Absorption and distribution patterns of oral phenoxymethyl penicillin (penicillin V) in the horse

Serum levels of phenoxymethyl penicillin (penicillin V; Pen V) were measured following oral administration of two different formulations of the drug to five healthy horses. The mean serum concentration profile was described by a two-compartment model with a first order rate of absorption. Half-lives of the absorption, distribution and elimination phases were, respectively, 0.199 +/- 0.035, 0.362 +/- 0.060 and 3.650 +/- 0.575 hours. The distribution of the drug to body fluid compartments other than serum was examined. Mean peak levels of Pen V in serum, synovial fluid, peritoneal fluid and urine were, respectively, 2.38 +/- 0.18, 0.35 +/- 0.02, 0.25 +/- 0.06 and 472 +/- 80 micrograms/ml with times to achieve peak concentration ranging from 30-360 minutes after oral administration. Steady state pharmacokinetic measurements predicted that minimum serum Pen V concentrations of 0.1 micrograms/ml would be maintained by oral administration of the drug (110,000 I.U./kg) at 8 hour intervals. Our data indicate that oral Pen V is a feasible approach to long-term penicillin therapy in the equine patient.