In most patients with systemic mastocytosis (SM) including aggressive SM and mast cell leukemia (MCL), neoplastic cells express the oncogenic c-KIT mutation D816V. KIT-D816V is associated with constitutive tyrosine kinase (TK) activity and thus represents an attractive target of drug therapy. However, most available TK inhibitors including STI571=imatinib, fail to block TK-activity of KIT D816V at pharmacologic concentrations. We provide evidence that the novel TK-targeting drugs PKC412 and AMN107 decrease TK-activity of D816V-mutated KIT and counteract growth of Ba/F3 cells with doxycycline-induced expression of KIT D816V as well as growth of the human mast cell leukemia cell line HMC-1 expressing this c-KIT mutation. PKC412 was found to be the superior drug with IC50 values of 50–250 nM and without differences seen between HMC-1 cells exhibiting or lacking KIT D816V. By contrast, AMN107 exhibited potent effects only in the absence of KIT D816V in HMC-1 cells. Corresponding results were obtained with Ba/F3 cells exhibiting wild-type or the D816V-mutated variant of KIT. Moreover, we found that PKC412 and AMN107 inhibit growth of primary neoplastic MC in a patient with KIT D816V+ SM. The growth-inhibitory effects of PKC412 and AMN107 on HMC-1 cells were associated with TK-inhibition of KIT and with induction of apoptosis. In addition, PKC412 was found to downregulate expression of CD2 and CD63, two cell surface antigens upregulated in SM. In co-incubation experiments, PKC412 was found to synergize with AMN107, imatinib, and 2CdA in producing growth inhibition in HMC-1 cells lacking KIT D816V, whereas in KIT D816V+ HMC-1 cells, drug-interactions were additive rather than synergistic. Together, PKC412 and AMN107 alone and in combination counteract growth of neoplastic mast cells. Both drugs may therefore be considered as novel promising agents for targeted therapy in patients with aggressive SM or MCL.

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