J. vet. Pharmacol. Therap. 37, 243--251. doi: 10.1111/jvp.12082.
Pharmacokinetics and safety of firocoxib after oral administration of repeated consecutive doses to neonatal foals N. HOVANESSIAN* J.L. DAVIS
†
H.C. MCKENZIE III*
Hovanessian, N., Davis, J.L., McKenzie, H.C. III, Hodgson, J.L., Hodgson, D.R., Crisman, M.V. Pharmacokinetics and safety of firocoxib after oral administration of repeated consecutive doses to neonatal foals. J. vet. Pharmacol. Therap. 37, 243–251.
J.L. HODGSON* D.R. HODGSON* & M.V. CRISMAN* *Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA, USA; † North Carolina State University College of Veterinary Medicine, Raleigh, NC, USA
The purpose of this study was to determine the pharmacokinetics and safety profile of firocoxib in neonatal foals. Seven healthy foals were administered 0.1 mg/kg firocoxib orally q24 h for nine consecutive days, commencing at 36 h of age. Blood was collected for firocoxib analysis using high-pressure liquid chromatography with fluorescence detection at 0 (dose #1 only), 0.25, 0.5, 1, 2, 4, 8, 16, and 24 h after doses 1, 5, and 9. For all other doses (2, 3, 4, 6, 7, and 8), blood was collected immediately prior to the next dose (24 h trough). Elimination samples (36, 48, 72, 96, 120, and 144 h) were collected after dose 9. Safety was assessed via physical examinations, body weight measurements, gastroscopy, complete blood count, plasma biochemistry and urinalysis. Firocoxib was rapidly absorbed following oral administration with minimal accumulation after repeat dosing. After the final dose, the terminal half-life was approximately 11 h. Firocoxib was below the limit of detection ( 0.99.
Monitoring
Pharmacokinetic analysis
Free catch urine samples were collected, when possible, on days 1, 3, 5, 7, 9, and 11 of the study. Urinalyses were performed, including specific gravity and urinary GGT/creatinine ratio. Gastroscopy was performed immediately after catheter placement (day 0) and on day 10 to evaluate for presence of gastric and esophageal ulceration. Foals were sedated with 0.5 mg/kg xylazine intravenously and gastroscopy performed (nasogastric) with a 1-meter flexible endoscope (Olympusâ Exera GIF-160; Olympus America, Center Valley, PA, USA). Gastric lesions were graded using the scoring system proposed by the Equine Gastric Ulcer Council (Anonymous, 1999; Bell et al., 2007). Blood was submitted in blood tubes containing EDTA for complete blood count and tubes containing lithium heparin for plasma biochemistry on days 1 and 11. On days 3 and 7, blood was submitted for evaluation of BUN and creatinine concentrations only.
Pharmacokinetic parameters for multidose firocoxib were calculated using the noncompartmental analysis model (log/linear trapezoidal) of commercially available pharmacokinetic software (WinNonlinâ version 5.0.1; Pharsight Corporation, Mountain View, CA, USA). Individual animal and average pharmacokinetic measures were taken. The maximum (Cmax) and minimum (Cmin) plasma concentrations and times (Tmax, Tmin) to those values were determined directly from the plasma concentrations of firocoxib. The average maximum and minimum concentrations at steadystate (Cssmax and Cssmin) and the terminal half-life (t1/2) were determined. Area under the curve (AUC) for 0–24 h (AUC0–24), and extrapolated to infinity (AUC0–∞), was established. Descriptive values were reported as Mean SD. An accumulation ratio was calculated from the following equation: s
RA ¼ 1=1 ekz Drug analysis Samples were analyzed at PKDM Department of Merial, Ltd. (Bridgewater, NJ, USA). Firocoxib concentrations in plasma were determined using a previously published HPLC method (Kvaternick et al., 2007b) that was modified for fluorescence detection with an excitation wavelength of 250 nm and emission wavelength of 375 nm. The current analysis method used fluorescence detection rather than UV detection for increased accuracy and to allow for a lower limit of quantitation at 5 ng/mL (vs. 25 ng/mL with UV detection). Additionally, a smaller sample size (0.2 vs. 1–2 mL plasma) was used. In brief, plasma samples were thawed at room temperature, vortexed, and centrifuged. Acetonitrile (1 mL) followed by 0.2 mL plasma was filled in a 96-well protein precipitation plate and left to sit for approximately 3 min. A vacuum was applied and the eluate collected in a 2-mL collection plate before being evaporated to dryness. The residue was reconstituted in 0.2 mL of 40% acetonitrile in water and a 0.05-mL aliquot injected onto the HPLC. Separation was accomplished on a Zorbax XBD precolumn (4.6 9 12.5 mm) and Zorbax RxC18 column (3 9 150 mm, 3.5 lm) (Agilent technologies, Santa Clara CA, USA) and maintained at 40 °C. The mobile phase consisted of 45:55 (v/v) acetonitrile and 0.25% trifluoroacetate anhydride (TFA) in water. Tandem HPLCs were used for this analysis, one set at a flow rate of 0.5 mL/min and the other at 0.8 mL/min. Acceptability of bioanalytical sets was based on the standard curves and fortified control sample results. The standard curve ranged from 2.5 to 250 ng/mL. Standards and quality control samples were prepared by adding an aliquot of standard solution to 200 lL of equine © 2013 John Wiley & Sons Ltd
where kz is the slope of the elimination phase and s is the dosing interval. Statistical analysis was performed using SigmaPlot 12.0 (SysStat Software, Inc, San Jose, CA, USA). Normality was assessed using the Shapiro–Wilk test. The one-way ANOVA was used to assess significant differences between the terminal half-life (T½), AUC0–24, Cmax, and Tmax for pharmacokinetic parameters calculated for days 1, 5, and 9. For non-normally distributed data, the Kruskal–Wallis one-way ANOVA on ranks was performed. Significance was set at P < 0.05. RESULTS Pharmacokinetics Following oral administration to neonatal foals, firocoxib was rapidly absorbed. After the initial dose, an average Cmax of 89.50 53.36 ng/mL (mean SD) was achieved at Tmax of 0.54 0.65 h. Figure 1 shows the average concentration-vs.time curve on days 1, 5, and 9 for neonatal foals (n = 7) treated with nine consecutive oral daily doses of firocoxib (0.1 mg/ kg). There was minimal accumulation after repeat dosing, with an average concentration (Cavg) in plasma of 39.1 8.4 ng/ mL. The actual average trough serum concentration of firocoxib for doses 1–9 was 14.1 3.2 ng/mL, whereas the calculated concentration was 16.3 8.3 ng/mL (Fig. 2). Near steady-state concentrations are achieved after the first dose, based on the fact that all average trough (or minimum) plasma concentrations are within one standard deviation of the calcu-
246 N. Hovanessian et al.
lated average minimum concentration. Based on the terminal half-life determined from the foals in this study, steady-state concentrations should be fully achieved after 55 h (2.3 doses). Figure 3 shows the measured daily firocoxib plasma concentrations for the seven foals after the ninth consecutive daily oral dose of the drug. Following the ninth dose, the harmonic mean of the terminal half-life (T½k) was 11.04 3.23 h. Relevant pharmacokinetic parameters after single (day 1) and multiple (days 5 and 9) consecutive oral daily doses of firocoxib are summarized in Table 1. A significant difference between the AUC0–24 values for day 1 and day 9 was noted. A difference was not detected for any other parameter. The final dose of firocoxib was administered at 192 h. Drug was still detected at 228 h in 6/7 foals and at 240 h in 4/7 foals. Drug was not detected past 264 h in any foal. Fig. 3. Plasma concentration vs. time for individual foals following the final ninth consecutive dose of firocoxib. The solid line represents the average concentrations.
Table 1. Mean SD values for pharmacokinetic parameters for single (day 1) and multiple oral doses of firocoxib (0.1 mg/kg) to neonatal foals
Fig. 1. Average plasma concentration–time curve from neonatal foals (n = 7) treated with nine consecutive oral daily doses of firocoxib (0.1 mg/kg) for doses 1, 5, and 9, and trough concentrations (taken immediately prior to dosing) for doses 2–4 and 6–8.
Day 1
Day 5
Day 9
(Mean SD)
(Mean SD)
(Mean SD)
kz (1/h)
0.08 0.03
0.07 0.02
0.07 0.03
T1/2kz (h) Tmax (h)
10.46 4.97 0.54 0.65
10.46 2.87 0.43 0.28
11.04 3.23 1.46 1.75
Cmax (ng/mL)
89.50 53.36
94.07 61.23
71.17 21.41
Cavg (ng/mL)
–
–
39.1 8.4
AUC0–24 (h*ng/mL)
629.16 178.09*
794.44 187.10
1162.45 326.98*
AUC0–∞ (h*ng/mL)
–
–
1255.68 372.76
MRT0–∞ (h) % Fluctuation
14.66 6.93 –
Accumulation
–
14.72 4.2 – –
16.22 4.09 147 72 1.29 0.15
index
kz, slope of the terminal phase; T½kz, half-life of terminal phase; Tmax, time to maximum concentration; Cmax, maximum concentration; Cavg, average concentration; AUC0–24, area under the concentration–time curve from 0 to 24 h; AUC0–∞, area under the concentration–time curve extrapolated to infinity; MRT, mean residence time. *Indicates a significant difference between parameters on different days (P < 0.05).
Gastroscopy
Fig. 2. Actual minimum plasma concentrations compared with average calculated minimum plasma concentrations.
No abnormalities were observed on initial gastroscopy, with all foals scoring grade 0 (intact epithelium). On day 11, Foal E displayed mild hyperkeratosis of the margo plicatus, giving it a score of grade 1 (intact mucosa, evidence of hyperkeratosis or hyperemia). All other evaluations of esophageal, gastric, and pyloric mucosa were unremarkable. © 2013 John Wiley & Sons Ltd
Firocoxib pharmacokinetics and safety in foals 247
Hematologic and biochemical variables Most of the foals had mild changes consistent with stress, evidenced by the presence of a mature neutrophilia, lymphopenia, and mild hyperglycemia, at one or more sampling periods. Six of the foals had BUN and plasma creatinine values that were below the reference interval. On day 11, five of the foals had mild to moderately elevated plasma GGT activities, with two also displaying mildly elevated AST activity. Urinalyses were within reference intervals for most of the foals throughout the study period. Four of the seven foals had traces of blood and/or protein in the urine on day 1. One foal (Foal 3) displayed trace proteinuria on day 11. Urinary GGT-to-creatinine ratios were determined for each foal on three to 5 days of the study period. The number of samples available depended on the ability to catch urine in the individual foals, since a free catch technique was employed. Urinary enzyme activities were highly variable before and after firocoxib administration but remained within reference intervals for adult horses (Rossier et al., 1995). Minor complications developed in three of the seven foals that were not related to firocoxib administration. On day 11 of the study, after the final dose of firocoxib, Foal 5 was lethargic, pyrexic (103.4°F), and lame in the left hind limb. The foal displayed an abnormal leukogram, with a total white cell count of 19.370 9 103/lL (reference interval 4.500–11.500 9 103/ lL), a segmented neutrophilia of 17.046 9 103/lL, (reference interval 3.040–9.570 9 103/lL), and monocytosis of 0.581 9 103/lL (reference interval 50–380 9 103/lL). This was accompanied by a mild hyperfibrinogenemia (500 mg/dL, reference interval 110–450 mg/dL). Further diagnostics identified mild cellulitis at the catheter site, a patent urachus and inflammation of the left metatarsophalangeal joint. The foal was treated with medical and surgical management including umbilical resection and joint lavage. These complications were unlikely due to firocoxib administration, and samples continued to be collected and evaluated until the end of the study period. The foal remained bright, continued to gain weight, and was healthy at discharge on day 22. One mare died 60 h after foaling due to uterine artery rupture, despite aggressive medical therapy. The foal (Foal 6) then was provided 25% of its body weight per day as a formulated mare’s milk replacer (Mare’s Matchâ; Land O’ Lakes Inc., Saint Paul’s, MN, USA). Freshly prepared milk replacer was offered every 6 h. In addition, the foal was administered with omeprazole (1 mg/kg p.o. q24 h). No adverse effects were noted in the
foal’s attitude, demeanor, or hydration status throughout the study period. Foal 7 dislodged its intravenous catheter on day 5 and then developed mild cellulitis at its second catheter site on day 10. The foal was treated with oral antimicrobial agents and topical therapy. The cellulitis resolved without complications.
DISCUSSION Firocoxib, given per os, to neonatal foals at the standard adult therapeutic dose achieves detectable concentrations in plasma without inducing clinical or laboratory evidence of toxicity. Similar to previous studies in which traditional NSAIDs were administered to equine neonates (Semrad et al., 1993; Wilcke et al., 1993, 1998; Crisman et al., 1996), firocoxib displays differences in the pharmacokinetics between adult horses and neonates. However, unlike traditional NSAIDs, firocoxib appears to be cleared faster and has a shorter half-life in neonatal foals compared with adults (Table 2). Interestingly, a similar finding in foals has been reported for another drug in the coxib class, meloxicam (Raidal et al., 2013). The time to peak serum concentration (Tmax) after a single oral dose of the drug is considerably shorter in foals compared with adults (Kvaternick et al., 2007a; Letendre et al., 2008). This phenomenon of more rapid absorption may be explained by differences in the diet of adults and foals. The drug may be binding to milk proteins present in the foal’s digestive tract, aiding its movement into the bloodstream. Alternatively, a lower gastric pH in foals (Baker & Gerring, 1993) may be altering absorption of the drug, potentially through pH-dependent solubility or effects on drug ionization. However, Kvaternick et al. (2007a) describe firocoxib as nonionizable, and solubility is not affected by pH, making it unlikely that this has an effect on absorption. Finally, it may be that the presence of feed material in the digestive tract is delaying firocoxib’s absorption in adults. After 9 days of dosing, the Tmax in foals increased from 0.54 0.65 h to 1.46 1.75 h. This may be indicative of a change in the foals’ feeding habits over that time and represent an increase in solid food within the stomach which may affect both pH and gastric emptying. The mean maximum serum concentration (Cmax) following a single oral dose of firocoxib is higher in neonates compared with adults (Kvaternick et al., 2007a; Letendre et al., 2008). The faster absorption in neonates may explain this increase in
Table 2. Comparative pharmacokinetic parameters for firocoxib in foals and adult horses following oral administration of single or multiple doses of 0.1 mg/kg AUC0–24 (h *lg/mL) Foal (single dose) Adult (single dose) Foal (multiple doses) Adult (multiple doses) See Table 1 for legend. © 2013 John Wiley & Sons Ltd
0.63 0.83 0.96 1.16 3.12
0.18 0.17 0.26 3.27 0.86
Cmax (ng/mL) 89.5 45.0 75.0 71.17 173.0
53.36 11.3 33.0 21.41 44.0
Tmax (h) 0.54 7.80 3.90 1.46 0.79
0.65 4.80 4.4 1.75 0.70
T½ (h) 10.46 ND 29.6 11.04 36.5
4.97 7.5 3.23 9.5
Reference Present study Letendre et al. (2008) Kvaternick et al. (2007a) Present study Letendre et al. (2008)
248 N. Hovanessian et al.
the Cmax as more of the drug is absorbed in a shorter period. After repeated daily oral doses, the average Cmax in adult horses is much higher than following a single dose (Letendre et al., 2008); however, this was not observed in the foals of the present study. Additionally, the terminal half-life after oral administration is shorter in neonates when compared with adults (Kvaternick et al., 2007a). This shorter terminal half-life leads to less accumulation in foals with a mean calculated accumulation index of 1.29 0.15 vs. 3.8 0.7 in adults. This lack of accumulation can explain the difference in Cmax following repeated doses. The shorter terminal half-life may be related to either a decreased volume of distribution or an increased clearance. Differences in volume of distribution can account for differences in half-life; however, foals typically have a larger volume of distribution compared with adults due to a higher percent body composition of water, and this would tend to increase the halflife. Therefore, the authors suggest a higher clearance due to either a faster hepatic metabolism or renal excretion may be occurring. Biotransformation of firocoxib in adult horses is via dealkylation and glucuronidation to at least three inactive metabolites which are partially excreted in urine (68%) and feces (15%) (Kvaternick et al., 2007a,b). Faster renal excretion of firocoxib due to an increased GFR in foals compared with adult horses (Gonda et al., 2003; Savage, 2008) may account for an increased clearance, at least to some degree. Potentially, the effect of ontogeny and maturation of drug-metabolizing pathways in the liver of neonatal animals could also account for a portion of the faster clearance. Raidal et al. (2013) speculated that clearance of meloxicam in neonatal foals was increased due to the fact that the cytochrome P450 enzyme systems are among the most abundant liver enzymes, and therefore, younger animals have a relative abundance of these enzymes when liver volume is normalized to body weight, which increases drug clearance (Blanco et al., 2000; BurgosVargas et al., 2004). Additionally, the gastrointestinal tract, blood cells and potentially the lungs may be affecting the rate of firocoxib metabolism in this age group (Allegaert et al., 2007). Regardless, the impact from the ontogeny of hepatic and renal clearance on drug metabolism in neonates remains largely unknown, although phenotypic variability is frequently observed (Alcorn & McNamara, 2002; Bartelink et al., 2006; Allegaert et al., 2007, 2009). The influence of clearance and volume of distribution can only be determined following an intravenous dose, and unfortunately, an intravenous formulation of firocoxib was not available at the time this study was performed. This lack of intravenous formulation also prevented the determination of the absolute bioavailability in neonatal foals in the present study. In adult horses, bioavailability of firocoxib is 79% (Kvaternick et al., 2007a). Based on previously published values in adult horses and those presented in the current study, a relative bioavailability for foals can be calculated as the ratio of the AUC0–24 in foals and adults. This value is 65.6–75.9% using single dose data, indicating that bioavailability in foals is lower than that reported for adults.
Other NSAIDs, such as ketoprofen and flunixin meglumine, have longer half-lives and reduced elimination in foals (Semrad et al., 1993; Wilcke et al., 1993; Crisman et al., 1996), which necessitates decreasing the dose or increasing the dosing interval. However, a report of the pharmacokinetics of another COX-2 selective NSAID, meloxicam, in equine neonates (Raidal et al., 2013) shows similar discrepancies to firocoxib when compared with other NSAIDs and that study concluded that a shorter dosing interval of 12 h (vs. 24 h in adults) should be used. The shorter half-life and faster elimination of firocoxib could result in more frequent and/or higher dosing being required to maintain the same serum concentrations as in adults. In vitro COX inhibition assays performed on whole blood from adult horses suggested a concentration that would inhibit 50% of COX-2 (IC50) for firocoxib of 0.09 lM (McCann et al., 2002). This correlates to a concentration of 30.276 ng/ mL, which is similar to the average concentration (Cavg) in plasma for the foals in this study on day 9 (39.7 ng/mL). However, whether similar serum concentrations in equine neonates would provide clinically appropriate anti-inflammatory, antipyretic, and analgesic therapy needs to be established. Currently, there are no published pharmacodynamic or efficacy data for firocoxib for equine neonates. Interestingly, the development of clinical signs in the foal with the patent urachus and joint effusion coincided with cessation of drug administration on day 10 of the study. This provides some anecdotal evidence of firocoxib’s anti-inflammatory, anti-pyretic, and analgesic effects in the foal at the administered dose. Evaluation of the toxicity of firocoxib in adults showed that toxicity was not induced until treatment at the recommended dose exceeded 30 days (Equioxxâ Package Insert, Merial; The Animal Health Division of Sanofi, Duluth, GA, USA). Toxic effects described were delayed healing of pre-existing oral ulcers and a higher incidence of oral ulceration when firocoxib was administered at the recommended dose for 42 days. No oral, esophageal or gastric ulceration was evident after nine consecutive days of oral dosing in any of the foals in this study. However, one foal was receiving omeprazole (foal 6), which may have masked the development of lesions. Flunixin meglumine, the current primary NSAID used in equine neonates, administered at the recommended therapeutic dose (1.1 mg/kg p.o. q 24 h) resulted in oral ulceration in all foals (n = 3) on days 10 or 11 with these lesions increasing in size and number over a 30-day study (Traub-Dargatz et al., 1988). All of these foals developed ulceration of the mucocutaneous junction of the nares bilaterally on days 11–13, attributed to flunixin meglumine being present in secretions draining down the nasal passages and accumulating at the nares. The ulceration in both circumstances was attributed to topical irritation of orally administered flunixin meglumine. Additionally, by day 30, all foals receiving oral flunixin meglumine had developed ulceration of both the glandular and squamous portions of the stomach, while only one foal in the control group (n = 3) developed gastric ulceration and then only in the squamous portion. Similar ulcers of the stomach and oral cavity, with the addition of lesions in the colon, are © 2013 John Wiley & Sons Ltd
Firocoxib pharmacokinetics and safety in foals 249
described in foals administered phenylbutazone orally at 10 mg/kg daily (Traub et al., 1983). In the current study, firocoxib did not cause any oral ulceration and only one foal had mild thickening of the margo plicatus on day 10. As the above-referenced studies were performed in different foals, direct comparisons between firocoxib and flunixin or phenylbutazone are not possible. However, the lack of any detectable gastric and oral ulceration in the foals of the present study suggests that firocoxib may be a safer therapeutic alternative in foals. Further studies evaluating longer and higher dosage regimens are warranted to confirm that firocoxib has a similar safety profile in the neonate to the adult. Proteinuria and/or hematuria was noted in four of the seven foals on day 1. These are considered a normal finding in healthy neonatal foals during the first 48 h of life (Edwards et al., 1990) and are likely consistent with closure of the urachus and possible umbilical trauma at foaling. Published reference intervals for urinary GGT-to-creatinine ratios for equine neonates are variable (Adams et al., 1985; Brewer et al., 1991). Mild elevations noted in the urinary GGT-to-creatinine ratios from the foals in the present study are within normal limits according to the most recent report in adult horses (Rossier et al., 1995). Established reference intervals for urinary GGT-to-creatinine ratios for the equine neonate were determined in 96 h old horse and pony foals (Brewer et al., 1991). Four of the six healthy neonates in the current study had values above those reported by Brewer et al. (1991) prior to administration of firocoxib, indicating the high variability in urinary GGT-to-creatinine ratios in healthy foals. However, these values were trending lower by day 11 in all four foals. Other renal biochemical variables (creatinine and BUN concentrations) were within or below established reference intervals in foals, and no clinically apparent renal abnormalities were noted. The low BUN values may be due to the foals remaining well hydrated by frequent suckling, as evidenced by low urine specific gravity measures throughout the study. Some of the foals displayed mild to moderately elevated GGT values during the study. Although biochemical profiles were not re-evaluated following the study, these foals remained part of a teaching herd at the V-MRCVM that was closely observed over the following 2 years and no long-term changes relating to a hepatopathy were reported. None of the other clinicopathologic changes from the study were considered significant in terms of their relationship to the administration of firocoxib. In the current study, orally administered firocoxib produced plasma concentrations that differed from adults. It is unknown at this time whether these concentrations provide therapeutic value in foals, and the drug’s clinical effectiveness for conditions affecting the neonate, including sepsis, endotoxemia, and musculoskeletal or gastrointestinal disease, has not been established. At the labeled dose (0.1 mg/kg q24 h), firocoxib provides substantial analgesia, without causing adverse effects, in experimentally induced lameness and in naturally occurring osteoarthritis in adult horses, with an effi© 2013 John Wiley & Sons Ltd
cacy that is comparable to phenylbutazone (Doucet et al., 2008; Back et al., 2009; Orsini et al., 2012). Although firocoxib’s clinical effectiveness for musculoskeletal pain in foals has not been reported, it is possible that the anti-inflammatory and analgesic effects would be comparable between the two drugs. Another frequent use of NSAIDs in neonates is for visceral inflammation and pain. In adult horses with experimentally induced jejunal ischemia, intravenous firocoxib administration provided effective visceral analgesia and improved recovery of mucosal barrier function in vitro faster than flunixin meglumine or the saline control (Cook et al., 2009). This was attributed to firocoxib’s selectivity for COX-2, which is upregulated after ischemic injury. The current anti-inflammatory of choice for visceral pain in neonates is flunixin meglumine. As a nonselective COX-inhibitor, flunixin also inhibits the production of homeostatic COX-1, which is required for prostaglandin-mediated intestinal repair. As a highly selective COX-2 inhibitor, with a COX-1/COX-2 IC50 ratio of 263-643 in the horse (Kvaternick et al., 2007a), it appears that firocoxib could be a more appropriate choice for visceral pain in neonates once a therapeutic dose is established. Currently, it appears the dose used in this study is safe and absorbed in the healthy neonate, but further work is required to determine a therapeutic dose for this age group. Intravenous dosing of the drug is likely to be more effective in this situation where gut motility and blood flow have been disrupted. Due to its clinical effectiveness in adults when compared with other NSAIDs, and reduced side effects as reported in toxicity studies, firocoxib provides promise as an effective and safer anti-inflammatory choice than the frequently utilized nonselective COX inhibitors. As no clinically apparent adverse side effects were found during the course of this study, the authors suggest that firocoxib can be safely administered per os to healthy neonatal foals for at least nine consecutive doses at the labeled dose (0.1 mg/kg q24 h). Safety with higher doses, more frequent dosing, or for longer dosing intervals remains to be determined, and more information needs to be collected prior to extrapolating results from these healthy foals to compromised neonates. However, more information is needed to determine the appropriate anti-inflammatory and analgesic dose in the neonate, as well as a more defined margin of safety, due to differences in the pharmacokinetic profile of firocoxib in neonates when compared with adults.
ACKNOWLEDGMENTS This study was supported by grants from the Stuart Equine Research Fund and Merial Animal Health Ltd.
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normal foals. American Journal of Veterinary Research, 64, 1486– 1490. Gunson, D.E. & Soma, L.R. (1983) Renal papillary necrosis in horses after phenylbutazone and water deprivation. Veterinary Pathology Online, 20, 603–610. Kvaternick, V., Pollmeier, M., Fischer, J. & Hanson, P.D. (2007a) Pharmacokinetics and metabolism of orally administered firocoxib, a novel second generation coxib, in horses. Journal of Veterinary Pharmacology and Therapeutics, 30, 208–217. Kvaternick, V., Malinski, T., Wortmann, J. & Fischer, J. (2007b) Quantitative HPLC-UV method for the determination of firocoxib from horse and dog plasma. Journal of Chromatography B-Analytical Technologies in the Biomedical and Life Sciences, 854, 313–319. Lees, P. & Toutain, P.L. (2013) Pharmacokinetics, pharmacodynamics, metabolism, toxicology and residues of phenylbutazone in humans and horses. Veterinary Journal, 196, 294–303. Letendre, L.T., Tessman, R.K., McClure, S.R., Kvaternick, V.J., Fischer, J.B. & Hanson, P.D. (2008) Pharmacokinetics of firocoxib after administration of multiple consecutive daily doses to horses. American Journal of Veterinary Research, 69, 1399–1405. Little, D., Jones, S.L., & Blikslager, A.T. (2007a) Cyclooxygenase (COX) inhibitors and the intestine. Journal of Veterinary Internal Medicine, 21, 367–377. Little, D., Brown, S.A., Campbell, N.B., Moeser, A.J., Davis, J.L. & Blikslager, A.T. (2007b) Effects of the cyclooxygenase inhibitor meloxicam on recovery of ischemia-injured equine jejunum. American Journal of Veterinary Research, 68, 614–624. Marshall, J.F. & Blikslager, A.T. (2011) The effect of nonsteroidal antiinflammatory drugs on the equine intestine. Equine Veterinary Journal, 43, 140–144. Martin, G.R. & Wallace, J.L. (2006) Gastrointestinal inflammation: a central component of mucosal defense and repair. Experimental Biology and Medicine, 231, 130–137. McCann, M.E., Anderson, D.R., Brideau, C, Black, W.C., Zhang, D.H. & Hickey, G.J. (2002) In vitro activity and in vivo efficacy of a novel CQX-2 inhibitor in the horse. Journal of Veterinary Internal Medicine, 16, 355. McConnico, R.S., Morgan, T.W., Williams, C.C., Hubert, J.D. & Moore, R.M. (2008) Pathophysiologic effects of phenylbutazone on the right dorsal colon in horses. American Journal of Veterinary Research, 69, 1496–1505. Murray, M.J. (1989) Endoscopic appearance of gastric lesions in foals: 94 cases (1987–1988). Journal of the American Veterinary Medical Association, 195, 1135–1141. Murray, M.J., Murray, C.M., Sweeney, H.J., Weld, J., Digby, N.J. & Stoneham, S.J. (1990a) Prevalence of gastric lesions in foals without signs of gastric disease: an endoscopic survey. Equine Veterinary Journal, 22, 6–8. Murray, M.J., Grodinsky, C., Cowles, R.R., Hawkins, W.L., Forfa, R.J. & Luba, N.K. (1990b) Endoscopic evaluation of changes in gastric lesions of Thoroughbred foals. Journal of American Veterinary Medical Association, 196, 1623–1627. Orsini, J.A., Ryan, W.G., Carithers, D.S. & Boston, R.C. (2012) Evaluation of oral administration of firocoxib for the management of musculoskeletal pain and lameness associated with osteoarthritis in horses. American Journal of Veterinary Research, 73, 664–671. Raidal, S.L., Edwards, S., Pippia, J., Boston, R. & Noble, G.K. (2013) Pharmacokinetics and safety of oral administration of meloxicam to foals. Journal of Veterinary Internal Medicine, 27, 300–307. Read, W.K. (1983) Renal medullary crest necrosis associated with phenylbutazone therapy in horses. Veterinary Pathology Online, 20, 662– 669.
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Firocoxib pharmacokinetics and safety in foals 251 Rossier, Y., Divers, T.J., & Sweeney, R.W. (1995) Variations in urinary gamma glutamyl transferase/urinary creatinine ratio in horses with or without pleuropneumonia treated with gentamicin. Equine Veterinary Journal, 27, 217–220. Ryan, C.A. & Sanchez, L.C. (2005) Nondiarrheal disorders of the gastrointestinal tract in neonatal foals. Veterinary Clinics of North America: Equine Practice, 21, 313–332. Savage, C.J. (2008) Urinary clinical pathologic findings and glomerular filtration rate in the horse. Veterinary Clinics of North America: Equine Practice, 24, 387–404. Semrad, S.D., Sams, R.A., & Ashcraft, S.M. (1993) Pharmacokinetics of and serum thromboxane suppression by flunixin meglumine in healthy foals during the first month of life. American Journal of Veterinary Research, 54, 2083–2087.
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Traub, J.L., Gallina, A.M., Grant, B.D., Reed, S.M., Gavin, P.R. & Paulsen, L.M. (1983) Phenylbutazone toxicosis in the foal. American Journal of Veterinary Research, 44, 1410–1418. Traub-Dargatz, J.L., Bertone, J.J., Gould, D.H., Wrigley, R.H., Weiser, M.G. & Forney, S.D. (1988) Chronic flunixin meglumine therapy in foals. American Journal of Veterinary Research, 49, 7–12. Vane, J.R. & Botting, R.M. (2003) The mechanism of action of aspirin. Thrombosis Research, 110, 255–258. Wilcke, J.R., Crisman, M.V., Sams, R.A. & Gerken, D.F. (1993) Pharmacokinetics of phenylbutazone in neonatal foals. American Journal of Veterinary Research, 54, 2064–2067. Wilcke, J.R., Crisman, M.V., Scarratt, W.K. & Sams, R.A. (1998) Pharmacokinetics of ketoprofen in healthy foals less than twenty-four hours old. American Journal of Veterinary Research, 59, 290–292.
Pharmacokinetics and safety of firocoxib after oral administration of repeated consecutive doses to neonatal foals N. HOVANESSIAN* J.L. DAVIS
†
H.C. MCKENZIE III*
Hovanessian, N., Davis, J.L., McKenzie, H.C. III, Hodgson, J.L., Hodgson, D.R., Crisman, M.V. Pharmacokinetics and safety of firocoxib after oral administration of repeated consecutive doses to neonatal foals. J. vet. Pharmacol. Therap. 37, 243–251.
J.L. HODGSON* D.R. HODGSON* & M.V. CRISMAN* *Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA, USA; † North Carolina State University College of Veterinary Medicine, Raleigh, NC, USA
The purpose of this study was to determine the pharmacokinetics and safety profile of firocoxib in neonatal foals. Seven healthy foals were administered 0.1 mg/kg firocoxib orally q24 h for nine consecutive days, commencing at 36 h of age. Blood was collected for firocoxib analysis using high-pressure liquid chromatography with fluorescence detection at 0 (dose #1 only), 0.25, 0.5, 1, 2, 4, 8, 16, and 24 h after doses 1, 5, and 9. For all other doses (2, 3, 4, 6, 7, and 8), blood was collected immediately prior to the next dose (24 h trough). Elimination samples (36, 48, 72, 96, 120, and 144 h) were collected after dose 9. Safety was assessed via physical examinations, body weight measurements, gastroscopy, complete blood count, plasma biochemistry and urinalysis. Firocoxib was rapidly absorbed following oral administration with minimal accumulation after repeat dosing. After the final dose, the terminal half-life was approximately 11 h. Firocoxib was below the limit of detection ( 0.99.
Monitoring
Pharmacokinetic analysis
Free catch urine samples were collected, when possible, on days 1, 3, 5, 7, 9, and 11 of the study. Urinalyses were performed, including specific gravity and urinary GGT/creatinine ratio. Gastroscopy was performed immediately after catheter placement (day 0) and on day 10 to evaluate for presence of gastric and esophageal ulceration. Foals were sedated with 0.5 mg/kg xylazine intravenously and gastroscopy performed (nasogastric) with a 1-meter flexible endoscope (Olympusâ Exera GIF-160; Olympus America, Center Valley, PA, USA). Gastric lesions were graded using the scoring system proposed by the Equine Gastric Ulcer Council (Anonymous, 1999; Bell et al., 2007). Blood was submitted in blood tubes containing EDTA for complete blood count and tubes containing lithium heparin for plasma biochemistry on days 1 and 11. On days 3 and 7, blood was submitted for evaluation of BUN and creatinine concentrations only.
Pharmacokinetic parameters for multidose firocoxib were calculated using the noncompartmental analysis model (log/linear trapezoidal) of commercially available pharmacokinetic software (WinNonlinâ version 5.0.1; Pharsight Corporation, Mountain View, CA, USA). Individual animal and average pharmacokinetic measures were taken. The maximum (Cmax) and minimum (Cmin) plasma concentrations and times (Tmax, Tmin) to those values were determined directly from the plasma concentrations of firocoxib. The average maximum and minimum concentrations at steadystate (Cssmax and Cssmin) and the terminal half-life (t1/2) were determined. Area under the curve (AUC) for 0–24 h (AUC0–24), and extrapolated to infinity (AUC0–∞), was established. Descriptive values were reported as Mean SD. An accumulation ratio was calculated from the following equation: s
RA ¼ 1=1 ekz Drug analysis Samples were analyzed at PKDM Department of Merial, Ltd. (Bridgewater, NJ, USA). Firocoxib concentrations in plasma were determined using a previously published HPLC method (Kvaternick et al., 2007b) that was modified for fluorescence detection with an excitation wavelength of 250 nm and emission wavelength of 375 nm. The current analysis method used fluorescence detection rather than UV detection for increased accuracy and to allow for a lower limit of quantitation at 5 ng/mL (vs. 25 ng/mL with UV detection). Additionally, a smaller sample size (0.2 vs. 1–2 mL plasma) was used. In brief, plasma samples were thawed at room temperature, vortexed, and centrifuged. Acetonitrile (1 mL) followed by 0.2 mL plasma was filled in a 96-well protein precipitation plate and left to sit for approximately 3 min. A vacuum was applied and the eluate collected in a 2-mL collection plate before being evaporated to dryness. The residue was reconstituted in 0.2 mL of 40% acetonitrile in water and a 0.05-mL aliquot injected onto the HPLC. Separation was accomplished on a Zorbax XBD precolumn (4.6 9 12.5 mm) and Zorbax RxC18 column (3 9 150 mm, 3.5 lm) (Agilent technologies, Santa Clara CA, USA) and maintained at 40 °C. The mobile phase consisted of 45:55 (v/v) acetonitrile and 0.25% trifluoroacetate anhydride (TFA) in water. Tandem HPLCs were used for this analysis, one set at a flow rate of 0.5 mL/min and the other at 0.8 mL/min. Acceptability of bioanalytical sets was based on the standard curves and fortified control sample results. The standard curve ranged from 2.5 to 250 ng/mL. Standards and quality control samples were prepared by adding an aliquot of standard solution to 200 lL of equine © 2013 John Wiley & Sons Ltd
where kz is the slope of the elimination phase and s is the dosing interval. Statistical analysis was performed using SigmaPlot 12.0 (SysStat Software, Inc, San Jose, CA, USA). Normality was assessed using the Shapiro–Wilk test. The one-way ANOVA was used to assess significant differences between the terminal half-life (T½), AUC0–24, Cmax, and Tmax for pharmacokinetic parameters calculated for days 1, 5, and 9. For non-normally distributed data, the Kruskal–Wallis one-way ANOVA on ranks was performed. Significance was set at P < 0.05. RESULTS Pharmacokinetics Following oral administration to neonatal foals, firocoxib was rapidly absorbed. After the initial dose, an average Cmax of 89.50 53.36 ng/mL (mean SD) was achieved at Tmax of 0.54 0.65 h. Figure 1 shows the average concentration-vs.time curve on days 1, 5, and 9 for neonatal foals (n = 7) treated with nine consecutive oral daily doses of firocoxib (0.1 mg/ kg). There was minimal accumulation after repeat dosing, with an average concentration (Cavg) in plasma of 39.1 8.4 ng/ mL. The actual average trough serum concentration of firocoxib for doses 1–9 was 14.1 3.2 ng/mL, whereas the calculated concentration was 16.3 8.3 ng/mL (Fig. 2). Near steady-state concentrations are achieved after the first dose, based on the fact that all average trough (or minimum) plasma concentrations are within one standard deviation of the calcu-
246 N. Hovanessian et al.
lated average minimum concentration. Based on the terminal half-life determined from the foals in this study, steady-state concentrations should be fully achieved after 55 h (2.3 doses). Figure 3 shows the measured daily firocoxib plasma concentrations for the seven foals after the ninth consecutive daily oral dose of the drug. Following the ninth dose, the harmonic mean of the terminal half-life (T½k) was 11.04 3.23 h. Relevant pharmacokinetic parameters after single (day 1) and multiple (days 5 and 9) consecutive oral daily doses of firocoxib are summarized in Table 1. A significant difference between the AUC0–24 values for day 1 and day 9 was noted. A difference was not detected for any other parameter. The final dose of firocoxib was administered at 192 h. Drug was still detected at 228 h in 6/7 foals and at 240 h in 4/7 foals. Drug was not detected past 264 h in any foal. Fig. 3. Plasma concentration vs. time for individual foals following the final ninth consecutive dose of firocoxib. The solid line represents the average concentrations.
Table 1. Mean SD values for pharmacokinetic parameters for single (day 1) and multiple oral doses of firocoxib (0.1 mg/kg) to neonatal foals
Fig. 1. Average plasma concentration–time curve from neonatal foals (n = 7) treated with nine consecutive oral daily doses of firocoxib (0.1 mg/kg) for doses 1, 5, and 9, and trough concentrations (taken immediately prior to dosing) for doses 2–4 and 6–8.
Day 1
Day 5
Day 9
(Mean SD)
(Mean SD)
(Mean SD)
kz (1/h)
0.08 0.03
0.07 0.02
0.07 0.03
T1/2kz (h) Tmax (h)
10.46 4.97 0.54 0.65
10.46 2.87 0.43 0.28
11.04 3.23 1.46 1.75
Cmax (ng/mL)
89.50 53.36
94.07 61.23
71.17 21.41
Cavg (ng/mL)
–
–
39.1 8.4
AUC0–24 (h*ng/mL)
629.16 178.09*
794.44 187.10
1162.45 326.98*
AUC0–∞ (h*ng/mL)
–
–
1255.68 372.76
MRT0–∞ (h) % Fluctuation
14.66 6.93 –
Accumulation
–
14.72 4.2 – –
16.22 4.09 147 72 1.29 0.15
index
kz, slope of the terminal phase; T½kz, half-life of terminal phase; Tmax, time to maximum concentration; Cmax, maximum concentration; Cavg, average concentration; AUC0–24, area under the concentration–time curve from 0 to 24 h; AUC0–∞, area under the concentration–time curve extrapolated to infinity; MRT, mean residence time. *Indicates a significant difference between parameters on different days (P < 0.05).
Gastroscopy
Fig. 2. Actual minimum plasma concentrations compared with average calculated minimum plasma concentrations.
No abnormalities were observed on initial gastroscopy, with all foals scoring grade 0 (intact epithelium). On day 11, Foal E displayed mild hyperkeratosis of the margo plicatus, giving it a score of grade 1 (intact mucosa, evidence of hyperkeratosis or hyperemia). All other evaluations of esophageal, gastric, and pyloric mucosa were unremarkable. © 2013 John Wiley & Sons Ltd
Firocoxib pharmacokinetics and safety in foals 247
Hematologic and biochemical variables Most of the foals had mild changes consistent with stress, evidenced by the presence of a mature neutrophilia, lymphopenia, and mild hyperglycemia, at one or more sampling periods. Six of the foals had BUN and plasma creatinine values that were below the reference interval. On day 11, five of the foals had mild to moderately elevated plasma GGT activities, with two also displaying mildly elevated AST activity. Urinalyses were within reference intervals for most of the foals throughout the study period. Four of the seven foals had traces of blood and/or protein in the urine on day 1. One foal (Foal 3) displayed trace proteinuria on day 11. Urinary GGT-to-creatinine ratios were determined for each foal on three to 5 days of the study period. The number of samples available depended on the ability to catch urine in the individual foals, since a free catch technique was employed. Urinary enzyme activities were highly variable before and after firocoxib administration but remained within reference intervals for adult horses (Rossier et al., 1995). Minor complications developed in three of the seven foals that were not related to firocoxib administration. On day 11 of the study, after the final dose of firocoxib, Foal 5 was lethargic, pyrexic (103.4°F), and lame in the left hind limb. The foal displayed an abnormal leukogram, with a total white cell count of 19.370 9 103/lL (reference interval 4.500–11.500 9 103/ lL), a segmented neutrophilia of 17.046 9 103/lL, (reference interval 3.040–9.570 9 103/lL), and monocytosis of 0.581 9 103/lL (reference interval 50–380 9 103/lL). This was accompanied by a mild hyperfibrinogenemia (500 mg/dL, reference interval 110–450 mg/dL). Further diagnostics identified mild cellulitis at the catheter site, a patent urachus and inflammation of the left metatarsophalangeal joint. The foal was treated with medical and surgical management including umbilical resection and joint lavage. These complications were unlikely due to firocoxib administration, and samples continued to be collected and evaluated until the end of the study period. The foal remained bright, continued to gain weight, and was healthy at discharge on day 22. One mare died 60 h after foaling due to uterine artery rupture, despite aggressive medical therapy. The foal (Foal 6) then was provided 25% of its body weight per day as a formulated mare’s milk replacer (Mare’s Matchâ; Land O’ Lakes Inc., Saint Paul’s, MN, USA). Freshly prepared milk replacer was offered every 6 h. In addition, the foal was administered with omeprazole (1 mg/kg p.o. q24 h). No adverse effects were noted in the
foal’s attitude, demeanor, or hydration status throughout the study period. Foal 7 dislodged its intravenous catheter on day 5 and then developed mild cellulitis at its second catheter site on day 10. The foal was treated with oral antimicrobial agents and topical therapy. The cellulitis resolved without complications.
DISCUSSION Firocoxib, given per os, to neonatal foals at the standard adult therapeutic dose achieves detectable concentrations in plasma without inducing clinical or laboratory evidence of toxicity. Similar to previous studies in which traditional NSAIDs were administered to equine neonates (Semrad et al., 1993; Wilcke et al., 1993, 1998; Crisman et al., 1996), firocoxib displays differences in the pharmacokinetics between adult horses and neonates. However, unlike traditional NSAIDs, firocoxib appears to be cleared faster and has a shorter half-life in neonatal foals compared with adults (Table 2). Interestingly, a similar finding in foals has been reported for another drug in the coxib class, meloxicam (Raidal et al., 2013). The time to peak serum concentration (Tmax) after a single oral dose of the drug is considerably shorter in foals compared with adults (Kvaternick et al., 2007a; Letendre et al., 2008). This phenomenon of more rapid absorption may be explained by differences in the diet of adults and foals. The drug may be binding to milk proteins present in the foal’s digestive tract, aiding its movement into the bloodstream. Alternatively, a lower gastric pH in foals (Baker & Gerring, 1993) may be altering absorption of the drug, potentially through pH-dependent solubility or effects on drug ionization. However, Kvaternick et al. (2007a) describe firocoxib as nonionizable, and solubility is not affected by pH, making it unlikely that this has an effect on absorption. Finally, it may be that the presence of feed material in the digestive tract is delaying firocoxib’s absorption in adults. After 9 days of dosing, the Tmax in foals increased from 0.54 0.65 h to 1.46 1.75 h. This may be indicative of a change in the foals’ feeding habits over that time and represent an increase in solid food within the stomach which may affect both pH and gastric emptying. The mean maximum serum concentration (Cmax) following a single oral dose of firocoxib is higher in neonates compared with adults (Kvaternick et al., 2007a; Letendre et al., 2008). The faster absorption in neonates may explain this increase in
Table 2. Comparative pharmacokinetic parameters for firocoxib in foals and adult horses following oral administration of single or multiple doses of 0.1 mg/kg AUC0–24 (h *lg/mL) Foal (single dose) Adult (single dose) Foal (multiple doses) Adult (multiple doses) See Table 1 for legend. © 2013 John Wiley & Sons Ltd
0.63 0.83 0.96 1.16 3.12
0.18 0.17 0.26 3.27 0.86
Cmax (ng/mL) 89.5 45.0 75.0 71.17 173.0
53.36 11.3 33.0 21.41 44.0
Tmax (h) 0.54 7.80 3.90 1.46 0.79
0.65 4.80 4.4 1.75 0.70
T½ (h) 10.46 ND 29.6 11.04 36.5
4.97 7.5 3.23 9.5
Reference Present study Letendre et al. (2008) Kvaternick et al. (2007a) Present study Letendre et al. (2008)
248 N. Hovanessian et al.
the Cmax as more of the drug is absorbed in a shorter period. After repeated daily oral doses, the average Cmax in adult horses is much higher than following a single dose (Letendre et al., 2008); however, this was not observed in the foals of the present study. Additionally, the terminal half-life after oral administration is shorter in neonates when compared with adults (Kvaternick et al., 2007a). This shorter terminal half-life leads to less accumulation in foals with a mean calculated accumulation index of 1.29 0.15 vs. 3.8 0.7 in adults. This lack of accumulation can explain the difference in Cmax following repeated doses. The shorter terminal half-life may be related to either a decreased volume of distribution or an increased clearance. Differences in volume of distribution can account for differences in half-life; however, foals typically have a larger volume of distribution compared with adults due to a higher percent body composition of water, and this would tend to increase the halflife. Therefore, the authors suggest a higher clearance due to either a faster hepatic metabolism or renal excretion may be occurring. Biotransformation of firocoxib in adult horses is via dealkylation and glucuronidation to at least three inactive metabolites which are partially excreted in urine (68%) and feces (15%) (Kvaternick et al., 2007a,b). Faster renal excretion of firocoxib due to an increased GFR in foals compared with adult horses (Gonda et al., 2003; Savage, 2008) may account for an increased clearance, at least to some degree. Potentially, the effect of ontogeny and maturation of drug-metabolizing pathways in the liver of neonatal animals could also account for a portion of the faster clearance. Raidal et al. (2013) speculated that clearance of meloxicam in neonatal foals was increased due to the fact that the cytochrome P450 enzyme systems are among the most abundant liver enzymes, and therefore, younger animals have a relative abundance of these enzymes when liver volume is normalized to body weight, which increases drug clearance (Blanco et al., 2000; BurgosVargas et al., 2004). Additionally, the gastrointestinal tract, blood cells and potentially the lungs may be affecting the rate of firocoxib metabolism in this age group (Allegaert et al., 2007). Regardless, the impact from the ontogeny of hepatic and renal clearance on drug metabolism in neonates remains largely unknown, although phenotypic variability is frequently observed (Alcorn & McNamara, 2002; Bartelink et al., 2006; Allegaert et al., 2007, 2009). The influence of clearance and volume of distribution can only be determined following an intravenous dose, and unfortunately, an intravenous formulation of firocoxib was not available at the time this study was performed. This lack of intravenous formulation also prevented the determination of the absolute bioavailability in neonatal foals in the present study. In adult horses, bioavailability of firocoxib is 79% (Kvaternick et al., 2007a). Based on previously published values in adult horses and those presented in the current study, a relative bioavailability for foals can be calculated as the ratio of the AUC0–24 in foals and adults. This value is 65.6–75.9% using single dose data, indicating that bioavailability in foals is lower than that reported for adults.
Other NSAIDs, such as ketoprofen and flunixin meglumine, have longer half-lives and reduced elimination in foals (Semrad et al., 1993; Wilcke et al., 1993; Crisman et al., 1996), which necessitates decreasing the dose or increasing the dosing interval. However, a report of the pharmacokinetics of another COX-2 selective NSAID, meloxicam, in equine neonates (Raidal et al., 2013) shows similar discrepancies to firocoxib when compared with other NSAIDs and that study concluded that a shorter dosing interval of 12 h (vs. 24 h in adults) should be used. The shorter half-life and faster elimination of firocoxib could result in more frequent and/or higher dosing being required to maintain the same serum concentrations as in adults. In vitro COX inhibition assays performed on whole blood from adult horses suggested a concentration that would inhibit 50% of COX-2 (IC50) for firocoxib of 0.09 lM (McCann et al., 2002). This correlates to a concentration of 30.276 ng/ mL, which is similar to the average concentration (Cavg) in plasma for the foals in this study on day 9 (39.7 ng/mL). However, whether similar serum concentrations in equine neonates would provide clinically appropriate anti-inflammatory, antipyretic, and analgesic therapy needs to be established. Currently, there are no published pharmacodynamic or efficacy data for firocoxib for equine neonates. Interestingly, the development of clinical signs in the foal with the patent urachus and joint effusion coincided with cessation of drug administration on day 10 of the study. This provides some anecdotal evidence of firocoxib’s anti-inflammatory, anti-pyretic, and analgesic effects in the foal at the administered dose. Evaluation of the toxicity of firocoxib in adults showed that toxicity was not induced until treatment at the recommended dose exceeded 30 days (Equioxxâ Package Insert, Merial; The Animal Health Division of Sanofi, Duluth, GA, USA). Toxic effects described were delayed healing of pre-existing oral ulcers and a higher incidence of oral ulceration when firocoxib was administered at the recommended dose for 42 days. No oral, esophageal or gastric ulceration was evident after nine consecutive days of oral dosing in any of the foals in this study. However, one foal was receiving omeprazole (foal 6), which may have masked the development of lesions. Flunixin meglumine, the current primary NSAID used in equine neonates, administered at the recommended therapeutic dose (1.1 mg/kg p.o. q 24 h) resulted in oral ulceration in all foals (n = 3) on days 10 or 11 with these lesions increasing in size and number over a 30-day study (Traub-Dargatz et al., 1988). All of these foals developed ulceration of the mucocutaneous junction of the nares bilaterally on days 11–13, attributed to flunixin meglumine being present in secretions draining down the nasal passages and accumulating at the nares. The ulceration in both circumstances was attributed to topical irritation of orally administered flunixin meglumine. Additionally, by day 30, all foals receiving oral flunixin meglumine had developed ulceration of both the glandular and squamous portions of the stomach, while only one foal in the control group (n = 3) developed gastric ulceration and then only in the squamous portion. Similar ulcers of the stomach and oral cavity, with the addition of lesions in the colon, are © 2013 John Wiley & Sons Ltd
Firocoxib pharmacokinetics and safety in foals 249
described in foals administered phenylbutazone orally at 10 mg/kg daily (Traub et al., 1983). In the current study, firocoxib did not cause any oral ulceration and only one foal had mild thickening of the margo plicatus on day 10. As the above-referenced studies were performed in different foals, direct comparisons between firocoxib and flunixin or phenylbutazone are not possible. However, the lack of any detectable gastric and oral ulceration in the foals of the present study suggests that firocoxib may be a safer therapeutic alternative in foals. Further studies evaluating longer and higher dosage regimens are warranted to confirm that firocoxib has a similar safety profile in the neonate to the adult. Proteinuria and/or hematuria was noted in four of the seven foals on day 1. These are considered a normal finding in healthy neonatal foals during the first 48 h of life (Edwards et al., 1990) and are likely consistent with closure of the urachus and possible umbilical trauma at foaling. Published reference intervals for urinary GGT-to-creatinine ratios for equine neonates are variable (Adams et al., 1985; Brewer et al., 1991). Mild elevations noted in the urinary GGT-to-creatinine ratios from the foals in the present study are within normal limits according to the most recent report in adult horses (Rossier et al., 1995). Established reference intervals for urinary GGT-to-creatinine ratios for the equine neonate were determined in 96 h old horse and pony foals (Brewer et al., 1991). Four of the six healthy neonates in the current study had values above those reported by Brewer et al. (1991) prior to administration of firocoxib, indicating the high variability in urinary GGT-to-creatinine ratios in healthy foals. However, these values were trending lower by day 11 in all four foals. Other renal biochemical variables (creatinine and BUN concentrations) were within or below established reference intervals in foals, and no clinically apparent renal abnormalities were noted. The low BUN values may be due to the foals remaining well hydrated by frequent suckling, as evidenced by low urine specific gravity measures throughout the study. Some of the foals displayed mild to moderately elevated GGT values during the study. Although biochemical profiles were not re-evaluated following the study, these foals remained part of a teaching herd at the V-MRCVM that was closely observed over the following 2 years and no long-term changes relating to a hepatopathy were reported. None of the other clinicopathologic changes from the study were considered significant in terms of their relationship to the administration of firocoxib. In the current study, orally administered firocoxib produced plasma concentrations that differed from adults. It is unknown at this time whether these concentrations provide therapeutic value in foals, and the drug’s clinical effectiveness for conditions affecting the neonate, including sepsis, endotoxemia, and musculoskeletal or gastrointestinal disease, has not been established. At the labeled dose (0.1 mg/kg q24 h), firocoxib provides substantial analgesia, without causing adverse effects, in experimentally induced lameness and in naturally occurring osteoarthritis in adult horses, with an effi© 2013 John Wiley & Sons Ltd
cacy that is comparable to phenylbutazone (Doucet et al., 2008; Back et al., 2009; Orsini et al., 2012). Although firocoxib’s clinical effectiveness for musculoskeletal pain in foals has not been reported, it is possible that the anti-inflammatory and analgesic effects would be comparable between the two drugs. Another frequent use of NSAIDs in neonates is for visceral inflammation and pain. In adult horses with experimentally induced jejunal ischemia, intravenous firocoxib administration provided effective visceral analgesia and improved recovery of mucosal barrier function in vitro faster than flunixin meglumine or the saline control (Cook et al., 2009). This was attributed to firocoxib’s selectivity for COX-2, which is upregulated after ischemic injury. The current anti-inflammatory of choice for visceral pain in neonates is flunixin meglumine. As a nonselective COX-inhibitor, flunixin also inhibits the production of homeostatic COX-1, which is required for prostaglandin-mediated intestinal repair. As a highly selective COX-2 inhibitor, with a COX-1/COX-2 IC50 ratio of 263-643 in the horse (Kvaternick et al., 2007a), it appears that firocoxib could be a more appropriate choice for visceral pain in neonates once a therapeutic dose is established. Currently, it appears the dose used in this study is safe and absorbed in the healthy neonate, but further work is required to determine a therapeutic dose for this age group. Intravenous dosing of the drug is likely to be more effective in this situation where gut motility and blood flow have been disrupted. Due to its clinical effectiveness in adults when compared with other NSAIDs, and reduced side effects as reported in toxicity studies, firocoxib provides promise as an effective and safer anti-inflammatory choice than the frequently utilized nonselective COX inhibitors. As no clinically apparent adverse side effects were found during the course of this study, the authors suggest that firocoxib can be safely administered per os to healthy neonatal foals for at least nine consecutive doses at the labeled dose (0.1 mg/kg q24 h). Safety with higher doses, more frequent dosing, or for longer dosing intervals remains to be determined, and more information needs to be collected prior to extrapolating results from these healthy foals to compromised neonates. However, more information is needed to determine the appropriate anti-inflammatory and analgesic dose in the neonate, as well as a more defined margin of safety, due to differences in the pharmacokinetic profile of firocoxib in neonates when compared with adults.
ACKNOWLEDGMENTS This study was supported by grants from the Stuart Equine Research Fund and Merial Animal Health Ltd.
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