Introduction

Osteoporosis (OP) is a systemic metabolic bone disease characterized by decreased bone mineral density (BMD) and the deterioration of bone microarchitecture, resulting in increased bone fragility and a significantly elevated risk of fractures.1 The underlying pathophysiological mechanism involves a dynamic imbalance in bone remodeling, where bone resorption exceeds bone formation.2 Under normal physiological conditions, the continuous process of bone remodeling is regulated by the coordinated activity of osteoclasts, which resorb bone, and osteoblasts, which form new bone, thereby maintaining skeletal homeostasis.3 However, in patients with OP, heightened osteoclast activity leads to excessive bone resorption, while osteoblast function is diminished, resulting in inadequate bone formation. This imbalance is often exacerbated by various factors, including hormonal changes, genetic predispositions, nutritional deficiencies, and aging.4 As bone mass progressively decreases, the structural integrity of bone is compromised, leading to a marked reduction in bone strength and an increased susceptibility to fractures.5 Understanding the complex pathophysiological mechanisms of OP not only sheds light on the disease’s multifactorial nature but also provides a solid theoretical foundation for the development of targeted therapeutic strategies.

Despite the progress made in slowing bone loss and reducing fracture risk, current therapeutic approaches for OP still face numerous challenges and limitations.6 The most commonly used treatments include anti-resorptive agents (such as bisphosphonates), anabolic agents (such as parathyroid hormone analogs), and hormone replacement therapies.7 However, long-term use of these medications often comes with significant adverse effects, such as gastrointestinal discomfort, osteonecrosis of the jaw, and cardiovascular complications, all of which severely affect the quality of life for patients.8 Additionally, the efficacy of these therapies in promoting bone regeneration is limited, making it difficult to fully repair damaged bone tissue. Furthermore, the complex and multifactorial pathophysiology of OP renders single-treatment strategies inadequate in addressing the diverse factors influencing disease progression.9 As a result, some patients continue to face a heightened risk of fractures despite ongoing treatment. These limitations underscore the urgent need for innovative therapeutic approaches that combine efficacy, safety, and targeted delivery. Multifunctional treatment strategies must be developed to overcome the current bottlenecks in OP management and provide more comprehensive and personalized care.

Functionalized nanoparticles, as an emerging therapeutic modality, have demonstrated immense potential and distinct advantages in the treatment of OP.10 Owing to their unique physicochemical properties, nanoparticles not only enable efficient drug loading and targeted delivery but also allow for controlled release through intelligent design, concentrating therapeutic effects at the site of bone lesions.11 This significantly enhances drug efficacy while minimizing systemic side effects. Through surface modification or multi-functionalization, nanoparticles can be engineered to carry a variety of therapeutic agents,12 such as anti-inflammatory drugs and bone-promoting factors, and can integrate imaging capabilities to enable theranostics (Figure 1). This enables real-time monitoring and precise control throughout the treatment process, thereby improving therapeutic accuracy and outcomes.13 Compared to conventional treatment methods, functionalized nanoparticles can overcome the challenges associated with poor targeting of bone tissue.14,15 Surface modifications enhance the binding affinity of drugs to bone surfaces, increasing drug retention time and local concentration in bone tissue, which greatly improves bone regeneration capacity.16 Furthermore, functionalized nanoparticles can regulate the dynamic balance between osteoclasts and osteoblasts, promoting bone restoration and addressing the underlying pathophysiology of OP.17 The high adaptability and multifunctionality of functionalized nanoparticles make them a promising breakthrough in the future of OP therapy, offering patients safer, more effective, and personalized treatment options, ultimately improving their quality of life.

Figure 1 Schematic illustration of the composition and structural framework of functionalized nanoparticles for OP treatment. (Some parts of the figure by Figdraw, www.figdraw.com).

Types and Architecture of Functionalized Nanoparticles

The diversity in the types and architecture of functionalized nanoparticles plays a pivotal role in determining their therapeutic efficacy, especially in the treatment of complex diseases like OP.18 The versatility of these nanoparticles allows for tailored modifications to enhance targeting precision, drug delivery, and controlled release mechanisms.19 From metal-based to polymer-based and even biomembrane-coated nanoparticles, each type brings unique advantages in terms of biocompatibility, mechanical properties, and drug-loading capabilities.20 By fine-tuning the surface characteristics and core materials, these nanoparticles can be optimized for specific biological environments, enabling a more effective and personalized therapeutic approach.

Surface Modification and Functionalization Techniques Targeting

In the treatment of bone-related diseases such as OP, the targeted delivery of functionalized nanoparticles is of paramount importance. The structural characteristics of bone tissue, including the complex network of trabecular and cortical bone, as well as the dynamic balance of bone cells in bone metabolism, necessitate precise drug targeting to the affected areas to achieve optimal therapeutic outcomes.21 Achieving bone-targeting primarily relies on the affinity between specific targeting molecules, modified on the surface of functionalized nanoparticles, and bone tissue—particularly their interaction with hydroxyapatite.22 This ensures the effective accumulation of functionalized nanoparticles at the site of bone lesions, thereby enhancing therapeutic efficacy.

Currently, commonly used bone-targeting agents include bisphosphonates (such as alendronate sodium23), peptides (such as Asp24), and novel targeting molecules (such as cell membranes overexpressing CXCR425). These targeting agents promote the accumulation of functionalized nanoparticles in bone tissue through their chemical affinity to the bone matrix.26 However, there are several notable drawbacks associated with these existing bone-targeting agents. First, most designed bone-targeting functionalized nanoparticles only generally target bone tissue, lacking precise targeting for specific types of bone cells, such as osteoclasts or osteoblasts, leading to suboptimal therapeutic outcomes.27 In response to this challenge, Zhang and colleagues28 proposed a novel dual-targeting functionalized nanoparticle system capable of effectively targeting different stages of osteoclasts (OCs). The functionalized nanoparticles designed in this study utilize calcitonin gene-related peptide receptor (CGRPR) and anti-tartrate-resistant acid phosphatase (TRAP) as targeting ligands to specifically address the characteristics of OCs at different developmental stages, thereby achieving more precise drug delivery. This innovative design not only enhances targeting efficiency toward OCs but also ensures that the drug exerts its effects during various functional stages of OCs, significantly improving treatment precision. Additionally, the stability of some targeting agents is insufficient, as they may rapidly degrade in vivo, thus compromising their targeting efficiency.29 Moreover, in complex physiological environments, targeting molecules are prone to competitive binding or nonspecific interactions, which can further reduce their effectiveness.30

Therefore, to overcome these challenges, future research should focus on optimizing the chemical stability and biocompatibility of targeting agents, while also developing more effective targeting strategies to achieve precise targeting of specific bone cell types. By pursuing these advanced design approaches, bone-targeting functionalized nanoparticles hold great promise for enhanced clinical applications, providing patients with more effective therapeutic options.

Biocompatibility and Cytocompatibility

In the treatment of OP, surface modification of functionalized nanoparticles is critical for enhancing their biocompatibility and cytocompatibility. Good biocompatibility provides the foundation for the long-term presence of functionalized nanoparticles in vivo, while cytocompatibility ensures effective interaction with bone cells, promoting endocytosis and drug release.31 Therefore, improving the biocompatibility and cytocompatibility of functionalized nanoparticles is key to successful OP treatment. Surface modification can effectively enhance nanoparticle stability in vivo, reduce immune system rejection, and ultimately improve both therapeutic efficacy and safety.32 Functionalized nanoparticles with excellent biocompatibility and cytocompatibility can increase drug bioavailability, enhance the efficiency of targeted delivery, reduce side effects, and promote the accumulation of drugs within specific bone cells.33

Commonly used materials to enhance the biocompatibility and cytocompatibility of functionalized nanoparticles include polyethylene glycol (PEG),34 chitosan,35 and polyethylenimine (PEI).36 These materials exhibit excellent biocompatibility and biodegradability, forming protective layers or increasing surface hydrophilicity, which can significantly reduce the immunogenicity of functionalized nanoparticles while promoting their uptake by bone cells.37 For instance, the incorporation of PEG can effectively minimize protein adsorption and prolong the circulation time of functionalized nanoparticles,38 whereas chitosan facilitates endocytosis by interacting with the bone cell membrane.39

Real-Time Monitoring

In the rapid advancement of nanomedicine, real-time monitoring technologies have become a crucial tool for the early diagnosis of diseases and the assessment of therapeutic outcomes.40 Particularly in OP research, functionalized nanoparticles allow for the real-time observation of bone cell behavior and the efficacy of therapeutic interventions.41 The core of this technology lies in the functional surface modifications of nanoparticles, which bestow them with specific targeting and sensing capabilities, enabling highly sensitive and specific detection of biomarkers.42 These functional modifications not only empower nanoparticles to recognize and bind to target cells but also enable them to sense specific physiological changes within cells.43 Through this approach, nanoparticles can track the status and functional alterations of mature bone cells in real-time, providing critical insights into cellular activity, drug responses, and disease progression.

Matrix metalloproteinase-13 (MMP13), a critical matrix metalloproteinase, plays a key role in bone remodeling, particularly in regulating the activity of mature osteoclasts.44 Yan and colleagues45 utilized a functionalized nanoparticle surface modified with an MMP13-responsive probe (BHQ) to monitor MMP13 expression, enabling real-time assessment of bone cell functional status and the dynamic process of bone remodeling. As illustrated in Figure 2, the surface of the functionalized nanoparticles is capped with a “hat” (BHQ), which quenches the fluorescence of the nanoparticles. Upon activation by 980 nm near-infrared light, the drug encapsulated within the functionalized nanoparticles is released, inducing osteogenic differentiation of mesenchymal stem cells (MSCs), leading to the production of MMP13. MMP13, in turn, cleaves the MMP13-sensitive peptide, removing the BHQ and restoring the fluorescence of the functionalized nanoparticles. Moreover, this real-time monitoring technology allows for tracking the impact of therapeutic interventions on bone cell activity throughout the treatment process, providing valuable data to support the development of personalized treatment strategies.46

Figure 2 (A) Synthesis and structural design of the UCNP nanocomplex; (B) Near-infrared (NIR) light-triggered osteogenic differentiation of MSCs coupled with real-time monitoring of cellular differentiation, demonstrated both in vitro and in vivo. Reproduced from Yan R, Guo Y, Wang X, et al. Near-infrared light-controlled and real-time detection of osteogenic differentiation in mesenchymal stem cells by upconversion nanoparticles for osteoporosis therapy. ACS Nano. 2022;16(5):8399–8418.45

Abbreviations: TEOS, tetraethyl orthosilicate; TEDATS, N-(3-trimethoxysilyl)propyl ethylenediamine triacetic acid trisodium salt; ONA, oligonucleotide; PEG, polyethylene glycol; BHQ, black hole quencher; RGD, Arg-Gly-Asp; ICA, Icariin; β-CD, β-Cyclodextrin; UCNP, upconversion nanoparticles; MMP13, matrix metalloproteinase 13; MSC, mesenchymal stem cells.

Real-time monitoring technology not only enhances our ability to gain deeper insights into the physiological state of bone cells but also opens new possibilities for the early diagnosis of OP and the assessment of therapeutic efficacy.47 Leveraging the advantages of functionalized nanoparticles, researchers can achieve highly sensitive and specific in vivo monitoring, enabling the timely detection of dynamic changes in bone cells under the influence of therapeutic agents.48 This innovative strategy marks a significant step forward in the evolution of nanomedicine toward more precise and personalized treatments.

Drug Loading and Controlled Release

In the treatment of OP, many drugs require modification through materials on the surface of functionalized nanoparticles due to poor water solubility, inadequate stability, or low bioavailability.49 This method of drug loading offers several advantages, including enhanced solubility and stability, prolonged circulation time in vivo, improved targeting, and controlled release at the diseased site.50 For instance, the use of polymers such as polylactic acid (PLA) or polyvinyl alcohol (PVA) as carriers helps form stable drug complexes, preventing rapid degradation in the body. However, this approach also presents certain limitations.51 The interactions between the drug and the carrier may reduce the drug’s activity, or result in uneven release rates in vivo, which can negatively impact therapeutic outcomes.

To overcome these challenges, controlled drug release technologies have emerged, playing a crucial role in the treatment of OP. These technologies ensure sustained drug release in the body, avoiding sharp fluctuations in plasma drug concentrations, thereby enhancing therapeutic outcomes.52 Functionalized nanoparticles exhibit unique advantages in this regard, as they can intelligently modulate drug release rates in response to varying physiological conditions such as pH, temperature, or light.53 For instance, Liu and colleagues54 demonstrated the application of carbon dots modified on the surface of nanoparticles, enabling phase transitions in hydrogels upon near-infrared light irradiation. This design mimics the natural pulsatile and sustained secretion pattern of PTHrP-2 (parathyroid hormone-related protein-2), achieving efficient drug release to bone cells. This light-responsive mechanism not only improves drug bioavailability but also allows for precise, time-controlled drug release, ensuring a stable therapeutic effect. Meanwhile, Cui and colleagues55 explored the use of polyethyleneimine-dimethyl methacrylamide (PEI-DMMA) modified on the surface of functionalized nanoparticles, conferring pH sensitivity. In neutral or alkaline environments, the PEI-DMMA layer remains hydrophobic, retaining the encapsulated drug. However, in acidic conditions, the PEI-DMMA layer undergoes protonation, transforming into a hydrophilic state. This protonation process leads to the expansion of the nanoparticle hydrogel core, disrupting the outer shell and triggering rapid drug release. This environment-responsive mechanism enables functionalized nanoparticles to release drugs precisely under specific pathological conditions, such as at sites of bone resorption, providing a more effective solution for OP treatment.56

Thus, functionalized nanoparticles not only effectively load OP therapeutic agents but also enable precise controlled drug release.57 These technological innovations have opened new possibilities for the application of nanomedicine in the treatment of diseases such as OP.58 Future research should continue to explore novel materials and modification strategies to further optimize drug loading and release mechanisms, providing stronger support for clinical applications.

Types of Material

To advance OP treatment, various multifunctional nanoparticles have been developed, each crafted from distinct materials that confer specific therapeutic properties.59 These materials—ranging from metals and polymers to biomembranes and carbon-based nanomaterials—exhibit unique characteristics that enhance their utility in drug delivery, bone regeneration, and targeted therapy.60,61 As summarized in Table 1, metal-based nanoparticles stand out for their exceptional physicochemical stability, tunable optical and magnetic properties, and diverse surface functionality, making them ideal for targeted delivery applications. In contrast, polymer-based nanoparticles offer superior biocompatibility and customizable release profiles,62 while membrane-based systems provide innate targeting abilities and intracellular delivery, effectively enhancing cellular uptake.63 Carbon-based nanoparticles, with their distinctive optical properties, not only facilitate targeted drug delivery but also enable simultaneous imaging, thereby supporting both diagnostic and therapeutic goals.64 Each material type brings particular advantages and challenges to OP treatment, necessitating careful selection based on the therapeutic requirements.

Table 1 Overview of the Types of Materials Used in Functionalized Nanoparticles for OP Treatment

Metal-Based Multifunctional Nanoparticles

Metal-based multifunctional nanoparticles have demonstrated significant advantages in the treatment of OP, primarily due to their excellent biocompatibility, superior mechanical strength, and tunable drug release properties.78 These multifunctional nanoparticles are typically synthesized through chemical reduction, sol-gel processes, or physical evaporation methods, with their formation relying on the reduction and aggregation of metal ions into nanoparticles.79 They can be categorized into several types, including iron-based nanoparticles,80 gold-based nanoparticles,69 copper-based nanoparticles,67 and other transition metal nanoparticles.81 Each type of metal nanoparticle has distinct characteristics in terms of functionalization and application, allowing for tailored designs to meet specific biological environments and therapeutic needs.

Metal-based multifunctional nanoparticles have shown unique advantages in the treatment of OP. For example, Fe3O4 nanoparticles synthesized by Lee and colleagues65 not only effectively promote the proliferation and differentiation of osteoblasts but also improve the bone microenvironment, enhancing the synthesis and mineralization of the bone matrix. These multifunctional nanoparticles interact with bone cells, stimulating osteoblast activity, thereby increasing bone density and strength. However, the potential risk of ferroptosis induced by Fe3O4 nanoparticles cannot be overlooked, as it may lead to apoptosis and further bone tissue damage.82 Despite their positive role in bone regeneration, caution is needed in their clinical application. In contrast, Fe2O3 nanoparticles offer advantages due to their lower iron load and better biocompatibility in vivo, effectively avoiding ferroptosis,83 which partially compensates for the drawbacks of Fe3O4 nanoparticles. Additionally, Fe2O3 enhances the activity of antioxidant enzymes, boosting bone cells’ resistance to oxidative stress, providing a new approach for OP treatment.84 However, the poor solubility of Fe2O3 nanoparticles in vivo may limit their therapeutic efficacy, suggesting future research should focus on improving their bioavailability to fully realize their potential in OP treatment.85 Optimizing a combination of Fe3O4 and Fe2O3 nanoparticles could lead to more effective outcomes for OP therapy. At the same time, in the field of optical therapy, upconversion nanoparticles (UCNPs) synthesized with different rare-earth metal ions demonstrate varying capabilities in spectral conversion.86 Ma and colleagues87 synthesized UCNPs-1, which can effectively convert near-infrared (NIR) light into visible light, significantly reducing potential harm to biological tissues. As shown in Figure 3A, the upconversion luminescence (UCL) spectrum of UCNPs-1 under 808 nm NIR excitation emits visible light peaks at around 540 nm (green) and 650 nm (red), both within the visible light range of 400 to 700 nm. Therefore, it is clear that UCNPs-1 emit visible light under 808 nm NIR excitation. This visible light exhibits lower phototoxicity and better biocompatibility, as its lower energy minimizes the risk of DNA damage or other photochemical reactions, thereby avoiding potential harm to cells and tissues.88 In contrast, the UCL spectrum of UCNPs-2, synthesized by Ye and colleagues,89 emits light peaks in the blue/ultraviolet region below 500 nm (Figure 3B). Although this higher-energy light can more effectively activate certain photosensitive molecules or processes, it also carries a higher risk, as blue or ultraviolet light has higher energy, potentially leading to DNA base dimerization or other photodamage reactions, increasing the risk of phototoxicity.90 Therefore, selecting appropriate rare-earth dopants and optimizing the shape and size of upconversion nanoparticles is crucial to achieving safer light conversion. In addition, cerium oxide nanoparticles (CeO2 NPs) are gaining attention in OP treatment due to their ability to modulate the intracellular redox state, striking a balance between antioxidation and oxidation.91 CeO2 nanoparticles possess dual oxidation states, allowing them to switch between oxidation and reduction, depending on the physiological conditions. In the acidic microenvironment of bone resorption, CeO2 promotes the expression of antioxidant enzymes (such as superoxide dismutase and glutathione peroxidase), effectively scavenging excess reactive oxygen species (ROS) and reducing oxidative stress-induced damage to bone cells.92 In contrast, in the neutral bone microenvironment, the oxidative properties of CeO2 stimulate osteoblast proliferation and differentiation, promoting bone matrix synthesis.93 This mechanism not only mitigates oxidative damage to bone cells but also promotes their self-renewal and repair, ultimately enhancing bone density. Therefore, combining and optimizing the design of CeO2 nanoparticles holds promise for providing more effective solutions for OP treatment, advancing clinical applications.

Figure 3 Upconversion luminescence (UCL) spectra of (A) UCNPs-1 and (B) UCNPs-2. Figure 3A Reproduced from Ma X, Luan Z, Zhao Q, et al. NIR-triggered release of nitric oxide by upconversion-based nanoplatforms to enhance osteogenic differentiation of mesenchymal stem cells for osteoporosis therapy. Biomater Res. 2024;28:0058. Distributed under a Creative Commons Attribution License 4.0 (CC BY 4.0). https://creativecommons.org/licenses/by/4.0/.87Figure 3B Reproduced from Ye J, Jiang J, Zhou Z, et al. Near-infrared light and upconversion nanoparticle defined nitric oxide-based osteoporosis targeting therapy. ACS Nano. 2021;15(8):13692–13702.89

Abbreviations: UCNP, upconversion nanoparticles; BHQ, black hole quencher; FL, fluorescence; UCPA, upconversion nanoparticle-based photosensitizing gas nanoplatform with alendronate modification; BNN, N,N’-Di-sec-butyl-N,N’-dinitroso-1,4-phenylenediamine.

The advantages of metal-based multifunctional nanoparticles lie in their excellent biocompatibility, controlled release properties, and targeted drug delivery capabilities, which can significantly enhance the therapeutic outcomes for OP. However, current studies have also revealed certain limitations, such as the potential for metal accumulation in the body, the risk of toxicity, and inconsistent release profiles.94,95 Future research should focus on optimizing the design of metal-based multifunctional nanoparticles to improve their stability in biological environments, ensuring both their safety and efficacy. These advancements will better position them for effective application in the treatment of OP.

Polymer-Based Multifunctional Nanoparticles

Polymer-based multifunctional nanoparticles, due to their excellent biocompatibility and tunability, have broad applications in the treatment of OP.96 Common types of polymer-based multifunctional nanoparticles include liposomes and hydrogel microspheres. Liposomes are favored for their superior drug-carrying capacity and biocompatibility,97 while hydrogel microspheres are widely studied for their controlled release properties and exceptional drug encapsulation capabilities.98 Both types of polymer-based multifunctional nanoparticles offer distinct advantages in OP treatment.

The synthesis of liposomes typically employs the thin-film hydration method, which efficiently encapsulates drugs of varying properties.99,100 Specifically, hydrophobic drugs are usually embedded in the hydrophobic region of the lipid bilayer, while hydrophilic drugs are encapsulated within the aqueous core.101,102 This dual drug-loading capability allows for the simultaneous transport of both hydrophobic and hydrophilic drugs, enhancing therapeutic diversity and efficacy. However, the design of conventional liposomes focuses primarily on drug encapsulation, with limited control over drug release, which restricts their flexibility and effectiveness in clinical applications.103 To address this limitation, functionalized liposomes have emerged, enabling more precise controlled drug release. One innovative example in this field is the thermosensitive liposome synthesized by Che and colleagues,104 which incorporates polydopamine modifications to create a temperature-sensitive system. As the ambient temperature increases, the structure of polydopamine changes, causing a phase transition in the liposomal membrane and leading to the rapid release of encapsulated drugs. This thermosensitive controlled-release mechanism allows for the timed release of drugs in response to body temperature changes, significantly improving therapeutic outcomes. On the other hand, Yang and colleagues105 introduced a pH-responsive functionalized liposome, which is modified with hexachlorocyclotriphosphazene, enabling structural changes in acidic environments. When the environmental pH decreases, the polymer chains of the liposome undergo a structural shift, increasing hydrophilicity and triggering rapid drug release. This pH-responsive mechanism facilitates precise drug release under bone resorption conditions, offering a novel approach for OP treatment. Hydrogel microspheres, on the other hand, are typically synthesized through polymer cross-linking, forming a three-dimensional network structure via physical or chemical cross-linking.106 In this structure, drugs can be effectively encapsulated through soaking or copolymerization. In OP applications, hydrogel microspheres maintain stability in physiological environments and can control drug release rates by adjusting cross-linking density and environmental conditions.107 This highly tunable release characteristic allows hydrogel microspheres to offer personalized drug release profiles based on the specific needs of the patient, thereby enhancing the therapeutic efficacy in OP treatment.

Membrane-Based Multifunctional Nanoparticles

Membrane-based multifunctional nanoparticles have shown unique advantages in the treatment of OP. These nanoparticles typically combine the properties of cell membranes and exosomes, enhancing their targeting ability, biocompatibility, and therapeutic efficacy.108,109 Tian and colleagues74 demonstrated a novel multifunctional nanoparticle system, where nanoparticles were coated with osteoblast membranes or exosome membranes, significantly improving their targeting capabilities toward specific bone cells. This design not only leverages the biocompatibility of cell membranes but also allows the nanoparticles to evade recognition by the immune system, thus extending their circulation time and enhancing their bioavailability. Furthermore, Hu and colleagues75 explored the critical role of exosomes in drug delivery. Their research demonstrated that exosomes can effectively encapsulate various types of therapeutic agents, including small-molecule drugs and nucleic acids such as siRNA and mRNA. Specifically, these exosomes deliver drugs to target cells through an endocytosis mechanism, facilitating efficient cellular uptake of siRNA, which subsequently exerts its function by significantly inhibiting the expression of specific genes within target cells. This exosome-mediated delivery of siRNA offers a novel strategy for OP treatment, as modulating the gene expression of osteoblasts can effectively improve bone metabolism.110

Carbon-Based Multifunctional Nanoparticles

Although research on carbon-based multifunctional nanoparticles for OP is still relatively limited, the application of carbon quantum dots is gradually becoming a focus in this field.111 As a novel nanomaterial, carbon quantum dots have garnered significant attention due to their excellent biocompatibility and tunable surface properties, which enable effective interactions with bone cells and promote the accumulation of drugs at the target site.112 Furthermore, the unique optical properties of carbon quantum dots allow them to function as imaging agents without the need for additional fluorescent markers, enabling real-time monitoring of bone cell status and drug release.113 This dual functionality, seamlessly integrating both therapeutic and diagnostic capabilities, offers new possibilities for the clinical management of OP.

Design Strategies for Multifunctional Nanoparticles

The design of functionalized nanoparticles plays a critical role in enabling precise and efficient treatment, especially for complex diseases like OP. These functionalized nanoparticles offer a flexible platform capable of addressing multiple therapeutic targets by integrating various functionalities such as drug encapsulation, controlled release, and targeted delivery.114,115 By incorporating a combination of therapeutic agents, functionalized nanoparticles can simultaneously modulate inflammation, promote bone regeneration, and inhibit bone resorption.116 Additionally, their structural adaptability allows for enhanced drug stability and targeting precision, ensuring that therapeutic agents are delivered directly to the site of action while minimizing side effects.117 Functionalized nanoparticles, through advanced design strategies, hold great potential for improving treatment outcomes and advancing the field of precision medicine.

Combined Drug Delivery

Functionalized nanoparticles offer vast potential in the field of drug delivery, with their core advantage being the ability to effectively encapsulate small-molecule drugs within their internal cavity or porous structure.118,119 Their nanoscale size and unique physicochemical properties enable them to protect drugs from degradation by external environmental factors, thus enhancing drug stability and bioavailability in vivo.120,121 Additionally, functionalized nanoparticles can be surface-modified with specific functional groups or ligands, allowing for targeted drug loading and controlled release. This process relies on multiple mechanisms, including chemical bonding, electrostatic adsorption, or hydrophobic interactions, ensuring precise drug targeting to specific pathological sites.122 However, the complexity of OP often requires more than single-drug delivery to address its multifaceted pathophysiology. A combined drug delivery strategy not only broadens the scope and depth of treatment but also reduces the required dosage of each individual drug, minimizing adverse reactions and enhancing overall therapeutic efficacy.

The combined drug delivery strategy enables the co-delivery of drugs with antioxidant, anti-inflammatory, bone formation-promoting, and bone resorption-inhibiting properties, providing a comprehensive therapeutic approach to OP.123 For instance, through the structural design of functionalized nanoparticles, antioxidant agents and bone-regenerating drugs can simultaneously target damaged sites, achieving synergistic effects.124 Beyond the delivery of single-function drugs, functionalized nanoparticles are also capable of delivering dual- or triple-function drugs, which possess both anti-inflammatory and bone regeneration-promoting properties.73 However, the therapeutic mechanisms and targeting specificity of such multifunctional drugs are often limited, making it challenging to address the full complexity of OP pathophysiology.125 Moreover, the advantage of combined drug delivery is not limited to the co-delivery of drugs with different therapeutic effects but also extends to the simultaneous delivery of drugs with different characteristics. Li and colleagues126 encapsulated both free vancomycin and vancomycin-loaded liposomes in a hydrogel, achieving a dual release profile of rapid and sustained drug release in vivo. This design significantly enhanced the effectiveness of anti-infective therapy, particularly in combating fracture-related infections associated with OP. Through this combined delivery strategy, nanoparticle carriers can flexibly meet various therapeutic needs, ensuring precise drug release at different time points, thereby providing more sustained and effective treatment.

Integrated Application of Imaging and Therapy

Functionalized nanoparticles not only efficiently deliver therapeutic agents but also offer the advantage of imaging, allowing real-time monitoring of drug distribution and release during treatment.127 In the context of OP therapy, the imaging capability of functionalized nanoparticles is particularly important, as it aids in precisely targeting bone tissue and tracking therapeutic outcomes.128 Different types of functionalized nanoparticles, with their unique optical or magnetic properties, enable the visualization of drug distribution within the bone. Cheng and colleagues129 developed a bone-targeting nanocarrier based on near-infrared (NIR) emission, utilizing the NIR-II region (>1000 nm) to achieve high-resolution whole-body skeletal imaging. This system leverages the luminescence properties of lanthanide elements, significantly enhancing imaging depth and spatial resolution, making it especially suitable for non-invasive bone imaging. As shown in Figure 4, a strong NIR-II signal was observed three hours after the in vivo injection of these multifunctional nanoparticles, demonstrating their successful accumulation in the bones and targeted delivery, which plays a crucial role in improving drug bioavailability and therapeutic efficacy. On the other hand, Chen and colleagues130 synthesized functionalized nanoparticles utilizing the principle of upconversion luminescence (UCL). These nanoparticles, doped with rare-earth ions, convert low-energy excitation into high-energy emission, enabling both in vivo and in vitro imaging. The key difference between these two methods lies in their luminescence mechanisms: the former provides deeper imaging through NIR emission, while the latter’s upconversion luminescence offers higher optical stability and lower background noise. Despite the differences in their light emission mechanisms, both techniques harness the unique optical properties of functionalized nanoparticles to integrate bone imaging and therapy, providing new possibilities for the diagnosis and treatment of OP.131,132 These methods not only enhance drug targeting but also enable real-time monitoring of therapeutic outcomes, advancing the development of precision medicine.

Figure 4 Bone-targeting effect and biotoxicity evaluation of LMEK. (a) Whole-body NIR-II imaging of OVX mice; (b) NIR-II imaging of the forelimbs of mice; (c) NIR-II imaging of the hindlimbs of mice; (d) Ex vivo NIR-II imaging of bones dissected from the LMEK group. Reprinted from Acta Biomaterialia, Cheng C, Xing Z, Hu Q, et al. A bone-targeting near-infrared luminescence nanocarrier facilitates alpha-ketoglutarate efficacy enhancement for osteoporosis therapy. 2024;173:442–456. With permission from Elsevier.129

Abbreviations: OVX, ovariectomy; LnNP, lanthanide nanoparticles; LMEK, β-NaYF4:7%Yb,60%Nd@NaLuF4@mSiO2-EDTA-AKG; EDTA, ethylenediaminetetraacetic acid; AKG, Alpha-ketoglutarate.

Multi-Layer Functional Modification

Multilayer functionalization of nanoparticles has shown immense potential in the treatment of OP. By applying multi-tiered modifications to the surface of functionalized nanoparticles, their biocompatibility, targeting specificity, and controlled release capabilities can be significantly enhanced.133 This multilayer structure enables nanoparticles to carry multiple drugs or bioactive factors simultaneously, achieving synergistic effects during therapy and improving therapeutic outcomes.134 Moreover, multilayer functionalization increases the stability of nanoparticles, prolonging their circulation time in vivo and reducing potential side effects.135

As a promising bone substitute material, calcium phosphate cement (CPC) is commonly used in the treatment of OP.136,137 However, its setting process can be disrupted by alendronate (ALN), making it difficult to use both simultaneously in OP therapy.138 To overcome this challenge, Huang and colleagues139 employed a multilayer functionalization strategy, incorporating a layer of poly(lactic-co-glycolic acid) (PLGA) between the CPC and ALN. This CPC-PLGA-ALN multilayer design not only enhanced the mechanical strength of the functionalized nanoparticles but also allowed CPC and ALN to act independently without interfering with each other. In contrast, Zhang and colleagues140 synthesized functionalized nanoparticles specifically designed for gene drug delivery. By constructing a polyelectrolyte core-shell structure, where the core is composed of cationic polymers for loading and releasing small interfering RNA (siRNA), and the outer shell is modified with polyethylene glycol (PEG) to improve biocompatibility and prolong circulation time in vivo, this design is particularly suited for precision gene therapy. It regulates bone resorption by inhibiting osteoclast maturation while promoting new bone formation, offering broad potential in OP treatment. Ma and colleagues141 developed a hyaluronic acid-gelatin composite hydrogel, forming microspheres with a hierarchical structure. These microspheres, featuring inverse-protein architecture with nano- and microscale pores, not only increased drug loading capacity but also promoted cell adhesion and osteogenesis. Xu and colleagues142 introduced a magnetic core-shell structured functionalized nanoparticle, where the core consists of magnetic nanoparticles, and the outer shell comprises extracellular vesicles loaded with miR-15b-5p. This core-shell design leverages the targeting ability of the magnetic core and the gene drug-loading capacity of the outer layer to achieve precise drug delivery. The system, enhanced by magnetic targeting, delivers miR-15b-5p directly to osteoclasts, downregulating GFAP expression, inhibiting osteoclast differentiation, and promoting bone formation.

Thus, it is evident that different multilayer functionalization strategies each offer unique advantages. The multilayer structure endows functionalized nanoparticles with remarkable flexibility and versatility, enabling them to meet various complex clinical need.143 On one hand, the enhanced mechanical strength provided by the multilayer design allows functionalized nanoparticles to play a crucial role in long-term support and stabilization in bone repair.144 On the other hand, multilayer modifications can load a variety of drugs, combining anti-inflammatory effects, promotion of osteogenesis, and inhibition of osteoclasts, providing multiple therapeutic effects.145 This characteristic demonstrates the strong synergistic potential of multilayer functionalization in the multidimensional treatment of OP. However, multilayer structures also present certain challenges. First, the increased complexity of the structure can complicate the synthesis process, raising both costs and technical barriers.146 Moreover, precisely controlling the release time and concentration of drugs from different layers, to avoid premature depletion or insufficient release during treatment, remains a key challenge in design.147 This is particularly true for gene therapy, where highly complex structures may introduce uncertainty, potentially affecting the final therapeutic outcome.148 Therefore, selecting the appropriate multilayer functionalization strategy must balance various functionalities based on therapeutic needs. If the goal is to enhance mechanical properties, prioritizing material strength is crucial. Conversely, if drug targeting and controlled release are paramount, the focus should be on layers that ensure precise release, ultimately providing the most personalized and effective treatment plan for the patient.

Application Mechanism of Functionalized Nanoparticles in the Treatment of OP

The treatment of OP presents significant challenges due to the complex interplay of inflammation, oxidative stress, and disrupted bone metabolism. Traditional therapeutic approaches often focus on isolated pathways, such as reducing inflammation or inhibiting bone resorption, but rarely address the multifactorial nature of the disease.149,150 The advent of functionalized nanoparticles offers a promising solution, providing a versatile platform that can simultaneously target multiple pathogenic mechanisms. By enhancing drug delivery precision, controlling release rates, and minimizing systemic side effects, these nanoparticles enable more effective management of osteoporosis.151 This chapter explores the mechanisms by which functionalized nanoparticles are employed to combat OP, focusing on their roles in modulating inflammation, combating oxidative stress, promoting bone formation, and inhibiting osteoclastogenesis, as detailed in Table 2. These strategies represent a significant advancement in osteoporosis treatment, opening doors to more comprehensive and personalized therapeutic approaches.

Table 2 Overview of the Therapeutic Efficacy of Functionalized Nanoparticles in OP Treatment

Anti-Inflammatory and Antioxidant

Inflammation and oxidative stress play critical roles in the onset and progression of OP, with their interplay exacerbating bone tissue damage and disrupting the balance between bone resorption and regeneration.168,169 Inflammation activates osteoclasts through the release of pro-inflammatory factors such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6), thereby increasing bone resorption.170 Simultaneously, oxidative stress generates excessive reactive oxygen species (ROS), directly damaging bone cells and inducing apoptosis.171 The combined effects of these two pathological states not only result in decreased bone mineral density and increased bone fragility but also significantly elevate the risk of fractures. Specifically, inflammation accelerates the formation of osteoclasts and inhibits the function of osteoblasts, aggravating the progression of OP. Meanwhile, oxidative stress impairs both osteoblast and osteoclast functions, disrupting normal bone remodeling and reducing the synthesis of the bone matrix.

Compounding this complexity is the fact that the presence of OP further intensifies inflammation and oxidative stress, creating a vicious cycle.172 Studies have shown that levels of inflammatory cytokines and oxidative stress markers are significantly elevated in patients with OP, leading to ongoing bone tissue damage.173 To address this issue, the delivery of antibiotics and/or antioxidants via multifunctional nanoparticles has emerged as a promising therapeutic approach.152,153 This strategy not only targets the underlying causes of the disease but also minimizes systemic drug exposure, reducing potential side effects. By functionalizing nanoparticles, drug concentrations can be increased at the site of damage, enhancing anti-inflammatory effects and ultimately promoting bone regeneration and repair.174 Chen and colleagues encapsulated quercetin into ROS-responsive nanoparticles to leverage its antioxidant, anti-inflammatory, and glucose metabolism-regulating properties, thereby significantly enhancing the osteogenic differentiation potential of mesenchymal stem cells (MSCs) for osteoporotic bone repair.124 Gao and colleagues ingeniously utilized disulfide bonds to attach the antioxidant α-lipoic acid (LA) to the surface of nanoparticles, thereby achieving efficient drug delivery.154 Yu and colleagues skillfully encapsulated Oroxylin A, which holds anti-inflammatory potential, within the core of pH-responsive nanoparticles to achieve precise treatment for OP.155 This precise drug delivery method offers a new perspective for the treatment of OP and holds potential to improve bone health and the overall quality of life for patients.

Promote Bone Formation and Inhibit Bone Resorption

Anti-inflammatory and antioxidant strategies play a crucial role in the treatment of OP, effectively promoting bone formation and inhibiting bone resorption. By alleviating the inflammatory response and reducing oxidative stress levels, these approaches create a favorable environment for the normal function of bone cells, thereby enhancing the synthesis and mineralization of the bone matrix.175 However, the use of anti-inflammatory drugs and antioxidants alone often falls short of fully achieving bone formation and/or resorption inhibition.176 This limitation arises from the fact that these agents typically lack targeted delivery and precise mechanistic control, which may prevent them from sufficiently activating osteoblasts or effectively suppressing osteoclast activity.

Thus, utilizing drugs specifically designed to promote bone formation and/or inhibit bone resorption can significantly enhance the efficacy of OP treatment. Hydroxyapatite, a widely used biomaterial, plays a crucial role in bone regeneration due to its unique chemical and physical properties.177 Studies have shown that hydroxyapatite promotes the adhesion and proliferation of osteoblasts, providing excellent biocompatibility and structural support, thereby further enhancing the synthesis and mineralization of the bone matrix.178 Its porous structure not only facilitates the infiltration of bone cells and angiogenesis but also provides ample space for mineral deposition, accelerating the bone healing process.157 Parathyroid hormone (PTH) promotes bone formation by activating osteoblastic signaling pathways, enhancing calcium absorption and redistribution, thereby improving bone density.159 Moreover, simvastatin, a statin commonly used for cholesterol reduction, has been found to regulate bone metabolism and promote bone formation.160 This effect may be related to its ability to improve osteoblast function and upregulate osteogenesis-related gene expression, offering a novel therapeutic option for OP. In terms of inhibiting bone resorption, alendronate sodium is a commonly used drug that binds to the surface of osteoclasts, preventing their activation and function, thereby reducing bone matrix degradation and maintaining bone density.179 Additionally, bicarbonate has been suggested to mitigate the negative effects of bone resorption by regulating the body’s acid-base balance, helping to preserve bone health.180

While anti-inflammatory and antioxidant strategies positively impact the treatment of OP, adopting therapeutic approaches that directly promote bone formation and inhibit bone resorption can provide a more effective means of addressing the disease and offering patients a comprehensive treatment plan.181 Future research should continue to explore the optimal combinations and applications of these therapies to achieve improved clinical outcomes.

Other

In the context of OP, inflammatory states and immune dysregulation can lead to the abnormal activation of T cells.182 These over-activated T cells not only secrete pro-inflammatory cytokines but also promote osteoclastogenesis, accelerating bone resorption and resulting in decreased bone density and increased bone fragility.183 To address this issue, Yang and colleagues184 developed multifunctional nanoparticles loaded with monocyte chemoattractant protein-1 (MCP-1) and Fas ligand (FasL) to enhance their targeting specificity and therapeutic efficacy. MCP-1 is a key chemokine that directs the migration of over-activated T cells to inflamed sites, while FasL regulates immune responses by inducing apoptosis in T cells.185,186 This design allows the nanoparticles to effectively target activated T cells and suppress their function, thereby reducing their contribution to bone resorption. The accumulation of senescent cells is also considered a major contributor to OP, as these cells release pro-inflammatory factors that induce local inflammation and inhibit normal bone remodeling.187 As shown in Figure 5, Xing and colleagues188 used multifunctional nanoparticles loaded with senolytic agents to selectively target and eliminate senescent cells, significantly improving bone density and strength. Once the underlying “crises” inducing OP have been mitigated, a healthy bone-vascular coupling is essential to ensure adequate nutrient and oxygen supply to bone tissue, which is necessary for maintaining its normal structure and function. Research has shown that the synergy between angiogenesis and bone remodeling is fundamental to bone health, and damage to either process can lead to decreased bone density and an increased risk of fractures.189 Therefore, strategies to promote bone-vascular coupling are critical for alleviating the symptoms of OP. Zheng and colleagues190 functionalized nanoparticles by modifying their surface with specific bioactive molecules such as vascular endothelial growth factor (VEGF) and bone-related factors to enhance their affinity for both bone tissue and vascular endothelial cells. These nanoparticles are capable of selectively delivering bioactive molecules, promoting both angiogenesis and the proliferation of osteoblasts. Through this strategy, functionalized nanoparticles not only improve the nutritional supply to bone tissue but also enhance the repair of the bone matrix, thereby alleviating the symptoms of OP.191 This innovative therapeutic approach provides a new perspective for managing OP, highlighting the importance of bone-vascular coupling in bone health, and pointing to new directions for future research.

Figure 5 Preparation and Characterization of Bone-Targeted Liposomes. (A) Schematic illustration of the binding of the bone-targeting peptide (DSS)6 to bone tissue; (B) Synthetic route of DSPE-PEG2000-(DSS)6; (C) Structural diagram of quercetin-loaded bone-targeted liposomes; (