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ISRIB

ISRIB (Integrated Stress Response InhiBitor) is a small-molecule compound that selectively inhibits the integrated stress response (ISR), a conserved cellular signaling pathway activated by stressors such as nutrient deprivation, viral infection, or proteotoxic challenges, which leads to phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) and subsequent global repression of protein synthesis to promote cell survival.^[1] By directly targeting the delta subunit of eIF2B, the guanine nucleotide exchange factor for eIF2, ISRIB enhances eIF2B's guanyl-nucleotide exchange activity with high potency (EC₅₀ of 27–35 nM), thereby restoring translation of most mRNAs independently of eIF2α dephosphorylation and counteracting ISR-mediated translational shutdown.^[1] This mechanism allows ISRIB to suppress ISR effectors like ATF4 without disrupting acute stress responses, operating effectively within a defined window of low-to-moderate eIF2α phosphorylation levels (below 45–70% of maximum).^[2] Discovered in 2013 through a high-throughput screen for molecules preventing ATF4 upregulation in response to eIF2α phosphorylation, ISRIB was initially noted for its ability to enhance cognitive memory processes in mice, including improved spatial and fear-associated learning without overt side effects.^[3] Subsequent genetic studies using ISRIB-resistant cell lines and CRISPR-Cas9 mutagenesis confirmed eIF2B as its direct target, with mutations in the N-terminal region of the eIF2Bδ subunit (e.g., at residues R171, V178, L180) conferring resistance by altering ISRIB binding and eIF2B decamer stability.^[1] Structural analyses have further illuminated how ISRIB stabilizes eIF2B's catalytic core, promoting its interaction with phosphorylated eIF2 and facilitating memory enhancement by modulating the unfolded protein response (UPR).^[4] ISRIB has shown therapeutic promise in preclinical models of neurological disorders driven by chronic ISR activation, including traumatic brain injury (TBI), where a single dose administered weeks post-concussion in mice fully reverses learning deficits, normalizes dendritic spine dynamics, and restores synaptic plasticity for over a month.^[5] It also ameliorates neurodegeneration in prion disease models by limiting ISR-induced synaptic loss and improves outcomes in spinal cord injury by attenuating apoptosis and neuroinflammation via P53 and JAK2/STAT3 pathways.^[2]^[6] Beyond the brain, ISRIB suppresses proteotoxic stress in intestinal inflammation and impairs ubiquitin-proteasome system function during acute protein overload, highlighting broader applications in stress-related pathologies.^[7]^[8] As of November 2025, ISRIB itself has not entered clinical trials due to challenges with solubility and bioavailability, though patient anecdotes in amyotrophic lateral sclerosis (ALS) report symptomatic benefits. A structurally related eIF2B activator, DNL343, advanced to Phase 2/3 trials for ALS but failed to meet its primary endpoint in early 2025; another eIF2B activator, fosigotifator, is in Phase 1b/2 trials for vanishing white matter disease.^[9]^[10]^[11]

Chemical Properties

Molecular Structure

ISRIB is a small-molecule compound with the chemical formula C_{22}H_{24}Cl_2N_2O_4 and a molecular weight of 451.34 g/mol. Its structure consists of a central 1,4-disubstituted cyclohexane ring bridged by two amide groups to 4-chlorophenoxyacetyl moieties, resulting in a symmetric bis-acetamide framework.^[12] The molecule exists as cis and trans diastereomers due to the cyclohexane substitution; the trans isomer, with axial-equatorial orientations in the chair conformation, is the biologically active form, exhibiting over 100-fold greater potency compared to the cis isomer, which adopts a less extended conformation unsuitable for effective target engagement.^[13] Key functional groups include the amide linkages, which enable hydrogen bonding interactions, and the para-chlorine substituents on the terminal phenyl rings, which contribute to hydrophobic stabilization in the binding pocket, as evidenced by structure-activity relationship studies showing that halogen variations or removal substantially reduce affinity.^[13] ISRIB serves as the lead compound in the class of bis-O-arylglycolamides targeting the integrated stress response; structurally related analogs, such as those replacing the cyclohexane with a 1,4-phenylene spacer or altering chlorine positions to meta-fluoro, display potencies ranging from 10-fold lower to picomolar levels, highlighting the importance of the rigid trans-cyclohexane core for optimal activity.^[13]

Synthesis and Availability

ISRIB was first synthesized in 2013 by researchers in Peter Walter's laboratory at the University of California, San Francisco, as part of efforts to identify small molecules that counteract the integrated stress response. The original synthesis route involves a straightforward amide coupling reaction starting from (1r,4r)-cyclohexane-1,4-diamine, reacting with two equivalents of 4-chlorophenoxyacetyl chloride in a tetrahydrofuran-water mixture (1:1) in the presence of potassium carbonate at ambient temperature. This single-step process yields the trans isomer of ISRIB, the active diastereomer, with a reported 65% yield after purification by sequential washes with water and diethyl ether, producing 51 mg of the white solid product. The synthesis highlights the compound's symmetric bisglycolamide structure, enabling efficient production without complex multi-step sequences. ISRIB is commercially available from suppliers such as Sigma-Aldrich for research purposes, offered as a white to beige powder with ≥98% purity by high-performance liquid chromatography (HPLC).^[14] It is typically packaged in 5 mg quantities, suitable for laboratory use, with solubility in dimethyl sulfoxide at 1 mg/mL when warmed, and recommended storage at 2-8°C.^[14] Pricing for the 5 mg vial is approximately $129, though bulk options may be available upon request.^[14] Other vendors, including Cayman Chemical, provide similar high-purity formulations in crystalline solid form for experimental applications.^[15] Intellectual property for ISRIB is covered by patents held by the Regents of the University of California, with Peter Walter listed as an inventor on key filings such as US Patent 9708247, which describes ISRIB and its analogs.^[16] Rights to these patents have been licensed by UCSF to Calico Life Sciences, a biotechnology company focused on aging and age-related diseases, enabling further development of ISRIB-based therapeutics.^[5] This licensing arrangement supports ongoing research while restricting commercial production to authorized entities for non-research uses.^[5]

Discovery and Development

Initial Discovery

ISRIB was discovered in 2013 by a team led by Peter Walter at the University of California, San Francisco (UCSF), through a phenotypic screen designed to identify compounds that reverse the integrated stress response (ISR), a cellular pathway that attenuates protein synthesis in response to stress via phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α).^[12] The discovery involved a high-throughput screen of approximately 106,000 small molecules using human embryonic kidney (HEK293T) cells stressed with thapsigargin, an endoplasmic reticulum stressor that activates the ISR; a secondary screen in U2OS cells confirmed hits, leading to the identification of ISRIB as a potent inhibitor with an IC50 of 5 nM for restoring translation of ISR-repressed mRNAs.^[12] This initial finding was reported in a 2013 eLife publication, which also provided early evidence of ISRIB's effects in vivo, showing that administration to mice enhanced spatial learning in the Morris water maze and fear-associated memory in contextual conditioning tasks, without altering baseline locomotion or anxiety.^[12] Subsequent characterization in a 2015 eLife study by the same group demonstrated that ISRIB acts downstream of eIF2α phosphorylation to antagonize the ISR regardless of the activating kinase (PERK, GCN2, PKR, or HRI), as evidenced by genome-wide ribosome profiling showing reversal of translational repression and rapid disassembly of stress granules in stressed mammalian cells.^[17]

Preclinical Development

Following initial screening, structure-activity relationship (SAR) studies optimized ISRIB by identifying the trans-isomer as the active form, which is over 100-fold more potent than the cis-isomer in rescuing translation inhibition (EC₅₀ = 5 nM versus >600 nM).^[18] These efforts focused on bis-O-arylglycolamide scaffolds, confirming that the trans configuration of N,N'-bis(2-(4-chlorophenoxy)acetamide) on a cyclohexane-1,4-diyl core enhances binding to eIF2B and promotes its dimerization for greater guanine nucleotide exchange factor activity. In rodents, trans-ISRIB demonstrates favorable pharmacokinetic properties, including excellent blood-brain barrier penetration with brain-to-plasma ratios reaching 1.62 at 24 hours post-dosing and a plasma half-life of approximately 8 hours.^[20] However, its poor aqueous solubility limits oral bioavailability, leading to primary use via intraperitoneal administration in preclinical models.^[3] Dose-response studies in mouse models established effective intraperitoneal doses ranging from 0.25 to 5 mg/kg, with 2.5 mg/kg commonly restoring protein synthesis rates to about 70% of controls and suppressing ISR markers like ATF4 without toxicity.^[20] For instance, in prion-diseased mice, 0.25 mg/kg daily dosing extended survival by 14% while preventing neurodegeneration.^[21] A 2019 study demonstrated that ISRIB selectively suppresses low-level ISR activation (when phosphorylated eIF2α is 45–70% of maximum) by stabilizing eIF2B complexes, but spares strong ISR signaling for cytoprotection, explaining its safety profile in vivo.^[2] Building on this, a 2020 UCSF investigation showed that three doses of 2.5 mg/kg reversed age-related memory deficits in mice, improving spatial working memory and hippocampal neuron excitability with effects persisting up to 20 days post-treatment.^[22] Further analog development has yielded compounds with enhanced potency, such as ISRIB-A17, which is nearly 10-fold more active (EC₅₀ ≈ 0.5 nM) by improving eIF2B binding affinity.^[13] Subsequent preclinical studies as of 2025 have explored ISRIB in additional models, including reversal of β-cell failure in Wolfram syndrome using stem cell-derived islets (2024) and synergy with WEE1 inhibitors to enhance efficacy in pancreatic cancer patient-derived xenografts (2025).^[23]^[24]

Mechanism of Action

Integrated Stress Response Pathway

The integrated stress response (ISR) is a conserved eukaryotic signaling pathway that enables cells to adapt to various environmental and pathological stresses, such as endoplasmic reticulum (ER) stress, nutrient deprivation, viral infection, and oxidative damage, by transiently halting global protein synthesis while selectively translating stress-response genes.^[25] This adaptive mechanism helps restore cellular homeostasis and promotes survival under adverse conditions.^[26] The ISR is initiated by the activation of four specialized serine/threonine kinases—PERK (protein kinase R-like ER kinase), GCN2 (general control nonderepressible 2), PKR (protein kinase R), and HRI (heme-regulated inhibitor)—each responsive to distinct stressors: PERK to unfolded proteins in the ER, GCN2 to amino acid starvation, PKR to double-stranded RNA from viruses, and HRI to heme deficiency or oxidative stress.^[26] Upon activation, these kinases phosphorylate the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α) at serine 51.^[25] This phosphorylation event inhibits the guanine nucleotide exchange factor eIF2B, which is essential for recycling eIF2 from its inactive GDP-bound form to the active GTP-bound form.^[26] Consequently, it reduces the formation of the ternary complex required for translation initiation:

\text{eIF2-GTP} + \text{Met-tRNA}_\text{i}^{\text{Met}} \rightarrow \text{ternary complex (eIF2-GTP-Met-tRNA}_\text{i}^{\text{Met}}\text{)}

where eIF2α phosphorylation (eIF2α-P) sequesters eIF2B, limiting GTP loading and ternary complex assembly, thereby attenuating cap-dependent translation of most mRNAs.^[26] Downstream of eIF2α phosphorylation, global protein synthesis is substantially suppressed in stressed cells (often by 50-80% or more depending on stress severity), conserving resources and reducing the protein folding burden.^[25]^[27] However, this also enables selective translation of transcripts with upstream open reading frames (uORFs), such as the transcription factor ATF4 (activating transcription factor 4), which is preferentially translated under ISR conditions.^[26] ATF4 translocates to the nucleus and induces expression of genes involved in amino acid metabolism, antioxidant defense, and autophagy, such as CHOP and GADD34, to facilitate stress adaptation.^[26] Prolonged or unresolved ISR activation can shift toward pro-apoptotic signaling, promoting cell death to eliminate damaged cells.^[25] Chronic ISR activation contributes to the pathogenesis of various diseases, including neurodegenerative disorders like Alzheimer's disease and amyotrophic lateral sclerosis (ALS), where persistent ER stress and protein misfolding lead to neuronal loss, and cancer, where ISR supports tumor cell survival under hypoxic or nutrient-poor conditions but can also sensitize cells to therapy-induced stress.^[28]^[29] In these contexts, dysregulated ISR signaling exacerbates proteostasis collapse and metabolic imbalances.^[25]

Specific Inhibition by ISRIB

ISRIB specifically targets the eukaryotic initiation factor 2B (eIF2B), a guanine nucleotide exchange factor (GEF) essential for translation initiation, by binding to it and restoring its activity even in the presence of phosphorylated eIF2α (eIF2(αP)). This binding counteracts the inhibitory effect of eIF2(αP), which normally sequesters eIF2B and impairs its GEF function, thereby allowing continued translation under stress conditions. ISRIB operates effectively within a defined window of low-to-moderate eIF2α phosphorylation levels (below 45–70% of maximum), suppressing ISR effectors like ATF4 without disrupting acute stress responses.^[2] The compound exhibits high potency, with an EC₅₀ of 27–35 nM for enhancing eIF2B guanyl-nucleotide exchange activity, as measured in cell-free translation and GEF assays.^[1] ISRIB demonstrates selectivity for eIF2B over other translation factors.^[1] Structural studies using cryo-electron microscopy (cryo-EM) at 2.8 Å resolution have revealed that ISRIB binds within a deep cleft at the interface between the β and δ subunits of the eIF2B decamer, acting as a molecular staple that stabilizes the active decameric assembly. This stabilization promotes the incorporation of the regulatory eIF2Bα subunits and prevents disassembly induced by eIF2(αP), thereby maintaining eIF2B's GEF activity.^[30] Unlike inhibitors targeting individual eIF2α kinases (such as PERK, GCN2, PKR, or HRI), ISRIB acts downstream of all four kinases, blocking ISR signaling regardless of the upstream activation mechanism. In non-stressed cells, ISRIB has no effect on baseline translation rates, but it potentiates protein synthesis specifically under stress by overcoming eIF2(αP)-mediated inhibition.

Biological Effects

Cellular Level Effects

ISRIB restores global protein synthesis in cells subjected to various stresses, such as endoplasmic reticulum (ER) stress induced by tunicamycin, by counteracting the translational repression caused by eIF2α phosphorylation.^[17] This restoration has been demonstrated through ribosome profiling in HEK293T cells, where ISRIB treatment normalized the translation of most mRNAs while specifically blocking the stress-induced translation of upstream open reading frame (uORF)-containing transcripts.^[17] Additionally, polysome profiling in stressed induced pluripotent stem cell (iPSC)-derived cells showed that ISRIB shifts mRNAs back to polysome fractions, enhancing overall protein synthesis rates without altering mRNA levels.^[31] A key cellular outcome of ISRIB is the selective reduction in ATF4 expression during stress, achieved without impacting the activity of upstream kinases like PERK that phosphorylate eIF2α.^[17] In ER-stressed HEK293T cells, ISRIB prevented the translational upregulation of ATF4 and related genes like CHOP, as evidenced by genome-wide ribosome-protected fragment sequencing, while eIF2α phosphorylation levels remained unchanged.^[17] This decoupling allows ISRIB to modulate ISR downstream effects precisely, promoting adaptive responses over pro-apoptotic signaling. Under proteotoxic stress, such as heat shock, ISRIB impairs ubiquitin-proteasome system (UPS) degradation, exacerbating protein aggregate accumulation in cells.^[8] A 2024 study in HEK293 cells demonstrated that ISRIB treatment during such stress reduced UPS-mediated protein turnover, as measured by cycloheximide chase assays and proteomics, potentially limiting the cell's ability to clear misfolded proteins despite restored synthesis.^[8] In vitro studies have shown that ISRIB inhibits ISR effectors like CHOP but can sensitize cells to apoptosis under prolonged ER stress by impairing adaptive responses, with effects varying by stress intensity.^[12] ISRIB exhibits no reported cytotoxicity at therapeutic concentrations up to 10 μM in neuronal models under basal or stressed conditions.^[32]

Animal Model Studies

ISRIB has demonstrated notable effects in various rodent models, particularly in enhancing cognitive functions impaired by stress, aging, or injury. In early preclinical investigations, administration of ISRIB to mice improved spatial memory performance in tasks such as the Morris water maze, with treated animals showing enhanced learning consolidation following training sessions compared to controls.^[12] This enhancement was observed at doses of 0.25 mg/kg intraperitoneally, without altering baseline anxiety or locomotion behaviors.^[12] In models of age-related cognitive decline, ISRIB rapidly restored hippocampal-dependent memory in aged mice. A study involving 18- to 24-month-old C57BL/6J mice treated with 2.5 mg/kg ISRIB intraperitoneally for three consecutive days showed reversal of spatial memory deficits in the Morris water maze within days, alongside improvements in working memory as assessed by the Y-maze spontaneous alternation task.^[33] These effects persisted for weeks post-treatment, correlating with normalized protein synthesis rates in hippocampal neurons, which had been suppressed due to chronic integrated stress response (ISR) activation.^[33] The treatment did not affect young mice, indicating specificity to age-related impairments.^[34] ISRIB also promoted functional recovery in mouse models of traumatic brain injury (TBI). In controlled cortical impact models, mice receiving 2.5 mg/kg ISRIB intraperitoneally starting one month post-injury exhibited full restoration of spatial learning and memory in the Barnes maze, even when treatment initiated weeks or months after the initial trauma.^[35] This recovery included normalized long-term potentiation in hippocampal slices and reduced ISR markers like phosphorylated eIF2α, demonstrating reversal of persistent synaptic and cognitive deficits.^[35] Similar benefits were observed across sexes, with no exacerbation of injury-related inflammation.^[36] Recent work in Fragile X syndrome (FXS) mouse models has highlighted ISRIB's potential to ameliorate behavioral phenotypes. In Fmr1 knockout mice, ISRIB treatment at 2.5 mg/kg improved social interaction behaviors in the three-chamber sociability test, increasing time spent with novel conspecifics compared to vehicle-treated FXS mice.^[16] These gains were linked to enhanced synaptic protein translation and reduced dendritic spine abnormalities in the prefrontal cortex.^[16] A FRAXA-funded study published in 2024 further supported these findings, showing social behavior improvements in adolescent FXS models following short-term ISRIB dosing.^[16]^[37] Across these studies, ISRIB exhibited a favorable safety profile in rodents at effective doses. No overt toxicity, such as significant weight loss, organ damage, or behavioral aberrations, was reported in mice or rats treated with 0.25–2.5 mg/kg ISRIB intraperitoneally or orally over acute or subchronic regimens, though higher doses (e.g., 5 mg/kg) occasionally induced transient hypoactivity.^[33]^[35]^[38]

Research Applications

Neurological Disorders

ISRIB has shown promise in preclinical research for several neurological disorders by modulating the integrated stress response to alleviate cognitive and synaptic impairments. In models of Alzheimer's disease, administration of ISRIB has rescued spatial memory deficits and reversed aberrant synaptic plasticity, such as Aβ-facilitated long-term depression in the hippocampus, without altering amyloid plaque load.^[39]^[40] While the integrated stress response contributes to tau pathology in Alzheimer's disease, ISRIB's direct impact on tau accumulation remains uncertain, with studies indicating no reduction in tau-related neuropathology in tauopathy mouse models despite improvements in protein synthesis and memory performance.^[28]^[41] In traumatic brain injury models, ISRIB has demonstrated robust recovery of long-term memory in rodents, fully reversing severe learning impairments caused by focal contusions or concussive injuries even when administered weeks after the initial trauma.^[35] For instance, in mice subjected to controlled cortical impact, a short course of ISRIB injections restored spatial and working memory to levels comparable to uninjured controls, with effects persisting for at least one week post-treatment and linked to normalized hippocampal long-term potentiation.^[42] Research on Fragile X syndrome, the most common inherited form of intellectual disability, has revealed that ISRIB enhances synaptic plasticity and ameliorates behavioral phenotypes in mouse models. A 2024 study reported in 2025 by the FRAXA Research Foundation showed that ISRIB treatment in Fmr1 knockout mice reduced immature dendritic spines, elevated AMPA receptor subunit GluA1 levels to improve synaptic transmission, and enhanced social recognition, thereby mitigating core deficits in social behavior associated with the disorder.^[43]^[37] Preliminary investigations into Parkinson's disease suggest ISRIB may mitigate aspects of pathology through suppression of the integrated stress response, which is activated by alpha-synuclein aggregates in dopaminergic neurons; however, specific data on reducing alpha-synuclein aggregation remain limited, with ISRIB positioned as a promising candidate for broader neurodegenerative applications.^[28] In healthy animal models, ISRIB has enhanced cognition without apparent side effects, improving learning efficiency and memory consolidation; for example, in aged mice, it reduced errors in novel object recognition and fear conditioning tasks, while in young mice, it accelerated acquisition in maze-based learning paradigms by restoring youthful levels of hippocampal protein synthesis and dendritic spine density.^[22] These findings align with general observations of ISRIB's ability to boost memory processes in uninjured rodents.^[34]

Other Therapeutic Areas

ISRIB has shown potential in oncology by inhibiting the integrated stress response (ISR), which cancer cells exploit for adaptive survival under chemotherapeutic stress. In pancreatic cancer models, ISRIB enhanced gemcitabine-induced cell death by reversing eIF2α phosphorylation and disrupting UPR-mediated resistance, thereby sensitizing tumors to the drug.^[44] Similarly, combining ISRIB with imatinib in chronic myeloid leukemia cells increased eradication both in vitro and in vivo, while in orthotopic pancreatic tumor models, ISRIB was well-tolerated and promoted apoptosis without reducing tumor size alone.^[45] These effects highlight ISRIB's role in blocking ISR-driven survival mechanisms that confer chemotherapy resistance across solid and hematologic malignancies. In infectious diseases, ISRIB counters pathogen-induced ISR activation, particularly via PKR signaling triggered by viral double-stranded RNA. For bacterial infections like tuberculosis, adjunctive ISRIB treatment in mouse models significantly lowered Mycobacterium tuberculosis burdens (P < 0.01) and accelerated sterility, reducing relapse rates to 0% after four months compared to standard antibiotics alone (P < 0.001), while tending to lessen lung inflammation.^[46] This suggests ISRIB protects host cells from ISR-mediated translational shutdown during infection, enhancing clearance without overt toxicity. ISRIB also mitigates metabolic stress in diabetes models by alleviating β-cell dysfunction. In Wolfram syndrome, a monogenic form of diabetes, ISRIB reversed WFS1 deficiency-induced ISR activation in human stem cell-derived islets, reducing stress granules, restoring protein synthesis, increasing β-cell proportions, and decreasing apoptosis. In vivo, it improved glucose homeostasis, insulin content, and secretion in Wfs1 knockout mice. Additionally, ISRIB blunted proinsulin decline under nutrient stress in β-cells, countering eIF2α-mediated repression and supporting secretory function.^[23] A 2019 study demonstrated ISRIB's capacity to reverse ISR activation across diverse stressors, including amino acid deprivation via GCN2 kinase, by enhancing eIF2B activity when eIF2α phosphorylation remains below 45–70% of maximum levels. This selective suppression preserved cytoprotective ISR effects at higher stress thresholds, explaining its tolerability, and extended to viral (PKR), oxidative (HRI), and ER (PERK) stresses without broad toxicity.^[2] In wound healing, ISRIB promotes tissue repair by dampening chronic inflammation linked to ISR. Topical ISRIB formulations attenuated lewisite-induced skin inflammation and injury in mice by targeting ISR pathways, reducing tissue damage.^[47] In zebrafish tailfin injury models, ISRIB decreased macrophage recruitment to the injury site and pro-inflammatory responses, facilitating faster resolution.^[48] Furthermore, ISRIB enhanced alveolar epithelial differentiation in lung repair models by resetting proteostasis and countering ISR barriers to regeneration, with broader anti-inflammatory promise in stress-related wounds.^[49]

Clinical Potential

Preclinical Promise

ISRIB demonstrates broad-spectrum reversal of the integrated stress response (ISR), enabling rapid cognitive recovery in preclinical models of age-related decline and brain injury. In aged mice, administration of ISRIB over just three days restored spatial memory performance to levels comparable to young animals, contrasting with the months-long recovery timelines observed in natural aging processes or other interventions.^[22] This swift enhancement extends to working and episodic memory, persisting for weeks after treatment cessation, highlighting ISRIB's potential to counteract chronic ISR activation underlying cognitive impairments.^[22] The compound exhibits a favorable safety profile in preclinical testing, with no evidence of genotoxicity, overt toxicity, or behavioral alterations in treated rodents. Studies in prion-diseased and aged mice showed no pancreatic damage, weight loss, or disruptions to normal learning and memory in young controls, even at therapeutic doses.^[21] Furthermore, ISRIB displays no significant impact on neuronal excitability or synaptic activity beyond reversing age-induced deficits.^[22] ISRIB shows synergy with existing therapies, such as kinase inhibitors like imatinib, enhancing their efficacy in eradicating leukemic cells by suppressing ISR-mediated resistance mechanisms. In vitro and in vivo models demonstrated that combining ISRIB with imatinib reduced cell proliferation and STAT5 signaling more effectively than either agent alone.^[45] Key pharmacological metrics underscore ISRIB's therapeutic promise, including 100-fold greater potency (IC₅₀ = 5 nM) compared to its cis-isomer and excellent blood-brain barrier penetration, allowing rapid accumulation in neural tissues to modulate ISR signaling. These attributes support its broad applicability without compromising systemic safety. In aging interventions, ISRIB reverses cognitive and neuronal decline—such as reduced spine density and inflammation—without adversely affecting overall lifespan in preclinical models, positioning it as a targeted modulator for age-related pathologies.^[22] Animal studies confirm these benefits across models of neurodegeneration and injury, with sustained cognitive improvements observed post-treatment.^[2]

Challenges and Future Directions

Despite promising preclinical results, ISRIB remains an experimental compound with no human clinical trials initiated as of 2025, lacking Phase I safety and dosing data in patients.^[9] This absence of human data stems from challenges in translating rodent model efficacy to clinical settings, where bioavailability and solubility issues have been noted as barriers to advancement.^[50] Without established pharmacokinetics in humans, regulatory approval for trials has been delayed, emphasizing the need for foundational safety assessments before broader therapeutic exploration. A key challenge lies in potential off-target effects, as concerns exist regarding ISRIB's long-term safety, including possible impacts on acute stress responses.^[9] While ISRIB demonstrates high specificity for eIF2B activation in controlled models, these concerns highlight the necessity for refined selectivity.^[51] The pharmacodynamics of ISRIB in humans, particularly long-term effects on the brain, remain largely unknown, raising uncertainties about sustained neuronal impacts and potential neurotoxicity. Preclinical studies indicate possible risks like enhanced cellular proliferation that could elevate cancer susceptibility over extended exposure, though direct human evidence is absent.^[52] Additionally, chronic inhibition might alter synaptic plasticity in unforeseen ways, necessitating longitudinal monitoring that current animal data cannot fully predict.^[53] Intellectual property considerations further complicate clinical development, as ISRIB's core patent is held by institutions like UCSF, with licensing agreements—such as Calico's 2019 acquisition—required for commercial progression.^[54] These arrangements demand partnerships with pharmaceutical entities to fund and execute trials, a process slowed by the compound's early-stage status and competing priorities in neurodegeneration research. Looking ahead, efforts focus on developing ISRIB analogs to enhance selectivity and mitigate off-target risks. For example, DNL343, an investigational eIF2B activator, completed Phase 2/3 testing in the HEALEY ALS Platform Trial in 2025 but did not meet its primary endpoints for slowing disease progression, though it demonstrated safety and tolerability; further analyses are ongoing.^[10] Similarly, fosigotifator (ABBV-CLS-7262), another related eIF2B activator from Calico and AbbVie, is in an ongoing Phase 1b/2 open-label study for vanishing white matter disease, with completion anticipated around November 2025.^[55] These developments, including IND filings completed prior to 2025 for such modulators, underscore the path toward human testing in neurological indications, though ISRIB itself awaits optimized formulations for regulatory submission.^[11]

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Small molecule cognitive enhancer reverses age-related memory ...
Here we explore the possibility of ISR inhibition by ISRIB as a potential strategy for modifying age-induced neuronal, immune, and cognitive dysfunction.
[29]
Drug Reverses Age-Related Mental Decline Within Days - UCSF
Dec 1, 2020 · ISRIB, discovered in 2013 in Walter's lab, works by rebooting cells' protein production machinery after it gets throttled by one of these ...Missing: route | Show results with:route
[30]
Inhibition of the integrated stress response reverses cognitive ...
Jul 10, 2017 · Our results indicate that ISR inhibition at time points late after injury can reverse memory deficits associated with TBI.
[31]
Integrated Stress Response Inhibitor Reverses Sex-Dependent ...
Jun 1, 2020 · Our results indicate that targeting the ISR even at late time points after injury can permanently reverse behavioral changes.
[32]
FRAXA-Funded Research Explores ISRIB as a Potential Treatment ...
Feb 26, 2025 · An experimental drug ISRIB (Integrated Stress Response Inhibitor) has potential to improve brain function and social behavior in a Fragile X mouse model.
[33]
Small-Molecule Integrated Stress Response Inhibitor Reduces ...
In the present study, we observed that the ISRIB reduced more than 50% of the fibrotic areas and inhibited the collagen type I deposition and α-SMA expression ...Cardiovascular · Results · Isrib Inhibits Af...
[34]
Inhibition of the ISR abrogates mGluR5-dependent long-term ...
Mar 8, 2022 · The small-molecule inhibitor of the ISR called ISRIB (trans-isomer) prevents Aβ-facilitated LTD and abrogates spatial learning and memory deficits in the ...
[35]
Correction of eIF2-dependent defects in brain protein synthesis ...
Feb 2, 2021 · For salubrinal experiments, mice were concomitantly injected daily with ISRIB (0.25 mg/kg) and salubrinal (1 mg/kg). Results obtained in this ...<|separator|>
[36]
Role of Endoplasmic Reticulum Stress in Learning and Memory Impairment and Alzheimer's Disease-Like Neuropathology in the PS19 and APPSwe Mouse Models of Tauopathy and Amyloidosis
- **Study Focus**: Investigates endoplasmic reticulum stress in PS19 (tauopathy) and APPSwe (amyloidosis) mouse models, focusing on learning, memory impairment, and Alzheimer’s disease-like neuropathology.
[37]
Archive: Drug Reverses Memory Failure Caused by Traumatic Brain ...
Jul 10, 2017 · Surprisingly, the drug fully restored the ability to learn and remember in the brain-injured mice even when the animals were first treated as ...
[38]
https://www.sciencedirect.com/science/article/pii/S0022356524257050
[39]
Targeting integrated stress response with ISRIB combined with ...
Dec 2, 2022 · The small molecule ISRIB blocks eIF2α-P-dependent downstream signaling with IC50 of 5 nM and inhibits the executive part of the ISR without ...
[40]
ALSUntangled #80: ISRIB (Integrated stress response InhiBitor)
Aug 5, 2025 · Currently, we do not have enough evidence to endorse the use of ISRIB for treating ALS. We support further research in disease models and ...
[41]
[PDF] ISRIB improves white matter injury following TBI by inhibiting ...
May 3, 2023 · One major concern is the potential for off-target effects, as the ISR is a crucial mechanism for maintaining cellular homeostasis ...
[42]
The small molecule ISRIB reverses the effects of eIF2α ...
ISRIB is the first reported antagonist of the ISR that blocks signaling downstream of all eIF2α kinases. It was shown to have good pharmacokinetic properties ...Missing: similarity | Show results with:similarity
[43]
Effects of the Small‐Molecule ISRIB on the Rapid and Efficient ...
Apr 28, 2025 · The results showed that ISRIB could reduce the overactivation of the UPR pathway and improve the function of astrocytes and oligodendrocytes in ...
[44]
Small-Molecule Integrated Stress Response Inhibitor Reduces ...
Fourth, further studies are needed to establish the long-term therapeutic effects of ISRIB on AF in the context of assessment of its potential toxic effects.Missing: pharmacodynamics | Show results with:pharmacodynamics
[45]
Calico licenses memory-enhancing drug ISRIB
Dec 3, 2019 · A new drug known as ISRIB which reverses Down's Syndrome memory deficits in mice has been licensed to Google's Calico for aging research.<|control11|><|separator|>
[46]
DNL343 is an investigational CNS penetrant eukaryotic initiation ...
They first assessed the pharmacokinetics of DNL343, demonstrating its ability to cross the blood-brain barrier and exhibit good bioavailability. In an acute ...<|separator|>
[47]
Clinical Trial: Vanishing White Matter - Calico Labs
Calico and AbbVie are currently conducting a Phase 1b/2 trial of fosigotifator to evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics and ...