Capitalizing on paradoxical activation of the MAPK pathway for treatment of Imatinib-resistant mast cell leukemia and chronic myelogenous leukemia

Prevention of fatal side effects during the therapy of cancer patients with high-dosed pharmacological inhibitors is to date a major challenge. Moreover, the development of drug resistance poses severe problems for the treatment of patients with leukemia or solid tumors. Particularly drug-mediated dimerization of RAF kinases can be the cause of acquired resistance, also called "paradoxical activation". By re-analyzing the effects of different tyrosine kinase inhibitors (TKIs) on the proliferation, metabolic activity, and survival of the Imatinib-resistant, KITV560G,D816V-expressing human mast cell leukemia (MCL) cell line HMC-1.2, low concentrations of the TKIs Nilotinib and Ponatinib resulted in enhanced proliferation, suggesting paradoxical activation of the MAPK pathway. Indeed, these TKIs caused BRAF-CRAF dimerization, resulting in ERK1/2 activation. This allowed then for the effective suppression of HMC-1.2 proliferation and metabolic activity, as well as induction of apoptotic cell death in the presence of nanomolar concentrations of a combination of Ponatinib and of the MEK inhibitor Trametinib. Effectiveness of this drug combination was recapitulated in the human MC line ROSA expressing KITD816V as well as in Imatinib-resistant Ba/F3-BCR-ABLT315I cells. In conclusion, Imatinib-resistance in cells expressing respective mutants of the tyrosine kinases KIT or BCR-ABL can be efficiently bypassed by a low-concentration combination of the TKI Ponatinib and the MEK inhibitor Trametinib, potentially reducing negative side-effects in the therapy of leukemia patients.