Effect of 1-hydroxyethylidene-1,1-bisphosphonate on membrane-mediated calcium phosphate formation in model liposomal suspensions

Biomimetic coating of bisphosphonate incorporated CDHA on Ti6Al4V

Bi-functional coatings of carbonated calcium deficient hydroxyapatite (CDHA) on Ti alloys were developed by using a biomimetic coating process. The bi-functionality was achieved by loading alendonate sodium (AS), an approved bisphosphonate drug used for the treatment of osteoporosis, into the inner layers of CDHA coatings. Three possible methods of loading AS into CDHA coatings were systematically studied and compared. The results indicated that the co-precipitation method had greater benefits and can modify the release profile of AS by incorporating AS in the inner layers of the coatings. As a preliminary study, the influences of applied AS dosage to CDHA coatings were evaluated using XRD and SEM. In vitro tests indicated that the AS content on CDHA coatings played a significant role, and optimum AS content in local area is beneficial for osteoblast cells proliferation. It is expected that the CDHA-AS coatings via the co-precipitation approach have potential for bone tissue engineering applications.

Chick embryo anchored alkaline phosphatase and mineralization process in vitro

Bone alkaline phosphatase with glycolipid anchor (GPI-bALP) from chick embryo femurs in a medium without exogenous inorganic phosphate, but containing calcium and GPI-bALP substrates, served as in vitro model of mineral formation. The mineralization process was initiated by the formation of inorganic phosphate, arising from the hydrolysis of a substrate by GPI-bALP. Several mineralization media containing different substrates were analysed after an incubation time ranging from 1.5 h to 144 h. The measurements of Ca/Pi ratio and infrared spectra permitted us to follow the presence of amorphous and noncrystalline structures, while the analysis of X-ray diffraction data allowed us to obtain the stoichiometry of crystals. The hydrolysis of phosphocreatine, glucose 1-phosphate, glucose 6-phosphate, glucose 1,6-bisphosphate by GPI-bALP produced hydroxyapatite in a manner similar to that of beta-glycerophosphate. Several distinct steps in the mineral formation were observed. Amorphous calcium phosphate was present at the onset of the mineral formation, then poorly formed hydroxyapatite crystalline structures were observed, followed by the presence of hydroxyapatite crystals after 6-12 h incubation time. However, the hydrolysis of either ATP or ADP, catalysed by GPI-bALP in calcium-containing medium, did not lead to the formation of any hydroxyapatite crystals, even after 144 h incubation time, when hydrolysis of both nucleotides was completed. In contrast, the hydrolysis of AMP by GPI-bALP led to the appearance of hydroxyapatite crystals after 12 h incubation time. The hydroxyapatite formation depends not only on the ability of GPI-bALP to hydrolyze the organic phosphate but also on the nature of substrates affecting the nucleation process or producing inhibitors of the mineralization.

In vitro inhibition of membrane-mediated calcification by novel phosphonates

The effects of a series of novel phosphonates on the kinetics of mineral development in an ionophore-primed 7:2:1 phosphatidylcholine (PC): dicetylphosphate (DCP): cholesterol (Chol) liposomal model system are reported. When present at 2.5 micromol/liter or 25 micromol/liter concentrations in the solution surrounding the liposomes, the investigated phosphonates did not significantly delay the initial formation of hydroxyapatite-like calcium phosphate salts (HAP) within the liposomes or the penetration of HAP crystals through the enclosing membranes. However, the phosphonates variably retarded the subsequent growth and proliferation of the HAP crystals once they became directly exposed to the phosphonate-containing solution. The effectiveness of phosphonates in inhibiting extraliposomal precipitation strongly depended on their structure. The inhibitory action on active surface growth sites of released intraliposomal crystals was found to be the most effective if the phosphonate molecule contained two phosphonic groups linked to the same C atom. At a phosphonate concentration of 25 micromol/liter, the following general order of effectiveness was established: geminal bisphosphonate >/= geminal tetrakisphosphonate > bisacylphosphonates > monoacylphosphonate > bisalkylphosphonate. Within the bisacylphosphonate family, the highest inhibitory action was observed when four or five -CH2- groups separated the ketophosphonic groups.