Comparison of Cephalexin, Penicillin V, and Ampicillin in Streptococcal Infections in Monkeys

Intravenous inoculation of a group A hemolytic streptococcus caused lethal infections in all of 11 untreated monkeys. Daily intragastric administration of either 25 or 50 mg per kg per day, given in two equal morning and afternoon doses, yielded similar results in monkeys treated with cephalexin, penicillin V, and ampicillin; all eight monkeys in each therapy group survived. At dose levels of 12.5 mg per kg per day, six of eight, four of eight, and one of eight receiving cephalexin, penicillin V, and ampicillin, respectively, died. The differences observed at the lower dose level between cephalexin and ampicillin could be attributed, in part, to differences in the minimal inhibitory concentrations (MIC) of cephalexin (MIC = 0.24 mug/ml) and ampicillin (MIC = 0.01 mug/ml). The differences in results between penicillin V, which had the same MIC as ampicillin, could perhaps be attributed, in part, to shorter duration of antibacterial activity and higher protein binding of penicillin V. These studies support previous observations that cephalexin at 25 to 50 mg/kg doses is effective in severe streptococcal sepsis in monkeys.