A combination of epigenetic BET and CDK9 inhibitors for treatment of human melanoma.

Dysregulation of epigenetic modifiers is a frequent event in melanoma and underlies many aspects of melanoma biology including resistance to targeted and immunotherapies. Here we report that dual targeting of BET and CDK9 proteins have synergistic effects against melanoma cells in vitro and in vivo. The BET inhibitor (IBET151) and CDK9 inhibitor (CDKI73) synergistically killed melanoma cells in vitro independent of their BRAF or NRAS mutation status. The combination of drugs markedly inhibited the growth of human melanoma C002M cells in vitro in 3D spheroids and in vivo in NSG mice compared to vehicle control and the individual drugs (p<0.05). Cell death was associated with mitochondrial depolarisation and caspase dependent apoptosis with cleavage of PARP1 as well as downregulation of anti-apoptotic proteins BCL2, BCLXL and MCL1. GSEA revealed downregulation of hallmark gene-sets associated with E2F, G2M checkpoint and c-MYC. Survival analysis showed worse prognosis with high G2M, E2F or c-MYC gene signatures suggesting biomarkers of response of BET and CDK9 inhibitors in melanoma. This combination of epigenetic inhibitors targets multiple downstream genes leading to cell death of melanoma cells in vitro and in vivo and warrant further investigation for treatment of melanoma in patients not responding to current therapies.