Volumetric absorptive microsampling as an alternative tool for therapeutic drug monitoring of first-generation anti-epileptic drugs.

Dosage adjustment of anti-epileptic drugs by therapeutic drug monitoring (TDM) is very useful, especially for the first-generation anti-epileptic drugs (AEDs). Microsampling-the collection of small volumes of blood-is increasingly considered a valuable alternative to conventional venous sampling for TDM. Volumetric absorptive microsampling (VAMS) allows accurate and precise collection of a fixed volume of blood, eliminating the volumetric blood hematocrit bias coupled to conventional dried blood spot collection. The aim of this study was to develop and validate an LC-MS/MS method for the determination and quantification of four anti-epileptic drugs (carbamazepine, valproic acid, phenobarbital, and phenytoin) and one active metabolite (carbamazepine-10,11-epoxide) in samples collected by VAMS. The method was fully validated based on international guidelines. Precision (%RSD) was below 10%, while, with a single exception, accuracy (%bias) met the acceptance criteria. Neither carry-over nor unacceptable interferences were observed, the method being able to distinguish between the isomers oxcarbazepine and carbamazepine-10,11-epoxide. All compounds were stable in VAMS samples for at least 1 month when stored at room temperature, 4 °C, and - 20 °C and for at least 1 week when stored at 60 °C. Internal standard-corrected matrix effects were below 10%, with %RSDs below 4%. High (> 85%) recovery values were obtained and the effect of the hematocrit on the recovery was overall limited. Successful application on external quality control materials and on left-over patient samples demonstrated the validity and applicability of the developed procedure. Graphical abstract Graphical representation of the sampling, chemical structures, and the resulting chromatogram for volumetric absorptive microsampling (VAMS)-based therapeutic drug monitoring of first-generation anti-epileptic drugs by liquid chromatography with tandem mass spectrometric detection.