Clinical pharmacology of penicillin in newborn infants

The clinical pharmacology of penicillin was studied in 47 newborn infants. Serum dose-response curves were determined after intramuscular injection of penicillin G in doses of 16,650, 25,000, and 50,000 units per kilogram. Peak serum concentrations in infants under one week of age were 22 to 24.8 μg per milliliter after 16,650 and 25,000 units per kilogram per dose and 35.8 μg per milliliter after 50,000 units per kilogram per dose; antimicrobial activity was measurable for 8 to 12 hours. The halflife of penicillin in serum correlated inversely with age and clearance of creatinine and ranged from 3.2 hours in infants 0 to 6 days of age to 1.4 hours in infants 14 days of age or older. Excretion of penicillin in urine correlated directly with clearance of creatinine. Mean serum levels after a single daily intramuscular dose of 50,000 units per kilogram of procaine penicillin G to infants under one week of age were 7 to 8 μg per milliliter for 12 hours and 1.5 μg per milliliter at 24 hours after the dose.