ISRIB

ISRIB (integrated stress response inhibitor) is a cell-permeable, brain-penetrant small molecule identified by Sidrauski et al. (2013) at the University of California, San Francisco, through a phenotypic screen for compounds that render cells resistant to the translational consequences of eukaryotic initiation factor 2 alpha (eIF2alpha) phosphorylation [1]. The compound blocks the integrated stress response (ISR) with an IC50 of approximately 5 nM in ATF4 reporter assays, operating downstream of all four eIF2alpha kinases (PERK, GCN2, HRI, PKR) at the level of the guanine nucleotide exchange factor eIF2B [2]. Cryo-electron microscopy studies (Tsai et al. 2018; Zyryanova et al. 2021) demonstrated that ISRIB binds at the symmetry interface of two eIF2B betagammadeltaepsilon tetrameric subcomplexes, acting as a molecular staple that promotes assembly of the catalytically active decameric holoenzyme and allosterically antagonizes the inhibitory effect of phosphorylated eIF2alpha on the nucleotide exchange reaction [3, 4]. The resulting restoration of ternary complex formation and global protein synthesis rates is partial rather than complete, reaching approximately 50 to 70 percent of unstressed control levels even at saturating compound concentrations, a feature that accounts for the favorable safety profile relative to direct PERK kinase inhibitors such as GSK2606414 that produce pancreatic exocrine destruction [5].

Preclinical pharmacology spans multiple disease-relevant models. In cognition, ISRIB enhances spatial and fear-associated learning in wild-type mice (Sidrauski et al. 2013) [1], reverses cognitive deficits weeks after traumatic brain injury (Chou et al. 2017) [6], restores age-related memory decline and hippocampal neuronal function within days of treatment in aged mice (Krukowski et al. 2020) [7], and rescues synaptic plasticity in a mouse model of Down syndrome [8]. In neurodegeneration, ISRIB prevents neuronal loss in prion-diseased mice without pancreatic toxicity (Halliday et al. 2015) [5] and stabilizes vanishing white matter disease eIF2B mutant complexes to wild-type catalytic activity (Wong et al. 2018) [9]. Additional preclinical activity has been reported in prostate cancer [10], noise-induced cochlear synaptopathy [11], postinfarct atrial fibrillation [12], and amyotrophic lateral sclerosis models [13]. Pharmacokinetically, ISRIB exhibits good blood-brain barrier penetration and achieves brain concentrations exceeding its IC50 at intraperitoneal doses of 0.25 to 2.5 mg/kg in mice, but is limited by poor aqueous solubility requiring vehicle formulations with dimethyl sulfoxide and polyethylene glycol 400 [5, 6].

No human clinical trials of ISRIB itself have been conducted. However, the compound served as the pharmacological prototype for two clinical-stage eIF2B activators: DNL343 (Denali Therapeutics), which completed a Phase 2/3 trial in amyotrophic lateral sclerosis (HEALEY platform trial) without meeting primary endpoints [14], and fosigotifator (Calico/AbbVie), which also failed to demonstrate significant slowing of disease progression in the same platform trial, although an exploratory high-dose arm showed signals on muscle strength preservation [15]. This monograph reviews the chemistry, stereochemistry, and synthesis of ISRIB; the molecular pharmacology of eIF2B activation and ISR inhibition; pharmacokinetic properties and formulation challenges; the preclinical evidence base across cognitive, neurodegenerative, oncologic, and inflammatory models; the clinical-translational status through derivative compounds; sourcing and quality verification for research use; reconstitution and handling; stack interactions; adverse events and safety signals; and a comparative assessment of five ISR-modulating alternatives against ISRIB on five competency standards.