Protein kinase A (PKA) regulates multiple signal transduction events via protein phosphorylation and is integral to all cellular responses involving the cyclic AMP second messenger system. KT 5720 is one of a family of compounds synthesized by the fungus Nocardiopsis sp. It blocks PKA signaling through competitive inhibition of ATP with a Ki value of 60 nM.[1] Reported IC50 values vary widely depending upon ATP concentration tested and can range from 56 nM (low ATP) to 3 μM (physiologic ATP).[2],[3] Non-specific effects of KT 5720 include inhibition of phosphorylase kinase, PDK1, MEK, MSK1, PKBα, and GSK3β at concentrations as effective as or more potent than that for inhibition of PKA.[2],[3]

Reference:
[1]. Kase, H., Iwahashi, K., Nakanishi, S., et al. K-252 compounds, novel and potent inhibitors of protein kinase C and cyclic nucleotide-dependent protein kinases. Biochemical and Biophysical Research Communications 142(2), 436-440 (1987).
[2]. Davies, S.P., Reddy, H., Caivano, M., et al. Specificity and mechanism of action of some commonly used protein kinase inhibitors. Biochem. J. 351(1), 95-105 (2000).
[3]. Murray, A.J. Pharmacological PKA inhibition: All may not be what it seems. Science Signaling 1(22), (2008).