Excretion of tacrolimus glucuronides in human bile

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.