A previously published application note in this series describes an LC/MS/MS method for the simultaneous determination of four immunosuppressant drugs: Cyclosporin A (CsA), Tacrolimus (TAC, aka FK-506), Sirolimus (SIR, aka Rapamycin) and Everolimus (EVE, aka RAD-001) (1, 2). The method is based on a simple and robust hardware configuration and shows to be fast and simple, without compromising precision and accuracy. Since this method was first described, there has been considerable interest in extending it to include a fifth immunosuppressant drug, Mycophenolic Acid (MPA, see structure in Figure 1).
MPA is the pharmacologically active form of the immunosuppressant prodrug Mycophenolate Mofetil. Because its pharmacological action differs significantly from CsA, TAC, SIR, and EVE, MPA is frequently co-prescribed as part of an immunosuppressant drug cocktail. Due to the chemical structure of MPA and the strong presence of some of its metabolites -most notably the Glucuronide metabolite, MPAGseveral authors have voiced their concern about the risk of overestimating the MPA concentration. It has been shown that chromatographic separation is essential to accurately measure MPA concentration in plasma by eliminating the interfering effect of MPAG (3, 4).
In this research note, minor modifications are described that extend the original method proposed in reference (1) to include MPA and determine the entire panel of 5 immunosuppressants on the Applied Biosystems API 3200TM LC/MS/MS Mass Spectrometer. With these modifications, the same hardware configuration can be used to determine CsA, TAC, SIR, and EVE in blood and MPA in plasma, without compromising sensitivity and specificity. Quantitation of MPA metabolites (mainly MPAG) can be included if desired.
Depending on the instrumental configuration and the up-stream sample