Identification of BRaf-Sparing Amino-Thienopyrimidines with Potent IRE1α Inhibitory Activity

Amino-quinazoline BRaf kinase inhibitor 2 was identified from a library screen as a modest inhibitor of the unfolded protein response (UPR) regulating potential anticancer target IRE1 alpha. A combination of crystallographic and conformational considerations were used to guide structure-based attenuation of BRaf activity and optimization of IRE1 alpha potency. Quinazoline 6position modifications were found to provide up to 100-fold improvement in IRE1 alpha cellular potency but were ineffective at reducing BRaf activity. A salt bridge contact with Glu651 in IRE1 alpha was then targeted to build in selectivity over BRaf which instead possesses a histidine in this position (His539). Torsional angle analysis revealed that the quinazoline hinge binder core was ill-suited to accommodate the required conformation to effectively reach Glu651, prompting a change to the thienopyrimidine hinge binder. Resulting analogues such as 25 demonstrated good IRE1 alpha cellular potency and imparted more than 1000-fold decrease in BRaf activity.