**4. Conclusions**

The analysis of phenolic-enriched extracts of Keitt and T. Atkins, the main commercial cultivars of *M. indica* in Costa Rica, using UPLC-DAD-ESI-MS techniques identified a total of 149 compounds, 82 of them in the Keitt cultivar, including 54 different gallotannins, which demonstrates the potential value of this fruit with a greater and more diverse number of compounds than cultivars from different countries and similar to previous important results reported for Keitt fruits from Spain [20,50]. Besides, our results for T. Atkins skin showing 59 compounds, including 30 gallotannins, constitutes the first report of such a high number and diversity of polyphenolic compounds for this cultivar. On the other hand, the TPC, DPPH, and ORAC antioxidant capacity showed high significant correlations (*p* < 0.05), with Keitt and T. Atkins skins exhibiting the highest values. Further, cytotoxicity results were also better for skin extracts in all three adenocarcinoma cell lines studied. For AGS and SW 620 cell lines, cytotoxicity activity showed a particularly high significant negative correlation with gallotannins (*r* = −0.977 and *r* = −0.940, respectively), while for the HepG2 cell line, the highest significant negative correlation was found with xanthonoid compounds (*r* = −0.921). These results and the presence of diverse xanthonoids and numerous gallotannins of a high polymerization degree, such as decamers (decagalloyl hexoses) and undecamers (undecagalloyl hexoses), which are reported for the first time for these *M. indica* cultivars, sugges<sup>t</sup> the potential of these extracts for further studies. For instance, xanthonoids have been linked with anti-inflammatory and anticancer activities, and gallotannins of a higher degree of polymerization have been found to enhance such properties [22]; hence, structure–activity relationship studies would contribute to increase the knowledge on the fruits as a source of dietary compounds and bioactivities associated with potential health benefits.

**Author Contributions:** Conceptualization, M.N., E.A. and I.M.; Formal analysis, S.Q., G.A., K.W., and P.C.; Funding acquisition, M.N.; Investigation, M.N.; Writing—original draft, M.N., S.Q., G.A. and F.V.; Writing—review and editing, M.N., E.A., I.M., S.Q., G.A., K.W., F.V. and P.C.

**Funding:** This work was partially funded by a gran<sup>t</sup> from the University of Costa Rica (115-B4-076).

**Acknowledgments:** Authors also thank the support from the Technological Institute of Costa Rica. Special thanks are due to Alonso Quesada from Costa Rican National Herbarium for his support with the vouchers.

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