The term multimodal analgesia describes the use of a combination of different analgesic drug classes to target different points in the pain pathway.1 When these combination regimens are prescribed, an assumption exists that the agents provide analgesia at the suggested dosages. However, when it comes to considering acute and chronic pain management in dogs, particularly for human medications used in an extralabel (off-label) manner, the optimal dosages are often unknown.
One drug combination suggested for analgesia in dogs is an oral opioid formulation plus acetaminophen.2–4 Opioids that have been combined with acetaminophen for this purpose include codeine, oxycodone, and hydrocodone.3,5 However, the clinical efficacy of these products has not been rigorously tested or supported in dogs.6 Indeed, a recent study5 yielded no data to support the use of acetaminophen-hydrocodone for the management of acute postoperative pain. Despite the lack of supportive clinical data, these products are used in small animal practice today, often to avoid administration of traditional NSAIDs when contraindicated. Additionally, AC has been used as a rescue analgesic in clinical trials to assess the success or failure of other compounds being tested for the treatment of osteoarthritis in dogs.7,8
The purpose of the study reported here was to examine the ability of AC to attenuate lameness attributable to SU-induced synovitis in the stifle joint of dogs and to compare those effects with the effects of an NSAID (carprofen) known to successfully attenuate signs of lameness in dogs that have this experimental condition.9–13 Measurement of plasma concentrations of the administered drugs was included in the protocol to ensure that the drugs had been administered in accordance with the protocol and had achieved measurable plasma drug concentrations. Intra-articular SU injection induces a consistent self-limiting lameness and provides a predictable method for detecting changes in lameness associated with transient inflammatory synovitis over a moderate period (36 to 48 hours). Our hypothesis was that AC would be as effective as the NSAID (carprofen) in the attenuation of SU-induced lameness as assessed by both subjective measurements and objective gait analysis.
Materials and Methods
Animals
Seven mixed-breed hound-type dogs were used in the study. An a priori sample size calculationa was performed (80% power) to detect a difference between treatments in primary outcome measurements (objective gait data) as evaluated in a crossover study design. Calculations were performed by use of data from previous studies9–12 that involved evaluation of ground reaction forces (PVF and VI) in dogs with SU-induced lameness. Sample size calculations indicated that 5 dogs would be needed to detect a clinically meaningful change in PVF of 4.0 units (percentage of body weight; SD = 1.5; α = 0.05) and in VI of 0.3 units (percentage of body weight × seconds; SD = 0.15; α = 0.05).
All dogs underwent physical and orthopedic examinations and bilateral radiographic evaluation of the hip and stifle joints to confirm that they were healthy. All dogs were housed in the study institution's climate-controlled animal facility, received routine vaccines and anthelmintic treatment, were fed a commercially available maintenance diet, and were offered water ad libitum. The study protocol was approved by the University of Georgia Institutional Animal Care and Use Committee.
Study design
In a blinded 2-way crossover study design, dogs were randomly assignedb to 2 groups by an individual who did not participate in the evaluation component of the study. Dogs received a single dose of ACc (acetaminophen dose, 15.5 to 18.5 mg/kg; codeine dose, 1.6 to 2.0 mg/kg; PO) or carprofend (2.1 to 2.3 mg/kg, PO) at 6 pm on day 0 (14 hours prior to SU injection). For dogs in the group receiving AC first, the same dose of AC was again administered at 6 am on day 1 (2 hours prior to SU injection) and then every 8 hours for an additional 48 hours. For dogs in the group receiving carprofen first, another dose (4.4 mg/kg, PO; actual dose range, 4.2 to 4.5 mg/kg) was administered at 6 am on day 1 (2 hours prior to SU injection), and that larger dose was continued every 24 hours for an additional 48 hours.
On day 1 of treatment, dogs were anesthetized with propofol and synovitis was induced by intra-articular injection of 2.0 mL of a 17.0-mg/mL solution of SU prepared as described elsewhere.9–13 The choice of right- or left-side stifle joint was randomly assigned for the first injection. A minimum washout period of 21 days was observed (21 days between SU injection and subsequent collection of baseline data for the subsequent treatment), and then dogs received the alternate treatment as described and lameness was induced in the opposite limb.
Lameness measurements
Lameness was assessed in each dog prior to SU injection (baseline) and at 3, 6, 9, 12, 24, 36, and 48 hours after SU injection by means of both subjective and objective methods. Clinical lameness scores were subjectively assigned by a single blinded investigator (SAK) who had extensive experience with a reported scoring system used for dogs with SU-induced lameness.9–11 This system is intended to measure lameness and pain associated with the induced synovitis, with higher scores indicating more limb dysfunction and pain. Briefly, stance was scored on a scale of 0 (normal) to 3 (unable to stand); lameness at a walk on a scale of 0 (no lameness and full weight bearing on all strides; normal gait) to 4 (full non–weight-bearing lameness; bears no weight on any strides); lameness at a trot on a scale of 0 to 4, as for lameness at a walk; and pain on a scale of 0 (no signs of pain elicited on palpation or movement of the affected joint) to 3 (signs of severe pain elicited on palpation or movement of the affected joint [ie, vocalizes, becomes aggressive, or will not allow palpation or movement of the affected joint]).11 Scores were summed to determine a total lameness score (possible range, 0 to 14), which was subsequently used in statistical analyses.
For objective lameness assessment, ground reaction force data were collected with 2 force platese mounted in series, with a dedicated computer and software,f while dogs were trotting over the plates at a speed of 1.70 to 2.10 m/s and an acceleration of −0.50 to 0.50 m/s/s. Each dog was trotted by the same handler for gait analyses. Trials were accepted only if there was a single rear limb footfall on each force platform while maintaining a standard trotting gait with no extraneous movements by the dog or handler. At each assessment point, 5 observations were recorded for both hind limbs (control and SU-injected stifle joints) of each dog. From the ground reaction force data, PVF, VI, CrCaBPF, CrCaBI, CrCaPPF, and CrCaPI were determined.
Dogs were also observed throughout the study for adverse events. The protocol was such that if a dog were to have a total lameness score ≥ 13 (scale, 0 to 14) or a score > 1 for pain (with 1 representing signs of mild pain elicited [turns head in recognition] on palpation or movement of the affected joint) at any point, rescue analgesia would be immediately instituted and the dog withdrawn from the study and treated with firocoxib (5.0 mg/kg).
Plasma drug concentrations
Blood samples for measurement of plasma drug concentrations were collected from each dog (via an 18-gauge catheter placed in a jugular vein) 2 hours prior to SU injection (baseline) and at 3, 6, 9, 12, 14, 22, 24, 30, 36, 38, 46, and 48 hours after SU injection. Plasma was harvested and stored at −80°C until the time of assay. Prior to assay performance, plasma samples were removed from the freezer and thawed for a minimum of 60 minutes.
Carprofen—Plasma carprofen concentrations were measured by reversed-phase HPLC with UV detection. Separation of the (R)- and (S)-enantiomers of carprofen was accomplished with a chiral HPLC column (4.6 × 150 mm; 5 μm)g kept at a constant temperature of 25°C. This column was specially designed for the separation of chiral isomers (enantiomeric compounds). The mobile phase consisted of 81% potassium monobasic phosphate buffer and 19% acetonitrile, run in isocratic mode at 1 mL/min. The wavelength for detection was 240 nm. Retention times were approximately 14 minutes for the (S)-enantiomer and 15 minutes for the (R)-enantiomer. Calibration samples and QC samples were prepared by fortifying (spiking) blank canine plasma with a solution containing reference standards of carprofen.h Reference solution was prepared by dissolving the analytic reference standard of carprofen in 100% methanol. Further dilutions were performed in a 50:50 methanol-water solution. These calibration solutions were used to prepare a range of 8 calibration and QC samples ranging in concentration from 0.05 to 50 μg/mL. Blank (control) plasma samples were also analyzed with each day's run to check for interfering peaks and estimate background noise. All calibration curves were linear with a value of R2 ≥ 0.99. The limit of quantification for carprofen isomers in canine plasma was 0.05 μg/mL, which was determined from the lowest point on a linear calibration curve that produced an acceptable signal-to-noise ratio and met acceptance criteria for our laboratory and the United States Pharmacopeia.i
Calibration plasma samples, QC samples, and all incurred samples were prepared in the same manner. A 3-mL solid-phase extraction cartridgej was conditioned with water and methanol, and 400 μL of the plasma sample was added. After a wash step with water and ammonium hydroxide (ratio, 95:5), the sample was eluted with methanol and formic acid (ratio, 98:2). The sample was evaporated to dryness, reconstituted with 200 μL of water, vortexed, and injected into the HPLC system. The system consisted of a quaternary solvent delivery pump, autosampler providing a 30-μL injection, UV detector, and related software.g
Acetaminophen—Plasma acetaminophen concentration was measured by use of a previously published method with modifications for use of canine plasma with HPLC and UV detection.14 The same solvent delivery system, detector, and autosampler were used, and computer integration of peaks was performed as reported for the carprofen analysis. The analytic column was a C18 reversed-phase column (4.6 × 15 cm)g maintained at a constant temperature of 35°C. The mobile phase was 95% distilled water and 5% acetonitrile, with trifluoroacetic acid added as a mobile phase modifier.
The acetaminophen analytic reference standardh was dissolved in 100% methanol to achieve a concentration of 1 mg/mL. Additional dilutions to prepare the calibration samples were performed in distilled water. Seven calibration samples were used, ranging from 0.05 to 10 μg/mL, as well as a blank (0 μg/mL) sample. To prepare the study samples, calibration curve samples, and QC samples, solid-phase extraction was used. A 400-μL plasma sample was added to a conditioned 1-mL solid-phase extraction cartridge.j The sample was cleaned with 1 mL of distilled water and eluted with 1 mL of 100% methanol. The methanol was evaporated to dryness to obtain a dry residue. The sample was then reconstituted with 200 μL of mobile phase and injected into the HPLC system at a flow rate of 1 mL/min and detected at a wavelength of 254 nm. The retention time for the peak of interest was approximately 4.2 minutes. The limit of quantification for acetaminophen in canine plasma was 0.05 μg/mL, which was determined from the lowest point on a linear calibration curve that produced an acceptable signal-to-noise ratio and met acceptance criteria for our laboratory and the United States Pharmacopeia.i
Codeine and morphine—Plasma codeine and morphine plasma concentrations were measured with a modified HPLC method.15,16 The solvent delivery system, detector, autosampler, and computer integration of peaks were the same as those used for carprofen analysis. A C18 analytic column (4.6 × 15 cm)g was used. The mobile phase consisted of 90% phosphate buffer (pH, 3) and 10% acetonitrile at a flow rate of 1 mL/min.
Codeine and morphine analytic reference standards were used to validate the assay.h Codeine monohydrate was dissolved in 100% methanol, and morphine sulfate was dissolved in distilled water. These stock solutions (1 mg/mL) were further diluted to prepare calibration curve and QC samples. The calibration curve samples ranged from 0.001 to 10 μg/mL for morphine and 0.01 to 10 μg/mL for codeine. Blank samples were included to measure background noise and ensure that there were no interfering peaks in the window of interest.
Extraction of the drugs from canine plasma was accomplished with a solid-phase extraction cartridge. A 200-μL plasma sample was added to a conditioned 1-mL solid-phase extraction cartridge.j The sample was cleaned with 1 mL of distilled water and eluted with 1 mL of methanol plus 0.25% phosphoric acid. The methanol was evaporated to dryness to obtain a dry residue. The sample was reconstituted with 200 μL of mobile phase and injected into the HPLC system at a flow rate of 1 mL/min and detected at a wavelength of 220 nm. The retention time for the peak of interest was approximately 2.4 minutes for morphine and 5.3 minutes for codeine. The limit of quantification for morphine and codeine in canine plasma was 0.001 and 0.01 μg/mL, respectively.
Statistical analysis
All statistical analyses were performed with statistical software.k The objectively measured data for vertical ground reaction forces (PVF and VI) were considered primary outcome variables. Secondary outcome variables were clinical lameness score and craniocaudal ground reaction force measurements.
A linear mixed model was used to test for differences in clinical lameness scores and force plate data between treatments (AC or carprofen) at each assessment point and within-treatment differences from baseline at each assessment point. The full linear mixed models included the fixed effects of treatment, assessment point, and interaction between treatment and assessment point and a random intercept to control for individual dog. Multiple comparisons were adjusted for by means of Tukey correction. All hypothesis tests were 2-sided, and values of P < 0.05 were considered significant. Only assessment points with nonzero data (ie, 3, 6, 9, and 12 hours) were analyzed for lameness scores. Lameness scores were compared with 0 by use of approximate t tests of the marginal means.
Results
Animals
All 7 dogs completed both treatment sessions. Following the washout period, no residual lameness was noted in the limb in which SU was injected in the first treatment session, as defined by a total clinical lameness score of 0 (scale of 0 to 14). Because none of the dogs had a total lameness score > 11 at any point, none required rescue analgesia. Additionally, no dog had a score > 1 in the pain section of the lameness scoring system.
Effects of treatments on lameness
When receiving carprofen, dogs had significantly higher clinical lameness scores than at baseline at 3 (P < 0.001), 6 (P = 0.002), and 9 (P = 0.027) hours after SU injection (Figure 1). When receiving AC, dogs had significantly higher clinical lameness scores than at baseline at 3 (P < 0.001), 6 (P < 0.001), and 9 (P < 0.001) hours. Clinical lameness scores at 3, 6, and 9 hours during AC treatment were significantly (P = 0.02 for all) higher than scores at the same assessment points during carprofen treatment.
Mean ± SD total clinical lameness scores before (baseline) and at various points after synovitis was induced in 1 stifle joint of healthy dogs (n = 7) by intra-articular injection of SU. Dogs orally received each of 2 treatments (AC [triangles] and carprofen [squares]), separated by a minimum 21-day washout period. The first dose of AC (acetaminophen dose, 15.5 to 18.5 mg/kg; codeine dose, 1.6 to 2.0 mg/kg) or carprofen (2.1 to 2.3 mg/kg) was given at 6 pm on day 0 (14 hours prior to SU injection). For dogs in the group receiving AC first, the same dose of AC was again administered at 6 am on day 1 (2 hours prior to SU injection) and then every 8 hours for an additional 48 hours. For dogs in the group receiving carprofen first, another dose (4.4 mg/kg, PO) was administered at 6 am on day 1 (2 hours prior to SU injection), and that larger dose was continued every 24 hours for an additional 48 hours. Note that some of the assessment points reflected by the x-axis are separated by different intervals. *Within the carprofen treatment, the indicated value differs significantly (P < 0.05) from the value at baseline. †Within the AC treatment, the indicated value differs significantly (P < 0.05) from the value at baseline. ‡Within an assessment point, the indicated value for carprofen differs significantly (P < 0.05) from the value for AC treatment.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD total clinical lameness scores before (baseline) and at various points after synovitis was induced in 1 stifle joint of healthy dogs (n = 7) by intra-articular injection of SU. Dogs orally received each of 2 treatments (AC [triangles] and carprofen [squares]), separated by a minimum 21-day washout period. The first dose of AC (acetaminophen dose, 15.5 to 18.5 mg/kg; codeine dose, 1.6 to 2.0 mg/kg) or carprofen (2.1 to 2.3 mg/kg) was given at 6 pm on day 0 (14 hours prior to SU injection). For dogs in the group receiving AC first, the same dose of AC was again administered at 6 am on day 1 (2 hours prior to SU injection) and then every 8 hours for an additional 48 hours. For dogs in the group receiving carprofen first, another dose (4.4 mg/kg, PO) was administered at 6 am on day 1 (2 hours prior to SU injection), and that larger dose was continued every 24 hours for an additional 48 hours. Note that some of the assessment points reflected by the x-axis are separated by different intervals. *Within the carprofen treatment, the indicated value differs significantly (P < 0.05) from the value at baseline. †Within the AC treatment, the indicated value differs significantly (P < 0.05) from the value at baseline. ‡Within an assessment point, the indicated value for carprofen differs significantly (P < 0.05) from the value for AC treatment.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD total clinical lameness scores before (baseline) and at various points after synovitis was induced in 1 stifle joint of healthy dogs (n = 7) by intra-articular injection of SU. Dogs orally received each of 2 treatments (AC [triangles] and carprofen [squares]), separated by a minimum 21-day washout period. The first dose of AC (acetaminophen dose, 15.5 to 18.5 mg/kg; codeine dose, 1.6 to 2.0 mg/kg) or carprofen (2.1 to 2.3 mg/kg) was given at 6 pm on day 0 (14 hours prior to SU injection). For dogs in the group receiving AC first, the same dose of AC was again administered at 6 am on day 1 (2 hours prior to SU injection) and then every 8 hours for an additional 48 hours. For dogs in the group receiving carprofen first, another dose (4.4 mg/kg, PO) was administered at 6 am on day 1 (2 hours prior to SU injection), and that larger dose was continued every 24 hours for an additional 48 hours. Note that some of the assessment points reflected by the x-axis are separated by different intervals. *Within the carprofen treatment, the indicated value differs significantly (P < 0.05) from the value at baseline. †Within the AC treatment, the indicated value differs significantly (P < 0.05) from the value at baseline. ‡Within an assessment point, the indicated value for carprofen differs significantly (P < 0.05) from the value for AC treatment.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Carprofen treatment resulted in a significantly lower PVF than at baseline at 3 (P < 0.01) and 6 (P < 0.01) hours after SU injection (Figure 2). The AC treatment resulted in a significantly lower PVF than at baseline at 3 (P < 0.001), 6 (P < 0.001), 9 (P < 0.001), and 12 (P = 0.002) hours. Mean PVF during AC treatment was lower than mean PVF during carprofen treatment at 3 (P < 0.001), 6 (P < 0.001), and 9 (P = 0.012) hours.
Mean ± SD PVF at various points for the dogs of Figure 1. BW = Body weight. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD PVF at various points for the dogs of Figure 1. BW = Body weight. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD PVF at various points for the dogs of Figure 1. BW = Body weight. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Carprofen treatment resulted in a significantly lower VI than at baseline at 3 (P < 0.01) and 6 (P < 0.01) hours after SU injection (Figure 3). The AC treatment resulted in a significantly (P < 0.001 for all) lower mean VI than at baseline at 3, 6, and 9 hours. Mean VI during AC treatment was significantly lower than mean VI during carprofen treatment at 3 (P < 0.001), 6 (P < 0.01), and 9 (P < 0.01) hours.
Mean ± SD VI at various points for the dogs of Figure 1. See Figures 1 and 2 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD VI at various points for the dogs of Figure 1. See Figures 1 and 2 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD VI at various points for the dogs of Figure 1. See Figures 1 and 2 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
During carprofen treatment, no significant differences from baseline in CrCaBPF were identified, whereas during AC treatment, significant (P < 0.001) differences were identified at 3 and 6 hours after SU injection. Mean CrCaBPF during AC treatment was significantly lower than mean CrCaBPF during carprofen treatment at 3 (P < 0.001), 6 (P < 0.001), 9 (P = 0.003), and 48 (P = 0.02) hours. Carprofen treatment resulted in no significant differences from baseline in CrCaBI. The AC treatment resulted in a significantly (P < 0.001 for both) lower mean CrCaBI than at baseline at 3 and 6 hours. Mean CrCaBI during AC treatment was significantly lower than mean CrCaBI during carprofen treatment at 3 (P < 0.001), 6 (P < 0.001), and 48 (P = 0.001) hours.
Carprofen treatment resulted in no significant differences from baseline in CrCaPPF. The AC treatment resulted in a significantly lower CrCaPPF than at baseline at 3 (P < 0.001), 6 (P < 0.001), and 9 (P = 0.03) hours after SU injection. Mean CrCaPPF during AC treatment was significantly lower than mean CrCaPPF during carprofen treatment at 3 (P = 0.001), 6 (P < 0.001), and 9 (P = 0.002) hours. Carprofen treatment resulted in no significant differences from baseline in CrCaPI. The AC treatment resulted in a significantly (P < 0.001) lower CrCaPI than at baseline at 3 and 6 hours. Mean CrCaPI during AC treatment was significantly lower than mean CrCaPI during carprofen treatment at 3 (P < 0.001), 6 (P < 0.001), and 9 (P = 0.001) hours.
Plasma drug concentrations
Plasma concentrations of carprofen (R)- and (S)-enantiomers during the assessment period ranged from 2.5 to 19.2 μg/mL and 4.6 to 25.0 μg/mL (Figure 4). Plasma acetaminophen concentrations ranged from 0.14 to 4.6 μg/mL (Figure 5). Plasma codeine concentrations ranged from 7.0 to 26.8 ng/mL, whereas plasma morphine concentrations ranged from 4.0 to 58.6 ng/mL (Figure 6).
Mean ± SD plasma concentrations of carprofen (R)-enantiomers (triangles) and (S)-enantiomers (circles) during carprofen treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma concentrations of carprofen (R)-enantiomers (triangles) and (S)-enantiomers (circles) during carprofen treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma concentrations of carprofen (R)-enantiomers (triangles) and (S)-enantiomers (circles) during carprofen treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma acetaminophen concentrations during AC treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma acetaminophen concentrations during AC treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma acetaminophen concentrations during AC treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma codeine (triangles) and morphine (circles) concentrations during AC treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma codeine (triangles) and morphine (circles) concentrations during AC treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Mean ± SD plasma codeine (triangles) and morphine (circles) concentrations during AC treatment for the dogs of Figure 1. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.627
Discussion
Intra-articular injection of SU in the dogs of the present study induced a consistent lameness as previously described.9–13,17,18 Given the results, we rejected our hypothesis that acetaminophen in combination with codeine phosphate would be as effective as carprofen in the attenuation of SU-induced lameness in dogs. Both objective and subjective outcome measurements indicated that lameness was significantly greater during the initial 9 to 12 hours after lameness induction when the dogs received AC rather than carprofen. Results of previous studies9–13,17,18 involving SU-induced lameness showed that dogs treated with NSAIDs (including carprofen) indicated that lameness was attenuated to a level similar to that achieved with carprofen in the present study. Additionally, although no direct comparisons can be made, the GRF and lameness score data for dogs when treated with AC were most consistent with the placebo group data in other studies11,12 involving SU-induced lameness. No placebo treatment was included in the present study because previous studies11,12 have established the severity and duration of the lameness induced with this method; thus, we believed it was unnecessary to include a placebo for ethical reasons. These AC data were also consistent with findings of 2 recent clinical studies5,19 that revealed limited efficacy of oral acetaminophen-hydrocodone or hydrocodone administration in the immediate postoperative period for dogs undergoing tibial plateau leveling procedures.
Plasma carprofen concentrations during carprofen treatment in the study reported here were consistent with those in previous studies20–24 in which carprofen was administered orally. We documented both the (R)-and (S)-enantiomers. The (S)-enantiomer is more active for inhibiting cyclooxygenase (eutomer). The aforementioned studies20–24 did not all involve the same carprofen dosages, but they showed that carprofen was adequately absorbed in all dogs, as was observed in the present study. The low plasma acetaminophen concentrations achieved in the present study were consistent with concentrations found in previous studies.25,26 The actual serum or plasma AC concentrations needed for effective analgesia in dogs are unknown, but in children, the effective concentration appears to be much higher (10 to 15 μg/mL)27,28 than the concentrations measured in the study dogs (0.14 to 4.6 μg/mL). Because acetaminophen has a brief terminal half-life of 0.94 hours, any concentrations attained are short-lived, which supports the low efficacy observed in the current SU model.26 A recent study29 involving IV administration of acetaminophen to dogs at similar doses did not yield dramatically higher plasma acetaminophen concentrations, compared with concentrations achieved with oral administration, over the dosing period.
In people, codeine is metabolized to morphine as the primary metabolite responsible for analgesia. The conversion of codeine to morphine occurs through O-demethylation of codeine into morphine, which is mediated by the genetically polymorphic enzyme cytochrome P450 2D6 (CYP2D6). This process is highly variable in people, depending on genetic polymorphisms.30 The enzymatic pathway is also likely variable in dogs. Greyhounds produce negligible amounts of morphine following oral codeine administration.25 Dogs do not have CYP2D6 activity, and it is unclear whether another metabolic pathway exists. Because the metabolism of Greyhounds may be different from that of other dogs, we were unsure whether codeine would be converted to morphine in the dogs of the present study and thus chose to measure both codeine and morphine concentrations in plasma samples. None of the various glucuronide metabolites were considered because these are not believed to be active. Interestingly, plasma morphine concentrations were higher in the dogs of the present study, with much greater conversion of codeine to morphine, than in the Greyhounds of previous reports.25,26 The reasons for this difference remain unknown. Breed-related differences in metabolism are possible in dogs, as are effects of coadministered drugs on the metabolism profile, which have been demonstrated for other analgesics.31 Regardless, the high plasma morphine concentrations observed in the present study did not translate into any attenuation of the SU-induced lameness. To our knowledge, no studies have been performed to determine the plasma morphine concentration effective for controlling pain associated with osteoarthritis or other types of joint pain.
The pharmacokinetics of codeine after oral administration to dogs has been reported.25 Codeine had only 4% oral bioavailability and a short half-life of 1.2 hours, with rapid clearance and negligible conversion of codeine to morphine. In a follow-up study,26 the half-life of codeine was also short (1.7 hours), with negligible conversion to morphine. Both studies25,26 were performed in Greyhounds, and it is unknown whether these data can be generalized to other breeds of dogs. In the present study, plasma codeine concentrations were consistent with previously published data.26
Limitations of the study reported here included the unknown clinical translation of lameness scores for SU-induced lameness to scores for acute pain associated with surgery or other types of trauma assigned with other scoring systems. The clinical lameness scoring system used here (and previously) focused more on signs of lameness in the dogs and less on indicators of pain.9–13 Thus, the use of a second pain-specific scoring system should be considered in future studies, particularly when such scoring is used to determine when rescue analgesia should be provided for dogs with SU-induced lameness. Additionally, the a priori establishment of a cutoff lameness score of 13/14 to prompt rescue analgesia may be concerning to individuals; however, no dog in our study scored > 1 in the pain subsection of the clinical lameness scale, and these findings were consistent with previously published results for SU-induced lameness in dogs.9–13 Finally, the dogs were pretreated with both drugs 14 hours and 2 hours prior to induction of the synovitis. The fact that AC was initially administered at a 12-hour interval and then every 8 hours for an additional 48 hours could be perceived as a bias, with a lower initial preemptive dose than carprofen; however, the published administration interval for AC is every 8 to 12 hours.1–4
Acknowledgments
No third-party funding or support was received in connection with this study. The authors declare that there were no conflicts of interest.
ABBREVIATIONS
| AC | Acetaminophen-codeine |
| CrCaBI | Craniocaudal braking impulse |
| CrCaBPF | Craniocaudal braking peak force |
| CrCaPI | Craniocaudal propulsion impulse |
| CrCaPPF | Craniocaudal propulsion peak force |
| HPLC | High-performance liquid chromatography |
| PVF | Peak vertical force |
| QC | Quality control |
| SU | Sodium urate |
| VI | Vertical impulse |
Footnotes
Schoenfeld D. Statistical considerations for clinical trials and scientific experiments. Available at: hedwig.mgh.harvard.edu/sample_size/js/js_crossover_quant.html. Accessed Jun 2, 2018.
Random integer generator, Randomness and Integrity Services Ltd, Dublin, Ireland Available at: www.random.org/integers. Accessed Jun 2, 2018.
Tylenol #3, Cardinal Health, Dublin, Ohio.
Rimadyl, Zoetis Animal Health, Parsippany, NJ.
Model OR-6-6, Advanced Mechanical Technology Inc, Newton, Mass.
Acquire, version 7.3, Sharon Software, East Lansing, Mich.
Agilent 1100 series, Agilent Technologies, Santa Clara, Calif.
Sigma-Aldrich Corp, St Louis, Mo.
USP chapter 1226: validation of compendial procedures United States Pharmacopeia Convention, Rockville, Md. Available at: www.USP.org. Accessed Mar 5, 2020.
Waters Associates, Milford, Mass.
SAS, version 9.3, SAS Institute Inc, Cary, NC.
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