The FIP1L1-PDGFR-α fusion kinase induces hypereosinophilic syndrome (HES) in a subset of patients (pts). Imatinib is a potent inhibitor of PDGFR-α and has been associated with durable hematologic responses in pts with HES. However, development of kinase domain mutations may hamper TKI activity, underscoring the need for novel agents to prevent or overcome resistance. We evaluated the efficacy of the novel TKI EXEL-0862 in FIP1L1-PDGFR-α-expressing cells. In 72-hour MTS assay, EXEL-0862 inhibited EOL-1 cell proliferation with an IC50 of 1.0 nM and markedly decreased phosphorylation of STAT3 and Erk1/2, suggesting inhibition of downstream targets of PDGFR-α. More important, EXEL-0862 induced dose-dependent inhibition of PDGFR-α phosphorylation in peripheral blood mononuclear cells from a pt with HES. Exposure of EOL-1 cells to EXEL-0862 10 nM for 24 hours caused apoptosis as evidenced by flow cytometry (annexin-V/PI staining) and transmission electron microscopy. This was coupled with time-dependent cleavage of caspases-3, caspase-9, and PARP and with mitochondrial damage as reflected by cytochrome c translocation into the cytosol, and a diminished mitochondrial transmembrane potential. Of note, apoptotic features were not evident after 16 hours of EXEL-0862 exposure. Immunoblots showed marked downregulation of Mcl-1, but not of Bcl-2, Bcl-XL, or Bax. To elucidate the mechanism of Mcl-1 cleavage, EOL-1 cells were exposed to 10 nM EXEL-0862 for 6 hours followed by cycloheximide (5 μg/ml), a potent translation inhibitor, which resulted in marked Mcl-1 turnover. Treatment with the proteasome inhibitor MG-132 failed to abrogate EXEL-0862-induced Mcl-1 cleavage, suggesting proteasome-independent degradation. Rather, a decrease of pro-form Mcl-1 (p42) and increase of the cleaved Mcl-1 peptide form of Mcl-1 (p28) coincided with caspase-3 activation. Pretreatment with 20 μM pan-caspase inhibitor Z-VAD abolished EXEL-0862-induced Mcl-1 cleavage, pointing to a caspase--dependent mechanism. Collectively, our results establish EXEL-0862 as a solid candidate for the targeted treatment of pts with FIP1L1-PDGFR-α-positive HES.

Disclosures: Exelixis, Inc.

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