Role of Phosphorus-Containing Molecules on the Formation of Nano-Sized Calcium Phosphate for Bone Therapy

Abstract

Calcium phosphate (CaP) is the principal inorganic constituent of bone and teeth in vertebrates and has various applications in biomedical areas. Among various types of CaPs, amorphous calcium phosphate (ACP) is considered to have superior bioactivity and biodegradability. With regard to the instability of ACP, the phosphorus-containing molecules are usually adopted to solve this issue, but the specific roles of the molecules in the formation of nano-sized CaP have not been clearly clarified yet. Herein, alendronate, cyclophosphamide, zoledronate, and foscarnet are selected as the model molecules, and theoretical calculations were performed to elucidate the interaction between calcium ions and different model molecules. Subsequently, CaPs were prepared with the addition of the phosphorus-containing molecules. It is found that cyclophosphamide has limited influence on the generation of CaPs due to their weak interaction. During the co-precipitation process of Ca²⁺ and PO₄ ^3-, the competitive relation among alendronate, zoledronate, and foscarnet plays critical roles in the produced inorganic-organic complex. Moreover, the biocompatibility of CaPs was also systematically evaluated. The DFT calculation provides a convincing strategy for predicting the structure of CaPs with various additives. This work is promising for designing CaP-based multifunctional drug delivery systems and tissue engineering materials.

Keywords: bone therapy; calcium phosphate; drug delivery; nanocomposites; phosphorus-containing molecules.

FIGURE 1

Optimized atomic structure showing the electrostatic density of (A) Adn⁻, (B) Adn⁻ + Ca²⁺, (C) Cpp, (D) Cpp + Ca²⁺, (E) Zda^-, (F) Zda⁻ + Ca²⁺, (G) Fss^3-, and (H) Fss^3- + Ca²⁺. Note: grey ball, carbon atom; white ball, hydrogen atom; blue ball, nitrogen atom; orange ball, phosphorus atom; lime-green ball, calcium atom; red ball, oxygen atom; and green ball, chlorine atom.

FIGURE 2

Chemical structural formula and the responding Gibbs free energy of (A) Adn⁻ and Adn⁻ + Ca²⁺, (B) Cpp and Cpp + Ca²⁺, (C) Zda^- and Zda⁻ + Ca²⁺, (D) Fss^3- and Fss^3- + Ca²⁺, (E) HPO₄ ^2- and HPO₄ ^2- + Ca²⁺, and (F) PO₄ ^3- and PO₄ ^3- + Ca²⁺.

FIGURE 3

TEM micrographs of (A) CaP Control, (B) CaP/Adn, (C) CaP/Cpp, (D) CaP/Zda, and (E) CaP/Fss; (F) XRD patterns of CaP Control, CaP/Adn, CaP/Cpp, CaP/Zda, and CaP/Fss products.

FIGURE 4

FTIR spectra of (A) CaP Control, Adn, CaP/Adn, Zda, CaP/Zda, Fss, and CaP/Fss; (B) CaP Control, Cpp, and CaP/Cpp.

FIGURE 5

Absorbance-concentration curve of (A) Zda and (B) Fss obtained from related UV-Vis absorption curves in Supplementary Figure S5A,B; (C) Zda-release and (D) Fss-release curves of CaP/Zda and CaP/Fss in PBS. TEM micrographs of (E) CaP Control—72 h, (F) CaP/Adn—72 h, (G) CaP/Cpp—72 h, (H) CaP/Zda—72 h, and (I) CaP/Fss—72 h.

FIGURE 6

Cell viability of BMSCs (A) and UMR-106 (B) cells co-cultured with different CaPs at various concentrations for 48 h.