This historic review (2019) examines the biosynthesis and pharmacology of psilocybin and summarizes the biotechnological routes of its synthesis.

Abstract

“The fungal genus Psilocybe and other genera comprise numerous mushroom species that biosynthesize psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine). It represents the prodrug to its dephosphorylated psychotropic analogue, psilocin. The colloquial term “magic mushrooms” for these fungi alludes to their hallucinogenic effects and to their use as recreational drugs. However, clinical trials have recognized psilocybin as a valuable candidate to be developed into a medication against depression and anxiety. We here highlight its recently elucidated biosynthesis, the concurrently developed concept of enzymatic in vitro and heterologous in vivo production, along with previous synthetic routes. The prospect of psilocybin as a promising therapeutic may entail an increased demand, which can be met by biotechnological production. Therefore, we also briefly touch on psilocybin’s therapeutic relevance and pharmacology.”

Authors: Janis Fricke, Claudius Lenz, Jonas Wick, Felix Blei & Dirk Hoffmeister

Summary

The genus Psilocybe comprises numerous mushroom species that biosynthesize psilocybin, a psychoactive compound used in medicine. It has been recognized as a valuable candidate to be developed into a medication against depression and anxiety.

Introduction

Pharmaceutical chemistry has recognized microbial natural products as a valuable source for new drugs. Psilocybin, a major metabolite of the hallucinogenic so-called magic mushrooms, is currently entering phase III clinical trials.

Bernardino de Sahagn’s Historia general de las cosas de la Nueva Espaa (General History of the Things of New Spain) documents the phenomenal pharmacological effects of psychotropic and hallucinogenic tryptamine-like alkaloids.

Richard E. Schultes investigated the lost and misinterpreted identity of the “plant” that caused the effects described in the ancient reports from Central America. He identified mushrooms of the genus Panaeolus as the source of the effects, and Roger Heim isolated 1 and 2 from the fungi.

These alkaloids are simple and achiral, and closely related to the neurotransmitter serotonin (5-hydroxytryptamine). One of them, 1, has a 4-hydroxyindole moiety, which is a very rare structural feature among natural products.

The pharmacology of psilocybin

The hallucinogenic effects of 1 are caused by cleavage of the phosphate ester, which converts the prodrug 1 into 2. The latter compound interferes with serotonergic neurotransmission, causing altered state of consciousness, enhanced introspect, decreased depression, and mystical experiences.

How mushrooms make psilocybin

The historically earliest and still popular way for humans to have access to 1 is to ingest the producing organism, but how does the mushroom produce 1?

A set of genes in an 11-22 kb portion of the genomes of various 1 producing species were reported, and the activities of these enzymes were confirmed in vitro and in vivo. This led to an emerging option on biotechnological 1 production.

PsiD catalyzes decarboxylation of 8 into 9 as initial step, but is also capable of decarboxylating 4-hydroxyL-tryptophan (12) as well.

PsiH and PsiK hydroxylate position 4 of 9 to produce 4-hydroxytryptamine (13) as second biosynthetic step. PsiM catalyzes the subsequent phosphotransfer step onto 13 to yield 3, which concludes the biosynthesis.

The pathway is designed to prevent 2 formation, yet 2 has been reported from Psilocybe species, yet the reported quantities represent an artefact generated during work-up.

The biosynthetic enzymes toward 1 were identified, and two different routes can be envisioned for its biotechnological synthesis.

Enzymatic production in vitro

PsiD accepts 12 as substrate, and a compatible P450 reductase was used to supply the monooxygenase with electrons. This resulted in the first time 1 was obtained neither from a mushroom nor synthetically.

A flexible tryptophan synthase, TrpB, was found in P. cubensis, which allowed for the production of 1-methylated tryptophan by offering 4-hydroxyindole as substrate. This allowed for the production of 12-hydroxyindole, which was further converted by PsiD, PsiK, and PsiM into 1.

Enzymatic production of psilocybin congeners

P. cubensis TrpB accepted 7-hydroxyindole as substrate and produced 7-hydroxyL-tryptophan in vitro. This result indicates that the PsiH/PsiK pair of enzymes represent the most plausible candidates to have catalyzed the biotransformation.

Heterologous in vivo production

A mold was genetically engineered to produce 1 from a single transcript. The mold produced 1 in standard small-volume Erlenmeyer shake flasks and without further optimization of culture conditions and media.

Synthetic routes toward psilocybin

Benzyl-protected 4-hydroxyindole is treated with oxalyl chloride and dimethylamine, followed by LiAlH4 reduction to 21 and finally deprotection to give 2. Later, Hofmann’s procedure was optimized by Nichols and Frescas.

The side chain of 2 can be built in one step via an iridium-catalyzed borrowing hydrogen procedure, or by reacting ortho-iodoanilin derivates with suitable unsaturated precursors. The side chain can also be generated prior to the indole ring formation.

The synthesis of 1 is completed by phosphate-esterification of 2 followed by deprotection via Pd-catalyzed hydrogenation, leading to 1 in acceptable yields (72 % from 2).

Scheme 3. Synthetic routes toward 2 and 1.

Presently, the demand for 1 for pharmaceutical purposes is met by organic synthesis. In vitro biotechnologically produced compound represents an attractive alternative, but requires effort to produce and purify the enzymes, as well as enzyme stability, solubility, and supply of cosubstrates.

Concluding remarks – psilocybin as a future therapeutic

The reason to produce a psychotropic compound stems from its (re)discovered therapeutic value. The mushroom soon developed into a popular recreational drug, and 1 was legally categorized as a Schedule I compound.

One generation of scholars later, research picked up momentum again on 1 as a prodrug of the therapeutic agent 2. The studies showed promising outcomes in the therapy of cancer-related psychiatric distress and anxiety, treatment-resistant depression, and substance addiction.

Acknowledgments

We acknowledge Robert Kargbo for valuable comments on the manuscript, and C.L. and D.H. are supported by the Deutsche Forschungsgemeinschaft and the Jena School for Microbial Communication.