Use of a MCL-1 inhibitor alone to de-bulk melanoma and in combination to kill melanoma initiating cells

// Nabanita Mukherjee 1 , Yan Lu 1 , Adam Almeida 1 , Karoline Lambert 1 , Chung-Wai Shiau 2 , Jung-Chen Su 2 , Yuchun Luo 1 , Mayumi Fujita 1 , William A. Robinson 3 , Steven E. Robinson 3 , David A. Norris 1, 4 and Yiqun G. Shellman 1 1 University of Colorado Anschutz Medical Campus, School of Medicine, Department of Dermatology, Aurora, CO, USA 2 Institute of Biopharmaceutical Sciences, National Yang-Ming University, Taipei, Taiwan 3 Division of Medical Oncology, University of Colorado Anschutz Medical Campus, School of Medicine, Aurora, CO, USA 4 Department of Veterans Affairs Medical Center, Dermatology Section, Denver, CO, USA Correspondence to: Yiqun G. Shellman, email: Yiqun.Shellman@ucdenver.edu David A. Norris, email: David.Norris@ucdenver.edu Keywords: melanoma stem cells, cancer stem cells, drug-induced cell death, melanoma, SC-2001 Received: December 22, 2015     Accepted: March 28, 2016     Published: April 12, 2016 ABSTRACT MCL-1 (BCL-2 family anti-apoptotic protein) is responsible for melanoma’s resistance to therapy. Cancer initiating cells also contribute to resistance and relapse from treatments. Here we examined the effects of the MCL-1 inhibitor SC-2001 in killing non melanoma-initiating-cells (bulk of melanoma), and melanoma-initiating-cells (MICs). By itself, SC-2001 significantly kills melanoma cells under monolayer conditions in vitro and in a conventional mouse xenograft model. However, even at high doses (10μM), SC-2001 does not effectively eliminate MICs. In contrast, the combination of SC-2001 with ABT-737 (a BCL-2/BCL-XL/BCL-W inhibitor) significantly decreases ALDH + cells, disrupts primary spheres, and inhibits the self-renewability of MICs. These results were observed in multiple melanomas, including short term cultures of relapsed tumors from current treatments, independent of the mutation status of BRAF or NRAS. Using a low-cell-number mouse xenograft model, we examined the effects of these treatments on the tumor initiating ability of MIC-enriched cultures. The combination therapy reduces tumor formation significantly compared to either drug alone. Mechanistic studies using shRNA and the CRISPR-Cas9 technology demonstrated that the upregulation of pro-apoptotic proteins NOXA and BIM contribute to the combination-induced cell death. These results indicate that the MCL-1 inhibitor SC-2001 combined with ABT-737 is a promising treatment strategy for targeting melanoma.