Pharmacokinetics of tobramycin in patients with cystic fibrosis. Implications for the dosing interval.

The pharmacokinetics of tobramycin were evaluated in 15 patients (8 to 22 years of age) with cystic fibrosis (CF). A dose of 3.0 to 3.3 mg/kg of body weight was given intravenously over 20 minutes, and concentrations in serum were followed up to eight hours after initiation of the infusion. In the calculation of pharmacokinetic parameters, a two-compartment open model was used. The elimination half-life of the drug was highly inversely correlated with age (p less than 0.0004), and body weight (p less than 0.00002). Total body clearance (TBC), and volume of distribution at steady state (VDSS) were directly correlated with age and body weight. However, when TBC and VDSS were corrected for BSA, no correlation could be demonstrated. The mean one-hour and eight-hour serum concentrations of tobramycin were 5.40 and 0.45 microgram X ml-1, respectively. Between patients, considerable differences were found in the time after administration at which the serum concentration decreased below 1 microgram X ml-1. This interpatient variation has clinical implications for tobramycin therapy in CF, in particular for the dosing interval.