– cis-ISRIB is a weak inhibitor of PERK signaling compared to trans-ISRIB , potently reverses the effects of eIF2? phosphorylation with IC50 of 600 nM.ISRIB reduces the viability of cells subjected to PERK-activation by chronic endoplasmic reticulum stress, displays significant enhancement in spatial and fear-associated learning.ISRIB is a potent inhibitor of the integrated stress response (ISR)integrated stress response (ISR).