Defining a model to predict the distribution of topically applied growth factors and other solutes in excisional full-thickness wounds

Mathematical Model Predicts that Acceleration of Diabetic Wound Healing is Dependent on Spatial Distribution of VEGF-A mRNA (AZD8601)

Introduction

Pharmacologic approaches for promoting angiogenesis have been utilized to accelerate healing of chronic wounds in diabetic patients with varying degrees of success. We hypothesize that the distribution of proangiogenic drugs in the wound area critically impacts the rate of closure of diabetic wounds. To evaluate this hypothesis, we developed a mathematical model that predicts how spatial distribution of VEGF-A produced by delivery of a modified mRNA (AZD8601) accelerates diabetic wound healing.

Methods

We modified a previously published model of cutaneous wound healing based on coupled partial differential equations that describe the density of sprouting capillary tips, chemoattractant concentration, and density of blood vessels in a circular wound. Key model parameters identified by a sensitivity analysis were fit to data obtained from an in vivo wound healing study performed in the dorsum of diabetic mice, and a pharmacokinetic model was used to simulate mRNA and VEGF-A distribution following injections with AZD8601. Due to the limited availability of data regarding the spatial distribution of AZD8601 in the wound bed, we performed simulations with perturbations to the location of injections and diffusion coefficient of mRNA to understand the impact of these spatial parameters on wound healing.

Results

When simulating injections delivered at the wound border, the model predicted that injections delivered on day 0 were more effective in accelerating wound healing than injections delivered at later time points. When the location of the injection was varied throughout the wound space, the model predicted that healing could be accelerated by delivering injections a distance of 1–2 mm inside the wound bed when compared to injections delivered on the same day at the wound border. Perturbations to the diffusivity of mRNA predicted that restricting diffusion of mRNA delayed wound healing by creating an accumulation of VEGF-A at the wound border. Alternatively, a high mRNA diffusivity had no effect on wound healing compared to a simulation with vehicle injection due to the rapid loss of mRNA at the wound border to surrounding tissue.

Conclusions

These findings highlight the critical need to consider the location of drug delivery and diffusivity of the drug, parameters not typically explored in pre-clinical experiments, when designing and testing drugs for treating diabetic wounds.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00678-9.

Predicting viable skin concentration: Diffusional and convective drug transport

Viable skin drug transport is an important concept to consider as it can have a significant impact on the local concentration of a drug. The concentration becomes even more critical for toxicological issues when implementing different permeability enhancement techniques. For this reason, it is important to develop models that can predict drug transport in the viable skin. This paper expands upon previous capillary modeling by representing the convective transport of a solute that has permeated into the capillary loops. As a result, convective transport caused the concentration profile to plateau within the deeper dermal layers, effectively matching the trend of previous experimental data. Furthermore, the new model also has a significantly quicker transient profile as the time required to reach steady-state is five-fold faster than predicted in previous homogenous models.

Modeling drug transport within the viable skin - a review

INTRODUCTION

In the past, mathematical modeling of the transport of transdermal drugs has been primarily focused on the stratum corneum. However, the development of pharmaceutical technologies, such as chemical enhancers, iontophoresis, and microneedles, has led to two outcomes; an increase in permeability in the stratum corneum or the ability to negate the layer entirely. As a result, these outcomes have made the transport of a solute in the viable skin far more critical when studying transdermal drug delivery.

AREAS COVERED

The review will explicitly show the various attempts to model drug transport within the viable skin. Furthermore, a brief review will be conducted on the different models that explain stratum corneum transport, microneedle dynamics and estimation of the diffusion coefficient.

EXPERT OPINION

Future development of mathematical models requires the focus to be changed from traditional diffusion-based tissue models to more sophisticated three-dimensional models that incorporate the physiology of the skin.

Sprayable and biodegradable, intrinsically adhesive wound dressing with antimicrobial properties

Conventional wound dressings are difficult to apply to large total body surface area (TBSA) wounds, as they typically are prefabricated, require a layer of adhesive coating for fixation, and need frequent replacement for entrapped exudate. Large TBSA wounds as well as orthopedic trauma and low-resource surgery also have a high risk of infection. In this report, a sprayable and intrinsically adhesive wound dressing loaded with antimicrobial silver is investigated that provides personalized fabrication with minimal patient contact. The dressing is composed of adhesive and biodegradable poly(lactic-co-glycolic acid) and poly(ethylene glycol) (PLGA/PEG) blend fibers with or without silver salt (AgNO3). in vitro studies demonstrate that the PLGA/PEG/Ag dressing has antimicrobial properties and low cytotoxicity, with antimicrobial silver controllably released over 7-14 days. In a porcine partial-thickness wound model, the wounds treated with both antimicrobial and nonantimicrobial PLGA/PEG dressings heal at rates similar to those of the clinical, thin film polyurethane wound dressing, with similar scarring. However, PLGA/PEG adds a number of features beneficial for wound healing: greater exudate absorption, integration into the wound, a 25% reduction in dressing changes, and tissue regeneration with greater vascularization. There is also modest improvement in epidermis thickness compared to the control wound dressing.

Physiologically based mathematical modelling of solute transport within the epidermis and dermis

The stratum corneum is the main barrier to transdermal drug delivery which has previously resulted in mathematical modelling of solute transport in the skin being primarily directed at this skin layer. However, for topical treatment and skin toxicity studies, the concentration in the epidermis and dermis is important and needs to be modelled mathematically. Hitherto, mathematical models for viable skin layers typically simplified the clearance of solute by blood, either assuming sink condition at the top of the skin capillary loops or assuming a distributed clearance in the dermis. This paper is an attempt to develop a physiologically based mathematical model of drug transport in the viable skin. It incorporates explicit modelling of the capillary loops within the dermis and employs COMSOL Multiphysics® software to model the transport in three dimensions. Previously derived simplified models were compared to the results from this new numerical model. The results of this comparison showed that the simplified model reasonably described the average concentration in the viable skin layers when parameters of the models were chosen appropriately. When the recruitment of the capillary loops in the dermis was full and the top of capillary loops was at a depth of 100μm, the effective depth to place a sink condition in the simpler models was found to be at 150μm. However, when there was only partial recruitment of the capillaries, the effective depth increased to 180μm. The presented modelling is also essential for determining a transdermal flux when the stratum corneum barrier is compromised by such methods as microporation, application of chemical enhancers or microneedles.

Keratinocyte Growth Factor Modified Messenger RNA Accelerating Cell Proliferation and Migration of Keratinocytes

Keratinocyte growth factor (KGF) plays a central role in wound healing as it induces cell proliferation and motility. The use of growth factors such as KGF is therefore viewed as a promising approach in wound therapy, although effective application remains a major problem because of inactivation and the resulting short half-life of applied growth factors in wound beds. Therefore, the rational of this study was to develop and investigate an innovative strategy to improve wound healing using an in vitro-transcribed modified KGF messenger RNA (mRNA). After transfection of cells, we evaluated the effects of the produced KGF protein on cell migration and reepithelialization of keratinocytes using a scratch assay. The results demonstrate that KGF-mRNA-transfected cells exhibited a high KGF protein release that is sufficient to significantly improve reepithelialization in the performed scratch assays. Transfection with growth factor mRNA therefore seems to be a promising therapeutic strategy, especially for difficult wounds, as it leads to a temporary increase of growth factor expression in the treated wound area without interfering with the DNA of the nucleus, as seen in gene therapeutic applications.

Diabetic Foot Ulcers and Epidermal Growth Factor: Revisiting the Local Delivery Route for a Successful Outcome

Soon after epidermal growth factor (EGF) discovery, some in vivo models appeared demonstrating its property to enhance cutaneous wound healing. EGF was the first growth factor (GF) introduced in the clinical arena as a healing enhancer, exerting its mitogenic effects on epithelial, fibroblastoid, and endothelial cells via a tyrosine kinase membrane receptor. Compelling evidences from the 90s documented that, for EGF, locally prolonged bioavailability and hourly interaction with the receptor were necessary for a successful tissue response. Eventually, the enthusiasm on the clinical use of EGF to steer the healing process was wiped out as the topical route to deliver proteins started to be questioned. The simultaneous in vivo experiments, emphasizing the impact of the parenterally administered EGF on epithelial and nonepithelial organs in terms of mitogenesis and cytoprotection, rendered the theoretical fundamentals for the injectable use of EGF and shaped the hypothesis that locally infiltrating the diabetic ulcers would lead to an effective healing. Although the diabetic chronic wounds microenvironment is hostile for local GFs bioavailability, EGF local infiltration circumvented the limitations of its topical application, thus expanding its therapeutic prospect. Our clinical pharmacovigilance and basic studies attest the significance of the GF local infiltration for chronic wounds healing.

The effects of insulin-like growth factor-1 gene therapy and cell transplantation on rat acute wound model

BACKGROUND

Wound healing is a complex process. Different types of skin cells, extracellular matrix and variety of growth factors are involved in wound healing. The use of recombinant growth factors in researches and production of skin substitutes are still a challenge.

OBJECTIVES

Much research has been done on the effects of gene therapy and cell therapy on wound healing. In this experimental study, the effect of insulin-like growth factor (IGF-1) gene transfer in fibroblast cells was assessed on acute dermal wound healing.

MATERIALS AND METHODS

Fibroblasts were cultured and transfected with IGF-1. Lipofectamine 2000 was used as a reagent of transfection. Transgene expression levels were measured by the enzyme linked immunosorbent assay (ELISA). To study in vivo, rats (weighing 170-200 g) were randomly divided into three groups (five/group) and full-thickness wounds were created on the dorsum region. Suspensions of transfected fibroblast cells were injected into the wound and were compared with wounds treated with native fibroblast cells and normal saline. For the microscopic examination, biopsy was performed on day seven.

RESULTS

In vitro, the maximum expression of IGF1 (96.95 pg/mL) in transfected fibroblast cells was 24 hours after gene transfer. In vivo, it was clear that IGF-1 gene therapy caused an increase in the number of keratinocyte cells during the wound healing process (mean of group A vs. group B with P value = 0.01, mean of group A vs. group C with P value = 0.000). Granulation of tissue formation in the transfected fibroblast group was more organized when compared with the normal saline group and native fibroblast cells.

CONCLUSIONS

This study indicated that the optimization of gene transfer increases the expression of IGF-1. High concentrations of IGF-1, in combination with cell therapy, have a significant effect on wound healing.

Microencapsulated VEGF gene-modified umbilical cord mesenchymal stromal cells promote the vascularization of tissue-engineered dermis: an experimental study

BACKGROUND AIMS

Tissue-engineered dermis (TED) is thought to be the best treatment for skin defect wounds; however, lack of vascular structures in these products can cause slow vascularization or even transplant failure. We assessed the therapeutic potential of microencapsulated human umbilical cord mesenchymal stromal cells (hUCMSCs) expressing vascular endothelial growth factor (VEGF) in vascularization of TED.

METHODS

hUCMSCs were isolated by means of enzymatic digestion and identified by means of testing biological characteristics. hUCMSCs were induced to differentiate into dermal fibroblasts in conditioned induction media. Collagen-chitosan laser drilling acellular dermal matrix (ADM) composite scaffold was prepared by means of the freeze dehydration and dehydrothermal cross-linking method. hUCMSC-derived fibroblasts were implanted on composite scaffolds to construct TED. TED with microencapsulated VEGF gene-modified hUCMSCs was then transplanted into skin defect wounds in pigs. The angiogenesis of TED at 1 week and status of wound healing at 3 weeks were observed.

RESULTS

The collagen-chitosan laser ADM composite has a uniform microporous structure. This composite has been used to grow hUCMSC-derived fibroblasts in vitro and to successfully construct stem cell-derived TED. Microencapsulated VEGF gene-modified hUCMSCs were prepared with the use of a sodium alginate-barium chloride one-step encapsulation technology. Seven days after the transplantation of the stem cell-derived TED and microencapsulated VEGF gene-modified hUCMSCs into the skin defect wounds on the backs of miniature pigs, the VEGF expression increased and the TED had a higher degree of vascularization. Re-epithelialization of the wound was completed after 3 weeks.

CONCLUSIONS

Microencapsulated VEGF gene-modified hUCMSCs can effectively improve the vascularization of TED and consequently the quality of wound healing.

Mathematical models for skin toxicology

INTRODUCTION

Our skin is exposed daily to substances; many of these are neutral and safe but others are potentially harmful. In order to estimate the degree of toxicity and damage to skin tissues when exposed to harmful substances, skin toxicology studies are required. If these studies are coupled with suitably designed mathematical models, they can provide a powerful tool that allows appropriate interpretation of data. This work reviews mathematical models that can be employed in skin toxicology studies.

AREAS COVERED

Two types of mathematical models and their suitability for assessing skin toxicology are covered in this review. The first is focused on predicting penetration rate through the skin from a solute's physicochemical properties, while the second type of models transport processes in skin layers using appropriate equations with the specific aim of predicting the concentration of a given solute in viable skin tissues.

EXPERT OPINION

Mathematical models are an important tool for accurate valuation of skin toxicity experiments, estimation of skin toxicity and for developing new formulations for skin disease therapy. Comprehensive mathematical models of drug transport in skin, especially those based on more physiologically detailed mechanistic considerations of transport processes, are required to further enhance their role in assessing skin toxicology.

Mathematical and pharmacokinetic modelling of epidermal and dermal transport processes

Topical delivery to the various regions of the skin and underlying tissues, transdermal drug delivery and dermal exposure to environmental chemicals are important areas of research. Mathematical models of epidermal and dermal transport, involving penetration of a solute through various layers of the skin, metabolism in the skin and its subsequent distribution and clearance into systemic circulation from underlying tissues, play an essential role in this research area and are reviewed in this work.

Convective transport of highly plasma protein bound drugs facilitates direct penetration into deep tissues after topical application

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Many products are applied to human skin for local effects in deeper tissues. Animal studies suggest that deep dermal and/or subcutaneous delivery may be facilitated by both dermal diffusion and transport via the cutaneous vasculature. However, the relationship between the extent and pathways of penetration, drug physicochemical properties and deeper tissue physiology is not well understood.

WHAT THIS STUDY ADDS

We have used a physiologically based pharmacokinetic model to analyze published human cutaneous microdialysis data, complemented by our own in vitro skin penetration studies. We found that convective blood, lymphatic and interstitial flow led to significant deep tissue concentrations for drugs that are highly plasma protein bound. In such cases, deeper tissue concentrations will occur earlier and may be several orders of magnitude greater than predicted by passive dermal diffusion alone.

AIMS

To relate the varying dermal, subcutaneous and muscle microdialysate concentrations found in man after topical application to the nature of the drug applied and to the underlying physiology.

METHODS

We developed a physiologically based pharmacokinetic model in which transport to deeper tissues was determined by tissue diffusion, blood, lymphatic and intersitial flow transport and drug properties. The model was applied to interpret published human microdialysis data, estimated in vitro dermal diffusion and protein binding affinity of drugs that have been previously applied topically in vivo and measured in deep cutaneous tissues over time.

RESULTS

Deeper tissue microdialysis concentrations for various drugs in vivo vary widely. Here, we show that carriage by the blood to the deeper tissues below topical application sites facilitates the transport of highly plasma protein bound drugs that penetrate the skin, leading to rapid and significant concentrations in those tissues. Hence, the fractional concentration for the highly plasma protein bound diclofenac in deeper tissues is 0.79 times that in a probe 4.5 mm below a superficial probe whereas the corresponding fractional concentration for the poorly protein bound nicotine is 0.02. Their corresponding estimated in vivo lag times for appearance of the drugs in the deeper probes were 1.1 min for diclofenac and 30 min for nicotine.

CONCLUSIONS

Poorly plasma protein bound drugs are mainly transported to deeper tissues after topical application by tissue diffusion whereas the transport of highly plasma protein bound drugs is additionally facilitated by convective blood, lymphatic and interstitial transport to deep tissues.

Diabetic lower extremity wounds: the rationale for growth factors-based infiltration treatment

Repair machinery and local infection control failure contribute to wound chronicity and lower extremity amputation in diabetic patients. In these wounds, inflammation is a proximal condition which disrupts wound matrix turnover and the local redox balance. Contemporary therapeutic interventions are relatively broad including drugs, devices and surgical procedures. However, clinical efficacy remains modest and recurrences are frequent. Recombinant growth factors advent was followed by their premature and empiric introduction in the clinical practice. Its topical administration is still challenged by local kinetic and pharmacodynamic limitations related to the hostile microenvironment of chronic wounds. The rationale of infiltrating epidermal growth factor (EGF) down inside complex diabetic wounds as an alternative treatment modality is described here. The concept emerged from two experimental evidences: (a) locally infiltrated EGF prevented trophic ulcers and limb necrosis upon denervation, (b) acute, controlled experimental wounds' exudate exhibited proteolytic activity. Depositing EGF in deep cells' responsive strata allows for two main pharmacological actions indispensable for chronic wounds healing: cyto-protection and proliferation of fibroblasts and endothelial cells, thus inducing progressive granulation. Ten years of clinical experience have validated laboratory and theoretical concepts, while most importantly have improved quality-of-life to thousands of diabetic patients.

Modelling dermal drug distribution after topical application in human

PURPOSE

To model and interpret drug distribution in the dermis and underlying tissues after topical application which is relevant to the treatment of local conditions.

METHODS

We created a new physiological pharmacokinetic model to describe the effect of blood flow, blood protein binding and dermal binding on the rate and depth of penetration of topical drugs into the underlying skin. We used this model to interpret literature in vivo human biopsy data on dermal drug concentration at various depths in the dermis after topical application of six substances. This interpretation was facilitated by our in vitro human dermal penetration studies in which dermal diffusion coefficient and binding were estimated.

RESULTS

The model shows that dermal diffusion alone cannot explain the in vivo data, and blood and/or lymphatic transport to deep tissues must be present for almost all of the drugs tested.

CONCLUSION

Topical drug delivery systems for deeper tissue delivery should recognise that blood/lymphatic transport may dominate over dermal diffusion for certain compounds.

Hypromellose films for the delivery of growth factors for wound healing

Hypromellose was investigated as a carrier for extending topical growth factor delivery to wounds. Films of hypromellose (E4M, K4M and E10M) containing a model protein horseradish peroxidase (1 % w/w HRP, MW 40 000) were cast from aqueous solutions and dried at 37°C. In-vitro release was determined using Franz-type diffusion cells and films were mounted directly into the Franz cell or cast onto a wound dressing (Melolin) backing. There was an initial burst release then an extended release over 5 h. The Melolin backing significantly reduced the burst but not the extended release rates (P < 0.05). Release of HRP was also determined from 7% w/v hypromellose gels and was significantly lower for E10M than E4M, suggesting that, once hydrated, the E10M hypromellose provides the greatest resistance to HRP release. The release profile of basic fibroblast growth factor from Melolin-backed films made from E4M hypromellose was not significantly different at any time point to that of HRP release from the same formulation. Hypromellose may be incorporated into a wound dressing such as Melolin to provide a prolonged release of an incorporated protein active.

Distribution of fibroblast growth factor-2 (FGF-2) within model excisional wounds following topical application

Objective

To characterise the magnitude and distribution of fibroblast growth factor-2 (FGF-2) following topical application in hypromellose gel and film formulations or a solution in an animal wound model, in order to assess the potential of this route for treatment of chronic wounds.

Method

Topical formulations of FGF-2 were applied to punch biopsy wounds, and FGF-2 levels within the wound measured. Each 12 mm diameter wound received 0.3 μg FGF-2 in solution, a 7% (w/w) hypromellose gel, a dried hypromellose film on Melolin-backing or a saline control. After 2, 5 or 8 h the wounds were horizontally dissected into four sections (surface granulation, subcutaneous fat, superficial muscle and deep muscle) which were then analysed for FGF-2 concentration using ELISA. Confocal microscopy was used to evaluate the distribution of FGF-2 within the wound.

Key findings

There were significant differences in the mean FGF-2 levels with respect to formulation and time following application (P < 0.05). FGF-2 penetrated faster into tissue when formulated as a solution than as a gel or a film. There did not appear to be a significant difference between the gel and the film with respect to total concentrations achieved in the tissue, although confocal microscopy showed differences in FGF-2 distribution within the wound.

Conclusions

Delivery of FGF-2 to wounds in a solution gave the greatest increase in tissue FGF-2 concentration when measured by ELISA and visualised using confocal microscopy. Gel and film formulations prolonged the release of FGF-2 into the wound, although FGF-2 levels were not significantly different from controls when measured by ELISA. Confocal microscopy highlighted the differences in the penetration and distribution of the FGF-2 within the wound when released from different formulations.

Insulin-like growth factor-1 gene therapy and cell transplantation in diabetic wounds

BACKGROUND

Impaired wound healing is a frequent phenomenon in diabetes mellitus. However, little is known of the fundamental cause of this pathology. The present study examined the effect of human insulin-like growth factor (hIGF)-1 overexpression in combination with autologous cell transplantation to diabetic wounds in a preclinical large-animal model.

METHODS

Diabetes was induced in Yorkshire pigs with streptozotocin. Keratinocytes were cultured and transfected with hIGF-1 or LacZ transgene. Plasmids were lipoplexed with either Lipofectin or Lipofectamin 2000. Transgene expression was assessed by enzyme-linked immunosorbent assay or X-gal staining. For in vivo studies, full-thickness wounds were created and dressed with a sealed chamber. Transfected cells were transplanted into the wounds. Wound contraction was monitored and biopsies were obtained for measurement of re-epithelialization. Wound fluid was collected and analysed for IGF-1 concentrations.

RESULTS

Quantification showed up to 740 ng/ml IGF-1 in vitro and significantly higher concentrations over 14 days compared to controls for the Lipofectamin 2000 group. Lipofectin-mediated gene transfer showed peak expression on day 2 with 68.5 ng/ml. In vivo, transfected cells showed peak expression of 457 ng/ml at day 1, followed by subsequent decline to 5 ng/ml on day 12 with Lipofectamin 2000. For Lipofectin, no significant IGF-1 expression could be detected. Gene therapy caused significantly faster wound closure (83%) than both controls (native-cell therapy = 57%; control wounds = 32%).

CONCLUSIONS

The present study demonstrates that optimized nonviral gene transfer increased IGF-1 expression in diabetic wounds by up to 900-fold. This high IGF-1 concentration in combination with cell therapy improved diabetic wound healing significantly.

Prevention and management of pressure ulcers

Pressure ulcers represent a major health problem causing a considerable amount of suffering for patients and a high financial burden for healthcare systems. The geriatric population with an increased risk of pressure ulcer development is rising constantly as a result of chronic degenerative diseases that can lead to prolonged immobilization and poor nutrition. Evidence clearly indicates that preventive measures are essential to reduce the prevalence rates of pressure ulcers; therefore healthcare professionals must be able to identify the appropriate strategies to adopt, in order to meet the individual patient's requirements. Important advances have taken place in the world of pressure ulcer treatment during the past decade. These advances are reflected in the high rate of cures being obtained. This, together with the implementation of prevention guidelines, the excellent cost/effectiveness relationship of the techniques described, and other factors, means that the field of pressure ulcers management is no more an isolated and self-administered issue in medical practice. The areas discussed here are those in which there will be linear or, in some cases, exponential growth in the decades to come.

SBD.4 stimulates regenerative processes in vitro, and wound healing in genetically diabetic mice and in human skin/severe-combined immunodeficiency mouse chimera

In search of novel angiostimulators, we performed a high-throughput screening of medicinal plants beneficial for blood circulation. From the panel of positive hits, Angelica sinensis was selected for further investigation. Purified down to a low-molecular-weight fraction and characterized by high-performance liquid chromatography-mass spectrometry, the material, named SBD.4A, revealed a particularly strong wound healing activity in the diabetic mouse wound-healing model, and in the human/severe combined immunodeficiency mouse chimera wound-healing model. In both models, SBD.4A compared favorably with the Food and Drug Administration-approved wound-healing drug becaplermin, suggesting that this botanical product could be a valuable treatment for difficult-to-heal wounds. Further high-performance liquid chromatography fractionation of SBD.4A yielded a hydrophilic fraction, which strongly stimulated endothelial cell proliferation, tridimensional endothelial cell network formation, as well as the proliferation of human dermal fibroblasts and type I collagen secretion. Because angiogenesis and fibroblast proliferation are essential for wound healing, we propose that this liquid chromatography-mass spectrometry-defined hydrophilic fraction is at least partially responsible for the wound-healing activity of SBD.4A.

Wound healing effect of silk fibroin/alginate-blended sponge in full thickness skin defect of rat

Silk fibroin (SF) and alginate (AA) have been proved to be invaluable natural materials in the field of biomedical engineering. This study was designed to compare the wound healing effect of SF, AA and SF/AA-blended sponge (SF/AA) with clinically used Nu Gauze(TM) (CONT) in a rat full thickness wound model. Two circular skin wounds on the back of rat were covered with either of CONT, SF, AA or SF/AA. On the postoperative days of 3, 7, 10 and 14, residual wound area was calculated, and skin wound tissues were biopsied to measure the area of regenerated epithelium and collagen deposition as well as the number of proliferating cell nuclear antigen (PCNA)-immunoreactive cells. Half healing time (HT(50)) of SF/AA was dramatically reduced as compared with that of SF, AA or CONT. Furthermore, SF/AA significantly increased the size of re-epithelialization and the number of PCNA positive cells, whereas the effect of SF/AA on collagen deposition was not significantly different as compared with that of SF or AA. These results demonstrate that the wound healing effect of SF/AA is the best among other treatments including SF and AA, and this synergic effect is mediated by re-epithelialization via rapid proliferation of epithelial cell.

Delivery of basic fibroblast growth factor (bFGF) from photoresponsive hydrogel scaffolds

Exogenous growth factor therapy has shown a notable promise in accelerating the healing of acute and chronic wounds. However, their susceptibility to enzymatic degradation and short contact time with the wound bed warrant the use of sophisticated delivery vehicles that stabilize the encapsulated peptides and control their rate of release. Herein, we describe the synthesis of a nitrocinnamate-derived polyethylene glycol (PEG-NC) hydrogel system and study the release kinetics of basic fibroblast growth factor (bFGF) as a function of hydrogel properties. Long-wave ultraviolet irradiation (365 nm) was used to alter the physical properties of the gel scaffold (i.e. degree of swelling) and consequently control the release rates of the encapsulated bFGF. The degree of swelling (DS) decreased from 10.7 to 8 as the length of irradiation increased from 5 to 30 min. Similarly, the DS decreased from 17.5 to 11.5 by increasing the initial PEG-NC concentration from 10 to 30 w/v% while keeping the crosslinking irradiation at 10 min. Radiolabeled I(125) studies were used to monitor the release of bFGF from PEG-NC hydrogels with variable swellabilities. By increasing the length of irradiation from 2 to 10 min the rate of bFGF release from PEG-NC gel scaffolds was decreased by 29% due to the enhanced crosslinking density. The bFGF-releasing PEG-NC hydrogels were not cytotoxic to human neonatal fibroblast cells and the released growth factor maintained its activity and induced fibroblast proliferation and collagen production in vitro. The addition of heparin within the gel scaffolds further increased the growth factor's activity.

Enhancing dermal matrix regeneration and biomechanical properties of 2nd degree-burn wounds by EGF-impregnated collagen sponge dressing

To better define the relationship between dermal regeneration and wound contraction and scar formation, the effects of epidermal growth factor (EGF) loaded in collagen sponge matrix on the fibroblast cell proliferation rate and the dermal mechanical strength were investigated. Collagen sponges with acid-soluble fraction of pig skin were prepared and incorporated with EGF at 0, 4, and 8 microg/1.7 cm2. Dermal fibroblasts were cultured to 80% confluence using DMEM, treated with the samples submerged, and the cell viability was estimated using MTT assay. A deep, 2nd degree- burn of diameter 1cm was prepared on the rabbit ear and the tested dressings were applied twice during the 15-day, post burn period. The processes of re-epithelialization and dermal regeneration were investigated until the complete wound closure day and histological analysis was performed with H-E staining. EGF increased the fibroblast cell proliferation rate. The histology showed well developed, weave-like collagen bundles and fibroblasts in EGF-treated wounds while open wounds showed irregular collagen bundles and impaired fibroblast growth. The breaking strength (944.1 +/- 35.6 vs. 411.5 +/- 57.0 Fmax, gmm(-2)) and skin resilience (11.3 +/- 1.4 vs. 6.5 +/- 0.6 mJ/mm2) were significantly increased with EGF-treated wounds as compared with open wounds, suggesting that EGF enhanced the dermal matrix formation and improved the wound mechanical strength. In conclusion, EGF-improved dermal matrix formation is related with a lower wound contraction rate. The impaired dermal regeneration observed in the open wounds could contribute to the formation of wound contraction and scar tissue development. An extraneous supply of EGF in the collagen dressing on deep, 2nd degree-burns enhanced the dermal matrix formation.

Topical epiregulin enhances repair of murine excisional wounds

Epiregulin is a broad specificity epidermal growth factor family member that activates ErbB1 and ErbB4 homodimers and all possible heterodimeric ErbB complexes. Our objective was to determine whether topical epiregulin enhanced repair of murine excisional wounds. Wounds were treated on days 0-4 with either topical epiregulin (1 micro g/ml), epidermal growth factor (10 micro g/ml), or vehicle. At day 5 postinjury, wounds receiving epiregulin were significantly smaller than those treated with epidermal growth factor or vehicle. Treatment with epiregulin promoted greater epidermal proliferation and thickening than epidermal growth factor or vehicle due to an expansion of the proliferative compartment of keratinocytes. Dermal thickness was also increased in epiregulin-treated wounds as compared to those treated with epidermal growth factor or vehicle. In day 5 wounds, matrix metalloproteinase-3 (stromelysin-1) mRNA levels were significantly lower in epiregulin- or epidermal growth factor-treated wounds than in vehicle-treated controls, suggesting that growth factor-treated wounds were more mature and required less ongoing proteolytic activity than their same-day vehicle-treated counterparts. This is the first report that topical epiregulin accelerates repair of full-thickness murine excisional wounds as compared to vehicle or epidermal growth factor. Furthermore, epiregulin is more potent and more effective than epidermal growth factor in promoting proliferation and maturation of the epidermis as well as enhancement of the neodermis.

Cellular, biochemical, and clinical aspects of wound healing

BACKGROUND

The response to tissue injury requires the symphonious interaction of immune cells, keratinocytes, fibroblasts, and endothelial cells, which unite to regenerate the damaged epithelium. Recent insights have elucidated the cellular and molecular mechanisms required for wound healing and have raised the prospect of novel therapeutic targets.

METHODS

Review of the pertinent literature.

RESULTS

The initial inflammatory response leads to the influx of macrophages and neutrophils, which release cytokines, growth factors, and nitric oxide, and induce nearby keratinocytes to migrate across the wounded epithelium. This process, known as re-epithelialization, requires integrin-mediated activation of Rho-GTPases. The subsequent influx of fibroblasts and endothelial cells results in the production of tissue stroma and formation of new blood vessels, which lead to the generation of functional tissue. Importantly, disease states associated with impaired or excessive wound healing can be attributed to defects in these responses, providing a rationale for the use of evidence-based biological therapies.

CONCLUSION

The elucidation of the cellular and biochemical response to wound healing is essential for an understanding to the treatment of clinical conditions during which impaired healing is encountered.

Gene therapy in wound repair and regeneration

The potential use of gene therapy to treat human disease increases with the development of various physical, chemical, and biological methods to deliver genes to mammalian cells, and with our rapidly expanding knowledge of the human genome. One area of therapeutic interest for gene therapy is the treatment of wound healing disorders. Most recently, recombinant human growth factor therapy has been examined as a means to treat problem wounds. However, this approach suffers from the difficulty in providing an accurate dose of growth factor and the expense of the recombinant proteins. Delivery of a gene that could be expressed within the wound is an attractive alternative to application of the protein. This review discusses several methods that have been used to deliver genes encoding growth factor proteins into wounds and the advantages/disadvantages of each approach. Novel methods to regulate the expression of the transgene are also presented, highlighting the ability of these unique vector systems to adjust gene dose as the wound heals. We expect that gene therapy will become a significant treatment modality for those wound healing pathologies refractory to other wound management approaches in the years ahead.