From the racemate to the eutomer: (S)-ketamine.
Renaissance of a substance?
by
Adams HA, Werner C.
Zentrum Anasthesiologie-Anasthesiologie I,
Medizinische Hannover.
Anaesthesist. 1997 Dec;46(12):1026-42.

ABSTRACT

The pharmacological profile of ketamine: Until recently, clinically available ketamine was a racemic mixture containing equal amounts of two enantiomers, (S)- and (R)-ketamine. The pharmacological profile of racemic ketamine is characterized by the so called dissociative anesthetic state and profound sympathomimetic properties. Among the different sites of action, N-methyl-D-aspartate (NMDA)-receptor antagonism is considered to be the most important neuropharmacological mechanism of ketamine. Effects on opiate receptors, monoaminergic and cholinergic transmitters, and local anesthetic effects are obvious as well. Following intravenous administration, a rapid onset of action is seen within 1 min, lasting for about 10 min. The anaesthetic state is terminated due to redistribution, followed by hepatic and renal elimination with a half-life period of 2-3 h. For alternative administration, the intramuscular and oral route is also appropriate. The most important adverse effects are hallucinations and excessive increases in blood pressure and heart rate. These reactions can be attenuated or avoided by combining of ketamine with sedative or hypnotic drugs like midazolam and/or propofol. During controlled ventilation, increases in intracranial pressure are unlikely to occur. The special pharmacological profile of (S)-ketamine: In general, the pharmacological properties of (S)-ketamine are comparable to the racemic compound. On the different sites of action, qualitatively comparable effects were found, but significant quantitative differences also became obvious. When compared with (R)-ketamine and the racmic mixture, the analgesic and anesthetic potency of (S)-ketamine is threefold or twofold higher. Thus, a 50% reduction of dosage is possible to achieve comparable clinical results. Because of the faster elimination of (S)-ketamine, better control of anesthesia will be provided. In summary, the pharmacokinetic improvements of (S)-ketamine are characterized by a reduced drug load, along with more rapid recovery. The clinical use of (S)-ketamine: The clinical use of (S)-ketamine depends on its analgesic and sympathomimetic properties, whereas the anaesthetic potency remains in the background. In clinical anesthesiology, (S)-ketamine, especially in combination with midazolam and/or propofol, can be used for short procedures with preserved spontaneous ventilation, for induction of anesthesia in patients with shock or asthmatic disorders, and for induction and maintenance of anesthesia in caesarean sections. Additional indications are repeated anesthesia, for example, in burn patients, analgesia during delivery and diagnostic procedures and intramuscular administration in uncooperative patients. The value of (S)-ketamine as an analgesic component for total intravenous anesthesia has not been defined yet. In comparison with opioides, the advantages are related to improved hemodynamic stability and reduced postoperative respiratory depression. When (S)-ketamine, especially in combination with midazolam, is used for analgosedation in intensive care medicine, a reduction of exogenous catecholamine demand can be expected. Moreover, the effects on intestinal motility are superior to opioids. In combination with midazolam and propofol, excellent control of analgosedation was found, making both combinations suitable for situations in which repeated neurological assessment of patients is necessary. In emergency and disaster medicine, (S)-ketamine is of outstanding importance because of its minimal logistic requirements, the chance for intramuscular administration and the broad range of use for analgesia, anaesthesia and analgosedation as well. Further perspectives of (S)-ketamine may be the treatment of chronic pain and the assumed neuroprotective action of the substance.

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