Abstract 1321: Identification of Markers of Sensitivity and Resistance to Palbociclib (PD0332991) in Melanoma

Abstract Background: Melanoma, a malignancy originating in pigment-producing melanocytes, is generally resistant to conventional treatments such as radiotherapy or chemotherapy. Molecular targeted therapeutics against BRAF and MEK have shown marked efficacy for patients with metastatic melanoma. However these targeted therapies have a limited duration of response. Palbociclib is a highly selective inhibitor of cyclin dependent kinases 4 and 6 (CDK4/6) that has been shown to inhibit growth of malignant cell lines in vitro and in vivo by preventing the phosphorylation of Rb and stopping the progression G1/S of cell cycle. Cyclin D-CDK4/6-Rb axis has been shown to be dysregulated in 85-90% of melanomas and may represent a new therapeutic target. Our main goal in this study was to evaluate the therapeutic potential of palbociclib in a large panel of human melanoma cell lines to identify potential biomarkers of sensitivity and resistance. Material and Methods: We evaluated the response to palbociclib treatment in 48 melanoma cell lines. A comprehensive biomarker screen was performed using mutation and gene expression (Agilent) data. The mechanism of action of the drug was investigated by western blot (WB) analysis measuring phospho-Rb and senescence markers. Effects on the cell cycle and apoptosis were study by flow cytometry. Results: A marked differential response to palbociclib was observed across the melanoma cell lines with IC50s ranging from 28nM to over 1µM. A subset (14/48) of cell lines were classified as sensitive at clinically achievable concentrations (IC50<150nM). BRAF and NRAS activating mutation did not correlate with sensitivity in vitro. Increased expression of genes correlating with cyclin D-CDK4/6 activation strongly predicted for palbociclib sensitivity. We also found that genomic and expression markers of activation of the hedgehog/smoothened (SHH) pathway strongly predicted for in vitro resistance to palbociclib. These markers included chromosomal amplification of SHH, and high baseline expression of SMO and GLI2. Protein analysis by WB showed a dramatic decrease in phospho-Rb protein only in sensitive cell lines and also it caused a decrease in FOXM1 protein, indicating palbociclib may partially act by inducing senescence in treated cells. Flow cytometry analysis confirmed that palbociclib induces a strong G1/S arrest, but not significant apoptosis. Conclusion: Nearly 30% of the melanoma cell lines evaluated were found to be sensitive to palbociclib in vitro. Response to palbociclib did not correlate with BRAF and NRAS mutational status. A gene expression signature for CDK 4/6 activation successfully identified those cell lines most sensitive to palbociclib. We also identified markers of hedgehog/smoothened/GLI activation that strongly predicted for resistance to palbociclib. These data support the development of CDK 4/6 inhibitors in melanoma and provide a hypothesis for patient enrichment in clinical trials. Note: This abstract was not presented at the meeting. Citation Format: Erika M. von Euw, Dylan Conklin, Hong-Mei Rong, Ke-Wei Gong, Richard S. Finn, Dennis J. Slamon. Identification of markers of sensitivity and resistance to palbociclib (PD0332991) in melanoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1321. doi:10.1158/1538-7445.AM2014-1321