Synergistic activity of S63845 and parthenolide to overcome acquired resistance to MEK1/2 inhibitor in melanoma cells: Mechanisms and therapeutic potential

Melanoma remains a global health challenge. While targeted therapy with BRAF^V600/MEK inhibitors largely improved the survival and quality of life for advanced melanoma patients, most of them progress due to acquired resistance. Dynamic heterogeneity and phenotypic plasticity of resistant melanoma prompted us to use parthenolide, an agent with an assorted bioactivity profile in combination with agents targeting enhanced pro-survival capacity of resistant melanoma cells. Parthenolide was used in combination with S63845, ABT-263 and ABT-199, BH3-mimetics targeting anti-/pro-apoptotic protein interactions. These drug combinations were investigated in patient-derived drug-naïve melanoma cell lines, their trametinib-resistant counterparts displaying either dedifferentiation neural-crest-like phenotype (MITF^low/NGFR^high) or differentiation phenotype (MITF^high/NGFR^low), and trametinib-resistant cell lines undergoing the phenotype transition after trametinib withdrawal. While parthenolide reduces proliferation and increases senescent cell fraction accompanied with reduction of NF-κB activity and MITF expression, it does not trigger apoptosis in high percentages of melanoma cells. Combination of parthenolide with BH3-mimetics induces externalization of phosphatidylserine, most efficiently with S63845, an inhibitor of pro-survival MCL-1. Parthenolide and S63845 synergize to cause massive apoptosis regardless of melanoma cell phenotypes, which is associated with caspase-3/7 activation, H2AX phosphorylation, and PARP cleavage. Mechanistically, parthenolide combined with S63845 reduces the protein level of MCL-1, upregulates pro-apoptotic NOXA, and prevents S63845-induced reduction of NOXA protein. Collectively, massive apoptosis induced synergistically by parthenolide combined with S63845 in trametinib-resistant melanoma cells displaying various phenotypes justifies exploring the potential of this strategy as a future treatment option for patients with melanoma resistant to therapies targeting MAPK signaling.