UHPLC-ESI-MS/MS assay for quantification of endocannabinoids in cerebrospinal fluid using surrogate calibrant and surrogate matrix approaches

Endocannabinoids are endogenous lipids with the main function recognized to act as neuromodulators through their cannabinoid receptors. Dysregulation of the endocannabinoid system is implicated in various pathologies, such as inflammatory and neurodegenerative diseases. In this study we describe a sensitive UHPLC-MS/MS method for the analysis of trace levels of 7 endocannabinoids in cerebrospinal fluid samples. The analytes covered comprised 1- and 2-arachidonoylglycerol 1- and 2-AG (which were analysed as sum due to their interconversion), 2-arachidonylglycerol ether 2-AGE, anandamide AEA, N-linoleoyl ethanolamide LEA, N-palmitoyl ethanolamide PEA and N-oleoyl ethanolamide OEA. Analytes were extracted from the biofluid by a simple monophasic procedure involving protein precipitation with acetonitrile (MeCN). The analytical method is based on chromatographic separation of the analytes with solid-core (core-shell, superficially porous) particle column Cortecs C18+ . Gradient elution with changing proportion of water and acetonitrile and constant concentration of formic acid provided reasonable separation of analytes, close elution of analytes and their internal standards and minimized matrix effects in biological samples. For specific detection of the endocannabinoids a triple-quadrupole tandem mass spectrometer with electrospray ionisation (ESI) and selected reaction monitoring (SRM) mode was used, and it provided good assay selectivity. The developed method required a minute volume of the biological samples (50 µL) and achieved excellent sensitivity (the lower limit of detection was between 4.15 and 30.18 pM of the biological sample). Linear calibration was achieved in the range from 25 to 10,545 pM for AEA, 90-3802 pM for 1-AG, 90-724 pM for 2-AG, 12-5226 pM for LEA, 33-13,942 for OEA, 34-23,850 pM for 2-AGE, 72-30,190 for PEA and 10-4218 for AEA-d4 in CSF. The method was validated and revealed relative errors in the range of - 14.7 to + 12.3% at LLOQ and - 14.1 to + 14.2% for the remaining validation range. Precisions were in the acceptable range (< 20% RSD at LLOQ, and