The simultaneous determination of lidocaine and its metabolites in serum by high performance liquid chromatography

Abstract

Lidocaine is a heart drug used widely in the management of ventricular arrhythmias. The deethylated metabolites of lidocaine (i.e., monoethylglycinexylidide and glycinexylidide) have been shown to have antiarrhythic potencies and central nervous system toxicities similar to those for lidocaine. In some patients receiving lidocaine there is evidence that these metabolites may accumulate to significant levels. Therefore it would be advantageous to analyze for monethylglycinexylidide and glycinexylidide simultaneously with lidocaine for the proper clinical evaluation of lidocaine therapy. The development of a high performance liquid chromatographic method suitable for this purpose is the subject of this report.

A reverse phase absorption liquid chromatographic system was used in this study to resolve and quantitate lidocaine and its metabolites. This system consisted of a Waters U Bondapak C-18 column stationary phase in conjunction with a phosphate-buffered acetonitrile moving phase of pH 6.1.

A charcoal absorption technique was employed to extract lidocaine from its metabolites from serum. The average extraction efficiency of this procedure was 72.3% for lidocaine, 64.3% for monoethylglycinexylidide and 49.8% for glycinexylidide. Partition extraction techniques using organic solvents immiscible with serum were also tested, but were found to have lower extraction efficiencies and to be more susceptible to interferences. Prilocaine was found to be a suitable internal standard for the assay procedure.

A spectrophotometer was used as the detector for the chromatographic system. A detection wavelength of 205 nm was used as this is near the absorption maximum of lidocaine and its metabolites. The limit of detection for lidocaine, monoethylglycinexylidide and glycinexylidide in serum was 0.4 ug/ml. Standard curves for all three substances were prepared and were found to be linear over the ranges tested ; i.e., 0.4-16.0 ug/ml for lidocaine and 0.4-8.0 ug/ml for monoethylglycinexylidide and glycinexylidide.

Four serum samples from three patients receiving lidocaine were analyzed. Three of the four samples were identified by retention time as having lidocaine concentrations of 2.9, 3.5 and 9.4 ug/ml. The first sample listed also had monoethylglycinexylidide and glycinexylidide concentrations of 0.5 ug/ml. Monoethylglycinexylidide was also indicated in another sample, but could not be quantified due to an interering peak.

Interference studies were performed which established that the drugs procainamide, ethosuximide, hydrochlorithiazide and primidone have elution times which interfer with the assay system used in this study.

For future work the use of a dual-wavelength detection system is strongly recommended as it would provide a greater degree of certainty in the identification of chromatographic peaks. Also the development of an alternative column separation procedure, e. g., ion-exchange, would provide another approach to providing greater certainty in peak identification when used in conjunction with the method described in this study.