| Systematic (IUPAC) name | |
|---|---|
| (RS)-2-(2-Chlorophenyl)-2-(methylamino)cyclohexanone | |
| Clinical data | |
| AHFS/Drugs.com | Consumer Drug Information |
| Pregnancy cat. | B |
| Legal status | Controlled (S8) (AU) Schedule I (CA) ? (UK) Schedule III (US) |
| Routes | IV, IM, Insufflated, oral, topical |
| Pharmacokinetic data | |
| Metabolism | Hepatic, primarily by CYP3A4[1] |
| Half-life | 2.5–3 hours. |
| Excretion | renal (>90%) |
| Identifiers | |
| CAS number | 6740-88-1 |
| ATC code | N01AX03 N01AX14 |
| PubChem | CID 3821 |
| DrugBank | DB01221 |
| ChemSpider | 3689 |
| UNII | 690G0D6V8H |
| KEGG | D08098 |
| ChEBI | CHEBI:6121 |
| ChEMBL | CHEMBL742 |
| Chemical data | |
| Formula | C13H16ClNO |
| Mol. mass | 237.725 g/mol |
| SMILES | eMolecules & PubChem |
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| Physical data | |
| Melt. point | 262 °C (504 °F) |
| (what is this?) (verify) |
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Ketamine is a drug used in human and veterinary medicine. Its hydrochloride salt is sold as Ketanest, Ketaset, and Ketalar. Pharmacologically, ketamine is classified as an NMDA receptor antagonist.[2] At high, fully anesthetic level doses, ketamine has also been found to bind to opioid μ receptors type 2 in cultured human neuroblastoma cells – however, without agonist activity[3] – and to sigma receptors in rats.[4] Also, ketamine interacts with muscarinic receptors, descending monoaminergic pain pathways and voltage-gated calcium channels.[5] Like other drugs of this class such as tiletamine and phencyclidine (PCP), it induces a state referred to as "dissociative anesthesia"[6] and is used as a recreational drug.
Ketamine has a wide range of effects in humans, including analgesia, anesthesia, hallucinations, elevated blood pressure, and bronchodilation.[7] Ketamine is primarily used for the induction and maintenance of general anesthesia, usually in combination with a sedative. Other uses include sedation in intensive care, analgesia (particularly in emergency medicine), and treatment of bronchospasm. It has been shown to be effective in treating depression in patients with bipolar disorder who have not responded to other anti-depressants.[8] In persons with major depressive disorder it produces a rapid antidepressant effect, acting within two hours as opposed to the several weeks taken by typical antidepressants to work.[9] It is also a popular anesthetic in veterinary medicine.
Ketamine is a chiral compound. Most pharmaceutical preparations of ketamine are racemic; however, some brands reportedly have (mostly undocumented) differences in enantiomeric proportions. The more active enantiomer, (S)-ketamine, is also available for medical use under the brand name Ketanest S.[10] (R)-ketamine, (S)-ketamine & racemic (R,S)-ketamine all have qualitatively separate distinct effect profiles, although S has the most active potency. Ketamine is a core medicine in the World Health Organization's "Essential Drugs List", a list of minimum medical needs for a basic health care system.[11]
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Indications for use as an anaesthetic:
In medical settings, ketamine is usually injected intravenously or intramuscularly.[13] Since it suppresses breathing much less than most other available anaesthetics,[14] ketamine is still used in human medicine as an anesthetic, however, due to the hallucinations which may be caused by ketamine, it is not typically used as a primary anesthetic, although it is the anaesthetic of choice when reliable ventilation equipment is not available. Ketamine tends to increase heart rate and blood pressure. Because ketamine tends to increase or maintain cardiac output, it is sometimes used in anesthesia for emergency surgery when the patient's fluid volume status is unknown (e.g., from traffic accidents). Ketamine can be used in podiatry and other minor surgery, and occasionally for the treatment of migraine. There is ongoing research in France, the Netherlands, Russia, Australia and the US into the drug's usefulness in pain therapy, depression suppression, and for the treatment of alcoholism[15] and heroin addiction.[16]
In veterinary anesthesia, ketamine is often used for its anesthetic and analgesic effects on cats, dogs, rabbits, rats, and other small animals. Veterinarians often use ketamine with sedative drugs to produce balanced anesthesia and analgesia, and as a constant rate infusion to help prevent pain wind-up. Ketamine is used to manage pain among large animals, though it has less effect on bovines. It is the primary intravenous anesthetic agent used in equine surgery, often in conjunction with detomidine and thiopental, or sometimes guaifenesin.
Ketamine may be used in small doses (0.1–0.5 mg/kg·h) as a local anesthetic, particularly for the treatment of pain associated with movement and neuropathic pain.[17] It may also be used as an intravenous co-analgesic together with opiates to manage otherwise intractable pain, particularly if this pain is neuropathic (pain due to vascular insufficiency or shingles are good examples). It has the added benefit of counter-acting spinal sensitization or wind-up phenomena experienced with chronic pain. At these doses, the psychotropic side effects are less apparent and well managed with benzodiazepines.[18] Ketamine is a co-analgesic, and so is most effective when used alongside a low-dose opioid; while it does have analgesic effects by itself, the higher doses required can cause disorienting side effects.[18] The combination of ketamine with an opioid is, however, particularly useful for pain caused by cancer.[19]
The effect of ketamine on the respiratory and circulatory systems is different from that of other anesthetics. When used at anesthetic doses, it will usually stimulate rather than depress the circulatory system.[20] It is sometimes possible to perform ketamine anesthesia without protective measures to the airways. Ketamine is also a potent analgesic and can be used in sub-anesthetic doses to relieve acute pain; however, its psychotropic properties must be taken into account. Patients have reported vivid hallucinations, "going into other worlds" or "seeing God" while anesthetized, and these unwanted psychological side-effects have reduced the use of ketamine in human medicine. They can, however, usually be avoided by concomitant application of a sedative such as a benzodiazepine.[18]
Low-dose ketamine is recognized for its potential effectiveness in the treatment of complex regional pain syndrome (CRPS), according to a retrospective review published in the October 2004 issue of Pain Medicine.[21] Although low-dose ketamine therapy is established as a generally safe procedure, reported side effects in some patients have included hallucinations, dizziness, lightheadedness and nausea. Therefore nurses administering ketamine to patients with CRPS should only do so in a setting where a trained physician is available if needed to assess potential adverse effects on patients.[22]
In some neurological ICUs, ketamine has been used in cases of prolonged status epilepticus. There has been some evidence that the NMDA-blocking effect of the drug protects neurons from glutamatergic damage during prolonged seizures.[23]
When treating patients suffering from complex regional pain syndrome (CRPS) with a low-dose (subanesthetic) ketamine infusion, it was observed that some patients made a significant recovery from associated depression. This recovery was not formally documented, as the primary concern was pain management. It was not possible to quantify to what degree depression recovery was secondary to the patient's recovery from CRPS.
One trial administered a short-term ketamine regimen to patients with severe depression, with the dose carefully monitored to prevent hallucinogenic side effects. The patients' normal medications were continued as it was feared that stopping them might result in severe depressive episodes. Before and following each treatment with ketamine, at patient clinic visits, the Beck Depression Inventory (BDI) and the Hamilton Rating Scale for Depression (HAMD-17) were obtained. Two of the patients demonstrated significant, long-term improvement.[24] Another small study found that ketamine significantly improved treatment-resistant major depression within hours of injection.[25] The improvement lasted up to one week after the single dose.[26] These patients were previously treatment resistant, having tried an average of six other treatments that failed. NIMH director Dr. Thomas Insel remarked:
"To my knowledge, this is the first report of any medication or other treatment that results in such a pronounced, rapid, prolonged response with a single dose. These were very treatment-resistant patients."
The researchers apparently attribute the effect to ketamine being an NMDA receptor antagonist.[27] Those findings of Zarate et al. corroborate earlier findings by Berman et al..[28] However Zarate et al. do raise some concerns about their results due to a possible lack of blinding, because of the inebriating effects of low dose ketamine infusion, and it is recommended that future studies include an active placebo.
These findings are corroborated by Liebrenz et al., who successfully, according to an attending doctor, treated a patient with a treatment-resistant major depression and a co-occurring alcohol and benzodiazepine dependence by giving an intravenous infusion of 0.5 mg/kg ketamine over a period of 50 minutes and Goforth et al. who helped a patient with severe, recurrent major depressive disorder that demonstrated marked improvement within 8 hours of receiving a preoperative dose of ketamine and one treatment of electroconvulsive therapy with bitemporal electrode placement.[29][30]
However, a new study in mice by Zarate et al. shows that blocking the NMDA receptor is an intermediate step. According to this study, blocking NMDA increases the activity of another receptor, AMPA, and this boost in AMPA activity is crucial for ketamine’s rapid antidepressant actions. NMDA and AMPA are receptors for the neurotransmitter glutamate. The glutamate system has been implicated in depression recently. This is a departure from previous thinking, which had focused on serotonin and norepinephrine. The glutamate system may represent a new avenue for treatment and research.[31]
Krystal et al. retrospectively compared the seizure duration, ictal EEG, and cognitive side effects of ketamine and methohexital anesthesia with ECT in 36 patients.[32] Ketamine was well tolerated and prolonged seizure duration overall, but particularly in those who had a seizure duration shorter than 25 seconds with methohexital at the maximum available stimulus intensity. Ketamine also increased midictal EEG slow-wave amplitude. Thus, a switch to ketamine may be useful when it is difficult to elicit a robust seizure. Faster post-treatment reorientation with ketamine may suggest a lower level of associated cognitive side effects.
Kudoh et al. investigated whether ketamine is suitable for depressed patients who had undergone orthopedic surgery.[33] Depressed mood, suicidal tendencies, somatic anxiety, and hypochondriasis significantly decreased in the active group as compared with the control. The group receiving ketamine also had significantly lower postoperative pain.
Acute administration of ketamine at the higher dose, but not imipramine, increased BDNF protein levels in the rat hippocampus. The increase of hippocampal BDNF protein levels induced by ketamine might be necessary to produce a rapid onset of antidepressant action in rats.[34]
The Russian doctor Evgeny Krupitsky (Clinical Director of Research for the Saint Petersburg Regional Center for Research in Addiction and Psychopharmacology) has claimed to have encouraging results by using ketamine as part of a treatment for alcohol addiction which combines psychedelic and aversive techniques.[35][36] This method involved psychotherapy, controlled ketamine use and group therapy, and resulted in 60 of the 86 alcoholic males selected for the study remaining fully abstinent through one year of treatment. For heroin addiction, the same researcher reached the conclusion that one ketamine-assisted psychotherapy session was significantly more effective than active placebo in promoting abstinence from heroin during one year without any adverse reactions. In a recently published study 59 detoxified inpatients with heroin dependence received a ketamine-assisted psychotherapy (KPT) session prior to their discharge from an addiction treatment hospital, and were then randomized into two treatment groups.
Participants in the first group received two addiction counseling sessions followed by two KPT sessions, (with a single im injection of 2 mg/kg ketamine) with sessions scheduled on a monthly interval (multiple KPT group). Participants in the second group received two addiction counseling sessions on a monthly interval, but no additional ketamine therapy sessions (single KPT group). At one-year follow-up, survival analysis demonstrated a significantly higher rate of abstinence in the multiple KPT group. Thirteen out of 26 subjects (50%) in the multiple KPT group remained abstinent, compared to 6 out of 27 subjects (22.2%) in the single KPT group (p < 0.05). No differences between groups were found in depression, anxiety, craving for heroin, or their understanding of the meaning of their lives. It was concluded that three sessions of ketamine-assisted psychotherapy are more effective than a single session for the treatment of heroin addiction.[37][38]
Krupitsky and Kolp summarized their work to date in 2007.[39]
Jovaisa et al. from Lithuania demonstrated attenuation of opiate withdrawal symptoms with ketamine. A total of 58 opiate-dependent patients were enrolled in a randomized, placebo-controlled, double-blind study. Patients underwent rapid opiate antagonist induction under general anesthesia. Prior to opiate antagonist induction patients were given either placebo (normal saline) or subanesthetic ketamine infusion of 0.5 mg/kg·h. Ketamine group presented better control of withdrawal symptoms, which lasted beyond ketamine infusion itself. Significant differences between ketamine and Control groups were noted in anesthetic and early postanesthetic phases. There were no differences in effects on outcome after 4 months.[16]
Ketamine is being used as an experimental and controversial treatment for Complex Regional Pain Syndrome (CRPS) also known as Reflex Sympathetic Dystrophy (RSD). CRPS/RSD is a severe chronic pain condition characterized by sensory, autonomic, motor and dystrophic signs and symptoms. The pain in CRPS is continuous, it worsens over time, and it is usually disproportionate to the severity and duration of the inciting event. The hypothesis is that ketamine manipulates NMDA receptors which might reboot aberrant brain activity. There are two treatment modalities, the first consist of a low dose ketamine infusion of between 25–90 mg per day, over five days either in hospital or as an outpatient. This is called the awake technique. Open label, prospective, pain journal evaluation of a 10-day infusion of intravenous ketamine (awake technique) in the CRPS patient concluded that "A four-hour ketamine infusion escalated from 40–80 mg over a 10-day period can result in a significant reduction of pain with increased mobility and a tendency to decreased autonomic dysregulation".[40]
Case notes of 33 patients whose CRPS pain was treated by the inpatient administration of a continuous subanesthetic intravenous infusion of ketamine were reviewed at Mackay Base Hospital, Queensland, Australia. A total of 33 patients with diagnoses of CRPS who had undergone ketamine treatment at least once were identified. Due to relapse, 12 of 33 patients received a second course of therapy, and two of 33 patients received a third. There was complete pain relief in 25 (76%), partial relief in six (18%), and no relief in two (6%) patients.
The degree of relief obtained following repeat therapy (N=12) appeared even better, as all 12 patients who received second courses of treatment experienced complete relief of their CRPS pain. The duration of relief was also impressive, as was the difference between the duration of relief obtained after the first and after the second courses of therapy. In this respect, following the first course of therapy, 54% of 33 individuals remained pain free for 3 months or more and 31% remained pain free for 6 months or more. After the second infusion, 58% of 12 patients experienced relief for a year or more, while almost 33% remained pain free for over 3 years. The most frequent side effect observed in patients receiving this treatment was a feeling of inebriation. Hallucinations occurred in six patients. Less frequent side effects also included complaints of light-headedness, dizziness, and nausea. In four patients, an alteration in hepatic enzyme profile was noted; the infusion was terminated and the abnormality resolved thereafter. No long-term side-effects were noted.[40] This procedure has only recently been allowed in the United States for the treatment of CRPS.
The second treatment modality consists of putting the patient into a medically-induced coma and given an extremely high dosage of ketamine; typically between 600–900 mg.[41] This version, currently not allowed in the United States, is most commonly done in Germany but some treatments are now also taking place in Monterrey, Mexico. According to Dr Schwartzman, 14 cases out of 41 patients in the coma induced ketamine experiments were completely cured. "We haven't cured the original injury", he says, "but we have cured the RSD or kept it in remission. The RSD pain is gone." He added that "No one ever cured it before... In 40 years, I have never seen anything like it. These are people who were disabled and in horrible pain. Most were completely incapacitated. They go back to work, back to school, and are doing everything they used to do. Most are on no medications at all. I have taken morphine pumps out of people. You turn off the pain and reset the whole system."[41]
In Tuebingen, Germany Dr Kiefer treated a patient presented with a rapidly progressing contiguous spread of CRPS from a severe ligamentous wrist injury. Standard pharmacological and interventional therapy successively failed to halt the spread of CRPS from the wrist to the entire right arm. Her pain was unmanageable with all standard therapy. As a last treatment option, the patient was transferred to the intensive care unit and treated on a compassionate care basis with anesthetic doses of ketamine in gradually increasing (3–5 mg/kg·h) doses in conjunction with midazolam over a period of 5 days. On the second day, edema, and discoloration began to resolve and increased spontaneous movement was noted. On day 6, symptoms completely resolved and infusions were tapered. The patient emerged from anesthesia completely free of pain and associated CRPS signs and symptoms. The patient has maintained this complete remission from CRPS for 8 years now. The psychiatric side effects of ketamine were successfully managed with the concomitant use of midazolam and resolved within 1 month of treatment.[42]
The dissociative anesthetic effects of ketamine have also been applied within the realm of postoperative pain management. Low doses of ketamine have been found to significantly reduce morphine consumption as well as reports of nausea following abdominal surgery.[43]
Ketamine sold illicitly comes either from diverted legitimate supplies and semi-legitimate suppliers, or from theft of legitimate suppliers.
In 2003, the US Drug Enforcement Agency conducted Operation TKO, a probe into the quality of ketamine being imported from Mexico.[44] As a result of operation TKO, US and Mexican authorities shut down the Mexico City company Laboratorios Ttokkyo, which was the biggest producer of ketamine in Mexico. According to the DEA, over 80% of ketamine seized in the US is of Mexican origin. In 2011 it is now mostly shipped from places like India as cheap as $5/gram.[44] The World Health Organization Expert Committee on Drug Dependence, in its thirty-third report (2003),[45] recommended research into its recreational use/misuse due to growing concerns about its rising popularity in Europe, Asia and North America.
In the 1993 book E for Ecstasy[46] (about the uses of the street drug Ecstasy in the UK) the writer, activist and Ecstasy advocate Nicholas Saunders highlighted test results showing that certain consignments of the drug also contained ketamine. Consignments of Ecstasy known as "Strawberry" contained what Saunders described as a "potentially dangerous combination of ketamine, ephedrine and selegiline", as did a consignment of "Sitting Duck" Ecstasy tablets.[47]
The former chairman of the British Advisory Council on the Misuse of Drugs, David Nutt, suggested that Ketamine should be upgraded from a class C drug due to the harm it can cause to users.[48]
Ketamine may be quantitated in blood or plasma to confirm a diagnosis of poisoning in hospitalized patients, provide evidence in an impaired driving arrest or to assist in a medicolegal death investigation. Blood or plasma ketamine concentrations are usually in a range of 0.5-5.0 mg/L in persons receiving the drug therapeutically (during general anesthesia), 1–2 mg/L in those arrested for impaired driving and 3–20 mg/L in victims of acute fatal overdosage. Urine is often the preferred specimen for routine drug use monitoring purposes. The presence of norketamine, a pharmacologically-active metabolite, is useful for confirmation of ketamine ingestion.[49][50][51]
Ketamine produces effects similar to phencyclidine (PCP) and dextromethorphan (DXM). Unlike the other well known dissociatives PCP and DXM, ketamine is very short acting, its hallucinatory effects lasting sixty minutes when insufflated or injected and up to two hours when ingested, the total experience lasting no more than a couple of hours.[52] Like other dissociative anaesthetics, hallucinations caused by ketamine are fundamentally different from those caused by classical hallucinogens. At low doses, hallucinations are only seen when one is in a dark room with one's eyes closed, while at medium to high doses the effects are far more intense and obvious.[53]
Ketamine produces a dissociative state, characterised by a sense of detachment from one's physical body and the external world which is known as depersonalization and derealization.[54] At sufficiently high doses (usually only when administered IM), users may experience what is called the "K-hole", a state of dissociation whose effects are thought to mimic the phenomenology of schizophrenia.[55] Users may experience worlds or dimensions that are ineffable, all the while being completely unaware of their individual identities or the external world. Users have reported intense hallucinations including visual hallucinations; perceptions of falling; fast and gradual movement and flying; "seeing God"; contact with non-human entities; experiencing divine forces; feeling connected to other users, objects and the cosmos; experiencing psychotic reactions; shared hallucinations and thoughts with adjacent users. John C. Lilly,[56] Marcia Moore[57] and D. M. Turner[58] (amongst others) have written extensively about their own spiritual/psychonautic use of ketamine. (Both Moore[59] and Turner[60] died prematurely in a way that has been linked to their ketamine use.)
Users may feel as though their perceptions are located so deep inside the mind that the real world seems distant. Some users may not remember this part of the experience after regaining consciousness, in the same way that a person may forget a dream. Owing to the role of the NMDA receptor in long-term potentiation, this may be due to disturbances in memory formation. The "re-integration" process is slow, and the user gradually becomes aware of surroundings. At first, users may not remember their own names, or even know that they are human, or what that means. Movement is extremely difficult, and a user may not be aware that he or she has a body at all.
Short term side effects of Ketamine are:
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Chronic use of ketamine may lead to cognitive impairments including memory problems.[61] In 1989, psychiatry professor John Olney reported that ketamine caused irreversible changes in two small areas of the rat brain, which however has significant differences in metabolism from the human brain and therefore may not occur in humans.[62][63][64]
The first large-scale, longitudinal study of ketamine users found that heavy ketamine users had impaired memory by several measures, including verbal, short-term memory and visual memory. However occasional (1-2 times per month) ketamine users and ex-ketamine users were not found to differ from controls in memory, attention and psychological well-being tests. This suggests that occasional use of ketamine does not lead to prolonged harm and that any damage that might occur may be reversible when ketamine use is stopped; however, depression worsened even in the abstinent user group over the period of the study (one year), along with dissociative symptoms still existing among infrequent users.[65]
Short-term exposure of cultures of GABAergic neurons to ketamine at high concentrations led to a significant loss of differentiated cells in one study, and non-cell-death-inducing concentrations of ketamine (10 μg/mL) may still initiate long-term alterations of dendritic arbor in differentiated neurons. The same study also demonstrated that chronic (>24 h) administration of ketamine at concentrations as low as 0.01 μg/mL can interfere with the maintenance of dendritic arbor architecture. These results raise the possibility that chronic exposure to low, subanesthetic concentrations of ketamine, while not affecting cell survival, could still impair neuronal morphology and thus might lead to dysfunctions of neural networks.[66][67]
There is a long list of medicines that could counteract these potential toxic effects , including clonidine, anticholinergics, benzodiazepines, barbiturates and risperidone.[63][64][68]
A study in Bristol reported in the British Medical Journal linked urinary tract disease with ketamine use. Symptoms reported by users included an increased need to urinate, passing blood in urine, leakage of urine and pain on urination. These symptoms may be associated with the scarification of the bladder lining, which leads to a shrunken bladder, erythema, and contact bleeding, and can then move to the ureters and damage the kidneys.[69]
Another small study found "marked thickening of the bladder wall, a small capacity, and perivesicular stranding, consistent with severe inflammation. At cystoscopy, all patients had severe ulcerative cystitis. Cessation of ketamine use, with the addition of pentosan polysulfate, appeared to provide some symptomatic relief."[70]
Many long term users report "K cramps".[71] The exact cause of these pains are unknown. The Ketamine induced abdominal pain is primarily limited to users of a gram or more of ketamine a day (route of administration does not seem to affect this symptom). It has been suggested that the amino acid Tyrosine may help alleviate the pain.[72]
In case reports of three patients treated with S(+)ketamine for relief of chronic pain, liver enzyme abnormalities occurred following repeat treatment with ketamine infusions, with the liver enzyme values returning below the upper reference limit of normal range on cessation of the drug. The result suggests that liver enzymes have to be monitored during such treament.[73]
Ketamine may increase the effects of other sedatives, including but not limited to: benzodiazepines, barbiturates, opiates/opioids, anesthetics, and alcoholic beverages.
Ketamine was long thought to act primarily by inhibiting NMDA receptors.[74][75] But another NMDA receptor antagonist, MK-801, does not exert the same hypnotic effects as ketamine in rats.[76] It appears more likely that the hypnotic effects of ketamine are produced by inhibiting hyperpolarization-activated cyclic nucleotide-modulated (HCN1) cation channels, which mediate the "sag" current (Ih ) in neurons.[77] Inhibition of Ih by ketamine in cultured neurons causes a hyperpolarizing shift in resting membrane potential and enhances summation of excitatory currents.[78] Such effects, if induced in vivo, would likely induce cortical oscillations reminiscent of sleep in rats.[79] Most importantly, knockout of HCN1 channels in mice eliminates the hypnotic actions of ketamine.[78]
Ketamine is a noncompetitive NMDA receptor antagonist.[80] This receptor opens in response to binding of the neurotransmitter glutamate, and blockade of this receptors is believed to mediate the analgesic (reduction of pain) effects of ketamine at low doses.[75] Evidence for this is reinforced by the fact that naloxone, an opioid antagonist, does not reverse the analgesia. Studies also seem to indicate that ketamine is "use dependent" meaning it only initiates its blocking action once a glutamate binds to the NMDA receptor.
At high, fully anesthetic level concentrations, ketamine has also been found to bind to opioid μ2 receptors in cultured human neuroblastoma cells without being an agonist on them [3] and sigma receptors.[81] It has also been shown to act as a weak D2 receptor partial agonist in rat brain cell homogenates,[82] as well as a dopamine reuptake inhibitor.
(rac)-ketamine is a noncompetitive inhibitor of human recombinant α7 nAChR expressed in frog eggs at clinically relevant concentrations.[83]
Ketamine is racemic, and its (R)- and (S)-stereoisomers have different binding affinities: (S)-ketamine has about four times greater affinity for the PCP site of the NMDA receptor than does (R)-ketamine (in guinea pig brain). (S)-ketamine seems to induce drowsiness more strongly than the (R)-enantiomer; it is probable that (R)-ketamine is the stronger sigma agonist and so this enantiomer is likely to be responsible for the lowering of the seizure threshold that can occur with ketamine. Since (S)-ketamine has greater analgesic effects than (R)-ketamine, the pure (S)-enantiomer is sometimes preferred to the racemic mix for use in medical procedures, especially when lower doses are used for minor surgical procedures where the patient remains conscious during the operation.[84]
The effects seem to take place mainly in the hippocampal formation and in the prefrontal cortex. This evidence, along with the NMDA receptor's connection with the memory formation process, explains ketamine's profound effects on memory and thought. These effects inhibit the filtering function of the brain and may mirror the sensory overload associated with schizophrenia and near death experiences.
The local anesthetic effects in frog nerve preparations are likely from the blocking action of ketamine on sodium channels.[85] Its in vitro blocking potency of rat muscle recombinant sodium channels, expressed in frog eggs, in the resting state is similar to that of lidocaine.[86]
Ketamine has a well-documented neuroprotective effect against ischemic brain-injury and glutamate induced brain injury in cortical cell cultures.[87] One hypothesis of its working mechanism in case of chronic pain management and depression is that it works as an antidote to an overactivity in glutamergic brain circuits.
Ketamine is synthesized from 2-chlorobenzonitrile, which reacts with cyclopentylmagnesium bromide to give 1-(2-chlorobenzoyl)cyclopentane.[88][89] The next step is bromination using bromine to the corresponding bromoketone, which upon reaction with an aqueous solution of methylamine forms the methylimino derivative. During this reaction a simultaneous hydrolysis of the tertiary bromine atom occurs. On further heating the reaction product in decalin, a ring expansion rearrangement occurs, causing formation of ketamine.
Ketamine was developed in 1962 as a derivative of phencyclidine (PCP), which was synthesized in 1956, a feat made possible by the discovery of the organic Grignard reaction by Parke-Davis scientist Harold Maddox.[90] Initially known as CI-581, ketamine was first synthesized by Parke-Davis scientist Calvin Stevens. Pharmacological investigations in human subjects began in 1964.[90] These investigations demonstrated that ketamine's shorter duration of action and lesser psychomimetic profile made it favorable over PCP as a "dissociative" anesthetic.[91] Following FDA approval in 1970, ketamine anesthesia was first given to American soldiers during the Vietnam War.
Nonmedical use of ketamine was documented in the early 1970s in underground literature (see The Fabulous Furry Freak Brothers). It was used in psychiatric and other academic research through the 1970s, culminating in 1978 with the publishing of psychonaut John Lilly's The Scientist and Marcia Moore and Howard Alltounian's Journeys into the Bright World, which documented the unusual phenomenology of ketamine intoxication.[92] The incidence of nonmedical ketamine use increased through the end of the century, especially in the context of raves and other parties.[93][94][95][96][97] However, its emergence as a club drug differs from other club drugs (e.g. MDMA) due to its anesthetic properties (e.g., slurred speech, immobilization) at higher doses;[97] in addition, reports of ketamine being sold as "ecstasy" are common.[98] The use of ketamine as part of a "post-clubbing experience" has also been documented.[99] Ketamine's rise in the dance culture was most rapid in Hong Kong by the end of the 1990s.[100]
The increase in illicit use prompted ketamine's placement in Schedule III of the United States Controlled Substance Act in August 1999.[101] In the United Kingdom, it became labeled a Class C drug on 1 January 2006.[102] In Canada ketamine is classified as a Schedule I narcotic, as of August 2005.[103] In Hong Kong, as of the year 2000, ketamine is regulated under Schedule 1 of Hong Kong Chapter 134 Dangerous Drugs Ordinance. It can only be used legally by health professionals, for university research purposes, or with a physician's prescription.[104]
Production for recreational use has been traced to 1967, when it was referred to as "mean green" and "rockmesc".[105] Recreational names for ketamine include "K",[106][107] "Ket",[108] "Special K",[107] "Kitties", "K2",[106] "Vitamin K",[108][107] "Super K",[107] "Jet" (Texas),[107][109] "Super acid",[107] "Mauve",[107] "Special LA coke",[107] "Purple",[107] "Cat Valium",[109] "Blind Squid"[110] "Keller",[110] "Kelly's Day",[110] "Kezzle", "Pony Powder", "Donkey Dust", "Tranx", "Horsey" and "Ney". A mixture of ketamine with cocaine is called "Calvin Klein" or "CK One".
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