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

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.