## 4 Conclusions  
This paper describes a study on the shielding properties of different types of glass for the photoprotection of dihydropyridine antihypertensives. The study aims to propose a valid alternative to the current, almost total formulation of these drugs in tablets, given their high photosensitivity in solution. Eleven DHPs, all currently used in antihypertensive therapy, have been exposed in hydroalcoholic solution to stressing light, according to international rules. Quartz and clear glass showed poor protection for all the substances. Amber glass demonstrated a good photoprotective effect towards most of the compounds, except for nifedipine and nisoldipine. The photodegradation test on these two substances was repeated by filtering the light through a transparent red glass, capable of absorbing the wavelengths up to 570 nm. Both drugs showed a marked decrease in the degradation process, with residual concentrations of unchanged drug of 84.33% and 93.15% for NF and NS, respectively, after 120 min of forced irradiation. Values higher than 95% were recorded when the degradation test with the aid of red glass was carried out on all the other DHPs. These results demonstrated the effectiveness of red glass in protecting drugs from light,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Standard Solutions**} & \multicolumn{2}{c}{**Sample Solutions**} \\ \hline
**DHP** & **Quartz** & **Transparent** & **Amber** & **Red** & **Red** \\ \hline AM & 19.09 & 31.6 & 97.41 & 98.32 & 97.68 \\ CL & 0.00 & 0.00 & 96.10 & 99.10 & 97.23 \\ FL & 46.72 & 62.56 & 94.37 & 94.84 & 96.21 \\ IS & 1.33 & 5.61 & 94.08 & 96.23 & 95.98 \\ LC & 0.00 & 0.00 & 94.33 & 99.28 & 97.56 \\ MN & 0.00 & 0.00 & 95.56 & 96.20 & 96.74 \\ NC & 0.00 & 0.00 & 95.23 & 95.47 & 95.30 \\ NF & 0.00 & 0.00 & 9.10 & 69.77 & 84.33 \\ NM & 0.00 & 0.00 & 97.52 & 98.51 & 96.32 \\ NS & 0.00 & 0.00 & 10.50 & 74.98 & 93.15 \\ NT & 0.00 & 0.00 & 95.24 & 97.89 & 95.49 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Residue percentage of drug in hydroalcoholic and PEG–ethanol solutions protected by different glasses after 120 min of forced irradiation.  
being able to absorb wavelengths above the values normally absorbed by amber glass, which is the glassy material usually used in pharmaceutical field.  
Conceptualization, M.D.L. and G.R.; methodology, M.D.L. and G.R.; software, M.D.L.; validation, G.I. and F.G.; formal analysis, G.I. and F.G.; investigation, M.D.L., G.I., F.G., and G.R.; resources, G.R.; data curation, M.D.L.; writing--original draft preparation, M.D.L., G.I., F.G., and G.R.; writing--review and editing, M.D.L. and G.R.; visualization, G.R.; supervision, G.R.; project administration, M.D.L. and G.R.; funding acquisition, M.D.L. and G.R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author (M.D.L.)  
The authors thank Ministero Istruzione Universita Ricerca (MIUR), Italy, for the financial support (grant 60% in 2019).  
The authors declare no conflict of interest.