Biomimetic porous calcium phosphate nanoparticles with lithium ions incorporation induce osteogenic stem cell differentiation in vitro and in vivo*

Calcium phosphates (CP) are inorganic compounds naturally found in approximately 60% of all human bones in form of semi crystalline carbonated hydroxyapatite nanocrystals, and responsible for bone regeneration. Due to their osteoconductive and chemical similarity to inorganic part of the bone, CP have been widely used as the backbone formulation to fabricate synthetic bone grafts for treatment of bone defects [1]. Here in, we have developed a novel type of CP nanoparticles (CaP), by mimicking bone forming precursors particles involved in bone mineralisation process. CaP are amorphous and porous with hospitable structure to be doped with various bioactive elements or incorporated with large and small biomolecules. Moreover, to promote vascularisation, which is a crucial step in bone repair, we have incorporated a trace amount of lithium ions between calcium and phosphate atoms of CaP, labelled as Li-CaP [2]. 

For accessing nanoparticle citotoxicity, human mesenchymal stem cells (hMSC) were seeded in presence of either CaP or Li-CaP at different concentrations. Since cell viability resulted to be dependent on CaP and Li-CaP concentrations, 80% metabolic activity was set as threshold for choosing the appropriate nanoparticles concentrations. When cells were co-cultured with nanoparticles up to 28 days, they maintained their elongated shape, typical of hMSC cultured in-vitro. Moreover, when osteogenic supplements were provided in culture media, an increased expression of ostegeonic markers, such as Runx2, osteocalcin andcollagen type I was noticed in CaP and Li-CaP groups. In addition, the in-vivo study confirmed the positive effect of Li-CaP in ectopic bone formation. Altogether, our results show the great potential of CaP and Li-CaP nanoparticles for bone regeneration. Further studies will be performed to investigate the angiogenic potential of Li-CaP.