Amorphous Ca-phosphate (ACP) particles stabilized by inorganic polyphosphate (polyP) were prepared by co-precipitation of calcium and phosphate in the presence of polyP (15% [w/w]). These hybrid nanoparticles showed no signs of crystallinity according to X-ray diffraction analysis, in contrast to the particles obtained at a lower (5% [w/w]) polyP concentration or to hydroxyapatite. The ACP/15% polyP particles proved to be a suitable matrix for cell growth and attachment and showed pronounced osteoblastic and vasculogenic activity in vitro. They strongly stimulated mineralization of the human osteosarcoma cell line SaOS-2, as well as cell migration/microvascularization, as demonstrated in the scratch assay and the in vitro angiogenesis tube forming assay. The possible involvement of an ATP gradient, generated by polyP during tube formation of human umbilical vein endothelial cells, was confirmed by ATP-depletion experiments. In order to assess the morphogenetic activity of the hybrid particles in vivo, experiments in rabbits using the calvarial bone defect model were performed. The particles were encapsulated in poly(d,l-lactide-co-glycolide) microspheres. In contrast, to crystalline Ca-phosphate (containing only 5% [w/w] polyP) or to crystalline β-tricalcium phosphate, amorphous ACP/15% polyP particles caused pronounced osteoinductive activity already after a six-week healing period. The synthesis of new bone tissue was accompanied by an intense vascularization and an increased expression of mineralization/vascularization marker genes. The data show that amorphous polyP-stabilized ACP, which combines osteoinductive activity with the ability to act as a precursor of hydroxyapatite formation both in vitro and in vivo, is a promising material for bone regeneration.
Keywords: Amorphous calcium carbonate; Bone regeneration; Calvarial bone defect; Neovascularization; Polyphosphates.
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