Oral noribogaine shows high brain uptake and anti-withdrawal effects not associated with place preference in rodents

Mash and colleagues situate this work in a historical and pharmacological context in which ibogaine, an alkaloid used traditionally in Central Africa and explored clinically, has been reported to reduce opioid withdrawal and craving after single large doses. Early clinical and animal reports suggested durable effects of ibogaine despite its relatively short half-life in rodents, prompting the hypothesis that a long-lived metabolite mediates the sustained actions. Noribogaine, the principal o‑demethylated metabolite of ibogaine, has since been identified and shown to have distinct pharmacology, including higher affinity for the serotonin transporter and differential activity at opioid and nicotinic receptors relative to the parent compound. The study aimed to test whether noribogaine can attenuate the somatic signs of opioid withdrawal in a mouse model when given orally, to characterise its brain penetration (brain/blood ratio), and to assess abuse liability using a conditioned place preference (CPP) assay in rats. The investigators also performed a retrospective review of prior animal studies of ibogaine to explore whether differences in administration route and timing could explain inconsistent findings about ibogaine's effects on morphine withdrawal.

The primary efficacy experiment used a naloxone-precipitated morphine withdrawal model in male Swiss Webster (CFW) mice (total N=52). Morphine was escalated over three days with three daily subcutaneous injections and a final dose on day 4; two hours after the last morphine dose mice received oral noribogaine or vehicle by gavage (10 mL/kg). Noribogaine HCl was prepared as a suspension in Tween 80, dextrose and methylcellulose and dosed as free base at 10, 30, 56 and 100 mg/kg (correction factor 1.12); group sizes were n=9 for 10, 30 and 100 mg/kg, n=5 for 56 mg/kg, and n=11 for vehicle. Two hours after noribogaine or vehicle, naloxone 3 mg/kg i.p. was administered and somatic withdrawal signs (jumps, paw tremors, body tremor, diarrhoea, wet dog shakes) were counted across 30 minutes. A weighted global opiate withdrawal score was calculated using pre‑specified weights for individual signs. Dose–response was fitted with a nonlinear regression to estimate an ED50. Statistical testing used one-way ANOVA with Bartlett's test for variance and Dunnett's post hoc comparisons; significance was set at p<0.05. To assess brain penetration, three mice per dose (10, 30, 56, 100 mg/kg) received a single oral noribogaine dose and whole blood and whole‑brain samples were collected two hours post dose (near anticipated Tmax). Samples were processed and quantified by liquid chromatography–tandem mass spectrometry (LC‑MS/MS) using d3‑noribogaine as an internal standard. Calibration curves covered 5–1000 ng/mL and QC samples were run; assay accuracy and precision met acceptance criteria. Brain/plasma partitioning was expressed as brain/blood ratios; plasma concentrations were estimated from whole blood using a 0.4 plasma/whole blood ratio. Abuse liability was evaluated with a biased conditioned place preference (CPP) protocol in experimentally naïve male Sprague‑Dawley rats (N=32; four groups of eight). Animals were habituated and the least preferred compartment was paired with noribogaine (10, 30, 100 mg/kg, oral gavage) while the alternate compartment was paired with vehicle; each animal received four noribogaine and four vehicle pairings, with two free‑choice tests (T1, T2). Preference scores (percentage time in the conditioned stimulus side) were analysed by repeated measures ANOVA including group, test interval (habituation, T1, T2) and their interaction; pairwise comparisons used Tukey adjustments at alpha 0.05 and 0.01. Finally, the investigators performed a retrospective collection and synthesis of published preclinical studies reporting ibogaine effects on antagonist‑precipitated morphine withdrawal, extracting endpoints and timing and noting route of administration to explore methodological drivers of disparate findings.

In the naloxone‑precipitated withdrawal assay, noribogaine produced dose‑dependent reductions in multiple somatic signs. Jumping and paw tremors showed dose-dependent inhibition with an estimated ED50 of approximately 16 mg/kg for those individual measures; maximal decreases in jumping and paw tremors were about 75% and 65% versus vehicle, respectively. Body tremors decreased by about 50% at 10 mg/kg and by about 80% at 100 mg/kg. Diarrhoea showed a non‑consistent decreasing trend at doses >10 mg/kg. One‑way ANOVA indicated a significant main effect of treatment, and Dunnett's post hoc tests found that 30 and 100 mg/kg significantly attenuated somatic withdrawal symptoms (reported p values ranging from <0.001 to <0.05). The 56 mg/kg group trended toward reductions but reached significance only for jumping, which the authors note may reflect the smaller sample size (n=5). Analysis of the weighted global opiate withdrawal score showed dose‑dependent suppression: 10 mg/kg produced a 35% reduction (not significant), while 30 mg/kg and 100 mg/kg produced significant reductions of 74% and 89%, respectively (both p<0.001). After omitting the 56 mg/kg data point for curve fitting, the sigmoidal dose–response for global scores yielded an apparent oral ED50 of 13.2 mg/kg (pED50 = 1.1±0.3) when noribogaine was administered two hours before naloxone. Pharmacokinetic sampling two hours after oral dosing found whole blood and brain concentrations that increased approximately linearly with dose except at the highest brain concentration. The mean brain/blood ratio across the four dose groups averaged 7±1; the lowest‑dose group (10 mg/kg) had a higher ratio (9.8±1.7) and the 100 mg/kg group a lower ratio (4.7±0.3). Overall the brain versus blood relationship was linear with a slope of 6.3±0.3. At 10 mg/kg, measured concentrations were 167±26 ng/mL in blood and 1727±533 ng/mL in brain, approximating 0.6 µM and 5.8 µM, respectively. The investigators note samples were taken near Tmax and prior to full equilibration. In the CPP experiment, noribogaine did not produce conditioned place preference at doses up to 100 mg/kg in rats. Mean CPP scores in the 100 mg/kg group rose from 37.5 in habituation to 45.2 (T1) and 45.7 (T2) but these changes were not statistically significant and were similar to vehicle controls; repeated measures ANOVA showed no significant drug effect. The retrospective review of published ibogaine studies indicated that ibogaine reduced somatic and affective signs of morphine withdrawal in studies where withdrawal was precipitated 45–240 minutes after ibogaine dosing, whereas studies that administered ibogaine subcutaneously or concurrently with the antagonist tended to report lesser or no efficacy. One study even reported aggravated jumping when ibogaine was given concomitantly with naloxone.

Mash and colleagues interpret the data as demonstrating that orally administered noribogaine produces robust, dose‑dependent attenuation of naloxone‑precipitated somatic morphine withdrawal in mice, with an apparent oral ED50 near 13 mg/kg and maximal suppression of global withdrawal scores up to about 89%. They emphasise that noribogaine achieves high brain penetration (mean brain/blood ratio ≈7) and that brain concentrations at behaviourally active doses are within ranges consistent with in vitro potencies at several central targets, including kappa and mu opioid receptors, nicotinic acetylcholine receptor subtypes and the serotonin transporter. The authors note that noribogaine did not produce conditioned place preference in rats up to 100 mg/kg, which they interpret as a lack of evidence for hedonic or rewarding subjective effects in rodents and therefore low preclinical abuse liability. They argue this profile suggests noribogaine is not acting as an opioid substitution therapy akin to methadone or buprenorphine. Mechanistically, the discussion highlights noribogaine's multimodal pharmacology — including recently reported biased agonism at kappa opioid receptors and inhibition of β‑arrestin2 signalling at mu and kappa receptors — as potential contributors to its anti‑withdrawal efficacy and to a possibly favourable side‑effect profile. The investigators position their findings relative to earlier inconsistent preclinical reports on ibogaine by proposing that pharmacokinetic factors — notably route of administration and timing relative to antagonist challenge — explain many discrepancies. Specifically, studies that allowed time for metabolism to noribogaine (for example 45 minutes to two hours post‑ibogaine) and used intraperitoneal rather than subcutaneous administration reported more robust withdrawal blockade. They also discuss an earlier negative noribogaine report and suggest formulation and bioavailability issues (vehicle suitability) may have limited exposure in that study. Key limitations and uncertainties acknowledged by the authors include variability in sample size for some dose groups (notably n=5 at 56 mg/kg), incomplete understanding of the relative contribution of each noribogaine target to the observed behavioural effects, and that the PK samples were collected near but not necessarily at full equilibrium. The authors recommend further preclinical work to assess effects in spontaneous withdrawal paradigms and affective measures such as conditioned place aversion, and to dissect the contribution of individual CNS targets. They also note translational considerations: an allometric conversion factor (mouse to human) suggests the active mouse doses correspond to substantially lower human doses, and they cite a separate study reporting single oral noribogaine doses of 3–60 mg were well tolerated in healthy volunteers. Overall, the authors present noribogaine as a metabolite with brain exposure and anti‑withdrawal efficacy in rodents that, in their view, merits further investigation as a potential treatment to aid opioid detoxification.