Pharmacokinetics of theophylline in premature infants on the first day of life.

The disposition of theophylline in premature infants on the first day of life was studied in nine preterm infants after intravenous administration of the drug. Theophylline concentrations in the blood were measured by means of high-pressure liquid chromatography. Theophylline's apparent volume of distribution (mean +/- SD) was 1.02 +/- 0.13 L/kg, a value similar to those previously reported, but the half-life (16.9 +/- 6.2 hr) was considerably shorter. The elimination rate constant (0.046 +/- 0.020/hr) and the clearance rate (46 +/- 14 ml/kg/hr) were faster than in older infants. Follow-up studies in three infants on the third day of life showed lower clearance rates than were obtained on the first day. The newborn preterm infants in this study eliminated theophylline at an unexpectedly rapid rate. Dosage calculations for therapy at this age should be based on these kinetic data.