Introduction
Amphetamines have been popular drugs of abuse for more than 50 years. They retain some slender clinical uses, such as in narcolepsy and some weight control effects. MDMA may well be their most popular manifestation in current usage (see below). Amphetamines are known by the usual litany of tedious street-names and the most popular variant of the more serious forms at the time of writing is methamphetamine, known mainly as ‘ice’ or ‘crystal meth’. Amphetamines and their derivatives can be dosed intravenously, orally or smoked, depending on the physical form of the drug and the speed of effects onset desired. They act to cause CNS and peripheral biogenic amine effects to be strongly potentiated by preventing their re-uptake and destruction. These sustained elevated amine levels, (particularly dopamine) lead to the characteristic stimulatory effects, which include feelings of well-being, euphoria, and boundless energy; they may also cause hallucinations. There is also evidence that methamphetamine addicts show impairment in seratonergic systems as well as dopaminergic areas. The general effects of amphetamines are similar to that of cocaine, but with an important difference. Cocaine is a very transient ‘high’, perhaps only a few minutes, whilst amphetamines can maintain their potent effects over more than half a day. The dysphoric effects of the drugs once they have been cleared are notoriously bad, due to severe synaptic biogenic amine depletion. Of course, the tight neuronal regulation of biogenic amine adapts rapidly through mechanisms such as receptor down-regulation. These erode the pharmacological effects over time, leading to tolerance and dependence with repeated usage. Addicts escalate their doses and try to beat the tolerance and the dysphoria by taking the drug for days alongside depressants such as ethanol or heroin. – Tweaking’. as it is known, leads to continuously wakeful states that may exceed two weeks and can make these individuals exceedingly dangerous to themselves and presumably they should definitely avoid operating machinery. Amphetamines can cause a long list of toxic effects, from hypertensive crises to strokes and seizures. They can even induce paranoid schizophrenia in some individuals, as well as repetitive stereotypic effects. The impact on the physical appearance of serious addicts over time is genuinely shocking.
The variety in amphetamine clearance routes reflects their closeness in structure to endogenous biogenic amines. Their main route of metabolism seems to be ring – hydroxylation by CYPs 3A4, 2D6 and 2B6. Potent inhibitors of 3A4 such as some of the HIV protease inhibitors are thought to be capable of causing fatal amphetamine accumulation from normally safe dosages as a consequence of inhibition. Amphetamines can also be N-oxidised by flavin monooxygenases (FMO-3) and deaminated by various enzyme systems such as the MAOs. MDMA is a good example of how the metabolic fate of an amphetamine has been gradually unravelled and how this might relate to the longterm consequences of its usage.
Figure B.7 Methylenedioxy amphetamine derivatives: MDMA (methylenedioxymethyl amphetamine: ecstasy), methylenedioxy amphetamine (MDA) and methylenedioxyethyl amphetamine (MDEA: Eve)
Ecstasy (MDMA) mode of action and acute toxicity
MDMA is well established as the illicit drug of choice for around 7 per cent of the male and 4 per cent of the female population in the 16-24 age group, although its use is widening in older people. The drug (Figure B.7) is the best-known representative of a group of N- substituted methylenedioxyamphetamine derivatives, which also include the N-ethyl derivative (‘Eve’) and MDA, the primary unsubstituted amine derivative. These agents are stimulants, although they are also reputed to induce feelings of empathy and warmth towards oneself and others; the word that has been coined to describe them is ‘entactogen’. MDMA analogue toxicity can be resolved into acute and chronic toxicity – acute toxicity is well understood and described. Overdose of these agents can lead to hyperthermia, high blood pressure, rhabdomyolysis and kidney failure. Deaths attributed to these drugs are sometimes exacerbated by repetitive violent physical activity (‘dancing’) and its attendant dehydration. Overall, deaths due to MDMA are rare, although the chronic toxicity is still far from completely understood.
Unfortunately, there is a strong perception among users that MDMA derivatives are generally much safer than other drugs. However, those who take MDMA, alongside most illicit drug users, are curiously trusting, believing that someone they just met in a nightclub will sell them high quality MDMA free from unpleasant impurities. The designer MDMA analogue 4-methylthioamphetamine (4-MTA) is often sold as ‘ecstasy’ and this agent is much more toxic than MDMA. 4-MTA is known on the street as ‘flatlin-ers’ and it is indeed far more likely to end it all for you than MDMA, according to the statistics. There are experimental data that suggest that CYP2D6 EMs could be at greatly increased risk of toxicity from 4-MTA. In response to these problems of purity and adulteration, in some European countries MDMA testing facilities are available where the drug is sold to guarantee a reasonable standard of purity.
Chronic toxicity and metabolism
There is some evidence that low and infrequent use of MDMA may not lead to permanent neural impairment, as some authors report it is not associated with any changes in cognitive brain function (memory, attention, associative memory). Other studies have shown worrying low dose-mediated changes in brain microvasculature and some neural damage. As with other amphetamines, a much more serious effect seems to occur at lower doses in younger users below the age of 18. However, it is difficult to evaluate the true risk of neurotoxicity caused by these drugs due to problems estimating accurately how much is taken and how often. It is generally agreed that around 10 per cent of MDMA users could experience serious long-term CNS problems as a result of their high regular intake of the drug.
The metabolism of MDMA has been hotly pursued (Figure B.8) as it was discovered in the mid 1990s that these agents were specifically demethylenated by CYP2D6, forming a catechol product. The ethyl side-chain in ‘Eve’ and the methyl group in MDMA can also be dealkylated by CYP2B6, although this route is a minor one in humans, as MDA levels are around only 5 per cent of parent drug in plasma. In man, the major route is CYP2D6-mediated demethylenation, and initially it was thought that catechol formation might occur within the brain and lead to reactive species formation. However, the parent drug itself is not directly neurotoxic and neither are any of its catechol metabolites (HHMA/HHA) when injected into rat brains. It appears that the drug must be given sys-temically and the metabolites make their way across the blood-brain barrier and undergo further metabolism to damage 5-HT neurones. It is believed that in rats the route of toxicity is due firstly to the formation of catechol metabolites (HHMA), followed by the formation of quinones, which then react with glutathione- derived thioethers, which can be transported into the brain to form reactive species. Some in vitro studies have also suggested that MDMA may be oxidized to nephrotoxic thioether metabolites.
In humans, the situation is more complex. There is now evidence in MDMA users that HHMA and HHA are further metabolized and they are found in urine as thioether products, which is indicative of a reactive intermediate being formed, as occurs in rats. Interestingly, the thioethers were found at only a very low level in urine at 4 hours post dosage (0.002 per cent of the dose). Whilst this adds weight to the contention that the neurotoxicity of the drug could be linked with CYP2D6-mediated reactive species formation, the low level excretion of the thioether could also be analogous to the situation with paracetamol, where mercapturates can be found in urine at therapeutic doses and the reactive species, though formed, are fully detoxified. It is still not exactly certain if catechol-related MDMA reactive species are indeed formed in the human brain. What makes the situation in man even more labyrinthine is that CYP2D6 undergoes a rapid mechanism-based irreversible inhibition with MDMA, which occurs after one or two consecutive doses. Clinical studies with the CYP2D6 substrate dextromethorphan have shown that MDMA turns nearly 70 per cent of users into CYP2D6 PMs. Its inhibitory effects can cause a 10-fold increase in dextromethorphan plasma levels and the effect lasts for around 10 days. This suggests that if CYP2D6-mediated demethylenation was the main route of toxicity through catechol formation, then bizarrely, repeated dosage of the drug could actually reduce the potential neurotoxicity. This does not appear to be the case and heavy use of MDMA is associated with CNS deficits, so CYP2D6 is not the only route of bioactivation. Indeed, although the CYP2D6 blockade causes MDMA accumulation on repeated dosage, several other CYPs such as 2B6, 3A4 and even 1A2 that can metabolize the drug, as well as MAO and conjugation systems that also clear it. It is difficult to estimate just how MDMA is toxic is likely to be in man, given the polymorphisms of CYP2D6 and other CYPs, as well as the differing activities of the other enzyme systems, such as COMT. There is also the variability of the dosage in each ecstasy pill to consider as well as whatever else the users are taking at the same time. What is certain, is that the inhibitory effects of MDMA on CYP2D6 are so potent that it is likely that the clearances of any prescription substrates of this isoform will be significantly extended: this might occur with antipsychotics, antidepressants (SSRIs and TCAs) and opiates.
Figure B.8 Metabolism of MDMA: the main route of clearance is initially via CYP2D6 to HHMA (3,4,dihydroxymethamphetamine), which is cleared either by conjugation or may be methylated by catechol-O-methyl transferase (COMT) to HMMA (4-hydroxy-3-methoxymethyl amphetamine). The demethylated MDA may then be demethylenated to HHA (3,4 dihydroxyamphetamine) which can also be methylated and undergo conjugation. MAO may also oxidize MDA, or MDMA
Animal studies have not been that helpful in the study of the relationship between MDMA, its metabolism and toxicity. Although 5-HT-system neurotoxicity caused by MDMA can be shown in animal models, in mice, this is thought to be due to dopaminemediated events, as the drug is not metabolized. In rats and monkeys, the profile of MDMA metabolism is markedly different again from man (mostly demethylation), so animal models appear to have limited value in the main objective of this type of research, which is to predict what exposure of MDMA will cause long-term neurological impairment. If anything, these models have been ‘too positive’ in underlining the apparent risks of these drugs, which patently flies in the face of the experience of the users. This leads to mistrust of official advice, no matter how well it is intentioned.
The multi- enzymatic complexity of MDMA metabolism and where it takes place in relation to potential sites of neural vulnerability means that accurate predictions of human toxicity may be some way away. However, it may be that factors such as the frequency of use, CYP2D6 status and age of the user might be greater determinants of possible longterm problems than dosage and co-administered drugs. It is likely that future epidemiological studies may determine the ultimate risks of MDMA and its relatives.
Piperazine derivatives (BZP, TFMPP)
Although amphetamines remain popular drugs of abuse, their illegality led to the promotion of alternatives with similar effects that could be sold commercially as they were not prohibited in many countries, although since they were banned in the US in 2004 this situation is changing. At the time of writing they remain legal in New Zealand, where they are made in quantity, although they were made illegal in the UK in 2009. There are several of these drugs (Figure B.9), although two are encountered most often, N-benzylpiperazine (BZP) and trifluormethylphenylpiperazine (TFMPP).
Figure B.9 Structures of dopamine, d-amphetamine, N-benzylpiperazine (BZP) and trifluorometh-ylphenylpiperazine (TFMPP)
These agents are either used singly or in combination, when they are intended to recreate the effects of ecstasy. Other popular derivatives include methylenedioxybenzyl-piperazine (MDBP) methoxyphenylpiperazine (MeOPP) as well as methylbenzylpiperazine (MPZP). Piperazines are often sold in pill form under a variety of jolly names, most usually, ‘party pills’. Curiously, BZP was originally synthesized commercially during the Second World War as a possible drug to be used against tapeworms, although this property is not an added bonus to regular users who enjoy raw meat, as it was not very effective. BZP and its relatives do have many amphet-amine-üke actions, including the usual stimulant effects (increased blood pressure and heart rate). BZP itself is a sympathomimetic like amphetamine, as it releases dopamine in the CNS through its effects on the dopamine transporter. The drug has shown some addictive potential in non- human primate studies, whilst TFMPP, which is a seratinonergic agonist, did not. The combination of BZP and TFMPP does appear to promote an MDMA-like ‘entactogen’ effect, where a feeling of general well-being is accompanied by a bonus of hallucinations at no extra charge. Whilst the combination causes seizures in rats, there are enough 17-year-old volunteers to determine in due course whether this occurs in man and at what dosage range. However, if you are 17 and overdose just on BZP, it is already established that there is an approximately 20 per cent chance of a seizure and a likely spell in intensive care. A small number of deaths have been recorded as linked with BZP, although as is the wont of habitual drug users, several other agents were also taken. Animal studies seem to reinforce the message that BZP and its related agents taken during adolescence may cause behavioural problems in maturity, although this remains to be discovered and experienced by lots of lucky ‘party animals’.
Piperazine metabolism
The possibility that BZP and its congeners may be problematic when taken with prescription drugs is borne out by preliminary studies on their human biotransformation and metabolism. -n vivo – relatively little is known of BZP’ s clearance, although one study suggested that its bioavailability is fairly low. The half-Hfe was estimated at 5.5 hours and it is cleared much more slowly than methamphetamine. Hydroxylation of the aromatic ring was noted at the 3 and 4 positions, which was followed by sulphation rather than glucuronidation. There is also sulphation at one of the nitrogens in the piperazine group. Using human liver microsomes, it was found that BZP and TFMPP were metabolized mainly by CYP2D6, CYP1A2 and CYP3A4, but not by CYP2C19 and CYP2C9, although BZP does not appear to block CYP2C19, it does inhibit the rest of the major CYPs. Indeed, these drugs as a class appear to be major CYP inhibitors. Interestingly, BZP and TFMPP also inhibit each others’ metabolism, which may partly explain their mutually enhanced pharmacodynamic effects on the CNS. Although more studies on the biotransformation of these drugs needs to be carried out, it seems certain that the piperazines will cause drug-drug interactions with CYP2D6 substrates such as antipsychotics and SSRIs as well as CYP3A4 substrates. The toxic interaction between MDMA and BZP is probably at least partly caused by irreversible CYP2D6 inhibition leading to BZP accumulation. It seems that the general propensity of drug users to take combinations of various agents may be particularly risky when this involves the benzylpiperazines.
