Pharmacokinetics and pharmacodynamics of hydromorphone after intravenous and intramuscular administration in horses.

The research article discusses the comparison of the effects, absorption, distribution, and excretion of hydromorphone in horses after getting administered either intravenously (IV) or intramuscularly (IM). It concludes that both H-IV and H-IM have similar analgesic effects, but hydromorphone has a greater bioavailability when administered IM.

Study Design and Methodology

• The researchers followed a randomized, masked, crossover design involving a total of six adult horses.
• Three treatment sessions were conducted with a seven-day washout period in between. The treatments included hydromorphone administered IV and saline solution administered IM (H-IV), hydromorphone administered IM and saline solution given IV (H-IM), and saline solution given both IV and IM (P).
• Blood samples were collected over a 24-hour period following each treatment to measure hydromorphone plasma concentration.
• Observations relating to thermal nociceptive threshold, heart rate, breathing rate, rectal temperature, and weight of feces were collected over the 24 hours post-treatment.

Results and Findings

• The results showed that the half-life of hydromorphone (the time taken for the body to eliminate half of the drug) was 19 minutes for the H-IV group and approximately 27 minutes for the H-IM group.
• The total amount of the drug in the body over time (Area Under the Curve) was 518 for the H-IV group and 1128 for the H-IM group, indicating that IM administration resulted in a higher amount of hydromorphone in the horses’ bodies over time.
• The level of absorption of the drug when administered IM was found to be 217%, indicating a high bioavailability of hydromorphone when administered intramuscularly.
• Both H-IV and H-IM significantly increased the thermal thresholds (indicating better pain control) compared to the saline group (p < 0.0001 for both groups) and the horses' baseline readings (p = 0.006). No differences were observed between H-IV and H-IM.
• Physical examination results (heart rate, breathing rate, rectal temperature) showed no difference among the groups or compared to the baseline.
• Both H-IV and H-IM groups experienced significantly less fecal weight compared to the saline group (p = 0.02).

Conclusion

• The results suggest that hydromorphone exhibits high bioavailability when administered intramuscularly and provides similar pain relief as when administered intravenously.
• The study indicates the effectiveness and similar degree and duration of pain relief of hydromorphone whether administered intramuscularly or intravenously in horses. This points to the flexibility in the administration method when using hydromorphone for pain relief in horses.