Bupivacaine in the horse: relationship of local anaesthetic responses and urinary concentrations of 3-hydroxybupivacaine.

This research paper investigates the relationship between the local anaesthetic responses of Bupivacaine, a potent painkiller used in horses, and the concentration of its metabolite, 3-hydroxybupivacaine, in horse urine. It reveals that the significant presence of Bupivacaine residues post-race might lead to regulatory penalties and henceforth provides a pharmacological database.

Experimental Analysis on Bupivacaine’s Impact

• The paper begins by establishing the maximum safe dose of Bupivacaine, known as the highest no-effect dose (HNED). This dosage determined to be 0.25mg, was identified using an abaxial sesamoid local anaesthetic model. The key objective of this analysis was to identify the dosage of Bupivacaine which would have the least significant impact on the horses.
• Once the HNED dosage was administered to a group of eight horses, urine samples were collected, and it was observed that the concentration of bupivacaine reached its peak, 23.3 ng/mL, two hours after injection.

Metabolite Analysis

• The next section of the paper deals with the analysis of Bupivacaine’s metabolite. It was observed that post the administration of Bupivacaine, the primary metabolite found in horse urine was hydroxybupivacaine – either 3-hydroxybupivacaine, 4-hydroxybupivacaine, or a mixture of the two. To determine which version of hydroxybupivacaine was present, 4-hydroxybupivacaine was procured, and 3-hydroxybupivacaine was synthesized.
• By utilizing a quantitative mass spectrometric technique, it was determined that the concentration of the hydroxybupivacaine reached its peak (27.4 ng/mL) four hours after Bupivacaine administration.
• Through further ion chromatography analysis it was unmistakably established, that the hydroxybupivacaine metabolite found in horse urine was exclusively 3-hydroxybupivacaine.

Understanding Parameters and Results

• The mean pH of the 4-hour urine samples was 7.21; the urine creatinine was approximately 209.5mg/dL; and urinary specific gravity was 1.028.
• Importantly it was observed that the pH, creatinine concentration, or specific gravity had no noticeable influence on the concentration of 3-hydroxybupivacaine detected.
• The concentration of bupivacaine or its metabolites post the administration of an HNED dose can be identified through mass spectrometric techniques. This research also indicates that the presence of less than approximately 30 ng/mL of 3-hydroxybupivacaine in post-race urine samples is unlikely to be linked with the recent administration of Bupivacaine.