THE ROLE OF PROCAINE IN ADVERSE REACTIONS TO PROCAINE PENICILLIN IN HORSES

Procaine penicillin is a commonly used antibiotic in equine medicine but its use is associated with a substantial incidence of adverse reactions. Soluble procaine concentrations were determined by HPLC in several commercially available procaine penicillin preparations, including some that were involved in adverse reactions. The mean (+/-SEM) soluble procaine concentrations in the veterinary preparations was 20.18 +/-5.07mg/ml, which was higher than the concentration in the only procaine penicillin preparation for use in humans in Australia of 7.3 mg/ml. Heating the veterinary procaine penicillin preparations to 50-degrees-C for 1 day led to a significant (P<0.01) increase in the amount of soluble procaine. Heating to 50-degrees-C for 7 days also produced a significant (P<0.02) increase. Soluble procaine tended to return to baseline concentrations when veterinary procaine penicillin preparations were heated to 50-degrees-C for 2 days then stored for 7 days at room temperature. Administration of procaine HCI intravenously (IV) at 2, 5, and 10 mg/kg produced behavioural, locomotor and vascular reactions, which were clinically similar to those reported in adverse reactions to procaine penicillin. The more severe reactions occurred at higher doses, although different horses responded variably at the same dose. Some adverse reactions lead to recumbency but none were fatal. The blood procaine concentrations 1 min after IV administration averaged 19.0 +/-12.6 and 25.3 +/-16-mu-g/ml at 2.5 mg/kg and 5 mg/kg, respectively. Ten min after administration, blood procaine concentrations were significantly higher (P<0.001) in the 5 mg/kg group than in the 2.5 mg/kg group. Intramuscular (IM) procaine HCI at 5 mg/kg produced significantly lower (P<0.001) blood concentrations than similar IV doses, and, in contrast to the IV doses, the amount of procaine in the blood was significantly higher 5 and 10 min after administration than it was after 1 min. Mild excitatory reactions in 4/5 horses were noted 5 to 10 min after IM administration. Administration of diazepam 20 s before procaine HCI prevented the excitatory adverse reaction in 2/2 horses, but administration after the procaine did not influence the outcome.