Nanomaterial Nitric Oxide Delivery in Traumatic Orthopedic Regenerative Medicine

Abstract

Achieving bone fracture union after trauma represents a major challenge for the orthopedic surgeon. Fracture non-healing has a multifactorial etiology and there are many risk factors for non-fusion. Environmental factors such as wound contamination, infection, and open fractures can contribute to non-healing, as can patient specific factors such as poor vascular status and improper immunologic response to fracture. Nitric oxide (NO) is a small, neutral, hydrophobic, highly reactive free radical that can diffuse across local cell membranes and exert paracrine functions in the vascular wall. This molecule plays a role in many biologic pathways, and participates in wound healing through decontamination, mediating inflammation, angiogenesis, and tissue remodeling. Additionally, NO is thought to play a role in fighting wound infection by mitigating growth of both Gram negative and Gram positive pathogens. Herein, we discuss recent developments in NO delivery mechanisms and potential implications for patients with bone fractures. NO donors are functional groups that store and release NO, independent of the enzymatic actions of NOS. Donor molecules include organic nitrates/nitrites, metal-NO complexes, and low molecular weight NO donors such as NONOates. Numerous advancements have also been made in developing mechanisms for localized nanomaterial delivery of nitric oxide to bone. NO-releasing aerogels, sol- gel derived nanomaterials, dendrimers, NO-releasing micelles, and core cross linked star (CCS) polymers are all discussed as potential avenues of NO delivery to bone. As a further target for improved fracture healing, 3d bone scaffolds have been developed to include potential for nanoparticulated NO release. These advancements are discussed in detail, and their potential therapeutic advantages are explored. This review aims to provide valuable insight for translational researchers who wish to improve the armamentarium of the feature trauma surgeon through use of NO mediated augmentation of bone healing.

Keywords: biologic; biologic therapy; bone; bone healing; fracture repair; nitric oxide; osteoinduction.

Figure 1

Stages of secondary bone healing (endochondral ossification). The stages of fracture healing are partially overlapping. (1) Coagulation and acute inflammatory response—initial stabilization, recruitment of inflammatory cells and cytokines. (2) Repair—(a) Revascularization, soft cartilage callus. Chondroblasts derived from MSCs deposit cartilage, mature to chondrocytes that express VEGF, inducing neovascularization; (b) Hard bony callus—hyperproliferative chondrocytes transdifferentiate to osteoblasts and osteocytes, tissue invaded by osteoblasts; (3) Remodeling—deposition of lamellar bone and restoration of pre-injury anatomic dimensions. This figure was created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by-sa/3.0/legalcode); https://smart.servier.com.

Figure 2

Immune cells of the fracture healing cascade for each stage of bone healing. The cellular contributions to fracture healing are described. Starting with platelets, a cascade of events that transition the bone from an inflammatory period to a proliferative period noted by soft and hard callus formation and predominated by reparative cells such as osteoblasts and osteoclasts. This phase then transitions to the remodeling phase where the healing site is structurally reinforced. Macrophage, being present throughout the healing cascade, release NO and play an important role throughout all stages of repair. Each stage of bone healing has a different spectrum of immune cells present at the fracture site. Reproduced with permission. This illustration was adapted by Martijn Hofman [licensed under Creative Commons CC-BY-SA 4.0] using the original illustration from Baht et al. (2018) [licensed under Creative commons CC-BY 4.0 (http://creativecommons.org/licenses/by/4.0/)] and images from Smart Servier Medical Art [licensed under Creative Commons CC-BY-SA 3.0 (https://smart.servier.com/)].

Figure 3

Interaction of NO with the immune cells of fracture repair. NO is involved in fracture repair through increasing blood vessel diameter, attracting immune cells, and mediating osteoblast/osteoclast differentiation and function. This figure was adapted using the original illustration from Medhat et al. (2019). [licensed under Creative commons CC-BY 4.0 (http://creativecommons.org/licenses/by/4.0/)] and images by Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by-sa/3.0/legalcode); https://smart.servier.com.

Figure 4

Effects of NO balance on bone remodeling. Low NO concentration stimulates osteoblast proliferation and bone formation. High NO concentration induces osteoblast apoptosis. This figure was created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by-sa/3.0/legalcode); https://smart.servier.com.

Figure 5

Chemical structures of NO donors studied in bone and tissue healing. There are a variety of NO donor molecules that store and release NO, independent of the enzymatic actions of NOS.

Figure 6

NO release mechanisms from NONOates and RSNOs. Primary release mechanisms of two major classes of NO donors, as described by Schmidt et al. (1997) and Nichols et al. (2012). Of note, in addition to releasing NO, RSNOs can also act through S-nitrosylation of protein thiols in tissues, a process called transnitrosation (Broniowska et al., 2013).

Figure 7

Synthesis of xerogels and sol gels. Both xerogels and sol-gels are nanomaterials that may function as NO-releasing coatings for implanted materials when NO donors are conjugated to the gel. This figure is a reproduction of an original image by Claudionico, which is licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by-sa/3.0/legalcode); https://commons.wikimedia.org/wiki/File:Sol-Gel_Technology_Scheme.png.

Figure 8

Dendrimers and Core Cross Linked Star Polymers for NO release. (Left) Chemical structure of a common NO-releasing polypropylenimine dendrimer. Reprinted (adapted) with permission from Stasko and Schoenfisch (2006). Copyright 2006 American Chemical Society. (Right) Formation of a CCS polymer and conjugation with NO. Reprinted (adapted) with permission from Duong et al. (2014). Copyright 2014 American Chemical Society.

Figure 9

Micelles prolong release of NO to bone. Compared to NONOates which simply release free NO that can be easily degraded by hemoglobin, micelles protect NO from degradation, thus prolonging the action of NO. This figure was reprinted (adapted) with permission from the original illustration by Lin et al. (2018).

Figure 10

Representation of a 3D Bone Scaffold releasing nano-NO. The biologic benefits of NO on bone healing can be realized through incorporation of NO into a 3D bone scaffold and implantation of that scaffold into a fracture site. This figure was created using Servier Medical Art templates, which are licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by-sa/3.0/legalcode); https://smart.servier.com.