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CLINICAL REVIEW

Growth Factors and Chronic Wound Healing: Past, Present, and Future

Goldman, Robert MD

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Growth factors exert potent and critical influence on normal wound healing. 1 Normal wounds, however, are the exception presenting to a typical chronic wound clinic; multiple comorbidities and aging contribute, making clinical growth factor therapy more complex than in the laboratory. For this reason, perhaps, therapeutic growth factor application has been somewhat of a clinical disappointment since its introduction. However, therapeutic potential for growth factor therapy remains immense. 2

Neuropathic wounds are currently the only chronic wound type for which growth factor efficacy can be claimed according to Food and Drug Administration (FDA) guidelines. Becaplermin (Regranex Gel; Ortho-McNeil Pharmaceutical, Inc, Raritan, NJ), a recombinant DNA platelet-derived growth factor (PDGF) product, has been approved for treatment of insensate diabetic ulcers. Under FDA guidance, other growth substance formulations have been in premarket phase 2 testing 3 or are ready for pilot clinical study. 2

GROWTH FACTOR TYPES

Three types of cell-cell signaling are relevant to wound healing:

  • autocrine, in which a cell signals itself
  • paracrine, in which a cell signals its immediate neighbors
  • endocrine, in which a cell (or cell group) signals remote cells via the bloodstream.

Autocrine and paracrine communication are important in coordinating the wound healing process; they are effective at extremely low concentrations (10-9 Molar). Low concentrations are effective because of the avid and precise binding of growth factor proteins to cell surface receptors. 4 Signaling proteins are traditionally called cytokines when they are expressed during the inflammatory phase of healing, and referred to as growth factors when expressed later in the repair phase (Table 1).

Table 1
Table 1:
CYTOKINES, GROWTH SUBSTANCES, AND OTHER BIOACTIVE MOLECULES

Growth substances are critical for wound healing; however, wound healing is only one small piece of a huge puzzle. These signaling proteins are important throughout the life cycle—in multiple tissue and organ systems and in health and disease. Cancer cells, for example, permanently overexpress growth factors, which helps explain their aggressiveness. 5 In a healthy state, certain signaling molecules (colony-stimulating factors) that stimulate bone marrow to produce blood cells also help direct wound inflammation.

The last several decades have seen an explosion of knowledge related to normal wound healing. Healing is understood to be a sequence of hundreds, if not thousands, of individual steps from wound formation to closure. Researchers have developed both in vitro and in vivo models to study healing. The scientific analysis of healing has resulted in a wide array of cell signaling molecules as important in vitro and in vivo. Although an alphabet soup of cytokines and growth factors is at first confusing, a chronologic sequence has emerged that ties different growth factors to different phases of healing.

NORMAL WOUND HEALING

At the clinical level, healing is defined as complete closing of the integument. 6 For normal (nonchronic) healing of a primary intention wound, the healing process is complete in about 3 to 14 days. The sequence of events includes wound formation, inflammation, provisional matrix formation, collagen syntheses, epithelialization, neoangiogenesis and, finally, wound closure.

Inflammation is characterized by the breakdown of preexisting tissue scaffolding and cleanup of cellular, extracellular, and pathogen debris. Matrix breakdown also enables migration of neutrophils, macrophages, epidermal cells, and fibroblasts to the injury site, enabling repair. Repair is analogous to the construction of a new building, with a “framework” of extracellular matrix components (provisional matrix of glycosaminoglycans—protein-sugar complexes and fibronectin) attached to “rivets,” or cell attachment sites (integrins). On this framework are reinforced “girders” of type I collagen, which are secreted in sections (fibrils) and self-assembled extracellularly. Over this structure, during construction, a “roof” of epidermal cells advances over the defect to effect a durable cover and a “plumbing” network of neovessels accrete to supply oxygen and nutrients. After closure, remodeling of dermal matrix occurs, during which collagen fibers are preferentially retained along lines of stress.

Each stage is governed by cytokines and growth factors communicating by autocrine and paracrine means (Figures 1, 2, and 3). Shortly after wounding, platelets within the fibrin plug release growth substances and cytokines, including PDGF, that are chemotactic for neutrophils. Neutrophils are substantially replaced within a few days by activated macrophages that release cytokines, including PDGF, tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), and interleukin-6 (IL-6). 7 IL-1 plays a key role in amplifying the inflammatory response by (1) inducing fibroblasts and endothelial cells to synthesize and secrete more IL-1 and other proinflammatory cytokines, including IL-6, granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF); and (2) inducing wound monocytes to release more interleukins and PDGF. 5

Figure 1.
Figure 1.:
INFLAMMATORY STAGE Normal wound healing, as directed by cytokines and growth factors, occurs in 4 stages: inflammation, epithelialization, neoangiogenisis, and provisional matrix formation and collagen matrix formation. For each stage to occur, communication must occur between different cell types. The simplified communication profile show that the inflammatory stage is directed by inflammatory cells (first neutrophils, then macrophages) that release multiple cytokines, including IL-1; cytokines induce expression of fibroblasts and endothelial cells and cause macrophage expression of more cytokines. These multiple, positive feedback loops amplify the inflammatory response. Macrophages also release NO, which kills pathogens, and proteases, which break down the damaged and necrotic matrix. As inflammation subsides, wound repair takes place
Figure 2.
Figure 2.:
EARLY WOUND REPAIR Early wound healing involves 3 simultaneous events: epithelialization, neoangiogenesis, and provisional matrix formation. Epithelialization (Figure 2A) is directed by fibroblasts that express KGF-2 and IL-6, which direct keratinocytes to proliferate and migrate. Initially, macrophages direct fibroblasts to express IL-1 and KGF-2. Later, keratinocytes self-express IL-6 and NO to perpetuate the process. Keratinocytes direct neoangiogenesis (Figure 2B) at the wound edge by expression of VEGF, which then causes proliferation of endothelial cells and formation of capillaries. Initially, macrophages direct this process by release of cytokines. Fibroblasts also express KGF and TGF-β to induce VEGF. VEGF expression is upregulated in the presence of NO, which may arise from endothelial cells and (speculatively) keratinocytes. Macrophages initially direct provisional matrix formation (Figure 2C; illustrated as a meshwork) by release of cytokines TNF-α and PDGF. Fibroblasts self-direct this process later partially by autocrine and paracrine expression of PDGF. TGF-β exists mainly in an inactive state in extracellular matrix and is activated by protease expression by macrophages and fibroblasts.
Figure 3.
Figure 3.:
LATER WOUND REPAIR AND COLLAGEN MATRIX FORMATION TGF-β directs collagen matrix expression. The illustrated meshwork is thicker, indicating deposition of collagen fibers over the provisional matrix network. TGF-β also upregulates tissue inhibitors of metalloproteinase (TIMP) to facilitate matrix construction. Collagen matrix and microvascular and epithelial components are in place at wound closure.

During the inflammatory stage, polymorphonuclear leukocytes (PMNs) phagocytize, invading organisms and debris and releasing proteolytic enzymes to destroy the invading organisms and digest nonviable tissue. There are several protease classes, depending on preferred target protein, amino acid, or metal ion within the enzyme. Serine proteases have broad substrate specificity (eg, elastase). Metalloproteinase, containing a zinc ion, is more specific; several specifically degrade collagen. Both protease types destroy preexisting extracellular matrix. Matrix in unwounded tissue is protected by an antiprotease shield comprised of many types of protease inhibitors. 8 This shield can be overwhelmed by the massive release of proteases by PMNs in the inflammatory phase of wound healing. Additionally, PMNs generate (via the myeloperoxidase pathway) reactive oxygen-free radicals that combine with chloride and assist in bacterial killing 8 within acute wounds. As acute wounds become longer lasting, PMNs are replaced by macrophages that lack myeloperoxidase but assist in proteolysis and pathogen killing.

Cytokines TNF and IL-1 may activate inducible nitric oxide synthase (iNOS) in macrophages to synthesize large amounts of nitric oxide (NO). 9 iNOS activity in a rat wound model peaks within 24 hours after experimental wounding and localizes to macrophages. Macrophage-synthesized NO reacts with peroxide ion O2 radicals to yield more toxic peroxynitrite and hydroxyl radicals for pathogen killing. NO helps kill Staphylococcus aureus, prevents the replication of DNA viruses within cells, and serves as an immune regulator. 9

Protease transcription is triggered by inflammatory cytokines; for example, TNF-α induces matrix metalloproteinase (MMP) transcription. 10 MMPs clear inflammatory debris and enable migration of individual wound cells through extracellular matrix. To enable the migration, MMPs are expressed by keratinocytes, fibroblasts, monocytes, and macrophages, even after inflammation and in the repair phase of healing.

It is not clear what factors enable the transition between inflammation and repair, although removal of noxious stimuli (eg, pathogens, eschar, hypoxia, repetitive trauma) is pivotal. As the general inflammatory response lessens, growth factors gradually replace cytokines in the wound fluid, 11 proteolytic activity diminishes, and the antiprotease shield is reestablished. 8 Epithelialization in normal wounds starts soon after wound formation and is partially initiated by inflammatory cytokines (Figure 2A). Cytokines IL-1 and TNF-α upregulate KGF gene expression in fibroblasts 12; fibroblast-secreted keratinocyte growth factors KGF-1, KGF-2, and IL-6 help keratinocytes proliferate, migrate into the defect, and then differentiate into the epidermis. 13,14 Recent studies suggest that KGF-2 is important in directing epithelialization of human wounds. 15,16 Keratinocytes and macrophages also express IL-6, which provides additional positive stimulation for keratinocytes to migrate and proliferate.

Nitric oxide is an important signaling molecule in epithelialization. It is hypothesized that NO synthesized by intact basal keratinocytes is a potent stimulus for keratinocyte proliferation and migration into the wound edge. As keratinocytes increase in density, NO concentration increases. A relatively high NO concentration signals keratinocytes to stop proliferating and start differentiating into the recognizable epidermis, with multiple layers. NO may be as potent a regulator of reepithelialization as KGF. 17

Another critical component of wound repair is neovascularization—granulation tissue engorges the repair site with blood and nutrients carried by capillaries and microvessels (Figure 2B). This microvascular network is induced by vascular endothelial growth factor (VEGF), synthesized mainly by keratinocytes at the wound edge. (IL-1, TNF-α, KGF, and TGF-β1 induce VEGF expression by keratinocytes.) When VEGF is present, subadjacent dermal endothelial cells proliferate and form capillary tubes. VEGF production is amplified in the presence of NO 18 synthesized by endothelial cells, with many important effects on microcirculation; endothelial cells express endothelial nitric oxide synthase (eNOS). eNOS-deficient mice show marked deficits in neovascularization during wound healing. 19 When capillaries form, eNOS-generated NO protects tissue from the toxic effects of ischemia by inducing vasodilatation and protecting against reperfusion injury. 20

One major step in early wound repair is proliferation of fibroblasts in response to PDGF (Figure 2C). PDGF is a signaling protein with a major role in the inflammation and repair phases of wound healing and an autocrine and paracrine amplifier of further PDGF expression by fibroblasts. In addition, fibroblasts synthesize provisional matrix comprised of glycosaminoglycans and fibronectin 21 in response to PDGF. Integrins, a matrix component, anchor cells to provisional matrix and are upregulated by TNF-α. 22 Finally, TGF-β, a potent mediator of collagen matrix formation, is expressed. TGF-β exists as an inactive precursor in extracellular matrix; proteolytic activity (eg, serine protease) liberates active TGF-β to direct cellular processes. 23

The later repair phase involves extracellular matrix production and a decrease in degradation, both directed by TGF-β23 (Figure 4). In normal wound repair, TGF-β peaks 7 to 14 days after formation of incisional wounds. 24 TGF-β is associated with fibroblast synthesis of type I collagen, decreased production of MMPs, enhanced expression of tissue inhibitors of metalloproteinase (TIMP), and enhanced expression of cell adhesion proteins, 5 all associated with granulation tissue formation. Large granulating wounds from pressure, neuropathy, or surgery (and, to a lesser extent, venous ulcers) heal by contraction that is, at least partially, directed by TGF-β. 25 Contraction and epithelialization directly lead to wound closure.

Figure 4.
Figure 4.:
BECAPLERMIN (PDGF-BB) Platelet-derived growth factor consists of 2 globular protein subunits connected by several molecular bonds. Each subunit attaches to a receptor positioned across the cell membrane with an intracellular domain. The intracellular domain contains an enzyme that initiates an intracellular signaling process (leading to a cell-specific wound response) only when 2 receptors are brought in close contact by attachment to the PDGF dimer. (A dimer is a protein with 2 subunits.) PDGF has an A or B subunit, depending on its amino acid sequence, that forms 1 of 3 isoforms: PDGF-AA, PDGF-AB, and PDGF-BB. All are active in wounds, however, PDGF-BB is the active ingredient of becaplermin. Becaplermin is PDGF-BB that has been produced from genetically engineered yeast cells into which the gene for the B-chain of PDGF has been inserted; the correct term is rh-PDGF-BB.

During the remodeling phase, the dermis becomes stronger, “turning over” collagen fibers so they are retained preferentially along lines of stress. Derangement of the remodeling mechanism occurs during keloid formation. Keloid-prone patients demonstrate an alteration of TGF growth factors; TGF-β1 and TGF-β2 (2 TGF-β isoforms) are both synthesized in excess by in vitro keloid fibroblasts. 26 Keloid fibroblasts also show increased sensitivity to TGF-β2 compared with normal skin fibroblasts. 27 Both mechanisms implicate TGF-β1 and TGF-β2 in keloid formation.

TGF-β also induces connective tissue growth factor (CTGF), a repair-related growth factor that facilitates wound repair but leads to cutaneous fibrosis in the pathologic condition. CTGF is diffusely active within keloids and other skin fibroses, such as scleroderma, systemic sclerosis, and Dupuytren contracture. 28

CHRONIC WOUND PATHOLOGY

Although a cycle of tissue damage and inflammation occurs in chronic wounds, tissue damage is primary to some external noxious stimulus, such as edema, mechanical injury, persistent eschar, local pathogens, debris, or infection. Eliminating the primary noxious factor by conservative wound care effectively heals the majority (up to 80%) of chronic wounds. In general, growth factor therapy is most effective in chronic wounds when combined with standard wound care and good wound bed preparation. 29–31

In contrast, inflammation is primary and tissue injury is secondary in autoimmune diseases such as rheumatoid arthritis (RA). RA now responds to treatment with a TNF-α blocker. 32 Cytokines are also important in the pathogenesis of cutaneous inflammatory disease, such as psoriasis. 33 Whether primary or secondary, chronic inflammation is associated at the molecular level with a persistence and excess of proinflammatory cytokines and proteases.

Venous stasis disease

The primary noxious stimulus in venous stasis disease is edema resulting from venous hypertension associated with pericapillary cuffing of fibrin, macroglobins, and other macromolecules. 34 It has been postulated that fibrin cuffs sequester growth substances, making them unavailable to trigger healing. With standard compression therapy, growth factors might become more available in the wound microenvironment. Most venous leg ulcers respond positively to standard compression therapy and a moist wound environment. After this treatment, venous ulcer fluid collected from beneath hydrocolloid dressings has been found to induce rapid proliferation of keratinocytes in cell culture, 35 suggesting that wound fluid from healing venous stasis lesions is rich in growth factors.

Even with growth factors present, wound cells might be unresponsive due to a lack of cell surface receptors. There is evidence that chronic untreated venous stasis ulcers lack intact receptors for TGF-β. 36 The missing receptor component signals the cell that TGF-β is bound to the receptor. When the ulcer is treated with compression and positive healing ensues, complete receptors are noted and signaling occurs, leading to wound repair.

For untreated venous stasis wounds, proteases are thought to overwhelm the antiproteinase shield, perpetuating tissue injury and deactivating growth factors 37 that could otherwise trigger healing. In healing wounds, proteases and antiproteases appear to become balanced. Levels of antiprotease tissue inhibitor of metalloproteinase-1 (TIMP-1) increase more than 10-fold 38 as chronic venous stasis ulcers began to heal. Proteases, abundant in chronic, nontreated wounds, decrease with leg elevation 39; elevated levels of MMP activity decrease significantly as wounds began to heal. It is believed that standard compression and moist wound healing techniques transform a chronic wound into an acute wound in terms of molecular biology and healing characteristics.

Neuropathic wounds

The primary noxious stimulus in neuropathic wounds is repeated axial pressure and shear. This noxious pathomechanic stimulus leads to a chronic inflammatory state (ie, excess cytokines and proteases) that delays healing. However, pressure and shear might delay healing by molecular mechanisms yet to be elaborated: Low levels of mechanical force (ie, fluid flow) augment growth factor effects by facilitating convection of growth factors to the exact cell location. 40,41 At higher levels, mechanical stimulation might directly cause matrix disassembly. Fibroblasts in the periodontal ligament, for example, express (ie, transcribe) the RNA for tissue plasminogen activator (t-PA). 42 The t-PA converts plasminogen to the protease plasmin, which degrades extracellular matrix.

Pressure ulcers

Pressure ulcers (PrUs), in addition to sources of chronicity, may be a result of direct necrosis from prolonged tissue hypoxia, such as PrUs that hypothetically develop on the operating table. A more subtle mechanism of tissue death, however, is ischemia/reperfusion injury. 43 After myocardial infarction and cerebrovascular accident, ischemia/reperfusion injury is an important process leading to tissue injury. Cyclical application and release of pressure are involved with PrUs, giving opportunity for reperfusion injury and facilitating wound formation. Reperfusion injury involves endothelial damage, neutrophil margination and extravasation, vasoconstriction, thrombosis, and hypoxia. 20 Neutrophils produce superoxide free radicals that contribute to vasoconstriction and hypoxia. Reperfusion injury is mitigated by tissue NO, a vasodilator. 20 Additionally, by augmenting endothelial response to VEGF, NO promotes neovascularization. 18

ANIMAL MODELS

Growth substances are externally applied to animal wounds (exogenous). In other research, investigators study naturally occurring (endogenous) growth substances. Normal animals are used for research on both endogenous and exogenous growth substances because they are straightforward to study.

Although the understanding of growth factor effects and normal wound healing has advanced dramatically, chronic wound healing remains obscure. There is a limited understanding of the influence of human comorbidities, such as hypoxia, neuropathy, and aging pathologies on chronic wounds. Acknowledging this, researchers are now more sophisticated in re-creating human comorbidities in animal models, including ischemia, aging, 44 diabetes, 45 and glucocorticoid (ie, steroid) treatment. 46 Animal models, however, do not represent the human comorbidities of spinal cord injury, poor nutrition, anemia, and end-stage renal disease because of the cost and complexity to develop and maintain these preparations.

Techniques similar to cloning are now used in wound healing research. For example, genetically engineered mice that have missing genes (ie, knockout mice) that overexpress genes for specific growth factors further our understanding of the mechanisms of action of growth substances. 47,48 Knockout mice, missing the gene for inducible NO synthase, exhibit marked deficits in wound healing. 48 For growth factors, animal testing has not focused on a specific animal or wound type, but on the wound process (eg, wound repair). 49

CELL SIGNALING

The effect of medical comorbidities on wound healing has been studied in animal models. A brief sampling of effects and cell signaling alterations are presented in Table 2.

Table 2
Table 2:
EXPERIMENTAL WOUNDING YIELDS INSIGHT INTO CHRONIC WOUND HEALING

EXOGENOUS GROWTH FACTORS

Preclinical studies

Preclinical studies have explored the effect of exogenous application of cytokines and growth factors on wounds in animal models. These studies have led to clinical trials investigating efficacy of GM-CSF and KGF for venous stasis ulcers and PDGF for pressure and neuropathic ulcers.

Colony-stimulating factors enhance a general wound healing response, working directly on macrophages and monocytes (not fibroblasts, keratinocytes, or endothelial cells). This is the basis for the rabbit ear chronic wound model study, which hypothesizes that M-CSF accelerates healing as a result of generalized macrophage activation. Wu et al 50 determined that wounds sustain a 30% increase in granulation volume and a 5-fold increase in TGF-β transcription.

One preclinical study hypothesized whether KGF-2 could induce faster epithelialization on human skin-mesh grafts on athymic nude rats. These rats retain human skin grafts without rejection, with grafts followed as they epithelialize. Epithelialization increased significantly but wound contraction did not increase, suggesting KGF-2 holds promise as a treatment for wounds with slow epithelialization rates, such as venous stasis ulcers. 16

Exogenous PDGF promotes wound repair; applied to wound models, it elicits a more typical histologic repair response than TGF-β and basic fibroblast growth factor (FGFb) 21 (Figure 4). The efficacy of PDGF is part of the motivation for premarket clinical trials involving pressure and neuropathic ulcer healing. Becaplermin (PDGF-BB) is FDA-approved for treatment of neuropathic ulcers.

Premarket clinical studies

In critically evaluating the results of clinical trials of wound healing agents and exogenous growth factors, it is important to: (1) acknowledge any significant improvement of growth factor over control; and (2) appreciate the outcome of the control arm by itself. If the healing rate for the control arm is much slower than benchmark criteria (ie, outcome employing accepted care standards), it might be questioned if the growth factor enhances the standard of care.

For venous stasis ulcers, the best conservative care involves maintenance of a moist wound environment 51 and adequate compression. A benchmark for the standard-of-care comes from Lyon et al. 52 After 12 weeks of observation of venous stasis ulcers treated with occlusive hydrocolloid dressings and standard compression, 55% of subjects in the control arm healed. These results are supported by other major observational studies of venous stasis ulcer healing employing compressive dressings. 53,54 McGuckin et al 53 developed a multidisciplinary guideline for venous ulcer treatment; using this algorithm prospectively, 100% healing was obtained for greater than 50% of patients in less than 12 weeks.

It could be argued that these excellent outcomes might be expected for community-based treatment of uncomplicated wounds and that patients with more chronic and larger wounds preferentially elect to participate in growth factor trials leading to relatively poor outcomes. However, local wound care is not a black box, but a critical quality standard against which growth factor clinical trial outcomes must be discussed and judged.

Marques da Costa et al 55 used compression dressings for a randomized, double-blind, placebo-controlled trial of recombinant human granulocyte colony-stimulating factor (rhGM-CSF) for treatment of venous stasis ulcers. The selected ulcers were less than 30 cm2 and well-perfused (ankle brachial index, 0.8). These ulcers were treated with an experimental agent with 4 equidistant injections near the poles of the target wound. The treatment was placebo (vehicle) or 200 μg or 400 μg of rhGM-CSF, administered once a week for 4 weeks or until wound healing occurred. Conventional wound care included 4-layer compression bandaging, debridement, and plain gauze dressings. Using this standard care, the percentage of ulcers with complete closure at 13 weeks was 61%, 57%, and 19% for 400 μg, 200 μg, and placebo, respectively. The healing rate in the placebo group (19% healed at 13 weeks) is much lower than the benchmark of 55% healed employing compression and moist wound environment. 52 If a moist wound environment (rather than plain gauze dressings) was employed, it is unclear if the relative outcome with GM-CSF would be as favorable.

In a retrospective uncontrolled trial, Jaschke et al, 56 applied GM-CSF to stasis ulcers, in addition to standard compression, and reported 91% of the treated ulcers healed compared with conventional care (typically, 70%). A controlled trial should follow this study to determine if GM-CSF actually performs better than the standard of care.

Robson et al 3 conducted a phase 2A multicenter, randomized, double-blind, placebo-controlled trial that investigated the effect of KGF-2 on chronic venous stasis ulcers of 3 to 36 months duration. The 94 study subjects were otherwise treated with standard compression therapy. Compression dressings were changed twice a week, with placebo or 20 μg/cm2 or 60 μg/cm2 of KGF-2 (repifermin) applied topically during dressing changes. Results were statistically analyzed by combining data at both active doses. Investigators found significantly improved healing rates during a 12-week observation period (ie, number of wounds that reached 25% of initial wound area). At 12 weeks, 29% of placebo-treated wounds had healed. The proportion of wounds healed in the placebo group increased to 40% (consistent with benchmark criteria) when results were stratified to include only wounds of less than 15 cm2 and of less than 18 months’ duration. For this subgroup, 60% of wounds receiving 60 μg/cm2 healed (not significant). However, if both active doses were pooled, there was a significant improvement in the proportion of wounds that were 75% and 90% healed at 12 weeks. Based on these promising results, a phase 2B trial was under taken. However, the company that manufactures repifermin (Human Genome Sciences, Rockville, MD) recently announced that the study did not meet its primary endpoint. The percentage of patients treated with repifermin who achieved complete wound closure within 20 weeks of treatment was not statistically significantly different from the placebo groups. Nor were there any favorable trends in the treatment group. As a result, the company has ceased development of repifermin for the treatment of chronic venous ulcers.

After an early phase 2 study of PrU treatment with FGFb yielded mixed results, 57 attention turned to PDGF, which caused a more balanced repair response than FGFb in a preclinical study. 27 The first PDGF study was a 4-week, phase 2, double-blind, prospective design investigating the effectiveness of rh-PDGF-BB (becaplermin) for treatment of Stage III spinal cord PrUs (patients aged <50 years). Although results were statistically significant, 58,59 clinical significance was less clear and a longer, 12-week clinical trial took place.

Rees et al 60 conducted a phase 2, multicenter, double-blind, parallel group, placebo-controlled trial involving 124 subjects (20 females and 104 males) with Stage III or IV chronic PrUs. The target ulcer was required to have a volume of 10 to 150 mL; change in wound volume was the primary outcome variable. “Good wound care” included debridement of fibrin and necrotic debris, systemic treatment of infections, and off-loading (although active support surfaces were not used). Topical care included saline-moistened gauze over becaplermin. The treatment arms (16 weeks) were becaplermin at 300 μg/g once a day, 100 μg/g twice a day, and placebo gel. The incidence of complete healing for the 300 μg/g, 100 μg/g, and control was 19%, 23%, and 0%, respectively. The fact that no wounds (0%) healed in the control group is below benchmark expectations.

In a prospective, longitudinal study of elderly nursing home residents, Brandeis et al 61 analyzed prospective data for 19,889 residents of 51 nursing homes in 11 states, all managed by 1 corporation. Strict quality standards were established for PrU definition, documentation, and treatment. After 3 months of conservative treatment, 18% to 23% of patients with Stage III PrUs and 25% to 32% of patients with Stage IV PrUs had healed. In another study, 62 after several months of treatment with hydrocolloid dressings, clean, small (<2 cm wide) Stage III PrUs resulted in a similar percentage healed. If similar outcomes occurred for the control arm of the Rees et al becaplermin study, 60 the relative effectiveness of PDGF-BB would be established. Due to continuing concerns about effectiveness, the FDA has not approved labeling supporting becaplermin for PrU treatment.

In a multicenter, randomized, prospective, double-blind, parallel-group, placebo-controlled trial, 63 significant overall improvement (20%) was seen in wound closure of lower extremity diabetic ulcers after 20 weeks of treatment with rh-PDGF-BB compared with placebo and good wound care alone, with adverse events (eg, infection) similar between groups. 64 Certain evidence suggests that the best results are obtained with PDGF-BB when wounds are also aggressively debrided, a process which removes senescent fibroblasts and pathogens. 65 Endogenous PDGF appears to cause paracrine amplification of exogenous PDGF. 66 Based on these studies, the FDA has allowed rh-PDGF-BB to be marketed for treatment of nonhypoxic neuropathic diabetic foot ulcers.

Steed et al 63 determined that, although 50% of growth factor-treated, uncomplicated neuropathic ulcers healed at 20 weeks, the proportion healed for control is 30%. The 30% unfavorably compares with total contact casting, a clinical benchmark for neuropathic ulcer healing. Casting has healing rates of 70%67 to 90%68 for healing uncomplicated neuropathic ulcers in retrospective trials. There has been no direct comparison of PDGF with total contact casting. Because total contact casts that are changed weekly preclude the recommended (daily) application of becaplermin, total contact casting may be the better choice for uncomplicated neuropathic wounds. However, if neuropathic wounds are not casted, PDGF is a reasonable choice.

EXOGENOUS TOPICAL APPLICATION

The first topical preparation available for periodic application was platelet-derived wound healing factor (PDWHF). PDWHF (Procuren) was a heterogenous, patient-specific mixture of growth substances originally developed by Curative Technologies and dispensed at a network of wound clinics under controlled conditions. Although 2 initial studies with PDWHF demonstrated efficacy, 69,70 a prospective, randomized, controlled trial could not replicate the results. 71 A more recent, large retrospective study of PDWHF, however, revealed increased effectiveness for healing diabetic neuropathic ulcers over standard treatment (ie, 15% to 50% increased chance of healing). 72 PDWHF was especially effective for larger wounds involving deeper structures (eg, tendon and fascia). Curative has since sold the manufacturing, sales, and distribution rights to Cytomedix. Cytomedix has renamed the substance AutoloGel Wound Care Therapy, which is currently available through licensed wound care centers.

PDGF-BB is available for neuropathic ulcers and, at physician discretion, off-label for other wound types. The cost of PDGF-BB ($400/tube) and the requirement for twice-a-day dressing changes raises questions about its appropriate use within wound care practice. Prudence suggests that PDGF-BB be reserved for wounds that fail optimum conservative treatment; however, this clinical approach is not evidence-based. Entry criteria for growth factor clinical trials have all been for chronic wounds of a particular type, not the 20% to 30% of chronic intractable wounds that fail optimum conservative treatment. Optimal conventional wound care is discussed at length in the literature. 73–76

PDGF-BB shows promise for treatment of complex wounds with exposed tendon or fascia (wounds, therefore, that are not candidates for casting) based on anecdotal data only. In case series, rh-PDGF-BB empirically promotes replacement of tendon or fascia with granulation tissue for complex leg wounds. 77,78 This unique niche for complex wound treatment has not yet been systematically studied in controlled trials.

ADJUNCTIVE MODALITIES

Treatment of complex intractable wounds requires consideration of growth factors in combination with adjunctive modalities. Adjunctive modalities, including electrical stimulation and hypberbaric oxygen (HBO), might be important for healing ischemic wounds. The HBO clinical protocol involves breathing 2 atmospheres pure oxygen for approximately 1 hour a day for about 1 month. The mechanism by which HBO augments ischemic wound healing involves NO expression. 79 In the ischemic rabbit ear model, the combination of HBO with either PDGF-BB or TGF-β1 has a synergistic effect that totally reverses the healing deficit induced by ischemia. 80 This suggests that future clinical protocols might empirically benefit from combined application of growth factors and hyperbaric oxygen.

There is an indication that short-pulse, high-amplitude electrotherapy (eg, high-voltage pulsed current [HVPC]) induces production of vasoactive substances by nerve endings (including vasoactive intestinal polypeptide). These vasoactive substances lead to increased NO release by endothelial cells, 81 presumably via the eNOS pathway. Endothelial cell synthesized NO promotes microperfusion and is thought to protect against reperfusion injury, 20 which is a reasonable hypothesis for why electrotherapy is reported effective in treatment of ischemic 82 and pressure ulcers. 83–86 The combination of growth substances and electrotherapy has yet to be studied in controlled trials.

THE FUTURE: GENE THERAPY

An important consideration implied from the above discussion is that repeated daily applications of topical growth factor produce, at best, a modest effect. If human growth factor genes could be inserted into wound cells, however, the response might be dramatic, efficient, and long lasting. Liechty and colleagues 87 have described this dramatic effect for the rabbit ischemic ear model. They introduced a weakened adenoviral vector of the transgene for human PDGF-AA into the wound environment. (A vector is the carrier of the DNA material into the human gene; in this case, an adenovirus similar to that which causes the common cold). At 3 days postinjection, viral vector was still present; however, it could not be detected at 1-week postinjection. Although the vector quickly disappeared, the wound healing effect was long-lasting and dramatic. A single vector application caused long-term granulation tissue volume, epithelialization rate, and wound closure rate that exceeded those of ischemic and nonischemic normal controls. This rapid, robust, and long-lasting healing response in a difficult-to-heal wound model, in response to 1 application, suggests a bright future for transgenic growth factors for the field of chronic wound healing.

Based on this success, Margolis et al 2 at the University of Pennsylvania have planned a phase 1 study of PDGF-BB injected as an adenoviral transgene (H5.020CMV.PDGF-b) into chronic neuropathic ulcers (up to 4 increasing doses) to determine maximum tolerated dose. If an acceptably low adverse effect profile occurs at a given dose, a subsequent pilot efficacy study will assess healing in response to H5.020CMV.PDGF-b. The potential clinical ramifications are exciting. For instance, transgene administration could be combined with total contact casting to effect rapid, reliable closure of intractable wounds on patients with multiple comorbidities.

However, safety must be assured. The gene therapy pathway will be long and arduous. Due to scrutiny by the FDA and National Institutes of Health, human gene therapy experiments will be carefully and appropriately monitored for safety; appropriate efforts to ensure safety might delay commercialization.

CONCLUSION

Certain growth factor preparations are FDA-approved for neuropathic ulcer treatment and for premarket clinical trials for venous stasis ulcers. Results are modest and should be applied only in the context of optimal conservative wound care. The long-term outlook remains promising. In 10 to 20 years, growth factor-based gene therapy might revolutionize treatment of chronic wounds.

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