Comparing the effect of isotopically labeled or structural analog internal standards on the performance of a LC-MS/MS method to determine ciclosporin A, everolimus, sirolimus and tacrolimus in whole blood

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is routinely used for analysis of immunosuppressive drugs. This study investigated whether replacing analog internal standards (ANISs) with isotopically labeled internal standards (ILISs) has an impact on the performance of a LC-MS/MS method for the quantification of tacrolimus (TAC), sirolimus (SIR), ciclosporin A (CsA) and everolimus (EVE) in whole blood. Following hemolysis, protein precipitation, and extraction with either ANISs (ascomycin, desmethoxy-rapamycin, CsD), or ILISs (TAC- C,D ; SIR- C,D ; CsA-D ; EVE-D ), samples were centrifuged and injected into a LC-MS/MS device equipped with a C18 reversed phase column. The effect of the two ISs on the linearity, precision, accuracy, trueness, matrix effects, and carryover was investigated by using the same patient-, proficiency testing-, and quality control samples. Statistical analysis of agreement between results includes a standard random effects model and Passing-Bablok regression. Within-day imprecision was