In vitro and in vivo binding of phenylbutazone and related drugs to equine feeds and digesta

The effects of breed and routes of administration on the plasma pharmacokinetics and faecal excretion of robenacoxib in goats

Robenacoxib (RX) is a non-steroidal anti-inflammatory drug (NSAID) of the coxib class. This study aimed to evaluate the plasma dispositions and faecal excretion profiles of RX in Alpine and Saanen goats following oral and subcutaneous routes. Two different goat breeds were allocated into two treatment groups concerning the breed. RX was administered subcutaneously to animals at a dose of 4 mg/kg b.w. Following a one-week washout period, RX was administered by oral route to the same animals at the same dose. Heparinized blood samples were collected from all animals before drug administration (0 h) and subsequently up to 24 h. Faecal samples were collected at various times between 8 h and 36 h. The concentrations of RX in plasma and faeces were determined by HPLC. The plasma half-life (T1/2λz) of RX in Saanen goats (1.21 h) was significantly longer (P < 0.017) than in Alpine goats (0.90 h) after subcutaneous administration. In both goat breeds, statistical differences were observed between subcutaneous and oral administration of RX for T1/2λz, Tlast, Cmax, AUC0-∞, and MRT0-∞. Faecal Cmax and Tmax parameters following oral administrations were 0.92 µg/g and 0.85 µg/g at 30 h and at 24 h in Alpine and Saanen goats, respectively. The difference in plasma protein ratio between Alpine and Saanen goats may have affected the T1/2λz of the drug. NSAIDs are among the drug groups frequently detected in aquatic and terrestrial ecosystems around the world and there are data on the effects of NSAID residues on wildlife and aquatic species. Therefore, revealing the excretion of NSAIDs, which are frequently used in the veterinary field, in faeces and urine should be considered for ecological sustainability.

The Clinical Pharmacology and Therapeutic Evaluation of Non-Steroidal Anti-Inflammatory Drugs in Adult Horses

This review firstly examines the underlying pathophysiology of pain and inflammation associated with orthopedic disease and endotoxemia. Then, it reviews the clinical pharmacology (pharmacokinetics and pharmacodynamics) of both conventional and non-conventional NSAIDs in the adult horse, and finally provides an overview of different modalities to evaluate the therapeutic efficacy of NSAIDs in research.

Pharmacokinetics and clinical efficacy of Acetaminophen (Paracetamol) in adult horses with mechanically induced lameness

BACKGROUND

Acetaminophen has been used clinically in horses alone or combined with traditional NSAIDs for treatment of musculoskeletal pain in horses.

OBJECTIVES

To determine the pharmacokinetics and efficacy of acetaminophen at two doses in horses with mechanically induced lameness compared to phenylbutazone or placebo control.

STUDY DESIGN

In vivo experiment.

METHODS

Nine healthy mares with mechanical lameness induced via a reversible sole pressure horseshoe model were treated with acetaminophen (20 mg/kg PO; A20), acetaminophen (30 mg/kg PO; A30), phenylbutazone (2.2 mg/kg, PO; PB) and oral placebo (C) in a randomised 4-way Latin square model. Plasma concentrations for A20 and A30 were analysed via LC-MS/MS and noncompartmental pharmacokinetic analysis. Heart rate and heart rate variability were measured using a portable telemetry. Lameness was scored by three blinded boarded equine surgeons using the AAEP and 10-point scales.

RESULTS

Mean maximum plasma concentration (C_max ) for A20 was 20.01 ug/mL within 0.66h (T_max ) after administration; The mean C_max for A30 was 30.02 ug/mL with a T_max of 0.43 h. Post-treatment heart rate for A30 was significantly lower than A20 at 1 and 7 h; lower than PB at 2, 3, 4.5, and 7 h; lower than C at 2, 3.5, 4.5, 6, 7, and 8 h. 10-point Lameness scores were significantly improved for A30 than C at 2 and 4 h post-treatment; PB was significantly improved than C at 8 h post treatment. There were no significant differences in lameness between A20, A30, and PB.

MAIN LIMITATIONS

Small sample size, lack of objective lameness measurement.

CONCLUSIONS

Acetaminophen at 30 mg/kg produced a more rapid improvement in lameness scores and heart rate compared to other treatments in this model. Further evaluation of the pharmacokinetics and safety of repeated oral dosing of acetaminophen at 30 mg/kg is needed to determine clinical utility.

Pharmacokinetics of three formulations of vitacoxib in horses

The pharmacokinetic properties of three formulations of vitacoxib were investigated in horses. To describe plasma concentrations and characterize the pharmacokinetics, 6 healthy adult Chinese Mongolian horses were administered a single dose of 0.1 mg/kg bodyweight intravenous (i.v.), oral paste, or oral tablet vitacoxib in a 3-way, randomized, parallel design. Blood samples were collected prior to and at various times up to 72 hr postadministration. Plasma vitacoxib concentrations were quantified using UPLC-MS/MS, and pharmacokinetic parameters were calculated using noncompartmental analysis. No complications resulting from the vitacoxib administration were noted on subsequent administrations, and all procedures were tolerated well by the horses throughout the study. The elimination half-life (T_1/2λz ) was 4.24 ± 1.98 hr (i.v.), 8.77 ± 0.91 hr (oral paste), and 8.12 ± 4.24 hr (oral tablet), respectively. Maximum plasma concentration (C_max ) was 28.61 ± 9.29 ng/ml (oral paste) and 19.64 ± 9.26 ng/ml (oral tablet), respectively. Area under the concentration-versus-time curve (AUC_last ) was 336 ± 229 ng hr/ml (i.v.), 221 ± 94 ng hr/ml (oral paste), and 203 ± 139 ng hr/ml, respectively. The results showed statistically significant differences between the 2 oral vitacoxib groups in T_max value. T_1/2λz (hr), AUC_last (ng hr/ml), and MRT (hr) were significantly different between i.v. and oral groups. The longer half-life observed following oral administration was consistent with the flip-flop phenomenon.

Anthelmintic drugs used in equine species

Internal parasites of horses comprise an intractable problem conferring disease, production and performance losses. Parasitism can rarely be controlled in grazing horses by management alone and anthelmintic drugs have formed the basis of therapy and prophylaxis for the last sixty years. The pharmacology of the anthelmintic drugs available dictate their spectrum of activity and degree of efficacy, their optimal routes of administration and characteristics which prevent some routes of administration, their safety tolerance and potential toxicities and as a consequence of their persistence in the body at effective concentrations their use in epidemiological control programmes. Their use has also resulted in the selection of parasites with genetically controlled characteristics which reduce their susceptibility to treatment, characteristics which are often common to whole chemical classes of anthelmintics. Pharmacological properties also confer compatibility in terms of safety and persistence with other anthelmintic drugs and thus the potential of combinations to treat parasites from different phylogenetic groups such as nematodes, cestodes and trematodes and also the potential by agency of their different molecular mechanisms of action to delay the selection of resistant genes. The major groups of anthelmintics now available, the benzimidazoles (BZD), macrocyclic lactones (MLs) and tetrahydropyrimidines are all highly effective against their targeted parasites (primarily nematodes for BZD's and ML's and cestodes for tetrahydropyrimidines) easily administered orally to horses and are well tolerated with wide margins of safety. Nevertheless, some parasitic stages are inherently less susceptible such as hypobiotic stages of the small strongyles (cyathostomins) and for some such as the adult stages of cyathostomins resistance has developed. Furthermore, for some less common parasites such as the liver fluke unlicensed drugs such as the salicylanilide, closantel have been used. A deep understanding of the pharmacology of anthelmintic drugs is essential to their optimal use in equine species.

Evaluation of pharmacokinetic properties of vitacoxib in fasted and fed horses

The pharmacokinetic properties of vitacoxib have not been established completely; current dosage recommendations are based on clinical experiences. The primary objective of this study was to describe plasma concentrations and characterize the pharmacokinetics of vitacoxib formulation following oral administrations in horses. Also, the effect of the state of stomach contents on the absorption of vitacoxib was investigated in fed/fasted horses. Blood samples were collected prior to and at various times up to 72 hr post-administration. Drug concentrations were measured using ultra high-performance liquid chromatography-tandem mass spectrometry. Pharmacokinetic parameters were calculated using Non-Compartmental Analysis Model 200 in WinNonlin™ software. No complications resulting from the vitacoxib administration were noted. All procedures were tolerated well by the horses throughout the study. C_max was 17.5 ± 9.36 ng/ml (fasted) and 9.47 ± 3.53 ng/ml (fed) following oral administrations. AUC_last was 173.7 ± 137.9 ng hr/ml (fasted) and 113.2 ± 70.8 ng h/ml (fed). No significant differences in pharmacokinetic parameters were noted and the results from the pharmacokinetic analysis were similar between the studies, regardless of precision of dosage and fasted and fed conditions. The study extends previous studies describing the pharmacokinetics of vitacoxib following p.o. administration to the horses. Further studies investigating the pharmacokinetics/pharmacodynamics of vitacoxib are necessary to establish adequate therapeutic protocols (optimal dosage and frequency of administration) in horses.

Nonsteroidal Anti-inflammatory Drug Use in Horses

Nonsteroidal anti-inflammatory drugs (NSAIDs) are effective anti-inflammatory and analgesic agents and are arguably the most commonly used class of drugs in equine medicine. This article provides a brief review of the mechanism of action, therapeutic uses, pharmacokinetics, and adverse effects associated with their use in horses. The use of COX-2 selective NSAIDs in veterinary medicine has increased over the past several years and special emphasis is given to the use of these drugs in horses. A brief discussion of the use of NSAIDs in performance horses is also included.

Pharmacokinetics of intravenously and orally administered meloxicam in sheep

OBJECTIVE

To determine the pharmacokinetics of meloxicam after IV and PO administration to 6 healthy sheep.

ANIMALS

6 healthy adult Dorset cross sheep (5 males and 1 female).

PROCEDURES

Meloxicam (0.5 mg/kg, IV, or 1.0 mg/kg, PO) was administered in a randomized crossover design with a 10-day washout period. Blood samples were collected at predetermined times over 96 hours. Serum drug concentrations were determined by high-pressure liquid chromatography with mass spectrometry. Computer software was used to estimate values of pharmacokinetic parameters through noncompartmental methods.

RESULTS

Following IV administration (n = 5), the geometric mean (range) elimination half-life was 14.0 hours (10.5 to 17.0 hours), volume of distribution was 0.204 L/kg (0.171 to 0.272 L/kg), and clearance was 0.17 mL/min/kg (0.12 to 0.27 mL/min/kg). Following oral administration (n = 6), maximum serum concentration was 1.72 μg/mL (1.45 to 1.93 μg/mL), time to maximum serum concentration was 19.0 hours (12.0 to 24.0 hours), clearance per bioavailability was 0.22 mL/min/kg (0.16 to 0.30 mL/min/kg), and terminal half-life was 15.4 hours (13.2 to 17.7 hours). Bioavailability of orally administered meloxicam was calculated as 72% (40% to 125%; n = 5). No adverse effects were evident following meloxicam administration via either route.

CONCLUSIONS AND CLINICAL RELEVANCE

Meloxicam administered PO at 1.0 mg/kg has good bioavailability with slow elimination kinetics in sheep. These data suggested that meloxicam may be clinically useful, provided the safety and analgesic efficacy of meloxicam as well as feed-related influences on its pharmacokinetics are established in ruminants.

Disposition of firocoxib in equine plasma after an oral loading dose and a multiple dose regimen

The objective of this study was to determine if a single loading dose (LD), 3× the label dose of firocoxib oral paste, followed by nine maintenance doses at the current label dose achieves and maintains near steady state concentrations. Six healthy, adult mares were administered 0.3mg/kg of firocoxib on Day 0, and 0.1 mg/kg 24 h later on Day 1, and at 24 h intervals from Day 2 to Day 9, for a total of 10 doses. Blood samples were collected throughout the study. The mean firocoxib maximum plasma concentration and standard deviation was 199±97 ng/mL, 175±44 ng/mL and 183±50 ng/mL after the LD, and first and last maintenance doses, respectively. The minimum mean concentration (C(min)) increased from 100±23 ng/mL after the LD to 132±38 ng/mL at Day 7. Then, the C(min) remained constant until Day 9. The average concentration at steady state (C(avg)) was 150±45 ng/mL, which compares well to the C(avg) (130±36 ng/mL) reported after multiple daily doses at 0.1 mg/kg. The administration of the single LD allowed achievement of the average steady state drug concentrations faster than a multi-dose regimen without a loading dose. After the LD, firocoxib at 0.1 mg/kg every 24 h was able to maintain a relatively constant average drug concentration which should produce less variability in onset of action and efficacy.

Establishing bioequivalence of veterinary premixes (Type A medicated articles)

a) Key issues concerning Premix (Type A medicated articles) Bioequivalence evaluations: 1) This is a complex issue concerning both route of administration and formulation. 2) If the animal is not at the bunk/trough, the animal is not self-administering (eating medicated feed), thus there can be no drug absorption. b) Differing opinions among scientists and regulatory authorities/expert bodies regarding: 1) No harmonization on how to design, conduct, and interpret in vivo studies. 2) Applicability of biowaivers to Type A (premix) products. 3) Why are topdress and complete feed considered differently? Are they different formulations or different routes of administration? 4) Single dose vs. multi-dose studies. 5) What is the final formulation? c) What are the next steps: 1) Harmonize current bioequivalence guidelines through the VICH process. 2) Determine the applicability/non-applicability of the Biopharmaceutical Classification System (BCS). 3) Establish the Total Mixed Ration (i.e. formulation) effects. 4) Define the test subject (individual, pen, etc.).

Bioavailability and pharmacokinetics of oral meloxicam in llamas

Background

South American camelids in the United States have rapidly developed into an important agricultural industry in need of veterinary services. Pain management is challenging in camelids because there are no drugs currently approved by the U.S. Food and Drug Administration for use in these species. Dosage regimens used for many therapeutic drugs have been extrapolated from other ruminants; however, the pharmacokinetics, in camelids, may differ from those of other species. Studies investigating the pharmacokinetics of cyclooxygenase-2 (COX-2) selective non-steroidal anti-inflammatory drugs in camelids are deficient in the published literature. Six adult llamas (121- 168 kg) were administered either a 1 mg/kg dose of oral or a 0.5 mg/kg dose of IV meloxicam in a randomized cross-over design with an 11 day washout period between treatments. Plasma samples collected up to 96 hours post-administration were analyzed by high pressure liquid chromatography and mass spectrometry detection (HPLC-MS) followed by non-compartmental pharmacokinetic analysis.

Results

A mean peak plasma concentration (C_MAX) of 1.314 μg/mL (Range: 0.826 – 1.776 μg/mL) was recorded at 21.4 hours (Range: 12.0 – 24.0 hours) with a half-life (T ½ λ_z) of 22.7 hours (Range: 18.0 – 30.8 hours) after oral meloxicam administration. In comparison, a half-life (T ½ λ_z) of 17.4 hours (Range: 16.2 – 20.7 hours) was demonstrated with IV meloxicam administration. The oral bioavailability (F) of meloxicam (dose normalized) was 76% (Range: 48 – 92%). No adverse effects associated with either treatment modality were observed in the llamas.

Conclusions

The mean bioavailability (F) of oral meloxicam was 76% indicating a high degree of gastrointestinal absorption. Plasma meloxicam concentrations >0.2 μg/mL were maintained for up to 72 h after oral administration; >0.2 μg/mL is considered to be the concentration of meloxicam required for analgesic effects in other species such as the horse. These data suggest that a single dosage of oral meloxicam at 1 mg/kg could potentially maintain therapeutic concentrations in plasma for up to 3 days in adult llamas.

Species differences in pharmacokinetics and pharmacodynamics

Veterinary medicine faces the unique challenge of having to treat many types of domestic animal species, including mammals, birds, and fishes. Moreover, these species have evolved into genetically unique breeds having certain distinguishable characteristics developed by artificial selection. The main challenge for veterinarians is not to select a drug but to determine, for the selected agent, a rational dosing regimen because the dosage regimen for a drug in a given species may depend on its anatomy, biochemistry, physiology, and behaviour as well as on the nature and causes of the condition requiring treatment. Both between- and within-species differences in drug response can be explained either by variations in drug pharmacokinetics (PK) or drug pharmacodynamics (PD), the magnitude of which varies from drug to drug. This chapter highlights selected aspects of species differences in PK and PD and considers underlying physiological and patho-physiological mechanisms in the main domestic species. Particular attention was paid to aspects of animal behaviour (food behaviour, social behavior, etc.) as a determinant of interspecies differences in PK or/and PD. Modalities of drug administration are many and result not only from anatomical, physiological and/or behavioural differences across species but also from management options. The latter is the case for collective/group treatment of food-producing animals, frequently dosed by the oral route at a herd or flock level. After drug administration, the main causes of observed inter-species differences arise from species differences in the handling of drugs (absorption, distribution, metabolism, and elimination). Such differences are most common and of greatest magnitude when functions which are phylogenetically divergent between species, such as digestive functions (ruminant vs. non-ruminant, carnivore vs. herbivore, etc.), are involved in drug absorption. Interspecies differences also exist in drug action but these are generally more limited, except when a particular targeted function has evolved, as is the case for reproductive physiology (mammals vs. birds vs. fishes; annual vs. seasonal reproductive cycle in mammals; etc.). In contrast, for antimicrobial and antiparasitic drugs, interspecies differences are more limited and rather reflect those of the pathogens than of the host. Interspecies difference in drug metabolism is a major factor accounting for species differences in PK and also in PD (production or not of active metabolites). Recent and future advances in molecular biology and pharmacogenetics will enable a more comprehensive view of interspecies differences and also between breeds with existing polymorphism. Finally, the main message of this review is that differences between species are not only numerous but also often unpredictable so that no generalisations are possible, even though for several drugs allometric approaches do allow some valuable interspecies extrapolations. Instead, each drug must be investigated on a species-by-species basis to guarantee its effective and safe use, thus ensuring the well-being of animals and safeguarding of the environment and human consumption of animal products.

Comparison of the effect of Sporobolus virginicus and Rhodes (Chloris gayana) hay diets on the absorption pattern of phenylbutazone in the camel (Camelus dromedarius)

The effect of feeding Sporobolus and Rhodes hay on phenylbutazone (4 g) relative absorption was examined in six camels using a two-period, two-sequence, two-treatment crossover design. Serum concentration of the drug was measured by high performance liquid chromatography. The measured values (means+/-SD) for Rhodes and Sporobolus hay, respectively, were Cmax 35.59+/-22.36 and 36.55+/-18.99 microg/mL, Tmax 26+/-2.53 and 26.3+/-1.97 h and AUC0-72 h 1552+/-872.6 and 1621+/-903.6 microg h/mL. Broad plateau concentrations of phenylbutazone in serum were observed between 12 and 36 h. There was no significant difference in any parameter between the two feeding regimens. Multiple peaks in serum concentration-time curve were observed, regardless of the type of grass available to and the animals prior to drug administration. It was concluded that the phasic absorption of phenylbutazone was a particular feature of hay feeding in camels, and the Sporobolus hay can be fed to camels without any effect on the rate and extent of phenylbutazone absorption compared to Rhodes grass hay.

Pharmacodynamics and pharmacokinetics of nonsteroidal anti-inflammatory drugs in species of veterinary interest

This review summarises selected aspects of the pharmacokinetics (PK) and pharmacodynamics (PD) of nonsteroidal anti-inflammatory drugs (NSAIDs). It is not intended to be comprehensive, in that it covers neither minor species nor several important aspects of NSAID PD. The limited objective of the review is to summarise those aspects of NSAID PK and PD, which are important to an understanding of PK-PD integration and PK-PD modelling (the subject of the next review in this issue). The general features of NSAID PK are: usually good bioavailability from oral, intramuscular and subcutaneous administration routes (but with delayed absorption in horses and ruminants after oral dosing), a high degree of binding to plasma protein, low volumes of distribution, limited excretion of administered dose as parent drug in urine, marked inter-species differences in clearance and elimination half-life and ready penetration into and slow clearance from acute inflammatory exudate. The therapeutic effects of NSAIDs are exerted both locally (at peripheral inflammatory sites) and centrally. There is widespread acceptance that the principal mechanism of action (both PD and toxicodynamics) of NSAIDs at the molecular level comprises inhibition of cyclooxygenase (COX), an enzyme in the arachidonic acid cascade, which generates inflammatory mediators of the prostaglandin group. However, NSAIDs possess also many other actions at the molecular level. Two isoforms of COX have been identified. Inhibition of COX-1 is likely to account for most of the side-effects of NSAIDs (gastrointestinal irritation, renotoxicity and inhibition of blood clotting) but a minor contribution also to some of the therapeutic effects (analgesic and anti-inflammatory actions) cannot be excluded. Inhibition of COX-2 accounts for most and possibly all of the therapeutic effects of NSAIDs. Consequently, there has been an intensive search to identify and develop drugs with selectivity for inhibition of COX-2. Whole blood in vitro assays are used to investigate quantitatively the three key PD parameters (efficacy, potency and sensitivity) for NSAID inhibition of COX isoforms, providing data on COX-1:COX-2 inhibition ratios. Limited published data point to species differences in NSAID-induced COX inhibition, for both potency and potency ratios. Members of the 2-arylpropionate sub-groups of NSAIDs exist in two enantiomeric forms [R-(-) and S-(+)] and are licensed as racemic mixtures. For these drugs there are marked enantiomeric differences in PK and PD properties of individual drugs in a given species, as well as important species differences in both PK and PD properties.

Pharmacology of drugs used to treat osteoarthritis in veterinary practice

As in humans, pain in animals may be associated with a wide range of conditions and circumstances, ranging from acute trauma to joint diseases. Joint diseases are common in companion animal medicine (horse, dog, cat) and at least 80% of cases are classified as osteoarthritis (OA). Several drug classes are available for OA therapy, including corticosteroids, non-steroidal antiinflammatory drugs (NSAIDs), agents with potential disease modifying properties and nutraceuticals. For long-term maintenance OA treatment, particularly in the horse and dog, NSAIDs are routinely and extensively used. This review outlines the pharmacokinetics (PK) and pharmacodynamics (PD) of NSAIDs in companion and farm animal species. NSAID PK and PD have been studied in models of acute inflammation, which enable use of PK-PD modeling to facilitate (a) studies of mechanism of action at the molecular level and (b) prediction of dosages for clinical use. The PK-PD approach is a powerful but underutilized tool which also facilitates inter-species comparisons.

Effect of composition and quality of diet and feeding time on the kinetics and efficacy of some anthelmintic drugs: a mini-review

This article reviews the literature dealing with the effects of composition and quality of diet and feeding time on the pharmacokinetics and efficacy of some anthelmintic drugs in ruminants. Studies have suggested that greater availability, and therefore improved anthelmintic activity, is possible through temporary feed restriction. It is also recommended that anthelmintic drugs should not be given to animals whilst they are maintained on large feed intakes, particularly of lush pasture that promotes rapid gastric transit, as this may reduce drug availability and anthelmintic efficacy. Generally, feeding animals low-quality fibrous diets reduces the passage rate of digesta and allows more time for absorption of several anthelmintic drugs and their metabolites from the gut. Some kinetic data of drugs given to animals on such diets may be slightly different, but this does not necessarily indicate alteration of the dosages of the anthelmintic drug. Nonetheless, due consideration should be given to anthelmintic dosages under various dietary regimes if optimum efficacy is to be achieved at all times.

In vitro anion transport alterations and apoptosis induced by phenylbutazone in the right dorsal colon of ponies

OBJECTIVES

To study the functional and structural responses of the right dorsal colon (RDC) of ponies to phenylbutazone (PBZ) in vitro at a concentration that could be achieved in vivo.

ANIMALS

8 adult ponies.

PROCEDURE

Short circuit current and conductance were measured in mucosa from the RDC. Tissues incubated with and without HCO3- were exposed to PBZ, bumetanide, or indomethacin. Bidirectional Cl- fluxes were determined. After a baseline flux period, prostaglandin E2 (PGE2) was added to the serosal surfaces and a second flux period followed. Light and transmission electron microscopy were performed.

RESULTS

Baseline short circuit current was diminished significantly by PBZ and indomethacin, and increased significantly after addictions of PGE2. After PGE2 was added, Cl- secretion increased significantly in tissues in HCO3- -free solutions and solutions with anti-inflammatory drugs, compared with corresponding baseline measurements and with control tissues exposed to PGE2. Bumetanide did not affect baseline short circuit current and Cl- fluxes. The predominant histologic change was apoptosis of surface epithelial cells treated with PBZ and to a lesser extent in those treated with indomethacin.

CONCLUSIONS AND CLINICAL RELEVANCE

Prostaglandin-induced Cl- secretion appeared to involve a transporter that might also secrete HCO3-. Both PBZ and indomethacin altered ion transport in RDC and caused apoptosis; PBZ can damage mucosa through a mechanism that could be important in vivo. The clinically harmful effect of PBZ on equine RDC in vivo could be mediated through its effects on Cl- and HCO3- secretion.

Gastrointestinal pharmacology

Diseases of the gastrointestinal tract that require pharmacologic management, usually in combination with other treatments, are gastric ulcers (omeprazole and others), colic (laxatives, analgesics), diarrhea (antibiotics, protectants and absorbents, glucocorticoids, motility inhibitors), reperfusion injury, postoperative ileus (prokinetic drugs), and adhesions. There is growing evidence that nonsteroidal anti-inflammatory drugs can alter important physiologic properties of the intestine; however, these drugs are valuable analgesics for horses and their use should be tempered with an awareness of their harmful effects. The role of antibiotics in treating gastrointestinal disease is controversial, but their ability to induce life-threatening diarrhea is well known and invites caution and defensible use of these drugs in horses.

Intramuscular bioavailability of ketoprofen lysine salt in horses

Lysine salts are often used in human pharmaceuticals to increase the solubility and absorption of acidic drugs when these are administered parenterally. In this study the intramuscular bioavailability of ketoprofen administered as the lysine salt was evaluated in horses (n = 5) treated intravenously and intramuscularly (2.2 mg/kg active substance) in a cross-over study. The absorption rate of ketoprofen administered as the lysine salt was rather low: the mean residence time increased from 31.7 min after IV injection to 128.9 min (after IM injection), and the bioavailability was high (mean 92.4%). The calculated steady state plasma concentrations of ketoprofen during multiple dosage were much higher after intramuscular (0.106 g/ml) than after intravenous (0.066 microgram/ml) administration. Intramuscular injections of the ketoprofen lysine salt can therefore be given to horses, which are particularly prone to develop soft tissue reactions, since use of the lysine salt markedly reduced local irritation at the injection site.

The effect of inflammation on the disposition of phenylbutazone in thoroughbred horses

The effect of inflammation on the disposition of phenylbutazone (PBZ) was investigated in Thoroughbred horses. An initial study (n = 5) in which PBZ (8.8 mg/kg) was injected intravenously twice, 5 weeks apart, suggested that the administration of PBZ would not affect the plasma kinetics of a subsequent dose. Two other groups of horses were given PBZ at either 8.8 mg/kg (n = 5) or 4.4 mg/kg (n = 4). Soft tissue inflammation was then induced by the injection of Freud's adjuvant and the administration of PBZ was repeated at a dose level equivalent to, but five weeks later than, the initial dose. Inflammation did not appear to affect the plasma kinetics or the urinary excretion of PBZ and its metabolites, oxyphenbutazone (OPBZ) or hydroxyphenylbutazone (OHPBZ) when PBZ was administered at 8.8 mg/kg. However, small but significant increases (P < 0.05) in total body clearance (CLB; 29.2 +/- 3.9 vs. 43.8 +/- 8.1 mL/ h.kg) and the volume of distribution, calculated by area (Vd(area); 0.18+/- 0.05 vs. 0.25 +/- 0.03 L/kg) or at steady-state (Vd(SS); 0.17 +/- 0.04 vs. 0.25 +/- 0.03 L/ kg), were obtained in horses after adjuvant injection, compared to controls, when PBZ was administered at 4.4 mg/kg which corresponded to relatively higher tissues concentrations and lower plasma concentrations (calculated) at the time of maximum peripheral PBZ concentration. Soft tissue inflammation also induced a significantly (P < 0.05) higher amount of OPBZ in the urine 18 h after PBZ administration but the total urinary excretion of analytes over 48 h was unchanged. These results have possible implications regarding the administration of PBZ to the horse close to race-day.

In vitro and in vivo binding of trimethoprim and sulphachlorpyridazine to equine food and digesta and their stability in caecal contents

Binding of antibiotics to food has received little attention in equine medicine, although such binding could potentially reduce the bioavailability and clinical efficacy. In the present study, binding of trimethoprim (TMP) and sulphachlorpyridazine (SCP) to hay, grass silage and concentrate was investigated in vitro in buffer at pH 6.8 at different concentrations. The binding of TMP and SCP to caecal contents was also studied. In addition, the degradation of TMP and SCP by the caecal microflora was investigated by incubating sterilized and non-sterilized caecal contents for 3 h at 37 degrees C under anaerobic conditions and comparing the TMP and SCP contents. Further, a TMP/SCP powder formulation was adminstered orally with concentrate at a dose rate of 5 mg/kg TMP and 25 mg/kg SCP to three ponies with a caecum fistula; the animals were deprived of food for 8 h before administration. Blood samples, caecal contents samples and faecal samples were collected and analysed for TMP and SCP concentrations by means of high performance liquid chromatography (HPLC). Three non-fistulated ponies, acting as control animals, were fed the same dose of TMP/SCP with concentrate after 8 h of food deprivation and blood samples were taken. The percentage of in vitro binding of TMP as well as SCP to hay, grass silage and concentrate at concentrations of 4 micrograms/mL to 10 micrograms/mL was high (60-90%). TMP and SCP were also extensively bound to caecal contents (50-70%). At spiking concentrations above 10 micrograms/mL the percentage of binding decreased. There was no evidence of biodegradation of TMP or SCP in caecal contents. In vivo, both drugs could be detected in the caecal contents and in the faeces of three fistulated ponies. However, the fistulated ponies differed from the control ponies in that their TMP and SCP plasma concentrations were higher, and two fistulated ponies did not show double peaks in their plasma concentration-time curves. Therefore, the fistulated ponies did not provide an optimal model for in vivo binding studies. Despite this limitation, it can be concluded that binding of TMP and SCP to food is a major cause of the limited bioavailability of these drugs in the horse. It is hypothesized that the binding is reversible, and that a second absorption phase occurs in the large intestine, but part of the administered dose remains bound as both drugs were found in the faeces.

Pharmacokinetics and pharmacodynamics of tolfenamic acid in calves

Pharmacokinetic/pharmacodynamic modelling was applied to a study in which tolfenamic acid was administered intravenously to calves at a dose rate of 2 mg kg-1. The drug had a shorter mean (SEM) elimination half-life (T1/2 beta) of 2.5 (0.95) hours and a larger volume of distribution (Vdarea) of 0.98 (0.28) litre kg-1 than other non-steroidal anti-inflammatory drugs. Its body clearance was high with a mean value of 0.30 (0.06) litre kg-1 h-1. It had inhibitory effects on inflammatory exudate PGE2 and beta-glucuronidase, serum TxB2 and bradykinin-induced swelling but it did not affect exudate LTB4 concentrations. Its mean EC50 values were lower for exudate PGE2, beta-glucuronidase and bradykinin-induced swelling inhibition (0.077 [0.018]; 0.040 [0.017] and 0.030 [0.020] microgram ml-1, respectively) than for serum TxB2 inhibition (0.137 (0.079) microgram ml-1). There were also differences in its equilibration halflife, which was short for the inhibition of serum TxB2, intermediate for exudate PGE2 and beta-glucuronidase and longer for bradykinin-induced swelling. These differences may be explained by the existence of three distribution compartments relating to the different sites of action of the drug.

Influence of closure of the reticular groove on the bioavailability and disposition kinetics of meclofenamate in sheep

Sodium meclofenamate is a non-steroidal anti-inflammatory drug with anaphylactic protective activity in cattle. The objectives of this study were to describe the pharmacokinetic behaviour of sodium meclofenamate after intravenous and oral administration to sheep and to determine the influence of closure of the reticular groove on the bioavailability of the drug. Sodium meclofenamate was administered by the intravenous (2.2 mg/kg) and oral (20 mg/kg) routes to sheep (n = 6). During the oral study the reticular groove was closed by intravenous administration of lysine vasopressin (0.3 IU/kg) or left open (saline solution). The closure of the reticular groove was assessed by determination of the blood glucose curves after oral administration of a glucose solution. After intravenous administration of meclofenamate, the distribution and elimination half-lives of the drug were 7.2 min and 542 min respectively, Vss was 1.68 L/kg and ClB was 2.47 mliter/min kg. Two different patterns of the plasma concentration curves were observed after oral administration of sodium meclofenamate. When the reticular groove was closed, two peaks were observed (tmax-1 12-15 min, Cmax-1 3.30-24.01 micrograms/mliter; and tmax-2' 52.50-75 min, Cmax-2, 6.45-11.08 micrograms/mliter).

Influence of formulation on the pharmacokinetics and bioavailability of racemic ketoprofen in horses

The bioavailability of S(+) and R(-) ketoprofen (KTP) in six horses was investigated after oral administration of the racemic (rac) mixture. Two oral formulations were studied, an oil-based paste containing micronised rac-KTP and powder from the same source in hard gelatin capsules, each at a dose rate of 2.2 mg/kg. For the oil-based paste two feeding schedules were used; horses were either allowed free access to food or access to food was restricted for 4 h before and 5 h after dosing. The drug in hard gelatin capsules was administered to horses with restricted access to food. After intravenous administration of rac-KTP, S(+) enantiomer concentrations exceeded those of the R(-) enantiomer. For S(+) and R(-)KTP, respectively, pharmacokinetic parameters were, t1/2 beta 0.99 +/- 0.14 h, 0.70 +/- 0.13 h; ClB 0.56 +/- 0.09, 0.92 +/- 0.20 L/h/kg; Vd(ss) 0.53 +/- 0.11, 0.61 +/- 0.10 L/kg. Following oral administration of rac-KTP as the oil-based paste to horses with free access to food, there were no detectable concentrations in plasma in three animals at any sampling time, while a fourth animal showed very low concentrations at two sampling times only. In the two remaining horses very low but detectable concentrations were present for 5 h. In the horses with restricted access to food, rac-KTP paste administration produced higher concentrations in plasma. However, bioavailability was very low, 2.67 +/- 0.43 and 5.75 +/- 1.48% for R(-) and S(+)KTP, respectively. When administered as pure drug substance in hard gelatin capsules, absorption of KTP was fairly rapid, but incomplete. Bioavailability was 50.55 +/- 10.95 and 54.17 +/- 9.9% for R(-) and S(+)KTP, respectively. This study demonstrates that rac-KTP had a modest bioavailability when administered as a micronised powder in hard gelatin capsules to horses with restricted access to food. When powder from the same source was administered as an oil-based paste, it was for practical purposes not bioavailable, regardless on the feeding schedule.

Pharmacokinetics of flunixin and its effect on prostaglandin F2 alpha metabolite concentrations after oral and intravenous administration in heifers

Flunixin meglumine (FM) was administered either orally as granules or intravenously to six heifers in a two period crossover study. Single doses of 2.2 mg/kg body weight were used. Pharmacokinetic variables were calculated using statistical moment methods. The effect exerted by flunixin was measured as changes in the basal plasma concentration of the main metabolite of prostaglandin (PG) F2 alpha. After oral FM the arithmetic means of pharmacokinetic variables were: MRT = 12.7 h; MAT = 6.3 h; Cmax = 0.9 microgram/mL; tmax = 3.5 h. The bioavailability was 60% and the mean half-life (harmonic mean) was 6.2 h. Oral administration of FM inhibited as effectively as intravenous administration the prostaglandin biosynthesis. The concentration of the PG metabolite decreased almost as rapidly as after intravenous administration. The duration of the effect was prolonged and the PG metabolite concentration was significantly lower between 10 and 30 h after oral than after intravenous administration. The results indicate that oral dosing of flunixin, in the form of granules, can be an alternative to intravenous administration for therapeutic use in cattle.

Pharmacokinetics of trimethoprim/sulphachlorpyridazine in horses after oral, nasogastric and intravenous administration

In the present study, the pharmacokinetic parameters of a trimethoprim/sulphachlorpyridazine preparation following intravenous administration, administration by nasogastric tube and administration with concentrate were determined in the horse. Eight adult horses were dosed at 1 week intervals in a sequentially designed study at a dose of 5 mg/kg trimethoprim (TMP) and 25 mg/kg sulphachlorpyridazine (SCP) on all occasions. Plasma concentrations of both drugs were measured serially for 48 h. Pharmacokinetic parameters of clinical importance (distribution and elimination half-lives, clearance, bioavailability, volume of distribution) were determined both for TMP and SCP. Following intravenous administration, the volume of distribution at steady-state (Vd(ss)) was significantly larger for TMP (1.51 +/- 0.25 L/kg than for SCP (0.26 +/- 0.05 L/kg. The clearance was 7.73 +/- 2.26 mL/min.kg for TMP and 2.64 +/- 0.48 mL/min.kg for SCP. For both TMP and SCP, mean peak plasma concentrations (Cmax) and the bioavailabilities (F) were reduced significantly when the drugs were mixed with concentrate (ct) as compared with those after nasogastric administration (ngt) (Fct = 44.3 +/- 10.7% vs. Fngt = 68.3 +/- 12.5% for TMP; Fct = 46.3 +/- 8.9% vs. Fngt = 67.3 +/- 13.7% for SCP). Following the administration of TMP and SCP mixed with concentrate, the plasma concentration-time curves showed a biphasic absorption pattern in all horses. The first peak occurred 1-2 h and the second peak 8-10 h after administration of the combination preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

The partition and fate of soluble and digesta particulate associated oxfendazole and its metabolites in the gastrointestinal tract of sheep

The disposition of oxfendazole (OFZ) containing a trace of [14C]OFZ was examined in the gastrointestinal tract and bloodstream of sheep fitted with rumen and abomasal cannulae. Within 2 h of intraruminal (IR) administration, OFZ and its metabolites were almost completely associated with rumen particulate digesta. The proportion of metabolites in digesta fluid increased with their passage from the rumen into the abomasum. To determine the fate of 14C-labelled metabolites after distribution throughout rumen digesta, the rumen particulate and fluid digesta phases from a donor sheep were separated and each transferred to the rumen of an untreated recipient sheep. The 14C-labelled metabolites which derived from the donor rumen fluid quickly associated with recipient rumen particulate material. The metabolites were then progressively desorbed, as were metabolites which were transferred already associated with rumen particulate digesta. Desorption occurred faster in the abomasum than in the rumen. There was no difference in uptake kinetics between administration routes, indicating rapid equilibrium. Consequently the disposition of [14C] OFZ and its metabolites in the bloodstream was similar in each group. It is suggested that the progressive desorption of particulate associated metabolites is a principal determinant of the duration of OFZ availability.

Nonsteroidal anti-inflammatory drugs

NSAIDs' mechanism of action by inhibiting the synthesis of prostanoids accounts for both their therapeutic and toxic effects. They are commonly used in acute and chronic musculoskeletal and soft-tissue conditions. Adverse reactions include gastrointestinal disturbances and hypoproteinemia. Their pharmacologic effect seems to have a longer duration than their plasma concentrations indicate. This gives implications on current regulations for competing horses and the relevance of permitted levels has been questioned. The NSAIDs do not appear to enhance performance, but rather allow the horse to run up to its potential by reducing pain and lameness. There is concern over the possible hazards to the horse by this kind of therapeutic use. In conclusion, NSAIDs have well justifiable therapeutic uses in equine practice. They should, however, be used when there is a clear clinical indication, in safe dose rates and without jeopardizing the welfare of the performance horse.

The effect of feed intake on the rate of flow of digesta and the disposition and activity of oxfendazole in sheep

Within 3 h of intra-ruminal (IR) administration oxfendazole (OFZ) was extensively associated with digesta particulate material. This association appeared to be a passive adsorption/desorption since sheep given dose equivalent OFZ which was contained solely in rumen particulate digesta (obtained from OFZ treated donor sheep) exhibited similar OFZ disposition to the IR treated animals. The association of OFZ with rumen particulate digesta is a major determinant of the pharmacokinetic behaviour of OFZ. The kinetic disposition of OFZ was compared in sheep fed 400 g and 800 g day-1 of 50:50 lucerne/wheaten chaff. The flow rate of rumen and abomasal fluid and particulate digesta increased with feed intake whilst the availability of OFZ and its metabolites in the digestive tract and bloodstream decreased. Compared to untreated sheep fed 800 g day-1, OFZ did not affect benzimidazole (BZ) resistant Trichostrongylus colubriformis, but reduced the numbers of BZ-resistant Haemonchus contortus by 61%. When sheep were fed 400 g day-1 36 h before and after OFZ administration, 60% of T. colubriformis and 94% of H. contortus were removed. Elevated anthelmintic efficacy in sheep can therefore be obtained when gastric flow has been slowed by temporarily reducing feed intake.

Pharmacokinetic disposition of benzimidazole drugs in the ruminant gastrointestinal tract

Orally administered benzimidazole (BZ) drugs are ideally deposited in the rumen where they associate extensively with particulate digesta material, the residence time of this drug-digesta complex being a major influence on the subsequent rate and duration of BZ availability. This duration is shortened if the dose should bypass the rumen due to oesophageal groove closure. Benzimidazole metabolites flow from the rumen primarily in association with particulate digesta. In the abomasum, the majority of soluble metabolites result from gastric secretions. These metabolites flow into the small intestine where they are absorbed into the systemic circulation. Depending on the chemical structure a significant portion are secreted in bile either in a free (ie. unconjugated) or conjugated form. Free biliary metabolites are absorbed from the upper small intestine whereas bacteria in the large intestine hydrolyse the conjugated biliary metabolites to promote further absorption. Biliary derived metabolites are enterohepatically recycled but contribute little to the peripheral plasma metabolite pool. In this review, Des Hennessy discusses these issues in relation to the pharmacology of BZ drugs in the gastrointestinal tract of ruminants.

Effect of feed type on the pharmacokinetic disposition of oxfendazole in sheep

The plasma concentration profiles of oxfendazole (OFZ), fenbendazole (FBZ) and FBZ sulphone (FBZ.SO2) were measured followed intraruminal administration of OFZ at 5 mg kg-1 to Merino weaners fed either dry forage or grazed on pasture lucerne clover. Plasma concentrations of OFZ and FBZ were significantly lower in sheep given the dry forage.

Influence of feeding schedule on the absorption of orally administered flunixin in the horse

The effects of access to hay and of restricted feeding on the pharmacokinetics of flunixin administered orally to six healthy ponies were compared in a cross-over study. No access to feed for a few hours before and after flunixin administration resulted in rapid absorption with a mean peak plasma concentration of 2.84 +/- 0.28 micrograms/ml attained in an average time of 0.76 +/- 0.18 h, followed by an exponential decline in plasma concentration. A lower peak plasma concentration was obtained when ponies had free access to hay before and after drug dosing. The mean maximum concentration (Cmax) was 1.30 +/- 0.23 micrograms/ml and maximum time (tmax) was prolonged to a mean time of 7.66 +/- 1.74 h. Free access to hay reduced and delayed the peak plasma concentration resulting in two or three separate concentration peaks in some ponies. The mean area under the plasma concentration-time curve was not significantly different for the two feeding regimens.