Physicochemical compatibility of mixtures of dornase alfa and tobramycin containing nebulizer solutions

Patients suffering from cystic fibrosis (CF) often need to inhale multiple doses of different nebulizable drugs per day. Patients attempt to shorten the time consuming administration procedure by mixing drug solutions/suspensions for simultaneous inhalation. The objective of this experimental study was to determine whether mixtures of the nebulizer solution dornase alfa (Pulmozyme) with tobramycin nebulizer solutions (TOBI and GERNEBCIN 80 mg) are physico-chemically compatible. Drug combinations were prepared by mixing the content of one respule Pulmozyme with either one respule TOBI or one ampoule GERNEBCIN 80 mg. Test solutions were stored at room temperature and exposed to light. Dornase alfa activity and tobramycin concentrations were determined by using a kinetic colorimetric DNase activity assay and a fluorescence immunoassay, respectively. Physical compatibility was determined by visual inspection and measurements of pH and osmolality. Tobramycin concentration was not affected by mixing the drug products. In spite of the high variability of the dornase alfa potency assay, it is obvious that activity is especially affected by sodium metabisulfite, used as excipient in GERNEBCIN. Patients should be advised, not to mix Pulmozyme with GERNEBCIN because of the incompatibility reaction. Further analytical studies are needed in order to determine the integrity and activity of dornase alfa in mixtures of Pulmozyme with TOBI. Finally clinical studies are necessary in order to demonstrate equivalent efficacy and safety of simultaneous inhalation in comparison to consecutive inhalation of both drugs.