Magnetic bar

Equilibrium azithromycin solubility of the raw azithromycin, the spray-dried azithromycin, physical mixtures (at the same molar composition as spray-dried formulations in Table 1) of azithromycin and colistin as well as the spray-dried composite formulations was determined (33 (link)). Briefly, an excess of azithromycin (i.e., 5 mg/mL equivalent of azithromycin of each formulation) was added to 5 mL of phosphate buffer saline (PBS, pH 7.4) maintained at room temperature (22 ± 2 °C) and 37 ±1 °C. The resultant suspensions were constantly stirred at 500 rpm (VWR International, Arlington Heights, IL, USA) with the help of a magnetic bar (12 mm diameter and 5 mm diameter, VWR International, Arlington Heights, IL, USA). After 24 h the suspensions were filtered using a 0.45 µm nylon syringe filter (VWR International, Arlington Heights, IL, USA) and the concentrations of azithromycin and colistin were determined by the validated HPLC method.

The equilibrium azithromycin solubility was determined. Briefly, an excess of azithromycin (i.e., > 5 mg/mL equivalent of azithromycin of each formulation) was added to 5 ml of dissolution media (phosphate buffer saline, PBS (pH 7.4), potassium chloride buffer (pH 2) and carbonate buffer (pH 11) maintained at 37 ± 1°C. Buffer media were used instead of water to reflect potential buffering effects of lung lining fluid. The resultant suspensions were constantly stirred at 500 rpm (VWR International, Arlington Heights, Illinois, USA) with a magnetic bar (12 mm, VWR, Arlington Heights, Illinois, USA). After 24 h the suspensions were filtered using a 0.45 μm nylon syringe filter (VWR, Arlington Heights, Illinois, USA) and the concentrations of azithromycin was determined by HPLC.