In vitro activity: MKC-3946 blocks XBP1 mRNA splicing and exhibits cytotoxicity against AML cells. MKC-3946 inhibits XBP1S expression induced by tunicamycin (TM) in NB4 cells (B) and AML sample from patients. MKC-3946 prevents the splicing of the XBP1 mRNA in response to ER stress caused by mutant proinsulin production. MKC-3946 is an IRE1α endoribonuclease domain inhibitor that blocks XBP1 mRNA splicing and triggers modest growth inhibition in MM cells. MKC-3946 inhibits XBP1s expression induced by Tm in a dose-dependent manner, but does not affect phosphorylation of IRE1α. MKC-3946 blocks XBP1 splicing and enhances cytotoxicity induced by bortezomib or 17-AAG. MKC-3946 (10μM) enhances ER stress-mediated apoptosis induced by bortezomib or 17-AAG, and enhances cytotoxicity of ER stressors, even in the presence of BMSCs or exogenous IL-6.

Cell Assay: Cell proliferation and viability are examined using MTT assay. For each assay, various number of cells (1,000 for cell proliferation and 10,000 for cell viability assays) are seeded in 96-well plates, followed by either vehicle (DMSO) or increasing concentrations of drug. For detection of relative numbers of living cells, 10 μL of MTT (5 mg/mL) is added to each well, placed in an incubator for four hours, followed by centrifugation (1,000 rpm, 5 min); 100 μL of supernatant media from each well are carefully removed and 100 μL of SDS buffer (20% in water) is added to dissolve the crystals. Results are further read on spectrophotometer machine at 570 nM wavelength. Half maximal inhibitory concentration (IC50) is calculated using the GraphPad Prism 5. Synergy of combination of two drugs is determined using the CalcuSyn software. The extent of drug interaction between the two drugs is determined using the combination index (CI) for mutually exclusive drugs. Different CI values are obtained when solving the equation for different concentrations of drugs. A CI of 1 indicates an additive effect, whereas a CI of<1denotes synergy. All experiments are repeated at least three times.