Preliminary study of the pharmacokinetics, tissue distribution, and behavioral and select physiological effects of morphine 6-glucuronide (M6G) following intravenous administration to horses

Although morphine has demonstrated antinociceptive effects in horses, its administration has been associated with dose-dependent adverse effects. In humans and rats, part of the analgesic effect of morphine has been attributed to the active metabolite, morphine-6-glucuronide (M6G). Although morphine can cause several undesirable effects, M6G has a more favorable safety profile. The objective of this study was to characterize the pharmacokinetics, tissue distribution, and behavioral and select physiological effects of M6G following intravenous administration to a small group of horses. In Part 1 of the study, 3 horses received a single intravenous administration of saline, 0.5 mg/kg body weight (BW) M6G, or 0.5 mg/kg BW morphine in a 3-way crossover design. Blood samples were collected up to 96 hours post-administration, concentrations of drug and metabolites measured, and pharmacokinetics determined. Behavioral and physiological effects were then recorded. In Part 2 of the study, 2 horses scheduled to be euthanized for other reasons, were administered 0.5 mg/kg BW M6G. Blood, cerebrospinal fluid (CSF), and various tissue samples were collected post-administration and concentrations of drug were determined. The clearance of M6G was more rapid and the volume of distribution at steady state was smaller for M6G compared to morphine. A reaction characterized by head shaking, pawing, and slight ataxia was observed immediately following administration of both morphine and M6G to horses. After M6G administration, these behaviors subsided rapidly and were followed by a longer period of sedation. Following administration, M6G was detected in the kidney, liver, CSF, and regions of the brain. Results of this study encourage further investigation of M6G in order to assess its clinical feasibility as an analgesic in horses.

Meperidine pharmacokinetics and effects on physiologic parameters and thermal threshold following intravenous administration of three doses to horses

Background

Meperidine is a synthetic opioid that belongs to the phenylpiperidine class and is a weak mu receptor agonist. In horses there are a limited number of published studies describing the analgesic effects of systemically administered meperidine in horses. The objective of this study was to describe the pharmacokinetics, behavioral and physiologic effects and effect on thermal threshold of three doses of intravenously administered meperidine to horses. Eight University owned horses (four mares and four geldings, aged 3–8 years were studied using a randomized balanced 4-way cross-over design. Horses received a single intravenous dose of saline, 0.25, 0.5 and 1.0 mg/kg meperidine. Blood was collected before administration and at various time points until 96 hours post administration. Plasma and urine samples were analyzed for meperidine and normeperidine by liquid chromatography-mass spectrometry and plasma pharmacokinetics determined. Behavioral and physiologic data (continuous heart rate, step counts, packed cell volume, total plasma protein and gastrointestinal sounds) were collected at baseline through 6 hours post administration. The effect of meperidine administration on thermal nociception was determined and thermal excursion calculated.

Results

Meperidine was rapidly converted to the metabolite normeperidine. The volume of distribution at steady state and systemic clearance (mean ± SD) ranged from 0.829 ± 0.138–1.58 ± 0.280 L/kg and 18.0 ± 1.4–22.8 ± 3.60 mL/min/kg, respectively for 0.5–1.0 mg/kg doses. Adverse effects included increased dose-dependent central nervous excitation, heart rate and cutaneous reactions. Significant effects on thermal nociception were short lived (up to 45 minutes at 0.5 mg/kg and 15 minutes at 1.0 mg/kg).

Conclusions

Results of the current study do not support routine clinical use of IV meperidine at a dose of 1 mg/kg to horses. Administration of 0.5 mg/kg may provide short-term analgesia, however, the associated inconsistent and/or short-term adverse effects suggest that its use as a sole agent at this dose, at best, must be cautiously considered.

Equine uridine diphospho-glucuronosyltransferase 1A1, 2A1, 2B4, 2B31: cDNA cloning, expression and initial characterization of morphine metabolism

OBJECTIVE

Uridine diphospho-glucuronosyltransferases (UGTs) are membrane-bound enzymes that catalyze the conjugation of glucuronic acid onto a diverse set of xenobiotics. Horses efficiently and extensively glucuronidate a number of xenobiotics, including opioids, making UGTs an important group of drug-metabolizing enzymes for the clearance of drugs. Recombinant enzymes have allowed researchers to characterize the metabolism of a variety of drugs. The primary objective was to clone, express and characterize equine UGTs using drugs characterized as UGT substrates in other species. A secondary objective was to characterize the in vitro metabolism of morphine in horses.

STUDY DESIGN

In vitro drug metabolism study using liver microsomes and recombinant enzyme systems.

ANIMALS

Liver microsomes and RNA from tissue collected from two Thoroughbred mares euthanized for other reasons.

METHODS

Based on homology to the human UGT2B7, four equine UGT variants were expressed: UGT1A1, UGT2A1, UGT2B31 and UGT2B4. cDNA sequences were cloned and resulting protein expressed in a baculovirus expression system. Functionality of the enzymes was assessed using 4-methylumbelliferone, testosterone, diclofenac and ketoprofen. Recombinant enzyme, control cells, equine liver microsomes and human UGT2B7 supersomes were then incubated with morphine. Concentrations of metabolites were measured using liquid chromatography-tandem mass spectrometry and enzyme kinetics determined.

RESULTS

4-Methylumbelliferone was glucuronidated by all expressed equine UGTs. Testosterone glucuronide was not produced by any of the expressed enzymes, and diclofenac glucuronide and ketoprofen glucuronide were produced by UG2A1 and UGT1A1, respectively. UGT2B31 metabolized morphine to morphine-3-glucuronide and low concentrations of morphine-6-glucuronide.

CONCLUSIONS AND CLINICAL RELEVANCE

This is the first successful expression of functional recombinant equine UGTs. UGT2B31 contributes to the glucuronidation of morphine; however, it is probably not the main metabolizing enzyme. These results warrant further investigation of equine UGTs, including expression of additional enzymes and further characterization of UGT2B31 as a contributor to morphine metabolism.

Pharmacokinetics and selected pharmacodynamics of morphine and its active metabolites in horses after intravenous administration of four doses

The objective of the current study was to describe and characterize the pharmacokinetics and selected pharmacodynamic effects of morphine and its two major metabolites in horses following several doses of morphine. A total of ten horses were administered a single intravenous dose of morphine: 0.05, 0.1, 0.2, or 0.5 mg/kg, or saline control. Blood samples were collected up to 72 hr, analyzed for morphine, and metabolites by LC/MS/MS, and pharmacokinetic parameters were determined. Step count, heart rate and rhythm, gastrointestinal borborygmi, fecal output, packed cell volume, and total protein were also assessed. Morphine-3 glucuronide (M3G) was the predominant metabolite detected, with concentrations exceeding those of morphine-6 glucuronide (M6G) at all time points. Maximal concentrations of M3G and M6G ranged from 55.1 to 504 and 6.2 to 28.4 ng/ml, respectively, across dose groups. The initial assessment of morphine pharmacokinetics was done using noncompartmental analysis (NCA). The volume of distribution at steady-state and systemic clearance ranged from 9.40 to 16.9 L/kg and 23.3 to 32.4 ml min^-1  kg^-1 , respectively. Adverse effects included signs of decreased gastrointestinal motility and increased central nervous excitation. There was a correlation between increasing doses of morphine, increases in M3G concentrations, and adverse effects. Findings from this study support direct administration of purified M3G and M6G to horses to better characterize the pharmacokinetics of morphine and its metabolites and to assess pharmacodynamic activity of these metabolites.