## 3 Results and Discussion
### Influence of Formulation Factors on the In Vitro Availability of Felodipine in Experimental Tablets
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**(a)**} \\ \hline \multicolumn{1}{c}{**Medium**} & \multicolumn{2}{c}{**Difference Factor**} & \multicolumn{2}{c}{**Similarity Factor**} \\  & & \(f_{1}\) & \(f_{2}\) \\ \hline LSNa 1.00\%-CTAB 0.40\% & REF & 32.24 & 37.85 \\ LSNa 1.00\%-CTAB 0.40\% & F1 & - & - \\ LSNa 1.00\%-CTAB 0.40\% & F2 & 37.89 & 36.99 \\ LSNa 1.00\%-CTAB 0.40\% & F3 & 22.90 & 36.76 \\ LSNa 1.00\%-CTAB 0.40\% & F4 & 128.12 & 14.32 \\ LSNa 1.00\%-CTAB 0.40\% & F5 & 23.82 & 58.34 \\ LSNa 1.00\%-CTAB 0.40\% & F6 & - & - \\ LSNa 1.00\%-CTAB 0.40\% & F7 & 36.22 & 35.46 \\ LSNa 1.00\%-CTAB 0.40\% & F8 & 15.50 & 57.20 \\ LSNa 1.00\%-CTAB 0.40\% & F9 & 17.62 & 55.69 \\ \hline \multicolumn{1}{c}{**(b)**} \\ \hline \multicolumn{1}{c}{**Medium**} & \multicolumn{2}{c}{**Difference Factor**} & \multicolumn{2}{c}{**Similarity Factor**} \\  & & \(f_{1}\) & \(f_{2}\) \\ \hline REF–F1 & LSNa 1.00\% & - & - \\ REF–F2 & LSNa 1.00\% & 12.53 & 58.87 \\ REF–F3 & LSNa 1.00\% & 31.56 & 37.43 \\ REF–F4 & LSNa 1.00\% & 92.45 & 4.86 \\ REF–F5 & LSNa 1.00\% & 35.68 & 40.47 \\ REF–F6 & LSNa 1.00\% & - & - \\ REF–F7 & LSNa 1.00\% & 11.79 & 56.67 \\ REF–F8 & LSNa 1.00\% & 33.60 & 40.49 \\ REF–F9 & LSNa 1.00\% & 14.55 & 58.74 \\ \hline REF–F1 & CTAB 0.40\% & 43.98 & 39.44 \\ REF–F2 & CTAB 0.40\% & 21.35 & 55.83 \\ REF–F3 & CTAB 0.40\% & 31.35 & 37.43 \\ REF–F4 & CTAB 0.40\% & 15.70 & 51.27 \\ REF–F5 & CTAB 0.40\% & 38.68 & 39.38 \\ REF–F6 & CTAB 0.40\% & - & - \\ REF–F7 & CTAB 0.40\% & 33.66 & 39.30 \\ REF–F8 & CTAB 0.40\% & 11.57 & 62.91 \\ REF–F9 & CTAB 0.40\% & 37.24 & 51.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: (**a**) The results obtained when comparing the dissolution profiles in the case of the innovative product and the developed formulations, in two dissolution media. (**b**) The results obtained when comparing the formulation profiles and the reference profile, in two different dissolution media. containing 30% HPMC K100, showed a release profile similar to the reference product in both dissolution media, in comparison with F1 which contained only 15% HPMC K100. When supplementing the 15% high viscosity HPMC K100 matrix with 25% HPMC of the lowest viscosity in F3, the felodipine release in 1% LSNa was significantly delayed and the release in CTAB at 6 h was slightly reduced; however, the obtained profiles are obviously different from the reference product ones. Replacing the high viscosity HPMC K100 matrix with 35% medium viscosity HPMC E50 and 15% HPMC E5, the felodipine release increased from 94.23% at two hours for F1 to 112.80% for F5 and 115.50% for F6, which are much higher values than the 11.10% achieved by the reference formula. In the CTAB medium, the dissolution rate increased to 30.05% for F5 and 100.50% for F6. The considerable differences in availability depend on the anionic or cationic surfactant. Practically, in the 1% LSNa medium, both F5 and F6 released the active substance in the first two hours, which is not in accordance with the dissolution requirements for extended-release products. In the CTAB 0.4% medium, dissolution was complete in two hours for the 35% HPMC E10 [34; 35; 36; 37]. Obviously, in the experimental formulations, the differences of dissolution in aqueous media were significant between surfactants of various categories (LSNa and CTAB). The use of HPMC E10 and HPMC E50 in F5 and F6 resulted in much higher felodipine availability compared to the reference product; in LSNa the release was almost complete in the first two hours of the study for both formulations, as opposed to dissolution in 0.4% CTAB medium, which was probably due to the anionic surfactant effect on the hydrophilic matrix. The inclusion of the low-polymerization HPMC matrix lead to much faster felodipine release from tablets, compared to the reference formulation, despite the large proportion used; 50% of the total mass. The felodipine dissolution in the investigated media was dependent on the type (viscosity) and on the proportions of the HPMC matrix.