Molecular Design of Bisphosphonate-Modified Proteins for Efficient Bone Targeting In Vivo

Optimizing Delivery of Therapeutic Growth Factors for Bone and Cartilage Regeneration

Bone- and cartilage-related diseases, such as osteoporosis and osteoarthritis, affect millions of people worldwide, impairing their quality of life and increasing mortality. Osteoporosis significantly increases the bone fracture risk of the spine, hip, and wrist. For successful fracture treatment and to facilitate proper healing in the most complicated cases, one of the most promising methods is to deliver a therapeutic protein to accelerate bone regeneration. Similarly, in the setting of osteoarthritis, where degraded cartilage does not regenerate, therapeutic proteins hold great promise to promote new cartilage formation. For both osteoporosis and osteoarthritis treatments, targeted delivery of therapeutic growth factors, with the aid of hydrogels, to bone and cartilage is a key to advance the field of regenerative medicine. In this review article, we propose five important aspects of therapeutic growth factor delivery for bone and cartilage regeneration: (1) protection of protein growth factors from physical and enzymatic degradation, (2) targeted growth factor delivery, (3) controlling GF release kinetics, (4) long-term stability of regenerated tissues, and (5) osteoimmunomodulatory effects of therapeutic growth factors and carriers/scaffolds.

Alendronate modified mPEG-PLGA nano-micelle drug delivery system loaded with astragaloside has anti-osteoporotic effect in rats

Astragaloside (AS) has an anti-osteoporotic effect, but its poor water solubility and low bioavailability limit its application. In this study, a novel nano-carrier with bone targeting was prepared by modifying mPEG-PLGA with alendronate (AL) before incorporation into astragaloside nano-micelles (AS-AL-mPEG-PLGA) to enhance the oral bioavailability, bone targeting and anti-osteoporosis effect of AS. The release behavior of AS-AL-mPEG-PLGA in vitro was investigated via dialysis. The pharmacokinetics of AS-AL-mPEG-PLGA was studied in Sprague-Dawley (SD) rats. The cytotoxicity of AS-AL-mPEG-PLGA in vitro (via MTT method), coupled with bone targeting ability in vitro and in vivo were evaluated. The therapeutic effects of free AS and AS-AL-mPEG-PLGA (ELISA, micro-CT, H&E staining) were compared in osteoporotic rats. AS-AL-mPEG-PLGA with smaller particle size (45.3 ± 3.8 nm) and high absolute zeta potential (−23.02 ± 0.51 mV) were successfully prepared, wherein it demonstrated higher entrapment efficiency (96.16 ± 0.18%), a significant sustained-release effect for 96 h and acceptable safety within 10–200 μg/mL. AS-AL-mPEG-PLGA could enhance the hydroxyapatite affinity and bone tissue concentration of AS. The relative bioavailability of AS-AL-mPEG-PLGA was 233.90% compared with free AS. In addition, the effect of AS in reducing serum levels of bone metabolism-related indicators, restoring the bone microarchitecture and improving bone injury could be enhanced by AS-AL-mPEG-PLGA. AS-AL-mPEG-PLGA with small particle size, good stability, remarkable sustained-release effect, safety and bone targeting was successfully constructed in this experiment to potentially improve the oral bioavailability and anti-osteoporosis effect of AS. Thus, AS-AL-mPEG-PLGA may be a promising strategy to prevent and treat osteoporosis.

L-Serine-Modified Poly-L-Lysine as a Biodegradable Kidney-Targeted Drug Carrier for the Efficient Radionuclide Therapy of Renal Cell Carcinoma

In the present study, L-serine (Ser)-modified poly-L-lysine (PLL) was synthesized to develop a biodegradable, kidney-targeted drug carrier for efficient radionuclide therapy in renal cell carcinoma (RCC). Ser-PLL was labeled with ¹¹¹In/⁹⁰Y via diethylenetriaminepentaacetic acid (DTPA) chelation for biodistribution analysis/radionuclide therapy. In mice, approximately 91% of the total dose accumulated in the kidney 3 h after intravenous injection of ¹¹¹In-labeled Ser-PLL. Single-photon emission computed tomography/computed tomography (SPECT/CT) imaging showed that ¹¹¹In-labeled Ser-PLL accumulated in the renal cortex following intravenous injection. An intrarenal distribution study showed that fluorescein isothiocyanate (FITC)-labeled Ser-PLL accumulated mainly in the renal proximal tubules. This pattern was associated with RCC pathogenesis. Moreover, ¹¹¹In-labeled Ser-PLL rapidly degraded and was eluted along with the low-molecular-weight fractions of the renal homogenate in gel filtration chromatography. Continuous Ser-PLL administration over five days had no significant effect on plasma creatinine, blood urea nitrogen (BUN), or renal histology. In a murine RCC model, kidney tumor growth was significantly inhibited by the administration of the beta-emitter ⁹⁰Y combined with Ser-PLL. The foregoing results indicate that Ser-PLL is promising as a biodegradable drug carrier for kidney-targeted drug delivery and efficient radionuclide therapy in RCC.

Induction of osteogenesis by bone-targeted Notch activation

Declining bone mass is associated with aging and osteoporosis, a disease characterized by progressive weakening of the skeleton and increased fracture incidence. Growth and lifelong homeostasis of bone rely on interactions between different cell types including vascular cells and mesenchymal stromal cells (MSCs). As these interactions involve Notch signaling, we have explored whether treatment with secreted Notch ligand proteins can enhance osteogenesis in adult mice. We show that a bone-targeting, high affinity version of the ligand Delta-like 4, termed Dll4_(E12), induces bone formation in male mice without causing adverse effects in other organs, which are known to rely on intact Notch signaling. Due to lower bone surface and thereby reduced retention of Dll4_(E12), the same approach failed to promote osteogenesis in female and ovariectomized mice but strongly enhanced trabecular bone formation in combination with parathyroid hormone. Single cell analysis of stromal cells indicates that Dll4_(E12) primarily acts on MSCs and has comparably minor effects on osteoblasts, endothelial cells, or chondrocytes. We propose that activation of Notch signaling by bone-targeted fusion proteins might be therapeutically useful and can avoid detrimental effects in Notch-dependent processes in other organs.

Advances in the occurrence and biotherapy of osteoporosis

Osteoporosis (OP) is a bone metabolic disease, is characterized by degeneration of bone structure and decreased bone mass. It happens in more than 1/3 women and 1/5 men of over than 50 years old, which affects the health and lives of people. The main mechanism of OP is mainly that the dynamic balance between the bone formation and resorption is broken, so that bone resorption is more than bone formation. It is prone to result in bone metabolism disorder. There are many precipitating factor such as elder age, low hormone level, genetic factors and bad hobbies. At the same time, the occurrence of the OP and its complications has different degrees of impact on people's quality of life. Based on the current understanding of the OP, we summarized the etiology, current clinical drugs and potential targeting therapy for OP. Although the research have made many progress in explore what is the novel mechanism and how to improve the effect, there are still many problems in the treatment method that limit its application prospects and need to be solved. In this review, we mainly focus on the mechanism of OP and related research on the targeted treatment of OP. Hopefully, our summary will provide a reference to develop some novel strategies for the target therapy of OP.

Using Alendronic Acid Coupled Fluorescently Labelled SM Liposomes as a Vehicle for Bone Targeting

Background:

We recently developed a liposomal nanoparticle system that can be used for drug delivery and simultaneously be monitored by optical or photoacoustic imaging devices. Here we tested the efficacy of alendronate as a homing molecule in SM-liposomes for bone targeting.

Methods:

Alendronate was immobilized covalently on the liposomal surface and the fluorescent dye indocyanine green was used as a payload in the liposomes. The indocyanine green delivery was analyzed by 3D optical tomography, optical fluorescence scanner, photoacoustic imaging, and by ex-vivo biodistribution studies.

Results:

The results show that the alendronate, coupled to the liposomal surface, increases sphingomyelin containing liposome targeting up to several-folds.

Conclusion:

The alendronate targeted liposomes open possibilities for an application in active bone targeting.

Dendrimer-based micelles with highly potent targeting to sites of active bone turnover for the treatment of bone metastasis

Bone-drug targeting therapies using nanoparticles based on targeting ligands remain challenging due to their uptake clearance at non-target sites such as the liver, kidney, and spleen. Furthermore, the distribution sites of nanoparticles in bones have not been fully investigated, thus halting the development of more effective bone metastasis treatment strategies. In this study, we developed nanoparticles self-assembled from cholesterol-terminated, polyethylene glycol-conjugated, aspartic acid (Asp)-modified polyamidoamine dendrimer (Asp-PAMAM-Micelles) with targeting to active bone turnover sites associated with bone metastasis pathogenesis. On analysis through whole-body single photon emission computed tomography/computed tomography (SPECT/CT) imaging, ¹¹¹In-Asp-PAMAM-Micelles showed high specificity to active bone turnover sites (especially the joints in the lower limbs, shoulder, and pelvis) after intravenous injection in mice. The lower limb bone uptake clearance for ¹¹¹In-Asp-PAMAM-Micelles encapsulating paclitaxel (PTX) was 3.5-fold higher than that for ¹¹¹In-unmodified PAMAM-Micelles (PTX). ³H-PTX encapsulated Asp-PAMAM-Micelles effectively accumulated in the lower limb bones in a similar manner as the ¹¹¹In-Asp-PAMAM-Micelles (PTX). In a bone metastatic tumor mouse model, the tumor growth in the lower limb bones was significantly inhibited by injection of Asp-PAMAM-Micelles (PTX) compared to unmodified PAMAM-Micelles (PTX). Our results demonstrate that Asp-PAMAM-Micelles are sophisticated drug delivery systems for highly potent targeting to active bone turnover sites.

Bone-Specific Drug Delivery for Osteoporosis and Rare Skeletal Disorders

PURPOSE OF REVIEW

The skeletal system provides an important role to support body structure and protect organs. The complexity of its architecture and components makes it challenging to deliver the right amount of the drug into bone regions, particularly avascular cartilage lesions. In this review, we describe the recent advance of bone-targeting methods using bisphosphonates, polymeric oligopeptides, and nanoparticles on osteoporosis and rare skeletal diseases.

RECENT FINDINGS

Hydroxyapatite (HA), a calcium phosphate with the formula Ca10(PO4)6(OH)2, is a primary matrix of bone mineral that includes a high concentration of positively charged calcium ion and is found only in the bone. This unique feature makes HA a general targeting moiety to the entire skeletal system. We have applied bone-targeting strategy using acidic amino acid oligopeptides into lysosomal enzymes, demonstrating the effects of bone-targeting enzyme replacement therapy and gene therapy on bone and cartilage lesions in inherited skeletal disorders. Virus or no-virus gene therapy using techniques of engineered capsid or nanomedicine has been studied preclinically for skeletal diseases. Efficient drug delivery into bone lesions remains an unmet challenge in clinical practice. Bone-targeting therapies based on gene transfer can be potential as new candidates for skeletal diseases.

Bone-Targeting Systems to Systemically Deliver Therapeutics to Bone Fractures for Accelerated Healing

PURPOSE OF REVIEW

Compared with the current standard of implanting bone anabolics for fracture repair, bone fracture-targeted anabolics would be more effective, less invasive, and less toxic and would allow for control over what phase of fracture healing is being affected. We therefore sought to identify the optimal bone-targeting molecule to allow for systemic administration of therapeutics to bone fractures.

RECENT FINDINGS

We found that many bone-targeting molecules exist, but most have been developed for the treatment of bone cancers, osteomyelitis, or osteoporosis. There are a few examples of bone-targeting ligands that have been developed for bone fractures that are selective for the bone fracture over the body and skeleton. Acidic oligopeptides have the ideal half-life, toxicity profile, and selectivity for a bone fracture-targeting ligand and are the most developed and promising of these bone fracture-targeting ligands. However, many other promising ligands have been developed that could be used for bone fractures.

Biomaterial-assisted local and systemic delivery of bioactive agents for bone repair

Although bone tissues possess an intrinsic capacity for repair, there are cases where bone healing is either impaired or insufficient, such as fracture non-union, osteoporosis, osteomyelitis, and cancers. In these cases, treatments like surgical interventions are used, either alone or in combination with bioactive agents, to promote tissue repair and manage associated clinical complications. Improving the efficacy of bioactive agents often requires carriers, with biomaterials being a pivotal player. In this review, we discuss the role of biomaterials in realizing the local and systemic delivery of biomolecules to the bone tissue. The versatility of biomaterials enables design of carriers with the desired loading efficiency, release profile, and on-demand delivery. Besides local administration, systemic administration of drugs is necessary to combat diseases like osteoporosis, warranting bone-targeting drug delivery systems. Thus, chemical moieties with the affinity towards bone extracellular matrix components like apatite minerals have been widely utilized to create bone-targeting carriers with better biodistribution, which cannot be achieved by the drugs alone. Bone-targeting carriers combined with the desired drugs or bioactive agents have been extensively investigated to enhance bone healing while minimizing off-target effects. Herein, these advancements in the field have been systematically reviewed. STATEMENT OF SIGNIFICANCE: Drug delivery is imperative when surgical interventions are not sufficient to address various bone diseases/defects. Biomaterial-assisted delivery systems have been designed to provide drugs with the desired loading efficiency, sustained release, and on-demand delivery to enhance bone healing. By surveying recent advances in the field, this review outlines the design of biomaterials as carriers for the local and systemic delivery of bioactive agents to the bone tissue. Particularly, biomaterials that bear chemical moieties with affinity to bone are attractive, as they can present the desired bioactive agents to the bone tissue efficiently and thus enhance the drug efficacy for bone repair.

Development of PEGylated aspartic acid-modified liposome as a bone-targeting carrier for the delivery of paclitaxel and treatment of bone metastasis

To prevent bone metastasis, we developed polyethylene glycol (PEG)-conjugated aspartic acid (Asp)-modified liposomes (PEG-Asp-Lipo) as a bone-targeting carrier of paclitaxel (PTX) by using Asp-modified 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE-Asp). The affinity of Asp-modified liposomes to hydroxyapatite increased as the concentration of DPPE-Asp increased. The bone accumulation of [³H]-labeled PEG(2)-Asp(33)-Lipo was approximately 24.6% 360 min after intravenous injection in mice, in contrast to 5.4% and 6.7% of [³H]-labeled normal Lipo and PEG(2)-Lipo, respectively. Similarly, [¹⁴C]-labeled PTX encapsulated into PEG(2)-Asp(33)-Lipo predominantly accumulated in the bone. Furthermore, using an in situ imaging experiment, we observed that near-infrared fluorescence-labeled PEG(2)-Asp(33)-Lipo selectively accumulated in the bone near the joint after intravenous injection in mice. We also found that FITC-labeled PEG(2)-Asp(33)-Lipo predominantly accumulated on eroded and quiescent bone surfaces. In a bone metastatic tumor mouse model, in which B16-BL6/Luc cells were injected into the left ventricle of female C57BL/6 mice, metastatic bone tumor growth was significantly inhibited by an intravenous injection of PEG(2)-Asp(33)-liposomal PTX. In contrast, PEGylated liposomal PTX hardly affected the growth of metastatic bone tumors. These findings indicate that PEG(2)-Asp(33)-Lipo is a promising bone-targeting carrier for the delivery of PTX and treatment of bone metastasis.

Development of Bone Targeting Drugs

The skeletal system, comprising bones, ligaments, cartilage and their connective tissues, is critical for the structure and support of the body. Diseases that affect the skeletal system can be difficult to treat, mainly because of the avascular cartilage region. Targeting drugs to the site of action can not only increase efficacy but also reduce toxicity. Bone-targeting drugs are designed with either of two general targeting moieties, aimed at the entire skeletal system or a specific cell type. Most bone-targeting drugs utilize an affinity to hydroxyapatite, a major component of the bone matrix that includes a high concentration of positively-charged Ca²⁺. The strategies for designing such targeting moieties can involve synthetic and/or biological components including negatively-charged amino acid peptides or bisphosphonates. Efficient delivery of bone-specific drugs provides significant impact in the treatment of skeletal related disorders including infectious diseases (osteoarthritis, osteomyelitis, etc.), osteoporosis, and metabolic skeletal dysplasia. Despite recent advances, however, both delivering the drug to its target without losing activity and avoiding adverse local effects remain a challenge. In this review, we investigate the current development of bone-targeting moieties, their efficacy and limitations, and discuss future directions for the development of these specific targeted treatments.