First Selective KIT D816V Inhibitor for Patients with Systemic Mastocytosis
Authors: Evans Erica K. , Hodous Brian L. , Gardino Alexandra , Zhu Julia , Shutes Adam , Davis Alison , Kim Joseph , Wilson Kevin , Wilson Doug , Zhang Yulian , Kohl Nancy E. , Guzi Timothy , Lengauer Christoph 
Published: November 2019

Abstract
Systemic mastocytosis is a disease characterized by the abnormal proliferation and accumulation of mast cells. In aggressive cases, these mast cells accumulate in organs such as bone marrow, liver and spleen and result in compromised organ function with average patient survival only 3 to 5 years after diagnosis. The mast cells of nearly all systemic mastocytosis patients harbor a heterozygous D816V mutation in the activation loop of KIT conferring constitutive, ligand-independent activation of this receptor tyrosine kinase, suggesting this mutation is a driver of disease. While KIT D816V can be targeted by small molecules such as dasatinib and midostaurin, these agents have activity against many human kinases resulting in dose limiting toxicities in the clinic that prevent complete suppression of KIT D816V activity in vivo. In vitro, their potent activity against multiple kinases leads to uncertainties regarding their mechanism of action. Thus far, selective inhibition of the KIT D816V mutation has not been achieved. However starting with a novel chemical library optimized for kinase selectivity, we have identified BLU-285, a small molecule inhibitor targeting KIT exon 17 mutants including the activated KIT D816V kinase. BLU-285 potently disrupts KIT D816V oncogenic signaling as measured by inhibition of both KIT D816V autophosphorylation and phosphorylation of the downstream substrates Akt and Stat3 in the human mast cell leukemia cell line HMC1.2. In vitro, BLU-285 inhibits proliferation and induces apoptosis in the mouse mastocytoma cell line P815. In vivo, BLU-285 is a well-tolerated, orally bioavailable agent that achieves dose dependent tumor growth inhibition in a P815 mouse xenograft model with tumor regression observed at 30 mg/kg once daily dosing. Tumor growth inhibition correlates with inhibition of KIT autophosphorylation; greater than 80% target suppression throughout the 24-hour dosing period is required for effective tumor growth inhibition. Prolonged target suppression is achievable with BLU-285 but not dasatinib, even when dosed at the MTD in mouse. Furthermore, to more closely mimic the nature of systemic mastocytosis, we have developed a disseminated model of disease whereby the in vivo growth of P815-luciferase expressing cells inoculated intravenously can be measured by whole body bioluminescence. Treatment of mice with systemic disease leads to dose dependent inhibition of disease, with a 3-fold increase in survival time when dosed 30 mg/kg QD. In addition, as anticipated by its selectivity profile, BLU-285 is very well tolerated in vivo with no impact on body weight at efficacious doses. Our data demonstrate that selective inhibition of KIT D816V with BLU-285 achieves complete and prolonged inactivation of the disease-driving kinase and suggests that BLU-285 may provide a compelling new therapy for patients with systemic mastocytosis.

Disclosures
Evans: Blueprint Medicines: Employment, Equity Ownership. Hodous:Blueprint Medicines: Employment, Equity Ownership. Gardino:Blueprint Medicines: Employment, Equity Ownership. Zhu:Blueprint Medicines: Employment, Equity Ownership. Shutes:Blueprint Medicines: Employment, Equity Ownership. Davis:Blueprint Medicines: Employment, Equity Ownership. Kim:Blueprint Medicines: Employment, Equity Ownership. Wilson:Blueprint Medicines: Employment, Equity Ownership. Wilson:Blueprint Medicines: Employment, Equity Ownership. Zhang:Blueprint Medicines: Employment, Equity Ownership. Kohl:Blueprint Medicines: Employment, Equity Ownership. Guzi:Blueprint Medicines: Employment, Equity Ownership. Lengauer:Blueprint Medicines: Employment, Equity Ownership.