Residue depletion of tritium-labeled ivermectin in rainbow trout following oral administration

Abstract

The residue depletion and metabolism of tritium-labeled ivermectin were studied in the muscle tissue of rainbow trout following oral treatment. Fish were administered 3H-ivermectin at the dose level of 0.1 mg/kg of body weight (9.25 μCi) in a gel-capsule via stomach tube. At each of the following withdrawal times 6 fish were sedated with MS-222, euthanatized, bled, scaled and fillets with adhering skin and bile collected: 1, 3, 7, 14, 28, 35 and 42 days. The muscle tissues were homogenized in dry ice and analyzed for total radioactive residues (TRR) by sample oxidation. The homogenized tissue was further extracted with acetonitrile for residue characterization by high pressure liquid chromatography (LC).Maximum TRR of 71 ng/g and 3857 ng/mL (ivermectin equivalent concentration) were detected in muscle and bile, respectively, by sample oxidation on post-dose day 3. LC analysis of the muscle extract revealed that the maximum TRR of 55 ng/g (representing ivermectin and metabolites) was also detected on post-dose day 3. By post-dose day 35, about 65% of the TRR in the muscle tissue was an unknown metabolite. Preliminary mass spectrometry results indicated that the unknown metabolite may be 3″-O-demethyl-ivermectin B1a. This metabolite could serve as a potential marker residue for ivermectin in rainbow trout, in contrast to cattle, swine and sheep where the parent ivermectin is known to be the marker residue.