Tissue distribution, metabolism, and residue depletion study in Atlantic salmon following oral administration of [3H]emamectin benzoate

Atlantic salmon (similar to1.3 kg) maintained in tanks of seawater at 5 +/- 1degreesC were dosed with [H-3]emamectin B, benzoate in feed at a nominal rate of 50 mug of emamectin benzoate/kg/day for 7 consecutive days. Tissues, blood, and bile were collected from 10 fish each at 3 and 12 h and at 1, 3, 7, 15, 30, 45, 60, and 90 days post final dose. Feces were collected daily from the tanks beginning just prior to dosing to 90 days post final dose. The total radioactive residues (TRR) of the daily feces samples during dosing were 0.25 ppm maximal, and >97% of the TRR in pooled feces covering the dosing period was emamectin B-1a. Feces TRR then rapidly declined to similar to0.05 ppm by 1 day post final dose. The ranges of mean TRR for tissues over the 90 days post dose period were as follows: kidney, 1.4-3 ppm; liver, 1.0-2.3 ppm; skin, 0.04-0.09 ppm; muscle, 0.02-0.06 ppm; and bone, <0.01 ppm. The residue components of liver, kidney, muscle, and skin samples pooled by post dose interval were emamectin B-1a (81-100% TRR) and desmethylemamectin B-1a (0-17% TRR) with N-formylemamectin Bia seen in trace amounts (<2%) in some muscle samples. The marker residue selected for regulatory surveillance of emamectin residues was emamectin B-1a. The emamectin B-1a level was quantified in individual samples of skin and muscle using HPLC-fluorometry and was below 85 ppb in all samples analyzed (3 h to 30 days post dose).