Perioperative analgesic effects of opiranserin in canine castration: a pilot study

Opiranserin, a non-opioid analgesic acting as a glycine transporter 2 inhibitor and 5-HT2A receptor antagonist, was evaluated for reducing intraoperative rescue analgesia and postoperative pain in dogs undergoing castration under isoflurane anesthesia.

Eight healthy male Beagle dogs were randomized to receive a continuous rate infusion of opiranserin (5 mg·kg^-1·h^-1 for 1 h before induction, then 0.33 mg·kg^-1·h^-1 for 9 h; n = 4) or 0.9% saline (n = 4). Anesthesia was induced with alfaxalone and maintained with isoflurane (1.5% end-tidal); all dogs received meloxicam and cefazolin preoperatively, without locoregional analgesia. Hemodynamic responses (ΔMAP, ΔHR) were recorded across 11 surgical events; rescue remifentanil was given when both increased ≥20% from baseline. Postoperative pain was scored with the Glasgow Composite Measure Pain Scale-Short Form (GCMPS-SF) from 30 min after extubation (T0) through T7, with rescue analgesia at scores ≥5. Primary outcomes were intraoperative rescue analgesia frequency and postoperative GCMPS-SF scores; secondary outcomes were intraoperative hemodynamic responses, postoperative rescue consumption, and adverse effects. Data were analyzed with Mann-Whitney U tests and linear mixed models with Bonferroni-adjusted comparisons.

The opiranserin group required fewer intraoperative rescue interventions than the normal saline group (median 2 [2-2] vs. 6.5 [3-9]; p = 0.021). GCMPS-SF scores showed significant effects of group (p = 0.030), time (p = 0.047), and group × time interaction (p = 0.034), with lower scores in the opiranserin group at T1 (adjusted p = 0.012) but not at other time points. Among secondary outcomes, significant overall effects of group were found for ΔMAP (p = 0.011) and ΔHR (p = 0.048), and of surgical event for both (p < 0.001 and p = 0.008), with no group × event interaction; no individual event differed significantly after correction. Cumulative postoperative rescue analgesic consumption and the incidence of adverse effects did not differ between groups.

These preliminary findings suggest that opiranserin may reduce intraoperative rescue analgesia requirements and attenuate postoperative pain in dogs, warranting confirmation in larger trials. The absence of significant adverse effects is reassuring but should be interpreted cautiously given the small sample size.