Effects of flunixin meglumine, phenylbutazone and a selective thromboxane synthetase inhibitor (UK-38,485) on thromboxane and prostacyclin production in healthy horses

Sparing the gut: COX-2 inhibitors herald a new era for treatment of horses with surgical colic

Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used to manage a wide variety of conditions in horses, including management of colic. Flunixin meglumine is by far the most commonly used drug in the control of colic pain and inflammation and has become a go-to for not only veterinarians but also horse-owners and nonmedical equine professionals. NSAID use, however, has always been controversial in critical cases due to a high risk of adverse effects associated with their potent cyclo-oxygenase (COX) inhibition. There are two important COX isoenzymes: COX-1 is generally beneficial for normal renal and gastrointestinal functions and COX-2 is associated with the pain and inflammation of disease. Newer selective NSAIDs can target COX-2-driven pathology while sparing important COX-1-driven physiology, which is of critical importance in horses with severe gastrointestinal disease. Emerging research suggests that firocoxib, a COX-2-selective NSAID labelled for use in horses, may be preferable for use in colic cases in spite of the decades-long dogma that flunixin saves lives.

Review of furosemide in horse racing: its effects and regulation

Furosemide has been used empirically and has been legally approved for many years by the US racing industry for the control of exercise-induced pulmonary haemorrhage (EIPH) or bleeding. Its use in horses for this purpose is highly controversial and has been criticized by organizations outside and inside of the racing industry. This review concentrates on its renal and extra-renal actions and the possible relationship of these actions to the modification of EIPH and changes in performance of horses. The existing literature references suggest that furosemide has the potential of increasing performance in horses without significantly changing the bleeding status. The pulmonary capillary transmural pressure in the exercising horse is estimated to be over 100 mmHg. The pressure reduction produced by the administration of furosemide is not of sufficient magnitude to reduce transmural pressures within the capillaries to a level where pressures resulting in rupture of the capillaries, and thus haemorrhage, would be completely prevented. This is substantiated by clinical observations that the administration of furosemide to horses with EIPH may reduce haemorrhage but does not completely stop it. The unanswered question is whether the improvement of racing times which have been shown in a number of studies are due to the reduction in bleeding or to other actions of furosemide. This review also discusses the difficulties encountered in furosemide regulation, in view of its diuretic actions and potential for the reduction in the ability of forensic laboratories to detect drugs and medications administered to a horse within days or hours before a race. Interactions between nonsteroidal anti-inflammatory drugs (NSAIDs) and furosemide have also been examined, and the results suggest that the effects of prior administration of NSAID may partially mitigate the renal and extra-renal effects which may contribute to the effects of furosemide on EIPH.

Effects of age on the pharmacodynamics of naproxen in the rat

The pharmacodynamics of naproxen were evaluated in 5- and 24-month-old male Fischer 344 rats. Plasma naproxen concentrations and thromboxane B2 (TxB2) concentrations were measured as a function of time after intravenous administration of 25 mg/kg naproxen. Age-dependent alterations in naproxen pharmacokinetics were attributed to significant reductions in free plasma clearance (CLfree) and free steady-state volume of distribution (VSSfree), suggesting a decline in metabolic activity and naproxen binding to tissue components in aged rats, respectively. The time course of TxB2 production as a function of unbound naproxen concentrations was described by a sigmoid Emax model. Age had no significant effect on the pharmacodynamic parameter Emax, the maximum percent inhibition of TxB2 formation. Age also had no statistically significant effect on EC50, the drug concentration producing 50% of the maximum effect, however, average EC50 values were 35% higher in the aged rats. The duration of TxB2 inhibition was unaffected by age, possibly owing to similar relative decreases in receptor sensitivity (increased EC50) and increases in free naproxen concentrations (decreased free clearance and volume of distribution). Alternatively, the age-related changes in pharmacokinetic parameters were not of sufficient magnitude to produce a significant change in drug response, naproxen, pharmacokinetics, pharmacodynamics, age, rats, thromboxane B2, nonsteroidal anti-inflammatory drugs.

Effect of phenylbutazone on the haemodynamic, acid-base and eicosanoid responses of horses to sustained submaximal exertion

The systemic haemodynamic and acid-base effects of the administration of phenylbutazone (4.4 mg kg-1 intravenously) to standing and running horses were investigated. Phenylbutazone, or a placebo, was administered to each of six mares either 15 minutes before, or after 30 minutes of a 60-minute submaximal exercise test which elicited heart rates approximately 55 per cent of maximal, and to the same horses at rest. The variables examined included the cardiac output, heart rate, systemic and pulmonary arterial pressures, right atrial and right ventricular pressures, and arterial and mixed venous blood gases and pH. Serum sodium, potassium and chloride concentrations, and plasma thromboxane B2, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), and prostaglandin E2 (PGE2) concentrations were measured in separate studies using similar protocols in the same horses. Running produced increases in heart rate, cardiac output, mean arterial and right ventricular pressure, and decreases in total peripheral resistance. The acid:base responses to exertion were characterised by respiratory alkalosis. Exertion did not significantly influence plasma 6-keto-PGF1 alpha or PGE2 concentrations but plasma thromboxane B2 concentrations were increased significantly by 60 minutes of exertion in the untreated horses. This exercise-induced increase in plasma thromboxane B2 concentration was inhibited by the previous administration of phenylbutazone, but phenylbutazone did not produce detectable changes in systemic haemodynamic or acid-base variables in either standing or running horses.

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.

Comparison of in vitro effects of flunixin and tolfenamic acid on human leukocyte and platelet functions

A study was made to compare the effects of two nonsteroidal antiinflammatory drugs (NSAIDs), flunixin and tolfenamic acid, on the leukotriene B4 (LTB4) production and migration of human polymorphonuclear leukocytes (PMNs) as well as on platelet aggregation and thromboxane B2 (TxB2) production during blood clotting. Tolfenamic acid inhibited LTB4 production in PMNs as well as FMLP- and LTB4-induced PMN migration (IC50 values 23 +/- 3, 39 +/- 11, and 68 +/- 13 microM, respectively), whereas flunixin inhibited these cell functions only with the highest concentration tested (100 microM). On the other hand, flunixin was clearly a more potent inhibitor of TxB2 production and adrenaline-induced platelet aggregation than tolfenamic acid, the IC50 values in TxB2 production being 0.28 +/- 0.02 microM and 2.6 +/- 0.3 microM for flunixin and tolfenamic acid, respectively. We suggest that inhibition of PMN functions may be an additional mechanism in the antiinflammatory action of tolfenamic acid. At least in human PMNs and platelets, flunixin seems to be only an inhibitor of cyclooxygenase.

Plasma concentrations of flunixin in the horse: its relationship to thromboxane B2 production

The effects of the intravenous (i.v.) administration of 1.1 mg/kg of flunixin meglumine on thromboxane B2 (TxB2) concentrations were studied in sedentary and 2-year-old horses in training. The baseline TxB2 serum concentrations generated during clotting were 2.89 +/- 0.81, 2.19 +/- 0.25 and 0.88 +/- 0.12 ng/ml for the 2-year-old Thoroughbreds in training, sedentary horses under 10 and over 10 years old, respectively. There was a significant difference in baseline TxB2 concentrations between older and younger horses (P less than 0.005). Significant reduction in TxB2 production from baseline were noted at 1 (P less than 0.01) and 4 h (P less than 0.01) but not at 8 h after flunixin administration. The percent reduction in serum TxB2 concentration at 1 h after the administration of flunixin was 68.6 +/- 7.3 and 45.2 +/- 6.8 for the training and sedentary horses, respectively; the differences were significant (P less than 0.04). Serum concentrations of TxB2 returned to baseline values by 12-16 h after flunixin administration. The results of this study indicate a difference in the TxB2 concentrations of older vs. younger horses and a difference in the suppression of TxB2 after the administration of flunixin in 2-year-old Thoroughbreds in training compared to sedentary horses. The results of this study suggest that the detection of low concentrations of flunixin in urine 24 h post-administration may not represent pharmacologic effective concentrations of flunixin in plasma.

Regulation of prostaglandin biosynthesis with flunixin meglumine in the bovine species

Flunixin meglumine (FM) was injected in 2 oophorectomized cows to follow changes in basal levels of the main circulating prostaglandin (PG)F2 alpha metabolite, 15-ketodihydro-PGF2 alpha. A rapid decrease in the levels was seen after FM and the effect was lasting for about 6 h. Thus, to obtain a full effect of the drug on prostaglandin synthesis it is recommended that FM should be injected 4 times daily. This concept was further studied in 3 cycling heifers which obtained FM 4 times daily from day 15 of the estrous cycle for 7 days (totally 28 injections). During the period of drug administration, prostaglandin metabolite levels were decreased and the expected pulsatile release seen during luteolysis was delayed. The pulsatile release started about one day after cessation of treatment and then luteolysis occurred. Progesterone levels were normal during the FM treatment and dropped concomitantly with the pulsatile release of PGF2 alpha. The levels of progesterone decreased to low levels before the heifers showed signs of estrus and ovulated. The administration of FM causes a situation resembling that seen during early pregnancy and FM can be a useful tool in understanding the mechanism behind maternal recognition of pregnancy.

Drugs that alter the hemostatic mechanism

This article offers a brief review of antiplatelet drugs, anticoagulants, fibrinolysins, and antifibrinolysins. Aspirin and heparin are discussed in detail. Warfarin, streptokinase, urokinase, and tissue-type plasminogen activator are also considered. Indications for use, mechanism of action, monitoring therapy, and current investigational studies are discussed.

Low dose flunixin meglumine: effects on eicosanoid production and clinical signs induced by experimental endotoxaemia in horses

The efficacy of low doses of flunixin meglumine in reducing eicosanoid generation and clinical signs in response to experimentally induced endotoxaemia was investigated. Thromboxane B2 and 6-keto-prostaglandin F1 alpha were measured in serum and plasma by radioimmunoassay. Plasma flunixin concentrations were determined by high performance liquid chromatography and pharmacokinetic parameters derived non-compartmentally. In horses administered flunixin meglumine before endotoxin challenge, a significant suppression in plasma thromboxane B2 and 6-keto-prostaglandin F1 alpha generation was observed. Elevations in blood lactate were significantly suppressed in horses pretreated with 0.25 mg/kg bodyweight flunixin meglumine. Reduction of the clinical signs of endotoxaemia by flunixin meglumine was dose dependent. Low doses of flunixin inhibited eicosanoid production without masking all of the physical manifestations of endotoxaemia necessary for accurate clinical evaluation of the horse's status.