**5. Conclusions**

In general, this study demonstrated that *P. o*ffi*cinalis* is a rich source of polyphenolic compounds with high antioxidant potential. Leaf extracts were stronger agents than root extracts, while methanol was a more effective solvent than water, as was demonstrated via its chemical antioxidant capacity and in α-amylase inhibitory assays. However, the selected water extract of leaves was a stronger antioxidant in the cellular antioxidant activity assay than the methanol extracts. These results confirm that plant extracts do not necessarily have the same response in a biological environment (in vitro) as chemical assays. Additionally, none of the extracts demonstrated associated cytotoxicity. UPLC-Q/TOF analysis supplemented with the on-line HPLC–DPPH•-scavenging method revealed 20 radical scavenging compounds, gallic acid derivatives, most likely, to be major contributors to the overall antioxidant potential of the extracts. Here, cellular antioxidant activity, cytotoxicity, and the inhibition of α-amylase *P*. *o*ffi*cinalis* extracts were reported for the first time. Therefore, *P. o*ffi*cinalis* may be a promising phytochemical to explore for its antidiabetic and antioxidant properties, with future applications in nutraceuticals, cosmeceuticals, and pharmaceuticals. For this purpose, further evaluation should focus on in vivo studies.

**Author Contributions:** Conceptualization, P.R.V., A.A.M.; methodology, L.D., M.P., A.P., C.V.P.; software: L.D.; validation, L.D., M.P., A.P., C.V.P.; formal analysis, L.D.; investigation, L.D., M.P., A.P., C.V.P.; resources, P.R.V., A.A.M..; data curation, P.R.V., A.A.M.; writing—original draft preparation, L.D.; writing—review and editing, P.R.V., A.A.M.; visualization, L.D.; supervision, P.R.V., A.A.M.; project administration, P.R.V., A.A.M.; funding acquisition, P.R.V., A.A.M.

**Funding:** The authors acknowledge the financial support received from the Portuguese Fundação para a Ciência e Tecnologia (FCT) through the PEst-OE/EQB/LA0004/2011 grant. The iNOVA4Health - UID/Multi/04462/2013 program is also acknowledged. A.A. Matias thanks for the IF Starting Grant – GRAPHYT (IF/00723/2014).

**Acknowledgments:** The authors would like to thank Kaunas Botanical Garden of Vytautas Magnus University (Kaunas, Lithuania) for supplying plant material.

**Conflicts of Interest:** The authors declare no conflict of interest.
