Enterohepatic circulation of lorazepam and acetaminophen conjugates in ponies.

This research study investigates how lorazepam and acetaminophen, two different drugs used for health purposes, are processed within the body systems of ponies, specifically focusing on their enterohepatic circulation (EHC). The authors explore the systemic pharmacokinetics, urinary and biliary excretion, and extent of EHC of these drugs, noting the changes that occur when EHC is interrupted.

Study Design and Methodology

• The study incorporated adult female ponies that weighed between 130 and 225 kilograms. Importantly, these ponies had chronically installed external biliary fistulas (medical tubes commonly referred to as T-tubes).
• Two types of drugs were used in this study: lorazepam (10 mg) and acetaminophen (2 g). Both of these drugs are primarily metabolized through hepatic conjugative reactions in the body.
• The research was conducted in a three-way cross-over design. There were three trial conditions identified as A, B, and C. In condition A, EHC was kept intact with blood and urine collection, but without bile collection after intravenous (i.v.) drug administration. In condition B, EHC was interrupted. Blood, urine, and bile were collected after the i.v. administration of the drug. In condition C, the collected bile from trial B was infused into the duodenum via the T-tube (without any i.v. drug administration) which allowed for EHC. In this trial, blood, urine, and bile were collected as well.
• The researchers ensured there was a gap of at least 2 weeks between each trial to prevent any crossover effects from prior drug administration.

Key Findings

• The study found that interrupting the EHC process caused changes in the plasma half-life, clearance rate, and total area under the plasma concentration curve for lorazepam glucuronide (a metabolite of lorazepam indicating drug breakdown).
• More specifically, the interruption of EHC shortened the plasma half-life of lorazepam (from 3.4 to 2.3 hours), increased its clearance (from 9.2 to 12.3 ml/min/kg), and decreased the total area under the plasma concentration curve for lorazepam glucuronide (from 210 to 310 ng/ml X hr).
• Recovery of lorazepam as its glucuronide in bile constituted 24.5% of the i.v. injected dose. Meanwhile, urinary elimination of lorazepam glucuronide decreased from 41% to 36% of the dose due to bile collection.
• Following the duodenal infusion of collected bile, which contained an average of 2.45 mg of lorazepam as glucuronide, urinary excretion of lorazepam glucuronide was 0.48 mg while 0.36 mg was re-excreted into bile.

Conclusion

The findings highlight the role and importance of enterohepatic circulation in the processing and metabolism of drugs like lorazepam and acetaminophen. ArrayListarrayListThis suggests that the EHC can significantly affect the pharmacokinetics of certain drugs, potentially modifying their therapeutic efficacy and toxicity. Therefore, an understanding of a drug’s EHC can enhance drug dosing and treatment strategies in veterinary medicine, improving outcomes and minimizing adverse effects.