Investigation of the Structure-Activity Relationships of Psilocybin Analogues

Abstract

The 5-HT_2A receptor is thought to be the primary target for psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine) and other serotonergic hallucinogens (psychedelic drugs). Although a large amount of experimental work has been conducted to characterize the pharmacology of psilocybin and its dephosphorylated metabolite psilocin (4-hydroxy-N,N-dimethyltryptamine), there has been little systematic investigation of the structure-activity relationships (SAR) of 4-substituted tryptamine derivatives. In addition, structural analogs of psilocybin containing a 4-acetoxy group, such as 4-acetoxy-N,N-dimethyltryptamine (4-AcO-DMT), have appeared as new designer drugs, but almost nothing is known about their pharmacological effects. To address the gap of information, studies were conducted with 17 tryptamines containing a variety of symmetrical and asymmetrical N,N-dialkyl substituents and either a 4-hydroxy or 4-acetoxy group. Calcium mobilization assays were conducted to assess functional activity at human and mouse 5-HT₂ subtypes. Head-twitch response (HTR) studies were conducted in C57BL/6J mice to assess 5-HT_2A activation in vivo. All of the compounds acted as full or partial agonists at 5-HT₂ subtypes, displaying similar potencies at 5-HT_2A and 5-HT_2B receptors, but some tryptamines with bulkier N-alkyl groups had lower potency at 5-HT_2C receptors and higher 5-HT_2B receptor efficacy. In addition, O-acetylation reduced the in vitro 5-HT_2A potency of 4-hydroxy-N,N-dialkyltryptamines by about 10- to 20-fold but did not alter agonist efficacy. All of the compounds induce head twitches in mice, consistent with an LSD-like behavioral profile. In contrast to the functional data, acetylation of the 4-hydroxy group had little effect on HTR potency, suggesting that O-acetylated tryptamines may be deacetylated in vivo, acting as prodrugs. In summary, the tryptamine derivatives have psilocybin-like pharmacological properties, supporting their classification as psychedelic drugs.

Figure 1

4-Substituted tryptamines induce the HTR in C57BL/6J mice. (A) Effect of psilocin (left) and 4-HO-MET (right) on the HTR. Data are presented as group means ± SEM for the entire test session. *, p < 0.05, significant difference from the vehicle control group (Tukey’s test). (B) Comparison of the potencies (ED₅₀ values, in μmol/kg) of 4-hydroxytryptamines (●) and 4-acetoxytryptamines (■) in HTR experiments.

Figure 2

Time-course of the HTR induced by 4-substituted tryptamines. Data are presented as group means during consecutive 2-min time bins. Drug doses are shown in mg/kg.

Figure 3

In vitro activity of 4-substituted tryptamines at 5-HT₂ receptors. All compounds were assayed in parallel measuring Gq-mediated calcium flux using the same drug dilutions for psilocin (A), 4-HO-DET (B), 4-HO-DPT (C), and 4-HO-DIPT (D) at h5-HT_2A (green), m5-HT_2A (orange), h5-HT_2B (red), and h5-HT_2C (blue) receptors expressing Flp-In T-REx 293 stable cell lines. Data represent concentration–response curves plotting mean and SEM of data points performed in triplicate from at least three independent experiments. (E) Relative activities of the compounds (log(E_MAX/EC₅₀) are plotted for h5-HT_2A (green), h5-HT_2B (red), and h5-HT_2C (blue) and represent data from at least three independent experiments.