Pharmacokinetic and Pharmacodynamic Assessments of Tapentadol in Yellow-Bellied Slider Turtles (Trachemys Scripta Scripta) after a Single Intramuscular Injection

In reptiles, administration of opioid drugs has yielded unexpected results with respect to analgesia. Tapentadol (TAP) is a novel atypical opioid drug labelled for human use. The objective of this study was to evaluate the pharmacokinetics and the pharmacodynamics of this drug in yellow-bellied sliders, after a single intramuscular (IM) injection of 5 mg/kg of TAP. Turtles (n = 9) were randomly assigned to 2 treatment groups, according to a single-dose, single-treatment, unpaired, 2-period crossover design. Group A (n = 5) received a single IM (proximal front limb) dose of TAP (5 mg/mL) at 5 mg/kg. Group B (n = 4) received a single IM injection of saline (equivalent volume to opioid volumes) of TAP at the same site. After a 1-month washout period, groups were rotated and the experiment repeated. TAP plasma concentrations were determined by a validated high-performance liquid chromatography-fluorescence method, and an infrared thermal stimuli was applied to the plantar surface of the turtles’ hind limbs to evaluate the thermal withdrawal latency (TWL). TAP plasma concentrations were detectable between 1 and 24 hour(s) (1619 and 37 ng/mL, respectively). The TAP-treated group showed an increase in TWL 1 hour after drug administration (13.32 ± 6.40 seconds). Subsequently, TWL decreased with time and significant differences between treatment and control groups were apparent up to 10 hours following treatment. A linear relationship (r2 = 0.99) between TAP plasma concentration and effect was found. Given these findings, TAP appears to be an attractive option for antinociception in turtles, owing to its rapid onset and acceptable duration of effect.