At a glance

1. What exactly is NSI-189?

NSI-189 is a small, lab-made molecule belonging to a family called benzylpiperazine-aminopyridines. That name is a mouthful, but the practical meaning is simple: it is a deliberately designed chemical “key” intended to fit a lock inside brain cells and turn on growth-related programs.

It came out of a specific idea in neuroscience: if depression involves a brain that has “shrunk back” — lost cells and connections in memory and mood centers — maybe a drug could help the brain re-grow rather than merely change its chemistry for a few hours.

That idea gave NSI-189 its distinctive identity. Most conventional antidepressants (SSRIs like fluoxetine/Prozac, for example) work by changing levels of a chemical messenger called serotonin. NSI-189 was designed to work by a different route entirely — one that isn’t primarily about serotonin at all.

The many names of one molecule

• NSI-189 — the original development code (“NeuralStem Inc., compound #189”).
• ALTO-100 — the name it received after the rights were acquired by Alto Neuroscience in 2021.
• Amdiglurax — its formal international nonproprietary name (INN), the standardized drug name assigned to a pharmaceutical substance.

All three names refer to the same underlying molecule.

2. “Free base” vs. “phosphate salt” — the friendly chemistry

This distinction trips up a lot of readers, so let’s make it intuitive.

The “free base” is the molecule in its neutral, unmodified form. Think of it as a bare key, with no handle attached.

A salt form (here, the phosphate salt) is that same key with a “handle” added — a partner molecule (phosphoric acid) snapped on. The handle changes practical properties:

Why this matters: Every published human clinical trial of NSI-189 used the phosphate salt, because it dissolves in water and forms stable capsules.

Because the free base dissolves better in fat, some users dissolve it under the tongue (“sublingually”) or in oils.

3. Why neuroscientists got excited: the “brain garden” idea

For most of the 20th century, the textbook said the adult brain couldn’t grow new neurons — you were born with what you had, and it was downhill from there. That turned out to be wrong. We now know that at least two regions of the adult brain keep producing new neurons throughout life. The most important is the hippocampus — a seahorse-shaped structure deep in the brain that acts like the brain’s memory hub and mood regulator.

Think of the hippocampus as a garden:

• Neural stem cells are the seeds.
• Neurogenesis is the process of seeds sprouting into new neurons.
• Synapses are the wiring that connects new neurons into the rest of the network.
• BDNF (brain-derived neurotrophic factor) is the fertilizer — a growth protein that helps neurons survive, grow, and connect.

Under chronic stress, depression, and aging, this garden tends to wither: fewer new cells, fewer connections, and sometimes a measurably smaller hippocampus. Conventional antidepressants can sometimes “re-fertilize” the garden indirectly — which is one reason they take weeks to work. NSI-189 was designed to act directly on the gardening process itself.

4. How NSI-189 is thought to work

Early on, researchers knew what NSI-189 did — it stimulated growth — but struggled to identify the exact molecular “lock” it fit. That’s not unusual in neuroscience; many psychiatric drugs work for reasons we only understand years later.

Here’s what the evidence suggests so far:

• It stimulates neurogenesis. In a dish, NSI-189 made human hippocampal neural stem cells multiply and mature. In live mice, it increased newborn neurons in the dentate gyrus (the hippocampus’s “nursery”) and enlarged hippocampal volume.
• It’s “monoamine-independent.” That’s jargon for “it doesn’t work through serotonin, dopamine, or norepinephrine the way SSRIs and similar drugs do.” It represents a genuinely different mechanism.
• It behaves like a “growth switch” in the cell nucleus. Recent work points to a receptor in the nucleus of cells called TLX (NR2E1) — a kind of master switch that activates many growth genes at once. Activating it is like turning on the brain-growth department’s lights, not just flipping one lamp.
• It boosts BDNF-related signaling “indirectly.” In animal studies, NSI-189 effects were linked to activation of the TrkB/Akt pathways — the same molecular cascade that BDNF (our “fertilizer”) uses. It also enhanced long-term potentiation (LTP), the cellular process underlying learning and memory.
• It has “pleiotropic” effects. Pleiotropic means “many effects from one cause.” Beyond growing new neurons, NSI-189 appears to help synapses form (synaptogenesis), protect against cell damage (neurotrophic/neuroprotective effects), and even support mitochondria (the cells’ energy factories).

5. What did the animal studies show?

The animal data are genuinely intriguing — this is where the enthusiasm came from. Among the published findings:

• Depression models: Improved behavior in a mouse model of depression and increased hippocampal neurogenesis and volume.
• Stroke (rats): Given after a stroke, 12 weeks of oral treatment led to faster, better motor recovery, with signs of increased synaptogenesis.
• Alzheimer’s models: Reduced learning deficits in two mouse models of Alzheimer’s disease.
• Radiation brain injury (rats): Ameliorated cognitive impairment caused by brain irradiation.
• Diabetes: In type 1 and type 2 diabetic mice, it prevented hippocampal shrinkage, protected memory, and partially reversed peripheral nerve damage (neuropathy).
• Angelman syndrome (mice): Reversed motor and cognitive deficits and improved synaptic plasticity, with effects dependent on new protein synthesis and TrkB/Akt activation.

These results justify clinical interest.

6. What do the human trials actually show?

Phase 1b (published 2016): small, safe, and cautiously interesting

• Design: 24 adults with major depressive disorder, given NSI-189 at 40 mg once, twice, or three times daily — or placebo — for 28 days, in an inpatient setting.
• Primary purpose: safety and dosing, not proof of effectiveness.
• Findings:
  • The drug was relatively well tolerated at all doses, with no serious adverse events.
  • On exploratory symptom questionnaires (the SDQ and CPFQ, among others), people on NSI-189 improved more than those on placebo, with some benefits persisting after treatment stopped.
  • Brain scans did not show a clear increase in hippocampal or amygdala volume, despite the dramatic volume increases seen in mice. There was a modest, non-significant numerical trend in the left hippocampus.
• Interpretation: a whisper, not a shout. The authors themselves framed the mood findings as exploratory. Small trials of this size frequently produce encouraging signals that don’t survive larger testing.

Phase 2 (announced 2017, published 2020): the decisive test — and a miss on the main goal

This was the study that mattered. About 220 outpatients with major depressive disorder were randomized to 40 mg/day, 80 mg/day, or placebo for 12 weeks.

• The headline result: On the primary outcome — the clinician-rated Montgomery–Åsberg Depression Rating Scale (MADRS) — neither dose beat placebo. The differences were small and not statistically significant (40 mg: −1.8 points, p = 0.22; 80 mg: −1.4 points, p = 0.34).
• Secondary signals: But the trial was not a clean washout. On a cluster of secondary measures — especially at the 40 mg dose — there were positive signals:
  • Patient-rated depression symptom questionnaires (SDQ, CPFQ, and QIDS-SR in part) showed statistically significant improvements over placebo. Specific items that improved included low mood, anxiety, crying, decision-making, ability to work, optimism, and outlook on life.
  • Objective computerized cognitive tests (the CogScreen battery, though notably not the Cogstate battery) showed advantages in attention, executive function (mental flexibility), and — most strikingly — delayed recall on a memory test. A later analysis suggested the largest effects were on tests of memory and on how people responded to negative feedback.
• It’s notable these cognitive signals replicated earlier signs from the Phase 1b trial, which lends them some credibility beyond pure noise.

A later post-hoc analysis (2020)

This analysis added more nuance: when researchers split the Phase 2 participants by depression severity, NSI-189’s benefits — both mood and cognitive — appeared concentrated in people with moderate depression (MADRS < 30), rather than severe depression. In that moderate subgroup, the 80 mg dose even beat placebo on a six-item subset of the MADRS.

Overview of Clinical Stages

7. Possible side effects

In the early trials, NSI-189 was generally well tolerated and no serious adverse events were reported in the drug groups. The most commonly noted side effects (small samples, so treat percentages cautiously):

• Headache (common in both drug and placebo groups — so not clearly drug-specific)
• Dizziness (more common at higher doses)
• Somnolence (sleepiness/drowsiness)
• Nausea
• Insomnia
• Vivid dreams or nightmares
• Paresthesia (tingling/”pins and needles” sensations)
• Dry mouth, palpitations, fatigue, rash (less common)

8. Beyond depression: the other conditions studied

One reason NSI-189 commands attention is that “grow and repair the brain” is a versatile idea with applications far beyond mood.

Preclinical (animal) work

• Stroke: In a rat model, NSI-189 improved motor and neurological function, with benefits reportedly maintained for six months after a three-month treatment course, alongside enhanced structural plasticity.
• Radiation-induced cognitive impairment: In rats given brain irradiation (a model for cancer patients who develop cognitive problems after radiotherapy), NSI-189 improved performance on multiple memory tasks, increased neurogenesis, and reduced activated microglia — cells associated with brain inflammation. Notably, the cognitive benefits persisted a month after treatment stopped.
• Angelman syndrome: A rare neurogenetic disorder causing severe cognitive and motor impairment. In a mouse model, NSI-189 enhanced long-term potentiation (a cellular measure of memory formation) and reversed some cognitive and motor deficits, via the TrkB/Akt pathway.

These results are encouraging in the specific ways that animal models can be — but they remain preclinical.