All compounds covered are laboratory research chemicals, not approved drugs.
Modern cognition-research literature splits "nootropics" into three mechanistically distinct families: ampakines (AMPA-receptor modulators), cholinergic / nicotinic agents (α7 nAChR partial agonists), and neurogenic compounds that grow new hippocampal neurons. This guide puts five of the most-searched research compounds — IDRA-21, NSI-189 free base, NSI-189 phosphate, PRL-8-53 and GTS-21 (DMXB-A) — side by side with CAS-verified identities, and shows how each differs from the racetam and ampakine benchmarks you already know.
Table of Contents
1. At-a-glance comparison table
All CAS numbers and formulas were verified against PubChem, Sigma-Aldrich and primary supplier monographs.
| Compound | CAS Number | Molecular Formula | MW (g/mol) | Chemical Class | Primary Research Target |
|---|---|---|---|---|---|
| IDRA-21 | 22503-72-6 |
C₈H₉ClN₂O₂S | 232.69 | Benzothiadiazine / Ampakine | AMPA receptor positive allosteric modulator (PAM) |
| PRL-8-53 (free base) | 51352-88-6 (HCl: 51352-87-5) |
C₁₈H₂₁NO₂ | 283.37 | Substituted phenethylamine | Cholinergic + dopamine; partial serotonin inhibition |
| GTS-21 (DMXB-A) | 148372-04-7 (diHCl: 156223-05-1) |
C₁₉H₂₀N₂O₂ | 308.38 | Anabaseine derivative | Selective α7 nicotinic ACh receptor (α7 nAChR) partial agonist |
| NSI-189 free base | 1270138-40-3 |
C₂₂H₃₀N₄O | 366.50 | Benzylpiperazine-aminopyridine | Hippocampal neurogenesis (neurogenic) |
| NSI-189 phosphate | 1270138-41-4 |
C₂₂H₃₃N₄O₅P | 464.50 | Phosphate salt of NSI-189 | Hippocampal neurogenesis (water-soluble form) |
2. Head-to-head: clinical progress & key limitations
| Compound | Primary Target | Key Expected Effect | Human Clinical Progress | Key Limitation | Salt Form Note |
|---|---|---|---|---|---|
| IDRA-21 | AMPA-R PAM | Learning & memory enhancement | No human clinical trial | Excitotoxic / seizure-threshold risk; no human safety data | Free base only |
| PRL-8-53 | Multi-transmitter (cholinergic / DA / 5HT) | Verbal memory retrieval enhancement | Single small 1978 human study; no modern follow-up | Mechanism unclear; lack of large-replication data | Mostly HCl salt for research |
| GTS-21 | α7-nAChR partial agonist | Cognition, sensory gating, anti-inflammation | Phase 2 completed; primary endpoints failed for AD / schizophrenia | Clinical CNS development discontinued | Dihydrochloride salt |
| NSI-189 Free Base | Neurogenesis / BDNF up-regulation | Anti-depressant, pro-cognitive | Zero human trials | Poor water solubility, inconsistent absorption | Free base, not for in-vivo dosing |
| NSI-189 Phosphate | Identical active moiety as free base | Anti-depressant, pro-cognitive | Phase 1b + Phase 2 MDD trials | No positive pivotal trial; only moderate-MDD subgroup signal | Phosphate salt, clinical-trial formulation |
3. Individual compound profiles
IDRA-21 — the benchmark ampakine
Chemical identity: 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide (synonym: S-18986). CAS 22503-72-6, formula C₈H₉ClN₂O₂S, MW 232.69 g/mol, melting point ~209 °C.
Mechanism: Blocks rapid desensitization of AMPA receptors and prolongs the decay of excitatory post-synaptic currents, facilitating long-term potentiation (LTP).
Research findings: In patas monkeys it reversed alprazolam- and scopolamine-induced cognitive deficits at doses 10–30× lower than aniracetam, with effects lasting up to 48 h. Only the (+)-enantiomer is active. Caveat: under ischemic (stroke) conditions, AMPA potentiation can worsen excitotoxic neuronal loss.
No formal human clinical trials have ever been completed; all human-related reports are self-experimenter anecdotes only. Theoretical excitotoxicity risk under high-dose glutamatergic over-stimulation; may lower seizure threshold. Soluble in DMSO; poor water solubility. Research-grade only. Related ampakine comparators: Cyclothiazide, Pesampator (PF-04958242), Aniracetam, Unifiram.
PRL-8-53 — the dopamine-cholinergic memory agent
Chemical identity: methyl 3-[2-[benzyl(methyl)amino]ethyl]benzoate. Free base CAS 51352-88-6 (HCl 51352-87-5), formula C₁₈H₂₁NO₂, MW 283.37 g/mol. HCl salt: C₁₈H₂₂ClNO₂, MW 319.83 g/mol.
Mechanism: Exact MoA unknown. Shows cholinergic activity, potentiates dopamine, partially inhibits serotonin; reverses reserpine-induced catalepsy/ptosis. Not a stimulant.
Research findings: Synthesized by Nikolaus Hansl at Creighton University (1970s). Only human trial (1978, double-blind, 47 volunteers, 5 mg oral) reported large recall gains in poor performers (+87–105%) and subjects over 30 (+108–152%), no reported side effects. Oral LD₅₀ mice 860 mg/kg.
A single small 1978 double-blind placebo-controlled human trial (n=47 healthy volunteers, 5 mg oral) reported significantly improved verbal-information retention and recall, with larger benefit in higher-IQ subjects and minimal gain in low-IQ participants. No modern large-scale replication exists; the long-term safety database for repeated administration is absent. Supplied almost exclusively as the HCl salt for research. Related comparators: Coluracetam, Noopept.
GTS-21 (DMXB-A) — the selective α7 nicotinic agonist
Chemical identity: 3-(2,4-dimethoxybenzylidene)anabaseine. Free base CAS 148372-04-7 (diHCl 156223-05-1), formula C₁₉H₂₀N₂O₂, MW 308.38 g/mol. diHCl: C₁₉H₂₀N₂O₂·2HCl, MW 381.30 g/mol.
Mechanism: Selective partial agonist at α7 nAChR; also binds α4β2 but activates α7 most. α7 activation links to neuroprotection and the cholinergic anti-inflammatory pathway.
Research findings: Investigated for schizophrenia (Phase I/II improved sensory gating/neurocognition), Alzheimer's and nicotine-dependence research. Active metabolite 4-OH-GTS-21 is also nootropic.
Phase 2 trials in schizophrenia and Alzheimer's disease were completed; cognitive metrics improved in healthy-volunteer tests but the program failed to meet primary endpoints in patient cohorts, with only a mild secondary benefit on schizophrenia negative symptoms. Human studies tolerated doses up to 450 mg/day, with mild nausea the most-reported adverse event. CNS clinical development was discontinued; GTS-21 remains a widely used preclinical tool for α7-nAChR pharmacology and neuro-inflammation. (It is also a weak antagonist at α4β2-nAChR and 5-HT3A.) Related α7 tool molecules: Encenicline (EVP-6124), PNU-282987, AVL-3288.
NSI-189 free base — the neurogenic scaffold
Chemical identity: (4-benzylpiperazin-1-yl)-[2-(3-methylbutylamino)pyridin-3-yl]methanone. CAS 1270138-40-3, formula C₂₂H₃₀N₄O, MW 366.50 g/mol. Neuralstem Inc.
Mechanism: Stimulates adult hippocampal neural stem-cell proliferation and increases hippocampal volume; upregulates BDNF/SCF via a monoamine-independent pathway.
Research findings: In chronic-stress rodents, 28-day oral dosing improved depression-like behavior and raised dentate-gyrus neurogenesis; advanced to Phase 1/2 MDD trials. Lipophilic, DMSO-soluble, not water-soluble.
Soluble in DMSO / ethanol; very low water solubility (~0.1 mg/mL), which causes inconsistent oral absorption in vivo. Not used in any human clinical trial. Store at −20 °C, dry and protected from light.
NSI-189 phosphate — the clinical salt form
Chemical identity: phosphate salt of NSI-189. CAS 1270138-41-4, formula C₂₂H₃₃N₄O₅P, MW 464.50 g/mol.
Mechanism: Identical neurogenic pharmacology; phosphate counterion raises aqueous solubility — the form used in NSI-189 Phase 1/2 human trials.
Phase 1b (n=24 MDD patients, 28-day dosing) was well tolerated with an exploratory antidepressant / pro-cognitive signal; half-life ~17.4–20.5 h, with effects persisting weeks after discontinuation. Phase 2 randomized double-blind placebo-controlled trial (n=220 MDD) did not meet the primary MADRS endpoint in the overall cohort, but a post-hoc analysis showed statistically meaningful improvement in the moderate-severity subgroup (MADRS < 30), including cognitive-function metrics. Pivotal positive data are absent; development is stalled and the compound remains research-only. Related neurogenic comparators: Dihexa, Noopept, P21 peptide, BDNF-mimetic small molecules.
4. NSI-189 free base vs phosphate: the salt-form question
| Property | NSI-189 Free Base | NSI-189 Phosphate |
|---|---|---|
| CAS | 1270138-40-3 | 1270138-41-4 |
| Formula | C₂₂H₃₀N₄O | C₂₂H₃₃N₄O₅P |
| MW | 366.50 g/mol | 464.50 g/mol |
| Water solubility | Low (DMSO vehicle) | Higher (aqueous formulations) |
| Lipophilicity | Higher | Lower |
| Used in clinical trials | No (mostly in vitro) | Yes (Phase 1/2 MDD) |
| Best for | Lipophilic vehicles, phosphorylation assays | Water-based dosing studies |
Bottom line: pharmacology is equivalent once absorbed; the choice is about formulation/solubility, not "which is stronger."
5. How these compounds work — three mechanism families
Family A — Ampakines (AMPA receptor PAMs). IDRA-21 is the purest example. By decelerating AMPA desensitization, ampakines strengthen fast excitatory transmission and LTP. Cousins: Aniracetam, Sunifiram, Unifiram.
Family B — Cholinergic / nicotinic agents. GTS-21 acts on α7 nAChR; PRL-8-53 works via cholinergic + dopaminergic pathways. Studied for sensory gating, attention and the cholinergic anti-inflammatory axis.
Family C — Neurogenic compounds. NSI-189 (both forms) promotes adult hippocampal neurogenesis and raises hippocampal volume — a structural endpoint, different from every racetam.
6. Related compounds: racetams & ampakines compared
| Compound | CAS | Formula | MW | Class | Primary Research Target |
|---|---|---|---|---|---|
| Piracetam | 7491-74-9 | C₆H₁₀N₂O₂ | 142.16 | Racetam (parent) | Membrane fluidity; ACh / AMPA support |
| Aniracetam | 72432-10-1 | C₁₂H₁₃NO₃ | 219.24 | Racetam (fat-soluble) | AMPA / kainate PAM; ACh release |
| Noopept | 157115-85-0 | C₁₇H₂₂N₂O₄ | 318.37 | Dipeptide / racetam-like | AMPA modulation + BDNF; ~1000× piracetam potency (animal) |
| Sunifiram | 314728-85-3 | C₁₄H₁₈N₂O₂ | 246.30 | Ampakine-like (piperazine) | AMPA PAM + NMDA glycine-site |
| Unifiram | 272786-64-8 | C₁₃H₁₅FN₂O₃S | 298.33 | Ampakine-like | AMPA-mediated anti-amnesic; ACh release |
| Fasoracetam | 110958-19-5 | C₁₀H₁₆N₂O₂ | 196.25 | Racetam | mGluR II/III modulator; GABA-B upregulator |
| Coluracetam | 135463-81-9 | C₁₉H₂₃N₃O₃ | 341.41 | Racetam (furoquinoline) | High-affinity choline uptake (HACU) enhancer |
| Phenylpiracetam | 77472-70-9 | C₁₂H₁₄N₂O₂ | 218.26 | Racetam / stimulant | Atypical dopamine reuptake inhibition; nicotinic AChR |
7. Frequently asked questions
IDRA-21 (CAS 22503-72-6) is a benzothiadiazine-derived ampakine — a positive allosteric modulator of AMPA receptors. It slows AMPA desensitization and prolongs excitatory currents, facilitating LTP. It was 10–30× more potent than aniracetam in animal cognition models.
IDRA-21 is reported as roughly 10–30× more potent than aniracetam (CAS 72432-10-1) in reversing induced cognitive deficits, with effects up to 48 h. Aniracetam has a shorter 1–2.5 h half-life and is fat-soluble.
No. NSI-189 is a benzylpiperazine-aminopyridine neurogenic agent from Neuralstem Inc. Its defining research activity is stimulation of adult hippocampal neurogenesis via a monoamine-independent pathway — not classic cholinergic or racetam mechanisms.
PRL-8-53 (free base CAS 51352-88-6) is a substituted phenethylamine nootropic. Its single human double-blind trial (5 mg, 47 volunteers) reported improved word recall, especially in poorer performers and subjects over 30. Mechanism involves cholinergic + dopaminergic activity.
GTS-21 / DMXB-A (CAS 148372-04-7) is a selective partial agonist at the α7 nicotinic acetylcholine receptor (α7 nAChR), derived from anabaseine. Studied in schizophrenia and cognitive-impairment research.
Preclinical literature describes them as CNS-directed: IDRA-21 crosses the BBB better than cyclothiazide; PRL-8-53 is orally active and BBB-permeable; GTS-21 is orally active; NSI-189 shows high BBB permeability. None are approved for human use.
No. All are laboratory research chemicals / reference materials. NSI-189 reached Phase 1/2 trials for MDD; GTS-21 reached Phase I/II in schizophrenia; IDRA-21 and PRL-8-53 remain preclinical tools.
Same parent molecule, different salt counter-ion. The phosphate salt dramatically improves water solubility and oral-absorption consistency. All human clinical research uses NSI-189 phosphate; the free base is mainly for in-vitro lab assays because of its poor water solubility. There is no difference in intracellular mechanism of action.
Positive pre-clinical results exist in animal and non-human-primate models, but there is zero formal human clinical data. Community anecdotes cannot replace clinical evidence, and the excitotoxic risk cannot be ignored for long-term use.
α7-nAChR receptor desensitization is a major challenge for agonists in vivo. While pre-clinical and healthy-volunteer cohorts showed cognitive improvement, patient-population heterogeneity and receptor-adaptation effects reduced clinical efficacy in schizophrenia and Alzheimer's patients, leading to discontinuation of CNS clinical programs.
One small 1978 human study reported verbal-retention improvement, but there are no modern independent large-sample replications. Interpret results cautiously; a modern safety dataset is absent.
In Phase 2 trials the overall cohort did not hit the primary endpoint. Post-hoc analysis shows a promising signal only for the moderate-severity major-depression subgroup (MADRS < 30), including cognitive-function metrics. It is not an approved anti-depressant drug and cannot substitute clinical-standard psychiatric treatment.
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All compounds described in this article are investigational research-grade chemicals. None are approved drugs for human use. All data are from published pre-clinical / clinical literature and are not medical advice. They are not for human ingestion, injection, or any in-vivo self-experimentation, and none are intended for human or veterinary consumption. IDRA-21 may worsen excitotoxic damage under ischemic (stroke) conditions; always follow institutional safety and MSDS protocols.