IUPAC: 3-[2-(dimethylamino)ethyl]-1H-indol-4-yl dihydrogen phosphate · 4-phosphoryloxy-N,N-dimethyltryptamine (4-PO-DMT) · C₁₂H₁₇N₂O₄P · MW 284.25 g/mol · CAS 520-52-5
Psilocybin. The phosphate-ester prodrug of psilocin (#004) — the principal alkaloid of Psilocybe mushrooms. Psilocybin itself is essentially inactive at 5-HT2A (Ki > 10 µM); it must be dephosphorylated by alkaline phosphatase to the active 4-hydroxy tryptamine psilocin before any receptor pharmacology occurs. Trade/context: COMP360, Indocybin, "magic mushrooms," teonanácatl.
Psilocybin is a bioactivation-gated prodrug. The molecule carries a 4-O-phosphate ester that renders it a zwitterion at physiological pH — water-soluble, orally stable, and, crucially, almost silent at the 5-HT2A receptor (measured Ki > 10,000 nM). Nothing psychedelic happens until endogenous alkaline phosphatase (and, to a lesser extent, nonspecific esterases) strips that phosphate to yield psilocin (4-hydroxy-DMT), which binds 5-HT2A with Ki ≈ 107 nM — roughly a 100-fold jump in affinity. Psilocybin is therefore the delivery vehicle; psilocin is the drug.
This is the key differentiator from its active metabolite, psilocin (#004): psilocin is the pharmacophore that actually docks the receptor, but it is chemically fragile and rapidly oxidized. The 4-phosphate protects the labile 4-hydroxyindole from first-pass oxidation, giving psilocybin a smoother, more reproducible oral pharmacokinetic profile — which is exactly why the phosphate ester, not free psilocin, is the molecule in the clinical trials.
Intact psilocybin has negligible intrinsic activity at 5-HT2A (Ki > 10 µM, ChEMBL). The anionic 4-phosphate blocks entry into the lipophilic orthosteric pocket — the receptor cannot productively bind the charged, bulky ester.
Alkaline phosphatase in the intestinal brush border, liver, kidney and blood hydrolyses the phosphate, unmasking the 4-OH. This single enzymatic step converts an inert zwitterion into a high-affinity tryptamine agonist — the rate-limiting activation event.
Psilocin occupies the deep orthosteric pocket of 5-HT2A (captured directly in PDB 7WC5), stabilizing the active-state conformation and driving Gq/PLC → IP₃/DAG signaling in layer V cortical pyramidal neurons.
5-HT2A activation on deep-layer pyramidal cells desynchronizes cortical activity, increasing neural signal diversity and relaxing high-level priors (the "REBUS" / entropic-brain account) — the proposed substrate of the acute psychedelic state.
Downstream of 5-HT2A, BDNF–TrkB and mTOR signaling promote transient dendritic spine formation in the cortex. This structural plasticity outlasts drug clearance and is a leading hypothesis for durable antidepressant effects after a single dose.
Psilocin's highest measured affinity is actually at 5-HT2B (Ki ≈ 4.6 nM). Agonism here is a theoretical valvulopathy flag with chronic/frequent dosing — largely irrelevant to single-session therapy, relevant to daily microdosing.
Because the whole cascade hinges on one hydrolysis step, oral psilocybin behaves as a slow-release front-end for psilocin: onset at 20–40 min, plateau near 2 h, resolution by 4–6 h. This is the pharmacological opposite of smoked/IV routes that deliver a free tryptamine to the brain in seconds.
Orally, psilocybin is largely dephosphorylated presystemically — much of the conversion to psilocin happens in the intestinal mucosa and on first pass through the liver, so measurable plasma psilocybin is low and transient while psilocin is the species that actually circulates and crosses the blood–brain barrier. Psilocin plasma Tmax is roughly 80–105 min, with a terminal half-life near 1.5–3 h; the subjective experience tracks psilocin exposure, not psilocybin.
Metabolism: after activation, psilocin is cleared by two competing routes — conjugation (dominant) and oxidation. The 4-OH is glucuronidated by UGT1A10 (intestine) and UGT1A9 (liver) to psilocin-4-O-glucuronide, the main urinary metabolite and the analyte most drug screens actually detect. A minor oxidative branch via MAO-A and aldehyde dehydrogenase yields 4-hydroxyindole-3-acetic acid (4-HIAA).
Psilocin ★ is the only pharmacologically meaningful entity in the chain. Everything upstream (psilocybin) is a pro-form and everything downstream (glucuronide, 4-HIAA) is inactive. Because MAO-A is on the oxidative branch, MAO inhibitors (including harmala alkaloids and prescription MAOIs) reduce psilocin clearance and potentiate and prolong the experience — a real and under-appreciated interaction, not a fatal one at typical psychedelic doses, but one that can push an intended moderate dose into an overwhelming one.
Tolerance is pharmacodynamic and rapid: 5-HT2A downregulates with repeated exposure, so daily dosing quickly stops working and cross-tolerance to LSD is near-complete. There is no physical dependence and no withdrawal syndrome; psilocybin is not self-administered by animals and has very low addictive liability.
Once psilocin engages cortical 5-HT2A, the acute effects unfold over 4–6 h: perceptual intensification and visual geometry, loss of the ordinary self/other boundary (ego dissolution), affective lability, and — at higher doses in supportive settings — the "mystical-type" or peak experience that repeatedly predicts clinical response. The subjective arc is inseparable from set and setting: the same receptor pharmacology yields healing or terror depending on preparation, environment, and support.
Dense 5-HT2A expression on layer V pyramidal cells in association cortex (visual, prefrontal, cingulate) is the anatomical bottleneck. Gq-driven depolarization increases spontaneous firing and neural signal diversity while weakening the normal top-down constraints of the default-mode network — the neuroimaging signature of the acute state.
BDNF–TrkB and mTOR signaling drive transient dendritic spine growth in prefrontal cortex. This "plasticity window" persisting days-to-weeks after a single dose is the leading mechanistic hypothesis for why one or two supervised sessions can produce sustained reductions in depression scores — a fundamentally different model from daily-dosed SSRIs.
The FDA granted Breakthrough Therapy status to COMPASS Pathways' synthetic psilocybin (COMP360) for treatment-resistant depression (2018) and to the Usona Institute's psilocybin for major depressive disorder (2019). COMPASS's Phase 3 TRD program (e.g. NCT05711940, ~572 participants across 116 sites) and Usona's MDD trials anchor the modern evidence base; ClinicalTrials.gov lists ~100+ psilocybin depression studies. Earlier Imperial College and Johns Hopkins work established the single-dose, therapy-supported paradigm.
Active or completed trials extend to psilocybin-assisted therapy for anxiety and existential distress in life-threatening illness, alcohol and tobacco use disorder, OCD, and depression in Parkinson's disease. The common thread is a small number of high-dose, supervised sessions embedded in structured psychotherapy rather than chronic self-administration.
Physiologically psilocybin is remarkably well tolerated — mild transient increases in blood pressure and heart rate, nausea, headache, and pupil dilation. The dominant risks are psychological, not organ-toxic: the acute "bad trip" and the rare precipitation of persistent difficulties. This is the inverse of the stimulant and opioid entries in this series, where the body is the thing at risk.
Evidence-based, non-moralistic. Psilocybin has one of the widest physiological safety margins of any controlled drug — no documented lethal overdose from psilocybin alone. The real risks are psychological, contextual, and combination-driven, and they are largely manageable with preparation, dose control, and set/setting.
91Q), X-ray, 3.2 Å (Cao et al., 2022 structural series). Active-species note:
the ligand shown is psilocin, the active metabolite — psilocybin itself is the inactive prodrug and does not co-crystallize productively here.
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| Target | Affinity | Rel. | Mechanism |
|---|---|---|---|
|
5-HT2A
Serotonin 2A · psilocin (active)
|
Ki = 107 nM
pKi 6.97
|
Gq partial agonist | |
|
5-HT2B
Serotonin 2B · psilocin
|
Ki = 4.6 nM
pKi 8.34
|
Agonist · cardiac flag | |
|
5-HT1A
Serotonin 1A · psilocin
|
Ki = 63 nM
pKi 7.20
|
Agonist | |
|
5-HT2C
Serotonin 2C · psilocin
|
Ki = 140 nM
pKi 6.85
|
Agonist | |
|
5-HT2A
Serotonin 2A · psilocybin (prodrug)
|
Ki > 10,000 nM
inactive intact
|
Inactive prodrug |