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

Analytical advances in horseracing medication and doping control from 2018 to 2023

The analytical approaches taken by laboratories to implement robust and efficient regulation of horseracing medication and doping control are complex and constantly evolving. Each laboratory's approach will be dictated by differences in regulatory, economic and scientific drivers specific to their local environment. However, in general, laboratories will all be undertaking developments and improvements to their screening strategies in order to meet new and emerging threats as well as provide improved service to their customers. In this paper, the published analytical advances in horseracing medication and doping control since the 22nd International Conference of Racing Analysts and Veterinarians will be reviewed. Due to the unprecedented impact of COVID-19 on the worldwide economy, the normal 2-year period of this review was extended to over 5 years. As such, there was considerable ground to cover, resulting in an increase in the number of relevant publications included from 107 to 307. Major trends in publications will be summarised and possible future directions highlighted. This will cover developments in the detection of 'small' and 'large' molecule drugs, sample preparation procedures and the use of alternative matrices, instrumental advances/applications, drug metabolism and pharmacokinetics, the detection and prevalence of 'endogenous' compounds and biomarker and OMICs approaches. Particular emphasis will be given to research into the potential threat of gene doping, which is a significant area of new and continued research for many laboratories. Furthermore, developments in analytical instrumentation relevant to equine medication and doping control will be discussed.

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.

Post-anesthetic CPS and EQUUS-FAP scores in surgical and non-surgical equine patients: an observational study

Background

Equine pain scoring may be affected by the residual effect of anesthetic drugs.

Objectives

To compare pain scores in the hours immediately following anesthetic recovery to baseline pre-anesthetic scores in equine patients undergoing surgical and non-surgical procedures.

Study design

Clinical observational study.

Methods

Fifty adult horses undergoing anesthesia for surgical or non-surgical procedures were enrolled. Horses underwent pain scoring using the Composite Pain Score (CPS) and Equine Utrecht University Scale for Facial Assessment of Pain (EQUUS-FAP) prior to anesthesia (T0) and following anesthetic recovery to standing, every hour for 5 h (T1-T5). Data were analyzed using a generalized linear mixed effects model. A post-hoc Dunnett's test for multiple comparisons was performed for variables where an effect was detected.

Results

Mean (95% confidence interval) CPS scores for T0-T5 were 1.6 (1.2-2.0), 6.8 (6.0-7.6), 5.1 (4.3-5.9), 4.3 (3.4-5.2), 3.7 (2.8-4.6), and 2.8 (2.0-3.6) and EQUUS-FAP scores were 0.6 (0.3-0.9), 3.0 (2.5-3.5), 1.9 (1.6-2.2), 1.1 (0.8-1.4), 0.6 (0.4-0.8), and 0.7 (0.4-1.0), respectively. For the CPS, scores greater than 5, and for the EQUUS-FAP scores greater than 3, are consistent with minor pain. There was no effect of type of procedure (surgical vs non-surgical) on CPS or EQUUS-FAP scores. There was an effect of time with CPS scores significantly greater than baseline at T1-T5 and EQUUS-FAP scores significantly greater than baseline at T1 and T2.

Main limitations

Discomfort caused by hoisting was not quantified and it was difficult to ascertain if this affected the results.

Conclusions

Post-anesthetic pain scores may be influenced by the residual effect of anesthetic agents for as long as 5 h and 2 h for the CPS and EQUUS-FAP, respectively.

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.

Pharmacokinetics and pharmacodynamics of meperidine after intramuscular and subcutaneous administration in horses

OBJECTIVE

To describe the pharmacokinetics and pharmacodynamics of meperidine after IM and subcutaneous administration in horses.

STUDY DESIGN

prospective, randomized, blinded, crossover trial.

ANIMALS

Six adult horses weighing 494 ± 33 kg.

METHODS

Treatments included meperidine 1 mg/kg IM with saline 6 mL subcutaneously, meperidine 1 mg/kg subcutaneously with saline 6 mL IM, and saline 6 mL subcutaneously and 6 mL IM, with a 7-day washout between treatments. Plasma meperidine concentrations and pharmacodynamic values (thermal and mechanical thresholds, physiological variables, fecal production) were collected at various time points for 24 hours. Accelerometry data were obtained for 8 hours to measure locomotor activity. Data were analyzed with a mixed effects model, and α was set at .05.

RESULTS

Meperidine terminal half-life (T_1/2 ), maximal plasma concentrations, and time to maximal concentration were 186 ± 59 and 164 ± 56 minutes, 265.7 ± 47.2 and 243.1 ± 80.1 ng/mL at 17 ± 6, and 24 ± 13 minutes for IM at subcutaneous administration, respectively. No effect of treatment or time was observed on thermal or mechanical thresholds, heart rate, respiratory rate, locomotor activity, frequency of defecations, or fecal weight (P > .2 for all).

CONCLUSION

Maximum meperidine concentrations were achieved quickly with a short T_1/2 in both treatment groups. Neither IM nor subcutaneous meperidine influenced thermal or mechanical threshold or physiological variables.

CLINICAL SIGNIFICANCE

The short half-life and lack of detectable antinociceptive effect do not support IM or subcutaneous administration meperidine at 1 mg/kg for analgesia in horses.