Abstract 1173: A novel selective Mcl-1 inhibitor exhibitsin vitroandin vivoefficacy in melanoma

Metastatic melanoma is the deadliest form of skin cancer that still has limited treatment options and dismal 5-year year survival rates as low as 15%. Only recently has there been significant progress in the treatment of metastatic melanoma, advent by molecular targeted drugs and immunotherapy, but their limitations are exposed as these cancers are quickly able to develop resistance. Dysregulation of apoptotic machinery in melanoma allows the cancer cells to evade cell death and contributes to treatment resistance. Up-regulation of Mcl-1, a member of the Bcl-2 family of anti-apoptotic proteins, has been correlated with melanoma progression and metastasis. Mcl-1 amplification is one of the most common genetic aberrations found in human cancers and has been labeled as a marker of aggressive oncogenesis and poor patient prognosis. Previous studies have pointed to Mcl-1 as a viable therapeutic target for the treatment of melanoma and conclude that small molecule Mcl-1 inhibitors may appeal this unmet medical need. We have discovered and characterized a new class of selective small molecule Mcl-1 inhibitors using various biochemical, functional, and cell based assays; further development of these compounds allowed us to achieve potent low-nanomolar binding affinity for Mcl-1 and more than 300-fold selectivity over Bcl-2/Bcl-xL. Our most potent inhibitor, 483-LM, was screened across a panel of human melanoma cell lines using a cell proliferation assay and revealed varying levels of sensitivity. A BH3 profiling assay demonstrates that C8161, the cell line most sensitive to our inhibitor, solely relies on Mcl-1 for survival. The Mcl-1 dependence of C8161 might contribute to the known metastatic nature of this cell line. Mechanistic studies revealed that 483-LM effectively engaged the endogenous Mcl-1 protein after treatment of C8161, as determined by a CETSA assay, and prompted disruption of protein-protein interactions between Mcl-1 and several pro-apoptotic proteins, including Bax, Bak and Bim. This was followed by induction of Bax/Bak dependent apoptosis and activation of hallmarks of the intrinsic apoptotic pathway, including mitochondrial outer membrane depolarization, caspase activation, and PARP cleavage. Importantly, treatment with 483-LM caused massive up-regulation of the pro-apoptotic BH3-only protein Noxa, an effect apparent after an 8 hour treatment, which contributes to the induction of cell death. The in vivo efficacy studies with 483-LM showed significant effect on tumor growth and caspase-3 activation in tumor samples. Overall, our data indicate that Mcl-1 inhibitors are a promising treatment option for aggressive metastatic melanoma and warrant further preclinical investigation of 483-LM as a promising selective Mcl-1 inhibitor to be used as a single agent and in combination with chemotherapy and immunotherapy. Citation Format: Karson J. Kump, Lei Miao, Ahmed S. Mady, Katherine Lev, William Giblin, Mary E. Skinner, David B. Lombard, Zaneta Nikolovska-Coleska. A novel selective Mcl-1 inhibitor exhibits in vitro and in vivo efficacy in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1173. doi:10.1158/1538-7445.AM2017-1173