Hydroxyapatite Nanoparticles for Improved Cancer Theranostics

Abstract

Beyond their well-known applications in bone tissue engineering, hydroxyapatite nanoparticles (HAp NPs) have also been showing great promise for improved cancer therapy. The chemical structure of HAp NPs offers excellent possibilities for loading and delivering a broad range of anticancer drugs in a sustained, prolonged, and targeted manner and thus eliciting lower complications than conventional chemotherapeutic strategies. The incorporation of specific therapeutic elements into the basic composition of HAp NPs is another approach, alone or synergistically with drug release, to provide advanced anticancer effects such as the capability to inhibit the growth and metastasis of cancer cells through activating specific cell signaling pathways. HAp NPs can be easily converted to smart anticancer agents by applying different surface modification treatments to facilitate the targeting and killing of cancer cells without significant adverse effects on normal healthy cells. The applications in cancer diagnosis for magnetic and nuclear in vivo imaging are also promising as the detection of solid tumor cells is now achievable by utilizing superparamagnetic HAp NPs. The ongoing research emphasizes the use of HAp NPs in fabricating three-dimensional scaffolds for the treatment of cancerous tissues or organs, promoting the regeneration of healthy tissue after cancer detection and removal. This review provides a summary of HAp NP applications in cancer theranostics, highlighting the current limitations and the challenges ahead for this field to open new avenues for research.

Keywords: bioceramics; cancer treatment; hydroxyapatite; nanomaterials.

Figure 1

Schematic representation of (a) the HAp unit cell (001) plan; (b) the distribution of octahedral site of PO₄³⁻ in the HAp structure; (c) the distribution of the tetrahedral sites of PO₄³⁻ and sequence of the octahedral sites; and (d) the tetrahedral and octahedral sites of PO₄³⁻ in the HAp structure. Reproduced with permission from [77].

Figure 2

(A) The optical behavior of HAp nanorods (HApnr): (a) the general model for self-activated fluorescence in the nano-sized particles, in which the energy levels in the forbidden zone are minimized while the presence of defects and the elimination of impurities lead to a higher disorder in the HAp structure, allowing radiative recombination of e’–h^• pairs at defect energy levels between the valence band (VB) and conduction band (CB); (b) the emission spectra of HApnr and HApnr350 samples by laser excitation at λ_exc = 350 nm; (c) the temperature-dependent photoluminescence of HApnr350 sample by laser excitation at λ_exc = 350 nm; (d) the excitation dependence of the HApnr350 sample probed by the spectrofluorometer at room temperature; and (e) the Commission Internationale de l’Éclairage (CIE) chromaticity diagram. (B) The optical response of the HAnr350 species obtained by confocal microscopy, monitored by exciting at λ_exc = 405, 458, 488, 514, 543, 594, and 633 nm (a, b, c, d, e, f, and g, respectively): (h) the bright-field micrograph of the power analyzed; scale bar = 200 mm and (i) the corresponding emission spectra obtained by exciting at distinct wavelengths. (C) The confocal imaging of human dermal fibroblast (HDFn) cells incubated with the HAnr350 sample (320 mg/mL) for 48 h: (a) the cytoskeleton labeled with Alexa Fluor 532 Phalloidin (λ_exc = 514 nm); (b) the fluorescence image exhibiting blue fluorescent HAp NPs (λ_exc = 405 nm); and (c) merged images. (d–g) The multiple excitation/emission imaging of intracellular the HAnr350 NPs (λ_exc = 405, 488, 543, and 594 nm, respectively). Reproduced with permission from ref [89].

Figure 3

(A) HAp NPs can induce cytotoxicity in cancer cells (adenocarcinoma human alveolar basal epithelial cells, A549 cells) through mitochondria-mediated apoptosis due to higher internalization as compared to normal cells (human bronchial epidermal cells, 16HBE cells). (B) Fluorescent micrographs of cells (a) before and after treatment with different doses of HAp NPs for 48 h (scale bar: 20 µm) and (b) Annexin V-FITC/PI double staining analysis of the cells showing apoptosis ratios in A549 and 16HBE cells post-treatment with HAp NPs. (C) In vivo tumor inhibition efficacy of HAp NPs: the effect of the HAp NPs treatments on changes in the body weight of healthy nude mice at a dosage of 40 mg/kg (three times a week) (a); histology evaluation (hematoxylin and eosin staining) of kidney and liver samples from healthy nude mice treated with saline or HAp NPs (40 mg/kg) on day 26 (magnification 200×) (b); tumor growth curves of nude mice bearing xenografted A549 cells treated with 40 mg/kg of HAp NPs three times a week (c); and macroscopic images of mice and excised tumors from saline or HAp NPs-treated groups at the time of sacrifice (d). Reproduced with permission from ref. [98].

Figure 4

Schematic illustration of erbium (Er)−doped hydroxyapatite (HAp) particles as a bioactive luminescent platform useful for cancer imaging. Reproduced with permission from ref. [128].

Figure 5

(A) Graphs reporting the emission spectra of non-calcined HAp: Eu/Gd excited at 273 and 394 nm ((a) and (b), respectively). (B) The in vivo fluorescent imaging of HAp: Eu/Gd (2:1.5) nanocrystals with no calcination in BALB/c-nu mice: (a) with no injection, (b) with injection in the enterocoelia, (c) five minutes post-vein injection, and (d) 1.5 h post-vein injection. Reproduced with permission from ref. [90].

Figure 6

(A) Photothermal therapy (PTT) mechanism. (B) Photodynamic therapy (PDT) mechanism. (C) The effect of NIR on increasing the temperature of HAp/gelatin/graphene scaffold and the role of increasing the concentration of graphene on increasing the temperature in PTT. (D) (a) The structure of the doped HAp-NPs with Hf for potential use in PDT; (b) the ROS generation of HAp-Hf in the non-irradiated and gamma rays (5 Gy) conditions. The DCF-fluorescence intensity significantly increased in the irradiated samples doped with Hf. Reproduced with permission from refs [50,142,144].

Figure 7

Schematic representation of a molecular targeted therapeutic mechanism for cancer treatment. This focuses on targeting specific cancer-associated molecules that are highly expressed in cancer cells or by modulating the tumor microenvironment (such as tumor vasculature, metastasis, or hypoxia). Reproduced with permission from [147].

Figure 8

Preparation of luminescent europium (III) complex-based hydroxyapatite nanocrystals (EHA) and evaluation of their cytocompatibility and cell imaging capability. The folic acid derivative, folate N−hydroxysuccinimidyl ester (FA−NHS), was immobilized on the EHA as the targeting ligand for the HeLa cancer cells. Both 3-aminopropyltriethoxysilane and methyltriethoxysilane molecules were utilized for mediation of the immobilization process. Reproduced with permission from [150].

Figure 9

(A) T2 weight magnetic resonance imaging (MRI) of commercial HAp (commerce) and the synthesized cobalt (Co)-doped HAp (CoHA) in three groups: synthesized CoHA via direct current (DC), pulse current (PC1) (same time as DC), and PC2 (same voltage as DC). Graph exhibiting the enhanced contrast of the MRI image along with an increase in cobalt concentrations. (B) Graph showing the results of the relaxation rate, R2 (1/T2), and T2-weighted spin-echo sequence in the MRI of the HAp doped with different Co concentrations. (C) Graph displaying the relaxation rate of CoHA particles, in which the relaxation rates of DC−CoHA, PC1−CoHA, and PC2−CoHA are about 283.4, 227.8, and 223.3 L/mmol s, respectively. Adapted from [166].

Figure 10

(A) Magnetization curves of Eu³⁺/Gd³⁺−HAp nanorods (a); T₁- and T₂- relativity plot of Eu³⁺/Gd³⁺−HAp nanorods (b and c, respectively); T₁- and T₂-weighted magnetic resonance (MR) micrographs of Eu³⁺/Gd³⁺−HAp nanorods dispersed in solution at different concentrations (0.5 to 10 mg mL⁻¹) (d). (B) In vivo photoluminescence (PL) imaging of the mice at (a) before and (b) after the subcutaneous of injection Eu³⁺/Gd³⁺−HAp (Eu³⁺:Gd³⁺ = 1:2) nanorods; (c) the PL emission micrographs of Eu³⁺/Gd³⁺−HAp nanorods at different concentrations, excited at a wavelength of 430 nm. (C) In vivo computed tomography (CT) images of the nude mice treated with Eu³⁺/Gd³⁺−HAp (Eu³⁺:Gd³⁺ = 1:2) nanorods suspended in phosphate-buffered saline solution (PBS); the transverse image of the back of the mouse at (a) before and (b) after the injection; the transverse micrographs of the buttock of the mouse at (c) before and (d) after the injection. Adapted with permission from [167].