Resistance mechanisms and therapeutic strategies of CDK4 and CDK6 kinase targeting in cancer

Abstract

Cyclin-dependent kinases (CDKs) 4 and 6 (CDK4/6) are important regulators of the cell cycle. Selective CDK4/6 small-molecule inhibitors have shown clinical activity in hormonal receptor-positive (HR⁺) metastatic breast cancer, but their effectiveness remains limited in other cancer types. CDK4/6 degradation and improved selectivity across CDK paralogs are approaches that could expand the effectiveness of CDK4/6 targeting. Recent studies also suggest the use of CDK4/6-targeting agents in cancer immunotherapy. In this Review, we highlight recent advancements in the mechanistic understanding and development of pharmacological approaches targeting CDK4/6. Collectively, these developments pose new challenges and opportunities for rationally designing more effective treatments.

Fig. 1 |. Binding models of select CDK inhibitors to CDK2 and CDK4/6 structures.

Interactions of inhibitors with key residues as formed in co-crystal structures (noted by Protein Data Bank (PDB) entry) are highlighted and compared with theoretical models to demonstrate differences that may explain inhibitor selectivity. a, Dinaciclib bound to the CDK2 (PDB 5L2W) structure. b, Dinaciclib, modeled on the CDK4 (PDB 7SJ3) and CDK6 (PDB 5L2I) structures. c, Palbociclib bound to the CDK6 (PDB 5L2I) structure and modeled on the CDK4 (PDB 7SJ3) structure. d, Palbociclib modeled on the CDK4 (PDB 7SJ3) and CDK2 (PDB 5L2W) structures. e, Ebvaciclib bound to the CDK2 (PDB 7KJS) structure. f, Ebvaciclib modeled on the CDK4 (PDB 7SJ3) and CDK6 (PDB 5L2I) structures. g, INX-315 modeled on the CDK2 (PDB 5L2W) structure. h, INX-315 modeled on the CDK4 (PDB 7SJ3) and CDK6 (PDB 5L2I) structures. Figures were prepared using PyMOL 3 (Schrödinger, 2023–2024).

Fig. 2 |. Major mechanisms of resistance to CDK4/6 inhibitors.

a, CDK4/6 inhibitors (CDK4/6i) suppress Rb phosphorylation and block Rb–E2F transcription by inhibiting the catalytic activity of CDK4–cyclin D (CycD) and CDK6–cyclin D complexes. Various mechanisms can cause resistance to CDK4/6 inhibitors. b, Downregulation or loss of Rb expression results in E2F transcription and cell cycle progression independent of CDK4/6 activity. c, Upregulation of expression and/or activity of CDK2–cyclin E results in Rb phosphorylation by CDK2, thereby bypassing CDK4/6 inhibition. d, Resistance to CDK4/6 inhibitors is also caused by reduced binding of drug to certain native conformations of CDK4 or CDK6. e, Activation of parallel growth factor signaling pathways (for example, PI3K–AKT–mechanistic target of rapamycin (mTOR)) promoting cell cycle progression by stabilizing cyclin D or cyclin E expression contributes to resistance to CDK4/6 inhibitors. RTK, receptor tyrosine kinase; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3-phosphate.

Fig. 3 |. Next-generation pharmacological strategies targeting the cell cycle machinery.

a,b, A large number of CDK4/6 inhibitor-based bifunctional degraders (PROTACs) have been developed, either degrading both CDK4 and CDK6 (a) or promoting selective degradation of CDK6 (b). CDK4/6-selective and CDK6-selective agents showed increased preclinical efficacy in preclinical models of HR⁺ breast cancer, MCL, acute myeloid leukemia and others. Ub, ubiquitin. c, Preclinical and clinical efforts explore combinatorial approaches targeting parallel growth factor signaling (PI3K–AKT, MAPK, mTOR) together with CDK4/6. MAPKi, MAPK inhibitor; RAF, rapidly accelerated fibrosarcoma. d, CDK2 is an established contributor to intrinsic and acquired tumor resistance to CDK4/6 inhibitors; inhibitors selective for CDK2 (CDK2i) or ‘dialing in’ CDK2 in addition to CDK4/6 are currently under early clinical development.

Fig. 4 |. CDK4/6 inhibitors promote tumor immunity.

a–c, In addition to their canonical role in blocking the cell cycle in the tumor (a), CDK4/6 inhibitors promote T cell infiltration in the tumor microenvironment by various mechanisms, including downregulation of the expression of PD-L1 (encoded by CD274) via cullin 3–speckle-type POZ protein (SPOP) E3 ligase through proteasome-mediated degradation (b), and secretion of type III IFNs and increased antigen presentation (c). DNMT, DNA methyltransferase (encoded by DNMT1); dsRNA, double-stranded RNA; ERV, endogenous retrovirus; MHC, major histocompatibility complex; NF-κB, nuclear factor κB. d,e, In the immune compartment, CDK4/6 inhibitors suppress regulatory T (T_reg) cell proliferation (d) through their canonical role of Rb–E2F suppression, while they stimulate effector T cells by upregulating NFAT activity (e) and promote a memory T cell phenotype.