Tackling resistance in chronic myeloid leukemia: Novel cell death modulators with improved efficacy.

The development of resistance poses a serious problem in the therapy of cancer due to the necessity of a multiple-drug and unlimited treatment of affected patients. In chronic myeloid leukemia (CML), the introduction of imatinib has revolutionized the therapy. The persistence of an untreatable cancer stem cell pool and other resistance-causing factors, however, also impede the cure of this malignancy. New therapeutic approaches are therefore essential to overcome current treatment drawbacks. In this regard, an intervention in the STAT5 signaling pathway can significantly improve drug response, as this central signaling node induces the formation of highly resistant CML cells. In the present study, we continued the design of efficient chemosensitizers derived from the partial PPARγ agonist telmisartan. The developed 2-carbonitriles or 2-carboxymethyl esters showed improved potency in sensitizing K562-resistant cells to imatinib treatment, even at concentrations, which are considered patient-relevant. At 5 μM, for instance, 2d sensitized the cells in such a manner that the resistance was fully overcome and the recovered efficacy of imatinib resulted in >76% cell death. Importantly, all compounds were non-cytotoxic per se. A transactivation experiment showed that only the carbonitriles are partial agonists of PPARγ, which does not seem to be involved in the mode of action. Yet, immunoassays revealed a suppression of the STAT5 phosphorylation status by co-application of the most active derivatives with imatinib. This mechanism consequently resulted in reduced cell proliferation and induction of cell death in resistant CML cells.