(P 25) PREDICTING IMATINIB RESPONSES USING IN VITRO MODELS OF PDGFRA-MUTANT GASTROINTESTINAL STROMAL TUMOR

Objective: Gastrointestinal stromal tumor (GIST) is the most common sarcoma with 4,000-6,000 annual cases diagnosed in the United States annually. The two most common GIST subtypes harbor oncogenic mutations in homologous receptor tyrosine kinases, KIT or platelet derived growth factor receptor alpha (PDGFRA), both of which are treated with tyrosine kinase inhibitors (TKIs). Type II TKIs like imatinib are successful in treating many patients with advanced KIT-mutant GIST, but are generally thought to be ineffective for PDGFRA-mutant GIST. Patients with the most common PDGFRA driver mutation, exon 18 D842V, historically had poor outcomes because their mutation is notoriously resistant to imatinib and other FDA-approved type II TKIs used for GIST. In 2020, the FDA approved avapritinib, a type I TKI, for the treatment of patients with advanced PDGFRA exon 18 mutant GIST. There are many other PDGFRA exon 18 mutations reported in GIST beyond D842V. We have curated data from a large cohort of PDGFRA-mutant GISTs, and over 80% of the mutations observed are in exon 18 (n=544), and include many unique non-D842V point mutations and in/dels (n=177/544). It is largely unknown how patients with these mutations respond to imatinib, however, currently most exon 18 mutations are assumed to be imatinib resistant like D842V. In previous work, we biochemically profiled some frequently observed patient mutations. Contrary to prior beliefs, we show that some of these exon 18 mutations were imatinib sensitive, and that sensitivity seemed to be dependent on the properties of the amino acid in the 842-codon position. Modeling each of the clinically reported mutations is not feasible, therefore based on the data above, we hypothesized that the imatinib sensitivity of PDGFRA exon 18 mutations could be predicted based on the 842 residue.

Methods: To test our hypothesis, we created mutagenic libraries in lentiviral vectors that included both point mutation as well as deletion mutation constructs that recapitulate patient mutations. Each mutation was expressed in Ba/F3 cells and underwent limiting dilution assays. From this, the oncogenic potential of each mutation along with imatinib sensitivity for –IL3 independent Ba/F3 cells was assessed using immunoblotting for phosphorylated-PDGFRA.

Results: In our assay, 13 out of the 20 amino acids when in the 842-codon position conferred imatinib sensitivity. This further debunks the belief that all PDGFRA exon 18 mutations are imatinib resistant. We found that the majority of hydrophobic residues (7 out of 8), conferred imatinib resistance (IC50 > 150nM), and amino acids from other classes (polar, negatively charged, positively charged, special case) were imatinib sensitive (IC50 < 150nM). Alanine, although hydrophobic, conferred imatinib sensitivity when at the 842-position. To understand the structural implications of these mutations on the kinase and on drug binding, we are currently doing molecular simulations and drug docking studies.

Conclusion: Our model has the potential to identify which PDGFRA exon 18 mutant GIST patients would benefit from front-line imatinib therapy, while subsequently identifying those that would require avapritinib treatment. Avapritinib was recently approved for first-line treatment for all PDGFRA exon 18 mutant GIST in the United States. Imatinib has been used clinically for over two decades, and is more cost-effective and has a superior safety and tolerability profile compared to avapritinib. If a patient progresses on imatinib, there are additional FDA-approved treatments (sunitinib, regorafenib, ripretinib), which are not currently available for those patients that progress on avapritinib. In addition, avapritinib is only approved for patients with D842V in Europe, therefore identifying those that have imatinib sensitive mutations is crucial, because many of these patients may receive no treatment at all. By using our paradigm shifting approach and optimizing TKI treatment sequencing for PDGFRA-mutant GIST, treating eligible patients with imatinib instead of avapritinib will transform patient care and clinical outcomes.