Mechanisms of CXL017: targeting drug resistant cancer

Abstract

Drug resistance represents a major challenge in the treatment of cancer, and agents with the capacity to overcome drug resistance are urgently needed. One of the most common pathways leading to multidrug resistance is through the overexpression of anti-apoptotic Bcl-2 family proteins. Previous work had identified sHA 14-1 as a lead compound with the ability to overcome drug resistance induced by overexpression of Bcl-2 proteins. In the current work, this compound was further characterized to inhibit the protein SERCA, a calcium pump located on the endoplasmic reticulum, and to overcome drug resistance in multiple cell lines. Further work led to the discovery of CXL017, a sHA 14-1 analog with improved potency and an IC50 value of ~2 µM in the multidrug resistant acute myeloid leukemia cell line HL60/MX2. This cell line is mitoxantrone resistant with cross-resistance to doxorubicin and etoposide; all three agents are topoisomerase II inhibitors. The new lead compound was then characterized in 9 pairs of drug resistant leukemia cell lines developed from exposure to standard therapies. CXL017 overcame the drug resistance in each cell line tested, revealing the compound's ability to widely overcome drug resistance. Chronic exposure of HL60/MX2 cells to CXL017 failed to result in the development of resistance to the compound and caused re-sensitization of the cell line to standard therapies. The new cell line, HL60/MX2/CXL017, was then studied along with HL60/MX2 cells in order to characterize the mechanism(s) of resistance in HL60/MX2 cells as well as the changes leading to re-sensitization. These efforts revealed that downregulation of topoisomerase IIβ in HL60/MX2 cells leads to slight resistance to mitoxantrone, but not to etoposide or doxorubicin. Rather, it is the overexpression of Mcl-1 observed in HL60/MX2 cells relative to HL60 cells that confers multidrug resistance. In HL60/MX2/CXL017 cells, topoisomerase IIβ levels are restored to the levels observed in HL60 cells along with a reduction of Mcl-1 compared to HL60/MX2 cells, leading to the re-sensitization. Overall, data from this work reveals that CXL017 has the potential to overcome drug resistance induced by various treatments, and elucidates the mechanisms leading to drug resistance in HL60/MX2 cells.