Studies were conducted to determine the effect of antibiotic combinations (lincomycin or bacitracin MD and roxarsone), length of dosing period, pretreatment with medicated feed, and dose vehicle on salinomycin residue levels in chicken tissues. Salinomycin was determined by high-performance liquid chromatography and thin-layer chromatography. Ionophoric activity was determined by a radiolabeled rubidium binding assay. Salinomycin residue levels were unaffected by treatment and assay method. Total residue levels were less than the established tolerance limits for muscle and skin/fat but approached the tolerance limit of 1.8 ppm for liver. Total drug residue levels in liver could be estimated by unchanged salinomycin level in skin/fat. Unchanged salinomycin did not account for all of the ionophoric activity present in liver. The affinity for rubidium binding by the salinomycin metabolites is approximately 20% of that of unchanged drug. Since liver metabolites have minimal ionophoric activity, the residues would have no toxicologic consequences.Sodium salinomycin ( Figure 1) is a polyether, carboxylic ionophore currently being marketed for its anticoccidial activity in chickens at a use level of 40-60 g/ton. Previous residue studies with unlabeled drug conducted at our facilities suggested that feeding salinomycin in combination with other antibiotics may affect the level of salinomycin in skin/fat, which was designated as the target tissue. The levels of salinomycin in skin/fat as determined by a thin-layer bioautographic method similar to that described by Dimenna et al. (1986b) were 0.039, 0.029, and 0.200 ppm, when salinomycin and roxarsone were fed in combination with bacitracin MD, zinc bacitracin, or virginiamycin, respectively. The levels of salinomycin in skin/fat as determined by a high-performance liquid chromatographic (HPLC) method described by Dimenna et al. (1986a) were 0.180, <0.100, and <0.100 ppm, when salinomycin and roxarsone were fed in combination with lincomycin, oxytetracycline, or chlortetracycline, respectively. Because of these results, residue studies were conducted to determine the effect of antibiotic combination and assay methodology on salinomycin residue levels in chicken tissues.In the first study (hereafter designated study A) unchanged salinomycin ,levels were determined in chicken tissues by two independent assay methods: HPLC and radiometric thin-layer chromatography (TLC). Salinomycin (75 g/ton) was administered by itself or in combination with roxarsone plus lincomycin for 20 days in the feed, followed by oral administration by gavage of [14C]-salinomycin at 6.7 mg/kg per day (equivalent to 75 g/ton) in sodium bicarbonate for 5 days. The HPLC method of Dimenna et al. (1986a) was modified in order to lower the quantifiable limit of detection. During the course of study A, it was discovered that the liver total drug residue levels were a t the tolerance limit of 1.8 ppm and were not affected by antibiotic combination. This was in contrast to an earlier residue study, in which c...

A high-performance liquid chromatographic (HPLC) method has been developed for the quantitative determination of salinomycin in chicken skin/fat. For this procedure skin/fat homogenate (10 mL, equivalent to 2 g of tissue) was extracted with methanol. The extract was partitioned with carbon tetrachloride, applied to a silica gel column, and, in turn, run through a C18 column. The purified salinomycin solution was then oxidized with pyridinium dichromate and washed with sodium bicarbonate. The derivatized product was purified by running it through a silica gel column. The HPLC system utilized an automated on-line column-switching system with UV detection at 225 nm; the minimum limit of detection was 100 ppb. Peak height ratios of salinomycin to internal standard were used for quantitation.

The effect of pretreatment with medicated feed on [14C] salinomycin residue levels in pig tissues was studied. Pigs were fed unmedicated feed or feed medicated with salinomycin at 41 ppm in the diet for 29 days and then dosed with [14C]salinomycin for 8 days. Total drug residue levels were below quantifiable limits of detection of kidney, fat, and muscle but at the tolerance limit of 1800 ppb for liver. In liver, pretreatment tended to lower total residue levels, and unchanged [14C] salinomycin accounted for <1% of the total drug residue. Approximately 15-20% of the total drug residue in liver was bound. Ionophoric activity in extracts of livers from the treated pigs was minimal, and only 2 of the 12 treated samples had ionophoric activity more than twice that obtained from the controls. Sodium salinomycin (Figure 1) is a polyether antibiotic possessing anticoccidial and growth-promoting activ-

A thin-layer bioautographic method was developed for the determination of salinomycin in chicken liver. An aliquot of liver homogenate equivalent to 2 g of liver was extracted with isooctane. The extract was then purified on a silica gel column and chromatographed on silica gel thin-layer plates. The salinomycin zones were quantitated by a bioautography technique that makes use of Bacillus subtilis. The limit of detection was 25 ppb, and the assay response was linear from 25.0 to at least 200 ppb. The mean recovery of salinomycin from spiked samples was 108.2 ± 15.8% (x + SD).

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