Relationship between gallamine plasma concentration and neuromuscular paralysis in surgical patients

Muscle distribution of the neuromuscular blocker gallamine using microdialysis

Measurement of drug concentrations in target tissue has the potential to provide insight into the pharmacokinetics and pharmacodynamics of a drug. In this study, the distribution of the neuromuscular blocker, gallamine, into muscle tissue was investigated in urethane-anesthetized rats after an intravenous bolus dose (6 mg/kg). Microdialysis sampling was used to continuously determine gallamine concentrations in muscle interstitial fluid (MIF). In vivo microdialysis recovery of gallamine was determined as the relative loss of gallamine from the perfusate into muscle tissue after perfusion with gallamine (2 microg/mL). Recovery was determined in each rat before the pharmacokinetic studies. Terminal muscle sampling followed by homogenization was also performed to examine gallamine distribution within muscle tissue. All samples were assayed for gallamine using a validated high-performance liquid chromatography assay. Gallamine was rapidly distributed into MIF with a MIF-plasma partition coefficient of 0.9 +/- 0.1 (n = 6). By contrast, the estimated gallamine concentration in muscle tissue homogenate was only 23 +/- 5% (n = 5) of the concentration in MIF as estimated by microdialysis sampling at the terminal sampling time. These findings suggest that gallamine is not distributed uniformly within muscle but selectively distributes into MIF. Simulations using a hybrid physiologically based pharmacokinetic model which describes uptake of drug only into the interstitial space showed good agreement between predicted and observed concentration data obtained from microdialysis sampling, supporting the findings that gallamine selectively distributes into MIF. These studies demonstrate microdialysis combined with conventional terminal tissue sampling provides valuable information on intra-tissue drug distribution.

Application of a high-performance liquid chromatographic assay for the neuromuscular blocker gallamine to analysis of rat plasma, muscle and microdialysate samples

A reliable high-performance liquid chromatographic method has been validated for determination of gallamine in rat plasma, muscle tissue and microdialysate samples. A C18 reversed-phase column with mobile phase of methanol and water containing 12.5 mM tetrabutyl ammonium (TBA) hydrogen sulphate (22:78, v/v) was used. The flow-rate was 1 ml/min with UV detection at 229 nm. Sample preparation involved protein precipitation with acetonitrile for plasma and muscle tissue homogenate samples. Microdialysate samples were injected into the HPLC system without any sample preparation. Intra-day and inter-day accuracy and precision of the assay were <13%. The limit of quantification was 1 microg/ml for plasma, 1.6 microg/g for muscle tissue and 0.5 microg/ml for microdialysate samples. The assay was applied successfully to analysis of samples obtained from a pharmacokinetic study in rats using the microdialysis technique.

Assessment of in vitro and in vivo recovery of gallamine using microdialysis

The application of microdialysis technique for the investigation of pharmacokinetics and pharmacodynamics of drugs requires careful assessment of probe performance to ensure validity of the data obtained using this technique. The aim of this study was to establish and validate the microdialysis technique for investigation of the pharmacokinetics and pharmacodynamics of the neuromuscular blocker, gallamine. In vitro recovery of gallamine from the microdialysis probe when different perfusion flow rates were employed was evaluated leading to selection of a flow rate of 2 microl/min with 15-min sampling intervals for the subsequent studies. In vitro recovery of gallamine from the microdialysis probe was independent of concentration, stable over an 8-h period and reproducible. Comparable in vitro recoveries were obtained by different established approaches including recovery estimation by gain, loss and the zero-net flux (ZNF) method. Recovery by loss was used to study the in vivo recovery of gallamine from rat muscle tissue. The in vivo recovery was stable over a 5.5-h sampling period. In vitro performance of the probe subsequent to the in vivo study remained stable supporting reusage of the probe. These data highlight the importance of a systematic examination of microdialysis probe validation.

Pharmacokinetic and pharmacodynamic modelling with pancuronium

The pharmacokinetics and pharmacodynamics of pancuronium were studied following intravenous infusion in eleven patients undergoing surgical anaesthesia. Measurement of the plasma concentrations (Cp) of the neuromuscular blocking agent ( NMBA ) and the concomitant intensities of paralysis allowed their simultaneous modelling. The pharmacokinetic parameters derived for pancuronium were in the range of previously reported values, except that the mean total systemic plasma clearance (0.79 +/- 0.28 ml X min-1 X kg-1) was reduced and the mean terminal phase half-life (169 min) was longer in these patients. Plasma concentration and % paralysis data were successfully fitted to a previously proposed pharmacodynamic model. This model assumes a separate effect compartment which exchanges drug directly with the central kinetic compartment (integrated effect model). The 'steady-state' Cp necessary to produce 50% paralysis ( ECpss (50] was estimated to be 0.21 +/- 0.08 micrograms X ml-1 (mechanical response) and 0.18 +/- 0.05 micrograms X ml-1 (EMG response). An analysis using the Hill equation of the Cp-response relationship, during and after the constant-rate infusion of pancuronium bromide, resulted in effective plasma concentrations for 50% paralysis ( ECp50 ) of 0.35 +/- 0.06 micrograms X ml-1 and 0.20 +/- 0.09 micrograms X ml-1, respectively, for mechanical twitch response. The corresponding values for EMG response were 0.32 +/- 0.06 micrograms X ml-1 and 0.17 +/- 0.06 micrograms X ml-1. Using this latter approach, the ECp50 estimated during onset of paralysis was significantly higher than that estimated during offset of paralysis (p less than 0.05); no such difference was apparent between this latter parameter and the ECpss (50) of the integrated effect model (p greater than 0.05). No significant differences were observed between any of the pharmacodynamic parameter estimates generated from the data obtained from the two methods of assessment of neuromuscular function (mechanical vs. EMG response) (p greater than 0.05).