Imatinib-Mesylate Blocks Recombinant T-Type Calcium Channels Expressed in Human Embryonic Kidney-293 Cells by a Protein Tyrosine Kinase-Independent Mechanism

The 2-phenylaminopyrimidine derivative imatinib-mesylate, a powerful protein tyrosine kinase (PTK) inhibitor that targets abl, c-kit, and the platelet-derived growth factor receptors, is rapidly gaining a relevant role in the treatment of several types of neoplasms. Because first generation PTK inhibitors affect the activity of a large number of voltage-dependent ion channels, the present study explored the possibility that imatinib-mesylate could interfere with the activity of T-type channels, a class of voltage-dependent Ca²⁺ channels that take part in the chain of events elicited by PTK activation. The effect of the drug on T-type channel activity was examined using the whole-cell patch-clamp technique with Ba ²⁺ (10 mM) as the permeant ion in human embryonic kidney-293 cells, stably expressing the rat Ca_v3.3 channels. Imatinib-mesylate concentrations, ranging from 30 to 300 μM, reversibly decreased Ca _v3.3 current amplitude with an IC₅₀ value of 56.9 μM. By contrast, when imatinib-mesylate (500 μM) was intracellularly dialyzed with the pipette solution, no reduction in Ba²⁺ current density was observed. The 2-phenylaminopyrimidine derivative modified neither the voltage dependence of activation nor the steady-state inactivation of Ca_v3.3 channels. The decrease in extracellular Ba²⁺ concentration from 10 to 2 mM and the substitution of Ca²⁺ for Ba²⁺ increased the extent of 30 μM imatinib-mesylate-induced percentage of channel blockade from 25.9 ± 2.4 to 36.3 ± 0.9% in 2 mM Ba²⁺ and 44.2 ± 2.3% in 2 mM Ca²⁺. In conclusion, imatinib-mesylate blocked the cloned Ca_v3.3 channels by a PTK-independent mechanism. Specifically, the drug did not affect the activation or the inactivation of the channel but interfered with the ion permeation process.