Functional Determinants of Cell Cycle Plasticity and Sensitivity to CDK4/6 Inhibition.

Intrinsic or acquired resistance to clinically approved CDK4/6 inhibitors has emerged as a major obstacle that hinders their utility beyond ER+ breast cancer. In this study, CDK4/6-dependent and -resistant models were employed to identify functional determinants of response to pharmacologic CDK4/6 inhibitors. In all models tested, the activation of RB and inhibition of CDK2 activity emerged as determinants of sensitivity. While depleting CDK4 and 6 was sufficient to limit proliferation in specific resistance settings, RB loss rendered cells completely independent of these kinases. The main downstream target in this context was the activation status of CDK2, which was suppressed with CDK4/6 inhibition in an RB-dependent fashion. Protein levels of p27 were associated with plasticity/rigidity of the cell cycle and correlated with sensitivity to CDK4/6 inhibition. Exogenous overexpression and pharmacologic induction of p27 via inhibition of SKP2 and targeting the MEK/ERK pathway enhanced the cytostatic effect of CDK4/6 inhibitors. Mice bearing ER+ xenografts displayed a durable antitumor response to palbociclib; however, over the course of treatment, few cells retained RB phosphorylation, which was associated with limited p27 protein levels as determined by multispectral imaging. Similarly, combination treatment of palbociclib with a MEK inhibitor in pancreatic cancer PDX models upregulated p27 and further enhanced the in vivo tumor response to palbociclib. Collectively, these results suggest that the cell cycle plasticity, which enables tumor models to evade palbociclib-mediated activation of RB, could be targeted using a clinically applicable CDK2 inhibitor. SIGNIFICANCE: This work provides a mechanistic insight toward understanding the functional roles of multiple cell cycle regulators that drive plasticity and sensitivity to CDK4/6 inhibition.

Kumarasamy V ，Vail P ，Nambiar R ，Witkiewicz AK ，Knudsen ES ... -  《-》

Biological specificity of CDK4/6 inhibitors: dose response relationship, in vivo signaling, and composite response signature.

Knudsen ES ，Hutcheson J ，Vail P ，Witkiewicz AK ... -  《Oncotarget》

Resistance to cyclin-dependent kinase (CDK) 4/6 inhibitors confers cross-resistance to other CDK inhibitors but not to chemotherapeutic agents in breast cancer cells.

Ogata R ，Kishino E ，Saitoh W ，Koike Y ，Kurebayashi J ... -  《-》

Regulation of PRMT5-MDM4 axis is critical in the response to CDK4/6 inhibitors in melanoma.

Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors are an established treatment in estrogen receptor-positive breast cancer and are currently in clinical development in melanoma, a tumor that exhibits high rates of CDK4 activation. We analyzed melanoma cells with acquired resistance to the CDK4/6 inhibitor palbociclib and demonstrate that the activity of PRMT5, a protein arginine methyltransferase and indirect target of CDK4, is essential for CDK4/6 inhibitor sensitivity. By indirectly suppressing PRMT5 activity, palbociclib alters the pre-mRNA splicing of MDM4, a negative regulator of p53, leading to decreased MDM4 protein expression and subsequent p53 activation. In turn, p53 induces p21, leading to inhibition of CDK2, the main kinase substituting for CDK4/6 and a key driver of resistance to palbociclib. Loss of the ability of palbociclib to regulate the PRMT5-MDM4 axis leads to resistance. Importantly, combining palbociclib with the PRMT5 inhibitor GSK3326595 enhances the efficacy of palbociclib in treating naive and resistant models and also delays the emergence of resistance. Our studies have uncovered a mechanism of action of CDK4/6 inhibitors in regulating the MDM4 oncogene and the tumor suppressor, p53. Furthermore, we have established that palbociclib inhibition of the PRMT5-MDM4 axis is essential for robust melanoma cell sensitivity and provide preclinical evidence that coinhibition of CDK4/6 and PRMT5 is an effective and well-tolerated therapeutic strategy. Overall, our data provide a strong rationale for further investigation of novel combinations of CDK4/6 and PRMT5 inhibitors, not only in melanoma but other tumor types, including breast, pancreatic, and esophageal carcinoma.

AbuHammad S ，Cullinane C ，Martin C ，Bacolas Z ，Ward T ，Chen H ，Slater A ，Ardley K ，Kirby L ，Chan KT ，Brajanovski N ，Smith LK ，Rao AD ，Lelliott EJ ，Kleinschmidt M ，Vergara IA ，Papenfuss AT ，Lau P ，Ghosh P ，Haupt S ，Haupt Y ，Sanij E ，Poortinga G ，Pearson RB ，Falk H ，Curtis DJ ，Stupple P ，Devlin M ，Street I ，Davies MA ，McArthur GA ，Sheppard KE ... -  《-》

Clinical CDK4/6 inhibitors induce selective and immediate dissociation of p21 from cyclin D-CDK4 to inhibit CDK2.

Pack LR ，Daigh LH ，Chung M ，Meyer T ... -  《Nature Communications》