Tissue residue depletion study

The pharmacokinetic characteristics of florfenicol have been described in goats (40), calves (41), and chickens (42). The efficacy of florfenicol in aquaculture has been also demonstrated against bacteria involved in some major fish pathologies, especially in salmon and trout (43, 44). Pharmacokinetic studies in Atlantic salmon indicated that the compound was well absorbed and distributed following oral administration (45). Tissue residue depletion studies after an oral daily administration of 10 mg/kg bw florfenicol in rainbow ftout at 10 C for 10 days showed that muscle/skin tissue contained 150 ppb drug at 15 days after the last dose (46). 

Intramammary use of bacitracin resulted in residues in milk, but not in plasma, udder, or any other tissue. Residue depletion studies in cows given intramammary bacitracin treatment showed that muscle, liver, kidney, fat, udder, and milk from untreated quarters did not contain detectable residues ( 0.003-0.005 lU/ml) after the end of treatment. In milk from treated quarters, however, residues of bacitracin could be detected during treatment, declining to around 0.04 lU/ml at the sixth milking after treatment. 

Because of its widespread therapeutic use and because the question of residues in food producing animals, SDM was selected for a study between species to compare its metabolism, the pharmacokinetics of the parent drug as well as its metabolites. Residue depletion studies were performed in edible tissues of calves, pigs and in the eggs of laying-hens. 

Residue depletion studies with piglets orally dosed with 30 mg spectinomycin/kg bw/day for 5 consecutive days showed that the average concentrations of the parent drug in liver, kidney, muscle, and skin/fat were 1030, 7700, less than 300, and less than 394 ppb, respectively, at 3 days, and less than 198, 500, 300, and 26 ppb, respectively, at 14 days postdosing. Broilers given water dosed at 50 mg spectinomycin/kg bw/day for 5 consecutive days were not found to contain detectable spectinomycin concentrations (limit of detection 500 ppb for liver and kidney and 250 ppb for muscle and skin/fat) in any edible tissue even only 1 day after the last dose. 

The presence of detectable levels in plasma and urine of treated cows indicates that nafcillin is absorbed systematically following intramammary administration. The major part of nafcillin is excreted in the milk, but a higher proportion of nafcillin is absorbed from the udder when nafcillin is administered at drying off. Residue depletion studies (70) with lactating cows showed that residues in all edible tissues were below 300 ppb at 72 h after cessation of treatment, while residues in milk were below 30 ppb from the fourth milking onwards, after cessation of the treatment. 

Residue depletion studies (87) after intramammary administration of cefquinome to lactating cows showed that residues in edible tissues could be detected only in kidney at 24 h posttreatment and at concentrations below 200 ppb. High concentrations of cefquinome were found in milk at the first milking posttreatment, while at the 10th milking the residue concentrations in all milk samples were below 20 ppb. 

Residue depletion studies in pigs fed 330 mg kitasamycin/kg feed for 14 days showed that at zero withdrawal only liver (100 ppb) and kidney (60 ppb) contained detectable residues. One day after withdrawal of the treatment, residual antibiotic activity was detectable only in the liver. Residue depletion studies in chickens fed 500 mg kitasamycin/kg feed for 14 days showed that at zero withdrawal only liver (70 ppb) contained detectable residues. One day after withdrawal, residual antibiotic activity could not be detected in any tissue. 

Residue depletion studies with radiolabeled furazolidone have shown that the almost complete degradation of the drug in the body resulted in formation of a variety of protein-bound metabolites that were not solvent-extractable. Thus, when pigs were given radiolabeled furazolidone orally at 16.5 mg/kg bw/day for 14 days (123), total residual radioactivity in liver, kidney, muscle, and fat accounted for 41.1 ppm, 34.4 ppm, 13.2 ppm, and 6.2 ppm furazolidone equivalents, respectively, at zero withdrawal (132). Total residues were substantially lower by 21 days withdrawal, but were still in the ppm range at 45 days withdrawal. Extraction of the incurred muscle tissue at 0 and 45 days withdrawal with organic solvents led to removal of 21.8 and 13.7% of the total radioactivity, respectively. In contrast, 44 and 8.3% of the total radioactivity was extracted from liver on days 0 and 45, respectively. 

This nonextractable radioactivity was probably the result of covalent binding of the furazolidone intermediates to endogenous macromolecules. The bioavailability of these bound tissue residues from the above pig residue depletion study was determined by feeding rats lyophilized samples of liver and muscle tissues from animals sacrificed at 0 and 45 days after the last treatment (132). Results showed that the fraction of the bound residues bioavailable to rats was in the range 16-41%. The toxicological impact of these bioavailable bound residues has not been yet determined. 

Residue depletion studies in cattle, swine, sheep, chickens, and turkeys given oral forms of oxytetracycline including feed premixes, soluble powders, and tablets showed that residues in all edible tissues, with the exception of kidney, were cleared of detectable amounts of oxytetracycline within 5 days postdose. Injectable forms of oxytetracycline yielded higher residue levels that persisted longer than the oral forms, while long-acting formulations of oxytetracycline required extended withdrawal periods (234). 

Following therapeutic treatment, thiophanate residues are higher in liver, with significant levels also being present in kidney relatively lower concentrations are detected in the other edible tissues (9). Residue depletion studies in sheep given a single oral dose of 100 mg thiophanate/kg bw showed that the mean thiophanate residue concentrations in liver, kidney, muscle, and fat were 930, 1,060, 670, and 2930 ppb, respectively, after 1 day, and below 100 ppb on days 3 and 7 after dosing. 

Residue depletion studies in swine given an infeed medication of 75 mg thiophanate/kg bw/day showed mean thiophanate concentrations in liver, kidney, muscle, and skin/fat of 5550, 6600, 2600, and 16,200 ppb, respectively, at 1 day after dosing, and 180, 100, less than 100, and 250 ppb, respectively, at 3 days after dosing. However, residue concentrations were below 100 ppb in all tissues at 7 day withdrawal. 

In a study on pigs treated with fenbendazole at 5 mg/kg bw, a concentration of 0.28 ppm of the parent drug was found in the liver at 7 day withdrawal other tissues were free of detectable fenbendazole residues. Residue depletion studies in fenbendazole-treated cattle at 10 mg/kg bw showed the presence of 8.4 ppm of the parent drug in liver, 1.04 ppm in kidney, 0.47 ppm in muscle, and 0.95 ppm in fat at 2 day withdrawal however, at 7 day withdrawal, only liver was found to contain residues at a level of 0.67 ppm (10). 

A residue depletion study (15) in rainbow trout given both oral and bath treatments of fenbendazole, at a water temperature of 12 C, showed that the drug was partly metabolized to fenbendazole sulfoxide. Both fenbendazole and fenbendazole sulfoxide were found to accumulate in fish skin. However, both the parent drug and its metabolite were largely depleted within 96 and 24 h, respectively, posttreatment. Formation of the sulfone metabolite was not detected in any fish tissue. 

Residue depletion studies in dairy cattle and sheep showed that the proportions of the major sulfoxide, sulfone, and 2-aminosulfone metabolites of albendazole change dramatically over a period of days in the case of tissues or hours in the case of milk, with albendazole sulfoxide predominating at early time points and albendazole sulfone and albendazole-2-aminosulfone appearing later (16). From day 4 onwards, more than 95% of the residues in bovine liver and kidney was in the bound form, but tissue binding in sheep tissues was less extensive. 

Residue depletion studies in cattle, sheep, swine, and horses showed that residue levels in tissues rapidly decreased with time (27). Total residue levels in muscle and fat, although higher than in liver and kidneys, were generally lower than 0.1 ppm at 4 days postdosing. Levels of extractable residues were also lower than 0.1 ppm at 7 days after treatment. 

In cattle and sheep, rafoxanide is not metabolized to any detectable degree and residues in liver are detectable for weeks after its administration. Residue depletion studies in cattle given a single oral dose of 15 mg rafoxanide/kg bw showed that edible tissues are free of drug residues at 28 days postdosing. 

Nitroxynil has a tendency to bind strongly to proteins and therefore is retained in animal tissues and milk for long periods after its administration (8). Residue depletion studies in cattle subcutaneously treated with nitroxynil showed that kidney contained 252, 107, and 90 ppb, muscle 149-587, 89-131, and 50, and the injection site 90-504, 90-207, and 90 ppb of the parent drug... 

Other residue depletion studies of dimetridazole in chickens, turkeys, and swine generally showed that the concentrations of dimetridazole residues decreased to less than 0.1 ppm in the edible tissues of chickens at 1 day withdrawal, and to less than 2 ppm in the edible tissues of turkeys and swine at 2-day withdrawal (9, 10). 

Pharmacokinetic studies have shown that most salinomycin appears in feces in the form of an inactive metabolite that is subsequently degraded with a half-life of less than 50 h. Residue depletion studies in chickens showed wide variation in the concentrations of residues appearing in edible tissues. Some workers have reported levels of salinomycin residues as high as 1100 ppb in liver of chickens at 0 withdrawal (34, 35). In contrast, other workers reported residue levels as low as 3 ppb in the same tissue under similar experimental conditions (36). 

Toltrazuril residue depletion studies in chickens orally given 14.1 mg/kg bw/day for 2 consecutive days per week, three times at a week interval, showed that the concentrations of the parent drug were 342, 1845, 870, 1332, and 1077 ppb in muscle, fat, skin, liver, and kidney, respectively, 1 day after the cessation of treatment. These concentrations declined thereafter to reach, at 6 days after dosing, 10, 81, 33, 22, and 15 ppb in muscle, fat, skin, liver, and kidney, respectively. At 10 days postdosing, 24 ppb were found in fat and 11 ppb in skin, whereas other tissues did not contain detectable residue levels. 

Residue depletion studies in young pigs fed carbadox-supplemented rations for 1 week showed the parent compound to be present at 20 ppb in blood, and at 26 ppb in muscle tissue at 24 h withdrawal residues were reduced to less than 2 ppb at 48 h, and eliminated at 72 h (15). Desoxycarbadox, although not detected in blood, could be detected in muscle at the 17 ppb level at 24 h withdrawal to be reduced, subsequently, to 9 ppb at 48 h and to below the detection limit at 72 h. Whereas only traces of carbadox were found in kidney at 24 h withdrawal. 

Unlike in calves, the nature of the major liver metabolites was identified in steers the -o-glucopyranoside of estradiol-17 was found to be a major metabolite, whereas the 3- -o-glucosiduronate of estradiol-17 and other 17-glucosides of estradiol-17 and estradiol-17 were found to be minor ones (3). Residue depletion studies in steers implanted for 70-180 days with controlled-release implants containing 24 mg estradiol-17 showed that 24 h after implant removal the concentrations of residual estradiol-17 and estrone were 4.0 and 4.0 ppt in muscle, 5.0 and 4.7 ppt in liver, 7.5 and 7.1 ppt in kidney, and 7.1 and 14.3 ppt in the fat, respectively. These concentrations of residual estradiol-17 and estrone in the incurred samples were very close to those in the control tissues, which accounted for 5.8 and 4.8 ppt in muscle, 4.0 and 6.5 ppt in liver,... 

Similarly to other endogenous sex steroids, residue levels of progesterone in edible tissues of treated animals were very low. Residue depletion studies (5) in steers showed progesterone levels of 0.4 ppb in muscle, liver, and kidney, and... 

Residue depletion studies (1) using melengesfiol acetate as marker residue and fat as marker tissue have demonstrated that residues in fat remained well... 

Melanin-binding studies have shown that the affinity of salbutamol for melanin-containing tissues should be lower than that of clenbuterol (40, 23). A residue depletion study in calves orally treated with 1 mg salbutamol/kg bw/day for 7 days showed that liver residues of salbutamol were almost 4 ppm at 0 withdrawal, falling to about 0.11 ppb after a 7 day withdrawal period (37). 

Residue depletion studies in pigs treated intramuscularly with 15 mg suxibuzone/kg bw for 3 days showed that suxibuzone was present in muscle and at the injection site only at 1 day after the last dose. Phenylbutazone was present in muscle tissue up to 3 days, in liver up to 6 days, and at the injection site up to day 10. Oxyphenbutazone was also present in all tissues up to 3 days after the last dose. 

Chemical analytical methods used in veterinary drug residue depletion studies in target animals constitute a potential source of suitable methods for determining compliance of tissue residues with established MRLs. In some situations. 

The development of veterinary products for cattle, pigs, sheep, poultry and other food producing animals also includes residue testing of all new pharmaceutical products, including adjuvants or other excipients in vaccines. Residue safety studies address the potential risk to humans due to the consumption of food from treated animals. Accordingly, the test substances in these toxicity studies are applied orally. Residue depletion studies (pharmacokinetic studies) in the target species must be carried out to define the occurence, concentration and elimination of the substance and its metabolites in edible tissues, milk and eggs. 

Radiolabeled residue depletion studies in target animals from zero withdrawal time to periods beyond the recommended withdrawal time (these studies should provide information on total residues, including free and bound residues, and major residue components in order to select a marker residue and target tissue). 

---