Background: The majority of GISTs are driven by constitutively activated KIT/PDGFRA kinases and susceptible to treatment with tyrosine kinase inhibitors such as imatinib, sunitinib and regorafenib. During treatment most tumors will develop heterogeneous secondary mutations in KIT or PDGFRA inducing drug resistance, so there is an unmet need for novel therapies. We tested the efficacy of M4205, a novel specific KIT inhibitor with high activity towards the most relevant KIT mutations, in patient-derived GIST xenograft models. Methods: NMRI nu/nu mice (n=146) were transplanted with patient-derived GIST xenograft models UZLX-GIST9 (KIT:p.P577del;W557LfsX5;D820G) known to be resistant to both imatinib and sunitinib, with the dose-dependent imatinib-sensitive and sunitinib-sensitive models UZLX-GIST2B (KIT:p.A502_Y503dup), UZLX-GIST25 (KIT: p.K642E) and the cell-line derived model GIST882 (KIT: p.K642E)*. Mice were treated daily with vehicle (control), imatinib (100mg/kg), avapritinib (5mg/kg), sunitinib (20mg/kg), or M4205 (10mg/kg, 25mg/kg). Efficacy was assessed by tumor volume evolution, histopathology [hematoxilin & eosine staining (H&E)] and immunohistochemistry [Ki-67, phospho-Histone H3 (pHH3), cleaved PARP]. Histologic response (HR) was graded as previously described°. Mann Whitney U and Wilcoxon Matched Pairs tests were used for statistical analysis, with p<0.05 considered as significant. Results: M4205 (25mg/kg) caused tumor volume shrinkage in UZLX-GIST2B, -GIST25 and GIST882 with relative decrease to 45.6%, 35.1% and 57.3% on the last day as compared to baseline. In UZLX-GIST9 tumor growth to 132.4% was observed in M4205 (25mg/kg)-treated tumors as compared to baseline. In all models we observed significant smaller tumor volumes in M4205 (25mg/kg)-treated tumors compared to control, and this antitumor activity was superior to imatinib in UZLX-GIST9, -GIST2B and GIST882, and to sunitinib in -GIST25. Compared to controls, M4205 (25mg/kg) induced a significant decrease in mitosis (H&E) in all models, confirmed on pHH3 and Ki-67 immunostainings in three models. In -GIST25 and GIST882 grade 2-4 HR with myxoid degeneration was observed in all tumors. All treatments were well tolerated. Conclusion: M4205 has significant antitumor activity in patient- and cell line-derived GIST xenograft models. The novel kinase inhibitor induces volumetric responses, decreases mitotic activity, has antiproliferative effects and in models with KIT exon 13 mutation leads to characteristic myxoid degeneration. * Cornillie et al. Mol Cancer Ther 2019; 18(6):1168-1178 ° Agaram et al. Clin Cancer Res 2007; 13:170-81 Citation Format: Luna De Sutter, Agnieszka Wozniak, Jasper Verreet, Ulla Vanleeuw, Lore De Cock, Nina Linde, Christine Drechsler, Christina Esdar, Raf Sciot, Patrick Schöffski. Anti-tumor effects of the novel KIT mutant inhibitor M4205 in patient-derived gastrointestinal stromal tumor (GIST) xenograft models [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 2666.

Background: The majority of GIST are driven by constitutively activated KIT/PDGFRA kinases and are susceptible to treatment with tyrosine kinase inhibitors. During treatment, most of these tumors will develop secondary mutations in KIT or PDGFRA inducing drug resistance, so there is an unmet need for novel therapies. We tested the efficacy of IDRX-42, a novel selective KIT inhibitor with high activity towards the most relevant KIT mutations, in 4 GIST xenograft models. Methods: NMRI nu/nu mice were transplanted with patient-derived GIST xenograft models UZLX-GIST9 (KIT:p.P577del;W557LfsX5;D820G), UZLX-GIST2B (KIT:p.A502_Y503dup), UZLX-GIST25 (KIT:p.K642E) and the cell-line derived model GIST882 (KIT:p.K642E). Mice were treated daily with vehicle (control), imatinib (100mg/kg), sunitinib (20mg/kg), avapritinib (5mg/kg), or IDRX-42 (10mg/kg, 25mg/kg). Efficacy was assessed by tumor volume evolution, histopathology, grading of histological response and immunohistochemistry. The Kruskal-Wallis and Wilcoxon Matched Pairs tests were used for statistical analysis, with p<0.05 considered as significant. Results: IDRX-42 (25mg/kg) caused tumor volume shrinkage in UZLX-GIST25, GIST882 and UZLX-GIST2B, with a relative decrease to 45.6%, 57.3% and 35.1% on the last day as compared to baseline, and tumor growth delay (160.9%) compared to control in UZLX-GIST9. Compared to controls, IDRX-42 (25mg/kg) induced a significant decrease in mitosis. In UZLX-GIST25 and GIST882 grade 2-4 HR with myxoid degeneration was observed in all IDRX-42 (25mg/kg)-treated tumors. Conclusion: IDRX-42 showed significant antitumor activity in patient- and cell line-derived GIST xenograft models. The novel kinase inhibitor induced volumetric responses, decreased mitotic activity and had antiproliferative effects. In models with KIT exon 13 mutation IDRX-42 induced characteristic myxoid degeneration.

<p>Supplementary Fig. 2. Individual absolute tumor volumes per experiment and per treatment group on the first and last day of experiment.</p>

<p>Supplementary Fig. 1. Histopathological characteristics.</p>

<p>Supplementary Fig. 2. Individual absolute tumor volumes per experiment and per treatment group on the first and last day of experiment.</p>