Bone-Fracture-Targeted Dasatinib-Oligoaspartic Acid Conjugate Potently Accelerates Fracture Repair

Biomimicking design of artificial periosteum for promoting bone healing

Background

Periosteum is a vascularized tissue membrane covering the bone surface and plays a decisive role in bone reconstruction process after fracture. Various artificial periosteum has been developed to assist the allografts or bionic bone scaffolds in accelerating bone healing. Recently, the biomimicking design of artificial periosteum has attracted increasing attention due to the recapitulation of the natural extracellular microenvironment of the periosteum and has presented unique capacity to modulate the cell fates and ultimately enhance the bone formation and improve neovascularization.

Methods

A systematic literature search is performed and relevant findings in biomimicking design of artificial periosteum have been reviewed and cited.

Results

We give a systematical overview of current development of biomimicking design of artificial periosteum. We first summarize the universal strategies for designing biomimicking artificial periosteum including biochemical biomimicry and biophysical biomimicry aspects. We then discuss three types of novel versatile biomimicking artificial periosteum including physical-chemical combined artificial periosteum, heterogeneous structured biomimicking periosteum, and healing phase-targeting biomimicking periosteum. Finally, we comment on the potential implications and prospects in the future design of biomimicking artificial periosteum.

Conclusion

This review summarizes the preparation strategies of biomimicking artificial periosteum in recent years with a discussion of material selection, animal model adoption, biophysical and biochemical cues to regulate the cell fates as well as three types of latest developed versatile biomimicking artificial periosteum. In future, integration of innervation, osteochondral regeneration, and osteoimmunomodulation, should be taken into consideration when fabricating multifunctional artificial periosteum.

The Translational Potential of this Article: This study provides a holistic view on the design strategy and the therapeutic potential of biomimicking artificial periosteum to promote bone healing. It is hoped to open a new avenue of artificial periosteum design with biomimicking considerations and reposition of the current strategy for accelerated bone healing.

Systemic therapy of MSCs in bone regeneration: a systematic review and meta-analysis

Objectives

Over the past decades, many studies focused on mesenchymal stem cells (MSCs) therapy for bone regeneration. Due to the efficiency of topical application has been widely dicussed and systemic application was also a feasible way for new bone formation, the aim of this study was to systematically review systemic therapy of MSCs for bone regeneration in pre-clinical studies.

Methods

The article search was conducted in PubMed and Embase databases. Original research articles that assessed potential effect of systemic application of MSCs for bone regeneration in vivo were selected and evaluated in this review, according to eligibility criteria. The efficacy of MSC systemic treatment was analyzed by random effects meta-analysis, and the outcomes were expressed in standard mean difference (SMD) and its 95% confidence interval. Subgroup analyses were conducted on animal species and gender, MSCs types, frequency and time of injection, and bone diseases.

Results

Twenty-three articles were selected in this review, of which 21 were included in meta-analysis. The results showed that systemic therapy increased bone mineral density (SMD 3.02 [1.84, 4.20]), bone volume to tissue volume ratio (2.10 [1.16, 3.03]), and the percentage of new bone area (7.03 [2.10, 11.96]). Bone loss caused by systemic disease tended to produce a better response to systemic treatment (p=0.05 in BMD, p=0.03 in BV/TV).

Conclusion

This study concluded that systemic therapy of MSCs promotes bone regeneration in preclinical experiments. These results provided important information for the systemic application of MSCs as a potential application of bone formation in further animal experiments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02456-w.

Osteoporosis: Current and Emerging Therapies Targeted to Immunological Checkpoints

Osteoporosis is a skeletal pathology characterized by compromised bone strength leading to increased risk of fracture, mainly the spine and hip fractures. Osteoporosis affects more than 200 million people worldwide and because of the skeletal fractures it causes, represents a major cause of morbidity, disability and mortality in older people. Recently, the new discoveries of osteoimmunology have clarified many of the pathogenetic mechanisms of osteoporosis, helping to identify new immunological targets for its treatment opening the way for new and effective therapies with biological drugs. Currently, there are basically two monoclonal antibodies for osteoporosis therapy: denosumab and romosozumab. Here, we focus on the modern approach to the osteoporosis management and in particular, on current and developing biologic drugs targeted to new immunological checkpoints, in the landscape of osteoimmunology.

Analysis of the bone fracture targeting properties of osteotropic ligands

PURPOSE

Although more than 18,000,000 fractures occur each year in the US, methods to promote fracture healing still rely primarily on fracture stabilization, with use of bone anabolic agents to accelerate fracture repair limited to rare occasions when the agent can be applied to the fracture surface. Because management of broken bones could be improved if bone anabolic agents could be continuously applied to a fracture over the entire course of the healing process, we undertook to identify strategies that would allow selective concentration of bone anabolic agents on a fracture surface following systemic administration. Moreover, because hydroxyapatite is uniquely exposed on a broken bone, we searched for molecules that would bind with high affinity and specificity for hydroxyapatite. We envisioned that by conjugating such osteotropic ligands to a bone anabolic agent, we could acquire the ability to continuously stimulate fracture healing.

RESULTS

Although bisphosphonates and tetracyclines were capable of localizing small amounts of peptidic payloads to fracture surfaces 2-fold over healthy bone, their specificities and capacities for drug delivery were significantly inferior to subsequent other ligands, and were therefore considered no further. In contrast, short oligopeptides of acidic amino acids were found to localize a peptide payload to a bone fracture 91.9 times more than the control untargeted peptide payload. Furthermore acidic oligopeptides were observed to be capable of targeting all classes of peptides, including hydrophobic, neutral, cationic, anionic, short oligopeptides, and long polypeptides. We further found that highly specific bone fracture targeting of multiple peptidic cargoes can be achieved by subcutaneous injection of the construct.

CONCLUSIONS

Using similar constructs, we anticipate that healing of bone fractures in humans that have relied on immobilization alone can be greately enhanced by continuous stimulation of bone growth using systemic administration of fracture-targeted bone anabolic agents.

Fluorescent Self-Threaded Peptide Probes for Biological Imaging

A general synthetic method creates a new class of covalently connected, self-threaded, fluorescent molecular probes with figure-eight topology, an encapsulated deep-red fluorophore, and two peripheral peptide loops. The globular molecular shape and rigidified peptide loops enhance imaging performance by promoting water solubility, eliminating probe self-aggregation, and increasing probe stability. Moreover, the peptide loops determine the affinity and selectivity for targets within complex biological samples such as cell culture, tissue histology slices, or living subjects. For example, a probe with cell-penetrating peptide loops targets the surface of cell plasma membranes, whereas, a probe with bone-targeting peptide loops selectively stains the skeleton within a living mouse. The unique combination of bright deep-red fluorescence, high stability, and predictable peptide-based targeting is ideal for photon intense fluorescence microscopy and biological imaging.

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.

Nanocomposites drug delivery systems for the healing of bone fractures

The skeletal system is fundamental for the structure and support of the body consisting of bones, cartilage, and connective tissues. Poor fracture healing is a chief clinical problem leading to disability, extended hospital stays and huge financial liability. Even though most fractures are cured using standard clinical methods, about 10% of fractures are delayed or non-union. Despite decades of progress, the bone-targeted delivery system is still restricted due to the distinctive anatomical bone features. Recently, various novel nanocomposite systems have been designed for the cell-specific targeting of bone, enhancing drug solubility, improving drug stability and inhibiting drug degradation so that it can reach its target site without being removed in the systemic circulation. Such targeting systems could consist of biological compounds i.e. bone marrow stem cells (BMSc), growth factors, RNAi, parathyroid hormone or synthetic compounds, i.e. bisphosphonates (BPs) and calcium phosphate cement. Hydrogels and nanoparticles are also being employed for fracture healing. In this review, we discussed the normal mechanism of bone healing and all the possible drug delivery systems being employed for the healing of the bone fracture.

Effects of 17β-HSD2 inhibition in bones on osteoporosis based on an animal rat model

Hormone replacement therapy is a viable option to protect bone from postmenopausal osteoporosis. Systemically elevated estrogen levels, however, are disadvantageous because of the risk of harmful side effects in other organs. The rationale of the study presented here is to target a key enzyme in estradiol (E2) and testosterone (T) metabolism to increase E2 levels in an organ-specific manner, thereby avoiding the disadvantages of systemically increased E2 levels. The 17ß-hydroxysteroid dehydrogenase (17β-HSD2), which is e.g. expressed in bone, catalyzes the oxidation of E2 and T into estrone (E1) and androstenedione. We postulate that inhibiting 17β-HSD2 should lead to elevated E2 and T levels in organs expressing the enzyme. Therefore, we can use the benefits of E2 directly, or those of T following aromatization into E2, in the bone without affecting systemic levels. We tested for the first time, the novel and potent 17β-HSD2 inhibitor, compound 24 (C24), to explore the therapeutic potential of a 17β-HSD2 inhibition in an ovariectomy (ovx)-induced rat model of bone loss. We tested the inhibitor alone and, together with low dose estrogen supplementation to model estrogen levels in the postmenopausal situation. Female mature Wistar-Hannover rats were treated for 8 weeks with doses of 2, 10, 50 mg C24 per kg body weight per day alone or in the presence of estradiol benzoate (E2B) supplementation to alleviate ovx-induced bone loss. Ovx placebo and sham operated animals served as negative and positive controls. The experiment was evaluated regarding aspects of efficacy and safety: Bone was analyzed to evaluate bone protective effects, and uterus for potential, unwanted E2-mediated side effects. We observed a good bioavailability of C24 as very high plasma concentrations were measured, up to a group mean of 15,412 nM for the ovx C24-high group. Histomorphometrical analyses and in vivo &ex vivo μCT revealed significant bone protective effects for the lowest inhibitor concentration used. Irrespective of the plasma concentration, no proliferative effects in the uterus could be observed. These results support our approach of intracellular targeting key enzymes of E2 and T metabolism to increase E2 and T levels in an organ specific manner.

Cyclooxygenase 2 augments osteoblastic but suppresses chondrocytic differentiation of CD90+ skeletal stem cells in fracture sites

Cyclooxygenase 2 (COX-2) is essential for normal tissue repair. Although COX-2 is known to enhance the differentiation of mesenchymal stem cells (MSCs), how COX-2 regulates MSC differentiation into different tissue-specific progenitors to promote tissue repair remains unknown. Because it has been shown that COX-2 is critical for normal bone repair and local COX-2 overexpression in fracture sites accelerates fracture repair, this study aimed to determine the MSC subsets that are targeted by COX-2. We showed that CD90⁺ mouse skeletal stem cells (mSSCs; i.e., CD45^-Tie2^-AlphaV⁺ MSCs) were selectively recruited by macrophage/monocyte chemoattractant protein 1 into fracture sites following local COX-2 overexpression. In addition, local COX-2 overexpression augmented osteoblast differentiation and suppressed chondrocyte differentiation in CD90⁺ mSSCs, which depended on canonical WNT signaling. CD90 depletion data demonstrated that local COX-2 overexpression targeted CD90⁺ mSSCs to accelerate fracture repair. In conclusion, CD90⁺ mSSCs are promising targets for the acceleration of bone repair.

Anabolic Therapies in Osteoporosis and Bone Regeneration

Osteoporosis represents the most common bone disease worldwide and results in a significantly increased fracture risk. Extrinsic and intrinsic factors implicated in the development of osteoporosis are also associated with delayed fracture healing and impaired bone regeneration. Based on a steadily increasing life expectancy in modern societies, the global implications of osteoporosis and impaired bone healing are substantial. Research in the last decades has revealed several molecular pathways that stimulate bone formation and could be targeted to treat both osteoporosis and impaired fracture healing. The identification and development of therapeutic approaches modulating bone formation, rather than bone resorption, fulfils an essential clinical need, as treatment options for reversing bone loss and promoting bone regeneration are limited. This review focuses on currently available and future approaches that may have the potential to achieve these aims.