A non-hallucinogenic psychedelic analogue with therapeutic potential

This paper describes an analogue to ibogaine (tabernanthalog) with similar therapeutic potential that is non-toxic, and non-psychedelic.

Abstract

“The psychedelic alkaloid ibogaine has anti-addictive properties in both humans and animals. Unlike most medications for the treatment of substance use disorders, anecdotal reports suggest that ibogaine has the potential to treat addiction to various substances, including opiates, alcohol and psychostimulants. The effects of ibogaine— like those of other psychedelic compounds—are long-lasting, which has been attributed to its ability to modify addiction-related neural circuitry through the activation of neurotrophic factor signalling. However, several safety concerns have hindered the clinical development of ibogaine, including its toxicity, hallucinogenic potential and tendency to induce cardiac arrhythmias. Here we apply the principles of function-oriented synthesis to identify the key structural elements of the potential therapeutic pharmacophore of ibogaine, and we use this information to engineer tabernanthalog—a water-soluble, non-hallucinogenic, non-toxic analogue of ibogaine that can be prepared in a single step. In rodents, tabernanthalog was found to promote structural neural plasticity, reduce alcohol- and heroin-seeking behaviour, and produce antidepressant-like effects. This work demonstrates that, through careful chemical design, it is possible to modify a psychedelic compound to produce a safer, non-hallucinogenic variant that has therapeutic potential.”

Authors: Lindsay P. Cameron, Robert J. Tombari, Ju Lu, Alexander J. Pell, Zefan Q. Hurley, Yann Ehinger, Maxemiliano V. Vargas, Matthew N. McCarroll, Jack C. Taylor, Douglas Myers-Turnbull, Taohui Liu, Bianca Yaghoobi, Lauren J. Laskowski, Emilie I. Anderson, Guoliang Zhang, Jayashri Viswanathan, Brandon M. Brown, Michelle Tjia, Lee E. Dunlap, Zachary T. Rabow, Oliver Fiehn, Heike Wulff, John D. McCorvy, Pamela J. Lein, David Kokel, Dorit Ron, Jamie Peters, Yi Zuo & David E. Olson

Notes

This paper is included in our ‘Top 10 Articles on Psychedelics in the Year 2020‘

The paper is also described in an accompanying article by the University of California, Davis, and Nature. The new molecule (tabernanthalog, TBG) is water-soluble and can be synthesized in one step. David Olson (the final author) has started a company, Delix Therapeutics, to commercialize this molecule.

Summary

Ibogaine, an alkaloid produced by Tabernanthe iboga, has been reported to reduce symptoms of drug withdrawal, reduce drug cravings and prevent relapse. However, the potential of ibogaine as a therapeutic is limited by several major issues, including the lack of a scalable, enantioselective, total synthesis and the high cardiotoxicity of ibogaine.

Although the exact mechanism of action of ibogaine has not yet been fully elucidated, evidence suggests that it may alter addiction-related circuitry by promoting neural plasticity. Moreover, psychoplastogens such as ibogaine may have the potential for broad use as anti-addictive agents.

We developed simplified analogues of ibogaine using function-oriented synthesis, which lack the toxicity and hallucinogenic effects of ibogaine but maintain its behavioural effects in rodent models of drug self-administration and relapse.

Function-oriented synthesis of ibogalogs

We synthesized a series of isoquinuclidine-containing compounds that lacked the tetrahydroazepine and/or indole moieties that are characteristic of ibogaine. These compounds differ from ibogaine only by the presence of the C2 – C16 bond.

Psychoplastogenic pharmacophore of ibogaine

Ibogainalog (IBG), a simplified version of ibogaine, showed comparable psychoplastogenic performance to ibogaine despite its simplified chemical structure. We prioritized IBG for further development because of its improved central nervous system multiparameter optimization (MPO) score18 and the fact that it can be synthesized in a single step.

TBG is a safer, non-hallucinogenic 5-HT2A agonist

The structure of IBG is similar to that of 5-MeO-DMT, a potent hallucinogen and serotonin 2A receptor (5-HT2A) agonist. However, unlike 5-MeO-DMT, 6-MeO-DMT does not substitute for the hallucinogen 2,5-dimethoxy-4-methylamphetamine at any dose in rodents.

We tested the hallucinogenic potential of IBG and TBG in the head-twitch response assay using 5-MeO-DMT as a positive control. IBG exhibits significantly reduced hallucinogenic potential, whereas TBG exhibits no hallucinogenic potential.

Ibogaine inhibits hERG channels with a half-maximal inhibitory concentration of 1 mM and increases the likelihood of inducing arrhythmias. IBG and TBG have a lower potential for cardiotoxicity.

We treated larval zebrafish with ibogaine, noribogaine, haloperidol, sertindole and terfenadine to compare the acute behavioural effects of these compounds. We found that ibogaine and noribogaine produced behavioural profiles similar to the lethal control.

Zebrafish expressing GCaMP5 did not show excessive neural activity when treated with ibogaine and TBG.

We compared the morphological effects of ibogaine, IBG and TBG on zebrafish using a well-established zebrafish developmental toxicity assay. Ibogaine significantly increased malformations and mortality, while noribogaine resulted in greater survival, but the majority of fish exhibited yolk sac and/or pericardial oedemas.

We performed a panel of serotonin (5-HT) and opioid receptor functional assays to assess canonical GPCR signalling. IBG and TBG showed potent agonist activity at human (Fig. 2f ) and mouse (5-HT2A) receptors, and act as antagonists at 5-HT2B receptors.

We performed a conditioned place preference assay in mice to determine whether TBG has rewarding effects. The results suggest that TBG has a low potential for abuse.

Effect of TBG on neural plasticity

TBG increased dendritic arbor complexity and spine density in rat embryonic cortical neurons and in mature cortical cultures, and increased spine formation with no effect on spine elimination in mouse sensory cortex.

Effect of TBG on forced swim test behaviour

After 7 days of unpredictable mild stress, mice spent more time immobile and needed more TBG to become mobile again.

Ketamine produced antidepressant-like effects in the forced swim test even in the absence of unpredictable mild stress, and TBG produced similar effects. Ketamine was more durable, however, and TBG had no effect on locomotion 24 h after administration.

TBG reduces alcohol- and heroin-seeking behaviour

We used an intermittent-access, two-bottle choice experiment to assess the effect of TBG on alcohol (ethanol) intake. TBG reduced alcohol intake without affecting water intake, and the effect lasted for at least 2 days.

Ibogaine can reduce opioid use in humans. In rodent models of opioid self-administration, ibogaine reduced heroin-seeking behaviour when administered during self-administration, before the first day of extinction, and immediately before cued reinstatement.

TBG reduced cue-induced relapse to sucrose-seeking behaviour long before cued reinstatement, and had anti-addictive effects lasting up to 12 – 14 days.

Discussion

Compounds that modify neural circuits controlling motivation, anxiety and drug-seeking behaviour have been proposed to be effective treatments for a diverse range of neuropsychiatric disorders. Psychedelic compounds might prove useful in this regard because they promote structural and functional neural plasticity in the prefrontal cortex of rodents.

Here we used the principles of function-oriented synthesis to develop a one-step synthesis of ibogaine analogues that promote structural neural plasticity both in cell culture and in vivo. These analogues are safe and effective, unlike 18-methoxycoronaridine (18-MC), which requires 13 steps to synthesize.

TBG promotes neuronal growth and inhibits heroin- and sucrose-seeking behaviour in mice and rats. It is unknown whether structural plasticity plays a role in these effects.

Data analysis and statistics

Treatments were randomized, and statistical analyses were performed by experimenters blinded to treatment conditions. Data are represented as mean s.e.m., with asterisks indicating *P 0.05, **P 0.01, ***P 0.001 and ****P 0.0001. To compare the effects of compound treatments on thigmotaxis, we used one-way ANOVA with Dunnett’s post hoc test, paired t-test, two-way repeated measures ANOVA with Sidak’s post hoc test, and two-tailed unpaired t-test.

Drugs

The NIDA Drug Supply Program provided ibogaine hydrochloride, noribogaine, heroin (diamorphine hydrochloride) and cocaine hydrochloride. Ketamine hydrochloride, ketanserin, eugenol and 5-hydroxytryptamine were purchased from commercial sources.

Calculation of CNS MPO score

CNS MPO scores were calculated using a previously published method18, and predicted pKa values were determined using Marvin Sketch (19.25.0).

Head-twitch response

The head-twitch response assay was performed using male and female C57BL/6J mice. Compounds were administered via intraperitoneal injection and behaviour was videotaped and scored by two blinded observers.

hERG inhibition studies

All experiments were conducted manually using a HEKA EPC-10 amplifier at room temperature in the whole-cell mode of the patch-clamp technique. Before experiments, cells were grown to 60 – 80% confluency, lifted using TrypLE, plated onto poly-l-lysine-coated coverslips, and patch pipettes with resistances of 2 – 4 M were used. The percentage reduction of tail current amplitude by the drugs was determined and data are shown as mean s.d.

Larval zebrafish heart rate experiments

Zebrafish express Zerg, an orthologue of hERG, and many hERG inhibitors induce bradycardia and arrhythmia in zebrafish. Heart rate was recorded and calculated as reported previously36 with slight modifications (n = 3 – 9).

Larval zebrafish behavioural experiments

Behavioural data were collected from 7 dpf zebrafish 1 h after treatment with 10 compounds. Classifiers were trained to predict responses to treatment and vehicle, and 95% confidence intervals were calculated over 1,000 bootstrap samples per comparison.

Larval zebrafish seizure experiments

Transgenic zebrafish larvae were anaesthetized with tricaine, immobilized with 1% LMA, treated with compounds, and recorded for 1 h. Fluorescence intensity was calculated and visualized using custom functions.

Larval zebrafish toxicity

5D wild-type larval zebrafish were obtained from Oregon State University and raised at UCD. Chemicals were diluted to 200 mM with embryo media and injected into individual wells of 96-well plates housing larval zebrafish.

Serotonin and opioid receptor functional assays

Functional assay screens were performed at 5-HT and opioid receptors using the same compound dilutions and 384-well-format high-throughput assay platforms.

HEK Flp-In 293 T-Rex stable cell lines were stimulated with compounds and read for baseline (0 – 10 s) and peak fold-over-basal fluorescence (5 min) at 25 °C on the FLIPR Tetra system. For 5-HT6 and 5-HT7a functional assays, cAMP accumulation was measured using the split-luciferase GloSensor assay in HEKT cells, and for 5-HT1A, 5-HT1B, 5-HT1F, MOR, KOR and DOR functional assays, -arrestin2 recruitment was measured using the Tango assay in HTLA cells.

Conditioned place preference assay

The conditioned place preference apparatus consisted of two chambers connected by a corridor. Male C57/BL6J mice (9 – 10 weeks old) were randomly sorted into treatment groups and administered an i.p. injection of either vehicle (saline) or TBG (counterbalanced) immediately before being confined to one of the two chambers for 30 min.

Pharmacokinetic studies

Male and female C57/BL6J mice were administered TBG via i.p. injection and euthanized 15 min or 3 h after injection. Brain and liver were collected and stored at 80 °C until metabolomic processing.

Sections were lyophilized, homogenized with 3.2-mm diameter stainless-steel beads, extracted with 225 l cold methanol, 190 l water, 750 l methyl tert-butyl ether, vortexed, sonicated and centrifuged before analysis for TBG. A six-point calibration curve was analysed after column equilibration using blank injections. Reconstituted sample (5 l) was injected onto a Waters Acquity UPLC CSH C18 column with an Acquity UPLC CSH C18 VanGuard precolumn using a Vanquish UHPLC coupled to a TSQ Altis triple quadrupole mass spectrometer. Mass spectrometer conditions were optimized for TBG by direct infusion, and data were processed with TraceFinder 4.1.

Spinogenesis experiments

Spinogenesis experiments were performed as previously described14, with the exception that cells were treated on DIV19 and fixed 24 h after treatment on DIV20.

In vivo spine dynamics

Male and female Thy1-GFP-M line mice were anaesthetized with ketamine and xylazine, and the apical dendrites of their primary sensory cortices were imaged using a Bruker Ultima IV two-photon microscope. The spine formation and elimination were quantified as percentages of spine numbers on day 0.

Mice (8 weeks old) were subjected to 7 d of unpredictable mild stress (UMS), which included restraint stress, exposure to a new room, sudden loud noise, tail suspension for 6 min, wet bedding, social isolation, tilted cage, island isolation, random puff of air, food deprivation, continual exposure to loud music).

Forced swim test in the absence of UMS

Male C57/BL6J mice were housed 4 – 5 mice per cage in a vivarium at UCD following an IACUC-approved protocol. They underwent a 6-min swim session in a clear Plexiglas cylinder 40 cm tall, 20 cm in diameter, and filled with 30 cm of 24 1 °C water. A week later, the FST was performed again to assess the sustained effects of the drugs. Immobility time was scored offline.

Alcohol consumption

Male C57/BL6J mice were housed in a reverse light/dark cycle room and given access to food and tap water ad libitum. Mice were given intermittent access to alcohol in their homecage for 7 weeks, and then were given TBG or vehicle 3 h before the beginning of a drinking session. Ethanol and water intake were monitored during the first 4 h, the first 24 h and the second 24 h.

Sucrose preference

Male C57/BL6J mice were administered TBG or vehicle via i.p. injection 3 h before the beginning of a two-bottle choice session. They were given one bottle of water and one bottle of water containing 5% sucrose.

Heroin self-administration behaviour

Wistar rats were age-matched and housed in a temperature- and humidity-controlled room with a 12 h light/dark cycle. Two rats were excluded from the final dataset owing to defective catheters. Heroin self-administration training began at least one week after surgery on a fixed ratio 1 (FR1) schedule of reinforcement. Each heroin infusion was coupled with a light cue and 3.5 kHz tone. After six self-administration sessions on FR1, rats progressed to a variable ratio 5 (VR5) for three sessions and continued to a final variable ratio 15 (VR15) for five sessions. Rats then underwent a cued reinstatement test (1 h, withdrawal day 10 – 12). On the third VR15 session, the first extinction session and the cued reinstatement test, TBG or vehicle was injected 30 min before placement in the chamber. Behavioural sessions were conducted daily.

Sucrose self-administration behaviour

Rats were given sucrose to self-administer to mimic heroin self-administration conditions. Eight rats were excluded due to failure to acquire sucrose self-administration.

Rats were trained to self-administer sucrose in operant chambers with an active (sucrose-delivering) and inactive lever. After 6 self-administration sessions on FR1, rats progressed to a variable ratio 5 (VR5) for 3 sessions and continued to a final variable ratio 15 (VR15) for 5 sessions. Injections of TBG were administered on the third VR15 session, the first extinction session, and the cued reinstatement test. The high dose was tested at all three test time points.

Open-field test

Naive male and female Wistar rats were allowed to acclimatize to the animal facility for at least one week after arrival. They were then allowed to move freely in a novel open field for 30 min, then were briefly removed from the apparatus to receive an injection of cocaine.