Selective inhibition of Ph-positive ALL cell growth through kinase-dependent and -independent effects by CDK6-specific PROTACs

Blood. 2020 Apr 30;135(18):1560-1573. doi: 10.1182/blood.2019003604.


Expression of the cell cycle regulatory gene CDK6 is required for Philadelphia-positive (Ph+) acute lymphoblastic leukemia (ALL) cell growth, whereas expression of the closely related CDK4 protein is dispensable. Moreover, CDK6 silencing is more effective than treatment with the dual CDK4/6 inhibitor palbociclib in suppressing Ph+ ALL in mice, suggesting that the growth-promoting effects of CDK6 are, in part, kinase-independent in Ph+ ALL. Accordingly, we developed CDK4/6-targeted proteolysis-targeting chimeras (PROTACs) that inhibit CDK6 enzymatic activity in vitro, promote the rapid and preferential degradation of CDK6 over CDK4 in Ph+ ALL cells, and markedly suppress S-phase cells concomitant with inhibition of CDK6-regulated phospho-RB and FOXM1 expression. No such effects were observed in CD34+ normal hematopoietic progenitors, although CDK6 was efficiently degraded. Treatment with the CDK6-degrading PROTAC YX-2-107 markedly suppressed leukemia burden in mice injected with de novo or tyrosine kinase inhibitor-resistant primary Ph+ ALL cells, and this effect was comparable or superior to that of the CDK4/6 enzymatic inhibitor palbociclib. These studies provide "proof of principle" that targeting CDK6 with PROTACs that inhibit its enzymatic activity and promote its degradation represents an effective strategy to exploit the "CDK6 dependence" of Ph+ ALL and, perhaps, of other hematologic malignancies. Moreover, they suggest that treatment of Ph+ ALL with CDK6-selective PROTACs would spare a high proportion of normal hematopoietic progenitors, preventing the neutropenia induced by treatment with dual CDK4/6 inhibitors.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Apoptosis / drug effects
  • Cell Cycle / drug effects
  • Cell Line, Tumor
  • Cell Proliferation / drug effects
  • Cyclin-Dependent Kinase 6 / antagonists & inhibitors*
  • Cyclin-Dependent Kinase 6 / metabolism
  • Disease Models, Animal
  • Enzyme Activation / drug effects
  • Gene Expression Profiling
  • Genes, cdc
  • Humans
  • Mice
  • Molecular Structure
  • Phosphorylation
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / drug therapy*
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / etiology
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / metabolism*
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / pathology
  • Protein Kinase Inhibitors / chemistry
  • Protein Kinase Inhibitors / pharmacology*
  • Protein Kinase Inhibitors / therapeutic use
  • Recombinant Fusion Proteins / chemistry
  • Recombinant Fusion Proteins / pharmacology*
  • Recombinant Fusion Proteins / therapeutic use
  • Treatment Outcome
  • Xenograft Model Antitumor Assays


  • Protein Kinase Inhibitors
  • Recombinant Fusion Proteins
  • CDK6 protein, human
  • Cyclin-Dependent Kinase 6