Minimizing aggregates when nebulizing antibody Fab fragments: high thermostability and high concentration are key

The therapeutic potential of inhaled biologics is limited by protein instability during aerosolization, which can cause aggregation and increase immunogenicity risks. This study aims to identify key molecular and formulation parameters that minimize the aggregation of antibody Fab fragments during vibrating mesh nebulization. A set of 14 engineered Fabs with a broad range of melting temperatures (T_m 60-90°C) were nebulized using two different commercial vibrating mesh devices. We systematically assessed the impact of Fab thermostability, protein concentration (10-80 mg/mL), formulation excipients, and nebulizer device on the formation of high molecular weight species (HMWS) and subvisible particles (SVP). Aerosol characteristics, including fine particle fraction (FPF) and output rate, were also evaluated. High intrinsic thermostability and high protein concentration were identified as the two most critical factors for preventing aggregation. Fabs with a T_m above 80°C demonstrated exceptional stability with negligible aggregation. We attribute this effect to a correlation of high T_m with a higher resistance against unfolding and therefore a better tolerance against nebulization-induced stresses, in particular air/liquid interfacial stress. Counterintuitively, increasing the protein concentration from 10 mg/mL to 80 mg/mL suppressed aggregation for all Fabs, which had the highest benefit for Fabs with lower thermostability. This effect can at least in part be explained by a saturation of the air/liquid interface at higher Fab concentrations. While higher concentrations modestly reduced nebulizer output rates, the overall inhalable protein output (mg protein/min) was significantly enhanced. In comparison, full-length monoclonal antibodies showed poor aerosolization performance at high concentrations. Our findings provide clear guidance for developing inhaled Fab therapeutics. To ensure stability and minimize aggregation, priority should be given to selecting Fabs with high thermostability (T_m > 80°C) and formulating them at high concentrations (≥ 40 mg/mL). These strategies are expected to mitigate aggregation-induced immunogenicity and concomitant safety risks, facilitating the development of the next generation of inhaled protein drugs.