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Asian Journal of Pharmaceutical Sciences 2012, 7 (3): 155-167

Novel topical drug delivery systems and their potential use in scars treatment

Xi Chen, Lihua Peng, Jianqing Gao*

College of Pharmaceutical Sciences, Zhejiang University, Hangzhou, ChinaReceived 1 August 2012; Revised 21 August 2012; Accepted 27 August 2012

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Abstract

Scars are resulted from wound healing processes with high prevalence. Among all kinds of scars, hypertrophic scars and keloids are notoriously difficult to eradicate with the high recurrence and side effects associated with the available treatment methods. Currently, topical drug delivery systems (TDDS) had attracted more and more attention. Direct delivery of therapeutic agents to the scar sites is desirable, particularly when systemic delivery could cause organ damage due to toxicological concerns associated with the preferred agents. However, the development of topical drug delivery was kept a challenge because of the barrier function of thick scar skin which inhibits the transport of drugs. Nowadays, pharmaceutical methods are extensively investigated to overcome the penetration barrier of the thick scar skin for topical drug delivery. Novel carriers, such as the liposomes and microsponges, with the advantages of biocompability, protecting drugs from the inactivation of external conditions, sustained release and the efficient permeation have been demonstrated for their benefits in hypertrophic scars and keloids treatment. After an introduction discussing the mechanism of the hypertrophic scars and keloids formation, a brief section on the drug treatment for scars was presented. Then, this article was focused on reviewing the emerged novel topical drug delivery systems, including hydrogels, solid lipid nanoparticles, liposomes, microemulsions, nanofibers and microsponges, as well as the frequently used materials for the topical drug delivery to scar sites were discussed.

Keywords: Hypertrophic scar; Keloid; Topical drug delivery system____________________________________________________________________________________________________________

1. Introduction

Wound healing is a complicated process involving the participation of many cytokines, extracellular matrix elements and various cell types [1]. Scars are resulted from wound healing processes with high prevalence. Mustoe et al. have classified clinical scars (Table 1) into the following categories: 1) normal mature scar, 2) immature scar, 3) linear hypertrophic scar, 4) widespread hypertrophic scar, 5) minor keloid, and 6) major keloid [2]. Among these scars, hypertrophic scars and keloids are notoriously difficult to heal with available managements. Bombaro et al. found that 62% of white race patients and 80% of the non-white patients had hypertrophic scars [3]. Hypertrophic scars and keloids are disfiguring and aesthetically unpleasant which frequently cause severe itching, tenderness, pain, sleep disturbance, anxiety, depression, and disruption of daily activities. Other psychosocial sequelas include development of post-traumatic stress reactions, loss of self esteem, and stigmatisation. In clinic, many methods, e.g.

pressure therapy, laser therapy, surgical management, etc have been investigated for the treatment of keloids and hypertrophic scars. However, because of the high recurrence rate and the incidence of side effects associated with the available treatment methods [4], it is still a big challenge to eradicate the keloids and hypertrophic scars [5].

Over the past years, topical delivery of drugs has caused more and more attention, of which, systemic side effects can be reduced compared to parenteral or oral drug administration. Drugs application to the skin surface circumvent hepatic first-pass metabolism and major fluctuations of plasma levels caused by repeated oral administration of rapidly eliminated drugs. Depending on the properties of the combined ingredients, a dispensing carrier to provide a stable physicochemical environment that protects the active compound(s) from chemical degradation should be designed, that can be a liquid or semi-solid, monophasic or multiphasic (e.g., oil-in-water or water-in-oil). However, the selection of the formulation is largely dependent on the characteristics of the active compound(s) and on the condition of the skin to be applied. So far, many drugs, such as TGF-β3 [6], bFGF [7], IL-10 [8], asiaticoside [9], genistein [10], mannose [11] have been reported for their effects in inhibiting the formation of hypertrophic scars and keloids after topical application.

__________*Corresponding author. Address: Institute of Pharmaceutics, College of Pharmaceutical Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China. Tel: +86-571-88208437 E-mail: [email protected]

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However, as the high thickness of hypertrophic scar and keloid skin forms a strong barrier for the skin penetration of drugs through topical delivery, to overcome this challenge, various formulations such as hydrogels, microemulsions, solid lipid nanoparticles, liposomes, dendrimers, and micro-sponges, etc have been investigated for the penetration enhancement and sustained release. These formulations are designed to be utilized not only as “drug transporters”, but also play a role in “drug reservoirs” for releasing active ingredients over a prolonged period of time or as receptacles for absorbing undesirable substances, such as the excess skin oil. In this review, the characteristics and benefits of novel topical drug delivery systems with the potential use for scars treatment are discussed based on the scars formation mechanism and the interaction of TDDS with scars. Especially, the TDDS efficacy for overcoming the barrier of scar thick skin was highlighted.

2. Mechanism and drug treatments of hypertrophic scars and keloids formation

2.1. Mechanism of hypertrophic scars and keloids formation

Wound healing process can be divided into three phases of inflammation, proliferation and maturation. In the stage of inflammation, neutrophils release elastases and proteases, and further vascular dilation and permeability causes inflammation. In proliferation phase, the first event is the migration of keratinocytes to the injured dermis. New blood vessels then form and the sprouts of capillaries replace the fibrin with granulation tissues (also known as angiogenesis). A proportion of the wound fibroblasts

differentiate into myofibroblasts [12]. Fibroblasts and myofibroblasts produce extracellular matrix (ECM), in the main form of collagen, which forms the bulk of mature scars [13]. The third stage of wound healing is remodelling and maturation, which begins after 2–3 weeks and lasts for one year or more. Most of the endothelial cells, macrophages and myofibroblasts undergo apoptosis or exit from the wound, leaving a mass that contains few cells and consists mostly of collagen and other extracellular-matrix proteins. In addition, over 6–12 months, the acellular matrix is actively remodelled from a mainly type III collagen backbone to one predominantly composed of type I collagen.

Both keloids and hypertrophic scars are raised from abnormal wound healing processes [14]. Hypertrophic scar represents an exaggerated fibroproliferative response of the dermis, which creates an imbalance of collagen synthesis and degradation, resulting in excess accumulation of dermal collagen [15-16]. Keloid is characterized as scar growing continuously and invasively beyond the boundaries of the original wound in contrast to hypertrophic scars [17]. The forming mechanism of hypertrophic scars and keloids are shown in Fig. 1. Keloids differ from hypertrophic scars clinically and histologically [18]. Clinically, keloids are deep red or purple with raised indurated tissue that extends beyond the original wound borders. Hypertrophic scars have a less impressive white or pink color, with firm tissue limited to original wound border. Histologically, keloids are composed of disorganized thick hyalinized collagen with a prominent mucoid matrix, whereas hypertrophic scars are characterized by fewer, more organized collagen

Table 1Classification of scars.

Scar name Definitions

Normal mature scar A light-colored, flat scar.

Immature scarA red, sometimes itchy or painful, and slightly elevated scar in the process of remodeling. Many of these will mature normally over time and become flat, and assume a pigmentation that is similar to the surrounding skin, although they can be paler or slightly darker.

Linear hypertrophic scar

A red, raised, sometimes itchy scar confined to the border of the original surgical incision. This usually occurs within weeks after surgery. These scars may increase in size rapidly for 3-6 months and then, after a static phase, begin to regress. They generally mature to have an elevated, slightly rope-like appearance with increased width, which is variable. The full maturation process may take up to 2 years.

Widespread hypertrophic scar A widespread red, raised, sometimes itchy scar that remains within the borders of the burn injury.

Minor keloidA focally raised, itchy scar extending over normal tissue. This may develop up to 1 year after injury and does not regress on its own. Simple surgical excision is often followed by recurrence. There may be a genetic abnormality involved in keloid scarring.

Major keloid A large, raised (0.5 cm) scar, possibly painful or pruritic and extending over normal tissue.

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fibers with a scanty mucoid matrix. The major difference of keloids and hypertrophic scars is the increasing rate of proline hydroxylase activity, a marker of collagen synthesis. Keloids exhibit 20 times the rate of collagen synthesis of normal skin and three times the rate of hypertrophic scars. This increased level normalizes after two to three years [19-20].

In summary, the major reasons for the formation of hypertrophic scars and keloids could be attributed to: 1) a dysregulated inflammatory phase; 2) dysregulated levels of cytokines; 3) the unbalance of collagen synthesis and collagen deposition; 4) the high ratio of type I/III collagen [21-22]. For examples, in keloids, the type I/III collagen ratio is approximately 17, which is significantly higher than the ratio of around 6 observed in normal scars [23]. Dysregulated levels of cytokines were found in the peripheral blood mononuclear cell fraction obtained from keloid patients, including up-regulation of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and interferon (IFN)-β, and the down-regulation of IFN-α, IFN-γ, and TNF-β [24].

2.2. Drug treatments on hypertrophic scars and keloids

At present, methods used in the scar treatment can be broadly categorized into two types: surgical therapy and nonsurgical therapy. Removal of scar by surgery alone, or combined with lipid graft, has been proved to be less effective because of the recurrence of adhesion postsurgery and the re-operation on the scar would be difficult and dangerous because of the risk of nerve root injury and dural tear. A wide range of drugs have been investigated to treat scar sites through the oral administration or topical delivery, including corticosteroids, silicones, antipro-liferative drugs, immune modulating agents, antimetabolic drugs, antiallergy drugs, calcium channel blockers, antioxidant agents, various enzymatic substances, cyto-kines, Chinese medicines. The products with topical delivery on the market for hypertrophic scars and keloids treatment were listed in Table 2.

Among them, cytokines are more and more popular in scar treatment nowadays. They can act by paracrine,

Fig. 1. The mechanism of hypertrophic scars and keloids formation. The injury process can be divided into three overlapping phases: inflammation, proliferation and maturation. Inflammation sets in within minutes of a skin injury. Proliferation phase is charactered as the proliferation and migration of different cell types. During this time, collagen begins to accumulate and wound contraction occurs. Maturation is descriptive of the continuing collagen accumulation and remodeling by the constituent cell types of the healing wound so that the scar can be eliminated. However, persistent inflammation, tissue necrosis and disrupted release of cytokines cause the unbalance of ECM formation and degration, which promotes the formation of hypertrophic scars and keloids.

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autocrine, juxtacrine, or endocrine mechanisms, and affect cell behavior as a consequence of their binding to specific cell surface receptors or ECM proteins. Binding to these receptors triggers a cascade of molecular events. The endpoint of this signaling is the binding of transcription factors to gene promoters that regulate the transcription of proteins controlling the cell cycle, motility, or differentiation patterns [25]. They have many functions such as antiproliferative properties, improving the pathologic features of dermal fibrosis directly or by antagonizing the effects of TGF-β and histamine [21-22]. At the same time, traditional Chinese medicines are warmly welcomed in scar inhibition. Traditional Chinese medicines may be extracted from tissues of terrestrial plants, marine organisms or microorganism fermentation broths. A crude extract from any one of these sources typically contains novel, structurally diverse chemical compounds, which the natural environment is a rich source. Recently, several purified natural products originated from traditional Chinese medicines have been applied in scar treatment, e.g. asiaticoside [9], genistein [10], mannose [11] and have demonstrated significant inhibition on scar formation.

3. Topical drug delivery systems for hypertrophic scars and keloids

The current therapeutic managements of hypertrophic scars and keloids include occlusive dressings, pressure therapy, intralesional corticosteroid injections, cryo-surgery, excision, radiation therapy, laser therapy, topical drug treatments, and other promising therapies. Among them, topical drug delivery systems had received increasing importance because of its small side effects and long term suppression in hypertrophic scars and keloids. The products with topical delivery on the market for hypertrophic scars and keloids treatment were listed in Table 2. With the TDDS, the active pharmaceutical ingredient makes contact with the target site before entering the systemic circulation. Systemic side effects can be greatly reduced, e.g. in circumventing hepatic first-pass metabolism and major fluctuations of plasma levels caused by oral administration. However, it was reported that the mean of scar thickness reached 4.91 mm among the patients with hypertrophic scar (the normal skin thickness is around 2 mm [26]. As the thickness of the hypertrophic scar and keloid skin is rather higher than the normal skin, so the drug permeation of the skins with hypertrophic scar or keloid is quite difficult. Since most of the current therapeutic products for anti-scar treatment are based on traditional carriers such as creams, gels and ointments, the biggest challenge for the TDDS is to ensure the effective penetration of drugs across the scar skin and play the anti-scar role. Besides that, problems of short

term effect and requirement of frequent administration existed widely in the current topical treatment of hypertrophic scar and keloid. Hengge et al. reported that most preparations of glucocorticosteroids are applied once or twice daily. Greater frequency of application may be necessary for the palms or soles, because the product is easily removed during normal activities such as walking and hand washing, and penetration is poor owing to a thick stratum corneum. With frequent application, many adverse effects occurred such as atrophic changes, steroid atrophy, telangiectasia, striae, purpura, stellate pseudoscars, ulceration, easy bruising, infections, masked microbial infections (tinea incognito), and aggravation of cutaneous candidiasis, herpes or demodex [27]. This report illustrated that sustained drug release efficacy and the permeation enhancement are fairly important in topical drug delivery system. With the sustained release and enhanced permeation could reduce the incidence of adverse effects caused by drugs. Recently, hydrogels, nanoparticles, microemulsions, liposomes, dendrimers, nanofibers and microsponges have been reported for the advantages of biocompability, protecting drugs from degradation by external conditions, sustained drug release efficacy and the permeation enhancement for the treatment of hypertrophic scars and keloids.

3.1. Hydrogels

The mechanism of hydrogel benefit to scar treatment was supposed to be that the natural adhesive and flexible properties of this water-based material allowed the sheeting to conform to the irregularities of the scar and stay in place with minimal additional attachment. Danielson et al. reported the results of a preliminary treatment regimen for hypertrophic scars combining topical 2% salicylic acid cream (Avosil) with an overlay of hydrogel dressing (Avogel) [28]. At the end of the 60-day treatment protocol, the area treated with 2% salicylic acid and hydrogel was asymptomatic. In contrast, the hydrogel-treated and untreated control areas remained erythematous and symptomatic for burning pain and pruritus. This study suggests the efficacy of combined salicylic acid and hydrogel therapy in the treatment of hypertrophic scars. Avogel reduced the hardness and size, while returning the scar to a more natural appearance. Hong et al. developed topical hydrogel which could increase the solubility of titrated extract of Centella asiatica (TECA) [29]. The hydrogel containing this mixture significantly decreased the size of wound area at the 9th day when applied to the wound area of rat dorsal skin. It ws thought that hydrogel preparations are normally preferred in TDDS because of their controlled release characterization, good tissue compatibility, easy manipulation of swelling level, and may improved targeting to the viable epidermis [30].

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Trade name Active agents Preparations Function

Juvidex Sugar mannose-6-phosphate (M-6-P)

Topicformulation

1) Accelerate wound healing; 2) Minimize the appearance of scars; 3) Prevent scab formation.

GHK-copper peptide EGF age-defying serum EGF Solution

1) An excellent skin renewal ability; 2) Promote wound healing; 3) Strengthen skin elasticity; 4) Increase layer of subcutaneous fat to prevent skin aging.

Moist exposed burn ointment Beta-sitosterol OintmentA moist environment for wound healing by simple ointment application have profound effects on mast cells, bFGF, TGF-β, interleukin-1, and nerve growth factor.

ADCON®-T/N glycosaminoglycan gel

Glycosaminoglycans Gel 1) Reduce levels of fibrosis in experimental wounds; 2) Inhibit adhesions.

Mederma skin care gel Allium cepa Gel1) Improve scar appearance and texture; 2) Downregulating the overproduction of collagen by fibroblasts; 3) Decreased inflammation.

Contractubex gel Aqueous onion extract, heparin, allantoin Gel

Inhibitory effects on inflammatory processes, fibroblast proliferation, and the synthesizing capacity of fibroblasts, influence scar development.

Recombinant human basic fibro-blast growth factor (rh-bFGF) Rh-bFGF Lyophilized

spray gel1) Accelerate wound healing; 2) Minimize the appearance of resulting scars; 3) Prevent scab formation.

Recombinant bovine basic　fibroblast growth factor (rb-bFGF) Rb-bFGF Spray 1) Accelerate wound healing; 2) Minimize the appearance

of resulting scars; 3) Prevent scab formation.

Asiaticoside ointment Asiaticoside Ointment 1) Skin condition; 2) Antioxidant.

Imiquimod cream Imiquimod Cream Activates immune cells through the toll-like receptor 7 (TLR7)

Talsyn-CI scar reduction cream Talsyn-CI Cream Used by plastic surgeons to minimize the appearance of scars after surgery.

Bio oil PurCellin oil Oil 1) Effective at hydrating skin; 2) Treats fine lines and wrinkles; 3) For use on major and minor scars.

Scarprin gelDimethicone, dimethiconol, cyclopentasiloxane

Gel

1) Effective for new and old scars; 2) Flattens and smoothes the scar to give an even skin appearance; 3) Reduces the color and thickness of scar; 4) Works on all types of scars including keloids; 5) Gel Based (Gels absorb more effectively then creams; 6) Can help with itching and pain associated with scars.

Zenmed scar treatment kit Ascorbic-glycolic-lactic Solution

1) Reduces the visibility of acne scars; 2) Minimizes the appearance of fine lines and wrinkles; 3) Diminishes hyperpigmentation caused by scarring.

Kelo-cote scar gel Silicone Gel

1) Helps to soften, flatten, and smooth scars while maintaining the moisture balance and elasticity of the adjacent skin; 2) Has also been shown to reduce the discoloration and itching associated with scars.

Table 2The products with topical delivery on the market for hypertrophic scars and keloids treatment.

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3.2. Solid lipid nanoparticles

In recent years, encapsulation of anti-scar drugs in nanoparticle systems has emerged as an innovative and promising alternative that enhances therapeutic effectiveness and minimizes undesirable side effects of the drugs. Solid lipid nanoparticles (SLN) are nanospheres or nanoplatelets made up from lipids solid at room and body temperature such as glycerol behenate (Compritol®), glycerol palmitostearate (Precirol®) or tristearin glyceride. Nanoparticles in particular, have unique physicochemical properties such as ultra small and controllable size, large surface area to mass ratio, high reactivity, and functionalizable structure. These properties can be applied to facilitate the administration of anti-scar drugs, thereby overcoming some of the limitations in traditional anti-scar therapeutics. Drug loading to the SLN can not only increase skin penetration rate but also can induce the epidermal drug targeting. It was shown that for the encapulation of podophyllotoxin in SLN signifcantly increased the selectivity in drug actions and reduced the unwanted side effects, with respect to the cutis and organ functions beyond the skin [31]. Although SLN has not been widely reported for the application in scar treatment yet, it has been shown a promising carrier for transdermal and topical drug delivery. Chen et al. reported that podophyllotoxin (POD) loaded SLN (P-SLN) had a strong localization of POD within the epidermis [31]. The penetration of P-SLN with low particle size into stratum corneum along the skin surface “furrow” and the consequent controlled release of POD might lead to the epidermal targeting. In summary, SLN possess good tolerability and stability, scaling-up feasibility, and the ability to incorporate hydrophobic/hydrophilic drugs. SLN are well suited for use on damaged or inflamed skin because they are made of non-irritant and non-toxic lipids.

However, the SLN dispersion possesses low viscosity and a yield value of practically zero, so it isn't convenient for use in skin [32].

3.3. Liposomes

Liposomes are spherical, self-closed structures formed by one or several concentric lipid bilayers with an aqueous phase inside and between the lipid bilayers [33]. In recent years, niosomes, ethosomes and highly flexible liposomes (also called transfersomes) have been studied to take the place of conventional liposomes. Niosomes offer higher chemical stability, lower costs, and great availability of surfactant classes when compared to conventional liposomes [34-36]. The ethosomal system is composed of ethanol, phospholipid and water [37]. The effect of ethanol on stratum corneum lipids and on vesicle fluidity as well as a dynamic interaction between ethosomes and the stratum corneum may contribute to the enhancement of skin permeation [38]. Transfersomes that follow the trans-epidermal water activity gradient in the skin can enhance the transdermal bioavailability of drugs. Vogt et al.reported that polyvinyl pyrrolidone-iodine (PPI) was carried by a new liposome hydrogel formulation (Betasom hydrogel) and was applied to the patients with meshed skin grafts after burns [39]. Clinical assessment indicated that better antiseptic condition and wound healing quality has been received with the Betasom hydrogel treatment. Compared to wounds treated with a conventional antiseptic chlorhexidine-gauze, PPI liposomal can provide higher moisture to the wound surface, release PVP-iodine at a low rate, and target the substance exactly by interaction with the cells surface. Yang et al. reported that the topical treatment of liposome-encapsulated hydroxycamptothecin (L-HCPT) could significantly reduce the epidural scar, compared with that of saline control group [40]. These

Table 2 (Continued)The products with topical delivery on the market for hypertrophic scars and keloids treatment.

Trade name Active agents Preparations Function

Scar esthetique cream Silicone Cream Contains dimethicone, an effective silicone gel used in reducing the appearance of scars.

Scar zone® topical scardiminishing cream

Dimethicone Cream 1) Can be used on new and old scars; 2) Contains dime-thicone, an effective silicone gel used in treating scars.

Wound be gone Hydroxyethylmethacrylate, cyclic amine Gel 1) Accelerate wound healing; 2) Minimize the appearance

of resulting scars; 3) Prevent scab formation.

Cimeosil scar gel Silicone Gel Diminish redness and appearance of scars.

Mederma scar cream Onion extract CreamInhibitory effects on inflammatory processes, fibroblast proliferation, and the synthesizing capacity of fibroblasts, influence scar development.

Vitamin E Alpha-tocopherol Oil Protect cells from oxidative stress

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results were further proved by the decreased concentration of hydroxyproline in the scar tissues. Other advantages of includes 1) liposomes could increase the hydrolysis half-life and observed equilibrium constant of drugs; 2) drugs can be slowly and continuously released from the lipo-some and kept a long-term effect; 3) the liposome could decrease the toxicity of free drugs.

3.4. Microemulsions

Microemulsions form spontaneously without high shear equipment. The active agents are solubilised and thus available for rapid penetration into the skin. Increasing thermodynamic activity, the presence of (co-)surfactants acting as penetration enhancers and occlusivity improve skin penetration to variable degrees [41]. Kitagawa et al. reported that genistein containing gel-like water-in-oil (W/O)-type microemulsion could prevent UV irradiation-induced erythema formation [42]. However, high concentration of surfactant and co-surfactant are necessary for stabilizing the nanodroplets. The solubilizing capacity for high-melting substances is therefore limited. The stability of microemulsions is also influenced by environmental parameters such as temperature and pH. There are several mechanisms for the enhancement of microemulsions on skin permeation, including: 1) the high drug loading capacity of microemulsions; 2) they can enhance the partitioning of drugs into the skin; 3) they are able to create high drug concentrations within the upper layers of the skin; 4) their lipophilic nature [43]. Microemulsion seems to be an ideal liquid vehicle for topical drug delivery since it provides all the possible requirements of a liquid system including thermodynamic stability, easy formation, low viscosity with newtonian behavior, high surface area, and very small droplet size. The small droplets of microemulsion have more chances to adhere to membranes and to transport bioactive molecules in a controlled fashion [43].

3.5. Nanofibers

Nanofibers are defined as fibers with diameters less than 1000 nanometers. They can be produced by interfacial polymerization and electrospinning. Nanofibers have characteristics of large specific surface area, high porosity, and small pore size. Nanofibers can be used as artificial organ components, tissue engineering, implant material, drug delivery, wound dressing, and medical textile materials. Schneider et al. made use of nanobiotechnology to augment the rate of wound reepithelialization by combining self-assembling peptide (SAP) nanofiber scaffold and epidermal growth factor (EGF) [44]. It was found that SAP underwent molecular

self-assembly to form unique 3D structures that stably covered the surface of the wound and EGF was only released when the scaffold was in direct contact with the bioengineered human skin equivalent. SAP scaffolds containing EGF accelerated the rate of wound coverage. Similarly, RADARADARADARADA (RADA16-I) peptide could form extremely stable β-pleated sheet structure and then self-assemble into nanofibers to produce high-order interwoven nanofiber scaffold hydrogel, which could speed up wound contraction. FGF and EGF were obviously expressed in nascent tissue such as epidermis and glands when wounds were treated with the RADA16-I after injury [45]. The large surface area and porosity of electrospun nanofibers enable good permeability for oxygen and water and the adsorption of liquids, and concomitantly protect the wound from bacterial penetration and dehydration. This feature demonstrated the electrospun nano-fibers to be a suitable material for wound dressing, especially for chronic wounds such as diabetic ulcers or burns. In addition to physical protection of the wound site, biomaterials composed of a biofunctionalized peptidic scaffold have properties that are well-suited for the treatment of cutaneous wounds in wound coverage, functionalization with bioactive molecules, localized growth factor release and activation of wound repair. The advantages of nanofibers are large specific surface area, high porosity, small pore size and controlled drug release. Although, the complex preparation procedures and the expensive costs limit the development of nanofibers, it is expected that nanofibers will be widely used in TDDS.

3.6. Microsponges

Controlled release of drugs onto the epidermis is with assurance that the drug remains primarily localized and does not enter the system in significant amounts. Although transdermal delivery systems (liposomes, nanoparticles, microemulsions, etc) can be efficient in supplying drugs for systemic effects, they are not practical for controlling the delivery of materials whose final target is the skin itself. And this is an area of research that has been recently addressed. Thus there is need to develop novel topical programmable delivery system. The Microsponge Delivery System (MDS) is a polymeric microsphere system uniquely fulfilling these requirements by providing topical controlled drug delivery systems. MDS is “highly cross-linked, porous, polymeric microspheres systems that can entrap wide range of actives and then release them onto the skin over a time and in response to triggers” [46]. It is a unique technology for the controlled release of topical agents and consists of microporous beads, typically 10–25 μm in diameter, loaded with active agent (Table 3). When applied to the skin, the MDS releases its active

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ingredients on a time mode and also in response to other stimulis (rubbing, temperature, pH, etc). MDS technology is being used in cosmetics, over-the-counter (OTC) skin care, sunscreens and prescription products. The MDS can prevent excessive accumulation of ingredients within the epidermis and the dermis. Potentially, the MDS can reduce significantly the irritation of effective drugs without decreasing their efficacy. These porous microspheres with active ingredients can be conveniently incorporated into other formulations such as creams, lotions and powders. Microsponges consisting of non-collapsible structures with porous surface through active ingredients are released in a controlled manner [47]. Depending upon the size the total pore length may range up to 10 ft and pore volume up to 1 ml/g. The microsponge particles have open structures and the active agent is free to move in and out from the particles into the vehicle until equilibrium is reached when the vehicle becomes saturated. Once the finished product is applied to the skin, the active agent that is already in the vehicles will be absorbed into the skin, depleting the vehicle, which will become unsaturated, therefore disturbing the equilibrium. This will start a flow of the active agent from the microsponge particle into the vehicle and from it, to the skin until the vehicle is either dried or absorbed. Even after that, the microsponge particles retained on the surface of the stratum corneum will continue to gradually release the active to the skin providing prolonged release over time (Fig. 2). The microsponge particles are too large to been absorbed in to the skin and this adds the safety of this materials.

The microsponge technology can also be used to bring two or more relatively incompatible materials together in the same preparation. It is a fact that hydroquinone and retinol are incompatible. However, the possibility

Active agents Applications References

Sunscreens 1) Long lasting product efficacy; 2) Improved protection against sunburns and sun related injuries even at elevated concentration; 3) Reduced irritancy and sensitization

[47]

Anti-acne e.g. benzoyl peroxide

1) Maintained efficacy 2) Decreased skin irritation and sensitization [53-54]

Anti-inflammatory e.g. hydrocortisone

1) Long lasting activity 2) Reduction of skin allergic response and dermatoses [55]

Anti-dandruffs e.g. zinc pyrithione, selenium sulfide

1) Reduced unpleasant odour 2) Lowered irritation 3) Extended safety and efficacy

[50]

Anti-pruritics 1) Extended and improved activity [47]

Skin depigmenting agents e.g. hydroquinone

1) Improved stabilization against oxidation with 2) Improved efficacy and aesthetic appeal [56]

Rubefacients 1) Prolonged activity 2) Reduced irritancy greasiness and odour [50]

Table 3Overview of research work published about MDS in topic drug delivery.

Fig. 2. Schematic showing the distribution of loaded drug on skin. The drug that is already in the vehicles will be absorbed into the skin, depleting the vehicle, which will become unsaturated, therefore disturbing the equilibrium. This will start a flow of the drug from the microsponge particle into the vehicle and from it, to the skin until the vehicle is either dried or absorbed.

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of incorporating both in the same formula could provide many benefits to the skin. The primary benefit from such a combination in the same formulation is the cosmetic effect of retinol for the reducing the appearance of fine lines and wrinkles. To demonstrate this capability, two different microsponge entrapments were used in the preparation: one of which contained hydroquinone while the other contained retinol. The microentrapped hydroquinone 4%/retinol 0.15% formulation produced improvement in the treatment of melasma and postinflammatory hyper-pigmentation. Improvement in disease severity and pig-mentation intensity was statistically significant at weeks 4, 8, and 12 compared with baseline. It demonstrated that microentrapped hydroquinone 4% with retinol 0.15% was safe and effective [48]. Nowadays, many microsponge products are designed and shown distinguished efficacies. Retin-A Micro (tretinoin gel) microsphere, 0.1% and 0.04%, is a formulation for topical treatment of acne vulgaris. This formulation uses patented methyl methacrylate/glycol dimethacrylate crosspolymer porous microspheres (Microsponge®) to enable inclusion of the tretinoin in an aqueous gel [49]. EpiQuin™ Micro and EpiQuin™ Micro XD contain hydroquinone USP 4% and retinol (vitamin A)incorporated into patented porous microspheres (Micro-sponge® system) composed of methyl methacrylate/glycol dimethacrylate crosspolymer. This polymeric system has been shown to provide gradual release of active ingredient into the skin [50]. Carac® cream (fluorouracil cream) con-tains 0.5% fluorouracil, with 0.35% being incorporated into a patented porous microsphere (Microsponge®) composed of methyl methacrylate/glycol dimethacrylate crosspolymer and dimethicone [51]. Stough et al. reported that Carac® cream was assessed in 277 participants with multiple actinic keratoses (AKs) on the face/anterior scalp and other body sites in a 18-month, open-label, multicenter study. Only 4 participants (7.4%) experienced a treatment-related adverse event (AE) that was not an application site reaction or eye irritation [52]. No unexpected AEs were reported; most were mild or moderate. After treatment cycle 1 (week 8), the number of AK lesions was significantly reduced on the face/anterior scalp and all other treated body sites (P < 0.0001). After treatment cycle 2 (week 60), face/anterior scalp AKs were significantly reduced (P < 0.0001) and the clearance rate was 33.3%. This study indicated that Carac® cream with a patented microsponge delivery system was well-tolerated and effective in treating and preventing recurrence of AK lesions up to 18 months after initial treatment. In summary, MDS provides a wide range of formulating advantages. Liquids can be transformed into free flowing powder. Formulations can be developed with otherwise incompatible ingredients with prolonged stability without use of preservatives. Safety of the irritating and sensitizing drugs can be increased and programmed release can control the amount of drug release to the targeted site.

3.7. Summary

To summarize, various drug delivery systems, for examples, the reviewed liposomes, nanofibers, and microsponges, loaded with drugs have been investigated as the carriers for topical drug delivery to the scar sites for treatment because of their efficiency in facilitating skin penetration, releasing drugs slowly, and keeping the moisture of scar skin, etc. Being similar to biological membranes, liposomes can navigate water soluble and lipophilic substances in different phases or domains. Liposomal preparations are reported to reduce the skin roughness because of their interaction with the corneocytes and intercellular lipids resulting in skin softening and smoothening [57]. However, liposomes are metastable systems and their pharmaceutical use is often limited by instability. Instability can be due to leakage of the vesicles, change in vesicle size due to aggregation or fusion, as well as ester hydrolysis and formation of oxidation products [58]. Therefore niosomes, ethosomes and highly flexible liposomes have been developed to overcome the shortcomings of instability of liposomes. Solid lipid nanoparticles (SLN) with a drug enriched shell show burst release whereas SLN with drug enriched core lead to a sustained release [59]. Due to the general adhesiveness of small particles, SLN applied to the skin form a film. This film of ultrafine particles has an occlusive effect, which promotes penetration of active ingredients into the upper part of the epidermis, mainly the stratum corneum, thus enhancing the pharmaceutical efficiency of incorporated ingredients. For porous nanofibers, they perform important functions, temporary substitute for the native ECM and potential carrier system for the controlled delivery of wound healing drugs. Because of their resemblance to the fibrillar highly porous structure and size scale of the native ECM, plain nanofibers inherently promote the hemostasis phase of wound healing and initiate tissue repair by facilitating cell attachment and proliferation [60]. As the microsponge® delivery system (MDS), it provides a wide range of formulating advantages. Liquids can be transformed into free flowing powder. Formulations can be developed with otherwise incompatible ingredients with prolonged stability without use of preservatives. Safety of the irritating and sensitizing drugs can be increased and programmed release can control the amount of drug release to the targeted site (50). To date, most evaluation studies have focused on comparing the safety and efficacy of the microparticles/nanoparticles with conventional formulations, however, little comparison among the microparicles or nanoparticles has been carried out. Considering the difference in composition and structure of these systems, their efficacy and tolerability are likely to vary. These delivery systems for topical scar treatment presented herein may reduce side effects and consequently

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increasing patient compliance. Taking these factors into account, for example, MDS presents clear advantages. Numerous studies have confirmed that MDS are non-irritating, non-mutagenic, non-allergenic, and non-toxic [50]. Nevertheless, further investigations are needed to make the large-scale production of microparticles or nanoparticles at lower costs to be practicable.

4. Biomaterials used in topical drug delivery systems preparation for scars treatment

Materials in TDDS are significantly important because they could influence the pH, elasticity, viscosity, degradation, water content, and drug release of drug carriers. Until now, many materials have been reported in the preparation of topical drug delivery systems, which comprise a class of synthetic polymers, e.g. poly (L-lactic acid) (PLLA), poly (L-glycolic acid) (PLGA), poly (vinyl alcohol) (PVA), natural polymers such as alginate, chitosan, collagen, and fibrin, and some novel biomaterials, e.g. dendrimers, self-assembling peptides (61). In biological polymers, chitosan was used widely in wound healing and scar inhibition. Chitosan was shown to influence all stages of wound repair, and the hemostatic activity of chitosan can be seen in the inflammatory phase; it also regulates the migration of neutrophils and macrophages acting on repair processes such as fibroplasias and re-epithelization [62-64].

Dendrimers are the new artificial macromolecules with the structure like a tree. They are hyperbranched and monodispersed three-dimensional molecules, and have defined molecular weights and host–guest entrapment properties [65]. Dendrimers are defined as highly ordered and regularly branched globular macromolecules produced by stepwise iterative approaches. The structure of dendrimers consists of three distinct architectural regions: a focal moiety or a core, layers of branched repeat units emerging from the core, and functional end groups on the outer layer of repeat units. Since dendrimers are synthesized from branched monomer units in a stepwise manner, it is possible to conduct a precise control on molecule size, shape, dimension, density, polarity, flexibility, and solubility by choosing different building/branching units and surface functional groups (66). Barata et al. reported that the generation (G) 3.5 polyamidoamine (PAMAM) dendrimer that was partially glycosylated with glucosamine could inhibit inflammation in a rabbit model of tissue scaring [66].

Nowadays, basic understanding of the three-dimensionalstructure of existing biological molecules is being applied to a ‘bottom-up’ approach to generate new, self-assembling supra-molecular architectures [67]. In particular, self-assembling peptides offer promise because of the large variety of sequences that can be made easily by automated chemical synthesis. The potential for bioactivity, the ability

to form nanofibers, and responsiveness to environmental cues are inherent in some of these materials such as self-complementary peptide RADARADARADARADA (RADA16-I) which we mentioned above. These peptides are characterized by their periodic repeats of alternating hydrophilic and hydrophobic amino acids in the form of β-sheet structures [68]. These peptides are packed together as anti-parallel β-sheet structures, with a hydrophobic inner and a hydrophilic outer site [69]. The peptide terminus can be chemically designed to incorporate functional ligands that further enhance scaffold performance, such as peptide epitopes containing integrin receptor-binding sites [70], bone marrow homing proteins [71], and insulin growth factor [72]. Tysseling-Mattiace et al. reported that self-assembling peptide amphiphile (PA) could inhibit glial scar formation and facilitate regeneration after spinal cord injury (SCI) using bioactive three-dimensional nanostructures displaying high densities of neuroactive epitopes on their surfaces [73].

5. Challenges and future perspectives

For adult tissues, healing of deep wounds often leads to excessive scarring of hypertrophic scars and keloids. The current therapies of hypertrophic scars and keloids contain occlusive dressings, pressure therapy, cryosurgery, excision, radiation therapy, laser therapy. The common side effects of these managements are poor compliance, high risk, hypopigmentation, ineffectiveness, eczema, and pruritus [74]. Upon the development much more new drugs for the hypertrophic scars and keloids inhibition, topical drug delivery systems possess increasing values for the drug treatments of scars. However, many anti-scar drugs are severely limited by poor skin penetration ability. Lack of effective delivery of drugs and undesirable skin interactions of the topical treatments are the main reasons for patient noncompliance. Nevertheless, newer developments in the formulation approaches have raised hopes in making topical therapy more useful and acceptable. Topical formulations offer significant advantages over systemic therapy, such as, ease of administration, fewer adverse effects and cost-effectiveness. Local treatment remains an attractive approach for scar treatment that does not pose a risk of developing complications which require systemic therapy. Much progress has been made to improve the performance of topical anti-scar care products in recent years. New excipients, refined processing techniques, and a better knowledge of the physicochemical properties of vehicles and drugs have led to the development of new delivery systems that may result in more advanced anti-scar therapies. Recently, emerging studies have demonstrated that novel micro/nano-particles based drug delivery systems, such as liposomes, solid lipid nanoparticles

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and microsponges are able to overcome these issues and facilitate anti-scar agents delivery to scar sites. Accordingly, the fundamental interest of such carriers lies on making the existing drugs more effective, safe, and patient-compliant. Many studies have demonstrated the importance of these systems for the enhancement in the skin penetration and accumulation of drug along with improved patient compliance. For example, the unique moisturization of the vesicles and their interactions with the skins is one of the possible reasons for the improvement in cutaneous transport of drugs.

However, several challenges remain that need to be taken into consideration in developing novel wound healing drug delivery formulations. For example, large variations in the physical-chemical properties of the drugs and the thickness of scar skins, suggest the difficulty in finding a single ideal formulation capable of application to all scar types. To make TDDS capable of penetrating across scar skins and at the same time allowing the modification in local intracellular transit, new materials bringing new properties may be beneficial when the modification of a drug’s chemical structure will be insufficient.

Additionally, it is known that, unlike oral administration, for example, where the blood level of a drug is a generally accepted ‘surrogate’ for its concentration at the site of action, topical drug delivery poses a more complex problem. Despite pressure on regulatory agencies, such as the FDA, there is no generally accepted method with which to evaluate the bioavailability and bioequivalence of topical drug pro-ducts. On the other hand, as a major obstacle to scar reduction is that scarring is a very complicated process involving many different factors, with activation and feed-back through multiple pathways. Therefore, another novel strategy for scar inhibition is gene therapy. Theoretically, delivery of an anti-scarring gene into fibroblasts, keratino-cytes even stem cells could potentially result in reduced scarring. Especially gene therapy with stem cells has caused wide attention in scar treatments. Stem cells could act as multiple players in re-epithelialization, revascularization and remolding. What’s more, they could contribute the regeneration of hair follicles, sweet glands and sebaceous glands [75]. As we know, an important feature of scar skin is lack of hair follicles, sweet glands and sebaceous glands. So stem cells reduce or inhibit the formation of scars at the wound healing processes.

Furthermore, in today’s self-image conscious world, patients are looking for topical products that are not only safe and effective, but also cosmetically acceptable and easy to apply. This is especially true in scar patients, where the esthetic aspect is one of the primary reasons why patients seek dermatologic consultation. Therefore, well controlled clinical trials are required to confirm the clinical benefits of these new formulations in terms of efficacy, tolerability, compliance, and cosmetic acceptability.

Acknowledgement

This study was supported in part by the 48th China Postdoctoral Grant (20100480091), the Fundamental Research Funds for the Central Universities, China, Zhejiang Provincial Program for the Cultivation of High-Level Innovative Health Talents, and National Natural Sciences Foundation of China (81102393).

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