Novel Class of Psychedelic Iboga Alkaloids Disrupts Opioid Use

Havel and colleagues situate their work against the backdrop of a persistent unmet need in treatments for substance use disorders: existing medications generally do not reverse the enduring neurocircuitry and psychological alterations that sustain addiction. Ibogaine, an alkaloid from Tabernanthe iboga, is highlighted as an influential prototype because of anecdotal and open‑label evidence that a single administration can rapidly and durably interrupt opioid dependence, but its clinical potential is limited by serious safety concerns, notably cardiac arrhythmias attributed to noribogaine, its long‑lived metabolite. This study set out to create and evaluate a new class of iboga alkaloids — termed "oxa‑iboga" — generated by a targeted structural edit that replaces the indole NH with an oxygen to produce benzofuran‑containing analogues. The investigators aimed to characterise the chemical synthesis, in vitro pharmacology, in vivo behavioural and analgesic profiles, pharmacokinetics and cardiac safety, and to test whether an oxa‑iboga lead compound, oxa‑noribogaine, could produce lasting suppression of opioid self‑administration in rodent models without the cardiotoxicity associated with noribogaine.

The study combined synthetic chemistry, in vitro receptor pharmacology, in vivo rodent behavioural pharmacology, pharmacokinetics and an ex vivo human cardiomyocyte assay. Chemically, the team developed a nickel‑catalysed coupling to assemble benzofuran‑containing iboga analogues (oxa‑iboga core) from an isoquinuclidine intermediate prepared via a Diels–Alder step. In vitro pharmacology included radioligand binding, bioluminescence resonance energy transfer (BRET) assays for G protein activation, and [35S]GTPγS functional assays to quantify activity at kappa (KOR), mu (MOR) and delta (DOR) opioid receptors, plus assays for serotonin transporter (SERT) inhibition and α3β4 nicotinic receptor activity. A broad receptor panel screen was also performed to characterise off‑target binding. Behavioural and physiological in vivo studies used mice and rats. Analgesic efficacy was measured using the tail‑flick thermal nociception assay in wild‑type and opioid receptor knockout mice (KOR‑KO and MOR‑KO) to infer receptor mediation; locomotor activity in an open field and sedation proxies were used as preclinical indicators of kappa psychedelic effects. Conditioned place preference/aversion (CPP/CPA) and forced swim tests assessed reward/aversion and acute depressive‑like effects. Pharmacokinetic (PK) profiling in mice and rats estimated brain penetration, plasma and brain free drug concentrations, and time to peak brain levels. Cardiac safety was assessed using adult human primary ventricular cardiomyocytes isolated from donor hearts and field‑stimulated to monitor contractility transients. This assay detects pro‑arrhythmic events such as after‑contractions and contraction failures and was used to compare noribogaine and oxa‑iboga analogues across clinically relevant concentrations (0.1–10 µM). Therapeutic potential for opioid use disorder was evaluated in a rat intravenous self‑administration (SA) model. Rats were trained to self‑administer morphine and received either noribogaine (40 mg/kg, i.p.) as a benchmark or oxa‑noribogaine at doses of 40 mg/kg and 10 mg/kg (i.p.). A repeated dosing regimen modelled clinical reset and tapering: two 40 mg/kg reset doses separated by 4 days, followed by maintenance and intermittent tapering doses (10 mg/kg and 5 mg/kg), totalling nine administrations over 26 days. Food‑maintained responding served as a natural‑reward control. The extracted text does not clearly report detailed statistical models or whether analyses were intention‑to‑treat.

Chemistry and in vitro pharmacology: The nickel‑catalysed route yielded benzofuran‑containing oxa‑iboga analogues. Oxa‑noribogaine displayed markedly enhanced KOR potency compared with noribogaine: binding to mouse KOR with Ki = 41 nM and activating G protein in the [35S]GTPγS assay (mKOR EC50 = 49 nM, Emax = 92%) and in the BRET assay (rat KOR EC50 = 43 nM, Emax = 78%). By contrast, noribogaine acted as a KOR partial agonist in BRET with EC50 = 6.2 µM and Emax = 54%. In the [35S] assay, oxa‑noribogaine was more than tenfold selective for KOR versus MOR and about sevenfold over DOR. Structure–activity observations included the critical role of the phenol at position 10 for agonist activity and modest modulation of potency by inversion at the C20 centre (epi‑oxa isomer). Off‑target activities relevant to iboga chemistry were largely retained but with altered potency: SERT inhibition was preserved though less potent for oxa‑noribogaine (IC50 = 711 nM) than for noribogaine (IC50 = 286 nM). Inhibition of α3β4 nicotinic receptors was comparable (IC50 ≈ 2.9 µM for oxa‑noribogaine versus 5.0 µM for noribogaine). Broad screening indicated >100‑fold separation between KOR binding potency and many non‑opioid human targets except SERT and α3β4. Analgesia and behavioural profile: In mice, oxa‑noribogaine produced potent antinociception in the tail‑flick assay with ED50 = 3.0 mg/kg (s.c.), comparable to the standard KOR agonist U50,488 (ED50 = 2.2 mg/kg). Analgesia was abolished or substantially shifted in KOR‑KO mice (ED50 WT 3.0 mg/kg to ED50 KOR‑KO 20.6 mg/kg), and showed a smaller rightward shift in MOR‑KO mice (ED50 MOR‑KO = 7.0 mg/kg), supporting KOR as the primary mediator. The epi‑oxa stereoisomer was also analgesic (ED50 WT = 1.3 mg/kg) but lost activity in KOR‑KO animals. Despite potent KOR engagement, oxa‑noribogaine differed from typical kappa psychedelics in behavioural assays. At near‑maximal analgesic doses (ED80 ≈ 5.4 mg/kg) oxa‑noribogaine did not induce locomotor suppression (a preclinical proxy for kappa psychedelic sedation), whereas epi‑oxa‑noribogaine produced strong sedation reversible by the KOR antagonist aticaprant. Supra‑analgesic doses of oxa‑noribogaine (>10 mg/kg) did induce sedation. In conditioned place paradigms, neither oxa‑noribogaine (10 mg/kg P = 0.84767; 40 mg/kg P = 0.2546) nor epi‑oxa (40 mg/kg P = 0.0768) produced conditioned place preference or aversion; cocaine and morphine produced significant place preference as expected. Forced swim testing showed no acute pro‑depressive‑like effects after oxa‑noribogaine at highly analgesic doses. Pharmacokinetics: Oxa‑noribogaine is highly brain penetrant with maximal brain concentration reached at about 30 minutes post‑injection and a brain/plasma ratio of ~5.5 in mice. Approximately 85% of compound is cleared in the first 2 hours with traces after 8 hours. Estimated free brain concentrations at analgesic doses aligned with in vitro KOR activation potencies (estimated Cmax(brain) at an ED50 analgesic dose ≈ 49 nM). Cardiac safety: In adult human primary ventricular cardiomyocytes, noribogaine produced concentration‑dependent pro‑arrhythmic effects (after‑contractions and contraction failures) with a substantial incidence at ≥1 µM, consistent with clinical concerns. In contrast, neither oxa‑noribogaine nor epi‑oxa‑noribogaine showed pro‑arrhythmic potential across the tested concentration range (0.1–10 µM). Estimated free plasma Cmax after a mouse 10 mg/kg dose was ~130 nM, suggesting a large safety margin versus concentrations at which noribogaine elicited pro‑arrhythmia in the assay. Self‑administration (SA) in rats: Noribogaine (40 mg/kg, i.p.) reproduced prior findings by suppressing morphine‑maintained responding in the session following administration and partially suppressing intake on days 2–3 before returning to baseline on days 4–5; food responding was suppressed acutely (day 1) but recovered by day 2. A single oxa‑noribogaine 40 mg/kg dose produced stronger and longer‑lasting suppression: statistically significant suppression for 7 days post‑dose (P = 0.0048) and clear suppression for at least 15 days in the group mean, with one subject showing >80% reduction on days 12–15. A single 10 mg/kg dose induced >85% acute suppression of morphine intake but generally returned to baseline thereafter; acute suppression at 10 mg/kg was largely morphine‑selective with only minor effects on food responding. Repeated dosing: A regimen modelled on clinical practice (two 40 mg/kg resets separated by 4 days, then maintenance/tapering with 10 mg/kg and 5 mg/kg intermittent doses for a total of nine doses over 26 days) significantly reduced morphine SA across sessions (P = 0.0054), produced progressive reductions in intake, allowed dose tapering while maintaining efficacy, and led to sustained suppression after treatment cessation (<30% of pretreatment morphine intake over 2.5 weeks in the group mean). Individual animals varied in responsiveness, with some achieving long‑lasting suppression after a single reset dose and others requiring multiple administrations. Food responding showed acute suppression at the high 40 mg/kg dose but returned to control levels on subsequent days, indicating the lasting effects were morphine‑selective.

Havel and colleagues interpret their findings as demonstrating that a single structural edit of the iboga scaffold yields a distinct class of compounds with enhanced therapeutic potential. Oxa‑iboga analogues, exemplified by oxa‑noribogaine, augment KOR agonism relative to noribogaine, deliver robust and sometimes long‑lasting suppression of opioid self‑administration in rats, and lack the pro‑arrhythmic liability observed for noribogaine in a human cardiomyocyte assay. These properties suggest the oxa‑iboga series preserves and amplifies iboga’s desirable anti‑addiction profile while removing a primary safety concern. The investigators situate their results within a mechanistic hypothesis that lasting interruption of addiction states may be driven by drug‑induced neuroplasticity and neuro‑restorative processes. They note previous reports and recent replication that ibogaine and noribogaine induce neurotrophic factors such as GDNF and BDNF, and propose that the oxa‑iboga constellation of molecular activities (notably KOR activation together with inherited iboga pharmacology) could trigger similar neurotrophic and circuit‑restorative pathways in reward and control circuits. The authors acknowledge limitations typical of preclinical addiction research, chiefly the uncertain predictive validity of animal self‑administration paradigms for clinical efficacy. They emphasise that their reverse‑translation strategy used noribogaine as a benchmark because it is the dominant circulating species after ibogaine and reproduces ibogaine’s rodent effects. Comparisons with other synthetic iboga analogues and 5‑HT receptor ligands are used to highlight that oxa‑iboga compounds appear to produce more persistent effects than some prior candidates. Remaining uncertainties include the precise molecular mechanisms that confer the atypical KOR profile (potent KOR agonism without acute aversion or depressive‑like effects) and the multi‑ion‑channel basis for the improved cardiac safety; the authors state that detailed mechanistic studies are required. They conclude that oxa‑iboga alkaloids constitute promising candidates for development as a novel class of anti‑substance use disorder pharmacotherapeutics, meriting further preclinical and translational investigation.