Stability of mycophenolic acid and glucuronide metabolites in human plasma and the impact of deproteinization methodology

BackgroundIn recent years there is growing interest in therapeutic drug monitoring of mycophenolic acid (MPA) and its glucuronide metabolites MPAG and AcMPAG. Like other acyl glucuronide metabolites, AcMPAG has a limited stability, but this aspect has received little attention.MethodsPlasma sample deproteinization with perchloric acid 2 M (method A) was compared to metaphosphoric acid 15% (method B). Stability of MPA, MPAG and AcMPAG in acidified and non-acidified plasma stored at room temperature, 4 °C, − 20 °C and − 80 °C was assessed over short and long time intervals using HPLC-UV methodology.ResultsThe area ratio of AcMPAG/IS on spiked plasma at pH 2.5 with method A was 63% of the respective ratio in water, in contrast to 102% with method B, suggesting partial deconjugation and/or incomplete release of AcMPAG from proteins with method A. At room temperature, AcMPAG concentrations in both whole blood and non-acidified plasma decreased significantly after 2–5 h. MPA, MPAG and AcMPAG concentrations remained stable in acidified plasma stored at − 20 °C and − 80 °C, but not longer than 5 months after collection.ConclusionsIt is concluded that adequate sample collection, storage measures and deproteinization methods should be applied in order to avoid deconjugation and hence underestimation of MPA, MPAG and AcMPAG concentrations.