Hormonal regulation of oxidative drug metabolism in the dwarf goat. The effect of sex and hormonal treatment on plasma disposition and metabolite formation of sulphadimidine

Oral pharmacokinetics of the acidic drugs, diclofenac and sulfamonomethoxine in male Shiba goats

In the present study, we examined the oral pharmacokinetics of the acidic drugs, diclofenac (DF) and sulfamonomethoxine (SMM), which have different physicochemical properties, in Shiba goats. DF and SMM were intravenously and orally administered to 5 male goats using a crossover design. The T_max of DF and SMM were reached 1.5 and 5.6 hr after they have been orally administered, respectively, and this was followed by their slow elimination. The elimination of both drugs was markedly faster after being intravenously rather than orally administered, which indicated flip-flop phenomena after the oral administration. The mean absorption times (MATs) of DF and SMM were 6 and 15 hr, respectively. This slow absorption may have been due to slow gastric emptying in goats. The large difference observed in MATs between DF and SMM may have been because DF, which is more lipophilic than SMM, was partly absorbed from the forestomach. Therefore, these results suggest that the absorption of highly lipophilic drugs from the forestomach may be markedly high in Shiba goats. In case of drugs whose elimination is quite fast, their efficacies may appear from the early stage after oral administration even in ruminants, because elimination rate is the determinant factor of T_max in flip-flop phenomena. Such drugs may be used orally even in ruminants.

Biotransformation Enzymes as Determinants of Xenobiotic Toxicity in Domestic Animals

After coming in contact with living organisms, the majority of foreign compounds undergo a number of chemical reactions known as biotransformations. These are performed by hepatic and extra-hepatic enzyme systems and usually yield more polar derivatives, referred to as 'metabolites', which may leave the body via the urinary and biliary routes or be excreted in animal products such as milk and eggs. Biotransformation does not always imply detoxification because in certain instances metabolites will be produced that are capable of reacting with tissue macromolecules or acquiring toxic properties different to or greater than those of the parent molecule. In this review, which is focused on domestic animals, the role played by oxidative, reductive, hydrolytic and conjugative biotransformation enzymes in the activation/detoxification of xenobiotics is examined. The relationship between extra-hepatic metabolism and target organ toxicity as well as the action of rumen microflora on feed additives, phytotoxins, and pesticides are then discussed. Some of the most important metabolic-based species-related susceptibilities to different poisons, and the influence of enzyme inducers or inhibitors on xenobiotic toxicity and drug safety are also reviewed.

Trends in veterinary clinical and fundamental pharmacology: past and future in The Netherlands

Veterinary pharmacology has undergone a gradual development in the Netherlands during this century. Starting from a historical perspective the paper aims to provide an overview of future trends and important issues in the area of veterinary pharmacology and toxicology. It is pointed out that this discipline comprises several subdisciplines as the comparative aspect of both, pharmacology and toxicology, is inherent to veterinary medicine which has to address a broad variety of animal species. Thus, the comparison of drug effects, side effects, and drug disposition as well as the comparison of the species-specific susceptibility to xenobiotics are obvious challenges in this discipline. Several areas in clinical pharmacology are highlighted to indicate future research needs. Finally, the principles of Good Veterinary Practice are presented as the 'golden standard' in veterinary clinical pharmacology.

Age-related differences in norfloxacin pharmacokinetic behaviour following intravenous and oral administration in sheep

The pharmacokinetics of norfloxacin after intravenous (i.v.) and oral (PO) administration in lambs (n = 5) and adult sheep (n = 5) were studied. After i.v. administration (10 mg.kg-1) plasma concentrations were best fitted by a three-compartment open model in both age groups. Distribution volumes were significantly larger in lambs (approximate 4.0 fold difference between 4 week old and adult sheep). There was no significant difference (p < 0.05) between the groups in terms of elimination halflife but plasma clearance was significantly higher in lambs. Norfloxacin was poorly absorbed after oral administration (60 mg.kg-1) in sheep (F = 4.04%). Mean oral bioavailability was 73.51% in lambs (30 mg.kg-1). Norfloxacin elimination was faster in lambs after oral administration. MRTt was significantly prolonged in both age groups when compared with the respective data for i.v. administration.

In vitro and in vivo oxidative biotransformation in the West-African dwarf goat (Caprus hircus aegagrus): substrate activities and effects of inducers

1. Cytochrome P450 activities in vivo and in vitro and enzyme induction by phenobarbital, beta-naphthoflavone, isoniazid and triacetyloleandomycin were investigated in the female dwarf goat. In vivo kinetics of antipyrine, sulphadimidine and caffeine were studied separately and as a combination ("cocktail'). After establishing a lack of interaction between these compounds the effects of the inducing agents were investigated. In vitro, hepatic microsomal enzyme activities and apoprotein levels were determined. 2. In the beta-naphthoflavone treated goat, the microsomal ethoxy-resorufin-O-deethylation rate was markedly increased. beta-naphthoflavone also induced caffeine plasma clearance but did not affect microsomal caffeine 1- and 3-demethylation rates. After phenobarbital treatment, caffeine plasma clearance was also increased. In contrast with beta-naphthoflavone treatment, phenobarbital treatment resulted in an increase of microsomal caffeine 1- and 3-demethylation rates. 3. Goat liver microsomes were able to hydroxylate tolbutamide, predominantly a CYP2C9 activity in man, and debrisoquine, a CYP2D activity in different species. These activities were not affected by either beta-naphthoflavone or phenobarbital. Sulphaphenazole was found to be a more potent inhibitor of tolbutamide hydroxylation than sulphadimethoxine. Quinine was a more potent inhibitor of debrisoquine hydroxylation than was quinidine. 4. As expected, the microsomal aniline-4-hydroxylation rate (CYP2E) was increased after isoniazid treatment. 5. The microsomal testosterone 6 beta-hydroxylation rate (CYP3A) was increased after phenobarbital and triacetyloleandomycin treatment. Antipyrine plasma clearance was also increased after phenobarbital treatment. 6. As cytochrome P450 activities and inducibility in the dwarf goat show many resemblances to those in man, they may be of value as a model for human biotransformation research.

Effect of ingesta and of tablets of different strengths on the systemic availability of digoxin in normal dogs

The plasma concentrations of digoxin were measured in eight normal dogs given digoxin on four occasions, using three different feeding regimens and tablets of two strengths. Although ingesta tended to slow the absorption of digoxin, the systemic availability of the drug, based on measurements of Cmax, tmax and AUC did not differ when digoxin tablets were given with canned food, with dry food, or without food. However, some of the pharmacokinetic characteristics and smaller individual variations with the dry food regimen would be considered advantageous for maintenance therapy. Tablets containing 62.5 micrograms or 250 micrograms of digoxin had a similar relative bioavailability. The peak plasma digoxin concentrations were higher in female dogs, and the trends in other data also suggested that the systemic availability of digoxin was better in female dogs.

Sulfadimidine metabolism in vitro: II. Comparative studies in cultured rat, goat, sheep and cattle hepatocytes

Hydroxylation and acetylation of sulphadimidine (SDD) and the deacetylation of N4-acetyl SDD was investigated in cultured hepatocytes from male and female rats, from male and female goats and from female sheep and cattle. Significant sex differences were observed for hydroxylation of SDD in hepatocytes from rat and goat. In goat, sheep and cow hepatocytes, the hydroxylation pathway is relatively important, whereas in rat hepatocytes, acetylation is predominant. Hepatocytes of all four species deacetylated N4-acetyl SDD. In ruminant hepatocytes, deacetylating activity was of considerable importance, whereas in rat hepatocytes, it appeared a minor pathway of metabolism. Similar to the in vivo situation, formation of N4-acetyl SDD in cultured hepatocytes results from an equilibrium of acetylation and deacetylation. A good correlation was found between results in isolated hepatocytes and previous findings in vivo, both in levels of species-related activities and in acetylation-hydroxylation ratios. In conclusion, cultured hepatocytes appear a useful in vitro model to study comparative sulfonamide metabolism.

Cytochrome P450 induction and metabolism of alkoxyresorufins, ethylmorphine and testosterone in cultured hepatocytes from goats, sheep and cattle

Very little is known of cytochrome P450 (P450) patterns and enzyme characteristics in food-producing animal species. Oxidative metabolism of alkoxyresorufins, ethylmorphine (EtM) and testosterone (TST) was used to monitor the effects of the P450 inducers phenobarbital (PB), beta-naphthoflavone (BNF), dexamethasone (DEX) and triacetyloleandomycin (TAO) in primary cultured hepatocytes from goats, sheep and cattle. BNF effectively and specifically induced ethoxyresorufin deethylase (> 20-fold), indicating the presence of an inducible P450 1A form, and down-regulated EtM demethylation and most selected TST hydroxylations. In non-induced hepatocyte cultures, TST was metabolized to 6 beta-, 2 beta-, 12 beta-, and 11 alpha-hydroxy-TST (OHT). PB and, to a lesser extent, DEX non-specifically induced all OHT formations, and EtM demethylation. TAO almost completely inhibited OHT formation and EtM demethylation. These results indicate the involvement of principally one P450 form, or a restricted number of related P450 forms, presumably belonging to the P450 3A subfamily. In western blot analysis, cross reactivity was found with rat anti-P450 3A1 and anti-sheep P450 3A. A more specific PB effect was observed for 16 alpha-OHT, which may be formed though a ruminant P450 2B form. None of the inducers influenced pentoxyresorufin depentylase (PROD) or EtM O-deethylation. Metabolite patterns and inducibility of selected activities in ruminant hepatocytes are in accordance with previous findings in goats in vivo. Cytochrome P450 characteristics in ruminants appear to differ from those in rats whereas similarities to the situation in humans appear to exist.

Sulphadimidine metabolism in vitro: I. Sex differences in acetylation and hydroxylation in cultured rat hepatocytes

The hydroxylation and acetylation of 0.5 mM sulphadimidine (SDD) was studied in primary cultures of hepatocytes from male and female rats, and from castrated male and sham operated male rats. In addition, SDD metabolism was investigated in hepatocytes from castrated male rats treated with testosterone, prior to liver cell isolation. In male rat hepatocytes a significantly higher hydroxylation activity was observed than in hepatocytes from female and castrated male rats. Acetylation activity was higher in females. Testosterone induced hydroxylation but did not affect acetylation. These results correlate well with data from previous in vivo studies, showing the relevance of this in vitro model.