J. vet. Pharmacol. Therap. 37, 374--381. doi: 10.1111/jvp.12098.
Preliminary pharmacokinetics of morphine and its major metabolites following intravenous administration of four doses to horses H. K. KNYCH* , † E. P. STEFFEY*
,‡
&
D. S. MCKEMIE* *K.L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA; † Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA; ‡ Department of Veterinary Surgery and Radiology, School of Veterinary Medicine, University of California, Davis, CA, USA
Knych, H. K., Steffey, E. P., McKemie, D. S. Preliminary pharmacokinetics of morphine and its major metabolites following intravenous administration of four doses to horses. J. vet. Pharmacol. Therap. 37, 374–381. The objective of the current study was to describe the pharmacokinetics of morphine and its metabolites following intravenous administration to the horse. A total of eight horses (two per dose group) received a single intravenous dose of 0.05, 0.1, 0.2, or 0.5 mg/kg morphine. Blood samples were collected up to 72 h postdrug administration, analyzed using LC-MS/MS and pharmacokinetic parameters determined. Behavior, step counts, and gastrointestinal activity were also assessed. The beta and gamma half-life for morphine ranged from 0.675 to 2.09 and 6.70 to 18.1 h, respectively, following administration of the four different IV doses. The volume of distribution at steady-state and systemic clearance ranged from 6.95 to 15.8 L/kg and 28.3 to 35.7 mLmin/kg, respectively. The only metabolites identified in blood samples were the primary metabolites identified in other species, 3-morphineglucuronide and 6-morphine-glucuronide. Muscle fasciculations were observed at 0.2 and 0.5 mg/kg and ataxia noted at 0.5 mg/kg. Gastrointestinal activity was decreased in all dose groups (for up to 8 h in 7/8 horses and 24 h in one horse). This study extends previous studies and is the first report describing the metabolites of morphine in the horse. Plasma concentrations of morphine-3-glucuronide, a metabolite with demonstrated neuro-excitatory activity in mice, far exceeded that of morphine-6-glucuronide. Further study is warranted to assess whether the high levels of the morphine-3-glucuronide contribute to the dose-dependent excitation observed at high morphine doses. (Paper received 29 September 2013; accepted for publication 14 November 2013) Dr. Heather K. Knych, K.L. Maddy Equine Analytical Chemistry Laboratory, University of California, Davis, School of Veterinary Medicine, West Health Science Drive, Davis, CA 95616, USA. E-mail: [email protected]
INTRODUCTION Morphine is a full mu-receptor agonist and a potent analgesic agent used in both human and veterinary medicine. In humans, morphine is rapidly metabolized to two primary metabolites identified as morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). Interestingly, although the metabolites are glucuronidated, which generally inactivates a compound and limits diffusion across lipid membranes, in the case of morphine, both glucuronide compounds are believed to elicit pharmacologic effects (Andersen et al., 2003). The M6G metabolite is reportedly a more potent analgesic agent than the parent compound and is able to cross the blood–brain barrier (BBB) (Murphey & Olsen, 1994). Conversely, the M3G metabolite has been associated with neuro-excitatory behavior in rats (Hemstapat et al., 2003). While the pharmacokinetics 374
and pharmacodynamics of morphine and its metabolites have been extensively studied in humans, less is known about its metabolic profile in horses. The identity of the metabolites produced following morphine biotransformation in horses has not been determined. However, if M3G is also a metabolic product of the horse and based on the reported pharmacologic effects of M3G, one could speculate that production of large quantities of M3G may contribute to the central nervous system (CNS) excitation observed following administration of high doses of morphine in horses (Combie et al., 1979, 1981). Therefore, the pharmacologic activity associated with the primary metabolites of morphine in other species, necessitates identification and characterization of the pharmacokinetics of metabolites in the horse. To that end, the goals of the current study were to (i) describe the pharmacokinetics of morphine following intravenous administration of © 2014 John Wiley & Sons Ltd
PK/PD of intravenous morphine in horses 375
multiple doses, (ii) identify the primary metabolites following morphine administration in the horse, and (iii) describe the pharmacokinetics of the primary metabolites of morphine in the horse. MATERIALS AND METHODS
side), and the remaining samples were collected by direct venipuncture. Blood samples were collected into EDTA blood tubes (Kendall/Tyco Healthcare, Mansfield, MA, USA) and were centrifuged at 3000 9 g for 10 min. Plasma was immediately transferred into storage cryovials (Phenix Research Products, Chandler, NC, USA) and stored at 20 °C until analysis (2 weeks).
Horses and drug administration Eight healthy, university owned adult research horses consisting of two Thoroughbreds, one Holsteiner and five Quarter Horses (two mares and six geldings with a mean SD weight of 566 49 kg and an age of 6.9 3.7 years) were studied. Food was withheld for 12 h prior to and for 12 h following drug administration. Water was available ad libitum throughout the study. Before beginning the study, horses were determined healthy and free of cardiovascular diseases by physical examination, complete blood count, and a serum biochemistry panel that included aspartate aminotransferase, creatinine phosphokinase, alkaline phosphatase, total bilirubin, sorbital dehydrogenase, blood urea nitrogen, and creatinine. Blood analyses were performed by the Clinical Pathology Laboratory of the William R. Pritchard Veterinary Medical Teaching Hospital of the University of California, Davis, using the laboratory’s standard protocols. Horses did not receive any other medications for at least 2 weeks prior to conducting this study. This study was approved by the Institutional Animal Care and Use Committee of the University of California at Davis. Instrumentation and drug administration A 14-gauge catheter was aseptically placed in each external jugular vein. The right jugular vein catheter was used for drug administration, while the contralateral catheter was used for sample collection. Each horse was weighed immediately prior to drug administration and was randomly assigned to one of four dose groups by a computerized random number generator. Two horses were assigned to each dose group and received a single intravenous dose of 0.05, 0.1, 0.2, or 0.5 mg/kg morphine sulfate (Hospira, Lake Forest, IL, USA). Following administration, the catheter was flushed with 10 mL of a dilute heparinized saline solution (10 IU/mL). The right jugular vein catheter was removed following dosing. Sample collection Blood samples were collected at time 0 (prior to drug administration) and at 5, 10, 15, 30, and 45 min, and 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 12, 18, 24, 36, 48, and 72 h postmorphine administration. Prior to drawing each sample of blood for analysis of drug concentrations, 10 mL of blood was aspirated from the catheter and T-Port extension set (combined internal volume
Preliminary pharmacokinetics of morphine and its major metabolites following intravenous administration of four doses to horses H. K. KNYCH* , † E. P. STEFFEY*
,‡
&
D. S. MCKEMIE* *K.L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA; † Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA; ‡ Department of Veterinary Surgery and Radiology, School of Veterinary Medicine, University of California, Davis, CA, USA
Knych, H. K., Steffey, E. P., McKemie, D. S. Preliminary pharmacokinetics of morphine and its major metabolites following intravenous administration of four doses to horses. J. vet. Pharmacol. Therap. 37, 374–381. The objective of the current study was to describe the pharmacokinetics of morphine and its metabolites following intravenous administration to the horse. A total of eight horses (two per dose group) received a single intravenous dose of 0.05, 0.1, 0.2, or 0.5 mg/kg morphine. Blood samples were collected up to 72 h postdrug administration, analyzed using LC-MS/MS and pharmacokinetic parameters determined. Behavior, step counts, and gastrointestinal activity were also assessed. The beta and gamma half-life for morphine ranged from 0.675 to 2.09 and 6.70 to 18.1 h, respectively, following administration of the four different IV doses. The volume of distribution at steady-state and systemic clearance ranged from 6.95 to 15.8 L/kg and 28.3 to 35.7 mLmin/kg, respectively. The only metabolites identified in blood samples were the primary metabolites identified in other species, 3-morphineglucuronide and 6-morphine-glucuronide. Muscle fasciculations were observed at 0.2 and 0.5 mg/kg and ataxia noted at 0.5 mg/kg. Gastrointestinal activity was decreased in all dose groups (for up to 8 h in 7/8 horses and 24 h in one horse). This study extends previous studies and is the first report describing the metabolites of morphine in the horse. Plasma concentrations of morphine-3-glucuronide, a metabolite with demonstrated neuro-excitatory activity in mice, far exceeded that of morphine-6-glucuronide. Further study is warranted to assess whether the high levels of the morphine-3-glucuronide contribute to the dose-dependent excitation observed at high morphine doses. (Paper received 29 September 2013; accepted for publication 14 November 2013) Dr. Heather K. Knych, K.L. Maddy Equine Analytical Chemistry Laboratory, University of California, Davis, School of Veterinary Medicine, West Health Science Drive, Davis, CA 95616, USA. E-mail: [email protected]
INTRODUCTION Morphine is a full mu-receptor agonist and a potent analgesic agent used in both human and veterinary medicine. In humans, morphine is rapidly metabolized to two primary metabolites identified as morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). Interestingly, although the metabolites are glucuronidated, which generally inactivates a compound and limits diffusion across lipid membranes, in the case of morphine, both glucuronide compounds are believed to elicit pharmacologic effects (Andersen et al., 2003). The M6G metabolite is reportedly a more potent analgesic agent than the parent compound and is able to cross the blood–brain barrier (BBB) (Murphey & Olsen, 1994). Conversely, the M3G metabolite has been associated with neuro-excitatory behavior in rats (Hemstapat et al., 2003). While the pharmacokinetics 374
and pharmacodynamics of morphine and its metabolites have been extensively studied in humans, less is known about its metabolic profile in horses. The identity of the metabolites produced following morphine biotransformation in horses has not been determined. However, if M3G is also a metabolic product of the horse and based on the reported pharmacologic effects of M3G, one could speculate that production of large quantities of M3G may contribute to the central nervous system (CNS) excitation observed following administration of high doses of morphine in horses (Combie et al., 1979, 1981). Therefore, the pharmacologic activity associated with the primary metabolites of morphine in other species, necessitates identification and characterization of the pharmacokinetics of metabolites in the horse. To that end, the goals of the current study were to (i) describe the pharmacokinetics of morphine following intravenous administration of © 2014 John Wiley & Sons Ltd
PK/PD of intravenous morphine in horses 375
multiple doses, (ii) identify the primary metabolites following morphine administration in the horse, and (iii) describe the pharmacokinetics of the primary metabolites of morphine in the horse. MATERIALS AND METHODS
side), and the remaining samples were collected by direct venipuncture. Blood samples were collected into EDTA blood tubes (Kendall/Tyco Healthcare, Mansfield, MA, USA) and were centrifuged at 3000 9 g for 10 min. Plasma was immediately transferred into storage cryovials (Phenix Research Products, Chandler, NC, USA) and stored at 20 °C until analysis (2 weeks).
Horses and drug administration Eight healthy, university owned adult research horses consisting of two Thoroughbreds, one Holsteiner and five Quarter Horses (two mares and six geldings with a mean SD weight of 566 49 kg and an age of 6.9 3.7 years) were studied. Food was withheld for 12 h prior to and for 12 h following drug administration. Water was available ad libitum throughout the study. Before beginning the study, horses were determined healthy and free of cardiovascular diseases by physical examination, complete blood count, and a serum biochemistry panel that included aspartate aminotransferase, creatinine phosphokinase, alkaline phosphatase, total bilirubin, sorbital dehydrogenase, blood urea nitrogen, and creatinine. Blood analyses were performed by the Clinical Pathology Laboratory of the William R. Pritchard Veterinary Medical Teaching Hospital of the University of California, Davis, using the laboratory’s standard protocols. Horses did not receive any other medications for at least 2 weeks prior to conducting this study. This study was approved by the Institutional Animal Care and Use Committee of the University of California at Davis. Instrumentation and drug administration A 14-gauge catheter was aseptically placed in each external jugular vein. The right jugular vein catheter was used for drug administration, while the contralateral catheter was used for sample collection. Each horse was weighed immediately prior to drug administration and was randomly assigned to one of four dose groups by a computerized random number generator. Two horses were assigned to each dose group and received a single intravenous dose of 0.05, 0.1, 0.2, or 0.5 mg/kg morphine sulfate (Hospira, Lake Forest, IL, USA). Following administration, the catheter was flushed with 10 mL of a dilute heparinized saline solution (10 IU/mL). The right jugular vein catheter was removed following dosing. Sample collection Blood samples were collected at time 0 (prior to drug administration) and at 5, 10, 15, 30, and 45 min, and 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 12, 18, 24, 36, 48, and 72 h postmorphine administration. Prior to drawing each sample of blood for analysis of drug concentrations, 10 mL of blood was aspirated from the catheter and T-Port extension set (combined internal volume
