BH3 mimetics potentiate pro-apoptotic activity of encorafenib in BRAFV600E melanoma cells

BRAF^V600- and MEK1/2-targeting therapies rarely produce durable response in melanoma patients. We investigated five BRAF^V600E melanoma cell lines derived from drug-naïve tumor specimens to assess cell death response to encorafenib (Braftovi), a recently FDA-approved BRAF^V600 inhibitor. Drug-naïve cell lines (i) did not harbor damaging alterations in genes encoding core apoptotic machinery, but they differed in (ii) mitochondrial priming as demonstrated by whole-cell BH3 profiling, and (iii) levels of selected anti-apoptotic proteins. Encorafenib modulated the balance between apoptosis-regulating proteins as it upregulated Bim and BMF, and attenuated NOXA, but did not affect the levels of pro-survival proteins except for Mcl-1 and BCL-X_L in selected cell lines. Induction of Apoptosis could be predicted using Dynamic BH3 profiling. The extent of Apoptosis was dependent on both (i) cell-intrinsic proximity to the apoptotic threshold (initial mitochondrial priming) and (ii) the abundance of encorafenib-induced Bim (iBIM; drug-induced change in priming). While co-inhibition of Mcl-1 and BCL-X_L/Bcl-2 was indispensable for Apoptosis in drug-naïve cells, encorafenib altered cell dependence to Mcl-1, and reliance on BCL-X_L/Bcl-2 was additionally found in cell lines that were highly primed to Apoptosis by encorafenib. This translated into robust Apoptosis when encorafenib was combined with selective BH3 mimetics. Our study provides a mechanistic insight into the role of proteins from the Bcl-2 Family in melanoma cell response to targeted therapy, and presents preclinical evidence that (i) Mcl-1 is a druggable target to potentiate encorafenib activity, whereas (ii) pharmacological inhibition of BCL-X_L/Bcl-2 might be relevant but only for a narrow group of encorafenib-treated patients.