We currently understand the mental effects of psychedelics to be caused by agonism or partial agonism of 5-HT 2A (and possibly 5-HT 2C ) receptors, and we understand that psychedelic drugs, especially phenylalkylamines, are fairly selective for these two receptors. This manuscript is a reference work on the receptor affinity pharmacology of psychedelic drugs. New data is presented on the affinity of twenty-five psychedelic drugs at fifty-one receptors, transporters, and ion channels, assayed by the National Institute of Mental Health – Psychoactive Drug Screening Program (NIMH-PDSP). In addition, comparable data gathered from the literature on ten additional drugs is also presented (mostly assayed by the NIMH-PDSP). A new method is introduced for normalizing affinity (K i ) data that factors out potency so that the multi-receptor affinity profiles of different drugs can be directly compared and contrasted. The method is then used to compare the thirty-five drugs in graphical and tabular form. It is shown that psychedelic drugs, especially phenylalkylamines, are not as selective as generally believed, interacting with forty-two of forty-nine broadly assayed sites. The thirty-five drugs of the study have very diverse patterns of interaction with different classes of receptors, emphasizing eighteen different receptors. This diversity of receptor interaction may underlie the qualitative diversity of these drugs. It should be possible to use this diverse set of drugs as probes into the roles played by the various receptor systems in the human mind.

Competing interests: The NIMH-PDSP actually produced the affinity data for twenty-five drugs, and provided it exclusively to the author. This does not alter the author's adherence to all the PLoS ONE policies on sharing data and materials.

Funding: This work resulted from a large set of receptor affinity assays performed by the NIMH-PDSP ( http://pdsp.med.unc.edu/ ). Although the project was specifically approved for the author by the NIMH and the NIMH-PDSP, it did not result in a grant in the sense of funds that flow through the author's institution. The funding went directly to the NIMH-PDSP which was at Case Western Reserve University at the time. Also, the National Institute on Drug Abuse Drug Supply Program ( http://www.nida.nih.gov/ ) provided many of the drugs, but they also went directly to the NIMH-PDSP at Case Western. There was no other funding for this research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. This statement is true except that the NIMH-PDSP actually produced the affinity data for twenty-five drugs, and provided it exclusively to the author.

Copyright: © 2010 Thomas S. Ray. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Twenty-five drugs assayed for this study by the NIMH-PDPS against fifty-one receptors, transporters and ion-channels. The twenty-five drugs include sixteen phenylalkylamines, eight tryptamines, and one ergoline. The three control drugs on the right include one representative from each structural class, and are believed to be non-psychedelic.

The objective of this paper is to present the receptor binding profiles of the thirty-five drugs ( Fig. 1 , Fig. 2 ) of this study in such a way that they can be easily compared in both their similarities and their differences. This is intended to serve as a reference work on the multi-receptor affinity pharmacology of psychedelic drugs. The tables and figures are the heart of this manuscript. Some of them have been included as “supporting information,” because they exceed the size limits of standard tables and figures. However, this supporting information is no less central to the manuscript than the standard tables and figures.

We currently understand the mental effects of psychedelics to be caused by agonism or partial agonism of 5-HT 2A (and possibly 5-HT 2C ) receptors (serotonin-2A and serotonin-2C receptors) [1] . This understanding was first developed in the 1980s [2] – [4] and has since been confirmed by a large body of evidence, as reviewed recently by Nichols [1] . However, many authors have commented that other receptors may also play a role [1] , [3] , [5] – [9] . In this post-genome era of high-throughput assays, it is time to take a broader view, move beyond the common-denominator approach [6] , and begin to explore the role of other receptors in shaping the mental effects of psychedelics, especially the qualitative differences among them.

Methods

New PDSP Binding Assays For this study, the NIMH-PDSP (http://pdsp.med.unc.edu/) has assayed sixteen phenylalkylamines, eight tryptamines and one ergoline (twenty-two psychedelics and three controls, Fig. 1) against a panel of fifty-one receptors, transporters, and ion channels. The methodology has been described previously by Glennon et al. [12]. Each compound is initially assayed at 10 µM against each receptor, transporter or ion channel (primary assay). Those that induce >50% inhibition (“hit”) are then assayed at 1, 10, 100, 1,000, and 10,000 nM to determine K i values (secondary assay). Each K i value (equilibrium dissociation constant, concentration at which 50% of the hot ligand is displaced by the test ligand) is calculated from at least three replicated assays. Details of how individual assays were conducted can be found at the NIMH-PDSP web site: http://pdsp.med.unc.edu/pdspw/binding.php. Table S2 shows raw K i data for the current study combined with data collected from the literature for the ten additional compounds; a total of thirty-five drugs and sixty-seven receptors, transporters and ion channels which were assayed. The table has been divided into three sections. The first section displays forty-two sites at which most compounds were assayed and at least one “hit” (K i <10,000 nm) was found: 5ht1a (5-HT 1A , serotonin-1A receptor), 5ht1b (5-HT 1B , serotonin-1B receptor), 5ht1d (5-HT 1D , serotonin-1D receptor), 5ht1e (5-HT 1E , serotonin-1E receptor), 5ht2a (5-HT 2A , serotonin-2A receptor), 5ht2b (5-HT 2B , serotonin-2B receptor), 5ht2c (5-HT 2C , serotonin-2C receptor), 5ht5a (5-HT 5A , serotonin-5A receptor), 5ht6 (5-HT 6 , serotonin-6 receptor), 5ht7 (5-HT 7 , serotonin-7 receptor), D1 (D 1 , dopamine-1 receptor), D2 (D 2 , dopamine-2 receptor), D3 (D 3 , dopamine-3 receptor), D4 (D 4 , dopamine-4 receptor), D5 (D 5 , dopamine-5 receptor), Alpha1A (α 1A , alpha-1A adrenergic receptor), Alpha1B (α 1B , alpha-1B adrenergic receptor), Alpha2A (α 2A , alpha-2A adrenergic receptor), Alpha2B (α 2B , alpha-2B adrenergic receptor), Alpha2C (α 2C , alpha-2C adrenergic receptor), Beta1 (β 1 , beta-1 adrenergic receptor), Beta2 (β 2 , beta-2 adrenergic receptor), SERT (serotonin transporter), DAT (dopamine transporter), NET (nor epinephrine transporter), Imidazoline1 (I 1 , imidazoline-1 receptor), Sigma1 (σ 1 , sigma-1 receptor), Sigma2 (σ 2 , sigma-2 receptor), DOR (delta opioid receptor), KOR (κ, kappa opioid receptor), MOR (μ, mu opioid receptor), M1 (M 1 , muscarinic-1 acetylcholine receptor), M2 (M 2 , muscarinic-2 acetylcholine receptor), M3 (M 3 , muscarinic-3 acetylcholine receptor), M4 (M 4 , muscarinic-4 acetylcholine receptor), M5 (M 5 , muscarinic-5 acetylcholine receptor), H1 (H 1 , histamine-1 receptor), H2 (H 2 , histamine-2 receptor), CB1 (CB 1 , cannabinoid-1 receptor), CB2 (CB 2 , cannabinoid-2 receptor), Ca+Channel (calcium+ ion channel), NMDA/MK801 (N-methyl D-aspartate glutamate receptor). The second section displays seven sites at which most compounds were assayed, but at which there were no hits: 5ht3 (serotonin-3 receptor), H3 (histamine-3 receptor), H4 (histamine-4 receptor), V1 (vasopressin-1 receptor), V2 (vasopressin-2 receptor), V3 (vasopressin-3 receptor), GabaA (GABA-A receptor). The third section displays the remaining eighteen sites, at which only a few compounds were assayed, and no hits were found: GabaB (GABA-B receptor), mGluR1a (mGluR1a metabotropic glutamate receptor), mGluR2 (mGluR2 metabotropic glutamate receptor), mGluR4 (mGluR4 metabotropic glutamate receptor), mGluR5 (mGluR5 metabotropic glutamate receptor), mGluR6 (mGluR6 metabotropic glutamate receptor), mGluR8 (mGluR8 metabotropic glutamate receptor), A2B2 (nicotinic a2/b2 acetylcholine receptor), A2B4 (nicotinic a2/b4 acetylcholine receptor), A3B2 (nicotinic a3/b2 acetylcholine receptor), A3B4 (nicotinic a3/b4 acetylcholine receptor), A4B2 (nicotinic a4/b2 acetylcholine receptor), A4B2** (nicotinic a4/b2** acetylcholine receptor), A4B4 (nicotinic a4/b4 acetylcholine receptor), BZP (a1) (GABA-BZP a1 receptor), EP3 (prostaglandin-3 receptor), MDR 1 (multidrug resistant p-Glycoprotein), PCP (PCP glutamate receptor).

Activity Assays For the twenty-five compounds of Fig. 1, the NIMH-PDSP also performed activity assays at 5-HT 2A and 5-HT 2C . The E max values (maximal activity) are relative to 5-HT (serotonin), measuring Ca++ mobilization. Ca++ flux assays were performed using a FLIPRTETRA. The activity assays were conducted with cell lines which have very high receptor expression levels (e.g. plenty of ‘spare receptors’). Under such conditions partial agonists will have considerable agonist activity. The data represent the mean ± variance of computer-derived estimates from single experiments done in quadruplicate. Thus, the four observations are averaged and a single estimate with error is provided (Table S3).

Sources The following compounds (Fig. 1) were used in the study: 2C-B, 4-Bromo-2,5-dimethoxyphenethylamine

2C-B-fly, 1-(8-Bromo-2,3,6,7-tetrahydrobenzo[1,2-b;4,5-b′]difuran-4-yl)2-aminoethane

2C-E, 4-Ethyl-2,5-dimethoxyphenethylamine

2C-T-2, 4-Ethylthio-2,5-dimethoxyphenethylamine

ALEPH-2, (±)-4-Ethylthio-2,5-dimethoxyamphetamine

4C-T-2, 4-Ethylthio-2,5-dimethoxyphenylbutylamine

MEM, (±)-2,5-Dimethoxy-4-ethoxyamphetamine

TMA-2: (±)-2,4,5-Trimethoxamphetamine

TMA: (±)-3,4,5-Trimethoxamphetamine

mescaline: 3,4,5-Trimethoxyphenethylamine

DOB: (±)-2,5-Dimethoxy-4-bromoamphetamine

DOI: (±)-2,5-Dimethoxy-4-iodoamphetamine

DOM: (±)-2,5-Dimethoxy-4-methylamphetamine

DOET: (±)-2,5-Dimethoxy-4-ethylamphetamine

MDA: (±)-3,4-Methylenedioxyamphetamine

MDMA: (±)-3,4-Methylenedioxymethamphetamine

DMT: N,N-Dimethyltryptamine

5-MeO-DMT: 5-Methoxy-N,N-dimethyltryptamine

DPT: N,N-Dipropyltryptamine

5-MeO-MIPT: 5-Methoxy-N-methyl-N-isopropyltryptamine

DIPT: N,N-Diisopropyltryptamine

5-MeO-DIPT: 5-Methoxy-N,N-diisopropyltryptamine

6-fluoro-DMT: 6-Fluoro-N,N-dimethyltryptamine

psilocin: 4-Hydroxy-N,N-dimethyltryptamine

lisuride 5-MeO-DMT, and DOI were purchased from Sigma. DOB, DOET, mescaline, TMA, MDA, MDMA, and psilocin were provided as gifts by the National Institute on Drug Abuse Drug Supply Program. 2C-B, 2C-B-fly, MEM, 4C-T-2, 5-MeO-MIPT, 6-fluoro-DMT, TMA-2, and lisuride were provided as gifts by Dave Nichols. DMT and DOM were provided as gifts by Richard Glennon. 2C-E, 2C-T-2, Aleph-2, DIPT, 5-MeO-DIPT, and DPT were provided as gifts by Alexander Shulgin.

Normalization The raw K i values are distributed over several orders of magnitude, thus a log transformation is a good first step in the analysis. In addition, higher affinities produce lower K i values, thus it is valuable to calculate: pK i = −log 10 (K i ). Higher affinities have higher pK i values, and each unit of pK i value corresponds to one order of magnitude of K i value. Table S4 presents the raw data transformed into pK i values. Generally, the highest K i value generated by NIMH-PDSP is 10,000, which produces a pK i value of −4 (although a value of 10,450 was reported for 5-MeO-TMT). For non-PDSP data gathered from the literature, some K i values greater than 10,000 are reported (i.e. 12,500, 14,142, 22,486, 39,409 and 70,000 for ibogaine). When the primary assay did not produce >50% inhibition, the K i value is treated as >10,000. When the primary assay hit, but the secondary assay was not performed, the K i value is also treated as >10,000. If a secondary assay produced a K i value significantly greater than 10,000, it is usually also reported as >10,000. The lowest K i value in the data set of this study is 0.3 (lisuride at 5-HT 1A ) and the highest value is 70,000 (ibogaine at D 3 ), thus collectively, the data in this study cover nearly six orders of magnitude of K i values. However, ignoring values reported as >10,000, the K i values for a single drug in this study never exceed four orders of magnitude in range. The goal of the normalization used in this study is to factor out potency, in order to allow easy comparison of the multi-receptor affinity profiles of different drugs. The normalization will adjust the highest pK i value for each drug to a value of 4, and set all K i values reported as >10,000 to a value of zero. K i values actually measured as greater than 10,000 are not set to zero (i.e. 5-MeO-TMT and ibogaine). We will call this normalized value npK i . Let the maximum pK i value for each drug be called pK iMax . For each individual drug: If K i treated as >10,000, then npK i = 0

treated as >10,000, then npK = 0 npK i = 4+pK i −pK iMax With this normalization: higher affinities have higher values

affinities too low to be measured will be reported as zero

for each drug, the highest affinity will be set to a value of 4

each unit of npK i value represents one order of magnitude of K i value

value represents one order of magnitude of K value potency is factored out so that drugs of different potencies can be directly compared This normalization effectively factors out the absolute potency of each drug, and allows us to focus on the relative affinities of each drug at each receptor.

Perceptibility It will also be seen that many psychedelic drugs interact with a large number of receptors. Fig. 3 shows the ranked distributions of npK i values for DOB and DOI, and the same data is listed below in numerical form (0.00 means K i >10,000, ND means the data is not available): PPT PowerPoint slide

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larger image TIFF original image Download: Figure 3. Receptor affinity profiles of DOB and DOI, ordered by decreasing affinity. The vertical axis is normalized pK i (npK i ). Horizontal axis is a list of forty-two receptors, arranged in order of decreasing affinity for each individual drug. Colors correspond to classes of receptors, and are the same as used in Fig. S1. The black vertical bars represent a 100-fold drop in affinity relative to the receptor with the highest affinity. As a rule of thumb, this is presumed to be the limit of perceptible receptor interaction. Receptors to the right of the black bar should be imperceptible, while receptors to the left of the black bar should be perceptible, increasingly so the further left they are. https://doi.org/10.1371/journal.pone.0009019.g003 DOB: 4.00 5ht2b, 3.23 5ht2a, 2.97 5ht2c, 2.11 Beta2, 1.89 5ht7, 1.82 Alpha2C, 1.79 5ht1d, 1.68 D3, 1.62 5ht1b, 1.53 M3, 1.44 5ht1e, 1.41 Alpha2B, 1.39 Imidazoline1, 1.25 Sigma1, 1.21 Beta1, 1.18 5ht1a, 0.96 Alpha2A, 0.87 5ht5a, 0.85 5ht6, 0.66 SERT, 0.63 H1; 0.00: D5, D2, D4, NET, D1, Alpha1B, Sigma2, DOR, KOR, MOR, M1, M2, DAT, M4, M5, Alpha1A, H2, CB2, CB1, Ca+Channel, NMDA DOI: 4.00 5ht2c, 3.79 Alpha2A, 3.52 Beta2, 3.44 5ht2a, 3.13 Alpha2B, 3.13 5ht2b, 3.00 5ht1d, 2.90 M4, 2.89 Beta1, 2.88 Alpha2C, 2.83 SERT, 2.66 5ht1e, 2.51 M3, 2.42 H1, 2.36 M2, 2.34 5ht6, 2.32 M5, 2.31 5ht1a, 2.23 M1, 1.90 5ht7, 1.73 Sigma1, 1.70 Sigma2, 1.67 D1; 0.00: 5ht1b, DAT, Imidazoline1, NET, 5ht5a, DOR, KOR, MOR, Alpha1B, D2, D3, D4, D5, Alpha1A, H2, CB2, CB1, NMDA; ND: Ca+Channel For potent compounds like DOB and DOI, it is possible to measure K i values over nearly a full four orders of magnitude range of affinity. However, not all of these affinities are able to produce perceptible mental effects. As a rule of thumb, 100-fold affinity is considered truly selective. Thus, receptors with npK i values below about 2.0 should not have perceptible mental effects. In Fig. 3, a black vertical bar represent a 100-fold drop in affinity relative to the receptor with the highest affinity, and divides those npK i values greater than 2.0 (on the left) from those 2.0 or less (on the right). This is presumed to be the limit of perceptible receptor interaction. Receptors to the right of the black bar should be imperceptible, while receptors to the left of the black bar should be perceptible, increasingly so the further left they are. In spite of the long tail of affinities, DOB is effectively selective for the three 5-HT 2 (serotonin-2) receptors (Beta2 falls at the approximate limit of perceptibility), while DOI by contrast has nineteen receptors in the presumed perceptible range, although they should not all be equally perceptible.

Breadth An index of the breadth (or inverse of selectivity), B, of the binding profiles of the individual drugs or receptors can be constructed by summing the forty-two npK i values for each drug, or the thirty-five npK i values for each receptor. If a drug were absolutely selective, binding at only one receptor (e.g. salvinorin A), it would have the minimal B value of 4, regardless of the absolute affinity of the drug for its one receptor. If a drug bound with equal affinity to all forty-two receptors, it would have the maximum B value of 4×42 = 168, regardless of its absolute receptor affinities. It is not clear that a simple sum of npK i values is the best index of breadth. In this method, four receptors with K i values of 1,000 collectively carry the same weight as one receptor with a K i value of 1. This may not be a realistic equivalence. Thus we will include three measures of breadth: B sq and B exp give greater weight to higher affinity (lower K i ) values. Regression analysis of receptor affinity vs. potency in humans suggests that B sq is the most meaningful breadth statistic. Table S5 and Table S6 present the raw K i data converted into npK i values, for both the individual receptors, and groups of receptors summed using the B sq statistic.

Proportional Breadth In addition to looking at the breadth of interaction of individual drugs with multiple receptors, it may be of value to look at an individual drug's interaction with one receptor or group of receptors, as a proportion of the drug's total interaction with all receptors. In order to compute the proportion for and individual receptor or a group of receptors, we divide the sum of squares of npK i values for the group of receptors, by the sum of squares of npK i values for all receptors: For example, to compute this proportion for “5-HT” receptors, we divide the squares of the values in the “5-HT” column of Table S5 (for LSD, 11.132 = 123.9), by the squares of the values in the center column (“B sq ”) of Table 3 (for LSD, 13.122 = 172.1); 123.9/172.1 = 0.719 for LSD. We will call this proportion B p . The proportional breadth data is displayed in Table S7 and Table S8. PPT PowerPoint slide

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larger image TIFF original image Download: Table 3. Thirty-five drugs arranged in order of decreasing breadth, increasing selectivity. https://doi.org/10.1371/journal.pone.0009019.t003