*2.2.1 Design of Experiment (DoE)*

DoE is a systematic approach to simultaneously evaluate the effects and interaction of multiple factors that influence the responses of a process. DoE is a component of Quality by Design (QbD), which is a recommended statistical practice in the formulation of drug products by the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human use (ICH) [17, 36].

Quality characteristics for each product must be pre-determined and must be able to be accurately measured. The measured quality characteristic is called a response. The quality of spray-dried products is influenced by several factors. *Arthur et al.* [37] used DoE to optimize the spray drying process of beer powder and found that moisture content, water activity, solubility, pH, and product yield were the most influential factors. *Ziaee et al.* [38] successfully spray-dried solid dispersions of ibuprofen and used DoE to identify that residual moisture content and particle size were the critical factors for the final yield, but the API/excipient ratio was critical to formulating samples. For our case study, we chose the following factors to investigate and optimize: Feed flow rate (FFR), spray gas, and outlet temperature. Preliminary operation tests assisted in designing the study. The following factors were kept constant between trials: Starting feed solution was identical (constant temperature and product dry weight % w/v), and the drying gas flow rate (aspiration) was kept at maximum capacity.

JMP Pro Version 17.0 (*SAS Institute Inc., Cary, NC*) was used for statistical analysis and model building. We used Response Surface Methodology (RSM) with an *I*-Optimal, custom, DoE design, consisting of 16 runs. RSM is an effective tool to optimize process parameters quickly and efficiently. It can explore the effects of multiple factors simultaneously, investigate factor interactions, and predict the resulting response.
