REGULATORY SIGNIFICANCE OF PROCAINE RESIDUES IN PLASMA AND URINE SAMPLES - PRELIMINARY COMMUNICATION

Plasma and urinary concentrations of procaine and the duration of resp onse to procaine after its administration as a local anaesthetic to ho rses were studied. Following injection of a clinical dose of procaine HCl (80 mg), the concentration of procaine in plasma was Less than the lower limit of quantitation and unsuitable for threshold determinatio n, Therefore, the urinary concentration of procaine was determined aft er injection of a dose of 5 mg procaine HCl, the highest no-effect dos e (HNED) of this agent. Free unconjugated procaine in equine urine rea ched a peak concentration of 23.7 ng/ml, while total (unconjugated plu s conjugated) procaine peaked at 37.9 ng/ml (mean urine pH of 8.5). Be cause a basic drug may concentrate substantially in acidic urine, a th reshold concentration of 25 ng/ml of unconjugated procaine is a reason able and conservative threshold for procaine at this time. Horses were administered abaxial sesamoid blocks containing 2% procaine HCl (40, 80, 160 and 320 mg) and 2% procaine HCl (40 and 320 mg) with epinephri ne (1:100,000) in local anaesthetic experiments, There was a significa nt local anaesthetic (LA) effect for all doses of procaine HCl with th e duration of effect ranging from 30 min (40 mg) to 60 min (320 mg). T he addition of epinephrine significantly increased the duration of loc al anaesthesia to 180 min for a 40 mg dose and 420 min for a 320 mg do se, Because epinephrine may extend the duration of local anaesthesia b eyond a reasonable period of confinement for horses before the startin g time of a race, the increased LA effect following the addition of ep inephrine to procaine has regulatory significance.