Chirality in anaesthesia - ropivacaine, ketamine and thiopentone

When Regional Anesthesia Met Pharmacokinetics

Pharmacokinetics of Local Anaesthetic Agents. By Tucker GT, Mather LE. Br J Anaesth 1975; 47(suppl 1):213-24 Information derived from measurements of blood concentrations of local anaesthetics can be extended by the application of pharmacokinetic analysis. A better understanding of quantitative aspects of the disposition and absorption of these drugs should assist the anaesthetist in deciding the optimal agent and dosage for regional block techniques.

STUDIES WITH KETAMINE AND ALFENTANIL FOLLOWING FREUND'S COMPLETE ADJUVANT-INDUCED INFLAMMATION IN RATS

1. N-Methyl-D-aspartate (NMDA) receptor antagonists suppress inflammatory hyperalgesia and the development of acute opioid tolerance. They may also enhance opioid-induced antinociception, while suppressing postopioid-induced hyperalgesia and opioid-enhanced inflammatory hyperalgesia. 2. The non-competitive NMDA receptor antagonist, ketamine, is a racemic chiral drug; its individual enantiomers have differing affinities for the NMDA receptor. The anaesthetic and antinociceptive potencies of (S)-ketamine are 1.5- and threefold higher, respectively, than those of (R)-ketamine in laboratory rodents. 3. The present study investigated the effects of racemic ketamine and enantiopure (S)-ketamine on inflammatory hyperalgesia in rats, 5 days after intraplantar injection of Freund's complete adjuvant (FCA) into one hind paw. First, racemic or (S)-ketamine was administered alone; second, racemic or (S)-ketamine was administered 30 min after initiation of i.v. infusions of the micro-opioid agonist, alfentanil. 4. Area under the curve (AUC) values for Von Frey paw withdrawal threshold (PWT) versus time curves were significantly increased (P < 0.05) for both inflamed and non-inflamed hind paws by racemic and (S)-ketamine (5 & 10 mg/kg, s.c.). Similarly, AUC values for reduction of hind paw volume versus time were significantly increased (P < 0.05) by racemic and (S)-ketamine (10 mg/kg, s.c.). 5. Alfentanil infusions significantly increased PWT in both hind paws, but neither racemic nor (S)-ketamine (5 mg/kg, s.c.) administered 30 min after initiation of alfentanil infusion produced further increases in PWT. 6. Racemic and (S)-ketamine produced antinociceptive effects in both hind paws, but an antihyperalgesic effect per se was not apparent. Additionally, there was a possible anti-inflammatory effect of both drugs in the inflamed hind paw. These findings complement previous studies in which non-competitive NMDA receptor antagonists suppressed behavioural hyperalgesia. 7. However, racemic and (S)-ketamine did not further enhance alfentanil's antinociceptive effects, although they appeared to prolong alfentanil's antinociceptive effects in the non-inflamed hind paw. These findings suggest that factors such as time-course, frequency and the mode of administration of NMDA receptor antagonists, in addition to the type of antinociceptive model (i.e. inflammatory compared with acute) and the nociceptive testing procedure (i.e. noxious mechanical compared with low threshold stimuli) may influence their effects on opioid-induced antinociception.

Stereochemistry in clinical medicine: a neurological perspective

Stereoisomers are compounds that have identical sets of atoms configured in the same positions but are arranged differently spatially. Approximately 25% of contemporary drugs are marketed and used as racemates (i.e., as equimolar mixtures of stereoisomers). This may have major clinical implications, as stereoisomers may possess qualitative and/or quantitative differences in pharmacological effects, plasma protein and tissue binding, metabolic and renal clearance. There are many examples of racemic drugs manufactured and used as single stereoisomers in the field of neurology including the anti-Parkinsonian drugs levodopa, selegiline, apomorphine and entacapone, the antiepileptic drugs tiagabine and levetiracetam, the secondary stroke prevention agent clopidogrel and the acetylcholinesterase inhibitor rivastigmine. The role of drug stereochemistry in the re-evaluation of established drugs and the production of new agents is becoming increasingly important as pharmaceutical companies endeavour to show proof of "no penalty" for the introduction of a racemic new drug over one or other of its single stereoisomers.

Electroencephalographic effects of thiopentone and its enantiomers in the rat: correlation with drug tissue distribution

1. To better understand the pharmacology of the thiopentone enantiomers, we studied their quantitative electroencephalographic effects and their distribution into vital tissues. 2. Adult Wistar rats were infused with rac-, R- or S-thiopentone at 4 mg kg(-1)min(-1) until death ensued. The EEG signal was acquired continuously; serial arterial plasma and terminal tissue thiopentone concentrations were measured enantiospecifically. Relevant drug tissue : plasma distribution coefficients and plasma concentration-EEG effect relationships were determined. 3. Doses (mg kg(-1)) (mean+/-s.e.mean) for anaesthesia (toe pinch) and lethality (respiratory failure), respectively, decreased in the order R-thiopentone (55.8+/-2.4 and 176.2+/-11.2)> rac-thiopentone (39.3+/-2.1 and 97.5+/-3.9)> S-thiopentone (35.6+/-1.9 and 74.2+/-5.2); plasma drug concentrations (microg ml(-1)) decreased in the order R-thiopentone (66.3+/-4.5 and 89.8+/-5.2)> rac-thiopentone (56.7+/-2.0 and 77. 8+/-2.8)> S-thiopentone (55.0+/-1.9 and 64.1+/-2.8). 4. Initial EEG activation was similar for all thiopentone forms. Plasma drug concentrations for the same extent of EEG deactivation reflected the potency order. 5. After infusion of rac-thiopentone, tissue : plasma distribution coefficients were higher for R- than for S-thiopentone in brain and visceral regions, but not in fat or muscle. After infusion of the separate enantiomers, the relative heart : brain distribution ratio was for S-thiopentone was double that for R-thiopentone. 6. The therapeutic index of R-thiopentone (3.16+/-0. 14) was more advantageous than either rac-thiopentone (2.52+/-0.13) or S-thiopentone (2.10+/-0.14), possibly due to the relatively greater distribution into CNS tissues than heart. The data suggest that R-thiopentone could make a satisfactory single enantiomer substitute for rac-thiopentone.