Tobramycin pharmacokinetics in premature identical twins during newborn period

Tobramycin is frequently used in premature infants but little is known about its pharmacokinetics in identical twins during the newborn period. We studied 6 twin infants (gestational age 29-31 weeks; postnatal age 3-4 days; birth weight 1.0-1.3 kg) receiving tobramycin 2.5 mg/kg i.v. over 20 min every 12-18 h. Steady-state peak and trough serum concentrations of tobramycin were in the range of 5.3-8.4 and 1.2-2.0 micrograms/ml, respectively. Total clearance (ClT) ranged from 0.74 to 1.19 ml/min/kg, distribution volume (V) from 0.74 to 0.94 liter/kg, and elimination half-life (t1/2) from 8.2 to 12.8 h. Comparison of data between infants in three identical twin pairs showed that despite a similar infusion method: (a) the time to achieve peak serum concentration ranged from 0.5 to 2.0 h; (b) the peak and trough concentrations normalized for dose varied from 0 to 50%, and (c) the ClT, V and t1/2 varied from 3 to 20%. These data should be considered in therapeutic drug monitoring of tobramycin in premature, identical twins during the newborn period.