Reasons for performing study
There are no refereed controlled documentations of the skeletal analgesic efficacy of different dosages of flunixin meglumine (FM).
There are no refereed controlled documentations of the skeletal analgesic efficacy of different dosages of flunixin meglumine (FM).
The objective of this experiment was to compare the efficacy of various dosages of FM with a negative control. The hypothesis was that higher doses would result in improved efficacy in a dose-dependent manner when tested in a reversible model of foot lameness.
Ten horses shod with adjustable heart bar shoes had weekly modified AAEP grade 4.0/5.0 lameness induced by tightening a set screw against the heart bar. Heart rate (HR) and lameness score (LS) were monitored by one double-blinded investigator at rest; every 20 min after lameness induction for 5 h and hourly for another 8 h. One hour after lameness induction, treatments were administered i.v. in a randomised order: negative control (isotonic saline: SAL) or FM at 0.55 (half-dose), 1.1 (single-dose) or 2.2 (double-dose) mg/kg bwt. Results were compared using RM ANOVA and Student–Newman–Keul's test with the level of significance set at P<0.05.
Compared to SAL, half-dose FM reduced HR at 2.33, 2.67, 4.0–8.0, and 10.0 h and LS at 1.33–12.0 h (P<0.05). Single- and double-dose FM reduced HR from 0.67 to 12.0 h and LS from 1.0 to 12.0 h post administration (P<0.05). Compared with half-dose FM, single- and double-dose LS were further decreased from 1.67 to 12.0 h post administration (P<0.05). Mean peak and decaying plasma FM concentrations were different between dosages in a dose-dependent manner through 6 h post administration (P<0.05).
Flunixin meglumine administration affected dependent variables in a dose-dependent manner with half-dose FM clinically effective for a shorter period. Higher dosages did not perform differently from one another.
Practitioners must be aware that half-doses of FM are less efficacious than single doses but double doses are not more efficacious and yet are potentially more toxic.
Flunixin meglumine (FM) has been used to induce analgesia in horses for a variety of conditions for over 30 years [1-4]. Initially, FM was characterised as primarily a visceral analgesic [1-4], whereas phenylbutazone was frequently touted as being better for musculoskeletal pain . However, more recent research has documented the value of FM as an analgesic agent for the relief of musculoskeletal pain [6-11]. Many of these recent studies have shown that FM given at 1.1 mg/kg bwt i.v. is similar in its ability to reduce lameness as standard single doses (4.4 mg/kg bwt i.v.) of phenylbutazone [8, 10, 11]. Unfortunately, there appear to be no refereed publications on the musculoskeletal analgesic efficacy of different dosages of FM in a controlled setting.
The objective of this study was to compare the clinical efficacy of various dosages of FM against a negative control. The hypothesis was that higher FM dosages would result in improved efficacy in a dose-dependent manner in a reversible model of equine foot lameness.
All materials and methods used for this study were used under the approval and authority of the University of Illinois Institutional Animal Care and Use Committee.
Ten horses (5 Thoroughbreds, 5 Quarter Horses) were studied for 4 weeks. The mean ± s.e. age of the horses was 9.1 ± 0.9 years (median 9.5 years, range 3–12 years) and mean body weight (bwt) 510.6 ± 13.8 kg (median 495.2 kg, range 450–550 kg). Complete physical, haematological, serum biochemical and lameness examinations were performed to ensure that each horse was normal before it was shod and experiments commenced. Subjects were documented to be negative for equine infectious anaemia using agar gel immunodiffusion and vaccinated against tetanus, eastern and western equine viral encephalitis, West Nile virus, rhinopneumonitis and influenza. All horses were dewormed with ivermectin administered orally. Each horse's left front foot had an adjustable heart bar shoe applied. The right front foot had a mid-foot bar shoe (without the heart bar but of a similar weight to the left shoe) applied for balance. A minimum of 7 days of stall rest were allowed after shoeing and before any trials commenced and horses confined to stalls throughout the duration of the experiment.
A hexagonal screw was temporarily tightened into a threaded centre hole in the stationary mid-foot bar, thereby causing the heart bar to apply pressure to the frog of the foot [10-16]. The same screw was re-used on each trial day for each individual horse. The number of turns of the screw was standardised for each horse each week to ensure that the same degree of lameness was created on each trial day. Previous research studies performed using this model have shown that heart rate (HR) increases with lameness during and after exercise when compared with negative controls, when the model was used either in laboratory [10-16] or in field training settings . Furthermore, the increase in HR is proportionate to the increasing degree of lameness in this model [12, 18].
The hexagonal set screw was tightened in the heart bar shoe to induce lameness one h prior to administration of treatment. Lameness was scored using a previously described decimal grading system [10-17]. Given that HR is a primary variable of interest in this model [10-16], movement or exercise outside the stall confounds the use of HR measurement as an objective quantifiable determinant of lameness or response to medication. All lameness grading in this model therefore occurs in the individual box stall without jogging the horse and without walking any individual horse other than observing spontaneous walking and turning movements within the stall [10-16].
Lameness grades were based originally on the AAEP Lameness Scale  but were modified for use solely in the stall, much like the Obel scale used to describe the degree of lameness associated with laminitis [10-17]. Lameness grades were: grade 0.0 (sound or undetectable lameness), grade 1.0 (barely detectable lameness; horse rarely looked lame at a walk in the stall, mainly when turning and/or pointed the lame toe forward intermittently and rarely), grade 2.0 (mild lameness; horse was more consistently lame at a walk in the stall, had a mild head bob when walking or turning in the stall and pointed its toe more consistently), grade 3.0 (moderate lameness but still weightbearing; horse had more obvious head bob at a walk, toe pointing more frequently), grade 4.0 (severe lameness, obvious head bob, toe pointing whenever not walking but not 3-legged lame at a walk) and grade 5.0 (nonweightbearing). Lameness that did not meet all the expressed criteria for a given grade was judged to be between 2 major points on the scale (e.g. a lameness might have been graded as 3.3 rather than grades 3 or 4 as the only ordinal possibilities for that horse at that point in time), thus rendering the scale continuous rather than a 6-point ordinal scale. A lameness score of grade 4.0 was achieved initially in each subject undergoing a lameness trial.
In the United States the standard FM dose is 500 mg to a 450 kg horse or 1.1 mg/kg bwt, either orally or i.v [8, 10, 11]. Treatments administered in this experiment once weekly therefore included: half-dose FM (0.55 mg/kg bwt i.v. [FlunixiJect, 50 mg/ml flunixin meglumine injectable])1 single-dose FM (1.1 mg/kg bwt i.v.), double-dose FM (2.2 mg/kg bwt i.v.) and isotonic saline as a negative control (1 ml/45 kg bwt i.v., yielding a volume similar to that when single-dose FM is administered: [0.9% Sodium Chloride Solution U.S.P., 1000 ml])2. Treatments were administered by a single coinvestigator according to a randomisation schedule so that all treatment order permutations were represented. Treatment periods were at least 7 days apart to ensure washout of the previously administered treatment; this washout period has been shown to be appropriate in previous studies in which one i.v. administered dose of nonsteroidal anti-inflammatory drugs (NSAIDs) have been evaluated using this model [10, 11, 13, 16] and based on an inability to detect FM in plasma less than one week after single i.v. doses (J.H. Foreman, unpublished data). The coinvestigator administering treatments was responsible for protecting the secrecy of the order of treatment assignments to ensure that the principal investigator measuring HR and lameness score remained blinded throughout each trial.
Heart rate (determined by indirect stethoscope auscultation) and lameness score were monitored in the stall each day before any experimentation commenced (resting sample at Hour -2), every 20 min for the first 5 h of each lameness trial and then hourly for an additional 8 h. Drug or saline was administered 1 h after lameness induction and horses monitored for an additional 12 h. Heparinised jugular venous blood samples were obtained from the jugular vein opposite to that into which treatment was administered at -1, 0 (immediately before treatment), 0:05 (peak after i.v. drug administration), 2, 4, 8, 12, 24, 48 and 72 h post drug administration. Throughout each trial, blood samples were placed on ice, centrifuged, separated and the plasma frozen. These monitoring and sampling techniques have been used successfully previously with this model [10-16].
Plasma FM concentrations were determined by high-performance liquid chromatography (HPLC) by a Fédération Equestre Internationale (FEI) reference laboratory. Samples were shipped frozen, on ice, overnight to the United States Equestrian Federation Equine Drug Testing and Research Laboratory, located in Ithaca, New York, USA at the time these analyses were completed (summer and autumn of 2010). Samples were masked by collection time and dose administered but not to drug sought (FM). Intra- and inter-assay coefficients of variation were <5% on a single column calibrated in triplicate in order to test each unknown sample once.
Mean (±s.e.) HR, lameness score and FM plasma concentration were determined for each treatment at each sampling interval (SigmaPlot 11.2)3. The lameness score was treated as a continuous variable since a fractionated, nonordinal, decimal grading scale was used. Significance of differences between treatments was tested by repeated measures multivariate analysis (SigmaPlot 11.2)3. When repeated measures analysis indicated significant time-by-treatment interactions, differences at specific sampling intervals were tested by Student–Newman–Keul's test (SigmaPlot 11.2)3. A value of P<0.05 was considered significant. In previous studies using this model, 6–8 horses have been sufficient for this repeated measures design and 10 were ultimately used in this study [10-16].
All treatments were administered at 0 h on the x axis (time) in Figures 1-3.
Half-dose FM reduced HR compared with the negative control at 2.33, 2.67, 4.0–8.0 and 10.0 h after administration (P<0.05). Compared with the negative control, single-dose and double-dose FM reduced HR from 0.67 to 12.0 h post administration (P<0.05). Double-dose HR also was decreased compared with the negative control at 0.33 h (20 min) post administration (P<0.05); otherwise, there was no difference in HR response between single- and double-dose FM. Compared with half-dose FM, single- and double-dose HR were further decreased at 3.0 and 8.0–12.0 h post administration (P<0.05).
Half-dose FM reduced lameness score compared with the negative control at 1.33 to 12.0 h after administration (P<0.05). Compared with the negative control, single-dose and double-dose FM reduced lameness score from 1.0 to 12.0 h post administration (P<0.05). Compared with half-dose FM, single- and double-dose lameness score were further decreased from 1.67 to 12.0 h post administration (P<0.05). There were no differences in lameness score response between single- and double-dose FM.
Plasma concentration of FM peaked at 5 min post injection and decremented similarly afterward regardless of dose. Mean (± s.e.) peak plasma concentrations for half-, single and double-doses were 6.3 ± 0.4 (range 3.7–8.1), 13.1 ± 0.5 (range 10.5–15.7) and 28.8 ± 0.9 (range 23.9–32.4) μg/ml, respectively. Mean peak and decaying concentrations were different between dosages in a dose-dependent manner through 6 h post administration (P<0.05).
There is a dearth of objective controlled data regarding the clinical efficacy of varying doses of FM in lame horses nor does there appear to be a dose titration study in horses in the literature. The results of this dose-titration study showed that the lower dosage (half-dose) of FM had clinical efficacy reflected in changes in HR and lameness score variables but the effects were intermittent and waning near the end of the 12 h monitoring period. However, the single- and double-doses demonstrated clinical efficacy throughout the 12 h monitoring period. Furthermore, there was greater efficacy with single- and double-doses than with half-doses but single- and double-doses were not different from one another in terms of measurable analgesia.
In previously published studies, FM has had good efficacy with 1.1 mg/kg bwt in providing visceral analgesia [1-4]. Visceral models such as intestinal balloon catheters  were originally used to document the visceral efficacy of FM. In contrast, few studies appear in the literature regarding the efficacy of FM as an analgesic for musculoskeletal pain. Erkert and co-workers (2005) used forceplate analysis to compare the efficacy of FM and phenylbutazone in horses with naturally occurring navicular disease  and reported no difference between FM (1.1 mg/kg bwt s.i.d. for 4 days) and phenylbutazone (4.4 mg/kg bwt s.i.d. for 4 days). Studies using a reversible model of foot lameness have also documented comparable efficacy of single i.v. injections of standard full doses of FM and phenylbutazone [10, 11]. In the current FM dose-titration study, the data clearly indicate that the analgesic effect of FM is dose-dependent to a point with single- and double-doses being more effective than half-doses. It is likely that the double-dose of FM did not perform any better than the single-dose because the single-dose must have completely saturated cyclo-oxygenase enzyme sites, making increasing dosages unlikely to provide greater efficacy. These data validate that the standard dosage (1.1 mg/kg bwt) is appropriate for effective management of musculoskeletal pain, whereas the lower dose (half-dose) did not work as well nor did it work as long and more (double-dose) was no more effective than single-dose FM.
It is important for veterinary practitioners to realise from these data that half-doses of FM, while commonly used, have more limited measurable clinical efficacy in equine foot lameness. Veterinarians often use quarter- or half-doses of FM in equine patients with gastrointestinal disturbances such as diarrhoea or post operative toxaemia. This technique is based on findings that smaller doses of FM have effects in vitro and in vivo similar to single-doses in preventing the adverse cellular and physiological effects of endotoxaemia [19-21]. These smaller FM doses are commonly referred to by the inaccurate sobriquet of ‘anti-endotoxic doses’. Unfortunately, many patients with gastrointestinal diseases progress to develop painful laminitic feet, yet these lower doses are clearly not as efficacious for that progressive skeletal pain. Practitioners should always be vigilant to observe horses on lower ‘anti-endotoxic’ FM doses carefully for the development of foot pain, which may require higher single-doses of FM for adequate and appropriate pain control.
Conversely, the use of double-doses of FM in this experiment was no more efficacious than were single-doses. This finding is also important for veterinary practitioners who often are challenged by patients or clients to provide more analgesia for horses with musculoskeletal soreness. While it may be tempting to use larger doses to provide more analgesia, clearly the highest dosage studied here was no more efficacious than the typical single-dose. However, the toxicity of NSAIDs is drug-, dosage- and duration-dependent [22-28], so use of a higher dosage should at least be more efficacious to justify the added risk for toxicity. Our data demonstrate that there is no analgesic benefit to use of the higher-than-standard single-dose. Practitioners must be cautious and judicious in the dosages of NSAIDs they choose to administer and, based on these data, there is no value to the use of highest dosage of FM in foot pain modelled using the shoeing system studied here.
From these data, we concluded that FM administration affected dependent variables in a dose-dependent manner, with the half-dose FM being clinically effective for a shorter period compared with single- or double-doses. The higher dosages did not perform differently from one another, indicating that the single-dose saturates cyclo-oxygenase binding sites and maximises clinical efficacy for at least 12 h in this lameness model. Further research with pharmacokinetic/pharmacodynamic modelling may help to delineate more clearly the minimum effective plasma concentration of FM.
No competing interests have been declared.
Funded by the Fédération Equestre Internationale.
Technical assistance was provided by Mr. Tom Lomangino of the United States Equestrian Federation Drug Testing Laboratory. The authors thank Dr. Pedro for treatment administrations.
Dr Foreman is responsible for all aspects of the paper and project. Dr Schumacher determined plasma drug concentrations. The rest of the authors are responsible for administering therapies, monitoring, patient care.
Butler Schein Animal Health, Dublin, Ohio, USA.
Hospira, Inc., Lake Forest, Illinois, USA.
Systat Software, Inc., San Jose, California, USA.