Abstract

Activating mutations of the activation loop (AL) of KIT are associated with certain human neoplasms, including a subset of patients with AML,systemic mast cell disorders (SM), seminoma, and Gastrointestinal Stromal Tumors (GIST). KIT AL mutations such as D816V that are typically found in AML and SM are resistant to imatinib (IM, IC50 > 5–10 μM). Dasatinib (BMS-354825) is a novel, oral, multi-targeted kinase inhibitor that targets BCR-ABL and SRC. Due to its potent inhibition of these kinases, dasatinib is currently being tested in clinical trials of patients with imatinib resistant/intolerant CML/Ph+ ALL. Based on previous observations of the ability of certain SRC/ABL inhibitors to also inhibit KIT kinase, we hypothesized that dasatinib might inhibit the kinase activity of both WT and mutant KIT isoforms. The inhibitory potential of dasatinib against WT KIT, KIT mutant isoforms and KIT-dependent downstream pathways was evaluated by immunoblotting. In addition, we evaluated the effects of dasatinib on cellular proliferation and induction of apoptosis. Dasatinib potently inhibited WT, juxtamembrane- (JM) and AL-mutant KIT autophosphorylation. Based on the ability of dasatinib to bind to BCR-ABL irrespective of the ATP AL conformation (inactive versus active), dasatinib was expected to be insensitive to KIT AL conformation. In contrast, we found that the IC50 for KIT autophosphorylation varied significantly among the various KIT mutant isoforms: WT KIT, D816Y, V560G (JM mutation) [IC50 1–10 nM] <D816F [IC50 100 nM] <D816V [IC50 200–250 nM]. These results indicate that the conformation of the KIT AL does influence dasatinib potency. Inhibition of KIT kinase activity by dasatinib reduced cellular proliferation and induced apoptosis in mast cell/leukemia cell lines expressing mutant KIT isoforms. In these cell lines, KIT activates downstream pathways important for cell viability and cell survival such as RAS/MAPK, JAK/STAT and PI3K/AKT. Dasatinib potently blocked activation of MAPK1/2 and STAT3. Inhibition of KIT by dasatinib abrogated phosphorylation of AKT S473, but not AKT T308. This partial inhibition of AKT activation was insufficient to inhibit phosphorylation of p70S6K, a kinase downstream of AKT and mTOR. Combining dasatinib with rapamycin, a known mTOR inhibitor, had an additive to synergistic anti-proliferative effect on cells expressing KIT D816V. Our studies suggest that dastatinib may have clinical activity against human neoplasms that are associated with gain-of-function KIT mutations such as AML, SM, seminoma, and GIST. Combining dasatinib with mTOR inhibitors may further increase efficacy against KIT-driven malignancies.

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