Efficacy of orally administered gabapentin in horses with chronic thoracic limb lameness

Uncontrolled pain: a call for better study design

Studies assessing animal pain in veterinary research are often performed primarily for the benefit of animals. Frequently, the goal of these studies is to determine whether the analgesic effect of a novel treatment is clinically meaningful, and therefore has the capacity to improve the welfare of treated animals. To determine the treatment effect of a potential analgesic, control groups are necessary to allow comparison. There are negative control groups (where pain is unattenuated) and positive control groups (where pain is attenuated). Arising out of animal welfare concerns, there is growing reluctance to use negative control groups in pain studies. But for studies where pain is experimentally induced, the absence of a negative control group removes the opportunity to demonstrate that the study methods could differentiate a positive control intervention from doing nothing at all. For studies that are controlled by a single comparison group, the capacity to distinguish treatment effects from experimental noise is more difficult; especially considering that pain studies often involve small sample sizes, small and variable treatment effects, systematic error and use pain assessment measures that are unreliable. Due to these limitations, and with a focus on farm animals, we argue that many pain studies would be enhanced by the simultaneous inclusion of positive and negative control groups. This would help provide study-specific definitions of pain and pain attenuation, thereby permitting more reliable estimates of treatment effects. Adoption of our suggested refinements could improve animal welfare outcomes for millions of animals globally.

Gabapentin: Clinical Use and Pharmacokinetics in Dogs, Cats, and Horses

Gabapentin is an anticonvulsant drug, which presents an established clinical efficacy in human patients for the management of refractory partial seizures, secondarily generalized tonic-clonic seizures, and for the control of chronic neuropathic pain. Gabapentin was synthesized as a structural analogue of the inhibitory neurotransmitter GABA, with GABA-mimetic effects, able to cross the blood-brain barrier. In veterinary medicine, is extra-label used in combination with other treatments to control seizures when other drugs are no longer effective or become toxic or for neuropathic pain treatment and anxiety. This review aimed to clarify gabapentin use and pharmacokinetic aspects to promote conscious use in dogs, cats, and horses. In dogs, gabapentin was beneficial in the treatment of epilepsy, as well as chronic, neuropathic, and post-operative pain, as well as anxiety. In cats, it showed efficacy in post-ovariohysterectomy-related pain and in anxiety management. In horses, gabapentin has been administered as an analgesic for chronic pain management. In conclusion, when used in combination with other drugs, gabapentin can be considered an interesting therapeutic option for the treatment of neuropathic diseases and analgesia in postoperative and chronic pain. However, despite its beneficial use in different clinical settings, further trials and pharmacokinetic studies are needed for the definition of an effective dosage regimen through proper pharmacokinetic/pharmacodynamic correlation in dogs, cats, and horses.

Synopsis of the pharmacokinetics, pharmacodynamics, applications, and safety of firocoxib in horses

According to in vitro and in vivo investigations, firocoxib (FX), a second-generation coxib, is a highly selective COX-2 inhibitor in horses. With a COX-1/COX-2 IC₅₀ ratio of 643 in horses, FX spares the COX-1 inhibitory effects. It is approved for the treatment of musculoskeletal problems and lameness in horses and dogs with osteoarthritis (OA). For the treatment of OA in horses, both an injectable formulation for IV administration at a dose of 0.09 mg/kg for five days and an oral paste formulation at a dose of 0.1 mg/kg for 14 days are licensed. Numerous analytical methods were reported in the literature to quantify FX in biological fluids, using HPLC and LC-MS. FX presents remarkable pharmacokinetics and pharmacodynamics compared to other coxibs. It has an oral bioavailability of 80% or higher and is effectively absorbed by horses. Its volume of distribution is around 2 L/kg, and it is slowly eliminated. Due to the long elimination half-life (around 2 days), which allows a once daily dosing, a single 0.3 mg/kg loading dose has been recommended. This enables the establishment of steady-state drug concentrations within 24 h, making it appropriate for acute treatment as well. Its IC₈₀ is equal to 103 ng/mL in whole blood and, with an EC₅₀ of 27 ng/mL, it has the highest affinity for its receptor compared to the other commonly administered NSAIDs in horses.

Pharmacokinetics and pharmacodynamics of repeat dosing of gabapentin in adult horses

Background

The repeated administration of high doses of gabapentin may provide better analgesia in horses than current clinical protocols.

Hypothesis and Objectives

Administration of gabapentin at 40 and 120 mg/kg PO q 12 h for 14 days will not alter serum biochemistry findings or cause adverse effects. Our objectives were to evaluate the effect of gabapentin on serum biochemistry, physical examination, and plasma pharmacokinetics of gabapentin.

Animals

Six healthy adult mares.

Methods

Horses received 40 and 120 mg/kg of gabapentin orally q 12 h for 14 days. Horses were examined and scored for ataxia and sedation daily. Serum biochemistry variables were analyzed before treatment and days 7 and 14 after gabapentin administration. Plasma disposition of gabapentin was evaluated after the first and last drug administration. Pharmacokinetic parameters were estimated using noncompartmental analysis.

Results

No changes occurred in physiologic or biochemical variables. Median (range) maximal plasma gabapentin concentrations (μg/mL) after the last dose (day 15) were 7.6 (6.2‐11) and 22 (14‐33) for 40 mg/kg and 120 mg/kg doses respectively. Maximal concentration of gabapentin was reached within 1 hour after drug administration. Repeated administration of gabapentin resulted in a median (range) area under the curve (AUC_{0‐12 hours}) last/first dose ratio of 1.5 (1.00‐2.63) and 2.92 (1.4‐3.8) for the 40 and 120 mg/kg regimens, respectively.

Conclusion and Clinical Importance

Our results suggest that horses tolerate gabapentin up to 120 mg/kg PO q 12 h for 14 days. The analgesic effect of the dosage regimens evaluated in our study warrants further research.

Pharmacology of the Equine Foot: Medical Pain Management for Laminitis

One of the biggest challenges in managing laminitis in horses remains the control of pain. The best analgesic approach is a multimodal approach, including nonsteroidal anti-inflammatory drugs, opioids, and/or constant rate infusions of α-2 agonists, ketamine, and lidocaine. Recent literature indicates that amitriptyline and soluble epoxide hydrolase inhibitor might be beneficial. Clinically oriented studies will be needed if they have a place in laminitis pain management. The systemic pain control can be combined with local techniques such as long-acting local anesthetics or epidural catheterization that allows for administration of potent analgesic therapy with a lower risk of negative side effects.