MKC8866, a salicylaldehyde analog, is a potent, selective IRE1 RNase inhibitor with an IC50 of 0.29 μM in human vitro. MKC8866 strongly inhibits Dithiothreitol-induced X-box-binding protein 1-spliced (XBP1s) expression with an EC50 of 0.52 μM and unstresses RPMI 8226 cells with an IC50 of 0.14 μM[1]. MKC8866 inhibits IRE1 RNase in breast cancer cells leading to the decreased production of pro-tumorigenic factors and it can inhibits prostate cancer (PCa) tumor growth[2].