Imatinib mesylate affects tyrosine kinase activity in both leukemic and normal primary mononuclear blood cells

Tyrosine kinase (TK) activity in primary mononuclear blood cells (MNBC) derived from chronic myelogenous leukemia (CML) patients in the chronic phase as well as from healthy donors was measured by a sensitive time-resolved fluorescence method using the Delfia® Tyrosine Kinase kit. The level of phosphotyrosine was assessed in parallel by flow-cytometry. The experimental protocol for Delfia® was optimized using a K562 cell line. A large part (20 to 50%) of the fluorescence signal from K562 cells was sensitive to Imatinib mesylate, an inhibitor of Bcr-Abl tyrosine kinase, which is currently the leading drug in CML treatment. In primary MNBC, the direct contribution of Bcr-Abl itself to the signal was low. However, a 48h treatment of MNBC with 5 μM Imatinib resulted in a significant reduction of the observed TK activity (mean TK activity value: 56% of control) paralleled by a decrease in the phosphotyrosine level in the CML group. Modification of TK activity by Imatinib was observed also in the donor group. Imatinib mesylate thus probably affects cell signalization even in Bcr-Abl negative cells.