Plasma ronidazole concentrations were examined after intravenous (i.v.) and oral administration of ronidazole in sheep (n = 6) at a dosage of 5 mg/kg body weight. In three sheep a ruminal and an abomasal fistula were inserted. The ronidazole determinations were performed by an HPLC method. Oral bioavailability in the fistulated sheep was only 5.5 +/- 1.8% (mean +/- SE). Somewhat lower values (4.6 +/- 1.4%) were obtained when the drug was administered through the ruminal fistula in the rumen. After administration of the same dose directly in the abomasum through the intraabomasal fistula, bioavailability was increased to 86.0 +/- 8.9%. In the non-fistulated sheep, oral biodisponibility was 2.6 +/- 0.5%. After water was restricted for 48 h before the oral ronidazole administration to these sheep, bioavailability was slightly increased (6.0 +/- 3.1%). When desmopressin acetate was injected i.v. before the oral ronidazole administration, bioavailability was 10.6 +/- 6.5%. When glypressin, another vasopressin analogue, was used, oral bioavailability was not influenced: 2.4 +/- 1.3%. Ronidazole was also incubated with ruminal contents and the ronidazole concentration decreased with a first order rate constant of 0.122 +/- 0.050 min-1 (mean +/- SE). These results suggest that oral administration of ronidazole to sheep is of little therapeutic use, because most is metabolised by the ruminal micro-organisms before it can reach the circulation. A second conclusion we can make is that it is very difficult, if not impossible, at least with the methods used, to influence gastro-intestinal motility in sheep to get a reproducible closure of the oesophageal groove.
A high-performance liquid chromatographic method to measure urinary indomethacin levels is described. In 0.5 ml urine, 1 micrograms/ml of indomethacin could be detected. Alkaline hydrolysis of urine resulted in the decomposition of indomethacin. When two suppositories of Indocid corresponding to 200 mg indomethacin were administered rectally to four horses the drug was rapidly absorbed and remained detectable in urine from 1 to 12 h. The excretion rate peaked after 2-3 h while the maximal concentration ranged from 18.5 to 80.6 micrograms/ml. Only 8 to 16% of the indomethacin dose was eliminated in urine after 12 h. A fraction of the dose was excreted as the glucuronide conjugate.
A high performance liquid chromatographic method is described to determine the anti-inflammatory drug suxibuzone (SXB) and its major metabolites phenylbutazone (PBZ) and oxyphenbutazone (OPBZ) in equine plasma and urine. When suxibuzone (6 mg/kg) was administered intravenously (i.v.) or orally (p.o.) no parent drug was detected in plasma or in urine. The disposition of the metabolite PBZ (i.v.) could be described by a 2 compartment model with a beta half-life varying from 7.40 to 8.35 h. Due to severe side effects the use of i.v. suxibuzone should not be encouraged in the horse. PBZ and OPBZ were detected in plasma and urine after p.o. SXB administration. Peak plasma PBZ concentrations (8.8 +/- 3.0 micrograms/ml) occurred 6 h after oral dosing and the terminal exponential constant was 0.11 +/- 0.01 h-1. Phenylbutazone and oxyphenbutazone were detectable in urine (> 1 microgram/ml) for at least 36 h, after p.o. administration. SXB was not hydrolyzed in vitro by horse plasma. Equine liver homogenates however appeared to have a very high capacity for hydrolysing SXB, indicating that first-pass effect could be responsible for the rapid disappearance of this NSAID in the horse.
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