Timing of inhaled tobramycin affects assessment of intravenous tobramycin pharmacokinetic monitoring.

Background: Aerosolized tobramycin inhalation solution (TIS) may be absorbed and result in measurable serum concentrations. We assessed the significance of TIS dosing in the latter portion of the IV dosing interval on the calculation of pharmacokinetic (PK) parameters and dosing.

Methods: Twenty adult CF patients admitted to the hospital for treatment of a pulmonary exacerbation were enrolled. PK parameters of tobramycin were calculated before and after introduction of TIS, which was given 5-9 h after the IV dose.

Results: Nine patients had a clinically significant change in tobramycin trough concentration. Fourteen patients had a reduced calculated elimination rate constant after TIS administration, which may be misinterpreted as a decreased clearance of IV tobramycin.

Conclusion: Trough tobramycin concentrations were significantly influenced in some CF patients (45%), suggesting that timing of the inhaled dose should be considered when interpreting PK measures of IV tobramycin dosing.