Liuba Taccola1, Vittoria Raffa2, Cristina Riggio2, Orazio Vittorio1,2, Maria Carla Iorio3, Renato Vanacore3, Andrea Pietrabissa4,5, Alfred Cuschieri21Department of Oncology, Transplantation and Advanced Technologies in Medicine, University of Pisa, Pisa; 2Medical Science Laboratory, Scuola Superiore Sant'Anna, Pisa; 3Laboratory of Immunogenetics, Immunohematology, Azienda Ospedaliera Universitaria Pisana, Pisa; 4Department of Surgical Science, Reanimation and Transplantation, University of Pavia, Pavia; 5Fondazione Policlinico San Matteo, Pavia, ItalyBackground: It has recently been demonstrated that zinc oxide nanoparticles (ZnO NPs) induce death of cancerous cells whilst having no cytotoxic effect on normal cells. However, there are several issues which need to be resolved before translation of zinc oxide nanoparticles into medical use, including lack of suitable biocompatible dispersion protocols and a better understanding being needed of the mechanism of their selective cytotoxic action. Methods: Nanoparticle dose affecting cell viability was evaluated in a model of proliferating cells both experimentally and mathematically. The key issue of selective toxicity of ZnO NPs toward proliferating cells was addressed by experiments using a biological model of noncancerous cells, ie, mesenchymal stem cells before and after cell differentiation to the osteogenic lineage.Results: In this paper, we report a biocompatible protocol for preparation of stable aqueous solutions of monodispersed zinc oxide nanoparticles. We found that the threshold of intracellular ZnO NP concentration required to induce cell death in proliferating cells is 0.4 ± 0.02 mM. Finally, flow cytometry analysis revealed that the threshold dose of zinc oxide nanoparticles was lethal to proliferating pluripotent mesenchymal stem cells but exhibited negligible cytotoxic effects to osteogenically differentiated mesenchymal stem cells.Conclusion: Results confirm the ZnO NP selective cytotoxic action on rapidly proliferating cells, whether benign or malignant.Keywords: zinc oxide nanoparticles, dispersion, selective cytotoxicity, mesenchymal stem cells