Excretion of tacrolimus glucuronides in human bile.
Firdaous I, Verbeeck RK, Hassoun A, Langrehr JM, Wallemacq PE
Department of Clinical Chemistry, University Hospital St Luc, Catholic University of Louvain, Brussels, Belgium.
Tacrolimus is extensively metabolized by the cytochrome P-450 system.
Hepatic metabolic phase I reactions of tacrolimus include mainly demethylation
and/or hydroxylation. No valid data have been published on phase II pathways
(glucuronide- or sulfo-conjugation). In order to investigate these pathways,
different beta-glucuronidase/sulfatase enzyme preparations were used to
hydrolyse the conjugates potentially present in human bile extracts. Two
analytical methods were used: a non-specific method, MEIA, and a specific
combined HPLC/MEIA method. The influence of the extraction pH was investigated.
After beta-glucuronidase hydrolysis and extraction at pH 5, tacrolimus
concentrations, obtained either from HPLC-MEIA or MEIA, always appeared
significantly higher, suggesting the presence of glucuronides in the bile.
When the extraction was performed at pH 1.5, only the HPLC-MEIA concentrations
appeared higher after hydrolysis. MEIA concentrations obtained before and
after hydrolysis were similar. These data are consistent with the fact
that glucuronides are extracted at pH 1.5 but not at pH 5 and suggest first
that, without hydrolysis, the extracted glucuronides are separated from
the tacrolimus fraction in the HPLC-MEIA procedure, and second, that the
glucuronides are cross-detected by the monoclonal antibody in the immunoassay.
From these data, it is concluded that clues have been found, suggesting
the presence in human bile of tacrolimus glucuronides, which cross-react
with the monoclonal antibody, provided they are extracted in the sample
tested.
PMID: 9358202, UI: 98023039