BACKGROUND
analgesic drugs including nonsteroidal anti-inflammatory drugs , opioids and local anaesthetic agents are used in dogs as peri- and post-surgery pain relievers.
nsaids are used in veterinary medicine for treating especially musculo-skeletal and abdominal pain  <cit> .
these drugs have antipyretic, anti-inflammatory and analgesic properties.
the common mechanism of action for this class of drugs can be attributed to a blockade of the biosynthesis of prostaglandins, resulting from the inhibitionof enzyme cyclooxygenase   <cit> .
meloxicam , belongs to the enolic acid class of nsaids and is also effective as antipyretic, analgesic or anti-inflammatory agent  <cit> .
it preferentially inhibits cox- <dig>  which is induced by inflammatory stimuli in pathophysiological conditions  <cit> .
mlx is about  <dig> times more potent at inhibiting cox- <dig> than cox- <dig> activity  <cit> .
surgery is a duration when there are some major stresses associated with pain, cardiovascular and possibly major haemodynamic changes peri- and post-operation, especially if there is a significant blood loss  <cit> .
in general, the pharmacokinetics of drugs in most species have been characterized in healthy animals.
however, the plasma disposition of the active compound and its metabolites may be markedly altered in diseased animals or during peri- or post-operative period.
the pharmacokinetics data are available for some nsaids in dogs undergoing surgery and can be compared with other published data  <cit> .
however, to the best of our knowledge, this is the first study in which the pharmacokinetic disposition of an nsaid in dogs has been evaluated in the same animal with and without surgery.
our hypothesis was that surgery alters the plasma disposition of certain drugs including analgesics and anaesthetics administered pre- or peri-operatively.
therefore, this study was designed to investigate the effects of surgery on the plasma disposition of mlx in dogs undergoing ovariohysterectomy following a single intravenous injection at  <dig>  mg/kg bodyweight.
RESULTS
clinically no adverse effects were observed during and after the mlx administration in dogs.
the pre-operative biochemical and haematological parameters were within the normal ranges.
mean age, bodyweight, duration of surgery and the selected surgical variables during anaesthesia are shown in table  <dig>  the heart rate, respiratory rate and end tidal co <dig> values were  <dig>  ±  <dig>  beats/min,  <dig>  ±  <dig>  breaths/min and  <dig>  ±  <dig>  mmhg during anaesthesia, respectively.
the albumin levels of each dog in the control group were similar to its own post-surgery plasma albumin concentration.
in addition, the mean albumin concentration in the control group  was not significantly different  compared with that observed in the surgery group .table  <dig> mean  values of general characteristics and selected surgical variables during anaesthesia of eight dogs undergoing ovariohysterectomy
the analysis yielded linear regression lines ranging from  <dig>  to 10 μg/ml of mlx with a correlation coefficient of  <dig> .
the limits of detection and quantification of the mlx assay were  <dig>  μg/ml and  <dig>  μg/ml, respectively.
the mean recovery was  <dig>  %  and the accuracy ranged from 96 % to 103 %.
pharmacokinetic parameters of mlx following intravenous administration  are given in table  <dig>  semi logarithmic plot of the mean plasma concentration vs. time curves are shown in fig  <dig>  in additions, the scatter plots of the pharmacokinetic parameters  in control vs. surgery group are presented in fig.
<dig> table  <dig> median pharmacokinetic parameters with lower and upper confidence intervals of meloxicam in control and surgery groups following intravenous  administration to bitches
c0: plasma concentration at time  <dig>  auc0→∞: area under the  curve from time  <dig> to infinity, t1/2λz: terminal half-life, aumc0→∞: area under the moment curve from time  <dig> to infinity; clb: total body clearance of drug; vdss: volume of distribution at steady-state; mrt0→∞: mean residence time
*the kinetic parameters in control group are significantly different  from the surgery group
fig  <dig> semi log plot of mean  plasma concentrations vs. time curves of meloxicam in control and surgery groups following intravenous  administration to bitches
fig  <dig> the scatter plots of some pharmacokinetic parameters  in control group vs. surgery group following intravenous  administration of meloxicam to dogs
the differences in most pharmacokinetic parameters between the control and surgery groups were not significant, however, variability was high for each phase.
nevertheless, the terminal half-life  and total body clearance  values were longer and higher, but the area under the curve  value was smaller in the control group  compared to the surgery group , respectively.
the volume of mlx distribution at steady-state  of the control group  was significantly larger  compared to that of the surgery group .
the vdss value of each individual animal decreased in phase i compared with that of the same animals in phase ii.
moreover, t1/2λz, auc and mrt values in  <dig> out of  <dig> dogs were altered in the post-surgery group compared to the control group.
discussion
the plasma concentration of mlx in dogs following intravenous administration exhibits slight fluctuations, especially in the early hours of the administration in both phases of the present study.
similar observations were also reported by studies performed recently in dogs, cats and horses subjected to ketorolac treatment  <cit> .
the reason of such characteristics in plasma concentrations is not clear and probably not associated with the anaesthetic agents and surgical procedures employed in the experiments, since similar fluctuations were observed in the animals of the control group.
there is a reasonable body of literature evaluating peri- or post-operative analgesic effects and plasma dispositions of some nsaids in dogs undergoing surgery  <cit> .
however, to our knowledge this is the first study in which the pharmacokinetics of an nsaid in dogs have been evaluated in the same animal with and without surgery.
although, all animals under surgery receive at least  <dig> drug during the operation for pre-anesthetic medication, anesthesia or analgesia in the surgery unit, there is no information available on the alterations in drug disposition following surgery in veterinary medicine.
during the surgery, many hemodynamic and metabolic alterations such as fluctuating hemodynamics, alteration of protein catabolism and anabolism may occur.
the changes in circulation, blood volume, extracellular fluid and blood protein levels, such as α1-acid glycoprotein and albumin may affect the distribution of drugs used during the peri- and post-operative periods  <cit> .
the kinetic parameters of mlx observed from the control group in the present study exhibits a low apparent volume of distribution  and a long plasma half-life  and residence time  in dogs following the intravenous administration.
these findings are in agreement with the results of a previous study  <cit> , regarding the values of vss , t1/ <dig> , mrt  and auc  in healthy dogs.
the differences in most pharmacokinetic parameters between the control and surgery groups were not significant, however, variability was high for each phase.
the dogs in the control group were demonstrated to have a prolonged half-life , smaller auc  and greater body clearance   when compared to themselves in the surgery group.
moreover, the volume of mlx distribution at steady-state in the control group  was significantly greater than that of the surgery group .
volume of distribution  is one of the most important parameters for the evaluation of the pharmacokinetic characteristics of the drugs and depends on many factors, including the physicochemical properties of the drug, binding strength of plasma proteins, binding capacity to the other tissues.
the reason for low levels of vdss measured in surgery group is probably associated with major haemodynamic changes during the surgery and/or drug-drug interaction.
the hepatic activity increases during the post-operative period.
serum albumin turnover elevates, drug-albumin binding characteristics alter and the level of serum proteins, including albumin decreases dramatically for a period following surgery  <cit> .
in addition, it has been reported that the serum albumin level was significantly reduced in dogs with inflammation compared with the healthy dogs  <cit> .
besides, high plasma albumin binding of mlx could be altered by the anaesthetic drugs, since it has been reported that the plasma proteins  decreases in under isoflurane anaesthesia in flying fox  <cit> .
both the duration and the intensity of drug action with the free drug concentration being the active compound are affected by the level of protein binding  <cit> .
in these circumstances, the protein binding is influenced, particularly for the drugs which have more than 90 % protein bound  <cit> .
the protein binding ratio of mlx in dogs was reported as 97 %  <cit>  and mlx exhibits a high degree of binding to albumin  <cit> .
generally, the reduction of plasma protein leads the increase of distribution volume and clearance of drugs via the decrease in the plasma protein bindings.
however, the mean albumin concentration in the control group  was not significantly different  compared with that observed in the surgery group  in the present study.
consequently, the difference on vdss of mlx is probably not related to the different plasma albumin level between the control and surgery groups.
the reason for lower vdss in the surgery group was however unclear.
it is probably related to the other factors such as drug-drug interaction which may have changed the drug-protein binding capacity in the surgery group since atropine sulphate, propofol and isoflurane were combinely used for premedication and as anaesthesia.
also ovariohysterectomy may have affected reproductive hormones and thus the disposition of the drugs.
in addition, the reason for lower vdss in the surgery group could be, at least in part, due to smaller clb and mrt as vdss = clb × mrt.
moreover, there was a limitation since the control and surgery phases  were arranged in a sequence, but not in a crossover fashion.
this experimental design may potentially induce a period effect.
however, this is judged to be unlikely as a 10-day washout period was considered to be sufficiently long  <cit> .
in the present study, the effect of surgical operation on the plasma disposition of mlx was investigated in only crossbred and female dogs.
however, the plasma disposition of mlx may be different in pure-bred and between male and female dogs.
the elimination half-life of naproxen in beagles  was much shorter  <cit> , compared with that of observed in mongrels   <cit> .
this difference probably reflects a breed difference, since it is known that beagles metabolise drugs more rapidly than other breeds  <cit> .
besides, it has been reported that, female dogs showed lower auc and elimination t1/ <dig> values of mlx, compared with male dogs  <cit> .
CONCLUSIONS
vdss of mlx in the control group is significantly greater compared to that of the surgery group.
generally limited or lowered distribution may decrease the drug exposure into tissues.
however, the consequent reduction in volume of distribution leads to higher plasma concentrations of mlx.
besides, higher plasma concentration of the increased unbound mlx fraction in plasma may increase risk of side effects such as gastrointestinal ulceration, unusual bleeding and loss of kidney function/failure when repeated doses are used.
the change in the disposition of some drugs pre-, peri- and post-operatively administered for pre-medication, anaesthesia or prevention of post-operative infections may result in changing efficacy and/or increasing side effects or toxicity.

 analgesic drugs including nonsteroidal anti-inflammatory drugs, opioids and local anaesthetic agents are used in dogs as peri- and post-surgery pain relievers. nsaids are used in veterinary medicine for treating especially musculo-skeletal and abdominal pain. these drugs have antipyretic, anti-inflammatory and analgesic properties. the common mechanism of action for this class of drugs can be attributed to a blockade of the biosynthesis of prostaglandins, resulting from the inhibitionof enzyme cyclooxygenase. meloxicam, belongs to the enolic acid class of nsaids and is also effective as antipyretic, analgesic or anti-inflammatory agent. it preferentially inhibits cox- which is induced by inflammatory stimuli in pathophysiological conditions. mlx is about times more potent at inhibiting cox- than cox- activity. surgery is a duration when there are some major stresses associated with pain, cardiovascular and possibly major haemodynamic changes peri- and post-operation, especially if there is a significant blood loss. in general, the pharmacokinetics of drugs in most species have been characterized in healthy animals. however, the plasma disposition of the active compound and its metabolites may be markedly altered in diseased animals or during peri- or post-operative period. the pharmacokinetics data are available for some nsaids in dogs undergoing surgery and can be compared with other published data. however, to the best of our knowledge, this is the first study in which the pharmacokinetic disposition of an nsaid in dogs has been evaluated in the same animal with and without surgery. our hypothesis was that surgery alters the plasma disposition of certain drugs including analgesics and anaesthetics administered pre- or peri-operatively. therefore, this study was designed to investigate the effects of surgery on the plasma disposition of mlx in dogs undergoing ovariohysterectomy following a single intravenous injection at  mg/kg bodyweight.  clinically no adverse effects were observed during and after the mlx administration in dogs. the pre-operative biochemical and haematological parameters were within the normal ranges. mean age, bodyweight, duration of surgery and the selected surgical variables during anaesthesia are shown in table  the heart rate, respiratory rate and end tidal co values were  ±  beats/min,  ±  breaths/min and  ±   mmhg during anaesthesia, respectively. the albumin levels of each dog in the control group were similar to its own post-surgery plasma albumin concentration. in addition, the mean albumin concentration in the control group was not significantly different compared with that observed in the surgery group.table mean values of general characteristics and selected surgical variables during anaesthesia of eight dogs undergoing ovariohysterectomy the analysis yielded linear regression lines ranging from to 10 μg/ml of mlx with a correlation coefficient of. the limits of detection and quantification of the mlx assay were  μg/ml and  μg/ml, respectively. the mean recovery was  % and the accuracy ranged from 96 % to 103 %. pharmacokinetic parameters of mlx following intravenous administration are given in table  semi logarithmic plot of the mean plasma concentration vs. time curves are shown in fig  in additions, the scatter plots of the pharmacokinetic parameters in control vs. surgery group are presented in fig.  table median pharmacokinetic parameters with lower and upper confidence intervals of meloxicam in control and surgery groups following intravenous administration to bitches c0: plasma concentration at time auc0→∞: area under the curve from time to infinity, t1/2λz: terminal half-life, aumc0→∞: area under the moment curve from time to infinity; clb: total body clearance of drug; vdss: volume of distribution at steady-state; mrt0→∞: mean residence time *the kinetic parameters in control group are significantly different from the surgery group fig semi log plot of mean plasma concentrations vs. time curves of meloxicam in control and surgery groups following intravenous administration to bitches fig the scatter plots of some pharmacokinetic parameters in control group vs. surgery group following intravenous administration of meloxicam to dogs the differences in most pharmacokinetic parameters between the control and surgery groups were not significant, however, variability was high for each phase. nevertheless, the terminal half-life and total body clearance values were longer and higher, but the area under the curve value was smaller in the control group compared to the surgery group, respectively. the volume of mlx distribution at steady-state of the control group was significantly larger compared to that of the surgery group. the vdss value of each individual animal decreased in phase i compared with that of the same animals in phase ii. moreover, t1/2λz, auc and mrt values in out of dogs were altered in the post-surgery group compared to the control group. discussion the plasma concentration of mlx in dogs following intravenous administration exhibits slight fluctuations, especially in the early hours of the administration in both phases of the present study. similar observations were also reported by studies performed recently in dogs, cats and horses subjected to ketorolac treatment. the reason of such characteristics in plasma concentrations is not clear and probably not associated with the anaesthetic agents and surgical procedures employed in the experiments, since similar fluctuations were observed in the animals of the control group. there is a reasonable body of literature evaluating peri- or post-operative analgesic effects and plasma dispositions of some nsaids in dogs undergoing surgery. however, to our knowledge this is the first study in which the pharmacokinetics of an nsaid in dogs have been evaluated in the same animal with and without surgery. although, all animals under surgery receive at least drug during the operation for pre-anesthetic medication, anesthesia or analgesia in the surgery unit, there is no information available on the alterations in drug disposition following surgery in veterinary medicine. during the surgery, many hemodynamic and metabolic alterations such as fluctuating hemodynamics, alteration of protein catabolism and anabolism may occur. the changes in circulation, blood volume, extracellular fluid and blood protein levels, such as α1-acid glycoprotein and albumin may affect the distribution of drugs used during the peri- and post-operative periods. the kinetic parameters of mlx observed from the control group in the present study exhibits a low apparent volume of distribution and a long plasma half-life and residence time in dogs following the intravenous administration. these findings are in agreement with the results of a previous study, regarding the values of vss, t1/, mrt and auc in healthy dogs. the differences in most pharmacokinetic parameters between the control and surgery groups were not significant, however, variability was high for each phase. the dogs in the control group were demonstrated to have a prolonged half-life, smaller auc and greater body clearance when compared to themselves in the surgery group. moreover, the volume of mlx distribution at steady-state in the control group was significantly greater than that of the surgery group. volume of distribution is one of the most important parameters for the evaluation of the pharmacokinetic characteristics of the drugs and depends on many factors, including the physicochemical properties of the drug, binding strength of plasma proteins, binding capacity to the other tissues. the reason for low levels of vdss measured in surgery group is probably associated with major haemodynamic changes during the surgery and/or drug-drug interaction. the hepatic activity increases during the post-operative period. serum albumin turnover elevates, drug-albumin binding characteristics alter and the level of serum proteins, including albumin decreases dramatically for a period following surgery. in addition, it has been reported that the serum albumin level was significantly reduced in dogs with inflammation compared with the healthy dogs. besides, high plasma albumin binding of mlx could be altered by the anaesthetic drugs, since it has been reported that the plasma proteins decreases in under isoflurane anaesthesia in flying fox. both the duration and the intensity of drug action with the free drug concentration being the active compound are affected by the level of protein binding. in these circumstances, the protein binding is influenced, particularly for the drugs which have more than 90 % protein bound. the protein binding ratio of mlx in dogs was reported as 97 % and mlx exhibits a high degree of binding to albumin. generally, the reduction of plasma protein leads the increase of distribution volume and clearance of drugs via the decrease in the plasma protein bindings. however, the mean albumin concentration in the control group was not significantly different compared with that observed in the surgery group in the present study. consequently, the difference on vdss of mlx is probably not related to the different plasma albumin level between the control and surgery groups. the reason for lower vdss in the surgery group was however unclear. it is probably related to the other factors such as drug-drug interaction which may have changed the drug-protein binding capacity in the surgery group since atropine sulphate, propofol and isoflurane were combinely used for premedication and as anaesthesia. also ovariohysterectomy may have affected reproductive hormones and thus the disposition of the drugs. in addition, the reason for lower vdss in the surgery group could be, at least in part, due to smaller clb and mrt as vdss = clb × mrt. moreover, there was a limitation since the control and surgery phases were arranged in a sequence, but not in a crossover fashion. this experimental design may potentially induce a period effect. however, this is judged to be unlikely as a 10-day washout period was considered to be sufficiently long. in the present study, the effect of surgical operation on the plasma disposition of mlx was investigated in only crossbred and female dogs. however, the plasma disposition of mlx may be different in pure-bred and between male and female dogs. the elimination half-life of naproxen in beagles was much shorter, compared with that of observed in mongrels. this difference probably reflects a breed difference, since it is known that beagles metabolise drugs more rapidly than other breeds. besides, it has been reported that, female dogs showed lower auc and elimination t1/ values of mlx, compared with male dogs.  vdss of mlx in the control group is significantly greater compared to that of the surgery group. generally limited or lowered distribution may decrease the drug exposure into tissues. however, the consequent reduction in volume of distribution leads to higher plasma concentrations of mlx. besides, higher plasma concentration of the increased unbound mlx fraction in plasma may increase risk of side effects such as gastrointestinal ulceration, unusual bleeding and loss of kidney function/failure when repeated doses are used. the change in the disposition of some drugs pre-, peri- and post-operatively administered for pre-medication, anaesthesia or prevention of post-operative infections may result in changing efficacy and/or increasing side effects or toxicity. 
meloxicam is a nonsteroidal anti-inflammatory drug used in the relief of postoperative pain for human and veterinary medicine. this study was designed to investigate the effect of surgery on the plasma disposition of mlx in dogs undergoing ovariohysterectomy following a single intravenous injection at a dose of  mg/kg bodyweight. the results of the present study indicated that surgery could alter the plasma disposition of mlx and thus the drug efficacy and side effects such as gastrointestinal ulceration, unusual bleeding and loss of kidney function/failure when repeated doses are used. a two-phase experimental design with a 10-day washout period was used. the volume of plasma mlx distribution at steady-state of the control group was significantly greater compared to that of the surgery group. the auc values were higher and the cl values were lower in the surgery group compared to the control group, respectively, but differences were not significant.
 meloxicam is a nonsteroidal anti-inflammatory drug used in the relief of postoperative pain for human and veterinary medicine. this study was designed to investigate the effect of surgery on the plasma disposition of mlx in dogs undergoing ovariohysterectomy following a single intravenous injection at a dose of  mg/kg bodyweight. eight crossbred bitches were used in the study. a two-phase experimental design with a 10-day washout period was used. pre-operative mlx was administered intravenously to bitches about 10 days before surgery at a dose of  mg/kg bodyweight and peri-operative mlx was administered intravenously after anaesthesia and 15 min before the start of surgery. blood samples were collected from all animals at various times between and 96 h after the drug administrations in both phases. the drug concentrations were analysed using high performance liquid chromatography.  the volume of plasma mlx distribution at steady-state of the control group was significantly greater compared to that of the surgery group. the auc values were higher and the cl values were lower in the surgery group compared to the control group, respectively, but differences were not significant.  the results of the present study indicated that surgery could alter the plasma disposition of mlx and thus the drug efficacy and side effects such as gastrointestinal ulceration, unusual bleeding and loss of kidney function/failure when repeated doses are used. keywords dogmeloxicampharmacokineticssurgeryissue-copyright-statement© the author 2016 