Cyclin-dependent kinase 6 (CDK6) is an important regulator of the cell cycle. Together with CDK4, it phosphorylates and inactivates
retinoblastoma (
Rb) protein. In tumour cells, CDK6 is frequently upregulated and CDK4/6
kinase inhibitors like
palbociclib possess high activity in
breast cancer and other
malignancies. Besides its crucial catalytic function,
kinase-independent roles of CDK6 have been described. Therefore, targeted degradation of CDK6 may be advantageous over
kinase inhibition.
Proteolysis targeting chimeras (
PROTACs) structurally based on the cereblon (CRBN)
ligand thalidomide have recently been described to degrade the targets CDK4/6. However, CRBN-based
PROTACs have several limitations including the remaining activity of
immunomodulatory drugs (
IMiDs) on Ikaros
transcription factors as well as CRBN inactivation as a resistance mechanism in
cancer. Here, we systematically explored the chemical space of CDK4/6
PROTACs by addressing different E3
ligases and connecting their respective small-molecule binders via various linkers to
palbociclib. The spectrum of CDK6-specific
PROTACs was extended to von Hippel Lindau (VHL) and cellular
inhibitor of apoptosis protein 1 (cIAP1) that are essential for most
cancer cells and therefore less likely to be inactivated. Our VHL-based
PROTAC series included compounds that were either specific for CDK6 or exhibited dual activity against CDK4 and CDK6.
IAP-based PROTACs caused a combined degradation of CDK4/6 and IAPs resulting in synergistic effects on
cancer cell growth. Our new degraders showed potent and long-lasting degrading activity in human and mouse cells and inhibited proliferation of several
leukemia, myeloma and
breast cancer cell lines. In conclusion, we show that VHL- and
IAP-based PROTACs are an attractive approach for targeted degradation of CDK4/6 in
cancer.