**5. Conclusions**

This in vitro study has demonstrated the biological effects of these unconventional exopolysaccharides. EPS derivatives (-DR or -DRS). EPS derivatives can slowdown cell proliferation kinetic, confirming their anti-proliferative effect on human osteosarcoma cells without significantly influencing the cell viability. Close results have been found also on human melanoma cells, while human glioblastoma cells have not been affected by those EPS derivatives. The anti-proliferative effect on other cell lines (i.e., human osteosarcoma, melanoma, breast cancer and lung cancer) is effective with polysaccharides but even more enhanced when EPS derivatives, and heparin, are complexed to scandium, in which the impact on the NCI is dose–response. The degree of complexation is represented by the different metal-to-ligand ratio. Different behaviors in the decrease of the proliferation kinetic have been evidenced depending on the metal-to-ligand ratio, that could be related to a conformation change of the polysaccharide when complexed with scandium. EPS-DRS:Sc with a 1:2 metal-to-ligand ratio has been shown to be the most effective compound in displaying a cytostatic effect. Our group has evidenced in a previous work [45] that the rheological behavior of EPS derivatives and heparin, notably the conformation, is influenced by the ionic strength of solvent, as well as at different concentrations of scandium. Thus, it is assumed that the change in conformation could modulate positively or negatively the antiproliferative effect. Concerning the cause of the antiproliferative or cytostatic effect, a damage in the cellular metabolism does not seem to be implicated since the cells remain viable, at least in the time frame of our study. The most probable explanation would be that an inhibition of growth factor-receptor signaling, as well as an interference of cell-cell adhesion mediated by specific transmembrane proteins.

These results are very encouraging in a theranostic perspective because of the intrinsic tropism of these EPS derivatives for cancer cells is maintained, even after the complexation. Additionally, the anti-proliferative effect on certain cell lines could be modulated by the degree of complexation. In the near future, the influence of the degree of complexation will be performed to investigate which are the best metal-to-ligand ratio in terms of efficacy. Besides, xCELLigence assay with EPS derivatives could be extended to other cell lines known to be sensible to GAG properties, while testing new assays to target other cellular mechanisms involved in cellular toxicity. Moreover, the biological effects of these EPS will be tested on normal cells as well in order to guarantee no cytostatic effect on healthy cells. The efficacy of these compounds will be evaluated with the PET tracer of 44m/44Sc.

**Author Contributions:** Conceptualization, S.H.-M., D.H., and S.C.-J.; methodology, D.H., J.M.-G., M.M. and S.C.-J.; software, J.M.-G.; validation, D.H., S.H.-M. and S.C.-J.; formal analysis, J.M.-G., M.M.; investigation, J.M.-G., M.M.; resources, C.S., S.C.-J.; data curation, M.M., J.M.-G.; writing— original draft preparation, M.M.; writing—review and editing, D.H., J.M.-G., S.C.-J., S.H.-M., C.A.;

visualization, D.H., S.H.-M.; supervision, D.H., S.H.-M.; project administration, S.H.-M.; funding acquisition, S.H.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been supported in part by grants from the French National Agency for Research, called "Investissements d'Avenir" IRON Labex No. ANR-11-LABX-0018-01; and Arronax-Plus Equipex No. ANR-11-EQPX-0004 and ISITE NExT (ANR-16-IDEX-0007). We acknowledge the Institut-Mines-Telecom and the Pays de la Loire council for the financial support of M. Mazza PhD grant.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data available in a publicly accessible repository. The data presented in this study are openly available at [10.3390/md19030174].

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