The dark side of protein kinase CK2 inhibition.

Casein kinase 2 (CK2) is a ubiquitous, highly pleiotropic and essential protein kinase whose abnormally high constitutive activity has been implicated in several human diseases. In the last decade, several ATP competitive inhibitors of CK2, characterized by an in vitro activity that ranges from micromolar to nanomolar, have been discovered. However, until now only one drug candidate has been entered in Phase I clinical trial as a potential anticancer drug. Why this constitutively active kinase is so undruggable? Can ATP competitive inhibitors be considered the most promising drug candidates for the near future? In this review, we would like to underline how targeting binding sites outside the conventional ATP-binding could represent a new promising strategy to inhibit CK2 activity and, consequently, bear a great potentiality in discovering new drug candidates.