Abstract B05: Targeting BRAF and CDK4 in BRAF mutant melanoma induces sustained tumor regression

Abstract Background: The p16-cyclinD-CDK4-RB1 pathway (CDK4 pathway) is deregulated in 90% of melanomas. The main outcome of CDK4 activation is the phosphorylation and thus, inhibition of RB1 leading to G1–S cell-cycle transition. In addition, CDK4 directly phosphorylates other proteins that promote cell-cycle progression and inhibit both cell senescence and apoptosis. The MAPK/ERK pathway is deregulated in 90% of melanoma and enhances CDK4 pathway signaling through increasing cyclin D1 expression. We have previously demonstrated in a panel of melanoma cell lines that the majority of cell lines were sensitive to a CDK4/6 inhibitor (Palbociclib) and consistent with the concept of CDK4 pathway-dependent oncogene addiction, cell lines with loss of functional p16INK4A were significantly more sensitive to Palbociclib than cells with wild-type p16INK4A. We hypothesized that dual targeting the MAPK/ERK and CDK4 pathways would lead to robust inhibition of the CDK4/Cyclin D complex and consequently induce greater tumor regression than single agent treatment. Results: In vitro, PLX4720, Palbociclib or their combination inhibited BRAF mutant melanoma cell line proliferation and clonogenic survival, however, only with the combination treatment group was a rapid sustained inhibition achieved. Enlarged flattened cells and a significant increase in SA-β-galactosidase staining indicated that the dual treatment resulted in robust cellular senescence. In a BRAFV600E mutant A375 xenograft model, Palbociclib and PLX4720 initially induced tumor regression and tumor stasis respectively, but resistance eventually developed. In contrast, in the combination treatment, tumors regressed and sustained inhibition of tumor growth was maintained. Preliminary biomarker studies indicate resistance to PLX4720 was due to reactivation MAPK/ERK pathway and resistance to Palbociclib to partial restoration of phosphorylated RB1. Conclusions: Combination therapy of MAPK/ERK pathway inhibitors with CDK4 inhibitors is likely to be an effective treatment for BRAF mutant melanoma and in addition delay resistance to single agent targeted therapy. Citation Format: Karen E. Sheppard, Cullinane Carleen, Laura Kirby, Kelly Waldeck, Richard Young, Claire Martin, Todd VanArsdale, Sophia Randolph, Grant A. McArthur. Targeting BRAF and CDK4 in BRAF mutant melanoma induces sustained tumor regression. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Melanoma: From Biology to Therapy; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(14 Suppl):Abstract nr B05.