Abstract 660: Predicting imatinib responses in exon 18 PDGFRA-mutant GIST

Abstract Gastrointestinal stromal tumor (GIST) is the most common GI sarcoma and frequently harbor oncogenic mutations in KIT or platelet derived growth factor receptor alpha (PDGFRA) receptor tyrosine kinases. While type II tyrosine kinase inhibitors (TKIs) revolutionized the treatment of KIT-mutant GIST, they are thought to be ineffective for many PDGFRA-mutant GIST cases. Over 80% of PDGFRA mutations in GIST occur in exon 18 and patients with the most common mutation, D842V, are resistant to imatinib and other type II TKIs, leading to poor outcomes. However, beyond D842V, numerous other PDGFRA exon 18 mutations have been observed. From curated data of a large cohort of PDGFRA-mutant GISTs, we have identified nearly 50 unique PDGFRA exon 18 mutations, including both point mutations and complex indels. Although it is largely unknown how these mutations respond to imatinib, currently most, if not all, PDGFRA exon 18 mutations are assumed to be imatinib resistant. Using cell models, we tested some of the most commonly observed mutations in this cohort. Contrary to dogma, we observed that some exon 18 mutations tested were imatinib sensitive. We also observed that imatinib sensitivity seemed to be dependent on the properties of the residue in the 842-codon position, which encodes a key auto-inhibitory residue. We then hypothesized that imatinib sensitivity of every known or future reported PDGFRA exon 18 mutation might be predicted based on the 842 residue. To test the predictive potential of the 842 residue, we created mutagenic libraries that included every amino acid substitution at the 842 position. We expressed each mutation in Ba/F3 cells and assessed imatinib sensitivity via immunoblotting for phosphorylated-PDGFRA. Overall, 40% of tested mutations were sensitive to imatinib, further debunking the belief that all PDGFRA exon 18 mutations are imatinib resistant. In agreement with our hypothesis, we found that any hydrophobic residue at the 842 position conferred imatinib resistance (IC50 > 100nM), and all other amino acids tested to date were imatinib sensitive (IC50 < 100nM). Based on these data alone, over 55% of the 50 unique mutations identified from our PDGFRA-mutant GIST patient cohort would be predicted to be imatinib sensitive, demonstrating the importance of understanding the biochemical properties of various exon 18 mutations to predict treatment outcomes. Our predictive model would help identify patients with PDGFRA exon 18 mutations that could benefit from using front-line imatinib as an alternative to avapritinib, which has been recently approved for first-line treatment for all PDGFRA exon 18 mutant GIST. Imatinib, which has been used clinically for over 20 years, has a superior safety and tolerability profile compared to avapritinib. This paradigm shifting approach has the potential to transform patient care and outcomes by optimizing the sequencing of TKIs for PDGFRA-mutant GIST patients. Citation Format: Homma M. Khosroyani, Lillian R. Klug, Johanna Falkenhorst, Sebastian Bauer, Michael C. Heinrich. Predicting imatinib responses in exon 18 PDGFRA-mutant GIST [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 660.