CDK4/6 Inhibitors: The Mechanism of Action May Not Be as Simple as Once Thought

In the paper

“In vitro studies using cyclin D1/CDK4 and various cyclin D/CDK6 kinases determined that both abemaciclib and ribociclib are more potent against CDK4 than CDK6 (Gelbert et al., 2014; Tripathy et al., 2017) (Table 1).”

What the sources say

• Not checked yetcites Gelbert et al., 2014 source
• Not checked yetcites Tripathy et al., 2017 source

In the paper

“Here we highlight the mechanisms by which CDK4/6 inhibitors can exert their anti-tumor activities beyond simply enforcing cytostatic growth arrest”

In the paper

“Such pan-CDK inhibitors had limited success, due in part to the dose limiting toxicities that hampered their use (Asghar et al., 2015).”

What the sources say

• Not checked yetcites Asghar et al., 2015 source

In the paper

“CDK4/6 activity bridges numerous extracellular signaling pathways to the cell cycle (Sherr and Roberts, 1999).”

What the sources say

• Not checked yetcites Sherr and Roberts, 1999 source

In the paper

“Although palbociclib was the first CDK4/6 inhibitor to demonstrate clinical efficacy (Finn et al., 2016), there are two others that soon followed.”

What the sources say

• Not checked yetcites Finn et al., 2016 source

In the paper

“This may translate to higher cerebrospinal fluid concentrations for ribocliclib compared to palbociclib and abemaciclib as noted in mouse models (DiPippo et al., 2016; Raub et al., 2015; Yin et al., 2017).”

What the sources say

• Not checked yetcites DiPippo et al., 2016 source
• Not checked yetcites Raub et al., 2015 source
• Not checked yetcites Yin et al., 2017 source

In the paper

“The growth suppressive properties of Rb are largely, but not completely, associated with its binding to the transcription factor E2F and repressing transcription at target promoters (Classon and Harlow, 2002; Harbour and Dean, 2000; Stevaux and Dyson, 2002).”

What the sources say

• Not checked yetcites Classon and Harlow, 2002 source
• Not checked yetcites Harbour and Dean, 2000 source
• Not checked yetcites Stevaux and Dyson, 2002

In the paper

“In such assays, abemaciclib also has modest activity, relative to its CDK4 inhibitory activity, against cyclin T1/CDK9, cyclin E2/CDK2, p25/CDK5 and p35/CDK5 (Gelbert et al., 2014) (Table 2).”

What the sources say

• Not checked yetcites Gelbert et al., 2014 source

In the paper

“CDK4/6 kinases can phosphorylate factors involved in cell differentiation affecting their transcriptional activity, apoptotic factors affecting their activity, and other factors that can directly affect mitochondrial activity (Hydbring et al., 2016; Lim and Kaldis, 2013).”

What the sources say

• Not checked yetcites Hydbring et al., 2016 source
• Not checked yetcites Lim and Kaldis, 2013

In the paper

“In virtually all human cancer cells, this circuit is dysregulated by either overexpression of cyclin D1, loss of p16Ink4a, the mutation of CDK4 to an Ink4-refractory state, or the loss of Rb itself (Classon and Harlow, 2002).”

What the sources say

• Not checked yetcites Classon and Harlow, 2002 source

In the paper

“However, the remarkable specificity of all of these drugs to inhibit the proliferation of Rb-positive tumor cells but not Rb-negative tumor cells, suggests that differences in the in vitro profiles might not contribute that much to their in vivo activity.”

In the paper

“Palbociclib on the other hand has similar potency when comparing its activity on cyclin D1/CDK4 and cyclin D2/CDK6 (Fry et al., 2004).”

What the sources say

• Not checked yetcites Fry et al., 2004 source

In the paper

“In normal cells, the phosphorylation of Rb is typically carried out by the sequential actions of the CDK4 or CDK6 kinases in complex with a positive regulatory D-type cyclin subunit, followed by cyclin E/CDK2 complexes (Harbour et al., 1999; Lundberg and Weinberg, 1998).”

What the sources say

• Not checked yetcites Harbour et al., 1999 source
• Not checked yetcites Lundberg and Weinberg, 1998

In the paper

“changing dosing schedules where cells are first induced to exit the cell cycle and are then treated with a second drug targeting such signaling events may even provide a therapeutic gain by minimizing the dose necessary and the subsequent toxicity associated with such inhibitors.”

In the paper

“Specifically, down-regulation of MDM2, redistribution of the chromatin remodeling enzyme ATRX, and repression of HRAS transcription are necessary for the transition of CDK4 inhibitor-induced quiescence into senescence in a number of mesenchymal and epithelial cell lines derived from different tumor types, including breast, non-small cell lung cancer, soft tissue sarcoma and glioma (Kovatcheva et a”

What the sources say

• Not checked yetcites Kovatcheva et al., 2017 source
• Not checked yetcites Kovatcheva et al., 2015 source

In the paper

“Outcomes can depend on the nature of the cytostatic effect in tumor cells, vis a vis whether it undergoes a reversible quiescence or a more stable senescence.”

In the paper

“In the few human patients receiving abemaciclib, in which drug levels in the central nervous system were measured, accumulations in the range of 2.2–14.7 nmol/L were achieved (Patnaik et al., 2016).”

What the sources say

• Not checked yetcites Patnaik et al., 2016 source

In the paper

“Collectively, these studies suggest that the cooperation of CDK4/6 inhibitors and signaling pathway inhibitors may be affecting sequential decisions in the tumor cell.”

In the paper

“Thus, increased therapeutic efficiency can occur by enforcing a more durable cell cycle exit (Figure 1A) as has been extensively reviewed (for example(Sherr et al., 2016)).”

What the sources say

• Not checked yetcites Sherr et al., 2016 source

In the paper

“When considering the cytostatic effect of CDK4/6 inhibitors, interrupted dosing can provide an opportunity for potent synergy with combination therapies dedicated to interfering with other cell cycle effects.”

In the paper

“On the other hand, the SASP can create a pro-tumorigenic environment (Coppe et al., 2010; Krtolica et al., 2001; Ruhland et al., 2016).”

What the sources say

• Not checked yetcites Coppe et al., 2010 source
• Not checked yetcites Krtolica et al., 2001 source
• Not checked yetcites Ruhland et al., 2016 source

In the paper

“In addition to creating a permissive senescence environment by fulfilling the requisite of cell cycle exit, CDK4/6 inhibition may elicit senescence via alternative CDK4/6 substrates.”

In the paper

“FOXM1 suppression following CDK4/6 inhibition leads to the accumulation of reactive oxygen species and senescence in transformed melanomas, but not in normal melanocytes.”

In the paper

“Inhibitors that have minimal effect in cycling tumor cells may have more of an impact in non-cycling CDK4/6 inhibitor treated cells, perhaps pushing them into senescence.”

In the paper

“Grade 3–4 neutropenia is observed in approximately 60% of patients taking palbociclib and ribociclib (Asghar et al., 2015; Hortobagyi et al., 2016).”

What the sources say

• Not checked yetcites Asghar et al., 2015 source
• Not checked yetcites Hortobagyi et al., 2016 source

In the paper

“Depending on the cell type and the transforming event, some Rb-positive cells undergo quiescence and others undergo senescence when treated with CDK4/6 inhibitors (Baughn et al., 2006; Choi et al., 2012; Kovatcheva et al., 2015; Michaud et al., 2010; Puyol et al., 2010).”

What the sources say

• Not checked yetcites Baughn et al., 2006 source
• Not checked yetcites Choi et al., 2012 source
• Not checked yetcites Kovatcheva et al., 2015 source

In the paper

“In some of these models, resistance can be overcome by including CDK4/6 inhibitors in the treatment (Finn et al., 2009; Goel et al., 2016; Kwong et al., 2012; Vora et al., 2014; Zhou et al., 2017).”

What the sources say

• Not checked yetcites Finn et al., 2009 source
• Not checked yetcites Goel et al., 2016 source
• Not checked yetcites Kwong et al., 2012 source

In the paper

“In cell lines and xenografts, intrinsic and acquired resistance to signaling pathway inhibitors against estrogen, RAF, EGFR, PI3K, and others is sometimes associated with mutations in p16Ink4a, upregulated expression of cyclin D1 or other D-type cyclins, or upregulation of CDK4 or CDK6 (Jiang et al., 2016; Long et al., 2014; Yadav et al., 2014).”

What the sources say

• Not checked yetcites Jiang et al., 2016 source
• Not checked yetcites Long et al., 2014 source
• Not checked yetcites Yadav et al., 2014 source

In the paper

“Senescent cells express a cell type and signal specific program of gene expression and cytokine secretion (the SASP), which can sculpt the immune response (Ohtani et al., 2012).”

What the sources say

• Not checked yetcites Ohtani et al., 2012 source

In the paper

“interrupted dosing may provide an additional opportunity to use cytotoxic DNA damaging and other cell cycle targeting therapies as the cells become somewhat synchronized upon release, resulting in increased susceptibility to those compounds (Francis et al., 2017; Huang et al., 2012; Yang et al., 2015).”

What the sources say

• Not checked yetcites Francis et al., 2017 source
• Not checked yetcites Huang et al., 2012 source
• Not checked yetcites Yang et al., 2015 source

In the paper

“However, a number of recent investigations provide an alternative mechanistic explanation for the clinical activity of CDK4/6 inhibitors in combination with other drugs.”

In the paper

“Recently, in a variety of breast cancer models, including patient-derived xenografts and an MMTV-HER2 mouse, it was demonstrated that abemaciclib or palbociclib induces growth arrest and up-regulation of antigen processing and presentation in the tumor cells (Goel et al., 2017).”

What the sources say

• Not checked yetcites Goel et al., 2017 source

In the paper

“Evidence that BCL2 inhibitors can directly eliminate senescent cells (Chang et al., 2016; Yosef et al., 2016) suggests that combining this with other cancer therapies in a sequential manner might be useful.”

What the sources say

• Not checked yetcites Chang et al., 2016 source
• Not checked yetcites Yosef et al., 2016 source

In the paper

“The importance of CDK4 activity within the microenvironment was first demonstrated by crossing an RCAS-PDGF/nestin-TvA mouse model of oligodendroglioma into a CDK4 deficient background (Ciznadija et al., 2011). CDK4 is required for the proliferation of the tumor cells; however, reconstituting incipient CDK4 deficient tumor cells with CDK4 expression vectors is not sufficient for tumors to progress”

What the sources say

• Not checked yetcites Ciznadija et al., 2011 source

In the paper

“This enhancement is driven at least in part by CDK6-induced phosphorylation of NFAT4.”

What the sources say

• Not checked yetcites Deng et al., 2017 source

In the paper

“CDK4 is required for the proliferation of the tumor cells; however, reconstituting incipient CDK4 deficient tumor cells with CDK4 expression vectors is not sufficient for tumors to progress to a more aggressive state when the rest of the animal is CDK4 deficient.”

What the sources say

• Not checked yetcites Ciznadija et al., 2011 source

In the paper

“The cyclin D3/CDK6 kinase complex can phosphorylate 6-phosphofructokinase and pyruvate kinase M2 (Wang et al., 2017).”

What the sources say

• Not checked yetcites Wang et al., 2017 source

In the paper

“The lack of progression in this model is associated with a defect in the maturation of tumor-associated microglia, which remain in a sentinel mode in the absence of CDK4.”

What the sources say

• Not checked yetcites Ciznadija et al., 2011 source

In the paper

“In both tumor bearing and non-tumor bearing animals, CDK4 deficiency is associated with a reduced number of infiltrating and circulating CD4+FOXP3+ Treg cells with minimal impact on other T-cell subsets (Chow et al., 2010; Goel et al., 2017).”

What the sources say

• Not checked yetcites Chow et al., 2010 source
• Not checked yetcites Goel et al., 2017 source

In the paper

“Inhibition of CDK4/6 with any of the three drugs in pancreatic ductal adenocarcinoma cell lines alters glycolytic and oxidative metabolism leading to an increase in ROS in an Rb-dependent manner (Franco et al., 2016).”

What the sources say

• Not checked yetcites Franco et al., 2016 source

In the paper

“Thus, by enhancing the antigenicity of the tumor cell and suppressing the negative regulatory cells, one can achieve a substantial effect on tumor growth using CDK4/6 inhibitors (Figure 1D).”

In the paper

“For example, although individually CDK4/6 inhibitors and PI3K inhibitors have little effect in a mouse model of triple negative breast cancer,”

What the sources say

• Not checked yetcites Teo et al., 2017 source

In the paper

“Interestingly, the action of CDK4/6 inhibitors on the microenvironment might not be only through its ability to block Rb phosphorylation and promote cell cycle exit.”

In the paper

“CDK4 was shown to have kinase activity towards SPOP, a cullin 3 E3 ubiquitin ligase adaptor protein that can interact with PD-L1 (Zhang et al., 2018).”

What the sources say

• Not checked yetcites Zhang et al., 2018 source

In the paper

“This enhances the immunogenicity of the tumor cells.”

In the paper

“Additionally, CDK4 is necessary for the development of CD4+FOXP3+ Treg cells that can suppress CTL responses.”

What the sources say

• Not checked yetcites Chow et al., 2010 source
• Not checked yetcites Goel et al., 2017 source

In the paper

“For example, phosphorylation of AMPKα2 by CDK4 is associated with increased glycolysis and decreased fatty acid oxidation in MEFS (Lopez-Mejia et al., 2017).”

What the sources say

• Not checked yetcites Lopez-Mejia et al., 2017 source

In the paper

“This is consistent with findings that the maturation of microglia from sentinel to reactive supports the progression of oligodendroglioma into its more aggressive form (Ghosh and Chaudhuri, 2010).”

What the sources say

• Not checked yetcites Ghosh and Chaudhuri, 2010

In the paper

“Furthermore, the SASP can induce cellular plasticity (Ritschka et al., 2017), and cancer cells that manage to escape senescence have a more aggressive, cancer stem cell-like identity (Milanovic et al., 2018).”

What the sources say

• Not checked yetcites Ritschka et al., 2017 source

In the paper

“These examples illustrate the profound effect that CDK4/6 inhibition can have on tumor growth by acting on the cells of the tumor microenvironment, either by affecting proliferation and maturation, or by affecting antigen processing and other immunological features of the cells, both tumor and normal.”

In the paper

“Interestingly, combined treatment with a MEK inhibitor uniquely drove these cells into senescence.”

What the sources say

• Not checked yetcites Franco et al., 2016 source

In the paper

“Furthermore, the SASP can induce cellular plasticity (Ritschka et al., 2017), and cancer cells that manage to escape senescence have a more aggressive, cancer stem cell-like identity (Milanovic et al., 2018).”

What the sources say

• Not checked yetcites Milanovic et al., 2018 source

In the paper

“In T-cell acute lymphoblastic leukemia (T-ALL), inhibition of CDK6 or genetic repression of cyclin D3 induces apoptosis (Choi et al., 2012; Sawai et al., 2012).”

What the sources say

• Not checked yetcites Choi et al., 2012 source
• Not checked yetcites Sawai et al., 2012 source

In the paper

“Combining palbociclib with a PD-1 blockade enhanced tumor regression and improved overall survival in mouse xenograft models of colon adenocarcinoma (Deng et al., 2017; Zhang et al., 2018).”

What the sources say

• Not checked yetcites Deng et al., 2017 source
• Not checked yetcites Zhang et al., 2018 source

In the paper

“While these investigators did not experimentally address why the combination was better, they further showed that adding an immune checkpoint blockade induces complete and durable regression of established tumors.”

What the sources say

• Not checked yetcites Teo et al., 2017 source

In the paper

“It is clear that the mechanisms by which CDK4/6 inhibitors can retard cancer progression are far more diverse than originally thought.”

In the paper

“Regardless, multiple cellular targets of the CDK4/6 inhibitors, in the tumor cell and in the tumor microenvironment, can also cooperate to yield optimal clinical success.”

In the paper

“The in vivo functions of CDK4/6 inhibition are likely to extend beyond simply enforcing reversible cytostasis.”