Gas Chromatographic Determination of Avilamycin Total Residues in Pig Tissues, Fat, Blood, Feces, and Urine

Abstract

A gas chromatographic (GC) procedure is presented for the determination of residues of avilamycin and all its metabolites/conjugates which can be converted to the common moiety dichloroisoeverninic acid (DIA). The method involves alkaline hydrolysis to DIA, cleanup by partitioning with chloroform, acidification of the aqueous phase, and partitioning of DIA into methylene chloride. After methylation of DIA, the product, 3,5-dicMoro-4,6-dimethoxy-2-methylbenzoic acid methyl ester, is cleaned up on a silica gel column prior to the final determination by electron capture GC. The method is sensitive to 0.1 mg/kg avilamycin equivalent. Overall average recoveries were 85.4%, with a standard deviation of 9.1 % for n = 20. Analyses of feces, urine, tissues, and fat of pigs treated with avilamycin demonstrated that 93% of the administered substance is excreted in feces and urine, within 72 h after treatment, and that no residues (<0.1 mg/kg) can be found in the tissues and fat of the animals at any time between 0 and 7 days after treatment with medicated feed.

Avilamycin is poorly absorbed, is extensively metabolized by swine and is not persistent in the environment

“…Contrary to statements made in the introduction by the authors, avilamycin is extensively metabolized in the gut of swine: less than 10% total radioactivity in swine feces was attributable to avilamycin. 2,3 It was perhaps not understood that the method reported by Formica and Giannone is not specific for unchanged avilamycin, but rather measures the total of avilamycin and any metabolites that include the dichloroisoeverninic acid moiety. The second report by Magnussen et al used carbon-14-labeled avilamycin to show that in fact very little avilamycin remains in the feces and that extensive metabolism has occurred.…”

“…The authors' assertion that 'the majority of the drug is excreted unmetabolized in the feces and urine' is therefore incorrect and does not reflect the conclusions of the very publications that were referenced. 2,3 The authors also refer to an evaluation by the Australian Pesticides and Veterinary Medicines Authority (APVMA) 4 to support an environmental safety justification for conducting the mass spectroscopic research on avilamycin. However, the reader may be drawn to the incorrect impression that an assessment of the environmental risk has not been conducted.…”