Understanding the impact of L-leucine on physical stability and aerosolization of spray-dried protein powder formulations containing trehalose

There is a growing interest in developing solid formulations for protein products due to enhanced physico-chemical stability. Spray drying is a widely used process for the manufacturing of solid-state protein therapeutics for pulmonary delivery. Stabilizers such as trehalose are commonly used and play a critical role in preventing protein degradation during the process of spray drying and upon storage. However, the hygroscopicity of spray-dried trehalose can result in moisture absorption, leading to crystallization and phase separation, and consequently impacting protein stability. Amino acids such as L-leucine have been studied as excipients that protect the hygroscopic spray-dried formulations against high humidity conditions. However, the interactions of L-leucine with protein and trehalose are less understood. This study systematically examined the role of L-leucine and trehalose on the physical and aerosol stabilities of the spray-dried protein formulations. Our findings revealed that a L-leucine ratio of 46.7% w/w was adequate to inhibit the crystallization of amorphous trehalose and prevent particle agglomeration even under humidity conditions such as 75% RH, without significantly impacting protein stability.