Introduction: GIST is the most common sarcoma of the GI tract, with most cases driven by activating mutations in KIT Ex11 or Ex9. Prognosis is poor for patients whose tumors progress following first-line (1L) imatinib treatment, despite availability of 3 approved drugs (sunitinib [2L], regorafenib [3L], ripretinib [4L]). In most patients, resistance is driven by secondary mutations in KIT, with nearly all occurring in either the ATP binding pocket (Ex13/14) or activation loop (Ex17/18). No approved drug has potent activity against both classes of resistance mutations and most patients appear to have multiple resistance mutations. There remains a need for a pan-mutant KIT inhibitor, which has potent activity against all classes of mutations.

Experimental Procedures: Drug activity was determined by assessing effects on tumor cell lines and BaF3 cells expressing mutant forms of KIT. Mice were dosed orally once (THE-630 and sunitinib) or twice (ripretinib) daily at doses that did not exceed the MTD.

Results: THE-630 potently (IC50 ~3 nM) inhibited KIT-driven viability and downstream signaling in GIST-T1 cells, which contain an activating deletion in Ex11 (Ex11Del). Similar results were observed in engineered BaF3 cells containing Ex11Del or an activating insertion in Ex9 (both IC50s ≤3 nM), whereas potency of imatinib, regorafenib, and ripretinib, was reduced by ≥5-fold against the Ex9 variant. Consistent with available clinical data, in engineered BaF3 cells containing a primary Ex11Del mutation and a resistance mutation, none of the 4 approved drugs had potent activity against both classes of resistance mutations. In contrast, THE-630 potently inhibited ATP binding pocket mutants (V654A and T670I) and activation loop mutants (D816G/H, D820A/G, N822K, Y823D, A829P), with IC50s all below 25 nM. THE-630 also had potent activity against V654A and D816H in the context of an Ex9 primary mutation (IC50s 10 and 33 nM). In vivo, THE-630 strongly inhibited tumor growth (by 86%) in a model containing the most common ATP binding pocket mutant (V654A) as a secondary mutation, while ripretinib achieved only modest inhibition (26%). THE-630 was also highly active in models expressing secondary activation loop mutants, regressing tumors containing N822K by 88% (compared to 25% tumor growth inhibition by sunitinib) and regressing tumors containing D820A by 59% (compared to 1% tumor regression by ripretinib).

Conclusions: We have identified a next-generation pan-KIT inhibitor that has potent activity against all major classes of activating and resistance mutations observed in KIT-mutant GIST patients. A phase 1 clinical trial of THE-630 is planned to begin in 2021.

Citation Format: Victor M. Rivera, Wei-Sheng Huang, Mengrou Lu, Justin R. Pritchard, David Dalgarno, William C. Shakespeare. Preclinical characterization of THE-630, a next-generation inhibitor for KIT-mutant gastrointestinal stromal tumors (GIST) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1292.