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.
The N-methyl-D-aspartatereceptor (also known as the NMDA receptor or NMDAR), is a glutamate receptor and ion channel found in neurons. The NMDA receptor is one of three types of ionotropic glutamate receptors, the other two being AMPA and kainate receptors. Depending on its subunit composition, its ligands are glutamate and glycine (or D-serine). However, the binding of the ligands is typically not sufficient to open the channel as it may be blocked by Mg2+ ions which are only removed when the neuron is sufficiently depolarized. Thus, the channel acts as a “coincidence detector” and only once both of these conditions are met, the channel opens and it allows positively charged ions (cations) to flow through the cell membrane. The NMDA receptor is thought to be very important for controlling synaptic plasticity and mediating learning and memory functions.
Memantine is a medication used to slow the progression of moderate-to-severe Alzheimer's disease. It is taken by mouth.
Neuroprotection refers to the relative preservation of neuronal structure and/or function. In the case of an ongoing insult the relative preservation of neuronal integrity implies a reduction in the rate of neuronal loss over time, which can be expressed as a differential equation. It is a widely explored treatment option for many central nervous system (CNS) disorders including neurodegenerative diseases, stroke, traumatic brain injury, spinal cord injury, and acute management of neurotoxin consumption. Neuroprotection aims to prevent or slow disease progression and secondary injuries by halting or at least slowing the loss of neurons. Despite differences in symptoms or injuries associated with CNS disorders, many of the mechanisms behind neurodegeneration are the same. Common mechanisms of neuronal injury include decreased delivery of oxygen and glucose to the brain, energy failure, increased levels in oxidative stress, mitochondrial dysfunction, excitotoxicity, inflammatory changes, iron accumulation, and protein aggregation. Of these mechanisms, neuroprotective treatments often target oxidative stress and excitotoxicity—both of which are highly associated with CNS disorders. Not only can oxidative stress and excitotoxicity trigger neuron cell death but when combined they have synergistic effects that cause even more degradation than on their own. Thus limiting excitotoxicity and oxidative stress is a very important aspect of neuroprotection. Common neuroprotective treatments are glutamate antagonists and antioxidants, which aim to limit excitotoxicity and oxidative stress respectively.
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.
Ditolylguanidine is a sigma receptor agonist. It is somewhat selective for sigma receptors, but non-selective between the two sigma receptor subtypes, binding to both σ1 and σ2 with equal affinity. It has neuroprotective and antidepressant effects, and potentiates the effects of NMDA antagonists.
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.
Lubeluzole (Prosynap) is a drug which acts as an indirect NMDA antagonist. It inhibits the release of glutamate, inhibits nitric oxide synthesis, and blocks calcium and sodium gated ion channels. It has neuroprotective effects particularly in hypoxic conditions, and was developed for the treatment of stroke. Trials showed it to be safe, effective and well tolerated at low doses, but unfortunately higher doses produced the dangerous cardiac side effect of lengthening the QTc interval, which could potentially lead to heart failure, and so this meant that subsequent trials were limited to using only the low dose range. Animal studies had shown lubeluzole to produce neuroprotective effects when administered for prolonged periods, but the aim of its developers was to produce a drug that would be effective for preventing damage from acute stroke, and so ultimately it failed to show sufficient efficacy in trials and development for medical use was halted.
Eliprodil is an NMDA antagonist drug candidate which selectively inhibits the NR2B (GLUN2B) subtype NMDA receptor at submicromolar concentrations. Eliprodil failed a Phase III clinical trial for the treatment of acute ischemic stroke in 1996, sponsored by Synthélabo Recherche.
Gacyclidine is a psychoactive drug which acts as a dissociative via functioning as a non-competitive NMDA receptor antagonist. It is closely related to phencyclidine (PCP), and specifically, is a derivative of tenocyclidine (TCP).
Remacemide is a drug which acts as a low-affinity NMDA antagonist with sodium channel blocking properties. It has been studied for the treatment of acute ischemic stroke, epilepsy, Huntington's disease, and Parkinson's disease.
Indeloxazine (INN) is an antidepressant and cerebral activator that was marketed in Japan and South Korea by Yamanouchi Pharmaceutical Co., Ltd for the treatment of psychiatric symptoms associated with cerebrovascular diseases, namely depression resulting from stroke, emotional disturbance, and avolition. It was marketed from 1988 to 1998, when it was removed from the market reportedly for lack of effectiveness.
Repinotan (BAYx3702), an aminomethylchroman derivative, is a selective 5-HT1A receptor full agonist with high potency and efficacy. It has neuroprotective effects in animal studies, and was trialed in humans for reducing brain injury following head trauma. It was subsequently trialed up to phase II for treatment of stroke, but while side effects were mild and consisted mainly of nausea, repinotan failed to demonstrate sufficient efficacy to justify further clinical trials. However, repinotan continues to be investigated for other applications, and was found to be effective at counteracting the respiratory depression produced by morphine, though with slight reduction in analgesic effects.
Traxoprodil is a drug developed by Pfizer which acts as an NMDA antagonist, selective for the NR2B subunit. It has neuroprotective, analgesic, and anti-Parkinsonian effects in animal studies. Traxoprodil has been researched in humans as a potential treatment to lessen the damage to the brain after stroke, but results from clinical trials showed only modest benefit. The drug was found to cause EKG abnormalities and its clinical development was stopped. More recent animal studies have suggested traxoprodil may exhibit rapid-acting antidepressant effects similar to those of ketamine, although there is some evidence for similar psychoactive side effects and abuse potential at higher doses, which might limit clinical acceptance of traxoprodil for this application.
Gavestinel (GV-150,526) was an investigational drug developed by GlaxoSmithKline for acute intracerebral hemorrhage, which in 2001 failed to show an effect in what was at the time, the largest clinical trial in stroke that had been conducted.
Anoxic depolarization is a progressive and uncontrollable depolarization of neurons during stroke or brain ischemia in which there is an inadequate supply of blood to the brain. Anoxic depolarization is induced by the loss of neuronal selective membrane permeability and the ion gradients across the membrane that are needed to support neuronal activity. Normally, the Na+/K+-ATPase pump maintains the transmembrane gradients of K+ and Na+ ions, but with anoxic brain injury, the supply of energy to drive this pump is lost. The hallmarks of anoxic depolarization are increased concentrations of extracellular K+ ions, intracellular Na+ and Ca2+ ions, and extracellular glutamate and aspartate. Glutamate and aspartate are normally present as the brain's primary excitatory neurotransmitters, but high concentrations activate a number of downstream apoptotic and necrotic pathways. This results in neuronal dysfunction and brain death.
Licostinel (INN) is a competitive, silent antagonist of the glycine site of the NMDA receptor. It was under investigation by Acea Pharmaceuticals as a neuroprotective agent for the treatment of cerebral ischemia associated with stroke and head injuries but was ultimately never marketed. In clinical trials, licostinel did not produce phencyclidine-like psychotomimetic effects at the doses tested, though transient sedation, dizziness, and nausea were observed. In addition to its actions at the NMDA receptor, licostinel also acts as an antagonist of the AMPA and kainate receptors at high concentrations.
Fanapanel, also known as MPQX, is a quinoxalinedione derivative drug which acts as a competitive antagonist of the AMPA receptor. It was under development by Schering AG for the treatment of cerebral ischemia associated with stroke and trauma, but clinical trials were halted for safety reasons related to possible glial cell toxicity and due to intolerable side effects such as excessive sedation, reduction in consciousness, and transient neurological deterioration. The drug was also observed to produce visual alteration and impairment, including blurred vision, strongly impaired color perception, and reduced visual acuity and dark vision, side effects thought to be caused by blockade of AMPA receptors in the retina.
L-4-Chlorokynurenine is an orally active small molecule prodrug of 7-chlorokynurenic acid, a NMDA receptor antagonist. It was investigated as a potential rapid-acting antidepressant.
PD-137889 (N-methylhexahydrofluorenamine) is a chemical compound that is active as an NMDA receptor antagonist in the central nervous system at roughly 30 times the potency of the "flagship" of its class, ketamine, and substitutes for phencyclidine in animal studies. Ki [3H]TCP binding = 27 nM versus ketamine's Ki = 860 nM.
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