Receptor interaction profiles of novel psychoactive tryptamines compared with classic hallucinogens

This study (2016) investigated the biochemical properties of a number of novel psychoactive drugs and found that their receptor interaction profiles predict effects similar to those of classic psychedelics and MDMA.

Abstract

“The present study investigated interactions between the novel psychoactive tryptamines DiPT, 4-OH-DiPT, 4-OH-MET, 5-MeO-AMT, and 5-MeO-MiPT at monoamine receptors and transporters compared with the classic hallucinogens lysergic acid diethylamide (LSD), psilocin, N,N-dimethyltryptamine (DMT), and mescaline. We investigated binding affinities at human monoamine receptors and determined functional serotonin (5-hydroxytryptamine [5-HT]) 5-HT2A and 5-HT2B receptor activation. Binding at and the inhibition of human monoamine uptake transporters and transporter-mediated monoamine release were also determined. All of the novel tryptamines interacted with 5-HT2A receptors and were partial or full 5-HT2A agonists. Binding affinity to the 5-HT2A receptor was lower for all of the tryptamines, including psilocin and DMT, compared with LSD and correlated with the reported psychoactive doses in humans. Several tryptamines, including psilocin, DMT, DiPT, 4-OH-DiPT, and 4-OH-MET, interacted with the serotonin transporter and partially the norepinephrine transporter, similar to 3,4-methylenedioxymethamphetamine but in contrast to LSD and mescaline. LSD but not the tryptamines interacted with adrenergic and dopaminergic receptors. In conclusion, the receptor interaction profiles of the tryptamines predict hallucinogenic effects that are similar to classic serotonergic hallucinogens but also MDMA-like psychoactive properties.”

Authors: Anna Rickli, Olivier D. Moning, Marius C. Hoener & Matthias E. Liechti

Summary

Abstract

The present study investigated the interactions between the novel psychoactive tryptamines DiPT, 4-OH-DiPT, 4-OH-MET, 5-MeO-AMT, and 5-MeO-MiPT and monoamine receptors and transporters. The results showed that the tryptamines have similar hallucinogenic effects to classic serotonergic hallucinogens but also MDMA-like psychoactive properties.

1. Introduction

Tryptamines are naturally found in toads, plants, and mushrooms, and are synthesized into novel psychoactive substances. Their psychoactive effects are thought to be mediated mainly by the 5-HT2A receptor, but may also be modulated by interactions with other 5-HT receptors, monoamine transporters, and trace amine-associated receptors. Different structural alterations of tryptamines have been shown to result in different pharmacological and psychoactive profiles, such as hallucinogenic effects with relative low potency in man, stimulant-type activation, and less visual perceptual alterations. Many tryptamines have been studied previously at selected targets, but new and pharmacologically unknown tryptamine derivatives are constantly emerging on the illicit drug market. The in vitro receptor interaction profiles of these novel substances are important to predict psychotropic effects and acute clinical toxicity.

The present study included recreationally used tryptamines, including DiPT, 4-OH-DiPT, 4-OH-MET, 5-MeO-AMT, and 5-MeO-MiPT. DiPT is an agonist at rat and human 5-HT2A receptors and also blocks the rat and human SERT.

4-OH-DiPT and 4-OH-MET, which are 4-substituted tryptamines, have been detected in the urine of substance users and have similar subjective effects to psilocybin.

5-MeO-AMT and 5-MeO-MiPT are 5-methoxy-substituted tryptamines that exert psychoactive effects at oral doses of 2 – 6mg. Data on other receptor interactions are lacking, and more comprehensive profiles of 5-MeO-AMT and 5-MeO-MiPT at human transporters and other receptors are needed.

Psilocin, DMT, and LSD are classic tryptamines that continue to be used recreationally. The current study examined the in vitro pharmacological effects of these substances on human receptors and allowed direct comparisons between classic tryptamines and other novel psychoactive substances within the same assays.

The radiolabeled chemicals [3H]NE and [3H]DA were purchased from Perkin-Elmer, and [3H]5-HT was obtained from Anawa. The data on LSD and mescaline were previously published.

2.2. Radioligand receptor and transporter binding assays

The radioligand binding assays were performed as described previously (Simmler et al., 2013). The IC50 values were determined by calculating nonlinear regression curves for a one-site model using three to five independent 10-point concentration-response curves for each compound.

2.3. Activity at the serotonin 5-HT2A receptor

Human 5-HT2A receptor-expressing mouse embryonic fibroblasts were incubated with the test substances for 1 h at 37 1C, and the increase in fluorescence was measured. The EC50 values were derived from the concentration-response curves using nonlinear regression.

2.4. Activity at the serotonin 5-HT2B receptor

Human 5-HT2B receptor-expressing HEK293 cells were incubated in growth medium, the calcium indicator Fluo-4 solution was added to each well, and the cells were incubated for 45 min at 31 1C. The increase in fluorescence was measured, and the EC50 values were derived using nonlinear regression.

2.5. Monoamine uptake transporter inhibition

We assessed the inhibition of human NET, DAT, and SERT in transfected HEK293 cells by adding radiolabeled monoamines at a final concentration of 5 nM at room temperature for 10 min. The cell pellets were lysed and the specific uptake was determined.

2.6. Transporter-mediated monoamine release

We added a single high dose of the test compounds to HEK293 cells that overexpressed the respective human monoamine transporter, and measured the release of NE, DA, and 5-HT. Substances that exhibited significant higher outflow than the controls were considered monoamine releasers.

2.7. Cytotoxicity

Cell toxicity was assessed using the ToxiLight bioassay, which measures the release of adenylate kinase from damaged cells. Dimethylsulfoxide was used as a negative control, and Triton X-100 was used as a positive control.

3.1. Interactions with serotonin receptors

All of the hallucinogens tested bound to or activated serotonin 5-HT receptors mostly at submicromolar concentrations, but 5-HT2A receptor binding affinity was not associated with 5-HT2A receptor activation potency, or with the estimated mean doses that produced hallucinogenic subjective effects.

The 5-HT2A receptor was the most potent receptor in the study, and all of the tryptamines were more potent agonists than LSD. However, 5-HT2A over 5-HT2C receptor binding selectivity was low, and LSD was the only compound to exhibit 5-HT1A receptor affinity at 1-digit nanomolar concentrations.

3.2. Binding to monoamine receptors and transporters

Table 2 shows that most compounds had weak binding affinities to monoamine transporters and receptors, with the exception of DiPT and 4-OH-MET, which exhibited submicromolar binding affinity to the SERT.

3.3. Monoamine uptake transporter inhibition

All substances inhibited at least one monoamine transporter, with the exception of mescaline and LSD. Psilocin, DMT, DiPT, and 4-OH-DiPT were weaker DAT inhibitors than MDMA.

4. Discussion

All of the tryptamines tested bound to and activated the 5-HT2A receptor, extending previous in vitro studies on this group of hallucinogens.

The present study confirms and extends previous characterizations of tryptamines, showing that 4-OH-DiPT and 4-OH-MET are 5-HT2A receptor partial agonists, SERT inhibitors, and weak NET inhibitors, exhibiting a similar profile to psilocin and consistent with their reportedly common clinical effects. 5-MeO-AMT was the most potent 5-HT2A receptor ligand and agonist among the tryptamines that were evaluated in the present study and previous studies. 5-MeO-AMT was a very weak monoamine transporter inhibitor, especially considering its much higher potency at the 5-HT2A receptor.

The pharmacological profiles of the tryptamines and classic and well-studied hallucinogen LSD were clearly different. LSD more potently bound to 5-HT2A receptors compared with all of the tryptamines, and LSD was absolutely and in most cases also relatively more potent than all of the tryptamines at the 5-HT1 receptor.

LSD, DMT, and psilocin were partial agonists at the 5-HT2A receptor, whereas all the novel tryptamines were full agonists. LSD also binds to adrenergic and dopaminergic receptors at submicromolar concentrations. All of the tryptamines inhibited the SERT at submicromolar concentrations, similar to MDMA, and DMT and 5-MeO-AMT released 5-HT through the human SERT, similar to our findings. LSD and mescaline did not interact with the monoamine transporters.

LSD is a more potent 5-HT2A receptor ligand compared with psilocin, but both substances are partial agonists at 5-HT receptors and 1- and 2-adrenergic and dopaminergic D1-3 receptors. Psilocin inhibits the SERT, but modern clinical studies that directly compare the effects of psilocybin and LSD are currently lacking.

The affinity of hallucinogens at the 5-HT2A receptor but not at the 5-HT1A receptor correlates with psychoactive potency in humans. Tryptamines can be expected to be psychoactive at higher doses than LSD.

In the present study, the highest 5-HT2A receptor binding affinity tryptamine (5-MeO-AMT) was psychoactive at 2 – 5mg, and the lowest 5-HT2A receptor binding affinity tryptamine (DiPT) was psychoactive at 20 – 100mg. However, 5-HT2A receptor activation potency did not correlate with the human doses.

Many psychoactive compounds bind to TAAR1, including LSD, DMT, 4-OH-MET, and 5-MeO-AMT. TAAR1 agonism may be linked to lower stimulant-type properties of tryptamines.

We observed several structure-activity relationships among tryptamines, including 5-MeO-AMT exhibiting the highest 5-HT2A receptor affinity and activation potency, and 4-hydroxylation increasing 5-HT2A receptor binding for psilocin vs. DMT and 4-OH-DiPT vs. DiPT. N-substitutions alter the receptor interaction profiles of psychoactive tryptamines and phenethylamines. N,N-isopropylation reduces binding affinity at 5-HT2 receptors compared with N,N-methylation, whereas asymmetrical N-methyl-N-isopropyl substitution results in greater activity compared with symmetrical N-alkyl substitution.

All of the tested tryptamines were partial or full 5-HT2A receptor agonists, similar to MDMA, and exerted hallucinogenic and MDMA-like effects in humans.