In melanoma the development of resistance to BRAF inhibitors limits clinical responses. Thus the search for novel single agent and combination therapies as well as sequencing strategies that overcome or delay the emergence of resistance is needed. The p16-CDK4- cyclinD-RB1 pathway (CDK4 pathway) is deregulated in approximately 90% of melanomas and most melanoma cell lines are sensitive to palbociclib a specific CDK4/6 inhibitor. 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. Proliferation and colony formation assays demonstrated that sequential and intermittent treatment with PLX4720 and palbociclib was not as effective as continuous combinational dosing. Only the combination of PLX4720 and palbociclib overcame the development of resistance leading to sustained inhibition of proliferation, cell death and induction of senescence. In A375 and HT144 human tumor xenograft models, palbociclib and PLX4720 initially induced tumor regression and tumor stasis, respectively, however resistance eventually developed to both agents. In contrast, the combination treatment induced rapid and sustained tumor regression. Biomarker studies indicate resistance to single agent PLX4720 was due to reactivation MAPK/ERK pathway and resistance to palbociclib to partial restoration of phosphorylated RB1. Tumor regression in the combination treatment was associated with the infiltration of leukocytes including activated natural killer cells indicating that these cells were involved in tumor clearance. These data demonstrate that dual targeting CDK4/6 and mutant BRAF evades resistance to single agents and leads to sustained tumor regression

Citation Format: Karen E. Sheppard, Carleen Cullinane, Claire Martin, Laura Kirby, Nicole Haynes, Kelly Waldeck, Richard Young, Todd VanArsdale, Grant McArthur. Sustained melanoma regression is achieved with continuous palbociclib and PLX4720 treatment but not with intermittent or sequential dosing. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2826.