Effects of transdermal lidocaine or lidocaine with prilocaine or tetracaine on mechanical superficial sensation and nociceptive thermal thresholds in horses

Pharmacokinetics and thermal anti-nociceptive effects of oral morphine in horses

Introduction

Morphine is an effective analgesic in horses, however, IV administration at therapeutic doses has been shown to produce dose-dependent neuroexcitation and unwanted gastrointestinal effects. The analgesic effects of morphine have, at least in part, been attributed to the morphine-6-glucuronide (M6G) metabolite. Oral administration to horses results in comparable M6G concentrations to that achieved following IV administration of a therapeutic dose without the adverse effects. The anti-nociceptive effects have not yet been reported. In the current study the thermal anti-nociceptive effects of single and multiple oral doses of morphine were assessed.

Methods

Six horses received a single 0.2 mg/kg IV dose of morphine and multiple oral doses of 0.8 mg/kg morphine every 12 h for 4.5 days. Blood samples were collected throughout administration, morphine, and metabolite concentrations determined and pharmacokinetic analysis performed. Drug related behavior and physiologic responses were recorded. Response to noxious stimuli was evaluated by determining thermal threshold latency in response to the application of heat.

Results

The maximum concentrations of M6G were higher following oral administration compared to IV and the combined morphine and M6G concentrations exceeded that of IV administration starting at 2 h. Oral administration of 0.8 mg/kg morphine provided and maintained comparable anti-nociception effects to IV morphine with less adverse effects, following single and multiple doses. Morphine was well tolerated following oral administration with less excitation and minimal effects on gastrointestinal borborygmi scores compared to IV administration.

Discussion

Results of the current study warrant further investigation of the anti-nociceptive effects of oral morphine administration to horses.

Pharmacokinetics, pharmacodynamics and antinociceptive effects of buprenorphine following transdermal administration to horses

OBJECTIVE

This study describes the pharmacokinetics and pharmacodynamics, including antinociceptive effects, of a transdermal buprenorphine solution in horses. It was hypothesized that transdermal application would lead to sustained blood concentrations and antinociceptive effects with fewer adverse effects compared with intravenous (IV) injection.

STUDY DESIGN

Prospective nonrandomized four-part parallel experimental study.

ANIMALS

A group of eight horses (three mares and five geldings) aged 6-12 years.

METHODS

Horses were administered incremental doses of 15, 30 and 45 μg kg-1 of buprenorphine transdermal solution and a single IV dose of 5 μg kg-1 of buprenorphine with a 2 week washout period between treatments. Concentrations of buprenorphine were determined in plasma using liquid chromatography-tandem mass spectrometry and modeled using a nonlinear mixed effects population pharmacokinetic model to determine pharmacokinetic parameters. Pharmacodynamic effects, including changes in locomotor activity, heart rate, body temperature, gastrointestinal borborygmi, thermal and mechanical nociceptive thresholds were recorded. Mixed effects analysis of variance and post hoc comparisons were performed using a Bonferroni multiple comparison adjustment to assess differences in pharmacodynamic parameters between baseline and each time point within each dose group and between dose groups at the same time point.

RESULTS

Transdermal application of buprenorphine resulted in low systemic concentrations relative to IV injection. Bioavailability after transdermal application was 11%. Thermal nociceptive thresholds were significantly (p < 0.05) increased (4.3-10.7% relative to baseline) for up to 72 hours in the IV dose group, but only sporadically in the transdermal dose groups (2.5-9.9% relative to baseline). Changes in locomotor activity, heart rate and borborygmi varied over time and with dose.

CONCLUSIONS AND CLINICAL RELEVANCE

Limited thermal antinociceptive effects were observed at the transdermal doses studied likely owing to limited absorption relative to IV dosing. Future studies may be directed toward investigating antinociceptive effects of higher transdermal doses and different application sites.

Pharmacokinetics and effects of codeine in combination with acetaminophen on thermal nociception in horses

Codeine and acetaminophen in combination have proven to be an effective analgesic treatment for moderate-to-severe and postoperative pain in humans. Studies have demonstrated that codeine and acetaminophen, when administered as sole agents, are well tolerated by horses. In the current study, we hypothesized that administration of the combination of codeine and acetaminophen would result in a significant thermal antinociceptive effect compared with administration of either alone. Six horses were administered oral doses of codeine (1.2 mg/kg), acetaminophen (20 mg/kg), and codeine plus acetaminophen (1.2 mg/kg codeine and 6-6.4 mg/kg acetaminophen) in a three-way balanced crossover design. Plasma samples were collected, concentrations of drug and metabolites determined via liquid chromatography-mass spectrometry, and pharmacokinetic analyses were performed. Pharmacodynamic outcomes, including effect on thermal thresholds, were assessed. Codeine Cmax and AUC were significantly different between the codeine and combination group. There was considerable inter-individual variation in the pharmacokinetic parameters for codeine, acetaminophen, and their metabolites in horses. All treatments were well tolerated with minimal significant adverse effects. An increase in the thermal threshold was noted at 1.5 and 2 h, from 15 min through 6 h and 0.5, 1, 1.5, and 3 h in the codeine, acetaminophen, and combination groups, respectively.

Effect of preemptive topical application of lidocaine or prilocaine-lidocaine on successful catheterization of ear veins in New Zealand White rabbits

OBJECTIVE

To compare 5% lidocaine ointment applied for 10 minutes before intravenous (IV) catheterization to a eutectic mixture of local anesthetics (2.5% lidocaine and 2.5% prilocaine; EMLA) cream applied for 10 or 60 minutes prior and a water-based placebo for 60 minutes for successful marginal ear IV catheterization in rabbits.

STUDY DESIGN

Randomized, blinded, complete crossover, controlled trial.

ANIMALS

A total of 10 intact male, New Zealand White rabbits aged 4 months.

METHODS

After application of one of the treatments on the marginal ear vein, an operator unaware of the treatment attempted IV catheter placement. Successful IV catheterization was considered as a catheter inserted at the first attempt, safely secured, with appropriate patency. Generalized linear mixed models were developed to identify relevant predictors of successful catheter placement.

RESULTS

Ears that had EMLA cream applied 60 minutes before the catheterization attempt had more than 10 times the odds of successful catheter placement [odds ratio (OR)=10.75; 95% confidence interval (CI), 1.92-60.16; p < 0.008] compared with placebo. Both the application of EMLA cream or lidocaine ointment 10 minutes before the catheterization attempt resulted in approximately three times the odds of successful catheter placement compared with placebo, but were not statistically significant.

CONCLUSIONS AND CLINICAL RELEVANCE

Application of EMLA cream on the marginal ear vein 60 minutes before catheterization increases the chance of successful IV catheter placement in conscious rabbits.

Pharmacokinetics, adverse effects and effects on thermal nociception following administration of three doses of codeine to horses

BACKGROUND

In humans, codeine is a commonly prescribed analgesic that produces its therapeutic effect largely through metabolism to morphine. In some species, analgesic effects of morphine have also been attributed to the morphine-6-glucuronide (M6G) metabolite. Although an effective analgesic, administration of morphine to horses produces dose-dependent neuroexcitation at therapeutic doses. Oral administration of codeine at a dose of 0.6 mg/kg has been shown to generate morphine and M6G concentrations comparable to that observed following administration of clinically effective doses of morphine, without the concomitant adverse effects observed with morphine administration. Based on these results, it was hypothesized that codeine administration would provide effective analgesia with decreased adverse excitatory effects compared to morphine. Seven horses received a single oral dose of saline or 0.3, 0.6 or 1.2 mg/kg codeine or 0.2 mg/kg morphine IV (positive control) in a randomized balanced 5-way cross-over design. Blood samples were collected up to 72 hours post administration, codeine, codeine 6-glucuronide, norcodeine morphine, morphine 3-glucuronide and M6G concentrations determined by liquid chromatography- mass spectrometry and pharmacokinetic analysis performed. Pre- and post-drug related behavior, locomotor activity, heart rate and gastrointestinal borborygmi were recorded. Response to noxious stimuli was evaluated by determining thermal threshold latency.

RESULTS

Morphine concentrations were highest in the morphine dose group at all times post administration, however, M6G concentrations were significantly higher in all the codeine dose groups compared to the morphine group starting at 1 hour post drug administration and up to 72-hours in the 1.2 mg/kg group. With the exception of one horse that exhibited signs of colic following administration of 0.3 and 0.6 mg/kg, codeine administration was well tolerated. Morphine administration, led to signs of agitation, tremors and excitation. There was not a significant effect on thermal nociception in any of the dose groups studied.

CONCLUSIONS

The current study describes the metabolic profile and pharmacokinetics of codeine in horses and provides information that can be utilized in the design of future studies to understand the anti-nociceptive and analgesic effects of opioids in this species with the goal of promoting judicious and safe use of this important class of drugs.

Transdermal Application of Anesthetic Preparations Is Effective in Increasing Mechanical Nociceptive Threshold at Perineural Injection Sites in Horses

Perineural anaesthesia is an essential part of lameness investigation in horses. Painful reactions to injections imply a high risk of accidents for veterinarians. To evaluate the efficacy of two topical anaesthetic creams on desensitization of the skin at two perineural injection sites and to evaluate the effects these formulations may have on pain perception distally to the application sites. Randomized, blind, prospective study. 14 healthy horses were randomly divided in two groups of 7 horses each, one treated with EMLA and one with PLIAGLIS preparations. The mechanical nociceptive threshold (MNT) was recorded via pressure algometer at the baseline and one hour after application (T1) on palmar and palmar digital nerve blocks sites. Skin sensation was also assessed on the distal limb, recording MNT on the coronary band and on both bulbs of the heels. Data were compared with a 3-way ANOVA statistical analyses. No significant difference was encountered in the MNT values between baseline records and records at T1 at the control sites. A significant difference of MNT values was detected between the two time points in the treated limbs. The significant sources of variation in our study were the time and the presence of anaesthetic cream. The type of treatment did not determine any differences. Furthermore, no significant difference of MNT values at sites distal to treatment was recorded between the two time points, both in the treated limb and control limbs. Both formulations are effective to obtain superficial skin desensitization.

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.