Comparison of the specificity of Trk inhibitors in recombinant and neuronal assays.

Neurotrophins are important mediators of neuronal development, survival and plasticity. They act via binding to Trk receptors, which results in the stimulation of the intracellular tyrosine kinase domain of the receptor leading to autophosphorylation of this domain. This in turn creates a scaffold that recruits various adapter proteins allowing the activation of intracellular signaling cascades including the PLCγ, MAPK and PI3K pathways. Compounds that specifically block the activity of the tyrosine kinase domain of Trk receptors would provide a powerful tool to study the role of these receptors in cells. K252a has previously been used for this purpose, however we show here that it can inhibit many tyrosine and serine/threonine kinases in vitro. Profiling of 3 newer inhibitors, referred to here as SHN-753, SHN-722 and GSK-Trk, demonstrate that they have significantly improved specificity for the kinase activity of TrkA in vitro compared to K252a. In addition these compounds were found to block the TrkB mediated activation of ERK1/2 by BDNF, but did not affect NMDA induced ERK1/2 activation. These compounds, while still not completely specific for Trk receptor kinase activity, do represent a considerable improvement over K252a and should prove valuable in the study of neurotrophin-mediated actions in the nervous system.