A Quantitative Study of in vitro Hepatic Metabolism of Tacrolimus (FK506) Using Secondary Ion and Matrix-assisted Laser Desorption/Ionization Mass Spectrometry

The identification and simultaneous quantification of Tacrolimus and its hepatic metabolites in baboons has been achieved using matrix‐assisted laser desorption/ionization (MALDI) time‐of‐flight (TOF) mass spectrometry and static secondary‐ion mass spectrometry (TOF‐SIMS). Little fragmentation, high sensitivity and tolerance to contamination are the major advantages of these methods, allowing facile identification and quantification of metabolites produced in vitro with minor analyte isolation. Based on the MALDI and TOF‐SIMS results, seven metabolites have been identified: de‐methylated, di de‐methylated, hydroxylated, di hydroxylated, de‐methylated hydroxylated, dihydrodiol, and di de‐methylated hydroxylated. The concentrations of the parent drug and its major metabolites (e.g. de‐methylated, di de‐methylated) were measured using Rapamycin as an internal standard. The time course of Tacrolimus and its major metabolites as a function of incubation time was calculated. Good correlation between SIMS and MALDI results was obtained.