MCL1 inhibitors S63845/MIK665 plus Navitoclax synergistically kill difficult-to-treat melanoma cells

Abstract

Current treatment for patients with metastatic melanoma include molecular-targeted therapies and immune checkpoint inhibitors. However, a subset of melanomas are difficult-to-treat. These melanomas include those without the genetic markers for targeted therapy, non-responsive to immunotherapy, and those who have relapsed or exhausted their therapeutic options. Therefore, it is necessary to understand and explore other biological processes that may provide new therapeutic approaches. One of most appealing is targeting the apoptotic/anti-apoptotic system that is effective against leukemia. We used genetic knockdown and pharmacologic approaches of BH3 mimetics to target anti-apoptotic BCL2 family members and identified MCL1 and BCLXL as crucial pro-survival members in melanoma. We then examined the effects of combining BH3 mimetics to target MCL1 and BCLXL in vitro and in vivo. These include clinical-trial-ready compounds such as ABT-263 (Navitoclax) and S63845/S64315 (MIK655). We used cell lines derived from patients with difficult-to-treat melanomas. In vitro, the combined inhibition of MCL1 and BCLXL resulted in significantly effective cell killing compared to single-agent treatment (p < 0.05) in multiple assays, including sphere assays. The combination-induced cell death was independent of BIM, and NOXA. Recapitulated in our mouse xenograft model, the combination inhibited tumor growth, reduced sphere-forming capacity (p < 0.01 and 0.05, respectively), and had tolerable toxicity (p > 0.40). Taken together, this study suggests that dual targeting of MCL1 and BCLXL should be considered as a treatment option for difficult-to-treat melanoma patients.

Simultaneously Inhibiting BCL2 and MCL1 Is a Therapeutic Option for Patients with Advanced Melanoma

“…We and others have shown that navitoclax plus MCL1 inhibitors can be effective to kill melanomas in vitro and in vivo [6,7]; however, in our experience, the potential toxicity was higher than ABT-199 plus MCL1 inhibitors. In our previously published study, we had to lower the drug concentrations of navitoclax and MCL1 inhibitors to minimize toxicity [6].…”

“…We and others have shown that navitoclax plus MCL1 inhibitors can be effective to kill melanomas in vitro and in vivo [6,7]; however, in our experience, the potential toxicity was higher than ABT-199 plus MCL1 inhibitors. In our previously published study, we had to lower the drug concentrations of navitoclax and MCL1 inhibitors to minimize toxicity [6]. In comparison, the combination of ABT-199 plus an MCL1 inhibitor has been tested in pre-clinical mouse studies and is considered safe even when administered at higher concentrations than what we demonstrate in this current study (S63845: 25 mg/kg twice weekly; ABT199: 50 mg/kg 2-3 times per week).…”

“…Previously published work has shown that single agent BH3 mimetics are not effective alone for melanoma, and that MCL1 is an essential anti-apoptotic protein [6,7]. The combination of MCL1 inhibition with ABT-199 displayed efficacy in neuroblastoma with high BCL2 expression in vitro and in vivo [15].…”

“…In acute myeloid leukemia (AML), BIM is an important mediator for S63845-induced apoptosis [33]. In melanoma, we and others have shown that NOXA and BIM mediate the killing effects of several combinations, including the BH3 mimetic ABT-737 and ABT-263 [6,[34][35][36][37][38][39][40][41]. Thus, we investigated the roles of BIM and NOXA in the S63845+ABT-199 mediated cell death with cell lines genetically modified with shRNA or CRISPR-Cas9 technology.…”

“…Cell viability was evaluated via a Cell Titer-Glo Luminescent cell viability assay (Promega Corp., Madison, WI, USA) according to the manufacturer's protocol. All sphere assays were completed as described in our previous publications [6,[36][37][38]40]. The experimental schematic for the primary and secondary sphere assays was described previously [38].…”