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The present work describes a new dry powder containing thymopentin (TP5) suitable for inhalation. A total of 21 dry powders were produced by co-spray drying TP5 with lactose or mannitol as a bulking agent, leucine as a dispersibility enhancer and poloxamer 188 as a drug stabilizer. Analyses by scanning electron microscopy, laser diffractometry, thermogravimetry, Twin Stage Impactor and HPLC were performed to characterize the manufactured powders. The results revealed that formulation compositions greatly influenced the physical characteristics of the powders, such as the angle of repose, tapped density, particle size and aerodynamic diameter which, in turn, affected their aerodynamic behavior. A higher loading of leucine in the formulations (> 63% by dry weight) improved the aerosolization properties of the powders by producing aerodynamically lighter particles. The optimum formulation, which had a tapped density of 0.31 g/cm3, an aerodynamic diameter of 1.9 μm and an in vitro deposition of 45%, was obtained by combining TP5/mannitol/leucine in the ratio of 10/18/72. In addition, it was interesting to find that poloxamer 188 had a significant impact on improving the powder flowability rather than stabilizing TP5. In conclusion, the chosen composition promises an enhanced aerosol performance for the new TP5 inhalation formulation, suitable for deep lung deposition.