Lipid nanoparticle: advanced drug delivery systems for promotion of angiogenesis in diabetic wounds

Abstract

Diabetic wound is one of the most challenge in healthcare, requiring innovative approaches to promote efficient healing. In recent years, lipid nanoparticle-based drug delivery systems have emerged as a promising strategy for enhancing diabetic wound repair by stimulating angiogenesis. These nanoparticles offer unique advantages, including improved drug stability, targeted delivery, and controlled release, making them promising in enhancing the formation of new blood vessels. In this review, we summarize the emerging advances in the utilization of lipid nanoparticles to deliver angiogenic agents and promote angiogenesis in diabetic wounds. Furthermore, we provide an in-depth exploration of key aspects, including the intricate design and fabrication of lipid nanoparticles, their underlying mechanisms of action, and a comprehensive overview of preclinical studies. Moreover, we address crucial considerations pertaining to safety and the translation of these innovative systems into clinical practice. By synthesizing and analyzing the available knowledge, our review offers valuable insights into the future prospects and challenges associated with utilizing the potential of lipid nanoparticle-based drug delivery systems for promoting robust angiogenesis in the intricate process of diabetic wound healing.

Keywords:

• Drug delivery
• nanoparticle
• diabetic wound
• lipid
• pharmacological therapy

Introduction

Diabetic wounds, with their substantial impact on healthcare, emerge as one of the most formidable burdens faced by the medical community, culminating in the relentless persistence of chronic non-healing ulcers and giving rise to a host of severe and potentially life-altering complications [1]. The impaired angiogenesis, which hinders the formation of new blood vessels, stands as a crucial factor contributing to the observed delays in wound healing among individuals with diabetes [2]. Recognizing the urgent need for innovative therapies to promote angiogenesis and accelerate the repair of diabetic wounds, researchers have increasingly focused on the development of advanced drug delivery systems (DDS) for facing the therapeutic challenge.

Lipid nanoparticles (LNPs) are bilayer vesicles formed spontaneously by phospholipids in water. LNPs are divided into aqueous core liposomes (ALNP) and lipid core liposomes (LLNP) according to their internal liquid composition. And LLNP are further classified into liquid lipid core (lipid nanoemulsion (LNE), lipid nanoencapsulation (LNC)), solid lipid nanoparticles (SLNs) and nanostructured lipid nanocarriers (NLCs) according to their internal properties [3]. The cavity structure is an inherent advantage for LNPs to load drugs, and the structure of lipid membrane is similar to the phospholipid bilayer of cells, which also enables LNPs to have good biocompatibility. Therefore, LNPs are often considered as high-quality drug delivery carriers [4, 5]. Meanwhile, the development of LNPs provides a promising option in wound healing strategies, offering unique and effective means of delivering angiogenic agents directly to the wound sites [6]. These nanoparticles possess functional properties, such as enhanced stability, efficient cellular uptake, and controlled release, rendering them ideal DDS for therapeutic molecules. By encapsulating and delivering angiogenic agents, LNPs can play a transformative role in facilitating the angiogenic process, thereby promoting the formation of new blood vessels and enhancing tissue regeneration in diabetic wounds [7].

Currently, emerging studies have underscored the considerable potential of LNPs-mediated drug delivery in promotion of angiogenesis for diabetic wound repair. For example, Kasiewicz and Whitehead [8] investigated the utilization of siRNA-loaded LNPs as a potential treatment for impaired wound healing. They formulated LNPs using an ionizable, degradable lipidoid and loaded them with TNFα-specific siRNA. These LNPs were topically applied to the wounds of both diabetic and nondiabetic mice. This study provides proof-of-concept evidence for the potential of using siRNA-loaded LNPs as a therapeutic approach to mitigate the complications of impaired wound healing in diabetes, particularly in the context of nonhealing diabetic foot ulcers.

In this comprehensive review, we aim to unravel the potential utilization of lipid nanoparticle-based DDS as advanced tools for promoting angiogenesis in the context of diabetic wound repair. First, we discuss the design and fabrication strategies employed in lipid LNPs, the selection and incorporation of angiogenic agents, the underlying mechanisms of action driving their effectiveness, and the outcomes from pertinent preclinical studies. Furthermore, we address critical considerations encompassing safety profiles, scalability, and the challenges associated with translating LNPs-based DDS into clinical practice. By shedding light on the future prospects and potential obstacles, our review aims to provide invaluable insights into the promising landscape of utilizing LNPs-based DDS to promote angiogenesis and revolutionize the treatment of diabetic wounds.

LNPs: design and fabrication

Functionality and characteristics of LNPs-based DDS

Owing to the excellent functions in tissue regeneration, such as high solubility potential, multifunctionality, excellent biocompatibility and biodegradability [9]. LNPs-based DDS have obtained considerable attention in diabetic wound therapy. These systems utilize lipids, particularly phospholipids, to form nanoparticles that exhibit remarkable capabilities in encapsulating and delivering therapeutic agents [9]. The unique properties and versatility of LNPs-based DDS make them a compelling choice for various applications, including diabetic wound repair.

One of the key characteristics of LNPs-based DDS lies in their inherent biocompatibility. Lipids are natural components of biological systems, making them well-tolerated by the human body [9]. This biocompatibility minimizes the risk of adverse reactions or toxicity, enhancing the safety profile of these systems when used for therapeutic purposes. By utilizing lipids as materials source of DDS, these systems can also provide increased stability to the encapsulated drugs, protecting them from degradation and preserving their pharmacological activity [10].

Another notable function of LNPs-based DDS is their ability to encapsulate a wide range of therapeutic agents, regardless of their solubility properties [11]. Lipids can accommodate both hydrophobic and hydrophilic drugs, allowing for the encapsulation of diverse therapeutic compounds. This versatility is particularly valuable in the context of diabetic wound repair, as it enables the delivery of various drugs that target different aspects of the wound healing process. For example, growth factors, antimicrobial agents, and anti-inflammatory drugs can all be encapsulated within lipid nanoparticles, providing a multifaceted approach to enhance wound healing [12].

Moreover, LNPs-based DDS can be modified to achieve specific release kinetics, enabling controlled and sustained drug release at the wound site [11, 12]. By carefully selecting the lipid composition and incorporating additional components, such as polymers or targeting ligands, researchers can control the release profile of the encapsulated drugs. This customization allows for optimal drug delivery, ensuring that therapeutic agents are released in a controlled manner, providing long-lasting effects and minimizing the need for frequent administration [12].

Additionally, LNPs-based systems offer the functional potential of targeted drug delivery. By incorporating ligands or antibodies onto the surface of the nanoparticles, these systems can be designed to specifically recognize and bind to certain cells or tissues relevant to diabetic wound repair [12]. This targeted approach enhances the accumulation of therapeutic agents at the site of action, maximizing their efficacy while minimizing off-target effects. By precisely delivering drugs to the wounded tissue, LNPs-based DDS hold significant potential in addressing the specific challenges associated with diabetic wound healing.

Advantages of LNPs-based DDS

LNPs provide a range of distinct advantages in targeted drug delivery for diabetic wound repair. Compared to other carriers used as lipophilic drug delivery systems, such as liposomes, microsponges and polymer nanoparticles, LNP is characterized by fewer side effects, better biocompatibility, and higher physical stability. Due to its lipid-based internal structure, LNP is considered to be an effective carrier for lipophilic drugs, and its advantages in siRNA delivery are becoming more and more generally accepted in the field of DDS [13, 14]. One notable advantage is their small size, which allows them to efficiently penetrate tissues and cells, reaching the desired site. Due to their nanoscale dimensions, LNPs can navigate through biological barriers and permeate the complex wound microenvironment, ensuring effective delivery of therapeutic agents to the target area. This property is particularly advantageous in diabetic wound repair, where precise localization of drugs is crucial for optimal therapeutic outcomes.

Another significant benefit of LNPs is their ability to protect encapsulated drugs from degradation. Lipids form a protective shell around the drugs, protecting them from enzymatic degradation and chemical breakdown. This encapsulation enhances the stability of the therapeutic agents, preserving their pharmacological activity during storage and transportation. By maintaining the integrity of the encapsulated drugs, LNPs ensure that the therapeutic agents remain intact until reaching the diabetic wound site, maximizing the therapeutic potential of the treatment.

Furthermore, LNPs can be functionalized with ligands or surface modifications to achieve specific targeting. By incorporating ligands or antibodies onto the surface of the nanoparticles, they can be engineered to recognize and bind to specific receptors or molecules that are overexpressed in diabetic wounds. This targeted approach increases the accumulation of therapeutic agents at the site or cell of action while minimizing their distribution to healthy tissue [15]. By precisely delivering drugs to the diabetic wound, LNPs reduce off-target effects and enhance the therapeutic efficacy of the treatment.

The versatility of LNPs also allows for the incorporation of complementary functionalities. For instance, in a prior study, keratinocyte-derived exosomes were genetically labeled with a GFP-reporter using tissue nanotransfection (TNT) and isolated from murine skin and wound-edge tissue through affinity selection using magnetic beads [16]. In vitro experiments demonstrated that the addition of wound-edge Exoκ-GFP to proinflammatory wound-macrophages led to their conversion into a proresolution phenotype, suggesting a role for these exosomes in promoting inflammation resolution. To further investigate the specific role of miRNA packaging within Exoκ-GFP, researchers utilized pH-responsive LNPs functionalized with siRNA targeting hnRNPA2B1 (a protein involved in miRNA packaging). These nanoparticles, labeled as TLNPκ/si-hnRNPA2B1, were designed to selectively inhibit miRNA packaging within Exoκ-GFP in vivo. The application of TLNPκ/si-hnRNPA2B1 to murine dorsal wounds significantly reduced hnRNPA2B1 expression by 80% in the epidermis compared to the control group (Figure 1).

Figure 1. (A-B) Schematic illustration of the keratinocyte-targeted LNPs DDS. (B) Mass spectrometric analysis of the nanoparticles. (C) In vivo results of the LNPs-based DDS in the application of wound repair. Reproduced with permission [16]. Copyright 2020, American Chemical Society.

Techniques and strategies for fabricating LNPs

Various techniques and strategies have been developed for the fabrication of LNPs tailored specifically for diabetic wound repair [17]. These techniques include the solvent evaporation technique, solvent diffusion method, microemulsion method, and lipid film hydration method [18]. Each method offers effective control over the size, surface properties, and drug-loading capacity of the nanoparticles [18] (Table 1).

Table 1. LNPs tailored specifically for diabetic wound repair.

Structure of LNP	Size of LNP	Angiogenic agents	Diabetes animal model	Main findings	Reference
LNE	150nm	HG-Exos	Mouse	LINC01435/YY1/HDAC8 may be an important signaling pathway affecting diabetic wound recovery.	[7]
LNE	110nm	siRNA	Mouse	RNA interference therapy with LNP reduces severity and duration of chronic diabetic wounds.	[8]
SLN	467nm	Fluoxetine	Rat	DWH gel-loaded solid LNP encapsulated with FX may be a potential vehicle for effective treatment and management of diabetic wounds.	[9]
LNC	101nm	VEGF-A mRNA	Mouse	Efficient delivery of VEGF-A mRNA via ionizable LNP for diabetic wound healing.	[12]
SLN	83nm	Retinoic acid	Mouse	Chitosan film-coated SLN-ATRA is a promising candidate for treating diabetic wounds and improving tissue healing.	[6]
LNC	110nm	SEC	Mouse	LNP as an effective platform to design ASCs with enhanced protein production to accelerate the development of ASC cell therapies.	[19]
NLC	150nm	siRNA	Mouse	Application of LNP-complexed siKeap1 restores the antioxidant function of Nrf2 and accelerates regeneration of diabetic tissues.	[20]
LNC	180nm	siRNA	Mouse	Potential of lipid-like nanoparticle RNAi therapy to inhibit positive feedback loops that promote diabetic foot ulcer pathogenesis.	[21]
LNC	80nm	VEGF-A and FGF1 mRNA	Mouse	Encapsulation of VEGF-A and FGF1 mRNA by f-LNP offers a promising approach to ameliorate the process of DFU scar formation	[22]
SLN	190nm	Sesamol	Rat	Sesamol encapsulated in solid LNP greatly enhances its wound healing potential.	[23]
LNE	104nm	PA-ELN	Mouse	Topical administration of PA-ELN significantly accelerated wound healing in a diabetic mouse model.	[24]

HG-exos, exos from high glucose-pretreated immortalized human epidermal; SEC, self-amplifying RNA (saRNA) and E3 mRNA complex (the combination of saRNA and E3 mRNA is named SEC); PA-ELN. Periplaneta americana-derived exosome-like nanoparticles.

The solvent evaporation technique involves dissolving lipids and the desired drug in an organic solvent [25]. The solvent is then evaporated, resulting in the formation of LNPs. This method allows for modulation of nanoparticle characteristics by adjusting the solvent composition and evaporation conditions [26]. This method allows the synthesis to be completed at low temperatures, but its disadvantage is the use of toxic organic solvents.

Similarly, the solvent diffusion method is the most widely used preparation method, which is simple and convenient. In this method, a water-miscible organic solvent containing lipids and drugs is injected into an aqueous solution [27]. The diffusion of the organic solvent into the aqueous medium leads to the formation of LNPs through the spontaneous self-assembly of lipids and drug molecules [28]. Although the synthesized LNPs inevitably aggregate, their stability is enhanced accordingly.

The microemulsion method, involves dissolving lipids and the desired lipophilic drug in an organic solvent and then removing the solvent by evaporation under reduced pressure to form a lipid film. This film self-assembles into a spherical structure to form LNPs with the addition of a suitable aqueous solvent [29]. This method allows for the control of nanoparticle size and drug loading by manipulating the composition of the microemulsion system and subsequent solvent evaporation or dilution [30]. The LNPs prepared by this method showed excellent cumulative drug penetration.

The lipid film hydration method also known as the Bangham method, is to add a small amount of aqueous solution proportionally to the organic solvent with dissolved lipids and drugs, and then mixing and emulsifying the contents by mechanical force to form a more stable emulsion. Subsequently, a large amount of aqueous solution is quickly added and mixed for secondary emulsification, and finally the residual organic solvent is removed by physical means to obtain the LNPs suspension [31]. This hydration process leads to the formation of LNPs with well-defined characteristics. The advantage of this technique is its simplicity, but a major disadvantage is that the size of the formulated granules.

Meanwhile, in order to produce LNPs with stable properties, microfluidic mixing is a contemporary and increasingly widespread technique. Valencia et al. [32] mixed an aqueous solution consisting of lecithin and DSPE-PEG (lecithin: DSPE-PEG, 8.4:1.6 (molar)) with a solution of PLGA dissolved in acetonitrile (1 mg/ml) in a 10:1 volume ratio. LNPs with PLGA-lecithin-PEG core-shell structures and properties were fabricated after the liquid was mixed intensively through the Tesla structure. Moreover, the size and charge of LNPs prepared by microfluidic mixing are found to be beneficial for the transitivity to and distribution within LN [33]. Continuous flow microfluidic synthesis allows for better control of NP formation than conventional synthesis and has the potential to tune NP properties in a reproducible manner, which is essential for determining the optimal NP formulation for any given application [32].

In addition to these fabrication techniques, various strategies have been developed to enhance the functionality and therapeutic potential of LNPs for angiogenesis promotion in diabetic wound healing [34]. Surface modification techniques can be employed to improve nanoparticle stability, biocompatibility, and targeting efficiency [35]. Co-encapsulation of multiple drugs within LNPs enables combination therapy, allowing for synergistic effects and improved wound healing outcomes [36]. Furthermore, the incorporation of stimuli-responsive materials in LNPs can facilitate controlled drug release in response to specific physiological cues or environmental stimuli [29].

Angiogenic agents for diabetic wound repair

Angiogenesis in diabetic wound healing

Angiogenesis, the process of new blood vessel formation, plays a pivotal and multifaceted role in wound healing, particularly in the context of diabetic wound repair [37]. In individuals with diabetes, impaired angiogenesis is a significant contributing factor to delayed wound healing and the development of chronic non-healing ulcers. The restoration of adequate blood supply is crucial for orchestrating the complex series of events required for successful wound healing [38].

Angiogenesis serves several critical functions during the wound healing process. Firstly, it promotes the formation of granulation tissue, a vital component for wound closure [39]. The formation of new blood vessels allows for the influx of fibroblasts, endothelial cells, and immune cells into the wound bed [39]. These cellular components work in harmony to remodel the extracellular matrix, facilitate tissue regeneration, and promote the tissue regeneration.

Furthermore, angiogenesis facilitates the recruitment of progenitor cells, including endothelial progenitor cells, which contribute to the development of functional blood vessels [37]. These progenitor cells play a fundamental role in angiogenesis by differentiating into mature endothelial cells and incorporating themselves into the growing vascular network. This process ensures the establishment of a well-organized and functional vasculature within the wound bed.

Angiogenesis also plays a vital role in delivering essential growth factors and cytokines necessary for wound healing. Key regulators of angiogenesis, such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF), stimulate the proliferation and migration of endothelial cells [40]. These growth factors not only promote angiogenesis but also contribute to the recruitment and activation of other cells involved in wound healing, including fibroblasts, keratinocytes, and immune cells. The orchestrated interplay among these cells is essential for effective tissue regeneration and the suppression of inflammation (Figure 2) [40].

Figure 2. The crucial role of angiogenesis in diabetic wound healing. During the process of wound healing, angiogenesis plays a vital role. Fibroblasts in the wound environment secrete VEGF, which activates endothelial cells (ECs). As a result, ECs increase their secretion of proteolytic proteins. Macrophages release elevated levels of matrix metalloproteinases (MMPs) and proteolytic enzymes, promoting the breakdown of the basement membrane. This breakdown enables the migration of ECs and the sprouting of new blood vessels into the wound site. Reproduced with permission [41]. Copyright 2022, MDPI.

Moreover, the newly formed blood vessels established through angiogenesis provide a continuous supply of nutrients and oxygen to support tissue regeneration within the wound bed. By supplying oxygen and nutrients, angiogenesis provides a microenvironment conducive to cell proliferation, migration, and tissue remodeling. Simultaneously, the vascular network helps remove metabolic waste products and inflammatory mediators from the wound site, contributing to a favorable microenvironment for healing [42].

Selecting appropriate angiogenic agents for LNPs delivery

The selection of appropriate angiogenic agents is critical in utilizing LNPs-based DDS to promote angiogenesis in diabetic wound repair [43]. Different types of angiogenic agents can be explored, including cytokines, Chinese traditional medicine, extracellular vesicles (EVs), and Food and Drug Administration (FDA)-approved drugs [44]. Each category offers unique advantages and therapeutic potential in promoting angiogenesis and facilitating wound healing.

Cytokines are signaling molecules that play crucial roles in regulating immune responses and cell communication. Several cytokines have shown beneficial in promoting angiogenesis and wound healing [45]. For example, VEGF is a potent angiogenic factor that stimulates the proliferation and migration of endothelial cells. FGF and PDGF also contribute to angiogenesis and tissue regeneration [11]. By encapsulating these cytokines within LNPs, their stability can be enhanced, controlled release can be achieved, and targeted delivery to the wound site can be facilitated [44].

Chinese traditional medicine, with its rich history and diverse herbal remedies, provides a valuable source of potential angiogenic agents for wound healing. Various herbal extracts and compounds derived from Chinese traditional medicine have demonstrated angiogenic properties [46]. For instance, ginsenosides derived from ginseng have shown pro-angiogenic effects by stimulating endothelial cell proliferation and migration [47]. Compounds such as salvianolic acid from Danshen (Salvia miltiorrhiza) and icariin from Horny Goat Weed (Epimedium spp.) have also been reported to promote angiogenesis [48]. LNPs can serve as effective carriers for these bioactive compounds, improving their bioavailability and targeting their release to the wound site.

EVs are small membrane-bound structures released by cells and play a crucial role in intercellular communication [49]. They contain various bioactive molecules, including microRNAs, proteins, and growth factors, which can modulate cellular functions, including angiogenesis [50]. EVs derived from stem cells, such as mesenchymal stem cells (MSCs), have shown promising in promoting angiogenesis and wound healing [51]. Loading these EVs into LNPs can protect and enhance the stability of the EV cargo and facilitate targeted delivery to the wound site.

In addition, FDA-approved drugs with angiogenic properties can be considered for LNP-based delivery [52]. These drugs have already undergone rigorous testing and approval processes, ensuring their safety and efficacy. For example, recombinant human vascular endothelial growth factor (rhVEGF) and its analogs, such as bevacizumab, have been approved for specific medical indications and have shown potential in promoting angiogenesis [52]. LNPs can serve as effective carriers for these drugs, improving their stability, enabling controlled release, and facilitating targeted delivery to the wound site.

To fully utilize the huge potential of LNP-based DDS in promoting angiogenesis, it is crucial to understand the underlying mechanisms of action. Angiogenic agents delivered by LNPs can act through various pathways to stimulate angiogenesis. These mechanisms may involve activating specific cell surface receptors, modulating signaling pathways, promoting cell migration and proliferation, and inducing the secretion of angiogenic factors [53]. Additionally, the sustained and controlled release of angiogenic agents provided by LNPs ensures a continuous supply of these agents, prolonging their effects and enhancing angiogenesis over an extended period [54].

Of note, the use of combination therapies involving multiple angiogenic agents delivered via LNPs has shown beneficial in enhancing angiogenesis and wound healing outcomes [11]. The synergistic effects of different angiogenic agents can target multiple pathways involved in angiogenesis, leading to a more comprehensive and effective therapeutic approach [31]. By carefully selecting and combining angiogenic agents, LNPs can be optimized to enhance angiogenesis and facilitate diabetic wound repair.

LNPs-mediated drug delivery for angiogenesis promotion

Encapsulation and loading strategies for angiogenic agents

LNPs serve as an efficient platform for encapsulating and delivering angiogenic agents precisely to the site of diabetic wounds, exerting their therapeutic effects by promoting angiogenesis and facilitating the wound repair process [55]. By employing encapsulation strategies, such as incorporating a diverse range of angiogenic agents, these agents receive comprehensive protection against degradation, enabling their stability in a controlled release manner.

To achieve effective encapsulation, a variety of loading strategies have been implemented. Among these strategies, physical encapsulation of angiogenic agents within the lipid matrix represents a commonly utilized approach [56]. Tejedor et al. [22] validated the ability of vascular endothelial growth factor (VEGF-A) and modified fibroblast growth factor (FGF1) on micro-vessel formation and wound healing. Subsequently, they packaged VEGF-A and FGF1 mRNA in LNPs by electrostatic interactions and found that the relevant genes were highly expressed in the wound tissue. Meanwhile, the wounds had significantly more neovascularisation on day 7 under treatment compared to the control group, and the wound healing time was greatly shortened. By capitalizing on the hydrophobic interactions between the lipid components and the angiogenic agents, this technique effectively encapsulates the agents within the core of the LNPs, protecting their integrity and preserving their bioactivity.

Furthermore, surface modification techniques have been also used to load angiogenic agents onto the outer surface of LNPs, bringing in numerous advantages [57]. By employing conjugation or adsorption methods, these agents can be effectively attached to the surface of LNPs, enabling targeted delivery to the specific site of the diabetic wound, thereby maximizing their therapeutic efficacy [57]. By selectively attaching targeting ligands or antibodies onto the LNPs surface, the nanoparticles can precisely interact with the receptors or cells involved in angiogenesis, ensuring the agents’ precise delivery to the desired location [53].

Li et al. [58] successfully constructed a lyophilized keratinocyte-targeted nanocarriers (TLNκ), which delivers anti-miRNAs to keratinocytes by modifying the surface of liposomes using the peptide sequence ASKAIQVFLALAG to upregulate dicer expression and accelerate wound healing. Through this targeted delivery approach, off-target effects can be minimized while simultaneously achieving a heightened concentration of the agents at the wound site, enhancing their therapeutic efficacy.

Moreover, surface modification empowers controlled release of the encapsulated angiogenic agents. Incorporating stimuli-responsive materials, such as pH-sensitive or temperature-sensitive polymers, into the LNP formulation enables the release of the agents modulated in response to specific cues present in the wound microenvironment [11]. For example, a prior study introduced a novel approach for promoting wound healing in diabetic individuals using all-trans retinoic acid (ATRA)-loaded solid lipid nanoparticles (SLN) surrounded by a chitosan film [6]. To promote controlled release and enhance the wound healing properties, the SLN-ATRA particles were incorporated into chitosan films. This combination facilitated a homogeneous distribution of the drug within the film and enabled controlled release of ATRA. In an in vivo assay using a diabetic mouse model, the chitosan films containing SLN-ATRA accelerated wound closure compared to control chitosan films without ATRA. Additionally, these films reduced leukocyte infiltrate in the wound bed, improved collagen deposition, and reduced scar tissue formation. Notably, no signs of skin irritation were observed during the treatment (Figure 3). These results indicated that this innovative approach enhanced tissue healing by promoting wound closure, reducing inflammation, enhancing collagen deposition, and minimizing scar formation.

Figure 3. The schematic illustration of the fabrication and application of SLN-ATRA particles in the treatment of diabetic wound. Reproduced with permission [6]. Copyright 2020, Elsevier.

The versatility of LNPs in encapsulating angiogenic agents, combined with their ability to incorporate various loading strategies, positions them as an exceptionally effective platform for targeted drug delivery in diabetic wound repair. LNPs offer indispensable advantages such as robust protection of the agents, enhanced stability, controlled release capabilities, and the potential for precise targeting.

Controlled release mechanisms for targeted and sustained delivery

Controlled release mechanisms play a vital role in the effective delivery of angiogenic agents using LNPs, contributing to the promotion of angiogenesis in diabetic wound repair [59].

For example, a study developed a self-gelling solid lipid nanoparticle (SLNs) dressing to incorporate simvastatin (SIM), a lipophilic, potential wound-healing agent, clinically limited due to poor solubility (0.03 mg/mL) and absorption. These mechanisms ensure targeted and sustained release of the encapsulated agents, allowing for their function at the wound site over an extended period, thus enhancing their therapeutic potential [59]. By manipulating various factors including LNPs composition, size, surface properties, and the interactions between the lipid matrix and the encapsulated agents, the release profiles can be effectively modulated.

One commonly employed approach for achieving controlled release is the diffusion-controlled mechanism [60]. In this mechanism, the angiogenic agents gradually diffuse through the lipid matrix of the LNPs, resulting in their sustained release [61]. The release rate can be modulated by adjusting the formulation parameters of the LNPs, such as the lipid composition, particle size, and surface modifications [61]. Through these modifications, researchers can effectively manipulate the release kinetics, ensuring a controlled and sustained supply of angiogenic agents at the wound site. This controlled release strategy allows for optimal utilization of the agents and promotes continuous angiogenesis, supporting the healing process.

In addition to diffusion-controlled release, other strategies such as stimuli-responsive release can be employed to achieve precise control over the release of angiogenic agents [62]. Stimuli-responsive LNPs incorporate specific elements that respond to changes in physiological conditions at the wound site [62]. For example, temperature-sensitive or pH-responsive components can be incorporated into the lipid matrix of the LNPs [16]. These components undergo structural changes in response to temperature or pH variations, triggering the release of angiogenic agents. By responding to the specific environmental cues present at the wound site, these stimuli-responsive LNPs enable on-demand release of the encapsulated agents, further enhancing the therapeutic efficacy.

The ability to achieve controlled release mechanisms in LNPs offers several advantages for diabetic wound repair [31]. Firstly, it ensures a sustained and prolonged presence of angiogenic agents at the wound site, providing a continuous stimulus for angiogenesis. This sustained release approach is particularly beneficial in diabetic wounds where impaired angiogenesis necessitates extended periods of treatment. Additionally, controlled release reduces the frequency of administration, enhancing patient compliance and convenience.

Enhanced stability and protection of angiogenic agents

Achieving and maintaining the stability and protection of angiogenic agents throughout the entire LNP-mediated drug delivery process is crucial for the successful promotion of angiogenesis in diabetic wound repair. Various strategies can be employed to enhance the stability and protection of these agents within LNPs, ensuring their structural integrity, biological activity, and therapeutic effectiveness [9]. The effectiveness of topical insulin in wound healing, for example, is hampered by proteases. Encapsulation in nanoparticles improves its stability in the wound and allows for its sustained release while providing good adhesion [63].

One effective approach involves the appropriate selection of lipid materials with inherent stability and compatibility with the encapsulated angiogenic agents [64]. Choosing lipids that exhibit high chemical stability, resistance to oxidation or hydrolysis, and low susceptibility to degradation is crucial for formulating robust LNPs [65]. These stable lipid components provide a protective environment, protecting the encapsulated agents from degradation and preserving their therapeutic properties during storage, transportation, and delivery. Waggoner et al. [66] demonstrated that LNP configured with different anchor lengths of polyethylene glycol (PEG)-lipids had significantly different accumulation and activities in mice. The increase of long anchor PEG-lipid significantly prolonged the circulation time of LNPs.

Furthermore, the incorporation of stabilizing agents into the LNP formulation can significantly enhance stability and protection [43]. Stabilizers, such as surfactants or polymers, can be included during the formulation process to improve the colloidal stability of LNPs. These stabilizing agents prevent particle aggregation, enhance dispersion, and maintain the integrity of the lipid matrix. By forming a protective layer around the LNPs, they prevent the encapsulated angiogenic agents from external stresses, such as temperature fluctuations or mechanical forces encountered during handling and administration [65]. Sharma et al. [67] prepared solid lipid nanoparticles contain Berberis extract by mixing phospholipids 90 G, tween 80 and water and heating them up to 82 °C, followed by a hot high-pressure homogenization technique in order to solve problems such as low bioavailability of drugs. This protective layer minimizes interactions with the surrounding environment, reducing the likelihood of agent degradation and ensuring their long-term stability.

In addition to stabilizers, the use of protective coatings or surface modifications can provide an additional layer of protection for LNPs and the encapsulated angiogenic agents [53]. Coating the surface of LNPs with biocompatible materials, such as polymers or PEGylation, offers enhanced stability and protection [68]. These coatings act as a physical barrier, shielding the encapsulated agents from enzymatic degradation and minimizing unwanted interactions with biological components. By reducing nonspecific interactions, these protective coatings help maintain the structural integrity and biological activity of the encapsulated agents, ensuring their therapeutic effectiveness upon reaching the wound site.

Future perspectives and challenges

LNPs-based drug delivery is a promising strategy for enhancing diabetic wound healing through promoting angiogenesis. Researchers are actively exploring novel approaches to enhance the efficacy, specificity, and clinical applicability of these systems [11].

One exciting avenue of advancement lies in the incorporation of targeting ligands or surface modifications on LNPs [69]. By introducing specific targeting ligands or modifying the surface properties of LNPs, researchers can improve their selectivity and affinity toward specific cell types or tissues. This targeted delivery approach enables the precise localization of angiogenic agents to the site of the diabetic wound, maximizing their therapeutic impact while minimizing off-target effects. By directing the therapeutic payload to the desired location, targeted LNPs enhance the efficiency and effectiveness of angiogenesis promotion in diabetic wound repair.

Another area of advancement is the integration of stimuli-responsive components within LNPs [43]. These components enable the development of LNPs that respond to specific environmental cues, such as pH, temperature, or enzymatic activity. By incorporating stimuli-responsive elements, LNPs can undergo controlled drug release in response to these cues, further optimizing therapeutic efficacy. This triggered release mechanism ensures that angiogenic agents are released at the right time and in the right place, enhancing their effectiveness and reducing the frequency of drug administration.

Despite the extensive progress in LNP-based drug delivery, several challenges and limitations must be addressed to facilitate successful clinical application. Clinical studies addressing LNPs in diabetic wound repair are lacking. One major concern is the immune response triggered by LNPs [57]. Thorough investigation and mitigation of immunogenicity are crucial to ensure the long-term safety and efficacy of these delivery systems. Researchers need to develop strategies to minimize immune responses and optimize the biocompatibility of LNPs.

Another challenge lies in maintaining the stability and integrity of LNPs during storage and administration. Issues such as premature drug release or instability of LNPs can compromise therapeutic efficacy and drug integrity [11]. To overcome these challenges, robust formulation strategies and optimized storage conditions should be developed. Meanwhile, production expansion is also a real challenge for late-stage clinical applications.

Conclusion

In conclusion, LNPs-based DDS represent a promising nanoplatform for promoting angiogenesis in diabetic wound. With their distinctive properties and adaptable design, LNPs offer precise and controlled delivery of angiogenic agents, providing a potential opportunity to revolutionize the treatment of diabetic wounds and enhance patient outcomes. The ongoing research, innovation, and collaborative endeavors in this field are crucial for advancing the translation of LNP-based therapies into clinical practice. By harnessing the potential of LNPs, we can bring about transformative changes in the management of diabetic wounds, ultimately improving the lives of countless individuals affected by this condition.

Authors’ contributions

All authors made a significant contribution to the work reported, gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure statement

The authors declare that there are no conflicts of interest.

Additional information

Funding

This work was supported by the National Science Foundation of China (No. 82002313).