Medication: a way forward from zero tolerance to irrelevant plasma concentrations Webbon 2002
This short editorial discusses the paper by Toutain and Lassourd (2002), which was published in the same edition of the journal. Webbon explains the problem associated with a ‘zero tolerance’ approach to drug testing, i.e. that modern analytical methods mean that legitimately used therapeutic substances may be detected for long after they cease to have an effect, thereby potentially impeding their valid therapeutic use in competition animals. By using data from studies that determine the concentration of a drug in plasma (1)  at which it has an effect, its ‘effective plasma concentration’, one can derive an ‘irrelevant drug concentration’ that can be used as a limit of detection. A knowledge of the relationship between plasma and urine concentrations of the drug then permits the calculation of an ‘irrelevant urine concentration’ for similar purposes. This approach can be used to control the use of therapeutic agents during competition without precluding their legitimate prescription for treatment. It must be remembered that the use of this method does not render drugs to which it is applied ‘permitted’ medications, since they have no therapeutic effect at and below the irrelevant concentration.

Link to relevant paper (pdf) - Medication: a way forward from zero tolerance to irrelevant plasma concentrations

(1) Blood may be considered to consist of a solution of many substances in water, also containing cells in suspension. The term ‘plasma’ refers to the liquid phase of the blood, i.e. blood - cells = plasma

Evaluation of the analgesic effects of phenylbutazone administered at a high or low dosage in horses with chronic lameness
Hu et al (2005)
Phenylbutazone was administered intravenously, at doses of 4.4 mg/kg or 8.8 mg/kg (2)  once daily for four days, to nine horses that were lame due to navicular disease (some had other orthopaedic conditions, too). Its effect on their lameness was assessed and compared to the effect of a saline placebo using both clinical observation and gait analysis (3).
A similar degree of improvement in lameness, as demonstrated by gait analysis, was seen for phenylbutazone at both dose rates six, twelve and twenty four hours after the final dose. Clinical improvement was seen at six and twelve hours with both doses and at twenty four hours with the higher dose.
This study demonstrates that phenylbutazone has an analgesic effect for at least twenty four hours after this dosage regime. There was no advantage in terms of analgesic effect in using the higher of the two doses.

Link to relevant paper (pdf) - Evaluation of the analgesic effects of phenylbutazone administered at a high or low dosage in horses with chronic lameness

(2) These doses are equivalent to 2g and 4g respectively when given to a 450 kg horse.
(3) The latter using a force plate to measure average peak vertical force (a measure of the amount of weight taken on the limb) on the lamer forelimb.

Effects of single-dose intravenous phenylbutazone on experimentally induced, reversible lameness in the horse
Foreman et al (2008)
The effect of a single intravenous injection of phenylbutazone at a dose rate of 4.4mg/kg was investigated in eight horses in which lameness had been induced using an adjustable heart bar shoe. The shoe, which was fitted to the left fore foot, could be adjusted so that it caused pressure on the sole, resulting in a temporary, marked lameness (grade four out of five).
The phenylbutazone was given one hour after induction of lameness and the degree of lameness, some blood biochemical factors and the plasma concentrations of phenylbutazone and one of its breakdown products, oxyphenbutazone, were measured over the next eight hours.
The degree of lameness was assessed only in the box since to walk and trot the horses for this would have affected their heart rates. After administration of phenylbutazone, lameness score was significantly reduced within an hour and remained so over the remainder of the eight hours of monitoring, whilst heart rate was reduced between two and six hours after treatment. Eight hours after administration of phenylbutazone, when there was still evidence of a clinical effect of treatment, the plasma concentrations of phenylbutazone and oxyphenbutazone were 7.2-7.5 and 1.6-1.9 μg/ml respectively. Administration of phenylbutazone to control horses that were not lame did not affect their heart rates.

Link to relevant paper (pdf) - Effects of single dose phenylbutazone on experimentally induced, reversible lameness in the horse

Effectiveness of administration of phenylbutazone alone or concurrent administration of phenylbutazone and flunixin meglumine to alleviate lameness in horses
Keegan et al 2008
The effect on naturally-occurring lamenesses of phenylbutazone given alone (2.2mg/kg by mouth twice daily for five days (4) ) was compared with that obtained using this dose of phenylbutazone together with flunixin meglumine (1.1 mg/kg twice daily by intravenous injection, also twice daily for five days). Twenty nine horses with a variety of orthopaedic conditions were used; their lameness was assessed using gait analysis during treadmill exercise (5).
One horse died of an acute inflammatory condition of the large intestine (6)  after receiving the combined treatment and this was consistent with the known toxic effects of treatment with non-steroidal anti-inflammatory agents such as the two used in the study. In a separate report, these authors found that combination therapy using the dosage regimen described here caused a greater degree of gastric ulceration than did phenylbutazone alone at the same dose.
There was some variability in response and five animals did not respond to either treatment regimen, in general however, the combination therapy using both drugs alleviated lameness more than did the use of phenylbutazone alone. Neither treatment regimen resulted in a complete alleviation of lameness.
Both drugs were detectable in blood samples obtained twelve hours after the final treatment in all cases except one horse in which flunixin meglumine was not detected. The plasma concentrations of the two drugs were highly variable.
The authors reported in discussion the study of Semrad et al (1993) in which it was found that administration of a combined single dose of phenylbutazone (2.2 mg/kg by intravenous injection) and flunixin meglumine (1.1 mg/kg also given intravenously) had a longer effect in decreasing the serum concentration of a chemical mediator of inflammation (7)  than did either drug given alone.

Link to relevant paper (pdf) - Effectiveness of administration of phenylbutazone alone or concurrent administration of phenylbutazone and flunixin meglumine to alleviate lameness in horses

(4) Equivalent to 1g twice daily for a 450 kg horse.
(5) The method of evaluation involved kinematic analysis in which the movement of markers attached to the horse was assessed to assess symmetry of gait.
(6) Acute necrotising colitis that was most severe in the right dorsal colon.
(7) Thromboxane-B.

Use of force plate analysis to compare the analgesic effects of intravenous administration of phenylbutazone and flunixin meglumine in horses with navicular syndrome
Erkert et al (2005)
The analgesic effects of phenylbutazone (PBZ) and flunixin meglumine (FXN) were compared in twelve horses with navicular syndrome. Horse received, on separate occasions, four day courses of treatment with phenylbutazone (given at 4.4 mg/kg per day (8) ), flunixin meglumine (1.1 mg/kg per day1) and saline (as a control), all given intravenously. The effects of treatment were assessed by clinical observation and gait analysis (9) .
The two treatments were associated with a similar degree of improvement in lameness at six, twelve and twenty four hours after the final dose was administered. It is of interest that when given at these commonly-used levels of administration, the two agents showed a similar degree of analgesia for lameness associated with navicular syndrome. It has previously been suggested that they have differing efficacies for the treatment of musculoskeletal conditions and conditions involving visceral pain (colic).

Link to relevant paper (pdf) - Use of force plate analysis to compare the analgesic effects of intravenous administration of phenylbutazone and flunixin meglumine on horses with navicular syndrome

(8) These dose rates are typically used in equine practice, the one for phenylbutazone corresponding to 2g for a 450kg horse.
(9) Gait analysis involved the use of a force plate to measure peak vertical force during forelimb footfall.

Effect of Flunixin Meglumine on Selected Physiologic and Performance Parameters of Athletically Conditioned Thoroughbred Horses Subjected to an Incremental Exercise Stress Test. Colahan et al
The effect of flunixin meglumine on exercising horses was investigated using twelve trained, clinically normal Thoroughbred animals. The horses performed a standardised treadmill exercise test with and without the administration of flunixin meglumine one hour prior to the test at a dose rate of 1.1 mg/kg (10). The drug did not improve exercise performance in this study.
Neither the average time for which horses could continue to exercise nor their average peak oxygen consumption (11) was affected by the administration of flunixin meglumine. Haematological parameters and the concentration of some blood constituents (12)  were also unaffected. Flunixin meglumine prevented the increases in blood concentrations of certain inflammatory mediators (13) (chemicals that can cause inflammation) that otherwise occurred during exercise. It was also associated with an increase in urine creatinine concentration four hours after exercise, although the practical significance of this is uncertain.
Flunixin was detected (14) in the blood samples of some horses for up to 36 hours after administration and in urine samples for up to 120 hours. Exercise had no effect on the rate of elimination of the drug. The authors cite other work that found flunixin to be detectable in urine for up to fifteen days in some animals.

Link to relevant paper (pdf) - Effect of Flunixin Meglumine on Selected Physiologic and Performance Parameters of Athletically Conditioned Thoroughbred Horses Subjected to an Incremental Exercise Stress Test

(10) A typically used dose rate in equine clinical practice.
(11) A measure of an animal's aerobic capacity.
(12) Including sodium, potassium, chloride, calcium, glucose, CO2 content, cortisol, ACTH, insulin, ß-endorphin and creatinine.
(13) Thromboxane-B2 and prostaglandin F1.
(14) Usin an ELISA assay.
 

Nonsteroidal anti-inflammatory agents and musculoskeletal injuries in Thoroughbred racehorses in Kentucky. Dirikolu et al 2008
The presence and concentration of three nonsteroidal anti-inflammatory drugs (NSAIDs) were determined in horses that sustained musculoskeletal injuries during racing in Kentucky in 1995 and 1996 and compared with those of the winning horses and random control horses in the races in which the injury occurred. After excluding horses in which the cause of injury was unrecognised or the data were incomplete, 161 cases were reported, 70 of which suffered injuries necessitating euthanasia.
The majority of horses in all groups had detectable plasma concentrations of phenylbutazone although these were less than 7 μg/ml (which the authors propose as the minimum effective plasma concentration) in most cases. The average plasma concentration of phenylbutazone in the injured horses was 5.84 μg/ml and this was significantly higher than the values for control and winning horses. The majority of horses also had detectable plasma concentrations of flunixin and these were all greater than 0.1 μg/ml (which is said to be a likely effective plasma concentration for this drug). Both injured and winning horses had significantly higher average plasma concentrations of flunixin than the control horses. Only small numbers of horses (less than eight percent in any group) had detectable plasma concentrations of naproxen. It is uncertain from this study whether the higher plasma concentrations of phenylbutazone or flunixin in the injured horses relate to a deleterious effect of these agents or whether they are indicative of a group with a higher prevalence of pre-existing injuries prior to racing.

Link to relevant paper (pdf) -Nonsteroidal anti-inflammatory agents and musculoskeletal injuries in Thoroughbred racehorses in Kentucky

Comparisons of efficacy and safety of paste formulations of firocoxib and phenylbutazone in horses with naturally occurring osteoarthritis
Doucet et al 2008
The efficacy and safety of phenylbutazone (given at a dose rate of 4.4 mg/kg by mouth daily (15) ) was compared with that of firocoxib (0.1 mg/kg by mouth daily) in 253 horses suffering from osteoarthritis or navicular syndrome.
Firocoxib is a member of a relatively new class of non-steroidal anti-inflammatory drugs (NSAIDS - the category that includes phenylbutazone, flunixin meglumine, etc.), which have been developed with the aim of producing drugs with fewer side effects than the older NSAIDs.
Horses were randomly assigned to two groups for treatment with one or other of the two drugs. After 14 days of treatment, around 85% of both groups of horses had shown clinical improvement, with no significant difference in the proportions that were improved clinically between the two groups, although there was greater improvement in some of the categories that were assessed for horses treated with firocoxib. It was concluded that overall firocoxib and phenylbutazone at these dosage regimens had comparable clinical efficacy for the conditions treated. No direct treatment-related adverse effects were detected for either drug.

Link to relevant paper (pdf) -Comparisons of efficacy and safety of paste formulations of firocoxib and phenylbutazone in horses with naturally occuring osteoarthritis

(15) Equivalent to a 2g dose to a 450 kg horse.

Initial opinions on the effects of phenylbutazone (PBZ) and historical prospective
(Soma)
This monograph provides a brief historical note on the use of phenylbutazone, which was introduced into veterinary practice in the 1950’s. It then goes on to review the literature regarding nonsteroidal anti-inflammatory drugs (NSAIDs) in the horse, with particular reference to phenylbutazone.
It is noted that NSAIDs can cross the blood-brain barrier and have central effects and that they can also enter synovial fluid. Most of the papers reviewed in this document that discuss the effects of phenylbutazone in naturally-occurring equine orthopaedic disorders and artificially-induced lameness appear elsewhere on this site. An exception is the study of Owens et al. (1995) that found that ketoprofen, given at a dose rate of 3.63 mg/kg, appeared more potent that phenylbutazone given at 4.4 mg/kg for the alleviation of pain in seven horses with chronic laminitis.
The use of methods of assessing the actions of NSAIDs other than those using naturally-occurring disorders is discussed and noteworthy here are the studies that have shown that phenylbutazone, its metabolite (breakdown product) oxyphenbutazone and flunixin are all cleared more slowly from inflammatory fluids than from plasma; this is likely to extend the effectiveness of these drugs in lesions.
Soma concludes, on the basis of the scientific literature and the clinical impressions of veterinarians, that at a dose rate of 4.4 mg/kg, phenylbutazone shows an effect at twenty four hours after administration.

Link to relevant paper (pdf) - Awaiting permission to offer full article

Pharmacokinetic/pharmacodynamic approach to assess irrelevant plasma or urine drug concentrations in postcompetition samples for drug control in the horse. Toutain and Lassourd 2002
The paper discusses a method for the determination of irrelevant plasma concentrations (IPC) and irrelevant urine concentrations (IUC) of drugs, i.e. ones at which they have no effect and which can, therefore, be used to set limits of detection for medication control procedures. For substances that are totally prohibited, analysing laboratories can report their presence in samples at all levels of detection. However, for substances that are legitimately used therapeutically such an approach could interfere with rational veterinary care and for them it is suggested that it is more appropriate to use calculated IPCs and IUCs to set limits of detection.

Link to relevant paper (pdf) - Pharmacokinetic/pharmacodynamic approach to assess irrelevant plasma or urine drug concentrations in postcompetition samples for drug control in the horse

Link to relevant paper (pdf) -Quantification and confirmation of flunixin in equine plasma by liquid chromatography-quadrupole tome-of-flight tandem mass spectrometry