Single oral dose MDMA neurotoxicity revisited
July 25, 2012
I have been awaiting this paper since I saw the poster a few meetings back, Dear Reader. It contributes to an ongoing theme of posts on MDMA (Ecstasy) that I have neglected for some time. Some of you may recall the topic of my third blog post which noted the current attempts to get MDMA approved as adjunctive treatment during PTSD psychotherapy. I have been somewhat critical of their approach, mostly because of my understanding of the MDMA-associated neurotoxicity literature in animal species.
To overview, very briefly, if you administer MDMA twice per day at approximately 6-12 hr intervals for four days to rats, monkeys and a few other species, you produce lasting decrements in many markers for serotonin neurons / serotonin function in the brain. Lasting as in, as long as 7 years in a monkey (Hatzidimitriou, 1999). There is a parallel literature identifying lasting affective and cognitive alterations in human consumers of MDMA / Ecstasy and some imaging evidence of similar serotonergic changes.
It is tempting to associate at least the affective disruptions with the lasting serotonergic alterations. (Three monkey studies failed to connect these serotonergic patterns to substantial changes in cognitive behavior so that one is a little more tenuous…you can find some hints in the rat literature and some mother’s eye stuff in the monkey studies but the gun ain’t smoking very much.) However, as you are aware DearReader, the human studies of drug users are fraught with complications. One doesn’t know anything about pre-existing differences (depressives more likely to use MDMA?), the precise dose and pattern folks exposed themselves to, the environmental conditions, co-administered psychoactive substances and even the identity of the drugs being consumed as “Ecstasy”.
This cycles the discussion back to the controlled animal models. The MDMA enthusiast is frequently found to contest the relevance of the animal models, primarily on the grounds of the dose. The typical animal model features 10-20 mg/kg of MDMA per injection in rats and 5-10 mg/kg in monkeys. Again, these are repeated twice daily for four days. In addition, the route of administration is typically intraperitoneal (rat) and either intramuscular or subcutaneous in monkeys. Naturally, the majority of human use is oral which Pharm101 tells us should reduce the peak brain exposure as well as the rapidity with which peak levels are attained compared with the injected routes. So there has been vigorous debate, including between animal-research and human-research scientists, as to whether the animal data should be taken as relevant to the human condition.
As I blogged before, there is another concept from Pharm101 that relates to this discussion, i.e., that of species-scaling of drug doses. The short version is that you need higher per-kilogram-of-bodyweight doses in smaller species to produce similar outcome on parameters such as peak plasma levels, Area Under the Curve as well as toxic outcomes for various body systems including the brain. That prior post lays out data which show that a 1.6 mg/kg oral MDMA dose in a human produces peak plasma levels similar to 2.8 mg/kg in a squirrel monkey but an AUC similar to 5.7 mg/kg in a squirrel monkey (with a higher peak, obviously).
All well and good but the evidence of lasting serotonin changes on the low-end of the dosing spectrum has not been all that good. There was an old Ricaurte paper from the early days that found serotonergic changes in a handful of brain regions after a single oral dose of 5 mg/kg MDMA in squirrel monkeys. The trouble is, it was never replicated by any other papers and it was only in 3 subjects. So…not quite as convincing as the data on the higher-dose, injected, repeated models which come from multiple labs, in several species of laboratory animals and in many (total) animals per species.
A new paper from the Ricaurte group,
Mueller M, Yuan J, McCann UD, Hatzidimitriou G, Ricaurte GA. Single oral doses of (±) 3,4-methylenedioxymethamphetamine (‘Ecstasy’) produce lasting serotonergic deficits in non-human primates: relationship to plasma drug and metabolite concentrations. Int J Neuropsychopharmacol. 2012 Jul 24:1-11. [Epub ahead of print] [PubMed]
provides a long-past-due update on their prior report (Ricaurte et al, 1988; PubMed).
The study tested single oral doses of 5.7 (N=8), 10.0 (N=6) and 14.3 (N=4) mg/kg MDMA and the brains were collected one week later for analysis. As with prior studies, significant reductions in brain content of serotonin and its major metabolite 5-HIAA were observed in multiple brain regions including frontal, temporal, parietal and occipital cortex, the hippocampus, caudate nucleus, putamen and thalamus. Importantly, these reductions were dose-dependent in magnitude with some differences from the vehicle control group (N=8) failing to reach statistical significance. The lowest dose, however, did produce significant reductions of serotonin in frontal and temporal cortex, hippocampus and caudate.
This last is the most critical contribution because it replicates the prior study in a larger sample.
The one oddest thing about the design was the collection of brains at one week instead of two. For the vast majority of studies in this area, two weeks seems to be the modal time for brain harvest. I think the choice of one week here is going to muddy the waters because there will be those that claim this is reflective of acute depletion of serotonin stores rather than the classic neurotoxicity profile. Concerns are partially alleviated by some serotonin transporter binding data provided suggesting reduced expression, but only a single brain slice per treatment group was shown. It would have been nicer if this had been a completed study with quantification from all animals. They authors have left some daylight for their critics and it is not really clear why they would have done this.
In the discussion, the authors continue their thesis that 5.7 mg/kg is equivalent to 1.6 mg/kg for a human. Therefore, they conclude that they have shown that lasting serotonergic deficits can be produced at doses that are unarguable “typical human doses” of MDMA. I have previously argued that this is a dose range that is being used in the clinical protocols even if you leave off notions of species scaling. So overall, yes I would say I agree with their basic contention that they have shown the expected serotonergic effects with MDMA exposure that is 1) oral, 2) single-dose and 3) within the range of expected human single-use episodes.
This study should further convince those who have previously argued that the animal data has no relevance because of dosing issues. This shows that there is no magic threshold of protection that happens to coincide with notions of “typical human use”.