High-performance liquid chromatographic determination of thiopentone enantiomers in sheep plasma

High-throughput assay for quantification of the plasma concentrations of thiopental using automated solid phase extraction (SPE) directly coupled to LC-MS/MS instrumentation

Most previous assays for thiopental are time-consuming due to laborious sample extraction steps prior to analysis using gas chromatography or high pressure liquid chromatography. Here, we describe the first high-throughput liquid chromatography - tandem mass spectrometry (LC-MS/MS) method for quantification of thiopental concentrations in samples of human plasma. Robotic on-line solid phase extraction (SPE) was used to elute the analytes of interest from samples of human plasma (50μL) loaded onto C18 SPE cartridges to which were added aliquots (50μL) of internal standard solution (thiopental-d5 100ng/mL) and 0.5% formic acid in water (100μL). Cartridges were washed using 10% methanol in ammonium acetate buffer (50mM, pH 7) before elution with mobile phase comprising 0.1% formic acid in water and acetonitrile with a flow rate of 0.55mL/min using a 7.2min run time. The analytes were separated on a C18 XTerra^® analytical column. Mass spectrometry detection was performed using a QTrap 5500 mass spectrometer (AB Sciex) with negative ionisation. The multiple reaction monitoring (MRM) transitions for thiopental and the internal standard were 241→58, and 246→58, respectively. The calibration curve was linear over a range of 6-600ng/mL. Thiopental was stable in human plasma samples for at least 36h in the autosampler, as well as after three cycles of freeze and thaw, and after 3h storage at room temperature. The absolute recovery and matrix effect were 102% and 6.9%, respectively, and the within-run and between-run precision and accuracy were ≤15%. Our method is fully-validated and satisfies the requirements of the 2012 European Medicines Agency (EMEA) guideline for Bioanalytical Method Validation.

Rapid in vitro elimination of anesthetic doses of thiopental in the isolated guinea pig brain

Electrophysiological and metabolic activities in brain tissue preparations maintained in vitro may be influenced by the persistent effect of anesthetic drugs utilized during tissue dissection. In order to clarify this issue, we studied elimination kinetics of the barbiturate thiopental from the brain parenchyma in the isolated guinea pig brain maintained in vitro, arterially perfused with a protein-free saline solution [M. de Curtis, G. Biella, C. Buccellati, G. Folco, Simultaneous investigation of the neuronal and vascular compartments in the guinea pig brain isolated in vitro, Brain Res. Protoc. 3 (1998) 21-28]. At the onset of anesthesia induced by a single i.p. injection of 125 mg/kg thiopental, the brain concentration of the drug, measured by high-performance liquid chromatographic assay, was 44.22+/-5.1 mg/L (mean+/-S.E.; n=7). After 30 min of arterial perfusion in vitro with a thiopental-free solution, the cerebral levels of the barbiturate decreased to 2.03+/-0.56 mg/L (n=3), and reached values close to zero within 1 h. No significant changes in thiopental elimination curve were observed when in vitro perfusion rate was either increased or decreased. The study demonstrates that thiopental is rapidly eliminated from the brain tissue with a mono-exponential kinetic. It can be concluded that barbiturate anesthesia utilized during brain dissection is not likely to influence activities recorded from the in vitro isolated brain preparation.

Barbiturates for acute neurological and neurosurgical emergencies--do they still have a role?

A number of clinical studies have reported poor clinical outcomes for patients treated with barbiturate therapy in acute neurological and neurosurgical emergencies. Barbiturate therapy, as currently practised with thiopentone and pentobarbitone at least, is also associated with a prolonged post-infusion period of clinical unresponsiveness. Hence, the popularity of barbiturate therapy for sedation of critically ill neurological and neurosurgical patients has declined over the past decade. A retrospective study of traumatic brain injury patients treated at the Royal North Shore Hospital, Sydney, with high-dose thiopentone therapy between 1987 and 1997 has found disappointing results with a 1-month mortality outcome of 50% (14 of 28 patients). Nevertheless, barbiturate therapy remains a consideration for patients with severe cranial trauma in whom preferred treatments have failed to control intracranial or cerebral perfusion pressures. More favourable results ( approximately 10% 1-month mortality rate) were encountered for patients with refractory vasospasm complicating subarachnoid haemorrhage or intracerebral haemorrhage complicating supratentorial arteriovenous malformation resection. A well designed, prospective and randomised controlled trial may be of value in further determining the role of barbiturate therapy in acute neurovascular emergencies refractory to standard therapy.

Prolonged thiopentone infusion for neurosurgical emergencies: usefulness of therapeutic drug monitoring

Serial serum thiopentone concentrations were measured during and following completion of an intravenous infusion of thiopentone in 20 patients with neurosurgical emergencies. The concentration data from a further 55 patients who had had some such measurements were reviewed retrospectively. The patients received an infusion for longer than 24 hours at a rate adjusted to maintain EEG burst suppression. The data were interpreted in terms of thiopentone pharmacokinetics and used to produce statistical models relating to clinical outcomes. In these patients, the one-month mortality rate following commencement of thiopentone treatment was 20%; the mean durations of pupillary and motor unresponsiveness following cessation of an infusion were 22 and 91 hours, respectively. Predictors of a prolonged duration of motor unresponsiveness included a prolonged duration of pupillary unresponsiveness, a low thiopentone clearance and a high maximum serum concentration of thiopentone. From pooled logistic regression, median effective serum thiopentone concentrations (EC50) were found to be 50 mg x l(-1) for recovery of pupillary responsiveness and 12 mg x l(-1) for the recovery of motor responsiveness. Because prolonged high-dose thiopentone leads to prolonged residual serum concentrations, it is difficult to distinguish the residual pharmacological effects of thiopentone from the clinical condition. This study suggests that, based on EC50 values for responses, monitoring of post-infusion serum thiopentone concentrations may help determine whether a patient's clinical state is due to residual thiopentone pharmacological effects.

Fast, simple and cost-effective determination of thiopental in human plasma by a new HPLC technique

BACKGROUND

Thiopental is an anaesthetic drug that is largely used in both short-term and long-term infusion. After long-term infusion of thiopental, non-linear and inter-individual-dependent pharmacokinetics occur because of the saturation and/or induction of the metabolism. Clinical monitoring is important so that therapeutic adjustments can be made in many of the different pharmacological treatments, especially when long-term infusion is required. We describe a new, rapid HPLC method for the determination of plasma thiopental.

METHODS

Sample preparation involved precipitation of plasma proteins using a mixture of methanol, zinc sulfate and ethylene glycol, and containing the internal standard 5-ethyl-5-p-tolyl-barbituric acid. After adding trichloroacetic acid, the sample was centrifuged and the supernatant was injected into a C(18) reversed-phase column. The mobile phase used was water-methanol-acetonitrile (50:40:10, v/v). The eluent was monitored at 290 nm.

RESULTS

The calibration curve was linear from 0.2 to 100 microg/mL. Precision, calculated as the coefficient of variation (%), was in the range of 3.62-0.70% for the within-day assay and 5.77-1.51% for the between-day assay. The absolute recoveries obtained from supplemented samples were never less than 100%.

CONCLUSIONS

This technique shows good reliability and seems to be suitable for a very fast and simple therapeutic monitoring of plasma thiopental.

Electroencephalographic effects of thiopentone and its enantiomers in the rat

Electrophysiological studies with some chiral barbiturates have shown that one enantiomer can be excitant while the other is depressant. Thiopentone, a chiral barbiturate, has both differences in potency between enantiomers and biphasic effects on the electroencephalogram (EEG). This study investigated whether a differential EEG activity between the enantiomers of thiopentone could account for the biphasic effects. Rats were administered rac-, R- or S-thiopentone to determine the nature and time course of quantitative EEG effects. Two studies using computer-controlled i.v. infusions of the three drugs were performed in groups of animals previously prepared with EEG electrodes and/or arterial blood sampling cannulae. Study 1 used several stepwise increments in plasma drug concentration over 35 min, followed by washout. Study 2 used a 4 min period of constant plasma drug concentration, followed by washout. In both studies, both enantiomers and racemate caused an initial EEG activation followed by deactivation. Quantitative enantioselectivity was found for depression. The extent of depression was significantly less for R-thiopentone (P=0.008) and racthiopentone (P=0.038) than for S-thiopentone; recovery from depression appeared to be faster for R-thiopentone than either rac- or S-thiopentone. Fatality was only found with S-thiopentone (3/7 animals in Study 2). R-thiopentone plasma concentrations were approximately 8% less than those of S-thiopentone in rats treated with racthiopentone. Although small differences in clearance between enantiomers were found that may influence recovery, they were not large enough to account for the reported differences in potency between the two enantiomers.

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.

Stereoselective interaction of thiopentone enantiomers with the GABA(A) receptor

1. As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the approximately 2 fold greater potency for CNS effects of (-)-S- over (+)-R-thiopentone, this study was performed to determine any enantioselectivity of thiopentone at the GABA(A) receptor, the primary receptor for barbiturate hypnotic effects. 2. Two electrode voltage clamp recording was performed on Xenopus laevis oocytes expressing human GABA(A) receptor subtype alpha1beta2gamma2 to determine relative differences in potentiation of the GABA response by rac-, (+)-R- and (-)-S-thiopentone, and rac-pentobarbitone. Changes in the cellular environment pH and in GABA concentrations were also evaluated. 3. With 3 microM GABA, the EC50 values were (-)-S-thiopentone (mean 26.0+/-s.e.mean 3.2 microM, n=9 cells) >rac-thiopentone (35.9+/-4.2 microM, n=6, P=0.1) >(+)-R-thiopentone (52.5+/-5.0 microM, n=8, P<0.02) >rac-pentobarbitone (97.0+/-11.2 microM, n=11, P<0.01). Adjustment of environment pH to 7.0 or 8.0 did not alter the EC50 values for (+)-R- or (-)-S-thiopentone. 4 Uninjected oocytes responded to >100 microM (-)-S- and R-thiopentone. This direct response was abolished by intracellular oocyte injection of 1,2-bis(2-aminophenoxy)ethane-N, N,N1,N1-tetraacetic acid (BAPTA), a Ca2+ chelating agent. With BAPTA, the EC50 values were (-)-S-thiopentone (20.6+/-3.2 microM, n=8) <(+)-R-thiopentone (36.2+/-3.2 microM, n=9, P<0.005). 5 (-)-S-thiopentone was found to be approximately 2 fold more potent than (+)-R-thiopentone in the potentiation of GABA at GABA(A) receptors expressed on Xenopus oocytes. This is consistent with the differences in potency for CNS depressant effects found in vivo.

Enantioselectivity of thiopental distribution into the central neural tissue of rats: an interaction with halothane

Thiopental is a racemate. In this study, we examined whether thiopental total body clearance and its distribution into central nervous system (CNS) tissue of rats was enantioselective. Rats, either anesthetized with halothane or conscious and restrained, were infused to stepwise steady-state targets of 5, 10, and 20 microg/mL thiopental by computer-controlled infusions. Serial arterial plasma and steady-state samples of brain and spinal cord were assayed enantiospecifically for thiopental. In both groups, concurrent total and unbound plasma concentrations of S-thiopental were approximately 10%-20% higher than those of R-thiopental, corresponding to its higher clearance. CNS tissue concentrations of S-thiopental were approximately 20% higher than those of R-thiopental. Spinal cord to plasma distribution coefficients were approximately 2 x those in the brain, with relative distribution coefficients approximately 10% greater for R-thiopental in both tissues. Plasma concentrations and distribution coefficients of both enantiomers were approximately 10%-20% lower in the halothane-anesthetized group, with a slightly greater effect on R-thiopental distribution. We conclude that the total body clearance of R-thiopental > S-thiopental, that halothane enantioselectively reduces the relative uptake of R-thiopental into brain tissue, and that composition is important in determining the CNS tissue concentrations of thiopental. The reported higher potency of S-thiopental did not seem to be due to its greater distribution into CNS tissues.

IMPLICATIONS

Because thiopental is a mixture of two forms (termed R-and S-enantiomers), correct interpretation of its distribution into, and clearance from, the body requires knowledge about both enantiomers. In this study, performed in rats, we showed that the two enantiomers of thiopental differed significantly, with the R-enantiomer having the preferred profile.

Characterization of the stereoselective metabolism of thiopental and its metabolite pentobarbital via analysis of their enantiomers in human plasma by capillary electrophoresis

Using capillary zone electrophoresis (CZE) with a 75 mM phosphate buffer at pH 8.5 containing 5 mM hydroxypropyl-gamma-cyclodextrin (OHP-gamma-CD) as chiral selector, the separation of the enantiomers of thiopental and its oxybarbiturate metabolite, pentobarbital, is reported. Enantiomer assignment was performed via preparation of enantiomerically enriched fractions using chiral recycling isotachophoresis (rITP) processing of racemic barbiturates and analysis of rITP fractions by chiral CZE and circular dichroism spectroscopy. Thiopental and pentobarbital enantiomers in plasma were extracted at low pH using dichloromethane and extracts were reconstituted in acetonitrile or 10-fold diluted, achiral running buffer. The stereoselectivity of the thiopental and pentobarbital metabolism was assessed via analysis of 12 plasma samples that stemmed from patients undergoing prolonged or having completed long-term racemic thiopental infusion. The data obtained revealed a modest stereoselectivity with R-(+)-thiopental/S-(-)-thiopental and R-(+)-pentobarbital/S-(-)-pentobarbital plasma ratios being < 1 (P < 0.05 compared to data obtained with racemic controls) and > 1 (P < 0.001), respectively. The total S-(-)-thiopental plasma concentration was found to be on average about 24% higher compared to the concentration of R-(+)-thiopental, whereas the total R-(+)-pentobarbital plasma level was observed to be on average 29% higher compared to the S-(-)-pentobarbital concentration.

Improved method of determining thiamylal enantiomers in human serum by high-performance liquid chromatography

Thiamylal, a widely used anesthetic drug, has two enantiomers. We developed a simple and rapid method for measuring the thiamylal enantiomers in human serum. The method involves a liquid-liquid extraction procedure followed by chiral resolution using a 5 microm silica-bonded alpha1-acid glycoprotein column (Chiral-AGP). The thiamylal enantiomers and internal standard were eluted within 15 min and were well-resolved. At concentrations of 1, 5 and 20 microg ml(-1), the relative standard deviations of R(+)- and S(-)-thiamylal were 1.35-2.88% and 1.37-3.01%, respectively, for the intra-day assay, and 2.93-4.46% and 2.46-4.84%, respectively, for the inter-day assay. This method facilitates the routine monitoring and pharmacokinetic studies of thiamylal enantiomers.

Chirality in anaesthesia - ropivacaine, ketamine and thiopentone

Drug chirality (molecular handedness) is a source of pharmacological differences between otherwise chemically identical molecules. Specific applications to the pharmacology of ropivacaine (single enantiomer), ketamine and thiopentone (both racemates) are discussed. Ropivacaine is produced as a single S-enantiomer homologue of the more toxic bupivacaine to preclude the higher central nervous system and heart toxicity found in the R-enantiomer. S-ketamine is presently undergoing trials as a potential replacement for the racemate, on the grounds that it optimizes anaesthesia and minimizes psychotomimetic phenomena. Thiopentone, previously known to have quantitative differences in the pharmacology of its enantiomers, has recently also been shown to have pharmacokinetic differences. The evidence for these claims is discussed in this review.