# Sildenafil Citrate Liposomes for Pulmonary Delivery by Ultrasonic Nebulization
## 3 Results
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Influence of lyophilization on the Characteristics of the Liposomes**} \\ \hline \multicolumn{2}{c}{**Liposomes**} & **Dh (nm)** & **PDI** & **Zeta Potential (mV)** & **EE\%** & **DL (mg/g Lipid)** \\ \hline \multirow{2}{*}{**Without**} & **Fresh** & 209.7 & 0.537 & \(-\)20.90 & 89.77 \(\pm\) 7.64 & 53.32 \(\pm\) 4.59 \\  & **S** & 408.4 & 0.572 & \(-\)21.0 & 89.65 \(\pm\) 1.02 & 53.84 \(\pm\) 0.61 \\
**Vit E** & **T** & 433.1 & 0.577 & \(-\)46.6 & 89.95 \(\pm\) 2.15 & 54.02 \(\pm\) 1.29 \\
**TPGS** & **S + T** & 471.7 & 0.701 & \(-\)38.9 & 88.95 \(\pm\) 0.30 & 53.42 \(\pm\) 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of the liposomes before (fresh liposomes; red curves) and after lyophilization with sucrose (S; green curves), trehalose (T; black curves), or sucrose and trehalose (S + T; blue curves). The effect of ultrasonic nebulization on the liposomes was evaluated, and the change of the EE% throughout the period of nebulization was measured for fresh and lyophilized liposomes. Table 3 shows the results of this assay. A progressive decrease in the EE% was observed as the nebulization period increased for both types of fresh liposomes. After 15 min of nebulization, the EE% decreased from 89.77 \(\pm\) 7.64% to 77.78 \(\pm\) 0.89% and from 80.30 \(\pm\) 11.03% to 59.24 \(\pm\) 1.61% for liposomes without and with Vit E TPGS, respectively. The differences in the EE% values before and after nebulization were significant in both cases (\(p\) = 0.0389 and \(p\) = 0.0201, respectively). In the case of the liposomes without Vit E TPGS, the lyophilized and fresh samples behaved similarly during the nebulization, and the presence of the lyoprotective agents did not affect the results obtained for the EE%. In contrast, the lyophilized samples with liposomes containing Vit E TPGS remained more stable than the fresh ones during nebulization, and only slight differences in the EE% were observed. As shown in Table 3, the EE% values were greater than 80% after 15 min of nebulization for this type of liposomes, irrespective of whether the suspension contained sucrose, trehalose, or sucrose and trehalose (EE = 82.2 \(\pm\) 1.13%, 81.12 \(\pm\) 1.43%, and 81.82 \(\pm\) 0.89%, respectively). The comparison of these values to the mean value obtained for fresh liposomes (59.24 \(\pm\) 1.61%) revealed statistically significant differences (\(p\) = 0.0126). Finally, the influence of the density and viscosity of the samples on the performance of the nebulizer was analyzed and the results are shown in Table 4. It was found that the addition of the lyoprotective agent did not modify the density of the liposome suspensions, but it did increase viscosity in all cases. The viscosity values after the addition of 4% sucrose, trehalose, or sucrose and trehalose were higher (\(\eta\) = 1.24 \(\pm\) 0.01 mm\({}^{2}\)/s) than those observed for the samples without the additives (\(\eta\) = 1.15 \(\pm\) 0.05 mm\({}^{2}\)/s), the differences being statistically significant (\(p\) = 0.0052).