The purpose of the present study was to investigate the in vitro and in vivo activity of PLX9486, a tyrosine kinase inhibitor (TKI) targeting both primary KIT exon 9 and 11 and secondary exon 17 and 18 mutations in gastrointestinal stromal tumors (GISTs). Imatinib, a potent inhibitor of mutated KIT, has revolutionized the clinical management of advanced, metastatic GIST. However, secondary resistance develops mainly through acquired mutations in KIT exons 13/14 or exons 17/18. Second-line sunitinib potently inhibits KIT exon 13/14 mutants but is ineffective against exon 17 mutations. In our study, PLX9486 demonstrated in vitro nanomolar potency in inhibiting the growth and KIT phosphorylation of engineered BaF3 cells transformed with KIT exon 17 mutations (p.D816V) and with the double KIT exon 11/17 mutations (p.V560G/D816V). The in vivo efficacy of PLX9486 was evaluated using two imatinib-resistant GIST patient-derived xenograft (PDX) models. In UZLX-GIST9 (KIT: p.P577del;W557LfsX5;D820G), PLX9486 100 mg/kg/day resulted in significant inhibition of proliferation. Pharmacodynamic analysis showed a pronounced reduction in mitogen-activated protein kinase (MAPK) activation and other downstream effects of the KIT signaling pathway but no significant effect on KIT Y703 and Y719 phosphorylation. Similarly, in MRL-GIST1 (KIT: p.W557_K558del;Y823D) PLX9486 treatment led to significant tumor regression and strong inhibition of MAPK activation. Interestingly, the inhibitory effect on MAPK activation was evident even after a single dose of PLX9486. In conclusion, PLX9486 showed anti-tumor efficacy in patient-derived imatinib-resistant GIST xenograft models, mainly through inhibition of KIT signaling. These preclinical efficacy data encourage further testing of PLX9486 in the clinical setting.
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Plexxikon (Berkeley, CA, USA) provided PLX9486 and financial support for the study.
Conflict of interest
Plexxikon provided PLX9486 and financial support for the study. EAB, BM, GH, JL, MN, GW, CZ and GB are employees of Plexxikon Inc. PS received institutional support from Plexxikon for advisory/consultancy, research funding and travel/accommodation/expenses.
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Growth curves of BaF3 KIT p.V560G/D816V cells, treated with PLX9486, showing PLX9486-sensitive (IC50 = 7.1 nM) KIT-dependent growth inhibition in the absence of IL3 (No IL3) and PLX9486-resistant (IC50 2,070 nM) KIT-independent growth in the presence of IL3. Data are shown as mean values ± standard error of the mean (TIFF 349 kb)
Body weight evolution during the treatment. UZLX-GIST9 (A). MRL-GIST1 (B) (TIFF 2155 kb)
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Gebreyohannes, Y.K., Burton, E.A., Wozniak, A. et al. PLX9486 shows anti-tumor efficacy in patient-derived, tyrosine kinase inhibitor-resistant KIT-mutant xenograft models of gastrointestinal stromal tumors. Clin Exp Med 19, 201–210 (2019). https://doi.org/10.1007/s10238-018-0541-2