Synthesis and structure-activity relationships of targeted protein degraders for the understudied kinase NEK9

Nek9 is a member of the understudied Nek family of dark kinases. Aberrant activation of Nek9 kinase signaling has been linked to poor keratinocyte differentiation phenotypes, and is a key driver of nevus comedonicus, a rare, localized form of acne. Nek9 also has essential scaffolding roles; during mitosis the non-catalytic C-terminal domain of Nek9 binds to Nek6 and Nek7, releasing them from an auto-inhibitory conformation, and enabling proper mitotic progression. Finally, Nek9 expression has been linked to cancer proliferation. SiRNA mediated Nek9 knock-down in a panel of cancer cell lines induces G1 cell cycle arrest and inhibits proliferation when p53 is also inactivated; cell lines with functional p53 are unaffected. Presently, no selective small molecule Nek9 chemical probes are available, though a subset of promiscuous kinase inhibitors have Nek9 activity. Recently described targeted protein degradation approaches have shown that degrader molecules based on multi-targeted kinase inhibitors may effect selective kinase degradation, despite binding to many targets. In this study we report the identification and SAR of potent degraders of the NEK9 kinase that represent attractive leads for further development. Future work is needed to optimize the selectivity of the compounds.