM plasma complete, 521 gene panel). A subset of the oncogenic alterations identified were selected for confirmation by ddPCR analysis in ctDNA samples collected longitudinally. Longitudinal plasma 2HG levels were also evaluated. Results: This analysis identified several EOT mutations not present at baseline. The most common mutations were in the MAPK pathway, including activating KRAS, NRAS, and MAP2K1 mutations and loss-of-function NF1 mutations (6/18 patients, 33%). In 2/6 patients, multiple MAPK pathway mutations were detected. There was a range of variant allele frequencies (VAFs) of RAS alterations, including some that had comparable levels to the truncal mIDH, suggesting that MAPK activation may have made a major contribution to the burden of resistant disease. ddPCR analyses from longitudinal plasma samples from selected cases confirmed these findings. In addition, 2 patients had mutations affecting transcription/epigenetics (WT1 and KMT2C). Finally, only 1 patient had acquired mutations in IDH genes (IDH2-R172K and IDH1-G131V). The WT1, KMT2C, and IDH variants had a low VAF. Accordingly, EOT plasma 2HG levels were reduced vs pre-treatment levels in the patient with secondary mIDH, indicating a limited role in the overall emergence of resistance. Conclusion: Analyses of paired baseline/EOT samples suggest that multiple acquired mutations could contribute to IVO resistance, with oncogenic MAPK pathway alterations being the most common. Consistent with previous clinical studies conducted in AML, IDH1 second site mutations and isotype switching to IDH2, rarely emerge during IVO treatment. The lack of detectable mutations in the majority of patients could reflect the sensitivity of the NGS assay or a role for non-genetic mechanisms of resistance. Citation Format: Hatice Duygu Saatcioglu, Haley Ellis, Jinkai Wan, Sam Lubner, Milind Javle, Ghassan K. Abou-Alfa, James M. Cleary, Robin Kate Kelley, Mitesh J. Borad, Teresa Macarulla, Do Youn Oh, Camelia Gliser, Susan Pandya, Sung Chloe, Nabeel Bardeesy, Adriana Tron. Circulating tumor DNA analysis demonstrates newly emerging alterations in previously treated IDH1 mutated cholangiocarcinoma patients enrolled in the ClarIDHy study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4648.