Pharmacokinetics of erythromycin ethylsuccinate and estolate in infants under 4 months of age.

We studied the pharmacokinetics of erythromycin estolate and ethylsuccinate suspensions in infants under 4 months of age who were being treated for chlamydial infections or pertussis. We conducted our studies after the initial dose of 10 mg/kg and subsequently during steady-state treatment. The estolate preparation resulted in higher peak concentrations in sera, and its absorption and elimination half-lives were longer. Peak concentrations occurred 3 h after a dose with the estolate preparation and 1 h after a dose with the ethylsuccinate preparation. The area under the curve for the estolate preparation was about three times greater than that for the ethylsuccinate preparation. Based on these findings, we recommend that erythromycin estolate suspensions be given to young infants at 8- or 12-h intervals (30 mg/kg per day in three divided doses or 20 mg/kg per day in two divided doses) and that erythromycin ethylsuccinate is best given at 6-h intervals (40 mg/kg per day in four divided doses).