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Transdermal deferoxamine prevents pressure-induced diabetic ulcers.

Author information

Affiliations

  • 1. Hagey Laboratory for Pediatric Regenerative Medicine, Division of Plastic and Reconstructive Surgery, Department of Surgery, Stanford University School of Medicine, Stanford, CA 94305;
      Authors
    • Duscher D 1
    • Neofytou E 1
    • Maan ZN 1
    • Rennert RC 1
    • Januszyk M 1
    • Rodrigues M 1
    • Whitmore AJ 1
    • Walmsley GG 1
    • Galvez MG 1
    • Whittam AJ 1
    • Gurtner GC 1
    (11 authors)
  • 2. Department of Plastic Surgery, Johns Hopkins University School of Medicine, Baltimore, MD 21201;
      Authors
    • Wong VW 2
    (1 author)
  • 3. Biomaterials and Advanced Drug Delivery Center, Stanford University School of Medicine, Stanford, CA 94305; and.
      Authors
    • Inayathullah M 3
    • Malkovskiy AV 3
    (2 authors)
  • 4. Diabetes Research Center, Albert Einstein College of Medicine, New York, NY 10461.
      Authors
    • Brownlee M 4
    (1 author)
  • 5. Biomaterials and Advanced Drug Delivery Center, Stanford University School of Medicine, Stanford, CA 94305; and
      Authors
    • Rajadas J 5
    (1 author)

ORCIDs linked to this article

Proceedings of the National Academy of Sciences of the United States of America, 22 Dec 2014, 112(1):94-99
https://doi.org/10.1073/pnas.1413445112 PMID: 25535360 PMCID: PMC4291638

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Abstract 

There is a high mortality in patients with diabetes and severe pressure ulcers. For example, chronic pressure sores of the heels often lead to limb loss in diabetic patients. A major factor underlying this is reduced neovascularization caused by impaired activity of the transcription factor hypoxia inducible factor-1 alpha (HIF-1α). In diabetes, HIF-1α function is compromised by a high glucose-induced and reactive oxygen species-mediated modification of its coactivator p300, leading to impaired HIF-1α transactivation. We examined whether local enhancement of HIF-1α activity would improve diabetic wound healing and minimize the severity of diabetic ulcers. To improve HIF-1α activity we designed a transdermal drug delivery system (TDDS) containing the FDA-approved small molecule deferoxamine (DFO), an iron chelator that increases HIF-1α transactivation in diabetes by preventing iron-catalyzed reactive oxygen stress. Applying this TDDS to a pressure-induced ulcer model in diabetic mice, we found that transdermal delivery of DFO significantly improved wound healing. Unexpectedly, prophylactic application of this transdermal delivery system also prevented diabetic ulcer formation. DFO-treated wounds demonstrated increased collagen density, improved neovascularization, and reduction of free radical formation, leading to decreased cell death. These findings suggest that transdermal delivery of DFO provides a targeted means to both prevent ulcer formation and accelerate diabetic wound healing with the potential for rapid clinical translation.

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Proc Natl Acad Sci U S A. 2015 Jan 6; 112(1): 94–99.
Published online 2014 Dec 22. https://doi.org/10.1073/pnas.1413445112
PMCID: PMC4291638
PMID: 25535360

Transdermal deferoxamine prevents pressure-induced diabetic ulcers

Dominik Duscher,a,1 Evgenios Neofytou,a,1 Victor W. Wong,b Zeshaan N. Maan,a Robert C. Rennert,a Mohammed Inayathullah,c Michael Januszyk,a Melanie Rodrigues,a Andrey V. Malkovskiy,c Arnetha J. Whitmore,a Graham G. Walmsley,a Michael G. Galvez,a Alexander J. Whittam,a Michael Brownlee,d Jayakumar Rajadas,c,2 and Geoffrey C. Gurtnera,2
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Associated Data

Supplementary Materials
Significance

Diabetes is the leading cause of nontraumatic amputations. There are no effective therapies to prevent diabetic ulcer formation and only modestly effective technologies to help with their healing. To enhance diabetic wound healing we designed a transdermal delivery system containing the FDA-approved small molecule deferoxamine, an iron chelator that increases defective hypoxia inducible factor-1 alpha transactivation in diabetes by preventing iron-catalyzed reactive oxygen stress. This system overcomes the challenge of delivering hydrophilic molecules through the normally impermeable stratum corneum and both prevents diabetic ulcer formation and improves the healing of existing diabetic wounds. This represents a prophylactic pharmacological agent to prevent ulcer formation that is rapidly translatable into the clinic and has the potential to ultimately transform the care and prevention of diabetic complications.

Keywords: wound healing, diabetes, drug delivery, small molecule, angiogenesis
Abstract

There is a high mortality in patients with diabetes and severe pressure ulcers. For example, chronic pressure sores of the heels often lead to limb loss in diabetic patients. A major factor underlying this is reduced neovascularization caused by impaired activity of the transcription factor hypoxia inducible factor-1 alpha (HIF-1α). In diabetes, HIF-1α function is compromised by a high glucose-induced and reactive oxygen species-mediated modification of its coactivator p300, leading to impaired HIF-1α transactivation. We examined whether local enhancement of HIF-1α activity would improve diabetic wound healing and minimize the severity of diabetic ulcers. To improve HIF-1α activity we designed a transdermal drug delivery system (TDDS) containing the FDA-approved small molecule deferoxamine (DFO), an iron chelator that increases HIF-1α transactivation in diabetes by preventing iron-catalyzed reactive oxygen stress. Applying this TDDS to a pressure-induced ulcer model in diabetic mice, we found that transdermal delivery of DFO significantly improved wound healing. Unexpectedly, prophylactic application of this transdermal delivery system also prevented diabetic ulcer formation. DFO-treated wounds demonstrated increased collagen density, improved neovascularization, and reduction of free radical formation, leading to decreased cell death. These findings suggest that transdermal delivery of DFO provides a targeted means to both prevent ulcer formation and accelerate diabetic wound healing with the potential for rapid clinical translation.

Diabetes mellitus affects over 25 million people in the United States (1, 2) and costs nearly $250 billion per year (3). Chronic diabetic wounds and decubiti are important long-term sequalae of both diabetes mellitus types 1 and 2 (4). There is a high mortality in diabetic patients who develop decubiti (5–7), and owing to prolonged disability and the high rates of recurrence these wounds represent an especially severe complication of diabetes (8). This is further underscored by the fact that diabetic nonhealing wounds are the leading cause of nontraumatic amputations in the United States (3, 9–11). As such, there is a clear need for new approaches to effectively manage and treat diabetic ulcers.

The propensity for wound development in diabetes is associated with a reduced capacity for ischemia-driven neovascularization (12, 13). Hypoxia inducible factor-1 (HIF-1), which consists of a highly regulated α-subunit and a constitutively expressed β-subunit, is a critical transcriptional regulator of the normal cellular response to hypoxia, promoting progenitor cell recruitment, proliferation, survival, and neovascularization (14, 15). In nondiabetics, hypoxia causes stabilization of HIF-1α protein by preventing the normal rapid proteasomal degradation of HIF-1α. It does this by inhibiting the prolyl hydroxylases (PHDs), which hydroxylate specific prolyl residues on HIF-1α. Without proline hydroxylation HIF-1α is not bound by the von Hippel–Lindau E3 ubiquitin ligase complex and is able to act as a transcription factor for expression of genes critical to vasculogenesis and wound healing (16). The HIF-1α–HIF-1β heterodimer binds to the hypoxia responsive element of oxygen-sensitive genes, including VEGF (14, 15). In diabetes, HIF-1α function is compromised by a high glucose-induced and reactive oxygen species-mediated modification of its coactivator p300, leading to impaired HIF-1α transactivation (17, 18).

Our laboratory has previously demonstrated that deferoxamine (DFO), an FDA-approved iron-chelating agent currently in clinical use for the treatment of hemochromatosis (19), corrects impaired HIF-1α–mediated transactivation in diabetes by preventing iron-catalyzed reactive oxygen stress. Moreover, DFO reduction of free radical formation decreased cell death (20, 21). Collectively, these effects promote wound healing and decrease tissue necrosis in the setting of diabetes (22, 23). Although systemic delivery of DFO is not a viable therapeutic option for diabetic patients owing to potential toxicity and short plasma half-life (24), local transdermal drug delivery systems (TDDS) would be very effective for clinical use.

Transdermal delivery of DFO is complicated by its relatively high atomic mass and hydrophilicity (25). This prevents it from penetrating the lipophilic outermost layer of the skin, the stratum corneum, without modification. We therefore developed a matrix-type TDDS that encapsulates DFO with nonionic surfactants and polymers for delivery enhancement (26, 27). Dispersed within a release-controlling polymer matrix, the polar DFO molecules are enclosed by reverse micelles, which permit delivery of DFO through the hydrophobic stratum corneum (28, 29). Applying this technology to a murine model of diabetic pressure sores we were able to prevent ulcer formation and improve diabetic wound healing through reduction of hyperglycemia-induced oxidative stress, which impairs HIF-1α transcriptional complex formation.

Results

Development of a TDDS for DFO.

To overcome the challenges of transdermal delivery, DFO was formulated into a monolithic polymer matrix-type TDDS (Fig. 1). This approach combines reverse micelle encapsulation of DFO by nonionic surfactants (29, 30) with dispersion in a degradable slow-release matrix (31), which allows for the targeted delivery of DFO molecules to the dermis (26, 32). Specifically, DFO migrates from the TDDS to the skin following application, as demonstrated by SEM (Fig. 2A). Once through the hydrophobic stratum corneum the reverse micelles can then disintegrate in the more hydrophilic, aqueous environment of the dermis.

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Development of a transdermal drug delivery system for DFO. DFO aggregates with PVP and surfactants to form reverse micelles (RMs). RMs are dispersed in the polymer ethyl cellulose. After release from the polymer matrix the RMs enter the stratum corneum and disintegrate. PVP dissolves and DFO is delivered to the dermis.

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Encapsulation and controlled release of DFO by a TDDS. (A) SEM images of the TDDS at time 0 (Left) and 48 h post skin application (Right). Porous structure remains within the polymer after the drug is released to murine skin (Right). (Scale bars, 100 and 20 µm.) (B) AFM showing the topography of formed RM. (C) AFM phase imaging demonstrating DFO particles inside the RM. (D) Raman spectroscopy showing the lipid shell of the RM. (E) Raman imaging specific for DFO. (Scale bars, 2 µm.) (F) DFO TDDS delivery demonstrated a sustained drug release in vitro (n = 3). (G) In vitro penetration profile showing the concentration and location of DFO in full-thickness human skin after 24 h TDDS application (n = 3). (H) Application of different TDDS formulations on the intact skin of diabetic mice revealed an increase in HIF-1α stabilization in a dose-dependent manner. n = 3. All values represent mean ± SEM; *P < 0.05.

To confirm the morphology of the DFO-encapsulating reverse micelles and to analyze their structural composition, atomic force microscopy (AFM) and Raman spectroscopy imaging of chemical functionalities were performed (Fig. 2 B–E). As expected, AFM analysis showed several topographically similar large objects with a spheroidal shape (Fig. 2B). Moreover, AFM phase imaging, which is sensitive to local sample stiffness, visualized objects in the middle of every spheroid with a stiffness much higher than that of the surrounding shell, representing encapsulated DFO molecules (Fig. 2C). On Raman spectroscopy, doughnut-shaped Raman maps of lipids with the overall shape of the micelle shell were detected (Fig. 2 B and D), whereas the DFO signal correlated with stiff clusters in AFM phase imaging (Fig. 2 C and E). Together, these data indicate successful micellar encapsulation of DFO particles.

DFO Release and Permeation Studies in Vitro and in Vivo.

We next evaluated the release and permeation abilities of the TDDS containing 1% DFO in vitro and in vivo (Fig. 2 F–H). Over 14 h of incubation in buffer solution under continuous shaking the cumulative amount of drug released by the TDDS gradually increased (n = 3), highlighting the ability of the TDDS to provide sustained delivery of DFO (Fig. 2F). To determine the dermal penetration of DFO delivered by the TDDS, in vitro skin permeation studies were performed (n = 3) using a Franz diffusion cell (33, 34). TDDS application to excised full-thickness human skin demonstrated penetration of DFO into the deep dermis within 24 h (Fig. 2G). To test the importance of the micellar delivery of DFO, the TDDS was compared with an otherwise identical formulation containing the established universal solvent DMSO (35) instead of the reverse micelle-forming surfactants. DMSO allowed for solubilization of the hydrophilic DFO in the hydrophobic chloroform, even in the absence of the surfactant molecules. Its use resulted in a homogenous mixture of all constituents and prevented any component precipitation. However, despite similar in vitro release profiles (Fig. S1), minimal DFO was delivered into the dermis by the TDDS with the altered formulation, confirming the importance of reverse micelle encapsulation for successful transdermal delivery (Fig. 2G). For both formulations no DFO could be detected in the receptor buffer of the Franz diffusion cell, consistent with the TDDS’s acting as an effective localized delivery system.

To further investigate skin permeation of DFO delivered by the TDDS in vivo (n = 3) we assessed the efficacy of two differently dosed TDDSs in uninjured diabetic mice. Using HIF-1α up-regulation as a surrogate for effective DFO delivery we found that transdermal DFO treatment resulted in a marginally increased HIF-1α transcriptional activation at 0.5% and a significant increase of HIF-1α at 1% DFO (P < 0.05) (Fig. 2H). These data support the efficacy of the TDDS as a local delivery method for DFO. TDDSs with 1% DFO were used for all further in vivo experiments.

DFO Transdermal Treatment Enhances Wound Healing in Diabetic Mice.

We adapted an established pressure-induced ulcer model (36) for use in diabetic mice (db/db leptin receptor-deficient). Pressure was applied intermittently by placing a ceramic magnet on both sides of a fold of dorsal skin (Fig. S2 A and B), with 6-h ischemia (magnets on)/reperfusion (magnets off) cycles resulting in the most consistent ulcer size and healing kinetics (Fig. S2C). With this protocol, skin ulcers with a thick eschar became apparent after 7 d, and the wounds completely healed by day 35 (Fig. S2D). No deaths, infections, or other complications occurred, and in subsequent experiments all diabetic ulcers were induced with 6-h ischemia/reperfusion intervals.

To examine the efficacy of transdermal DFO application in diabetic wounds we applied either DFO TDDS or vehicle controls onto pressure-induced ulcers on the dorsum of diabetic mice. Transdermal treatment was begun 24 h after the last ischemia/reperfusion cycle and the TDDS was changed every 48 h until complete ulcer healing (Fig. S3A). TDDS delivery of DFO resulted in significantly accelerated healing (Fig. 3 A and B). Complete resurfacing of ulcers occurred by 27 d in DFO-treated mice versus 39 d in untreated mice (P < 0.01, Fig. 3B).

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DFO TDDS improves healing of diabetic ulcers. (A) Full-thickness ulcer wounds of diabetic mice treated with a transdermal DFO TDDS formulation or vehicle control (n = 10). TDDS were replaced every 48 h. (B) Wound-healing kinetics (wound area as a function of time). Wound closure occurred significantly faster at day 27 in the DFO-treated group versus day 39 in the vehicle-treated controls (n = 10). (C) VEGF protein levels in ulcers of diabetic mice after transdermal DFO treatment for 1 and 2 d, respectively (n = 3). (D) Evaluation of VEGF protein expression in skin directly underneath the TDDS, adjacent to it, and 5 mm distant (n = 3). *P < 0.01.

Transdermal DFO Delivery Increases VEGF Expression.

One of the main characteristics of the ischemic environment of diabetic wounds is impaired VEGF production, which directly compromises neovascularization necessary for proper wound healing (18, 37). Therefore, we evaluated whether sustained DFO delivery increased VEGF expression in diabetic ulcers. Following application of DFO TDDS to fully developed diabetic ulcers (Fig. S3B) we assessed VEGF protein levels after 24 and 48 h and observed significantly increased VEGF expression at both time points (Fig. 3C, P < 0.01). Furthermore, consistent with its efficacy as a local drug delivery system, VEGF up-regulation was limited to the treated area, with adjacent and distant skin being unaffected (Fig. 3D, P < 0.01).

DFO TDDS Treatment Enhances Neovascularization and Dermal Thickness.

To further evaluate the positive effects of DFO TDDS treatment on ulcer healing, histological samples were taken upon complete wound closure. Healed DFO-treated diabetic ulcers exhibited significantly increased neovascularization compared with the vehicle control group, demonstrated by increased CD31 immonostaining (greater than threefold, P < 0.01, Fig. 4 A and B). Further histological examination of the healed wounds showed that DFO TDDS treatment also significantly improved the dermal thickness of healed diabetic ulcers, visualized as increased picrosirius red staining on polarized light images (greater than threefold, P < 0.01, Fig. 4 C and D). These data indicate that DFO not only accelerates wound closure by increasing neovascularization but also effectively improves the quality of the healed skin.

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Localized DFO treatment enhances neovascularization and dermal thickness. (A) Upon complete healing, immunohistochemistry was performed for the capillary endothelial cell marker CD31 (red). Increased vascularity was seen in transdermal DFO-treated diabetic mice. Blue indicates DAPI staining (Scale bars, 10 µm.) (B) Quantification of CD31-positive pixels per high-power field (HPF) (n = 10). (C) Dermal thickness of completely healed wounds was assessed by polarized light microscopy after picrosirius red staining. (Scale bars, 10 µm.) (D) Quantification of picrosirius red-positive pixels per HPF (n = 10). *P < 0.01.

Localized DFO Treatment Effectively Prevents Diabetic Ulcer Formation.

To investigate the prophylactic efficacy of DFO we pretreated the dorsal skin of diabetic mice for 48 h with a DFO TDDS, followed by removal of the TDDS and ulcer induction as described above (Fig. S3C). Macroscopic monitoring of ulcer formation showed that skin pretreatment resulted in prevention of ulcer formation and skin necrosis compared with untreated controls (Fig. 5 A and B, P < 0.01). Histologic analysis confirmed loss of epithelial integrity, destruction of dermal architecture, and a profound inflammatory response in controls compared with the minimal tissue destruction observed in DFO TDDS-treated skin (Fig. 5C).

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Transdermal DFO treatment prevents ulcer formation in diabetic mice. (A) Representative photographs of skin after ulcer induction in diabetic mice pretreated with either DFO or control TDDS. No severe ulcer formation in the DFO-treated group. (B) Quantification of control and DFO-treated necrotic area (n = 10). (C) Representative histological H&E-stained tissue sections showing ulcer formation in the vehicle control group (n = 10). *P < 0.01. (Scale bars, 10 µm.)

DFO TDDS Attenuates Tissue Necrosis by Decreasing Apoptosis and Reactive Oxygen Species Stress.

Previous evidence suggests that apoptosis contributes significantly to cell death following ischemia/reperfusion injury (38, 39). To investigate whether DFO treatment attenuates these apoptotic effects we performed analysis of protein levels of the apoptotic markers cleaved caspase 3 and Bax in DFO pretreated and control wounds. DFO-pretreated mice showed a significant reduction of both apoptotic markers. (Fig. 6 A–C, P < 0.05).

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DFO TDDS attenuates tissue necrosis by decreasing apoptosis and ROS stress. (A) Western blot analysis of Cleaved Caspase-3 (Cl Casp.3) and Bax proteins. DFO-pretreated mice show a significant reduction of both apoptotic markers. (B and C) Quantification of Western blot (n = 3). (D) DHE immunofluorescent stain for oxidative stress reveals decreased ROS accumulation (red) in DFO-treated wounds (n = 3). *P < 0.05.

In ischemic tissues, DFO is known to reduce levels of reactive oxygen species (ROS), which play a major role in diabetic ulcer pathogenesis and persistence (40, 41). Therefore, we evaluated the influence of transdermal DFO treatment on superoxide levels using DHE immunofluorescent staining (42). We found that transdermal delivery of DFO resulted in a dramatic decrease of ROS accumulation, consistent with the observed reduction of apoptosis, skin necrosis, and ulcer formation (Fig. 6D).

In summary, these findings support the feasibility of using a transdermal DFO delivery system to address impaired diabetic wound healing. We demonstrate the ability to modulate established biologic pathways, which are impaired in diabetic ulcer healing, and effectively augment tissue repair and restoration. Further, by prophylactically preloading skin with DFO we demonstrate the ability to prevent pressure ulcer formation in a diabetic wound model.

Discussion

In this paper we describe the development of a highly effective TDDS for the treatment and prevention of diabetic wounds. Specifically, transdermal DFO delivery was found to effectively up-regulate VEGF secretion and accelerate diabetic wound healing. Moreover, DFO-treated mice exhibited significantly increased angiogenesis and dermal thickness as well as reduced apoptosis and ROS formation in a pressure-induced diabetic ulcer model. More interestingly, pretreatment with a DFO TDDS effectively prevented ulcer formation in diabetic mice. Because DFO is already FDA- approved, this TDDS has the potential for rapid translation into clinical application for the prevention and management of diabetic ulcers.

There are currently no available pharmacologic agents for the prevention of wound development and only one available to accelerate healing in existing wounds (becaplermin, PDGF-BB) (43). Unfortunately, an increased cancer risk has been reported in patients treated with becaplermin, and it is not widely used for this and other reasons (44). Surprisingly, simple pressure offloading remains one of the mainstays of both treatment and prevention of diabetic decubiti and ulcers, but the compliance with these approaches in the long term remains low (45). Several other technologies such as silicone-coated foam and hydrocolloids have been used in attempts to reduce the risk of ulcer formation, but none have demonstrated significant efficacy (46). There is thus an eminent need for effective pharmacological approaches to address the tremendous health-care burden of chronic diabetic wounds.

The therapy of a chronic disease such as a diabetic nonhealing wound requires repeated treatment administration. In the case of a substance with a short biological half-life such as DFO, the drug would have to be administered within short intervals up to several times daily. Frequent wound dressing changes have been shown to result in increased pain, irritation, and infection risk (47). To reduce application frequency and thereby increase patient safety and comfort as well as drug efficacy, formulations with sustained drug delivery are ideal. Moreover, this approach could easily be adopted because both diabetic foot ulcers and pressure sores occur in very stereotypic locations and are in general 1–10 cm2 in size. Current clinical care for these wounds includes placing of adhesive sheets (i.e., duoderm) to “cushion” these areas (48), so the application of a therapeutic patch would not be a significant departure from current practice.

Oxidative stress plays a pivotal role in the development of diabetes complications. The metabolic abnormalities of diabetes cause mitochondrial superoxide overproduction, resulting in defective angiogenesis in response to ischemia (49). The underlying mechanism of diabetes-induced superoxide-mediated impairments in neovascularization is a dysfunction of HIF-1α (17, 18). A molecular option to therapeutically modulate HIF-1α activity is the iron chelator DFO (50–52). DFO has a crucial advantage over drugs, which purely up-regulate HIF-1α levels, such as the PHD inhibitor dimethyloxalylglycine, in that it also has a direct antioxidant effect and is capable of reducing the oxidative stress associated with ischemia (53). In keeping with this mechanism, DFO has been found to play a protective role during hypoxic preconditioning in brain (51) and heart tissue (54) as well as in cutaneous ischemic preconditioning (23).

Consistent with its predicted therapeutic potential we have previously demonstrated the efficacy of topical administration of DFO in healing diabetic wounds (18). However, in order for DFO to be used for ulcer prevention it needs to penetrate the unwounded stratum corneum, a difficult process owing to the innate properties of the skin. The stratum corneum is composed of nonliving corneocytes and a mixture of lipids organized in bilayers (55). The drug transport across the stratum corneum barrier is limited by the structural and solubility requirements for solution and diffusion within the lipid bilayers (55). Transdermal drug delivery can be classified into two categories: passive delivery and active, physically enhanced delivery (56). Passive delivery approaches include ointments, creams, gels, and transdermal patches. The drugs suitable for passive delivery are usually hydrophobic and have a molecular weight <500 Da, allowing them to pass through the hydrophobic stratum corneum (57). Unfortunately, DFO has a molecular weight of 560 Da and consists of three hydrophilic bidentate oxygen-containing groups, which reduces its lipid solubility (58). Collectively, these physical properties prevent it from penetrating intact stratum corneum.

Therefore, to use DFO to prevent and treat diabetic ulcers in “at risk” patients the development of a novel drug delivery platform was necessary. Recent innovative approaches for transdermal drug delivery include both chemical and physical enhancement (55). More aggressive chemical enhancers such as sulphoxides or alcohols improve the delivery efficiency for hydrophilic molecules but are known to cause skin irritation and erythema (59). Active physical enhancers include iontophoresis (60), sonophoresis (61), electroporation (62), or more invasive approaches such as microneedles (63), thermal ablation (64), or skin abrasion (65), but owing to device complexity and high cost they are still experimental (57). A less invasive, passive method of augmenting stratum corneum permeation involves the use of nonionic surfactants (59). Tween 80 and Span 20 are widely regarded as safe (59), and chemical enhanced matrix-type transdermal patches are a very cost-effective way to deliver molecules across the skin barrier (57), making them an attractive approach with high clinical translatability.

Nanoscale vehicles have undergone rapid development in recent years and include liposomes, micelles, and reverse micelles (66). Reverse micelles consist of a hydrophilic core containing the hydrophilic drug and a hydrophobic outer shell, which combined with their small particle size makes them an ideal vehicle for penetrating the hydrophobic stratum corneum (67, 68). The penetration of the skin barrier is dependent on multiple factors, including particle polarity and size. Nanosized particles are more likely to enter more deeply into the skin than larger ones. Zheng et al. (69) have shown that gold cores coated with highly oriented, covalently immobilized, spherical nucleic acid nanoparticle conjugates of siRNA completely penetrate keratinocytes in vitro, as well as mouse skin and human epidermis only hours after application. If nanoparticle or microparticle movement were based strictly on molecular weight, then this would not be possible. Presumably the particles in that report move through skin via unclear mechanisms not entirely restrained by Stokes–Einstein principles, as would be the case in the present study.

One particular feature of our approach is the specific targeting of well-studied molecular pathways in diabetic pathophysiology to not only treat but also prevent ulcer formation. Our data suggest that preconditioning potential areas of ischemia/reperfusion can prevent ulcer formation in susceptible tissues. These findings have immense clinical importance, because the prevention of ulcer formation via a simple, topical TDDS would significantly reduce patient morbidity and health-care costs associated with chronic diabetic wounds.

Conclusion

Dysregulation of the HIF-1α–VEGF axis owing to ROS stress has been identified as a central problem in diabetic wound healing. We describe the formulation of a biodegradable polymeric TDDS that allows for efficient delivery of DFO to diabetic wounds. Transdermal delivery of DFO was found to prevent diabetic ulcer formation when used prophylactically and to decrease tissue necrosis and improve wound healing in preexisting ulcers by decreasing oxidative stress. DFO application reduced cellular apoptosis and tissue destruction while increasing VEGF expression and neovascularization. This represents, to our knowledge, the first description of pharmacological prevention of diabetic wounds. Given the status of DFO as an FDA-approved molecule in clinical use for over three decades, its transdermal application would be an effective and translatable addition to the armamentarium for chronic wound treatment.

Materials and Methods

See SI Materials and Methods for further information.

Design of the TDDS.

A monolithic matrix-type transdermal drug delivery system containing DFO dispersed within a biodegradable polymer was designed. DFO mesylate salt powder was purchased from Sigma-Aldrich. All reagents used were analytic grade. Owing to its hydrophilicity and tendency to crystallize, DFO is especially well suited for delivery complexed with the polymer polyvinylpyrrolidone (PVP). PVP is known to stabilize drugs in an amorphous form (27, 33) and to promote permeation of hydrophilic molecules (59). To facilitate dermal penetration of the DFO/PVP complexes, reverse micelle-forming nonionic surfactants polysorbate 80 (Tween 80) and sorbitan monolaurete 20 (Span 20) (26) were added to the formulation. Finally, ethyl cellulose was added to form a slow-releasing matrix (27). For the preparation of the drug-release layer we dissolved the two polymers ethyl cellulose (3.5% by weight) and PVP (0.5% by weight) with 1% DFO (by weight) in chloroform (27) and added the nonionic surfactants Tween 80 and Span 20 (1% each, by weight) for reverse micelle formation (26, 59). Di-n-butylphthalate was used as a plasticizer (30% weight-in-weight of polymers) (27). The solution was stirred vigorously until a fine suspension was achieved. This solution was then poured onto a sterile glass Petri dish and dried at room temperature. The uniform dispersion was cast onto a 2% (wt/vol) polyvinyl alcohol backing membrane, dried at 40 °C for 6 h, and cut with a 16-mm circular biopsy punch in equal-sized discs. Finally, the finished transdermal delivery system was attached to a contact adhesive (Tegaderm; 3M). For comparison, an alternative TDDS was formulated using the permeation enhancer DMSO instead of nonionic surfactants, and a control formulation containing only vehicle was prepared by making a suspension of the polymers and surfactants without the addition of DFO.

AFM and Raman Spectroscopy Imaging.

Both Raman and AFM were performed using NTEGRA Spectra combined AFM-Raman system (NT-MDT) (70). AFM imaging was performed in tapping mode with commercial high-durability rounded cantilevers (k = 5.4 N/m, R ~40 nm) at 0.7 Hz. This provided surface topography and phase-contrast images to discern stiffness of different areas within the micelle particles. Raman confocal scanning was performed in backscattering geometry with a long-working Mitutoyo objective (100, 0.7 N.A.). The illumination light was 473 nm, and the power was kept at ~2 mW to lower the possibility of sample damage. Raman maps were produced with a step size of 0.5 mm and 1-s exposure. Gratings (600 g/mm) were used for optimal signal and spectral resolution. The peaks at ~1,625 cm−1 (integrated spectral intensities 1,575–1,675 cm−1) were attributed to DFO molecules, whereas the CH bands at 2,800–3,050 cm−1, less the DFO CH peak at 2,927–2,952 cm−1, were attributed to the lipid molecules (70).

In Vitro Drug Release.

DFO TDDS was placed into 10 mL PBS (pH 7.4) maintained at a temperature of 37 °C and shaken continuously for 14 h (n = 3). The concentration of DFO was measured by LC-MS (Shimadzu; AB SCIEX) every hour as previously described (71) (See SI Materials and Methods for details).

In Vitro Skin Permeation.

For in vitro skin permeation studies a vertical Franz diffusion cell model was used (n = 3) as previously described (34) (see SI Materials and Methods for details).

Pressure Ulcer Model and TDDS Application.

Twelve-week-old male C57BL/6 db/db mice (BKS. CG-M+/+Lepr<db>/J; Jackson Laboratories) were randomized into the following groups: DFO TDDS-treated versus vehicle TDDS control (n = 10). Pressure ulcers on the dorsum of db/db mice were induced as previously described (36) (see SI Materials and Methods for details).

Ulcer Wound Analysis.

Digital photographs were taken before ulcer initiation, the day after, and every other day until closure. Ulcer closure was defined as the time at which the wound was completely re-epithelialized. Ulcer wound area was determined using ImageJ software (National Institutes of Health).

Histology.

After the mice were euthanized wounds were harvested with a 2-mm rim of unwounded skin. Skin tissues were fixed in 4% paraformaldehyde overnight followed by serial dehydration in ethanol and embedding in paraffin. Five-micrometer sections were stained with H&E or picrosirius red. Frozen tissue samples for CD31 immunohistochemistry and DHE stain were prepared by immediate optimum cutting temperature (OCT) embedding (Sakura Finetek) (see SI Materials and Methods for details).

Statistical Analysis.

Statistical analysis was performed using either ANOVA or an unpaired Student t test (MATLAB). Values are presented as means ± SEM. P values < 0.05 were considered statistically significant.

Supplementary Material

Supplementary File

Acknowledgments

The authors thank Prof. Jürgen Stampfl (Technical University Vienna) for insightful discussion of the manuscript and Revanth Kosaraju for proofreading. Funding for wound healing research in our laboratory has been provided by the National Institutes of Health (R01-DK074095, R01-AG025016, and R03-DK094521), the Harrington Discovery Institute, the Hagey Family Endowed Fund in Stem Cell Research and Regenerative Medicine, and The Oak Foundation.

Footnotes

Conflict of interest statement: G.C.G., J.R., E.N., and M.G.G. are listed on the following patent assigned to Stanford University: Topical and Transdermal Delivery of HIF-1 Modulators to Prevent and Treat Chronic Wounds (20100092546).

*This Direct Submission article had a prearranged editor.

This article contains supporting information online at www.pnas.org/lookup/suppl/10.1073/pnas.1413445112/-/DCSupplemental.

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