Pharmacokinetic disposition of an immediate-release aminophylline and a sustained-release theophylline formulation in the horse

Systematic analysis to assess the scientific validity of the international residue limits for caffeine and theophylline in horse-racing

Based on their performance-enhancing potential, caffeine and theophylline are prohibited substances in equine sports. Residues in horses can be caused by wilful application or by unintended uptake of contaminated feed. The International Federation of Horseracing Authorities recently introduced international residue limits (IRLs) to facilitate the discrimination between pharmacological relevant and irrelevant concentrations in doping samples. The objective of this study was to investigate the scientific validity of these IRLs. A systematic analysis was performed to assess the IRLs by different statistical approaches using published pharmacokinetic data. 31 out of 218 potentially relevant publications met the inclusion criteria. Thereby, both IRLs were found to be appropriate for the exclusion of the presence of a relevant pharmacological effect after a wilful application. The IRL of theophylline was also determined to be suitable for the prevention of positive doping tests caused by the ingestion of contaminated feed. In contrast, the IRL of caffeine is not suitable to prevent positive doping test caused by the ingestion of more than 10 mg caffeine per day per horse with contaminated feed. The lack of corresponding regulation for paraxanthine, a major active metabolite of caffeine and theophylline, was recognised as a substantial shortcoming of the current system, rendering both IRLs incomplete.

Effects of hydrocortisone and aminophylline on the aggregation of equine platelets in vitro

The purpose of this study was to evaluate in vitro the effects of hydrocortisone and aminophylline on adenosine diphosphate (ADP)-induced platelet aggregation in horses. Blood samples from 30 healthy Thoroughbred horses were collected by via jugular venipuncture to assess platelet aggregation. Platelet-rich and platelet-poor plasma were prepared from all samples by centrifugation and divided into three different aliquots. In the first aliquot, platelet aggregation was measured after platelet activation with 1 µM and 0.5 µM ADP (Group A). In the other two aliquots, the effect of a 10 min preincubation with hydrocortisone (Group B) or aminophylline (Group C) on ADP-induced aggregation at final ADP concentrations of 1 µM and 0.5 µM was observed. Platelet aggregation, recorded by an aggregometer, was evaluated by measuring the maximum degree of platelet aggregation and the initial velocities of platelet aggregation were obtained. Our results demonstrated the inhibitory effect of hydrocortisone and the induction effect of aminophylline on equine platelet responses in vitro.

Theophylline does not potentiate the effects of a low dose of dexamethasone in horses with recurrent airway obstruction

REASONS FOR PERFORMING STUDY

Theophylline has been shown to have corticosteroid-sparing effects for the treatment of human asthma. A similar effect, if present in horses, would allow diminishing the dose of corticosteroids administered to equine patients with inflammatory airway diseases.

OBJECTIVES

To evaluate whether theophylline potentiates the effects of a low dose of dexamethasone when treating horses with recurrent airway obstruction (RAO).

HYPOTHESIS

Theophylline has steroid-sparing effects in horses with RAO.

METHODS

Ten mature mixed breed horses in clinical exacerbation of RAO were studied. Using an incomplete crossover design and 3 experimental periods of 7 days duration, horses were distributed randomly in 5 treatment groups; and administered dexamethasone s.i.d., at either 0.05 mg/kg bwt i.v. or per os, or 0.02 mg/kg bwt alone or combined with theophylline at 5 mg/kg bwt per os b.i.d. A fifth group was treated with theophylline alone at the above dosage. Lung function was evaluated prior to drug administration and then 3 and 7 days later.

RESULTS

Oral administration of dexamethasone alone or combined with theophylline failed to improve lung function significantly in RAO affected horses. Theophylline alone also failed to improve lung function in all treated horses. Conversely, dexamethasone administration at 0.05 mg/kg bwt i.v. resulted in a significant improvement in lung function starting on Day 3.

CONCLUSIONS AND POTENTIAL RELEVANCE

Oral theophylline for 7 days did not improve the effects of a low dose of dexamethasone for the treatment of horses with RAO.

Equine respiratory pharmacology

Differentiation of diseases of the equine respiratory tract is based on history, clinical signs, auscultation, endoscopy, imaging, and sampling of airway exudate. Upper respiratory therapies include surgical correction of airway obstructions; flushing of localized abscesses (strangles), guttural pouch disease, or sinusitis; and oral or parenteral antibiotic and anti-inflammatory therapy if deemed necessary. Pneumonia usually is treated with antimicrobials, anti-inflammatories, and bronchodilators. Pleural drainage is indicated if significant pleural effusion is present. The most commonly used therapies for early inflammatory and chronic allergic obstructive conditions include bronchodilators and anti-inflammatories. Acute respiratory distress, particularly acute pulmonary edema, is treated with diuretics (usually furosemide), intranasal oxygen, bronchodilators, corticosteroids, and alleviation of the underlying cause. Furosemide also had been used in North America as a race-day preventative for exercise-induced pulmonary hemorrhage (EIPH), but recent data have shown that furosemide may be a performance-enhancing agent itself.

Absorption and dosage of theophylline in the horse after single and repeated administration of a microencapsulated preparation

The kinetics of 2 formulations of theophylline were studied in horses. In an initial cross-over study (Phase I) serum concentration-time curves were determined for granulated and microencapsulated theophylline after a single oral administration (5 mg/kg bwt). In Phase II microencapsulated theophylline was administered at 5 mg/kg bwt/12 h for 10 days at feeding time, as in normal clinical practice. Although no significant differences between the 2 preparations were found with respect to the main kinetic parameters, the microencapsulated form was more evenly and completely absorbed from the digestive tract; furthermore, after the repeated treatment, its trough-peak serum concentrations were always within the therapeutic window and no toxic effects were observed in treated animals.

Drug therapy in the neonatal foal

The neonatal period in foals refers to the first 7 days of postnatal life. The effects of drugs (pharmacologic agents) may be different in neonatal foals, particularly during the first 3 days of postnatal life, from those in older foals and adult horses. The changed drug effects decrease as the physiologic processes that affect absorption, distribution, and elimination (metabolism and excretion) of drugs mature. Dosage regimens should take into account the altered pharmacokinetic profiles of drugs, and because of wide individual variation, the response to therapy should be closely monitored for signs of toxicity. In conjunction with the prudent use of drugs, good nursing care and the provision of supportive therapy are critical in the management of neonatal foal diseases. Over-crowding imposes stress upon young foals and predisposes them to an increased incidence of bacterial and parasitic infections. The collection of specimens for precise microbiologic diagnosis and correction of deficits in serum immunoglobulins should precede antimicrobial therapy. Although E. coli is by far the most common cause of bacterial infections in neonatal foals, other bacterial pathogens of unpredictable susceptibility often cause infection. The selection of an antimicrobial drug for specific therapy should be based on both the microbiologic (quantitative susceptibility) and pharmacologic (pharmacokinetic) properties of the drug. The use of an antimicrobial drug or combination of drugs that will produce a bactericidal effect is highly desirable. Whenever possible, a parenteral preparation that can be administered intravenously should be chosen. The bioavailability and selectivity of action of pharmacologic agents are influenced by the dosage form and route of administration. Diazepam is the sedative drug of choice for neonatal foals. Cimetidine, an H2-receptor antagonist, may be indicated in foals diagnosed to have gastric ulcers; hepatic microsomal oxidative metabolism of drugs administered concurrently with cimetidine is decreased. Nonsteroidal anti-inflammatory drugs (flunixin, phenylbutazone) have a higher incidence of toxicity in foals and, when indicated, should be used at lower dosage than in adult horses. Even though it is highly important to maintain hydration status and electrolyte balance, intravenous infusion should always be performed slowly. Immature renal function decreases the ability of the neonatal animal to excrete excess fluid. The use of drugs in neonatal foals requires greater precision in dosage, more attention to the route and rate of administration, and close monitoring of pharmacologic effects.

Bioavailability and bioequivalence of veterinary drug dosage forms, with particular reference to horses: an overview

The route of administration and formulation of the dosage form affect the bioavailability (rate and extent of absorption) of a drug and may thereby influence the intensity and duration of the pharmacological effect. Location of injection site may affect the plasma concentration profile of drugs administered as aqueous suspensions or sustained release parenteral preparations (procaine penicillin G). When absorption influences the rate of elimination ('flip-flop' phenomenon), the apparent half-life of the drug will be increased (cefazolin sodium, i.m.; meclofenamic acid, p.o.). Absorption generally approximates a first-order process and either the absorption half-life or the mean absorption time (statistical moment term) will provide an estimate of the rate of absorption. The method of corresponding areas is the usual technique employed in estimating the extent of absorption (systemic availability). Inherent in this technique is the assumption that clearance of the drug remains unchanged. In horses, the time of feeding relative to oral dosing has been shown to affect systemic availability (rifampin, trimethoprim) and pattern of absorption (phenylbutazone). Oral paste formulations (trimethoprim-sulphadiazine, ivermectin) are convenient to administer, allow precision in dosage compared with powders or granules added to feed, and could provide sustained release. Assessment of bioequivalence is based on relative bioavailability, using a reference dosage form, together with a measure of the uncertainty (variance) of the estimate. Bioequivalence relies on the concept that preparations of a drug which provide essentially equivalent plasma concentration profiles should produce the same therapeutic effect.