Therapeutic drug monitoring of everolimus using the dried blood spot method in combination with liquid chromatography–mass spectrometry

An assay of everolimus based on finger prick sampling and consecutive application as a blood spot on sampling paper has been developed. We explored several methods [K. Hoogtanders, J. van der Heijden, M. Christiaans, P. Edelbroek, J. van Hooff, L. Stolk, J. Pharm. Biomed. Anal. 44 (2006) 658–664; A. Allanson, M. Cotton, J. Tettey, et al., J. Pharm. Biomed. Anal. 44 (2007) 963–969] and developed a new method, namely the impregnation of sampling paper with a solution of plasma–protein, formic acid and ammonium acetate, in combination with the extraction of the blood spot by filter filtration. This kind of sample preparation provides new possibilities for blood spot sampling especially if analytes are adsorbed to the paper. The dried blood spot was analysed using the HPLC–electrospray-tandem mass spectrometry method, with 32-desmethoxyrapamycin as the internal standard. The working range of our study was 2–30 μg/l. Within this range, intra-and inter-assay variability for precision and accuracy was