us is variable. Results of a single dose bioequivalence study with adult healthy volunteers showed that Modigraf granules were approximately 20% more bioavailable than the Prograf capsules. Mean oral bioavailability of tacrolimus (investigated with the Prograf capsules formulation) is in the range of 20 - 25% (individual range in adult patients 6 - 43%, in paediatric kidney transplant patients 3 - 77%). The oral bioavailability of tacrolimus was reduced when it was administered after a meal.
Bile flow does not influence the absorption of tacrolimus and therefore treatment with Modigraf granules may commence orally.
In some patients, tacrolimus appears to be continuously absorbed over a prolonged period yielding a relatively flat absorption profile.
The rate and extent of absorption of tacrolimus is greatest under fasted conditions. The presence of food decreases both the rate and extent of absorption of tacrolimus, the effect being most pronounced after a high-fat meal. The effect of a high-carbohydrate meal is less pronounced.
In stable liver transplant patients, the oral bioavailability of tacrolimus was reduced when it was administered after a meal of moderate fat (34% of calories) content. Decreases in AUC (27%) and Cmax (50%), and an increase in tmax (173%) in whole blood were evident.
In a study of stable renal transplant patients who were administered tacrolimus immediately after a standard continental breakfast the effect on oral bioavailability was less pronounced. Decreases in AUC (2 to 12%) and Cmax (15 to 38%), and an increase in tmax (38 to 80%) in whole blood were evident.
A strong correlation exists between AUC and whole blood trough levels at steady-state for Modigraf. Monitoring of whole blood trough levels therefore provides a good estimate of systemic exposure.
Distribution
In man, the disposition of tacrolimus after intravenous infusion may be described as biphasic.
In the systemic circulation, tacrolimus binds strongly to erythrocytes resulting in an approximate 20:1 distribution ratio of whole blood/plasma concentrations. In plasma, tacrolimus is highly bound (> 98.8%) to plasma proteins, mainly to serum albumin and α-1-acid glycoprotein.
Tacrolimus is extensively distributed in the body. The steady-state volume of distribution based on plasma concentrations is approximately 1300 l (healthy subjects). Corresponding data based on whole blood averaged 47.6 l.
Metabolism
Tacrolimus is widely metabolised in the liver, primarily by the cytochrome P450-3A4. Tacrolimus is also considerably metabolised in the intestinal wall. There are several metabolites identified. Only one of these has been shown in vitro to have immunosuppressive activity similar to that of tacrolimus. The other metabolites have only weak or no immunosuppressive activity. In systemic circulation only one of the inactive metabolites is present at low concentrations. Therefore, metabolites do not contribute to pharmacological activity of tacrolimus.
Excretion
Tacrolimus is a low-clearance substance. In healthy subjects, the average total body clearance estimated from whole blood concentrations was 2.25 l/h. In adult liver, kidney and heart transplant patients, values of 4.1 l/h, 6.7 l/h and 3.9 l/h, respectively, have been observed. Factors such as low haematocrit and protein levels, which result in an increase in the unbound fraction of tacrolimus, or corticosteroid-induced increased metabolism, are considered to be |
