Abstracts Biological Sciences

Methyltestosterone: pharmacokinetics, oral bioavailability, and residue depletion in rainbow trout

by Andrew M. Vick

Methyltestosterone has potential for use in finfish aquaculture for its anabolic and androgenic action. However, ingestion of methyltestosterone residue is potentially hazardous to consumers, and knowledge of its disposition in market-size rainbow trout was sought to determine a safe depletion period. In addition to characterization of the pharmacokinetics and total residue depletion, the following precursor studies were undertaken: analytical method development, characterization of analyte stability in stored fluid and tissue samples, formulation of a methyltestosterone-medicated diet, and pilot studies. Methods for extraction and gradient HPLC quantification were developed and validated for methyltestosterone and two of its metabolites, and used in conjunction with a reverse isotope dilution technique. At -20°C methyltestosterone and its metabolites were stable for 2 weeks in liver and bile samples, and for at least 4 weeks in kidney, plasma, muscle, and skin. Plasma concentration-time profiles after intraarterial dosage (2 and 20 mg/kg) declined biexponentially with a distribution half-life of 4.13 and 8.23 hr and a terminal elimination half-life of 54.9 and 58.6 hr for the 2 and 20 mg/kg doses, respectively. Two-compartment model parameter estimates for the 2 and 20 mg/kg doses were: area under the curve (11.2 ± 3.89 and 82.3 ± 33.0 µmol <sup>*</sup> hr/L), total body clearance (0.640 ± 0.187 and 0.903 ± 0.317 L/hr/kg), volume of the central compartment (3.83 ± 0.906 and 13.9 ± 6.30 L/kg), steady state volume of distribution (6.06 ± 1.61 and 26.8 ± 15.5 L/kg), and mean residence time (9.57 ± 0.727 and 22.7 ± 12.6 hr), respectively. After a 30 mg/kg oral dose, the average peak plasma concentration was 3.03 ± 0.345 µmol/L, at 8.80 ± 3.35 hr, and the area under the curve was 90.2 ± 44.3 µmol<sup>*</sup>hr/L. The oral bioavailability was 73% and 68%, using model-independent and model-dependent methods, respectively. The tissue distribution and metabolite profiles were determined during and after dietary exposure of fish to <sup>14</sup>C-methyltestosterone (30 mg/kg), and autoradiographs of sagittal sections were prepared. At 4 days (steady state), the concentrations of total radioactivity (ppm equivalents) were highest in bile (3,520 ± 1,220), followed in descending order by liver (26.9 ± 18.2), kidney (8.76 ± 3.01), plasma (4.43 ± 0.726), skin (2.52 ± 0.481), and muscle (2.18 ± 0.181). A biphasic elimination profile was observed, and at 21 days the total concentration of radioactivity was = 0.1 ppm in the edible tissues. A suitable depletion period was devised based upon a consumption rate of 83.0 kg of methyltestosterone-treated trout per year, and no-observable effect levels (NOEL) for hepatotoxicity and fetotoxicity of 2.5 and 0.4 mg/day, respectively. An acceptable daily intake (ADI) for methyltestosterone was then derived by applying a safety factor of 100 to the NOEL. Edible tissues of methyltestosterone-treated trout were found to deplete to acceptable levels of 0.11 and 0.018…