A BÜCHI B-290 spray dryer (BÜCHI Labortechnik AG, Flawil, Switzerland) with a two-fluid nozzle (diameter 0.7 mm) was used to produce powder formulations. The spray drying parameters were chosen based on previous studies [29 (link), 34 (link)]. Feed solutions were prepared in purified water with a total solid content of 21.7 mg/mL. The feed solutions of combination formulations were obtained by mixing solutions of colistin and tobramycin at molar ratios of 1:1 and 1:5 (equivalent mass ratios of 7:3 and 3:7 respectively). These ratios were chosen because colistin and tobramycin at molar ratio 1:5 led to a significantly reduced toxicity to lung epithelial A549 cells [11 ]. The feed solutions were atomized into the drying chamber at 2 mL/min and dried using hot air at 120 ± 5°C that was aspirated at 35 m3/h. The air temperature at the chamber outlet was approximately 68 ± 3°C. The atomizing air flow rate was 0.7 m3/h.