Impact of Natural-Based Viscosity Modifiers of Inhalation Drugs on the Dynamic Surface Properties of the Pulmonary Surfactant

The effectiveness of inhalation therapy depends on aerosol size distribution, which determines the penetration and regional deposition of drug in the lungs. As the size of droplets inhaled from medical nebulizers varies depending on the physicochemical properties of the nebulized liquid, it can be adjusted by adding some compounds as viscosity modifiers (VMs) of a liquid drug. Natural polysaccharides have been recently proposed for this purpose and while they are biocompatible and generally recognized as safe (GRAS), their direct influence of the pulmonary structures is unknown. This work studied the direct influence of three natural VMs (sodium hyaluronate, xanthan gum, and agar) on the surface activity of the pulmonary surfactant (PS) measured in vitro using the oscillating drop method. The results allowed for comparing the variations of the dynamic surface tension during breathing-like oscillations of the gas/liquid interface with the PS, and the viscoelastic response of this system, as reflected by the hysteresis of the surface tension. The analysis was done using quantitative parameters, i.e., stability index (SI), normalized hysteresis area (HAn), and loss angle (φ), depending on the oscillation frequency (f). It was also found that, typically, SI is in the range of 0.15–0.3 and increases nonlinearly with f, while φ slightly decreases. The effect of NaCl ions on the interfacial properties of PS was noted, which was usually positive for the size of hysteresis with an HAn value up to 2.5 mN/m. All VMs in general were shown to have only a minor effect on the dynamic interfacial properties of PS, suggesting the potential safety of the tested compounds as functional additives in medical nebulization. The results also demonstrated relationships between the parameters typically used in the analysis of PS dynamics (i.e., HAn and SI) and dilatational rheological properties of the interface, allowing for easier interpretation of such data.