Development and validation of an analytical method for the determination of thiopental for a physicochemical stability study

Introduction

Thiopental is a hypnotic used in the induction of general anaesthesia with or without intubation. In the gynaecological unit, in order to guarantee the effective case management of a red code caesarean, thiopental syringes at 25 mg/mL are daily reconstituted by the nurses and renewed every 24 hours. The objective of this work was to develop a ‘stability indicating’ analytical method to control the reconstitution and physicochemical stability of the syringes over 24 hours.

Materials and Methods
An analytical method for determination by DAD-HPLC was developed and validated. The mobile phase was a mixture of acetonitrile and ultrapure water (45/55; v/v) at a flow rate of 1 mL/min and the column a Purospher® STAR RP-18 (5 µm, 150 × 4.6 mm). The detector wavelength was set at 290 nm for the quantification. The calibration range of the method varied from 3.125 to 50 µg/mL and three quality control levels were defined (3.125, 12.5 and 50 µg/mL, respectively low, medium and high). Validation of the method was performed according to ICH Q2(R1) guidelines for the following criteria: linearity and accuracy (precision and trueness). Forced degradation (pH variations, temperature, oxidation reactions and photolysis) was performed. The analytical method was used to assay thiopental syringes at the time of their preparation (T0) and 24 hours later (T24) under block storage conditions.

Results and Discussion
The retention time of thiopental was 4.30 min. The calibration curves provide satisfactory linearity with correlation coefficients greater than 0.999 and a residual value lower than 8.35%. The average equation of the linear regression curve was y = 63,498x + 18,288. The relative standard deviation obtained in the repeatability and intermediate fidelity studies were lower than 2.48% and 2.75%, respectively, regardless of the quality control level. The mean percent recovery obtained in the fidelity study remained close to 100% of the expected value. Forced degradation showed degradation products with different retention times compared to thiopental except for light. Storage of the syringes in a dark place is therefore considered. The analysis of the thiopental syringes at T0 showed no significant difference with the theoretical concentration (p = 0.204), as did the comparison of the average concentration observed at T0 and T24 (p = 0.39), demonstrating to satisfactory reconstitution of the nurses and physicochemical stability at 24 hours.

Conclusion
A simple, accurate and rapid analytical method has been developed and validated allowing the realization of an awareness campaign on the syringe’s reconstitution in the gynaecological unit. Moreover, the physicochemical stability of thiopental over 24 hours allows to consider a longer stability study, although the control of the microbiological stability of the syringes is essential in order to carry out hospital preparations.

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