Sir Peter MacCallum Department of Oncology - Theses

Permanent URI for this collection

Search Results

Now showing 1 - 1 of 1
  • Item
    Thumbnail Image
    Targeting cyclin dependent kinase 4 (CDK4) in melanoma: mechanisms of resistance and novel combination therapies
    AbuHammad, Shatha ( 2018)
    Activation of the Cyclin-Dependent Kinase 4 (CDK4) pathway is a frequent occurrence in cancer and is a major risk factor for the development of melanoma. Activation of this pathway occurs predominantly as a result of mutations in the CDKN2A locus that encodes for p16, a potent inhibitor of CDK4. To a lesser degree, mutations in CDK4 itself can also result in activation of this pathway. These mutations lead to loss of control over the cell cycle and to uninhibited growth of melanoma cells. The frequency of CDK4 pathway-activating events in melanoma and other cancers made this pathway a promising therapeutic target. CDK4 small molecule inhibitors have shown potent antitumour activity in several cancers including breast cancer and melanoma, especially in combination with other targeted or hormonal therapeutics. Despite the remarkable advancements in targeted therapeutics, the initial efficacy of these agents is impaired by the emergence of acquired resistance. Recognizing the mechanisms of resistance will allow the development of novel combination strategies to delay or prevent the emergence of resistance to CDK4/6 inhibitors, it can also assist in identifying predictive biomarkers of response to these agents. In order to identify the mechanism of acquired resistance to the CDK4/6 inhibitor palbociclib in melanoma, a panel of in vitro resistant melanoma cell lines was developed by subjecting the cells to constant drug pressure. All palbociclib-resistant cell lines, irrespective of genomic background, showed elevated levels of cyclin E1, an activator of CDK2, compared to their parental counterparts. Further analysis identified CDK2 as a driver of resistance via its ability to phosphorylate RB upon CDK4 inhibition. High throughput drug screening revealed that the development of resistance to palbociclib was associated with alteration in the sensitivity to other therapeutic agents. Several palbociclib-resistant cell lines showed heightened sensitivity to inhibitors of protein arginine methyltransferase 5 (PRMT5). Further investigation revealed that loss of inhibition of PRMT5 in the resistant cells mediated the resistance to palbociclib via modulating CDK2 activity. Inhibition of PRMT5 suppressed CDK2 activity by increasing levels of p21, a CDK2 inhibitor, or reducing levels of Cyclin E1. Combining palbociclib with GSK3326595, a PRMT5 inhibitor in clinical development, restored sensitivity to palbociclib and potently prevented cell proliferation in vitro. In vivo, the combination was well tolerated and induced robust tumour growth inhibition. Global analysis of gene expression demonstrated that the resistance to palbociclib was accompanied by vast changes in signalling pathways and cellular processes. These changes suggest that the resistance is a complex mechanism that requires altering multiple pathways to support the cells ability to escape the inhibitory effect of the drug. Additionally, those changes in expression provide explanation for the altered sensitivity to various targeted therapeutics observed in the resistant cells. These findings demonstrate the activation of a PRMT5-CDK2 axis as a mediator of resistance to CDK4/6 inhibitors in melanoma. And indicate that the combination of CDK4 and PRMT5 inhibitors is a promising therapeutic approach that can overcome or delay the resistance to CDK4/6 inhibitors.