Phencyclidine or phenylcyclohexyl piperidine (PCP), also known in its use as a street drug as angel dust among other names, is a dissociative anesthetic mainly used recreationally for its significant mind-altering effects. PCP may cause hallucinations, distorted perceptions of sounds, and violent behavior. As a recreational drug, it is typically smoked, but may be taken by mouth, snorted, or injected. It may also be mixed with cannabis or tobacco.
Dissociatives, colloquially dissos, are a subclass of hallucinogens which distort perception of sight and sound and produce feelings of detachment – dissociation – from the environment and/or self. Although many kinds of drugs are capable of such action, dissociatives are unique in that they do so in such a way that they produce hallucinogenic effects, which may include dissociation, a general decrease in sensory experience, hallucinations, dream-like states or anesthesia. Despite most dissociatives' main mechanism of action being tied to NMDA receptor antagonism, some of these substances, which are nonselective in action and affect the dopamine and/or opioid systems, may be capable of inducing more direct and repeatable euphoria or symptoms which are more akin to the effects of typical "hard drugs" or common drugs of abuse. This is likely why dissociatives are considered to be addictive with a fair to moderate potential for abuse, unlike psychedelics. Despite some dissociatives, such as phencyclidine (PCP) possessing stimulating properties, most dissociatives seem to have a general depressant effect and can produce sedation, respiratory depression, nausea, disorientation, analgesia, anesthesia, ataxia, cognitive and memory impairment as well as amnesia.
Dizocilpine (INN), also known as MK-801, is a pore blocker of the NMDA receptor, a glutamate receptor, discovered by a team at Merck in 1982. Glutamate is the brain's primary excitatory neurotransmitter. The channel is normally blocked with a magnesium ion and requires depolarization of the neuron to remove the magnesium and allow the glutamate to open the channel, causing an influx of calcium, which then leads to subsequent depolarization. Dizocilpine binds inside the ion channel of the receptor at several of PCP's binding sites thus preventing the flow of ions, including calcium (Ca2+), through the channel. Dizocilpine blocks NMDA receptors in a use- and voltage-dependent manner, since the channel must open for the drug to bind inside it. The drug acts as a potent anti-convulsant and probably has dissociative anesthetic properties, but it is not used clinically for this purpose because of the discovery of brain lesions, called Olney's lesions (see below), in laboratory rats. Dizocilpine is also associated with a number of negative side effects, including cognitive disruption and psychotic-spectrum reactions. It inhibits the induction of long term potentiation and has been found to impair the acquisition of difficult, but not easy, learning tasks in rats and primates. Because of these effects of dizocilpine, the NMDA receptor pore blocker ketamine is used instead as a dissociative anesthetic in human medical procedures. While ketamine may also trigger temporary psychosis in certain individuals, its short half-life and lower potency make it a much safer clinical option. However, dizocilpine is the most frequently used uncompetitive NMDA receptor antagonist in animal models to mimic psychosis for experimental purposes.
Dextromethorphan (DXM) is a cough suppressant used in many cough and cold medicines. It affects serotonin, norepinephrine, NMDA, and sigma-1 receptors in the brain, all of which have been implicated in the pathophysiology of depression. In 2022, the FDA approved a formulation of it combined with bupropion named Auvelity to serve as a rapid acting antidepressant in patients with major depressive disorder.
NMDA receptor antagonists are a class of drugs that work to antagonize, or inhibit the action of, the N-Methyl-D-aspartate receptor (NMDAR). They are commonly used as anesthetics for human and non-human animals; the state of anesthesia they induce is referred to as dissociative anesthesia.
The μ-opioid receptors (MOR) are a class of opioid receptors with a high affinity for enkephalins and beta-endorphin, but a low affinity for dynorphins. They are also referred to as μ(mu)-opioid peptide (MOP) receptors. The prototypical μ-opioid receptor agonist is morphine, the primary psychoactive alkaloid in opium and for which the receptor was named, with mu being the first letter of Morpheus, the compound's namesake in the original Greek. It is an inhibitory G-protein coupled receptor that activates the Gi alpha subunit, inhibiting adenylate cyclase activity, lowering cAMP levels.
Etoxadrol (CL-1848C) is a dissociative anaesthetic drug that has been found to be an NMDA antagonist and produce similar effects to PCP in animals. Etoxadrol, along with another related drug dexoxadrol, were developed as analgesics for use in humans, but development was discontinued in the late 1970s after patients reported side effects such as nightmares and hallucinations.
Reuptake inhibitors (RIs) are a type of reuptake modulators. It is a drug that inhibits the plasmalemmal transporter-mediated reuptake of a neurotransmitter from the synapse into the pre-synaptic neuron. This leads to an increase in extracellular concentrations of the neurotransmitter and an increase in neurotransmission. Various drugs exert their psychological and physiological effects through reuptake inhibition, including many antidepressants and psychostimulants.
Selfotel (CGS-19755) is a drug which acts as a competitive NMDA antagonist, directly competing with glutamate for binding to the receptor. Initial studies showed it to have anticonvulsant, anxiolytic, analgesic and neuroprotective effects, and it was originally researched for the treatment of stroke, but subsequent animal and human studies showed phencyclidine-like effects, as well as limited efficacy and evidence for possible neurotoxicity under some conditions, and so clinical development was ultimately discontinued.
LY-379,268 is a drug that is used in neuroscience research, which acts as a potent and selective agonist for the group II metabotropic glutamate receptors (mGluR2/3).
Arylcyclohexylamines, also known as arylcyclohexamines or arylcyclohexanamines, are a chemical class of pharmaceutical, designer, and experimental drugs.
3-Methoxyphencyclidine (3-MeO-PCP) is a dissociative hallucinogen of the arylcyclohexylamine class related to phencyclidine (PCP) which has been sold online as a designer drug. It acts mainly as an NMDA receptor antagonist, though it has also been found to interact with the sigma σ1 receptor and the serotonin transporter. The drug does not possess any opioid activity nor does it act as a dopamine reuptake inhibitor.
4-Methoxyphencyclidine is a dissociative anesthetic drug that has been sold online as a research chemical. The synthesis of 4-MeO-PCP was first reported in 1965 by the Parke-Davis medicinal chemist Victor Maddox. A 1999 review published by a chemist using the pseudonym John Q. Beagle suggested the potency of 4-MeO-PCP in man was reduced relative to PCP, two years later Beagle published a detailed description of the synthesis and qualitative effects of 4-MeO-PCP, which he said possessed 70% the potency of PCP. 4-MeO-PCP was the first arylcyclohexylamine research chemical to be sold online, it was introduced in late 2008 by a company trading under the name CBAY and was followed by several related compounds such as 3-MeO-PCP and methoxetamine. 4-MeO-PCP has lower affinity for the NMDA receptor than PCP, but higher affinity than ketamine, it is orally active in a dosage range similar to ketamine, with some users requiring doses in excess of 100 mg for desired effects. Users have reported substantial differences in active dose, these discrepancies can be partially explained by the presence of unreacted PCC and other impurities in samples sold on the grey market. 4-MeO-PCP has Ki values of 404 nM for the NMDA receptor, 713 nM for the norepinephrine transporter, 844 nM for the serotonin transporter, 296 nM for the σ1 receptor and 143 nM for the σ2 receptor.
Methoxetamine, abbreviated as MXE, is a dissociative hallucinogen that has been sold as a designer drug. It differs from many dissociatives such as ketamine and phencyclidine (PCP) that were developed as pharmaceutical drugs for use as general anesthetics in that it was designed specifically to increase the antidepressant effects of ketamine.
Diphenidine is a dissociative anesthetic that has been sold as a designer drug. The synthesis of diphenidine was first reported in 1924, and employed a Bruylants reaction analogous to the one that would later be used to discover phencyclidine in 1956. Shortly after the 2013 UK ban on arylcyclohexylamines, diphenidine and the related compound methoxphenidine became available on the grey market. Anecdotal reports describe high doses of diphenidine producing "bizarre somatosensory phenomena and transient anterograde amnesia." Diphenidine and related diarylethylamines have been studied in vitro as treatments for neurotoxic injury and are antagonists of the NMDA receptor. In dogs diphenidine exhibits greater antitussive potency than codeine phosphate.
Methoxphenidine is a dissociative of the diarylethylamine class that has been sold online as a designer drug. Methoxphenidine was first reported in a 1989 patent where it was tested as a treatment for neurotoxic injury. Shortly after the 2013 UK ban on arylcyclohexylamines methoxphenidine and the related compound diphenidine became available on the gray market, where it has been encountered as a powder and in tablet form. Though diphenidine possesses higher affinity for the NMDA receptor, anecdotal reports suggest methoxphenidine has greater oral potency. Of the three isomeric anisyl-substituents methoxphenidine has affinity for the NMDA receptor that is higher than 4-MeO-Diphenidine but lower than 3-MeO-Diphenidine, a structure–activity relationship shared by the arylcyclohexylamines.
3-Methoxyeticyclidine (3-MeO-PCE), also known as methoxieticyclidine, is a dissociative anesthetic that is qualitatively similar to PCE and PCP and has been sold online as a designer drug.
3-Hydroxyphencyclidine (3-HO-PCP) is a dissociative of the arylcyclohexylamine class related to phencyclidine (PCP) that has been sold online as a designer drug.
3-Methyl-PCPy (3-Me-PCPy) is an arylcyclohexylamine derivative with an unusual spectrum of pharmacological effects, acting as both a potent NMDA antagonist and also a triple reuptake inhibitor which inhibits reuptake of all three monoamine neurotransmitters serotonin, dopamine and noradrenaline. It also acts as a high affinity sigma receptor ligand, selective for the σ2 subtype. It produces both stimulant and dissociative effects in animal behavioural studies.
