Synergistic Growth-Inhibitory Effects of Two Tyrosine Kinase Inhibitors, Dasatinib and PKC412, on Neoplastic Mast Cells Expressing the D816V-Mutated Oncogenic Variant of KIT.

In a majority of all patients with systemic mastocytosis (SM) including aggressive SM and mast cell leukemia (MCL), neoplastic cells display the D816V-mutated variant of KIT. The respective oncoprotein, KIT-D816V, exhibits constitutive tyrosine kinase (TK) activity and has been implicated in malignant cell growth. Therefore, several attempts have been made to identify KIT-D816V-targeting drugs. We found that the TK-inhibitor dasatinib (BMS-354825) inhibits TK activity of wild type (wt) KIT and KIT-D816V in Ba/F3 cells with doxycycline-inducible KIT-expression. In addition, dasatinib was found to inhibit KIT D816V-induced cluster formation and viability in Ba/F3 cells as well as growth of HMC-1.1 cells (KIT-D816V-negative) and HMC-1.2 cells (KIT-D816V-positive). The effects of dasatinib on growth of HMC-1 cells were dose-dependent, with 100–1,000-fold higher IC₅₀-values in cells harbouring KIT-D816V compared to cells lacking KIT-D816V. Furthermore, dasatinib was found to inhibit the growth of primary neoplastic mast cells in SM in all patients examined. The inhibitory effects of dasatinib in HMC-1 cells were found to be associated with apoptosis and a decrease in expression of CD2 and CD63 as determined by flow cytometry. In addition, dasatinib was found to cooperate with the tyrosine kinase inhibitors PKC412 (midostaurin), AMN107 (nilotinib), and STI571 (imatinib), as well as with 2CdA (cladribine) in producing growth-inhibition in neoplastic mast cells. In HMC-1.1 cells, all drug-interactions applied were found to be synergistic. By contrast, in HMC-1.2 cells, only the combinations “dasatinib+PKC412” and “dasatinib+2CdA” were found to produce synergistic effects. These drug-combinations may thus represent an interesting pharmacologic approach for the treatment of patients with aggressive systemic mastocytosis or mast cell leukemia.