Measurement of neosaxitoxin in human plasma using liquid-chromatography tandem mass spectrometry: Proof of concept for a pharmacokinetic application

•Neosaxitoxin has shown potential as an effective, long-acting, anesthetic.•We have developed a sensitive method for measurement of neosaxitoxin in plasma LC-MS/MS.•We have demonstrated the application of our method to a pharmacokinetic study.

Neosaxitoxin, a member of the saxitoxin family of paralytic shellfish poisoning toxins, has shown potential as an effective, long-acting, anesthetic. We describe the development and validation of a highly sensitive method for measurement of neosaxitoxin in human plasma using liquid chromatography tandem mass spectrometry (LC-MS/MS) and provide evidence for its use in a human pharmacokinetic study. Samples were prepared using cation exchange solid phase extraction followed by hydrophilic interaction liquid chromatography and MS/MS detection in positive electrospray ionization mode. Multiple reaction monitoring was used to monitor neosaxitoxin (m/z 316.17 > 220.07) and the internal standard analogue decarbamoylneosaxitoxin (m/z 273.12 > 180.00). The method was validated for lower limit of quantification, precision, accuracy, linearity and matrix effect. The stability of neosaxitoxin in plasma matrix at various storage conditions was also investigated. Standard curves for calibration were linear (r > 0.995) across the assay calibration range, 10 to 1000 pg/mL. The analytical measurable range of the assay was 10-10,000 pg/mL in plasma matrix. This method has demonstrated excellent sensitivity demonstrating a lower limit of quantification in human plasma of 10 pg/mL. The mean, inter-batch variation was <5.2% across the concentration range 30 to 800 pg/mL. This method was successfully used in a phase 1 trial to investigate the pharmacokinetic profile of neosaxitoxin in humans following the intravenous administration of the drug at a range of doses up to 40 μg. We conclude that our high-sensitivity method for measurement of neosaxitoxin in human plasma is capable of supporting future clinical trials.