Class of 2017 Abstract / Objectives: Milk protein allergy is estimated to affect 1.2% to as much as 17% of people of all ages. Advair® Diskus® (FP/SX) utilizes lactose as an excipient which limits the utility of this product for this population. Furthermore, Advair® Diskus® is formulated as an interactive physical mixture via a micronization process. Alternatively, spray dried engineering achieves narrow particle size distribution, allowing greater deposition in the targeted respiratory bronchioles. The purpose of this dry powder inhaler (DPI) study was to conduct an in vitro comparative analysis of the aerodynamic performance of a co-spray dried lactose-free formulation of FP/SX with a mannitol excipient as a molecular mixture versus the Advair® Diskus® 250/50 (FP/SX) interactive physical mixture product. Methods: Utilizing mannitol as an excipient, a co-spray dried FP/SX powder was prepared using the Buchi Mini-Spray Dryer B-290 under closed system configuration. The resulting feed solution was spray dried at pump rates of 25%, 50%, and 100% with all other parameters remaining constant (aspiration, atomization rate, nitrogen gas rate). The primary outcome measure, aerodynamic performance, was assessed using the Copley Next-Generation Impactor (NGI). NGI data for the DPIs was used to calculate mass median aerodynamic diameter (MMAD), geometric standard deviation (GSD), and fine particle fraction (FPF) of each powder, including the Advair® Diskus®. Residual water content was quantified by Karl Fischer titration. Particle characteristics were visualized by scanning electron microscopy. Results: FPF, MMAD, and GSD were calculated from NGI data; Wolfram Alpha software was used to calculate MMAD and GSD. T-test regression was used for comparative analysis of spray-dried and Advair® Diskus® powders. MMAD for each spray dried sample was analyzed using a t-test regression against the MMAD values from the Advair® Diskus®. Using aerodynamic analysis studies triplicated for each powder, there was no significant difference between the spray dried powder and Advair® Diskus® for MMAD and GSD (p-values >0.05). The 50% and 100% pump rate samples had similar FPF to the Advair® Diskus® (p-values >0.05). However, the 25% pump rate sample had a significantly improved FPF compared to the Advair® Diskus® (p <0.01). Conclusions: A co-spray-dried lactose-free formulation of FP/SX with a mannitol excipient demonstrated similar aerodynamic performance to the Advair® Diskus® which consists of a physical mixture of two drugs with lactose. Of significance, 25% pump rate spray-dry conditions demonstrated an improved FPF compared to the Advair® Diskus®.