Tacrolimus metabolite formation in intestinal enterocytes exposed to 1, 3, or 7, 5 μmol l−1 tacrolimus for 2 h. Metabolites were analysed by LC/particle beam-MS. Data is shown as mean±s.d. of at least four dishes (2×106 cells). Tacrolimus was added to the culture medium 4 h post isolation. (a) demethyl-tacrolimus (M1), (b) bisdemethyl-tacrolimus (M2), demethyl-hydroxy-tacrolimus (M3) and hydroxy-tacrolimus (M4).