The Disposition of C-labeled Tacrolimus after Intravenous and Oral Administration in Healthy Human Subjects

Tacrolimus is a macrolide lactone with potent immunosuppressive properties. It has been shown in clinical studies to prevent allograft rejection. The pharmacokinetics of tacrolimus in healthy subjects and transplant patients has been described in earlier studies using immunoassay methods; however, detailed information on the absorption, distribution, metabolism, and excretion of tacrolimus using a radiolabeled drug is lacking. The objective of the present study was to characterize the disposition of tacrolimus after single i.v. (0.01 mg/kg) and oral (0.05 mg/kg) administration of C-labeled drug in six healthy subjects. Tacrolimus was absorbed rapidly after oral dosing with a mean Cmax and Tmax of 42 ng/ml and 1 h, respectively. The oral bioavailability was about 20%. After i.v. and oral dosing, most of the administered dose was recovered in feces, suggesting that bile is the principal route of elimination. Urinary excretion accounted for less than 3% of total administered dose. In systemic circulation, unchanged parent compound accounted for nearly all the radioactivity; however, less than 0.5% of unchanged drug was detectable in feces and urine. The excretion of the metabolites was formation-rate-limited. The mean total body clearance at 37.5 ml/min was equivalent to about 3% of the liver blood flow. Renal clearance was less than 1% of the total body clearance. The mean elimination half-life was 44 h. Tacrolimus, [3S-[3R*[E(1S*,3S*,4S*)]4S*,5R*,8S*,9E,12R*, 14R*,15S*,16R*,18S*,19S*,26aR*]]-5,6,8,11,12,13,14,15,16, 17,18,19,24,25,26,26a-hexadecahydro-5,19-dihydroxy-3-[2-(4hydroxy-3-methoxycyclohexyl)-1-methylethylenyl]-14,16-dimethoxy4,10,12,18-tetramethyl-8-(2-propenyl)-15,19-epoxy-3H-pyrido[2,1-c] [1,4]oxaazacyclotricosine-1,7,20,21(4H,23H)-tetrone, monohydrate (FK506; Prograf), is a potent immunosuppressive agent developed by Fujisawa, Japan. Tacrolimus is a macrolide obtained from Streptomyces tsukubaensis, which has been shown in preclinical and clinical studies to prevent allograft rejection (Yoshimura and Oka, 1990; European FK506 Multicentre Liver Study Group, 1994; European Tacrolimus Multicentre Renal Study Group, 1997). After binding to the intracellular immunophilin FK506-binding protein, tacrolimus blocks intermediate steps in the pathway that links early membrane-associated events with gene expression and inhibits T and B cell proliferation. Its selective effect on the immune system appears to be due to selectivity for a subset of calciumassociated signal transduction pathways that may predominate in the T cell receptor-mediated cascade leading to cytokine production. The pharmacokinetics of tacrolimus in healthy volunteers and transplant patients has been described in studies using enzyme immunoassays to determine concentrations of tacrolimus in blood and plasma. These studies indicate that tacrolimus is bound strongly to plasma proteins and erythrocytes, that the ratio of blood/plasma concentrations is 20:1 (Venkataramanan et al., 1990; Beysens et al., 1991), and that pharmacokinetic parameters calculated from measurement of whole-blood concentrations are different from those derived from plasma concentrations. The terminal elimination half-life calculated from whole blood and from plasma was 23 to 40 h and 27 to 65 h, respectively. Estimates of total body clearance also differ greatly (1.2–2.2 liters/h in whole blood and 18–49 liters/h in plasma). Detailed information on the absorption, distribution, metabolism, and excretion of tacrolimus using a radiolabel assay is lacking. The objectives of the present study were to characterize the pharmacokinetics of tacrolimus in whole blood after i.v. and oral administration of C-labeled drug in healthy volunteers. Materials and Methods Subjects. After being informed of the purpose, design, and potential risks of the study, six subjects gave written informed consent to participate. The study was approved by the local Ethics Committee. The subjects were healthy, male, and white nonsmokers, aged 49 to 67 years, with no evidence of metabolic or other disease or drug abuse, who had alcohol consumption of ,1 liter of beer/day or the equivalent and had not received a radioisotope within the previous 12 months. Medication other than paracetamol was not permitted; however, one subject received treatment for facial paresis (not drug-related) from day 8 of the study. All subjects were negative for HIV and hepatitis B surface antigen. Subjects were randomized in a nonblinded crossover study design to receive single doses of C-labeled tacrolimus i.v. and orally separated by a washout period of 14 days. Each was admitted on the evening before administration and remained in the unit until the morning of the 12th day thereafter. Samples of blood and feces were taken until discharge from the unit. The subjects’ mean age was 59.0 years (S.D. 5 6.2; range, 49–62) and their mean weight was 77.5 kg (S.D. 5 3.6; range, 72–82). Laboratory and ECG parameters were appropriate for this age group in all subjects and did not differ significantly from normal. There was no difference between preand poststudy parameters. No subject withdrew from the study. Dosage and Administration. The i.v. dose was administered as a 4-h infusion (5% dextrose solution) at a nominal dose of 20 mg/kg C-labeled This study was supported by Fujisawa GmbH, Munich. Send reprint requests to: Dr. N. A. Undre, Fujisawa GmbH, Levelingstrasse 12, D-81606, Munich, Germany. E-mail: [email protected] 0090-9556/99/2706-0633–636$02.00/0 DRUG METABOLISM AND DISPOSITION Vol. 27, No. 6 Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. 633 at A PE T Jornals on Jne 3, 2017 dm d.aspurnals.org D ow nladed from tacrolimus adjusted to a maximum dose of radioactivity of 30 mCi. Blood samples were taken from an appropriate vein in the opposite arm. The oral dose was formulated in polyethylene glycol solution and administered in a single hard-gelatin capsule with 100 ml of water while the subject was in an upright sitting position that was maintained for 30 min after administration. The oral dose of C-labeled tacrolimus was approximately 50 mg/kg (range, 44–51 mg/kg), adjusted to a maximum dose of radioactivity of 70 mCi. The volunteers had fasted for at least 10 h before and for 4 h after the dose, at which time they received a light lunch; they received decaffeinated coffee or rose hip tea after 8 h and dinner after 10 h. On the remaining days, they received standardized hospital meals. Assays. Total radioactivity (tacrolimus and metabolites) was measured by liquid scintillation counting (LSC) (Iwasaki et al., 1995); the concentration of tacrolimus was measured by enzyme-linked immunosorbent assay (ELISA) in blood, plasma, feces, and urine (Iwasaki et al., 1991). Pharmacokinetics. Pharmacokinetic parameters were calculated by a compartment-independent model. The area under the concentration–time profile (AUC) was estimated using the log/linear trapezoidal rule. The AUC to infinite time (AUC0-`) was defined as the area under the curve to the time of the last measured concentration above the limit of quantification (AUC0-t) plus the remaining area extrapolated to infinity (AUCt-`) estimated using the following equation: