盐酸克伦特罗在羊主要脏器中残留量消除规律的研究

本试验对盐酸克伦特罗在休药期肉羊眼睛、心脏、肾脏、肺脏、脾脏等组织中的残留规律进行了研究。选择24头体重为(30±5)kg健康肉羊进行试验,在饲料中添加50 μg/kg盐酸克伦特罗,连续饲喂35 d后休药,通过液相色谱—质谱联用/质谱检测休药期肉羊组织中克伦特罗含量,研究其残留量消除规律。试验结果表明,肉羊眼睛中有高浓度的盐酸克伦特罗残留且其在肝脏中消除较慢;停药第14天眼睛中盐酸克伦特罗的浓度仍为42.42～63.48 μg/kg,脾脏中盐酸克伦特罗消除速度是最快的;停药3 d时,检测不到盐酸克伦特罗的残留量(低于检出限0.07 μg/kg),故眼睛可用作检测盐酸克伦特罗在肉羊生产上非法使用的靶标。     

超高效液相色谱-串联质谱法研究克伦特罗在绒山羊体内消除规律

建立了超高效液相色谱-串联质谱法并对克伦特罗（Clenbuterol）在绒山羊体内残留消除规律进行了研究。选取60只健康绒山羊（30±4．8）kg,平均分成2组,分别在饲料中添加150μg／kgb．w．（A组）和300μg／kgb．w．（B组）克伦特罗,连续饲喂25d后停药。建立UPLC—MS／MS检测方法监测喂药期尿液、休药期尿液和组织中克伦特罗含量。克伦特罗在饲喂期间,A组尿液中平均含量为289～5313μg／L,B组尿液中平均含量为1323～11378μg／L。停药后尿液中克伦特罗含量迅速下降,停药40d后,A、B两组均下降至10μg／L以下;停药后,取绒山羊肝脏、肾脏、肺脏、肌肉测定其残留量。A、B两组停药后组织中克伦特罗含量持续下降,停药40d后,肝脏含量33μg／kg,肾脏约5μg／kg,其余组织均未检出。     

Clenbuterol in the horse: urinary concentrations determined by ELISA and GC/MS after clinical doses

Clenbuterol is a beta2 agonist/antagonist bronchodilator marketed as Ventipulmin and is the only member of this group of drugs approved by the US Food and Drug Administration (FDA) for use in horses. Clenbuterol is a class 3 drug in the Association of Racing Commissioners International (ARCI) classification system; therefore, its identification in postrace samples may lead to sanctions. Recently, the sensitivity of postrace testing for clenbuterol has been substantially increased. The objective of this study was to determine the 'detection times' for clenbuterol after administration of an oral clinical dose (0.8 g/kg, b.i.d.) of Ventipulmin syrup. Five horses received oral clenbuterol (0.8 g/kg, b.i.d.) for 10 days, and urine concentrations of clenbuterol were determined by an enhanced enzyme-linked immunoabsorbent assay (ELISA) test and gas chromatography/mass spectrometric (GC/MS) analysis by two different methods for 30 days after administration. Twenty-four hours after the last administration, urine concentrations of apparent clenbuterol, as measured by ELISA, averaged about 500 ng/mL, dropping to about 1 ng/mL by day 5 posttreatment. However, there was a later transient increase in the mean concentrations of apparent clenbuterol in urine, peaking at 7 ng/mL on day 10 postadministration. The urine samples were also analysed using mass spectral quantification of both the trimethylsilyl (TMS) and methane boronic acid (MBA) derivatives of clenbuterol. Analysis using the TMS method showed that, at 24 h after the last administration, the mean concentration of recovered clenbuterol was about 22 ng/mL. Thereafter, clenbuterol concentrations fell below the limit of detection of the TMS-method by day 5 after administration but became transiently detectable again at day 10, with a mean concentration of about 1 ng/mL. Derivatization with MBA offers significant advantages over TMS for the mass spectral detection of clenbuterol, primarily because MBA derivatization yields a high molecular weight base peak of 243 m/z, which is ideal for quantitative purposes. Therefore, mass spectral analyses of selected urine samples, including the transient peak on day 10, were repeated using MBA derivatization, and comparable results were obtained. The results show that clenbuterol was undetectable in horse urine by day 5 after administration. However, an unexpected secondary peak of clenbuterol was observed at day 10 after administration that averaged approximately 1 ng/mL. Because of this secondary peak, the detection time for clenbuterol (0.8 g/kg, b.i.d. x 10 days) is at least 11 days if the threshold for detection is set at 1 ng/mL.     

Distribution and elimination of clenbuterol in tissues and fluids of calves following prolonged oral administration at a growth-promoting dose

A pharmacokinetic study is described in which Friesian calves (n = 30) were treated orally with clenbuterol at 10 times the therapeutic dose. The study was designed to establish the distribution and elimination of clenbuterol from edible tissues, the major compartments of the eye and body fluids. Animals (n= 24) were dosed (10 μg/kg body weight) twice daily with clenbuterol for 21 days and slaughtered in groups of five (one untreated control animal per group) at 6 h and 1, 2, 4, 8 and 16 days after cessation of treatment At slaughter, samples of diaphragm muscle, liver, kidney, bile, urine and both eyes were obtained. One of the eyes was separated into constituent tissues: aqueous humour, vitreous humour, comea, lens, retina (without pigmented epithelium), choroid (with pigmented retinal epithelium; choroid/PRE) and sclera. All samples were stored at -20°C. Clenbuterol concentrations were higher in liver than kidney, bile and urine from day 2 of withdrawal onwards. Concentrations in choroid/PRE were at least 10 times higher than in liver at all periods following cessation of treatment and 52 times higher 16 days after treatment The concentrations of clenbuterol in the constituent tissues of the eye were in the order choroid/PRE > comea > sclera > retina > aqueous humour/vitreous humour ≥ lens. Concentrations of clenbuterol in choroid/PRE taken from eyes frozen whole were generally lower than those in choroid/PRE separated before storage. Choroid/PRE stored by either method contained clenbuterol at more than 100 ng/g 16 days following cessation of treatment These data suggest that analysis of choroid taken at the abattoir would provide reliable surveillance information on the use or abuse of clenbuterol within the slaughter population. It is proposed that liver samples taken concurrently could be used in the event of a positive result from choroid/PRE analysis to indicate whether the maximum residue limit for edible tissues has been exceeded.     

Tissue concentrations and pharmacokinetics of florfenicol in male veal calves given repeated doses

The pharmacokinetic disposition of florfenicol was studied in male veal calves given 11 mg of florfenicol/kg of body weight, IV and 11 mg of florfenicol/kg PO every 12 hours for 7 doses. After florfenicol administration IV, the median elimination half-life was 222.8 minutes, whereas the median half-life of the distribution phase was 7.94 minutes. Median body clearance and apparent volume of distribution were 2.87 ml/kg/min and 0.907 L/kg, respectively. After florfenicol administration, PO, there was a wide variation in the calculated half-life, which was attributed to variation in the rate of florfenicol absorption. The half-life was 167.4 to 534.9 minutes after the first oral dose and 190 to 808.8 minutes after the seventh dose. The median bioavailability after the first oral dose was 0.8888. Peak and trough concentrations of florfenicol were increased after subsequent doses were administered, compared with those after the first oral dose. The percentage of protein binding in serum from one adult cow was 22% to 26%. Florfenicol concentrations in tissues and body fluids of male veal calves were studied after the seventh dose of 11 mg of florfenicol/kg. High concentrations of florfenicol were measured in the urine, kidney, and bile. Low concentrations were measured in the brain, CSF, and aqueous humor. Concentrations in all other tissues and fluids studied were similar to the concurrent serum concentration.     

Pharmacokinetics of florfenicol in veal calves

The pharmacokinetic disposition of florfenicol was described in veal calves after administration of a single 22-mg/kg dose intravenously, orally after a 12-h fast and orally 5 min post feeding. Both serum concentrations and urinary excretion were studied. After intravenous administration the median elimination half-life was 171.9 min while the half-life of the distribution phase was 5.9 min. The median body clearance (Cl) and apparent volume of distribution (Vz) were 2.85 ml/kg/min and 0.78 l/kg, respectively. Following oral administration the median bio-availability (f) was 0.88 for calves dosed after a 12-h fast and 0.65 for calves dosed 5 min post feeding. Calves given the oral doses had a complex absorption pattern with delayed absorption. Slightly more than 50% of the administered dose both orally and intravenously was recovered as unchanged florfenicol in the urine by 30 h.     

Kinetics and disposition of orally dosed sodium chlorate in sheep

Experiments were conducted in sheep to determine excretory characteristics of sodium chlorate after a single oral dose. In Exp. 1, lambs (n = 16; age = 8.1 ± 1.7 d; BW = 8.2 ± 1.1 kg; mean ± SD) were dosed orally with 0, 30, 60, or 90 mg/kg BW of sodium chlorate. Twenty-four hours after exposure chlorate residues were dose dependent (P < 0.05) in small intestinal contents, serum, and urine, but chlorate residues were not consistently detected in cecal or colonic contents. In Exp. 2, non-pregnant yearling ewes (BW = 74.8 ± 5.6 kg; mean ± SD) were orally dosed with 0, 150, 300, or 450 mg/kg BW of sodium chlorate. Across dose, chlorate residues averaged from 47 to 114, 0.6 to 4.5, and were not detectable to 0.2 μg/mL at 24, 48, and 72 h, respectively, in serum of treated animals; in feces, residues averaged 29 to 82, 0.8 to 14, and were not detectable to 1.2 μg/mL at the same respective time periods. In Exp. 3, six lactating ewes (BW = 76.3 ± 8.0 kg) were dosed orally with 450 mg/kg BW of sodium chlorate; residues were measured in serum, milk, urine and feces in periods encompassing 0 to 8, 8 to 16, 16 to 24, 24 to 32, 32 to 40, and 40 to 48 h. Chlorate residues in milk were detectable at all time periods with concentrations averaging from 287 ± 67 to 26 ± 13 μg/mL during the first and last collection periods, respectively. Urine contained the greatest concentration of chlorate at each time point and averaged 480 ± 268 μg/mL at 40 to 48 h. Depletion half-lives in serum, milk, urine, and feces were estimated to be 6.2, 27, 19, and 10 h, respectively; milk, urinary and fecal half-lives are likely overestimated due to the fact that 8-h sample pools were used in half-life estimations. In Exp. 4, three wethers (BW = 87.1 ± 5.3 kg) each were orally dosed with 14 or 42 mg/kg BW of sodium chlorate; blood samples were serially collected for 48 h, and urine samples were collected at 0 to 8, 8 to 16, 16 to 24, 24 to 36, and 36 to 48 h. Estimates of absorption and elimination half-lives based on serum chlorate concentrations were about 0.4 and 2.5 h, respectively. Urine collected during the 6 h immediately following dosing contained the greatest concentrations of chlorate residues relative to subsequent collection periods. Rapid removal of chlorate from the gastrointestinal lumen suggests that effects of chlorate on colonic and fecal gastrointestinal bacteria may occur through mechanisms other than direct luminal contact between microbe and chlorate salts.     

Pharmacokinetics of phenoxymethyl penicillin (penicillin V) in calves

Phenoxymethyl penicillin (penicillin V) was administered intravenously (i.v.) and orally to pre-ruminant calves and the distribution and elimination kinetics, as well as the oral bioavailability, were determined. After i.v. injection, the drug was distributed rapidly in the body, the elimination half-life (t1/2 beta) was 34 min and the apparent volume of distribution at steady-state (Vd ss) was 0.30 l/kg. Mean peak serum drug concentrations were directly related to the oral dose administered, i.e. 0.22 microgram/ml, 1.06 micrograms/ml and 2.14 micrograms/ml after dosing at 10, 20 and 40 mg/kg, respectively. The elimination t1/2 of the drug after oral dosing varied between 90 and 110 min, and the oral bioavailability was approximately 30% of the dose. The co-administration of phenoxymethyl penicillin and probenecid resulted in elevation and prolongation of serum drug concentration. The percentage of drug bound to serum proteins was 78.8% +/- 8.2%. Phenoxymethyl penicillin was probably inactivated and degraded in the gastrointestinal tract of 6-week-old calves fed exclusively hay, silage and concentrates as very low and erratic serum drug concentrations were measured after these calves were dosed orally with the drug at 40 mg/kg. In view of the narrow antibacterial spectrum of the drug and the relatively high dose required, it appears that phenoxymethyl penicillin can only be of limited practical value for the treatment of bacterial infections in preruminant calves.     

Tissue disposition and depletion of penicillin G after oral administration with milk in unweaned dairy calves

OBJECTIVE: To determine tissue depletion of penicillin G in calves after oral ingestion with milk replacer and estimate a withdrawal period. DESIGN: Longitudinal controlled trial. ANIMALS: 26 Holstein calves. PROCEDURE: Once daily, 24 calves were fed milk replacer containing procaine penicillin G (0.68 mg/kg [0.31 mg/lb] of body weight); 2 calves served as controls. After 1 feeding, 12 calves were euthanatized in groups of 3 each 4, 6.5, 9.5, and 13 hours after feeding. After 14 days, 12 calves were euthanatized in groups of 3 each 4, 6.5, 9.5, and 13 hours after the final feeding. Concentrations of penicillin G were determined in tissues, blood, and urine by use of high-performance liquid chromatography. RESULTS: Penicillin G was not detected in muscle samples of treated calves. The highest concentrations of penicillin G in plasma, kidney, and liver were 13 ng/ml, 92 ng/g, and 142 ng/g, respectively. Thirteen carcasses had violative drug residues; 12 had violative residues in the liver only, and 1 had violative residues in the liver and kidney. A 21-hour withdrawal period was estimated. CONCLUSIONS AND CLINICAL RELEVANCE: Liver had the highest concentration of penicillin G and was most likely to have violative residues. Feeding calves milk containing penicillin G has the potential to cause violative drug residues in tissues. It is recommended to observe an appropriate withdrawal time prior to slaughter if calves are fed milk from cows treated with penicillin G.     

Oxytetracycline pharmacokinetics, tissue depletion, and toxicity after administration of a long-acting preparation at double the label dosage

Oxytetracycline (OTC) concentration in plasma and tissues, plasma pharmacokinetics, depletion from tissue, and toxicity were studied in 30 healthy calves after IM administration of a long-acting OTC preparation (40 mg/kg of body weight) at double the label dosage (20 mg/kg). Plasma OTC concentration increased rapidly after drug administration, and by 2 hours, mean (+/- SD) values were 7.4 +/- 2.6 micrograms/ml, Peak plasma OTC concentration was 9.6 +/- 2.6 micrograms/ml, and the time to peak plasma concentration was 7.6 +/- 4.0 hours. Plasma OTC concentration decreased slowly for 168 hours (elimination phase) after drug administration, and the elimination half-life was 23.9 hours. Plasma OTC concentration exceeded 3.8 micrograms/ml at 48 hours after drug administration. From 168 to 240 hours after drug administration, plasma OTC concentration decreased at a slower rate than that seen during the elimination phase. This slower phase was termed the depletion phase, and the depletion half-life was 280.7 hours. Tissue OTC concentration was highest in kidneys and liver. Lung OTC concentration exceeded 4.4 micrograms/g of tissue and 2.0 micrograms/g of tissue at 12 and 48 hours after drug administration, respectively. The drug persisted the longest in kidneys and liver. At 42 days after drug administration, 0.1 micrograms of OTC/g of kidney was detected. At 49 days after drug administration, all OTC tissue concentrations were below the detectable limit. Reactions and toxicosis after drug administration were limited to an anaphylaxis-like reaction (n = 1) and injection site swellings (n = 2).     

Changes in lymphocyte glucocorticoid and beta-adrenergic receptors in veal calves treated with clenbuterol and steroid hormones for growth-promoting purposes

In order to identify possible peripheral markers of illegal treatments with growth-promoting agents in veal calves, beta-adrenergic receptor (beta-AR) and glucocorticoid receptor (GR) concentrations were measured in lymphocytes of 12 male Friesian crossbred calves (six controls and six treated). The animals received a cocktail of anabolic and re-partitioning agents [17beta-oestradiol: 3 x 10 mg intramuscular (i.m.) doses at 17-day intervals; dexamethasone sodium phosphate: 4 mg/day for 6 days and 5 mg/day for six further days dissolved in milk; and clenbuterol: 20 microg/kg/day dissolved in milk for the last 40 days before slaughter]. Blood samples were collected by venipuncture at different time points and lymphocytes were isolated by density gradient centrifugation. Lymphocyte beta-AR and GR levels were measured by binding assays. Treatment with re-partitioning agents caused a significant down-regulation of lymphocyte beta-ARs 19 days after the beginning of clenbuterol administration and at day 55 (after dexamethasone withdrawal, just before slaughter). This phenomenon was partially reversed at day 50, after dexamethasone administration, at which time a significant decrease in GR concentrations also occurred. For both types of receptors, no significant changes in the dissociation constant values were observed at any time point. Lymphocytes express measurable concentrations of beta-ARs and GRs and the measurement of receptor levels highlights the fluctuation of receptor expression due to the dynamic interaction of the drugs used in combination. Lymphocyte receptor determination could therefore be included in a battery of biological assays to detect illegal treatments with anabolic agents in veal calves in the light of a multivariate approach.     

Effect of human growth hormone-releasing factor and(or) thyrotropin-releasing factor on growth, carcass composition, diet digestibility, nutrient balance, and plasma constituents in dairy calves

Sixty male dairy grain-fed calves, raised from 70 to 223 kg BW in individual crates, were used in a 2 X 2 factorial arrangement to determine the effect of administration of human growth hormone-releasing factor (1-29)NH2 (GRF) and(or) thyrotropin-releasing factor (TRF). Calves received twice-daily s.c. injections of .9% NaCl (control), GRF (5 micrograms/kg BW), TRF (1 micrograms/kg BW) or GRF (5 micrograms/kg BW) plus TRF (1 micrograms/kg GTRF). Average daily gain and days on feed were not affected by treatments, but TRF treatment increased (P less than .05) total intake of dry matter (DM) and feed conversion ratio: 3.00, 3.02, 3.08, and 3.22 kg DM/kg weight gain for control, GRF, TRF, and GTRF, respectively. During two 7-d periods, after 66 and 75 d of treatment, feces and urine were collected from 40 calves (5 per treatment per period). Treatment with GRF increased (P less than .05) digestibility of DM, nitrogen (N), and energy and tended (P less than .20) to increase N retention. At slaughter, withers height was increased (P = .05) by GRF and carcass length was increased (P less than .05) by TRF. Pituitary and liver weights were increased (P less than .05) by TRF. The combination of GRF and TRF slightly increased (P less than .10) protein content and decreased (P less than .05) fat content of the 9-10-11th rib section. After d 1, GRF treatment chronically increased (P less than .05) insulin concentrations and also increased (P less than .10) IGF-I concentrations on d 29 and 57. In summary, chronic treatment with GRF and(or) TRF did not improve growth or efficiency, although GRF increased digestibility of DM, N, and energy and the GRF plus TRF combination resulted in slightly leaner carcasses.     

Amikacin sulfate in mares: pharmacokinetics and body fluid and endometrial concentrations after repeated intramuscular administration

Six mares were given 5 IM injections (at 12-hour intervals between doses) of amikacin sulfate at a dosage of 7 mg/kg of body weight. Serum amikacin concentrations were measured serially throughout the study; synovial, peritoneal, endometrial, and urine concentrations were determined after the last injection. Amikacin concentrations of the CSF were measured serially in 3 of the 6 mares; 1 of the 3 mares had septic meningitis. Mean serum amikacin concentrations peaked at 1 to 2 hours after IM injection. The highest mean serum concentration was 19.2 micrograms/ml (1.5 hours after the 5th injection). The highest mean synovial concentration was 10.8 micrograms/ml at 2 hours after the 5th injection; the highest mean peritoneal concentration was 16.2 micrograms/ml at 3 hours after the 5th injection. The mean endometrial amikacin concentration was 2.5 micrograms/g (1.5 hours after the 5th injection). Amikacin reached a CSF concentration of 0.97 micrograms/ml in the mare with meningitis, but amikacin was not detected in CSF of healthy mares. Urine concentrations reached 1,458 micrograms/ml. Pharmacokinetic values were estimated after the 1st injection (elimination rate constant = 0.31/hour; half-life = 2.3 hours; apparent volume of distribution = 0.26 L/kg), and after the 5th injection (elimination rate constant = 0.28/hour; half-life = 2.6 hours; apparent volume of distribution = 0.30 L/kg); significant differences were not observed.     

The effect of age and diet on sulfadiazine/trimethoprim disposition following oral and subcutaneous administration to calves

Thirty milligrams per kilogram of sulfadiazine/trimethoprim (SDZ/TMP, Tribrissen) was given orally and subcutaneously (s.c.) to two groups of male, Holstein calves. One group was fed milk-replacer throughout the 13-week period of the study while the second group was weaned onto a chopped grain-fiber mixture when 5 weeks old. Serum and urine were assayed for concentrations of unchanged drug. Trimethoprim bioavailability, following oral administration at 1, 6 and 12 weeks of age, is higher in milk-fed calves (non-ruminants) than in grain-fiber-fed calves (ruminants); bioavailability decreases with increasing age in both groups of calves. Serum concentrations above 0.1 micrograms/ml (the level of sensitivity of the assay) could not be obtained in ruminating calves. The rate of SDZ absorption following oral administration, as determined by the Wagner-Nelson method, was very slow in all the calves in this study with average half-life values ranging from 8.2-12.67 h; absorption was slightly faster in ruminating calves. Absorption of SDZ is rate-limiting and determines the biological half-life of the drug; SDZ serum concentrations above 2 micrograms/ml were maintained in all calves for at least 24 h. Following s.c. administration of Tribrissen to 7-and 13-week-old calves, urinary excretion patterns indicated that TMP was slowly released from the injection site; serum concentrations were below 0.1 micrograms/ml. In contrast, absorption of SDZ was very rapid; values for tmax were 1.5-1.8 h. The pharmacokinetic parameters for SDZ were calculated according to a one-compartment open model; neither diet nor age had a significant effect on SDZ disposition following s.c. injection. Subcutaneous administration of 30 mg/kg Tribrissen, b.i.d., may be the best therapeutic regimen; even though measureable concentrations of TMP cannot be achieved in the serum following a single s.c. dose, TMP concentrations should accumulate and, because of its sustained release, provide almost continual potentiation of SDZ.     

Pharmacokinetics and body fluid and endometrial concentrations of cephapirin in mares

Six healthy adult horse mares were each given a single injection of sodium cephapirin (20 mg/kg of body weight, IV), and serum cephapirin concentrations were measured serially over a 6-hour period. The mean elimination rate constant was 0.78 hour-1 and the elimination half-life was 0.92 hours. The apparent volume of distribution (at steady state) and the clearance of the drug were estimated at 0.17 L/kg and 598 ml/hour/kg, respectively. Each mare was then given 4 consecutive IM injections of sodium cephapirin (400 mg/ml) at a dosage level of 20 mg/kg. Cephapirin concentrations in serum, synovial fluid, peritoneal fluid, CSF, urine, and endometrium were measured serially. After IM administration, the highest mean serum concentration was 14.8 micrograms/ml 25 minutes after the 4th injection. The highest mean synovial and peritoneal concentrations were 4.6 micrograms/ml and 5.0 micrograms/ml, respectively, 2 hours after the 4th injection. The highest mean endometrial concentration was 2.2 micrograms/g 4 hours after the 4th injection. Mean urine concentrations reached 7,421 micrograms/ml. Cephapirin did not readily penetrate the CSF. When cephapirin was given IM at the same dose, but in a less concentrated solution (250 mg/ml), serum concentrations peaked at 25.0 micrograms/ml 20 minutes after injection, but the area under the serum concentration-time curve was not significantly different (P greater than 0.05). The bioavailability of the drug was greater than or equal to 95% after IM injection.     

Pharmacokinetics of sulphanilamide and its three acetyl metabolites in dairy cows and calves

An intravenous low dosage of sulphanilamide (SAA) (14.0 mg/kg) to 6 pre-ruminant calves revealed a biphasic SAA plasma disposition with a mean elimination half-life of 4.1 h. The main metabolite in plasma was N4-acetylsulphanilamide (N4), which 4 hours after injection exceeded the parent SAA plasma concentration. Urinary recovery of SAA was 10 to 16% of the dose; of N4, it was at least 69%. Traces of the N1-acetyl (N1) metabolite and the doubly acetylated derivative (N1N4) were present in urine. The renal clearances of the N1 and N4 metabolites showed a tubular secretion pattern, which was at least 2 to 6 times higher than that of SAA. A single high oral SAA dose of 200 mg/kg to 3 dairy cows resulted in extensive metabolism of SAA into N4, N1, and N1N4 metabolites; their mean maximum plasma concentrations were 64, 48, 0.72 and 24 micrograms/ml, respectively. The mean disposition half-life of SAA in plasma and milk was 10 h. In milk the metabolite concentrations exceeded those in plasma; the N4 and N1N4 metabolite concentrations in milk exceeded that of SAA. The mean maximum concentrations of SAA, N4, N1, and N1N4 in milk were 52, 89, 2.3, and 98 micrograms/ml, respectively. For SAA and its metabolites, the binding to plasma and milk proteins was determined. No glucuronide or sulphate conjugates of SAA and its acetyl metabolites could be found in plasma, milk, or urine. Based on the sensitivity of the bioassay (0.2 micrograms SAA/ml), a withholding time of 5 days was suggested for milk following single oral SAA dosage of 200 mg/kg.     

Pharmacokinetics of chloramphenicol in calves during the first weeks of life

The pharmacokinetics of chloramphenicol, either administered as the monosuccinate ester or as a veterinary formulation, were studied in calves from the first day of life to the age of 10–12 weeks and compared with the results obtained in adult cattle. (1) After intravenous injection of 0.15 mmol/kg chloramphenicol monosuccinate, the plasma elimination half life of intact ester fell from a value of 33 min on the first day of life to 15 min at the age of 10–12 weeks (value in cows = 14 min). Free chloramphenicol reached maximal plasma concentrations after 2–3 h on the first day of life, but in less than 15 min in cows. The elimination half-life fell from about 15 h on day 1 to 4.8 h at the age of 10–12 weeks (4.2 h in cows). The bioavailability of the ester was more than 90% on Day 1, but declined to 50–60% from Day 7 on account of rapid renal excretion: 21–28% of the total dose was excreted as intact ester in a 2 h period following injection in calves aged 10–12 weeks. (2) The veterinary formulation of chloramphenicol proved toxic when administered intravenously at a dose of 0.15 mmol/kg, and even a dose of 0.093 mmol/kg was less well tolerated than 0.15 mmol/kg of the monosuccinate ester. (3) The pharmacokinetics of chloramphenicol fitted an open two-compartment model, the half-life of the elimination phase corresponded well to the values determined in the experiments with the monosuccinate ester. (4) The intramuscular injection of 0.15 mmol/kg of the ester or 0.093 mmol/kg of chloramphenicol provided ‘therapeutic’ plasma concentrations (≥ 5 μg/ml) within 15–30 min and for about 24 h in calves aged 7 days. (5) Chloramphenicol crossed the placenta when given to cows shortly before a Caesarian section, but equilibrium was not reached within 50–100 min. (6) The binding of chloramphenicol to serum proteins was dependent both on total protein and drug concentrations. It rose from less than 30% on day 1 to about 40% in adult cattle. (7) Recommendations for a dosage regime for chloramphenicol in calves are made on the basis of the pharmacokinetic data.     

Pharmacokinetics and body fluid and endometrial concentrations of cefoxitin in mares

Four healthy adult mares were each given a single injection of sodium cefoxitin (20 mg/kg of body weight, IV), and serum cefoxitin concentrations were measured serially during a 6-hour period. The mean elimination rate constant was 1.08/hour and the elimination half-life was 0.82 hour. The apparent volume of distribution (at steady state) and the clearance of the drug were estimated at 0.12 L/kg and 259 ml/hr/kg, respectively. Each mare and 2 additional mares were then given 4 consecutive IM injections of sodium cefoxitin (400 mg/ml) at a dosage of 20 mg/kg. Cefoxitin concentrations in serum, synovial fluid, peritoneal fluid, CSF, urine, and endometrium were measured serially. After IM administration, the highest mean serum concentration was 23.1 micrograms/ml 30 minutes after the 2nd injection. The highest mean synovial concentration was 11.4 micrograms/ml 1 hour after the 4th injection. The highest mean peritoneal concentration was 10.4 micrograms/ml 2 hours after the 4th injection. The highest mean endometrial concentration was 4.5 micrograms/g 4 hours after the 4th injection. Mean urine concentrations reached 11,645 micrograms/ml. Cefoxitin did not readily penetrate the CSF. Bioavailability of cefoxitin given IM was 65% to 89% (mean +/- SEM = 77% +/- 5.9%). One of the 6 mares developed acute laminitis during the IM experiment.     

Pharmacokinetics of metronidazole and its concentration in body fluids and endometrial tissues of mares.

Serum concentrations of metronidazole were determined in 6 healthy adult mares after a single IV injection of metronidazole (15 mg/kg of body weight). The mean elimination rate (K) was 0.23 h-1, and the mean elimination half-life (t1/2) was 3.1 hours. The apparent volume of distribution at steady state was 0.69 L/kg, and the clearance was 168 ml/h/kg. Each mare was then given a loading dose (15 mg/kg) of metronidazole at time 0, followed by 4 maintenance doses (7.5 mg/kg, q 6 h) by nasogastric tube. Metronidazole concentrations were measured in serial samples of serum, synovia, peritoneal fluid, and urine. Metronidazole concentrations in CSF and endometrial tissues were measured after the fourth maintenance dose. The highest mean concentration in serum was 13.9 +/- 2.18 micrograms/ml at 40 minutes after the loading dose (time 0). The highest mean synovial and peritoneal fluid concentrations were 8.9 +/- 1.31 micrograms/ml and 12.8 +/- 3.21 micrograms/ml, respectively, 2 hours after the loading dose. The lowest mean trough concentration in urine was 32 micrograms/ml. Mean concentration of metronidazole in CSF was 4.3 +/- 2.51 micrograms/ml and the mean concentration in endometrial tissues was 0.9 +/- 0.48 micrograms/g at 3 hours after the fourth maintenance dose. Two mares hospitalized for treatment of bacterial pleuropneumonia were given metronidazole (15.0 mg/kg, PO, initially then 7.5 mg/kg, PO, q 6 h), while concurrently receiving gentamicin, potassium penicillin, and flunixin meglumine IV. Metronidazole pharmacokinetics and serum concentrations in the sick mares were similar to those obtained in the healthy mares.     

Single intravenous and multiple intramuscular dose pharmacokinetics and tissue residue profile of gentamicin in sheep

Single and multiple dose gentamicin regimens were compared in sheep to determine the relevant pharmacokinetic differences. Seven mature sheep were given 10 mg/kg of gentamicin by IV bolus. Serum concentrations were monitored for 19 days. Four weeks after the initial bolus, gentamicin was administered IM (3 mg/kg every 8 hours) for 7 days. Ewes were euthanatized and necropsied at 1, 8, and 15 days after termination of the IM regimen and the tissues were assayed for gentamicin. Serum concentrations were analyzed using a triexponential equation. The IV kinetic studies revealed an alpha half-life (t1/2) of 0.31 +/- 0.14 hours, beta t1/2 of 2.4 +/- 0.5 hours, and gamma t1/2 of 30.4 +/- 18.9 hours. Multiple IM dose kinetic studies revealed a beta t1/2 of 2.8 +/- 0.6 hours and gamma t1/2 of 82.1 +/- 17.8 hours. After multiple dosing, gamma t1/2 was significantly longer than after the single IV bolus (P less than 0.05). Twenty-four hour urine collection accounted for 75% to 80% of the total IV dose. Renal cortical gentamicin concentration reached 224 micrograms/g of tissue and then decreased, with a 90-hour t1/2. Renal medullary gentamicin concentration reached 18 micrograms/g with a 42-day t1/2. After multiple dosing, liver gentamicin concentration reached 11 micrograms/g and skeletal muscle concentrations were less than or equal to 0.6 micrograms/g. Route or duration of administration significantly affected the gamma-phase serum concentrations, which may influence gentamicin nephrotoxicosis. The present study also illustrated the complexities in predicting aminoglycoside withdrawal times for food-producing animals before slaughter.