Suggestions for the optimization of the initial tobramycin dose in adolescent and adult patients with cystic fibrosis

Clinical pharmacokinetic data of intravenously administered tobramycin in 34 patients with cystic fibrosis (CF) were correlated with patient parameters. Patients began tobramycin therapy with 10 mg/kg/day in three divided doses. Peak and trough serum concentrations were measured. Tobramycin dose was adjusted to a 30 min postdose peak of 10 mg/L and a predose trough of 1 mg/L. Pharmacokinetic data were calculated according to a one-compartment open model and were correlated with clinical data. Tobramycin half-life and total body clearance did not correlate with age, actual body weight, lean body mass, height, or body surface area. Tobramycin volume of distribution correlated with actual body weight (p < 0.02), lean body mass (p < 0.002), height (p < 0.05), and body surface area (p < 0.01), but not with age. The required daily dose after adjustment to a peak serum concentration of 10 mg/L and a trough level of 1 mg/L correlated with lean body mass (p < 0.02) and body surface area (p < 0.05). Based on our findings, the initial daily dose of tobramycin in patients with CF should be calculated by lean body mass rather than actual body weight or body surface area. A formula is presented to calculate the initial daily dose of tobramycin in CF patients who have normal renal function. Monitoring of tobramycin serum levels remains, however, necessary.