# Sildenafil Citrate Liposomes for Pulmonary Delivery by Ultrasonic Nebulization
## 4 Discussion

Table 4: Performance of the ultrasonic nebulizer based on the discharge rate (DR) and the nebulization efficiency (NE) for liposomes before (Fresh) and after lyophilization with sucrose (S), trehalose (T), or the mixture (S + T). PDI = 0.682). The zeta potential was also affected by lyophilization, likely due to the negative charge of disaccharides. Ultrasonic nebulizers are frequently used for pulmonary delivery of liquid formulations; therefore, the impact of this type of nebulization on the stability of SC-loaded liposomes was studied. Very interesting results were obtained when comparing the behavior of liposomes with or without Vit E TPGS, and these are illustrated in the next figure. On one hand, in absence of lyoprotectants, ultrasonic nebulization slightly affected the EE% of the liposomes without Vit E TPGS (Figure 4A) but did produce a relevant effect on the liposomes with Vit E TPGS (Figure 4B), with 12.39% versus 26.23% reduction observed, respectively. The surfactant properties of Vit E TPGS may promote the lipid bilayer destabilization induced by the ultrasonic vibration involved in aerosol generation. On the other hand, the additives used as lyoprotectants affect both types of liposomes in different ways. The presence of sucrose, trehalose, or the mixture did not modify the results of the EE% registered during nebulization of the liposomes without Vit E TPGS, but it did produce a beneficial effect on liposomes containing Vit E TPGS (see Figure 4). Reductions of 7.40%, 8.41% and 7.43% in EE were observed when samples of Vit E TPGS liposomes containing sucrose, trehalose, or the mixture, respectively, were nebulized. The viscous damping force in samples with additives might counteract the synergic effects of Vit E TPGS and ultrasonic nebulization, contributing to the maintenance of the integrity of these type of liposomes during nebulization. Regarding the influence of viscosity on the nebulizer performance, opposing effects on DR and NE were observed, as shown in Figures 5 and 6. Figure 4: Influence of the lyoprotective agents on the EE% of the nebulized liposomes without Vit E TPGS (**A**) and with Vit E TPGS (**B**). The influence of sample viscosity on nebulizer performance has been previously reported [44; 45], and the results showing that higher viscosity leads to lower DR were predictable, since ultrasonic nebulizers have been found to be less efficient for use with viscous solutions [44]. The beneficial effects of viscosity on NE, however, have not been reported before. Samples containing less SC than the nebulized suspensions (NE \(<\) 100%) were discharged by the ultrasonic nebulizer, irrespective of whether the suspension contained a lyoprotective agent or not, and irrespective of whether the liposomes contained Vit E TPGS or not. Nevertheless, lower values of NE were obtained for suspensions without the additives. Viscous damping force might be considered again to explain the positive effect of the additives on NE. For liposome suspensions without the lyoprotective agent, the less viscous external medium facilitates its own nebulization, and the result is higher discharged volume (higher DR) of samples with lower liposome content (lower NE). When lyoprotectants were added, the viscosity of the external medium increased and its nebulization became slower, resulting in lower discharged volume (lower DR) of samples with higher liposome content (higher NE). Despite the additives being beneficial, the discharged samples contained, in all cases, fewer liposomes and less SC than original liposome suspensions. Figure 5: Effect of viscosity on DR for the nebulized liposomes with and without Vit E TPGS. Figure 6: Effect of viscosity on NE for the nebulized liposomes with and without Vit E TPGS.