Lasers as an approach for promoting drug delivery via skin.

Review

Lasers as an approach for promoting drug delivery via skin Chih-Hung Lin, Ibrahim A Aljuffali & Jia-You Fang† †

1.

Introduction

2.

Drug penetration pathways via the skin

3.

Mechanisms of drug permeation enhanced by the

Expert Opin. Drug Deliv. Downloaded from informahealthcare.com by Ondokuz Mayis Univ. on 05/23/14 For personal use only.

laser 4.

Drug permeation enhanced by different laser types

5.

Conclusion

6.

Expert opinion

Chang Gung University, Graduate Institute of Natural Products, Pharmaceutics Laboratory, Taoyuan, Taiwan

Introduction: Using lasers can be an effective drug permeation-enhancement approach for facilitating drug delivery into or across the skin. The controlled disruption and ablation of the stratum corneum (SC), the predominant barrier for drug delivery, is achieved by the use of lasers. The possible mechanisms of laser-assisted drug permeation are the direct ablation of the skin barrier, optical breakdown by a photomechanical wave and a photothermal effect. It has been demonstrated that ablative approaches for enhancing drug transport provide some advantages, including increased bioavailability, fast treatment time, quick recovery of SC integrity and the fact that skin surface contact is not needed. In recent years, the concept of using laser techniques to treat the skin has attracted increasing attention. Areas covered: This review describes recent developments in using nonablative and ablative lasers for drug absorption enhancement. This review systematically introduces the concepts and enhancement mechanisms of lasers, highlighting the potential of this technique for greatly increasing drug absorption via the skin. Lasers with different wavelengths and types are employed to increase drug permeation. These include the ruby laser, the erbium:yttrium-gallium-garnet laser, the neodymium-doped yttrium-aluminumgarnet laser and the CO2 laser. Fractional modality is a novel concept for promoting topical/transdermal drug delivery. The laser is useful in enhancing the permeation of a wide variety of permeants, such as small-molecule drugs, macromolecules and nanoparticles. Expert opinion: This potential use of the laser affords a new treatment for topical/transdermal application with significant efficacy. Further studies using a large group of humans or patients are needed to confirm and clarify the findings in animal studies. Although the laser fluence or output energy used for enhancing drug absorption is much lower than for treatment of skin disorders and rejuvenation, the safety of using lasers is still an issue. Caution should be used in optimizing the feasible conditions of the lasers in balancing the effectiveness of permeation enhancement and skin damage. Keywords: drug delivery, laser, percutaneous absorption, photomechanical wave, skin ablation Expert Opin. Drug Deliv. (2014) 11(4):599-614

1.

Introduction

The term ‘laser’ is an acronym for Light Amplification by Stimulated Emission of Radiation. A laser is a photogenic device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. Lasers exhibit a wide application in biomedical uses, especially in dermatology. They can act as a modality of therapy for treating skin disorders, such as acne scars, rosacea, rhytides, port wine stains, warts, vitiligo and melasma, as well as for hair removal [1-3]. Over the past 20 years, lasers have been a predominant advancement in cosmetic dermatology for facial skin resurfacing and rejuvenation [4]. By using

10.1517/17425247.2014.885501 © 2014 Informa UK, Ltd. ISSN 1742-5247, e-ISSN 1744-7593 All rights reserved: reproduction in whole or in part not permitted

599

C.-H. Lin et al.

Article highlights. . . . .


.

This review describes recent developments in topical/ transdermal delivery using laser strategies. Laser has provided new insights into enhancing capabilities of drug absorption via skin. Both the skin permeation of small-molecule drugs and macromolecules can be enhanced by laser approach. The permeation level of the laser-mediated drug delivery can be precisely controlled by modulating the laser fluence and pulse numbers. More results in clinical studies will encourage future application of laser-assisted drug delivery.

This box summarizes key points contained in the article.

lasers, great success can be achieved in treating wrinkles, photoaging and pigmentation. The main advantages of laser treatment over other approaches, such as chemical peeling and dermabrasion, are the ease of application, precise control of tissue ablation and minimal damage to the adjacent normal tissue [5]. Laser irradiation can interact with skin tissues, such as the stratum corneum (SC), epidermis and dermis. The skin is a semipermeable barrier that protects the body from the external environment and prevents water loss [6]. The outermost layer of the skin is the SC, which has a rigid structure for resisting invasion by external elements. Dermal and transdermal drug delivery as an easy and targeted administration method has attracted extensive investigation [7]. Nevertheless, the SC blocks drug permeation via the skin, which limits the outcome of therapy. The SC, with a thickness of 10 -20 µm, is composed of nonviable corneocytes surrounded by a lipid extracellular matrix. Due to the firm structure, only small molecular weight (< 500 Da) lipophilic permeants can passively diffuse across intact skin [8]. Even the most rapidly permeating drugs diffuse through the SC very slowly. To improve drug permeation via the SC by breaching the SC barrier function, some chemical and physical strategies are introduced. These include penetration enhancers, prodrugs, nanomedicine, SC stripping, ultrasound, iontophoresis, electroporation and microneedles [9]. The SC barrier can be partially conquered by tape stripping. Nevertheless, the area and ablated depth cannot be predicted by tape stripping. The recovery and safety of this technique are also controversial. These limitations can be overcome by laser modality, which precisely removes the SC layer by controlled energy. Only minimal laser fluence is needed for peeling the SC without affecting the structures of viable skin, thus assuring a fast recovery to normal status. Furthermore, lasers can irradiate the SC using noncontact behavior, thus avoiding the risk of cross contamination [10]. Another advantage of laser-assisted drug permeation is the very short time required for operation within nanoseconds or microseconds [11]. The first article about topical/transdermal drug delivery enhanced by lasers was published in 1991. Nelson et al. [12] 600

utilized a mid-infrared laser for enhancing skin permeation of hydrocortisone and g-interferon. Excised swine skin was irradiated with 1 J/cm2 fluence. Histological observation showed a linear correlation between the etched depth and the number of pulses. The in vitro Franz cell was employed to examine cumulative penetration. Laser ablation of 12.6% of the SC area elevated the permeation of hydrocortisone and g-interferon by 2.8- and 2.1-fold, respectively. This work has encouraged the subsequent investigations involved in laser-assisted drug delivery via the skin. Lasers can be categorized into two types, ablative and nonablative, depending on the affinity of the wavelength for water [13]. The devices with wavelengths highly absorbed by water are categorized as ablative lasers, which include erbium:yttrium-galliumgarnet (Er:YAG, 2940 nm), yttrium-scandium-gallium-garnet (YSGG, 2790 nm) and CO2 (10,600 nm) lasers. The major chromophore for absorbance of these lasers is water. Those with only moderately absorbed wavelengths are called nonablative lasers (1410, 1440, 1540 and 1550 nm). Recently, a fractional laser has been developed as a novel modality to improve the safety of the traditional laser. It creates numerous microscopic treatment zones (MTZs) with controlled depth, area and density that are surrounded by a reservoir of spared tissues [14]. The surrounding tissue is relatively unaffected by the fractional laser and serves as a resource involved in the wound-healing process [15]. A rapid recovery of skin barrier function and epidermal healing can be achieved with the fractional laser within 1 -- 2 days [16]. In this review article, we would systematically introduce different laser types that demonstrate the mechanisms of SC ablation or disruption for assisting drug permeation via the skin. This approach is successfully used in promoting skin delivery of small molecules, hydrophilic molecules, peptides, vaccines, DNA and smallinterfering RNA (siRNA). The mechanisms of laser enhancement on the skin permeation of these drugs or permeants are also elucidated in this review. 2.

Drug penetration pathways via the skin

The SC is the main absorption barrier for most skinpermeating drugs. The SC consists of lipids and proteins, with the primary lipids being phospholipids, glycosphingolipids, cholesterol sulfate and neutral lipids. Stratum granulosum, stratum spinosum and stratum basale are different layers in the viable epidermis. Although the SC is regarded as the predominant barrier for topical/transdermal drug penetration, recent investigations [17,18] have shown that the viable epidermis plays an essential role as a drug permeation barrier. The tight junction in the stratum granulosum constitutes an important barrier for drug permeation. Therefore, the SC and tight junction combine to form the functional skin barrier [19]. For successful permeation via the skin, drug molecules should enter into the SC and then penetrate through it. Three routes have been suggested for permeation across the SC: the

Expert Opin. Drug Deliv. (2014) 11(4)

Lasers as an approach for promoting drug delivery via skin

Transcellular route

Intercellular route

Transappendageal route

Stratum corneum

Lipid

Cell cytoplasm Intercellular place


Minimal lipid

Keratin

Water molecules

Plasma membrane

Hair follicles

Figure 1. The possible transport routes for drug delivery via the stratum corneum are shown.

intercellular, transcellular and transappendageal pathways (Figure 1). Transport between the corneocytes (the intercellular route) is a route by which most drugs penetrate the skin, especially compounds with small molecular weights. This pathway is considered to allow free-volume diffusion through the lipid bilayers present among the corneocytes [20]. The transcellular path is considered to be hydrophilic in nature. It is composed of aqueous areas surrounded by polar lipids that create the walls of channels. Drug molecules penetrating across this path diffuse between corneocyte clusters across imperfections that create openings composed of water. Intracellular keratins predominantly provide this track. Hair follicles and other appendages such as sebaceous glands are also likely to play an important role in dermal/transdermal delivery, as follicles are an efficient route for delivering drugs into deeper skin strata. Follicles also provide an interesting therapeutic target as they represent complex and dynamic three-dimensional structures [21]. Follicular delivery enables rapid and direct transport into the infundibulum region, where permeation into living tissues is facilitated by reduced barrier characteristics. In addition, follicles appear to be an efficient long-term reservoir for drug delivery. Hair follicles, in particular, are significant for skin absorption of large molecules and nanoparticles [22].

Mechanisms of drug permeation enhanced by the laser

promote the subsequent passage of topically applied drugs through the skin. In ablation, laser irradiation elicits decomposition of the skin target into small fragments, which move away from the skin surface at a supersonic speed [23]. This ablation can largely reduce the barrier function of the SC, thus facilitating drug transport across the skin. PW is a broadband, unipolar, compressive wave generated by lasers. PW transiently permeabilizes the cell membrane and skin surface but cannot peel the SC. Lipid disruption in the SC is induced by PW to allow drug diffusion into the deeper strata. Electron microscopy demonstrates an expansion of lacunar space within SC lipids. This expansion produces transient pores that qualify drug delivery through the SC and into viable skin [24]. PW also affects the cell plasma membrane and thus opens up transcellular routes for facilitating drug transport across the skin [25]. PW produced by the lasers can efficiently enhance skin delivery of macromolecules, such as insulin, plasmid DNA and proteins. Some laser modalities, such as the CO2 laser, create a significant photothermal effect. The CO2 laser possesses a wavelength of 10,600 nm and is well absorbed by water. The light energy absorbed can be converted into heat, which quickly heats the skin tissue and causes vaporization if delivered in a high quantity during a very short time [26]. The thermal effect disrupts the skin barrier function, leading to easier penetration of the drugs into the skin.

3.

Three mechanisms, including direct ablation, optical breakdown by photomechanical wave (PW) and a photothermal effect, are related to laser-skin interactions for promoting percutaneous absorption of drugs (Figure 2). These interactions

Drug permeation enhanced by different laser types

4.

Ruby laser The ruby laser emits a 694-nm wavelength of laser light (Table 1). It is used for treating freckles, pigmented skin lesions, 4.1


601

C.-H. Lin et al.

Laser

Laser

Laser

Enlargement of lacunar space Thermal damage

Ablation Stratum corneum

Epidermis


Dermis

Photomechanical wave

Direct ablation

Photothermal effect

Figure 2. The possible mechanisms of laser for enhancing dermal/transdermal drug delivery are shown.

Table 1. The wavelengths of various laser types. Laser type

Wavelength (nm)

Ruby YSGG Er:YAG Nd:YAG CO2

694 2790 2940 355, 532, 1064 and 1320 10,600

Er:YAG: Erbium:yttrium-gallium-garnet; Nd:YAG: Neodymium-doped yttrium-aluminum-garnet; YSGG: Yttrium-scandium-gallium-garnet.

tattoo removal, lentigos and melasma, in clinical dermatology [27,28]. This laser can generate PW, which is advantageous for topical/transdermal drug delivery (Figure 3). The PW is applied for only a very short period of 10 ns -- 1 µs. The permeabilization of the SC by PW is transient and can be recovered after few minutes [23]. Lee et al. [29] examined the skin transport of 5-aminolevulinic acid (ALA) in humans assisted by the Qswitched ruby laser. ALA is a photosensitizing agent used for photodynamic therapy (PDT) to treat skin cancers. Topical delivery of ALA is very low due to its hydrophilicity [30,31]. In vivo fluorescence spectroscopy was utilized for a noninvasive assay of ALA accumulation in the skin. The barrier function of SC could be modulated by PW generated from the ruby laser without affecting viable skin. A 30% increase in peak pressure of PW led to a 6.8-fold enhancement of the amount delivered. The SC barrier always recovered within minutes after a PW application. This was approved by applying ALA to a site 15 -- 30 min after PW application. Very little or no ALA could be detected in skin after this duration [29]. Lee et al. [32] also evaluated PW-mediated topical delivery of macromolecules, including 40 kDa dextran and 20 nm latex nanoparticles. Hairy rats were used as the animal model. PW was generated by the ruby laser with a 23-ns pulse to achieve a fluence of 7 J/cm2. According to the imaging of fluorescence microscopy, dextran 602

showed a broad distribution from the SC to the dermis in the site exposed to PW. In contrast, dextran was located in the SC and follicles in the untreated site. The fluorescent latex nanoparticles could also be delivered through the SC by the ruby laser. Lee et al. [33] further investigated the skin delivery of 40 kDa dextran by the ruby laser at a different fluence (5 J/cm2) and pulse duration (490 ns) compared to their previous study. The penetration of dextran assisted by PW had reached a depth of 400 µm into the dermis of rats. The broad and continuous distribution of dextran in viable skin suggested that the delivery pathways were either intercellular or intracellular. Lee et al. [34] investigated the combined activity of PW and sodium lauryl sulfate (SLS) for enhancing topical dextran delivery. SLS is an anionic surfactant used as a penetration enhancer for percutaneous absorption of drugs. The use of SLS delayed the recovery of the SC barrier function; thus, the time available for dextran delivery was prolonged. The penetration of dextran extended to a 600-µm depth into the dermis. A synergistic enhancement was obtained by combining SLS and PW treatments. A single PW from the ruby laser was applied onto rat skin in vivo to determine the topical delivery of microspheres with a diameter of ~ 100 nm [35]. PW exposure to the skin allowed the diffusion of microspheres to the epidermis, which was visualized by a fiber-based fluorescence spectroscopy. This study demonstrated that PW can facilitate topical delivery of very large permeants. Er:YAG laser The Er:YAG laser emits a wavelength of 2940 nm (Table 1), which is efficiently absorbed by water molecules. Because of its shallow irradiation into the skin, this laser can precisely ablate the SC with reduced thermal injury (Figure 3). The benefits of the Er:YAG laser for ablation are quicker healing time, less erythema and fewer pigmentation problems [36,37]. The Er:YAG laser is now extensively employed as an ablative device for scar treatment, skin tumor removal and skin resurfacing [38]. With respect to enhancing drug delivery via the skin, a low 4.2



SC


Viable skin

Ruby laser: Lacunar space expansion by photomechanical wave

Er:YAG laser: Efficient ablation and photomechanical wave

Q-switched Nd:YAG laser: SC rupture, comeocyte disruption and micropore perforating

Long-pulsed Nd:YAG laser: Thinning and fragile comeocytes with photothermal effect

CO2 laser: SC ablation with photothermal effect

Fractional laser: Creation of microchannels

Figure 3. The influence of different laser types on interaction with skin structure is shown.

fluence (< 5 J/cm2) is needed to ablate the SC layer. This fluence is much lower than that used for skin resurfacing in clinical dermatology. The fluence of enhancing drug permeation is at least 2 times lower than that of rejuvenation. The effect of the Er:YAG laser on the percutaneous transport of nalbuphine and indomethacin was compared [39]. These two permeants have the same molecular weight but different lipophilicity. Nalbuphine and indomethacin can be regarded as hydrophilic and lipophilic drugs, respectively. The pulse duration of this laser was 250 µs. The output energy was set to 0.9 -- 17.5 J/cm2. The laser had greater enhancement for nalbuphine than for indomethacin. For example, a fluence of 3.6 J/cm2 increased the flux of nalbuphine and indomethacin by 194.4and 9.9-fold, respectively. This is because lipophilic SC contributes a significant barrier for hydrophilic drugs. The ablation of SC, thus, can largely facilitate the permeation of hydrophilic drugs. Lee et al. [40] evaluated the feasibility of using the Er:YAG laser to control and promote transdermal delivery of narcotic drugs, including morphine, nalbuphine and buprenorphine. The in vitro Franz cell with pig skin was used as the experimental platform. The three drugs showed similar flux across intact skin (1.4 -- 1.5 nmol/cm2/h). Er:

YAG irradiation at 2.6 J/cm2 resulted in the increase of flux to 25.2, 25.3 and 10.9 nmol/cm2/h for morphine, nalbuphine and buprenorphine, respectively. The skin transport of morphine and nalbuphine showed higher enhancement with the laser treatment as compared to that of buprenorphine. This may have been due to the higher lipophilicity and molecular mass of buprenorphine as compared to the other two drugs. Although the drug polarity and molecular weight generally play important roles on passive permeation, these two factors also govern laser-assisted delivery. PW may generate micropores in SC, which size allowed facile penetration of morphine and nalbuphine but not buprenorphine with larger molecular volume. In this case, the physicochemical characteristics of the permeants are important for laser-mediated delivery but not passive diffusion. A PW was generated by filtering laser radiation through a polystyrene target. The morphine permeation enhancement (sevenfold) was significantly lower than that produced by direct laser irradiation (35-fold), indicating the predominant mechanism of the SC ablation but not PW by the Er:YAG laser for transdermal drug delivery. Fang et al. [41] optimized and enhanced in vitro ALA permeation by Er:YAG laser irradiation. Another resurfacing


603


C.-H. Lin et al.

tool, microdermabrasion, was also used for comparison. A 247-fold increase of ALA flux was achieved by the laser at 2.6 J/cm2 based on Franz cell experiments with pig skin as the permeation barrier. Skin penetration of ALA via microdermabrasion-treated skin was 5- to 15-fold greater than that via untreated skin, indicating a superior role of the laser for increasing ALA delivery. The dose of ALA for clinical use is usually 20%. This high dose may produce skin lesions [42]. It is expected that the use of lasers can enhance ALA absorption, thus reducing the dose required. The cost of PDT can also be reduced. ALA is converted to protoporphyrin IX (PpIX) after penetration into the skin for eliciting pharmacological activity. Shen et al. [43] evaluated in vivo PpIX accumulation in the skin and underlying tumors after topical ALA administration combined with the Er:YAG laser in nude mice. ALA penetration after laser ablation produced higher PpIX accumulation within subcutaneous tumors compared with the accumulation in the control group. The enhancement ratios of the laser-treated group ranged from 1.5-fold to 4.9-fold depending on the fluence used. The skin needed 3 -- 4 days to recover to a normal status after laser irradiation by measuring transepidermal water loss (TEWL). Methotrexate is a drug used for treating skin cancers and psoriasis. The high hydrophilicity of methotrexate makes it difficult for it to penetrate the skin. Lee et al. [44] demonstrated methotrexate permeation by the application of the Er:YAG laser. The nude mouse skin was used as a permeation barrier. The fluencies of 1.4, 1.7 and 1.9 J/cm2 produced the enhancement of methotrexate flux by 3-, 54- and 81-fold, respectively. Hyperproliferative skin was also induced as the barrier simulating psoriatic skin. The fluence of 1.7 J/cm2 increased methotrexate flux by 40-fold, which was less than that across normal skin. Yun et al. [45] evaluated the anesthetic efficacy of 5% lidocaine cream administered after Er:YAG laser ablation of the SC. Randomized and controlled comparison of local anesthesia was carried out on 12 human subjects. The face was covered with the cream for 60 min after ablation. The visual analog pain scores (0 -- 10) were recorded. Subjects treated with laser-mediated topical lidocaine exhibited scores reflecting significantly less pain compared to the group without laser treatment. The peeling after laser-assisted lidocaine absorption was well tolerated by 72% of the volunteers. Besides use with small-molecular drugs, the Er:YAG laser has been widely employed for promoting skin delivery of macromolecules and nanoparticles. Fang et al. [46] assessed the influence of the molecular size of dextran (4.4 -77 kDa) on laser-assisted skin permeation. The dextran distribution in pig skin was observed using skin slices under fluorescence microscopy. The experimental results indicated a significant increase of dextran permeation by Er:YAG laser irradiation. Skin penetration of dextran with a molecular weight of 77 kDa could be accomplished with laser treatment. Follicles seemed to be an important pathway after ablation. The laser irradiation can also propagate to epithelium around 604

the follicles, reducing barrier function of follicular epithelium. Insulin as a model macromolecule was also used in this study. Insulin forms a hexamer at therapeutically relevant concentrations to reach a molecular weight of ~ 36 kDa [47]. The permeation of hexameric insulin by laser treatment was greater than that of dextran with 38 kDa. The conformation of the macromolecules can govern laser-assisted delivery. Dextran shows a linear conformation, whereas insulin represents a globular conformation with a radius of only 2.5 nm. The laser ablation for macromolecular permeation may favor a globular structure compared to a linear one. Lee et al. [48] studied skin absorption of gene-based drugs enhanced by the Er:YAG laser. Nude mouse skin was used as a permeation barrier. The antisense phosphorothioate oligonucleotides of 15-mers (5036 Da) and 25-mers (8103 Da) were used as model permeants in the in vitro Franz cell experiments. Laser ablation with 1.4 -- 1.9 J/cm2 resulted in a respective permeation enhancement ratio of 4:30 and 4:9 for 15- and 25-mer oligonucleotides. In vivo topical application in nude mice showed a localization of oligonucleotides in the epidermis and follicles after laser exposure. The in vivo expression of reporter gene coding for b-galactosidase and green fluorescent protein (GFP) in intact and laser-treated skin was observed by X-gal staining and confocal laser scanning microscopy (CLSM). SC ablation contributed to the wide distribution of DNA expression from the epidermis to the lower dermis. GFP expression in laser-ablated skin was 160-fold greater as compared to that in untreated skin. The use of vaccine may not be appropriate for penetration into the skin due to the large size. The laser adjuvant for vaccine delivery can be induced by brief irradiation of a small area of the skin with a safe and noninvasive approach prior to intradermal injection [49]. Transdermal delivery of peptides with the related vaccine was evaluated under the assistance of Er:YAG laser ablation [50]. The number of peptides transported across the skin was examined using the Franz cell. The molecular mass, lipophilicity and sequence of peptides were found to play important roles in modulating laser enhancement on skin delivery. The peptides with a larger molecular weight (2190 and 2864 Da) led to higher enhancement compared to a smaller one (716 Da). The permeation of hydrophilic peptides was more effective for laser ablation than that of lipophilic peptides. The mouse skin was treated with laser followed by in vivo skin vaccination with a lysozyme antigen. Laser exposure could enhance antibody production in serum by threefold. This indicates that the Er:YAG laser induced a systemic effect on the macromolecules. Lee et al. [51] reported on the enhancing method for delivering siRNA and its plasmid vector using the Er:YAG laser. Both in vitro and in vivo skin permeation experiments were performed. In vitro permeation profiles demonstrated a significant improvement of siRNA (9266 Da) transport with laser application at 1.7 J/cm2, which showed a 10-fold increase compared to the untreated group. In vivo siRNA distribution examined by CLSM had revealed intense fluorescence within the epidermis



Table 2. Erbium:yttrium-gallium-garnet laser as an enhancing approach for dermal/transdermal drug delivery reviewed in this article. Output energy 0.91 -- 17.51 J/cm


1.7 and 2.6 J/cm2

Nalbuphine and indomethacin

1.2 -- 3.5 J/cm2 0.9 -- 3.1 J/cm2

1.4 -- 1.9 J/cm2

Methotrexate

1.3 J/cm2

Lidocaine

1.2 -- 3.5 J/cm2 1.4 -- 1.9 J/cm2

Dextran Oligonucleotides and plasmid DNA Peptides and vaccine

1.2 -- 1.7 J/cm2 3J 0.9 and 1.2 J/cm2 2.5 -- 6.3 J/cm2

Ref.

Dorsal skin of nude mouse (6 weeks old) Dorsal skin of pig (1 week old)

In vitro

[39]

In vitro

[40]

Dorsal skin of pig (1 week old) Dorsal skin of nude mouse (7 -- 8 weeks old) Nude mouse with basal cell carcinoma Dorsal skin of nude mouse (8 weeks old) Facial skin of human subjects (> 18 years old) Dorsal skin of pig (1 week old) Dorsal skin of nude mouse (6 -- 8 weeks old) Dorsal skin of nude mouse and ICR CD-1 mouse (8 weeks old) Dorsal skin of nude mouse (8 weeks old) Forearm skin of human subjects

In vitro In vitro/in vivo

[41] [43]

In vitro

[44]

In vivo

[45]

In vitro In vitro/in vivo

[46] [48]

In vitro/in vivo

[50]

In vitro/in vivo

[51]

In vitro/in vivo

[53]

Dorsal skin of nude mouse (6 -- 8 weeks old) Dorsal skin of nude mouse

In vitro

[65]

In vitro

[66]

Animal or skin model

Morphine, nalbuphine and buprenorphine 5-Aminolevulinic acid 5-Aminolevulinic acid

1.2 -- 1.7 J/cm2

In vitro or in vivo

Drug 2

Small interfering RNA and plasmid vector TiO2 nanoparticles (100 nm) and Al2O3 microparticles (27 µm) Vitamin C and magnesium ascorbyl phosphate 3-O-ethyl ascorbic acid and ascorbic acid 2-glucoside

and the upper dermis after laser treatment. The increased fluorescence of the plasmid vector after laser exposure was mainly in the dermis, which was fourfold greater than the signal from the skin that underwent vector permeation without laser treatment. It is possible to use nanoparticles and microparticles as drug carriers for efficient skin penetration and storage into the appendages or skin reservoirs [52]. Genina et al. [53] presented the results of absorption of TiO2 nanoparticles (100 nm) and Al2O3 microparticles (27 µm) into laser-treated skin. Ex vivo and in vivo human skin was used in this work. The output energy and pulse duration of the Er:YAG laser were set at 3 J and 20 ms, respectively. This pulse energy allowed for nanoparticle and microparticle permeation up to 230 µm deep into the dermis. The spectral measurement demonstrated that these particles stayed in the dermis longer than 1 month. Table 2 summarizes the data for Er:YAG laser-assisted drug delivery.

Neodymium-doped yttrium-aluminum-garnet laser

4.3

Use of the neodymium-doped yttrium-aluminum-garnet (Nd: YAG) laser is another resurfacing technique with skin-ablative capability (Figure 3). In clinical conditions, Nd:YAG laser is effective for skin disorder treatment and cosmetic rejuvenation, such as pigmentation removal, scar treatment, hair

removal, dermal remodeling and lipolysis [54,55]. The Nd: YAG laser displays a wide variety of emitted wavelengths, including 355, 532, 1064 and 1320 nm [56,57]. Liu et al. [58] compared glycerol permeation by treatment of a 1064 nmNd:YAG laser with the long-pulsed (15 J/cm2) and Qswitched (0.5 J/cm2) output modes. The pulse duration for long-pulsed and Q-switched modes was 40 ms and 8 ns, respectively. Long-pulsed irradiation increased skin temperature (+13 C) and loosened keratin, making corneocytes fragile and exfoliative. The Q-switched mode completely disrupted the keratin and corneocytes, punching micropores in the SC layer with a minimal temperature rise (+1 C). The experimental profiles of Wistar rat skin permeation showed that both modes possessed a similar enhancing capacity as glycerol permeation. Long-pulsed laser could make the SC exfoliated and thinned, which might be SC disruption caused by photothermal effect. The Q-switched laser broke down the local layer of SC, which could be due to giant pressure within ultrashort impulse irradiation on skin with high-energy peak. The in vivo skin delivery was 12 times greater that of the control group on the first day. This enhancement could persist for at least 1 week without infection. Go´mez et al. [59] examined 5-fluorouracil permeation via skin treated by the Nd:YAG laser at 355, 532 and 1064 nm. The pinna skin of the inner side of a rabbit ear was used for in vitro Franz cell penetration. Without laser treatment, the permeability of 5-fluorouracil


605


C.-H. Lin et al.

was negligible. On irradiation at 355 nm, increasing the fluence from 0.3 to 2 J/cm2 resulted in an enhancement ratio of drug permeation from 2.36 to 429. Laser radiation at 532 and 1064 nm also showed enhancement on drug flux, whereas the fluence should be increased (2.5 -- 9 J/cm2) to obtain the similar enhancement level as at 355 nm. The ablative effect of the Nd:YAG laser removed SC and some epidermal layers. The nanosecond range of pulse duration was shorter than the thermal relaxation time of the skin (1 ms), thus avoiding thermal injury to the skin. Ogura et al. [60] investigated the effect of combining the Nd:YAG laser and skin heating for percutaneous absorption of porfimer sodium, a photosensitizer for PDT. The laser fluence was set at 1.4 J/cm2. A mild heating (47 C) of the rat skin before the creation of PW on the skin from the Nd: YAG laser had enhanced in vivo drug permeation. The heating increased lipid bilayer fluidity and corneocyte swelling; thus, the PW could facilely form micropores for porfimer delivery across the skin according to CLSM images. Ogura et al. [61,62] further demonstrated that in vivo gene targeting of rat skin could be achieved by a 532-nm Nd:YAG laser without significant skin irritation. For analysis of plasmid DNA encoding enhanced GFP (EGFP), both the surface and cross-section of the rat skin were visualized by fluorescence microscopy. Expression of EGFP was observed in the area irradiated with the laser. The epidermal cells were selectively transfected. It can be proposed that PW of the Nd: YAG laser increased cell permeabilization. Chen et al. [63] developed Nd:YAG laser-based vaccine delivery for enhancing the immune response in skin. Mouse skin was exposed to a Q-switched Nd:YAG laser at 532 nm. Following illumination, immunogens were intradermally administered into the exposed skin. Nd:YAG laser enhanced ovalbumin-specific antibody production by threefold to fivefold over the nontreatment group. A similar result was shown for influenza vaccine. CO2 laser The CO2 laser has an emission wavelength of 10,600 nm, where the major chromophore is water (Table 1). The CO2 laser creates instant heating of water, resulting in tissue vaporization (Figure 3). This residual thermal effect yields coagulation of skin and necrosis. The CO2 laser generally ablates less skin thickness than the Er:YAG laser at the same fluences. On the other hand, this thermal effect can rejuvenate the skin for cosmetic purposes after optimizing the feasible laser energies [38,64]. The ability of the Er:YAG and CO2 lasers to promote skin diffusion of vitamin C was examined [65]. The flux of vitamin C across nude mouse skin dramatically increased 351-fold after stripping the SC, thus suggesting SC to be the major barrier to vitamin C permeation. The Er:YAG laser exposure resulted in an enhancement ratio of about 100 for fluences of 0.9 and 1.2 J/cm2 (Table 2). The more significant changes in skin structure with the 1.4 J/cm2 intensity led to a higher enhancement ratio (142) of vitamin C flux compared 4.4

606

with the non-irradiated control. A single pass of 1.4 J/cm2 from the CO2 laser was clearly sufficient to increase the transport of vitamin C (8.2-fold), although the enhancement was lower than that by the Er:YAG laser. This indicates the negligible influence of thermolysis by the lasers on drugpermeation enhancement. The weaker SC ablation capability of the CO2 laser compared to the Er:YAG laser contributes to lower vitamin C transport. The CO2 laser at higher fluences (4 and 7 J/cm2) increased vitamin C delivery by 49- and 65-fold, respectively. Hsiao et al. [66] investigated the effects of the Er:YAG and CO2 lasers on the skin-delivery enhancement of two vitamin C derivatives, 3-O-ethyl ascorbic acid (EAC) and ascorbic acid 2-glucoside (AA2G). The in vitro permeation was performed in the Franz cell. The flux of EAC and AA2G across the Er:YAG laser (2.5 -- 6.3 J/cm2)-treated skin was 105- to 189-fold and 35- to 78-fold higher, respectively, than their flux across intact skin. On the other hand, the flux of EAC and AA2G across CO2 laser-treated skin at 5 W was 181-fold and 82-fold higher than the flux across intact skin. Fractional laser Although the conventional ablative lasers are commonly used for skin rejuvenation nowadays, it is associated with delayed reepithelialization and erythema because of the lengthy recovery time. Fractional modality has been developed as an alternative to the conventional ablative laser, which is designed to minimize the side effects with quicker healing [67]. The concept of the fractional laser is the production of MTZs at a specific depth in the skin without affecting the neighboring tissues (Figure 3). Due to the uninjured tissue around MTZs, intact epidermis/dermis rapidly heals beneath the laser-treated zones [68]. The nonablative fractional photothermolysis was first described in 2004 [69]. In this technique, selective and controlled skin heating without epidermal damage is achieved by combining laser treatment with properly timed superficial skin cooling. In 2007, this idea was further developed with respect to ablative fractional resurfacing [70]. Both Er:YAG and CO2 lasers were utilized to design ablative fractional modalities. Besides the quick reepithelialization within 1 day [69], the main advantage of the fractional type over the conventional laser is the extensive adjustment of laser--skin interaction by the shape, depth and pattern of the MTZs [71]. The fractional laser has a broad application for treating acne scars, photodamaged skin, pigmented lesions, age spots, keloids, melasma and fine wrinkles [72,73]. Due to its ability to remove a part of superficial layers, the fractional laser has been used for peeling SC and/or viable skin for subsequent drug permeation enhancement in recent years. The fractional modality of ablative lasers has been recently verified as an efficient method for facilitating drug absorption. Fractional irradiation produces cylinder-shaped microchannels in skin. In addition to vertical diffusion of permeants into deeper strata, the permeants can diffuse into surrounding skin tissues by a lateral route, as illustrated in Figure 4. The increase in 4.5



Drug Co-solvent molecules

Drug partitioning into epidermis

Vehicle

SC


Solvation of co-solvent molecules in SC

Epidermis

Intact skin

Vehicle

Drug partitioning into epidermis

Ablated epidermis with disruption

Solvation of co-solvent molecules in epidermis

Conventional laser-treated skin

Solvation of vehicle molecules in remnant SC and epidermis

Vehicle

SC Lateral and downward drug diffusion Epidermis

Fractional laser-treated skin

Figure 4. A scheme revealing possible processes and mechanisms of drug permeation via skin treated with full-surface and fractional ablative lasers.

diffusion area with MTZ formation can lead to more efficient delivery of drugs. A portable fractional Er:YAG laser called the Painless Laser Epidermal System (P.L.E.A.S.E.) was developed to create a user-defined array of microchannels in a few seconds [74]. Bachhav et al. [75] investigated the effect of the pore number and the depth of P.L.E.A.S.E. on lidocaine delivery into the

skin. Histological sections of porcine skin confirmed that fluence could effectively control the pore depth: low energies (4.53 and 13.59 J/cm2) resulted in SC removal; intermediate energy (22.65 J/cm2) produced pores extended to the viable epidermis; high energy (45.30 J/cm2) created pores that reached the dermis. The cumulative lidocaine penetration was 1180, 1350, 1240 and 1653 µg/cm2 at fluences of 22.65,


607


C.-H. Lin et al.

45.30, 90.60 and 135.9 J/cm2, respectively. This suggests that increasing the fluence did not produce a statistically significant increase in lidocaine permeation. Cumulative lidocaine penetration with 0 (nontreatment), 150, 300, 450 and 900 micropores was 107, 774, 1400, 1653 and 1811 µg/cm2, respectively. Oni et al. [76] demonstrated in vivo lidocaine absorption by the fractional Er:YAG laser using pig as the animal model. The drug level in the serum was detected after topical application of 4% lidocaine. The serum concentration was undetectable in the nontreated group. The serum level was detectable following laser treatment. Peak levels of lidocaine were significantly greater at 250-µm pore depth (0.62 mg/l), compared to 500 µm (0.45 mg/l), 50 µm (0.48 mg/l) and 25 µm (0.3 mg/l). The greater depth of the microchannels did not guarantee greater enhancement. The microchannels are lined by coagulated cells. The deeper channels produced by higher fluence formed a thicker coagulated lining, which acted as a barrier to drug transport. Bachhav et al. [77] have explored the use of P.L.E.A.S.E. for skin delivery of diclofenac, a nonsteroidal anti-inflammatory drug. The in vitro porcine skin permeation was performed by varying the number of micropores, while the fluence was fixed at 22.65 J/cm2. Cumulative diclofenac permeation across the skin with 150, 300, 450 and 900 shallow pores was 3.7-, 7.5-, 9.2-, and 13-fold superior to that across nontreated skin. The influence of the drug vehicle was also explored by using aqueous solution and propylene glycol (PG). Diclofenac permeation was significantly higher with the aqueous solution than with the formulation of PG. The higher viscosity of PG prevented the drug carrier from creating an interface with interstitial fluid released from the bulk skin into the microchannels. Yu et al. [78] demonstrated that P.L.E.A.S.E. could increase transdermal delivery of prednisone. Prednisone absorption was investigated as a function of pore number (450, 900 and 1800 pores) and fluence (22.65, 45.3 and 90.6 J/cm2). Prednisone delivery via laser-treated pig ear skin (1800 pores) after 24 h was 197 µg/cm2. In contrast, no penetration was revealed via intact skin. An incremental increase of pore depth by higher fluence resulted in an increase in drug absorption. Lee et al. [79] applied the fractional Er:YAG laser to improve the skin’s uptake of ALA. The flux of ALA across laser-treated nude mouse skin and porcine skin exhibited higher levels than those through intact skin by 27- to 124-fold and 3- to 260-fold depending on various fluences (2 and 3 J/cm2) and the number of passes (1 -- 6 passes). The in vivo ALA diffusion depth was raised after fractional laser treatment, according to CLSM profiles. Reepithelialization determined by TEWL was completed within 1 day postoperation, which was at least 2 days earlier than the recovery by traditional laser. The confocal images also showed that PpIX was predominantly accumulated in the follicles after laser treatment. This suggests that the fractional laser may also propagate the PW and ablation effect around the appendages. The fractional Er:YAG laser also can be an efficient tool for elevating macromolecular permeation. Lee et al. [80] evaluated 608

the capability of the fractional Er:YAG laser in enhancing skin transport of imiquimod, peptides (716 -- 2354 Da) and dextrans (4 -- 150 kDa). Increases of 46- and 127-fold in imiquimod flux were detectable in employing respective fluencies of 2 and 3 J/cm2. Imiquimod concentration of 0.4% (w/v) in aqueous solution, with laser assistance, was sufficient to reach the flux from commercial cream (Aldara) with a drug dose of 5%. Thus, the applied dose can be largely reduced by fractional laser utilization. Peptide delivery assisted by laser was size- and sequence-dependent, with the smaller molecular weight and more hydrophilic entities exhibiting greater enhancement. CLSM observation of in vivo skin deposition showed an intense fluorescence signal of dextrans for all molecular sizes tested. The follicular route was significant in the laser-irradiated group. Weiss et al. [81] studied the ability of transcutaneous immunization via P.L.E.A.S.E.-induced microchannels for the elicitation of immune responses. Both BALB/c and C57BL/6 mice were used for in vivo skin vaccination. The topical administration of ovalbumin to lasertreated skin resulted in a dose-dependent proliferation of ovalbumin-specific T cells. This proliferation increased with the number of pulses. The maximal T-cell stimulation was usually obtained with 4 -- 8 pulses. Application of ovalbumin to the microchannels mainly activated CD4+ DO11.10 or OT-II T cells and to a lesser extent CD8+ OT-I cells in skin draining lymph nodes. Yu et al. [82] investigated the use of P.L.E.A.S.E. in enhancing the skin delivery of functional immunosuppressive antibodies, anti-thymocyte globulin (ATG) and basiliximab. Increasing laser fluence from 0 to 22.65, 45.3 and 135.9 J/cm2 increased the total ATG permeation from 0.06 to 1.70, 4.97 and 8.70 µg/cm2 by using pig ear skin as the barrier. Increasing the laser fluence and pore number led to a substantial increase in ATG delivery. A similar trend was observed for basiliximab permeation. Hessenberger et al. [83] tested P.L.E.A.S.E. application for allergen-specific immunotherapy. Four pulses with a fluence of 1.9 J/cm2 per pulse were applied with a pore density of 500 pores/cm2. With this setting, a microchannel depth of 30 -- 40 µm penetrating well into the dermis of BALB/c mice was achieved. The grass pollen allergen Phl p 5 was administered by patch with or without T-helper 1-promoting CpG as an adjuvant. Protection from allergic immune responses was tested by sensitization via injection of allergen adjuvated with alum. The application of the antigen via microchannels induced T-helper 2-biased immune responses. The therapeutic efficacy of transcutaneous immunotherapy was equal to subcutaneous administration but was superior with respect to the suppression of already established IgE responses. The results indicate that immunotherapy assisted by P.L.E.A.S.E. offers an efficient way to treat type 1 allergic diseases. Genina et al. [84] demonstrated that the creation of microchannels by the fractional Er:YAG laser promoted deeper and targeted delivery of nanoparticles. The pulse energy was set to 0.5 -- 3.0 J. The visualization of microchannels and insertion of TiO2 nanoparticles (100 nm) was implemented by optical multiphoton




tomography. Different fluencies provided the penetration of nanoparticles from the skin surface to the depth of 150 -- 400 µm of minipig skin. The histological examination showed that the nanoparticles stayed in the dermis at the depth of 400 µm for no < 3 weeks. The conventional CO2 laser always produces thermal injury to the skin structure, leading to concern about the safety of this laser modality. The fractional type of the CO2 laser may lessen this drawback due to there being less damage to the skin compared to that with the full-ablative type. Letada et al. [85] have enhanced ALA delivery into the subcorneal layers of the palmar skin of healthy subjects. Six palms from three volunteers were treated with the fractional CO2 laser with or without Er:YAG surface ablation. The relative efficacy was determined using Wood’s lamp illumination for the presence of PpIX. The depth of the ablative micropores formed by the fractional laser was ~ 1500 µm, which allowed ALA to bypass the thick SC of the palm and penetrate into the subcorneal tissue. The palm treated with intradermal ALA injection and the palm pretreated with the fractional CO2 laser exhibited fluorescence in the treated areas. The fractional laser provided an effective means for targeting ALA to the subcorneal region. Lee et al. [86] had utilized full-surface ablative and fractional CO2 lasers to promote skin delivery of ALA and imiquimod. Vehicles for carrying the drugs were water with 40% polyethylene glycol 400 (PEG400), PG and ethanol. Lipid nanoparticles were also used as drug carriers. In vitro permeation profiles demonstrated improvement in imiquimod flux with a 2.5-J/cm2 conventional laser producing a 12-, 9- and 5-fold increase when loading this drug in PEG400, PG and ethanol, respectively, as compared with intact skin. Nanoparticulate delivery by conventional laser generated a sixfold enhancement. The fractional laser produced fewer enhancements on imiquimod permeation than the conventional laser. ALA delivery from aqueous buffer, PEG400 and PG with full-ablative laser was 641-, 445-, and 104-fold superior to passive control. In vivo skin accumulation examined by CLSM indicated the same trend in in vitro experiments. It is inferred that diffusion of cosolvent molecules into ablated skin and drug partitioning from vehicles are two factors governing laser-assisted delivery. Lateral drug diffusion from the microchannels of fractional laser-irradiated skin is expected (Figure 4). Hædersdal et al. [87] evaluated skin delivery assisted by the fractional CO2 laser by using methyl ALA as the model permeant. Methyl ALA is an ester prodrug of ALA. Yorkshire swine were treated with the fractional CO2 laser and were subsequently applied with methyl ALA. Fluorescence derived from PpIX was measured by fluorescence microscopy at a skin depth of 1800 µm. The fractional laser created coneshaped channels of 300 µm in diameter and 1850 µm in depth. This ablation enhanced drug delivery with higher fluorescence in the hair follicles and dermis as compared to intact skin. The fractional laser irradiation facilitates topical delivery of porphyrin precursor deep into the skin. Hædersdal et al.

[88] also investigated the methyl ALA permeation enhanced by fractional CO2 laser quantifying PpIX skin distribution and PDT-induced photobleaching from the skin surface to the depth of 1800 µm in Yorkshire swine. The red light for creating photobleaching was light-emitting diode arrays delivered at fluencies of 37 and 200 J/cm2. The fraction of porphyrin fluorescence lost by photobleaching was less after 37 J/cm2 than after 200 J/cm2. The fractional laser greatly facilitated the skin PpIX, and the fraction of photobleached porphyrins was similar for the superficial and deep skin layers. Distribution of PpIX into laser-treated skin may depend on the microchannel depth and the drug incubation period. Haak et al. [89] further evaluated whether the depth of the fractional laser and the incubation time affect methyl ALA permeation. Yorkshire swine were treated with the fractional CO2 laser at 37, 190 and 380 mJ to create microchannel depths of 300 (superficial), 1400 (mid) and 2100 µm (deep dermis/subcutaneous), respectively. The incubation times for methyl ALA cream were 30, 60, 120 and 180 min. Similar fluorescence of PpIX was induced throughout the skin layers independent of the laser channel depth by a 180-min incubation. Laser irradiation and the following methyl ALA incubation for 60 min increased fluorescence in the skin surface compared to intact skin. Laser exposure and the subsequent methyl ALA incubation for 120 min increased fluorescence in the follicles and the dermis compared to intact skin. Hsiao et al. [90] compared skin histology and AA2G permeation after fractional and conventional CO2 laser irradiation on porcine skin. Fractional laser irradiation with four or fewer passes produced less skin disruption than conventional laser treatment at the same fluence (5, 7 or 9 W). The diffusion of AA2G increased as the number of passes increased. When the number of passes reached four, the flux (79 µg/cm2/h) approximated that of the conventional modality at 5 W (84 µg/cm2/h). Haak et al. [91] studied the influence of fractional CO2 laser density (25 -- 400 pores/cm2) on the skin delivery of PEG with different molecular weights (240 -4300 Da). The molecular size of PEG can be classified from small molecular to macromolecular permeants. Fractional laser substantially increased PEG permeation into and through human abdominal skin for all molecular sizes. Increasing the laser density from 1 to 20% led to increased permeation, but densities > 1% resulted in reduced delivery per microchannel. Chen et al. [92] indicated that the fractional CO2 laser sufficiently delivered protein vaccine into the skin and elicited a systemic immune response. A laser energy of 2.5 or 5.0 mJ with skin coverage at 5 or 15% was irradiated into the dorsal skin of a BALB/c mouse. The molecular weight of ovalbumin, a vaccine antigen, was ~ 45 kDa. Following entry into the microchannels created by the laser, the ovalbumin diffused quickly to the dermis via the lateral surface of the pores. The vaccine was taken up by Langerhans and dendritic cells after entering the microchannels, leading to the transport to the draining lymph nodes. The strong immune response was thus elicited. The efficiency by laser-


609


C.-H. Lin et al.

assisted delivery was much better than that by tape stripping. The laser-treated skin could be recovered for maintaining integrity within 1 -- 2 days, which was observed by skin histological analysis. A comparison of the fractional Er:YAG laser and the CO2 laser was conducted to examine skin absorption of siRNA and plasmid DNA vectors [93]. The nude mouse skin received Er:YAG laser fluence of 2, 3 or 5 J/cm2 with a diameter of MTZ of 250 µm. The fluence and MTZ diameter of the CO2 laser was 2 or 4 mJ and 120 µm, respectively. The siRNA permeation by Er:YAG laser irradiation at 5 J/cm2 was 11-fold superior to that of the control skin. The CO2 laser at 4 mJ increased siRNA permeation by 12-fold. The siRNA uptake in follicles with the Er:YAG laser and the CO2 laser was 9and 3-times higher than that without treatment. According to fluorescence microscopy, there was a widespread distribution of fluorescence from siRNA and plasmid DNA throughout the epidermis after irradiation of both fractional lasers. Lee et al. [94] evaluated the skin transport enhancement mediated by the fractional CO2 laser for dextran and quantum dots. Quantum dots are metal nanoparticles made with CdSe and ZnS. The quantum dots used in this work had a diameter of 18 nm [95]. A laser fluence of 4 mJ promoted the flux of dextran with 20 and 40 kDa from 0 (intact skin) to 0.72 and 0.43 nmol/cm2/h, respectively. The penetration of quantum dots via intact skin was limited in the outermost SC. The imaging of laser-treated skin demonstrated a clear fluorescence increase in both intensity and depth for the nanoparticles. Predominant routes for laser-assisted delivery of macromolecules and nanoparticles were intercellular and follicular transport based on the observation of fluorescence microscopy and CLSM. The skin barrier function was completed 12 h after fractional laser irradiation, which was much quicker than full-surface ablation (4 days). Table 3 summarizes the drug permeation by fractional laser. 5.

Conclusion

Increasing attention has been paid to laser irradiation as an enhancing method of applying drugs topically or transdermally due to its advantages over other approaches, such as increased bioavailability, fast treatment time, quick recovery of SC integrity and the fact that skin surface contact is not being needed. The permeation level of the laser-mediated drug delivery can be controlled by modulating the laser fluence and pulse numbers. SC ablation and PWs were the main mechanisms that enhance skin permeation. Several laser modalities can be employed for enhancing topical/ transdermal administration of drugs. Er:YAG and CO2 lasers are the modalities frequently used for laser-assisted drug delivery because of their ablative effect on the superficial skin layers. The fractional laser has recently been used for promoting skin permeation of drugs due to its causing less irritation and having a faster recovery time as compared with the fullablative laser. The major pathways for laser-mediated delivery 610

were intercellular and follicular penetration. The laser-assisted delivery is effective in facilitating the skin transport of not only the small-molecule drugs but also macromolecules and nanoparticles. The prospective benefits of laser-assisted permeation are that it can provide a less-invasive method to deliver macromolecules such as proteins, vaccines and genes compared to a parenteral injection. This potential use of the laser affords a new treatment for topical/transdermal application with significant efficacy. This treatment modality deserves further exploration to maximize benefits. 6.

Expert opinion

Investigation into the in vitro/in vivo effectiveness of laser devices for facilitating drug permeation in animals is abundant. However, few studies have examined clinical studies of the laser-assisted drug delivery in humans. Topical anesthesia and PDT are two indications worthy of development for clinical use of laser-assisted drug delivery due to the abundant and successful data of skin absorption experiments in the previous studies. Further studies on a large group of human patients are needed to approve and clarify the findings in animal studies. The basic studies are also necessary in order to elucidate the detailed mechanisms of drug delivery with specific techniques and to achieve better therapeutic results. The development of suitable animal models for lasermediated delivery is important as well. The large and expensive device is another concern for the application of lasers. The development of hand-held portable devices is in progress and shows successful results, including P.L.E.A.S.E. (Pantec Biosolutions) and LAD-01 (Norwood Abbey Ltd). It is anticipated that the cost of these methods can be reduced in the near future. Possible skin irritation from the lasers is another concern. Although the laser fluence or output energy used for enhancing drug absorption is much lower than for treatment of skin disorders and rejuvenation, there are apparent safety-related issues of using lasers that must be considered. Caution should be used in optimizing the feasible conditions of lasers for balancing the effectiveness of permeation enhancement and skin damage. Consequently, in spite of lasers’ therapeutic benefits, scientific investigators should pay attention to the adverse effects of the lasers utilized. All laser tools applied for enhancing topical/ transdermal drug delivery are employed directly from the devices for clinical dermatology. The fluence range of these lasers is usually higher than the fluence needed for promoting drug permeation. New devices designed with low and precise fluence can be a specific aim for use in laser-assisted drug absorption. The development of new devices should be accomplished under the cooperation of dermatologists, engineers and researches. The selection of feasible drug candidates and/or vehicles for laser-assisted topical delivery is important as well. Generally, the hydrophilic drugs are suitable candidates for laser-assisted skin permeation due to the greater enhancement of drug



Table 3. Fractional laser as an enhancing approach for dermal/transdermal drug delivery reviewed in this article.


Laser type

Output energy

Drug

Animal or skin model

Er:YAG (P.L.E.A.S.E.) Er:YAG Er:YAG (P.L.E.A.S.E.) Er:YAG (P.L.E.A.S.E.) Er:YAG

4.53 -- 135.9 J/cm2; 150 -- 900 pores Ablative depth: 25 -- 500 µm 22.65 J/cm2; 150 -- 900 pores

Er:YAG

2 and 3 J/cm2; 1 -- 6 passes

Er:YAG (P.L.E.A.S.E.) Er:YAG (P.L.E.A.S.E.) Er:YAG (P.L.E.A.S.E.) Er:YAG CO2 CO2

0.76 J/cm2; 900 pores; 1 -- 8 passes 22.65, 45.3 and 135.9 J/cm2; 300 -- 900 pores 1.9 J/cm2; 500 pores/cm2; 4 passes 0.5 -- 3.0 J 50 mJ; 5% density 2 and 4 mJ; 400 pores/cm2

ATG and basiliximab CpG-adjuvanted allergen TiO2 nanoparticles ALA ALA and imiquimod

CO2

91.6 mJ (30.5 W)

Methyl ALA

CO2

91.6 mJ (30.5 W)

Methyl ALA

CO2

37, 190 and 380 mJ

Methyl ALA

CO2

5, 7 and 9 W

Ascorbic acid 2glucoside

CO2

180 mJ; 25, 100, 225 and 400 pores/cm2; 1, 5, 11 and 20% coverage 2.5 and 5.0 mJ; 5 and 15% coverage 2, 3 or 5 J/cm2for Er:YAG; 2 or 4 mJ for CO2 2 or 4 mJ

Polyethylene glycol (400, 1000, 2050 and 3350 Da) Ovalbumin

CO2 Er:YAG and CO2 CO2

22.65 -- 90.6 J/cm2; 450 -- 1800 pores 2 and 3 J/cm2; 1 -- 6 passes

In vitro or in vivo

Ref.

Lidocaine

Ear skin of pig

In vitro

[75]

Lidocaine Diclofenac

Abdominal skin of pig Full-thickness porcine skin (1.0 -- 1.5 mm) Ear skin of pig

In vivo In vitro

[76] [77]

In vitro

[78]

Dorsal skin of nude mouse (8 weeks old) and pathogen-free pig (1 week old) Dorsal skin of nude mouse (8 weeks old) and pathogen-free pig (1 week old) BALB/c and C57BL/6 mice (6 -- 8 weeks old) Dermatomed pig ear skin and human abdominal skin BALB/c mouse (6 -- 8 weeks old)

In vitro/in vivo

[79]

In vitro/in vivo

[80]

In vitro/in vivo

[81]

In vitro/in vivo

[82]

In vivo

[83]

Human skin and minipig skin Palmar skin of human Dorsal skin of nude mouse (8 weeks old) Yorkshire swine (12 -- 13 weeks old) Yorkshire swine (12 -- 13 weeks old) Yorkshire swine (12 -- 13 weeks old) Dorsal skin of nude mouse (8 week old) and pathogen-free pig (1 week old) Abdominal skin of human

In vitro/in vivo In vivo In vitro/in vivo

[84] [85] [86]

In vitro/in vivo

[87]

In vivo

[88]

In vitro/in vivo

[89]

In vitro/in vivo

[90]

In vitro

[91]

Dorsal skin of BALB/c mouse (6 -- 8 weeks old) Dorsal skin of nude mouse (8 weeks old) Dorsal skin of nude mouse (8 weeks old)

In vivo

[92]

In vitro/in vivo

[93]

In vitro/in vivo

[94]

Prednisone ALA

Imiquimod, peptides and dextrans Ovalbumin

siRNA and plasmid DNA Dextran and quantum dots

ALA: Aminolevulinic acid; ATG: Anti-thymocyte globulin; Er:YAG: Erbium:yttrium-gallium-garnet; P.L.E.A.S.E.: Painless Laser Epidermal System; siRNA: Small-interfering RNA.

absorption compared to lipophilic drugs. The predominant reason is that the ablative lasers can remove the lipophilic proportions of SC, thus reducing the main barrier for hydrophilic permeants. This does not mean that lipophilic drugs are infeasible for laser-mediated delivery. The removal of intracellular route and the effect on follicular transport by the lasers contribute to the permeation enhancement of lipophilic permeants. Vehicle types can modify drug permeation via laser-treated skin. According to previous studies, the hydrophilic carriers such as buffer, PEG and hydrogel are used in providing a satisfied permeation of the permeants into laser-

irradiated skin. On the other hand, the lipophilic vehicles offer lower drug delivery as compared to hydrophilic ones. Further investigations are necessary to verify the effect of vehicles on laser-assisted drug delivery. Nanoparticles are a case which can be utilized as a carrier for drug permeation after the assistance of laser exposure.

Declaration of interest The authors state no conflict of interest and have received no payment in preparation of this manuscript.


611

C.-H. Lin et al.

Bibliography Papers of special note have been highlighted as either of interest () or of considerable interest () to readers. 1.


2.

3.

.

4.

5.

6.

..

Vrijman C, van Drooge AM, Limpens J, et al. Laser and intense pulsed light therapy for the treatment of hypertrophic scars: a systematic review. Br J Dermatol 2011;165:934-42 Ong MWS, Bashir SJ. Fractional laser resurfacing for acne scars. Br J Dermatol 2012;166:1160-9 Anderson RR. Lasers for dermatology and skin biology. J Invest Dermatol 2013;133:E21-3 The introduction of laser usefulness on dermatology. Sadick NS, Karcher C, Palmisano L. Cosmetic dermatology of the aging face. Clin Dermatol 2009;27:S3-12 Marcells GN, Ellis DAF. Laser facial skin resurfacing: discussion on erbium:YAG and CO2 lasers. J Otolaryngol 2000;29:78-82

Subedi RK, Oh SY, Chun MK, et al. Recent advances in transdermal drug delivery. Arch Pharm Res 2010;33:339-51

8.

Brown MB, Martin GP, Jones SA, et al. Dermal and transdermal drug delivery systems: current and future prospects. Drug Deliv 2006;13:175-87 A complete review of topical/ transdermal drug delivery system mediated by chemical and physical enhancement approaches.

9.

10.

11.

612

12.

.

Paudel KS, Milewski M, Swadley CL, et al. Challenges and opportunities in dermal/transdermal delivery. Ther Deliv 2010;1:109-31 Lapidoth M, Adatto M, Halachmi S. Treatment of actinic keratoses and photodamage with non-contact fractional 1540-nm laser quasi-ablation: an ex vivo and clinical evaluation. Lasers Med Sci 2013;28:537-42 Habbema L, Verhagen R, Van Hal R, et al. Minimally invasive non-thermal laser technology using laser-induced

Nelson JS, McCullough JL, Glenn TC, et al. Mid-infrared laser ablation of stratum corneum enhances in vitro percutaneous transport of drugs. J Invest Dermatol 1991;97:874-9 The first article utilizing laser for enhancing percutaneous absorption of drugs.

13.

Allemann IB, Kaufman J. Fractional photothermolysis- an update. Lasers Med Sci 2010;25:137-44

14.

Paasch U, Hædersdal M. Laser systems for ablative fractional resurfacing. Expert Rev Med Devices 2011;8:67-83 Complete introduction of fractional lasers.

..

15.

16.

Prausnitz MR, Langer R. Transdermal drug delivery. Nat Biotechnol 2008;26:1261-8 An update review for drug delivery via skin.

7.

.

optical breakdown for skin rejuvenation. J Biophotonics 2012;5:194-9

17.

18.

19.

20.

21.

22.

Carniol PJ, Harirchian S, Kelly E. Fractional CO2 laser resurfacing. Facial Plast Surg Clin N Am 2011;19:247-51 Prignano F, Bonciani D, Campolmi P, et al. A study of fractional CO2 laser resurfacing: the best fluencies through a clinical, histological, and ultrastructural evaluation. J Cosmet Dermatol 2011;10:210-16 Yamamoto T, Kurasawa M, Hattori T, et al. Relationship between expression of tight junction-related molecules and perturbed epidermal barrier function in UVB-irradiated hairless mice. Arch Dermatol Res 2008;300:61-8 Liu KS, Sung KC, Al-Suwayeh SA, et al. Enhancement of transdermal apomorphine delivery with a diester prodrug strategy. Eur J Pharm Biopharm 2011;78:422-31 Niessen CM. Tight junctions/adherins junctions: basic structure and function. J Invest Dermatol 2007;127:2525-32 Fang JY, Leu YL. Prodrug strategy for enhancing drug delivery via skin. Curr Drug Discov Technol 2006;3:211-24 Liu X, Grice JE, Lademann J, et al. Hair follicles contribute significantly to penetration through human skin only at times soon after application as a solvent deposited solid in man. Br J Clin Pharmacol 2011;72:768-74 Baroli B. Penetration of nanoparticles and nanomaterials in the skin: fiction or reality? J Pharm Sci 2010;99:21-50


23.

.

Doukas AG, Kollias N. Transdermal drug delivery with a pressure wave. Adv Drug Deliv Rev 2004;56:559-79 The introduction of PWs generated from lasers.

24.

Menon GK, Kollias N, Doukas AG. Ultrastructural evidence of stratum corneum permeabilization induced by photomechanical waves. J Invest Dermatol 2003;121:104-9

25.

Lee S, Doukas AG. Laser-generated stress waves and their effects on the cell membrane. IEEE J Sel Top Quantum Electron 1999;5:997-1003

26.

Alexiades-Armenakas MR, Dover JS, Arndt KA. The spectrum of laser skin resurfacing: nonablative, fractional, and ablative laser resurfacing. J Am Acad Dermatol 2008;58:719-37

27.

Wanner M. Laser hair removal. Dermatol Ther 2005;18:209-16

28.

Bernstein EF. Laser treatment of tattoos. Clin Dermatol 2006;24:43-55

29.

Lee S, Kollias N, McAuliffe DJ, et al. Topical drug delivery in humans with a single photomechanical wave. Pharm Res 1999;16:1717-21

30.

De Rosa FS, Bentley MVL. Photodynamic therapy of skin cancers: sensitizers, clinical studies and future directives. Pharm Res 2000;17:1447-55

31.

Zhang LW, Fang YP, Fang JY. Enhancement techniques for improving 5-aminolevulinic acid delivery through the skin. Dermatol Sin 2011;29:1-7

32.

Lee S, McAuliffe DJ, Flotte TJ, et al. Photomechanical transcutaneous delivery of macromolecules. J Invest Dermatol 1998;111:925-9

33.

Lee S, McAuliffe DJ, Flotte TJ, et al. Photomechanical transdermal delivery: the effect of laser confinement. Lasers Surg Med 2001;28:344-7

34.

Lee S, McAuliffe DJ, Flotte TJ, et al. Permeabilization and recovery of the stratum corneum in vivo: the synergy of photomechanical waves and sodium lauryl sulfate. Lasers Surg Med 2001;29:145-50

35.

Lee S, McAuliffe DJ, Kollias N, et al. Photomechanical delivery of 100-nm microspheres through the stratum corneum: implications for transdermal drug delivery. Lasers Surg Med 2002;31:207-10


36.

37.


38.

.

39.

40.

41.

42.

43.

44.

45.

El-Domyati M, Abd-El-Raheem T, Abdel-Wahab H, et al. Fractional versus ablative erbium:yttrium-aluminum-garnet laser resurfacing for facial rejuvenation: an objective evaluation. J Am Acad Dermatol 2013;68:103-12 Tanzi EL, Alster TS. Single-pass carbon dioxide versus multiple-pass Er:YAG laser skin resurfacing: a comparison of postoperative wound healing and sideeffect rates. Dermatol Surg, 2003;29:80-4 Tsai TH, Jee SH, Chan JY, et al. Visualizing laser-skin interaction in vivo by multiphoton microscopy. J Biomed Opt 2009;14:024034 Investigation about the laser--skin interaction. Lee WR, Shen SC, Lai HH, et al. Transdermal drug delivery enhanced and controlled by erbium:YAG laser: a comparative study of lipophilic and hydrophilic drugs. J Control Release 2001;75:155-66 Lee WR, Shen SC, Fang CL, et al. Skin pretreatment with an Er:YAG laser promotes the transdermal delivery of three narcotic analgesics. Lasers Med Sci 2007;22:271-8 Fang JY, Lee WR, Shen SC, et al. Enhancement of topical 5-aminolevulinic acid delivery by erbium:YAG laser and microdermabrasion: a comparison to iontophoresis and electroporation. Br J Dermatol 2004;151:132-40 Hædersdal M, Togsverd-Bo K, Wulf HC. Evidence-based review of lasers, light sources and photodynamic therapy in the treatment of acne vulgaris. J Eur Acad Dermatol Venereol 2008;22:267-78 Shen SC, Lee WR, Fang YP, et al. In vitro percutaneous absorption and in vivo protoporphyrin IX accumulation in skin and tumors after topical 5-aminolevulinic acid application with enhancement using an erbium:YAG laser. J Pharm Sci 2006;95:929-38 Lee WR, Shen SC, Fang CL, et al. Topical delivery of methotrexate via skin pretreated with physical enhancement techniques: low-fluence erbium:YAG laser and electroporation. Lasers Surg Med 2008;40:468-76 Yun PL, Tachihara R, Anderson RR. Efficacy of erbium:yttrium-aluminumgarnet laser-assisted delivery of topical anesthetic. J Am Acad Dermatol 2002;47:542-7

46.

Fang JY, Lee WR, Shen SC, et al. Transdermal delivery of macromolecules by erbium:YAG laser. J Control Release 2004;100:75-85

47.

Jacoby E, Hua QX, Stern AS, et al. Structure and dynamics of a protein assembly.1H-NMR studies of the 36 kDaR6insulin hexamer. J Mol Biol 1996;258:136-57

48.

Lee WR, Shen SC, Liu CJ, et al. Erbium:YAG laser-mediated oligonucleotide and DNA delivery via the skin: an animal study. J Control Release 2006;115:344-53 Ablative laser for enhancing oligonucleotide and DNA permeation via skin.

..

49.

50.

51.

52.

scaling. Photomed Laser Surg 2006;24:366-76 58.

59.

Liu C, Zhang J, Yue Y, et al. 1064 nm-Nd:YAG lasers with different output modes enhancing transdermal delivery: physical and physiological mechanisms. J Biomed Opt 2013;18:061228 Go´mez C, Costela A´, Garcı´a-Moreno I, et al. Laser treatments on skin enhancing and controlling transdermal delivery of 5-fluorouracil. Lasers Surg Med 2008;40:6-12

60.

Ogura M, Sato S, Kuroki M, et al. Transdermal delivery of photosensitizer by the laser-induced stress wave in combination with skin heating. Jpn J Appl Phys 2002;41:814-16

61.

Ogura M, Sato S, Ashida H, et al. In vivo targeted gene transfer by direct irradiation with nanosecond pulsed laser. Jpn J Appl Phys 2004;43:1268-70

62.

Lee WR, Shen SC, Zhuo RZ, et al. Enhancement of topical small interfering RNA delivery and expression by low-fluence erbium:YAG laser pretreatment of skin. Hum Gene Ther 2009;20:580-8

Ogura M, Sato S, Nakanishi K, et al. In vivo targeted gene transfer in skin by the use of laser-induced stress waves. Lasers Surg Med 2004;34:242-8

63.

Cevc G, Vierl U. Nanotechnology and the transdermal route. A state of the art review and critical appraisal. J Control Release 2010;141:277-99

Chen X, Kim P, Farinelli B, et al. A novel laser vaccine adjuvant increases the motility of antigen presenting cells. PLoS One 2010;5:e13776

64.

Griffin R. Periorbital rejuvenation using CO2 laser skin resurfacing. Prime 2012;2:30-40

Chen X, Wu MX. Laser vaccine adjuvant for cutaneous immunization. Expert Rev Vaccines 2011;10:1397-403 Lee WR, Pan TL, Wang PW, et al. Erbium:YAG laser enhances transdermal peptide delivery and skin vaccination. J Control Release 2008;128:200-8

53.

Genina EA, Bashkatov AN, Dolotov LE, et al. Transcutaneous delivery of microand nanoparticles with laser microporation. J Biomed Opt 2013;18:111406

65.

Lee WR, Shen SC, Wang KH, et al. Lasers and microdermabrasion enhance and control topical delivery of vitamin C. J Invest Dermatol 2003;121:1118-25

54.

Tanzi EL, Alster TS. Long-pulsed 1064-nm Nd:YAG laser-assisted hair removal in all skin types. Dermatol Surg 2004;30:13-17

66.

55.

Ichikawa K, Miyasaka M, Tanaka R, et al. Histologic evaluation of the pulsed Nd:YAG laser for laser lipolysis. Lasers Surg Med 2005;36:43-6

Hsiao CY, Huang CH, Hu S, et al. Skin pretreatment with lasers promotes the transdermal delivery of vitamin C derivatives. Lasers Med Sci 2011;26:369-76

67.

Fatemi A, Weiss MA, Weiss RA. Short-term histologic effects of nonablative resurfacing: results with a dynamically cooled millisecond-domain 1320 nm Nd:YAG laser. Dermatol Surg 2002;28:172-6 Go´mez C, Costela A´, Garcı´a-Moreno I,

Trelles MA, Mordon S, Velez M, et al. Results of fractional ablative skin surfacing with the erbium:yttrium-gallium-garnet laser 1 week and 2 months after one single treatment in 30 patients. Lasers Med Sci 2009;24:186-94

68.

Kim SG, Kim EY, Kim YJ, et al. The efficacy and safety of ablative fractional resurfacing using a 2,940-nm Er:YAG laser for traumatic scars in the early posttraumatic period. Arch Plast Surg 2012;39:232-7

69.

Manstein D, Herron GS, Sink RK, et al. Fractional photothermolysis: a new concept for cutaneous remodeling using

56.

57.

et al. In vitro evaluation of Nd:YAG laser radiation at three different wavelengths (1064, 532, and 355 nm) on calculus removal in comparison with ultrasonic


613

C.-H. Lin et al.

..

70.


71.

microscopic patterns of thermal injury. Lasers Surg Med 2004;34:426-38 The first article to introduce the concept of fractional laser. Hantash BH, Bedi VP, Kapadia B, et al. In vivo histological evaluation of a novel ablative fractional resurfacing device. Lasers Surg Med 2007;39:96-107

Goldberg DJ, Berlin AL, Phelps R. Histologic and ultrastructural analysis of melasma after fractional resurfacing. Lasers Surg Med 2008;40:134-8

73.

Ortiz AE, Tremaine AM, Zachary CB. Long-term efficacy of a fractional resurfacing device. Lasers Surg Med 2010;42:168-70 Kalia YN, Bachhav YG, Bragagna T, et al. P.L.E.A.S.E. (Painless Laser Epidermal System). A new laser microporation technology. Drug Deliv Technol 2008;8:26-31

75.

Bachhav YG, Summer S, Heinrich A, et al. Effect of controlled laser microporation on drug transport kinetics into and across the skin. J Control Release 2010;146:31-6

76.

Oni G, Brown SA, Kenkel JM. Can fractional lasers enhance transdermal absorption of topical lidocaine in an in vivo animal model. Lasers Surg Med 2012;44:168-74

77.

Bachhav YG, Heinrich A, Kalia YN. Using laser microporation to improve transdermal transdermal delivery of diclofenac: increasing bioavailability and the range of therapeutic applications. Eur J Pharm Biopharm 2011;78:408-14

78.

Yu J, Bachhav YG, Summer S, et al. Using controlled laser-microporation to increase transdermal delivery of prednisone. J Control Release 2010;148:e71-3

79.

Lee WR, Shen SC, Pai MH, et al. Fractional laser as a tool to enhance the skin permeation of 5-aminolevulinic acid with minimal skin disruption: a comparison with conventional erbium: YAG laser. J Control Release 2010;145:124-33

80.

614

.

Lee WR, Shen SC, Al-Suwayeh SA, et al. Laser-assisted topical drug delivery by using a low-fluence fractional laser: imiquimod and macromolecules. J Control Release 2011;153:240-8

Weiss R, Hessenberger M, Kitzmu¨ller S, et al. Transcutaneous vaccination via laser microporation. J Control Release 2012;162:391-9 Laser-assisted delivery for skin vaccination.

82.

Yu J, Kalaria DR, Kalia YN. Erbium: YAG fractional laser ablation for the percutaneous delivery of intact functional therapeutic antibodies. J Control Release 2011;156:53-9

83.

Hessenberger M, Weiss R, Weinberger EE, et al. Transcutaneous delivery of CpG-adjuvanted allergen via laser-generated micropores. Vaccine 2013;31:3427-34

84.

Genina EA, Dolotov LE, Bashkatov AN, et al. Fractional laser microablation of skin aimed at enhancing its permeability for nanoparticles. Quantum Electron 2011;41:396-401

85.

Letada PR, Schumaker PR, Uebelhoer NS. Demonstration of protoporphyrin IX (PpIX) localized to areas of palmar skin injected with 5-aminolevulinic acid (ALA) and pre-treated with a fractionated CO2 laser prior to topically applied ALA. Photodiagn Photodyn Ther 2010;7:120-2

Allemann IB, Kaufman J. Fractional photothermolysis- an update. Lasers Med Sci 2010;25:137-44

72.

74.

81.

86.

.

Lee WR, Shen SC, Aljuffali IA, et al. Impact of different vehicles for laser-assisted drug permeation via skin: full-surface versus fractional ablation. Pharm Res 2013; Epub ahead of print The impact of vehicles/carriers on laser-assisted drug delivery.

87.

Hædersdal M, Sakamoto FH, Fairinelli WA, et al. Fractional CO2 laser-assisted drug delivery. Lasers Surg Med 2010;42:113-22

88.

Hædersdal M, Katsnelson J, Sakamoto FH, et al. Enhanced uptake and photoactivation of topical methyl aminolevulinate after fractional CO2 laser pretreatment. Lasers Surg Med 2011;43:804-13

89.

Haak CS, Farinelli WA, Tam J, et al. Fractional laser-assisted delivery of methyl aminolevulinate: impact of laser channel depth and incubation time. Lasers Surg Med 2012;44:787-95

90.

Hsiao CY, Huang CH, Hu S, et al. Fractional carbon dioxide laser treatment to enhance skin permeation of ascorbic acid 2-glucoside with minimal skin disruption. Dermatol Surg 2012;38:1284-93


91.

Haak CS, Bhayana B, Farinelli WA, et al. The impact of treatment density and molecular weight for fractional laserassisted drug delivery. J Control Release 2012;163:335-41

92.

Chen X, Shah D, Kositratna G, et al. Facilitation of transcutaneous drug delivery and vaccine immunization by a safe laser technology. J Control Release 2012;159:43-51

93.

Lee WR, Shen SC, Chen WY, et al. Noninvasive delivery of siRNA and plasmid DNA into skin by fractional ablation: erbium:YAG laser versus CO2 laser. Eur J Phar Biopharm 2013; Epub ahead of print

94.

Lee WR, Shen SC, Al-Suwayeh SA, et al. Skin permeation of small-molecule drugs, macromolecules, and nanoparticles mediated by a fractional carbon dioxide laser: the role of hair follicles. Pharm Res 2013;30:792-802 Investigation of the role of hair follicles on laser-assisted drug delivery.

.

95.

Hsu SH, Wen CJ, Al-Suwayeh SA, et al. Formulation design and evaluation of quantum dot-loaded nanostructured lipid carriers for integrating bioimaging and anticancer therapy. Nanomedicine 2013;8:1253-69

Affiliation Chih-Hung Lin1,2, Ibrahim A Aljuffali3, Jia-You Fang†4,5,6 † Author for correspondence 1 Chang Gung University of Science and Technology, Center for General Education, Kweishan, Taoyuan, Taiwan 2 Chang Gung University of Science and Technology, Chronic Diseases and Health Promotion Research Center, Kweishan, Taoyuan, Taiwan 3 King Saud University, College of Pharmacy, Department of Pharmaceutics, Riyadh, Saudi Arabia 4 Chang Gung University, Graduate Institute of Natural Products, Pharmaceutics Laboratory, 259 Wen-Hwa 1st Road, Kweishan, Taoyuan 333, Taiwan Tel: +886 3211 8800; Fax: +886 3211 8236; E-mail: [email protected] 5 Chang Gung University, Healthy Aging Research Center, Chinese Herbal Medicine Research Team, Kweishan, Taoyuan, Taiwan 6 Chang Gung University of Science and Technology, Research Center for Industry of Human Ecology, Kweishan, Taoyuan, Taiwan

Self-nanoemulsifying drug delivery systems as novel approach for pDNA drug delivery.

Promoting quality of program delivery via an internet message delivery system.

Drug delivery nanoparticles in skin cancers.

Conjugation of squalene to gemcitabine as unique approach exploiting endogenous lipoproteins for drug delivery.

PAMAM dendrimers as aerosol drug nanocarriers for pulmonary delivery via nebulization.

Modern methods of promoting drug absorption through the skin.

Ovalbumin Delivery by Guanidine-Terminated Dendrimers Bearing an Amyloid-Promoting Peptide via Nanoparticle Formulation.

A novel device for chronic intracranial drug delivery via microdialysis.

Microneedles for drug delivery via the gastrointestinal tract.

A hydrophilic matrix approach for controlled vaginal drug delivery.

Promoting siRNA delivery via enhanced cellular uptake using an arginine-decorated amphiphilic dendrimer.

Carbon nanotubes: a promising approach for drug delivery.

Nano-niosomes as nanoscale drug delivery systems: an illustrated review.

A nanomedicine-promising approach to provide an appropriate colon-targeted drug delivery system for 5-fluorouracil.

Liposomes containing lipids from Sulfolobus islandicus withstand intestinal bile salts: An approach for oral drug delivery?

Skin delivery of kojic acid-loaded nanotechnology-based drug delivery systems for the treatment of skin aging.

Cutaneous penetration of soft nanoparticles via photodamaged skin: Lipid-based and polymer-based nanocarriers for drug delivery.

Aptamers as drug delivery vehicles.

Lasers are not Effective for Melasma in Darkly Pigmented Skin.

Zebrafish models for functional and toxicological screening of nanoscale drug delivery systems: promoting preclinical applications.

Collagen synthesis promoting pullulan-PEI-ascorbic acid conjugate as an efficient anti-cancer gene delivery vector.

DNA-carbon dots function as fluorescent vehicles for drug delivery.

Heat effects on drug delivery across human skin.

DNA origami as an in vivo drug delivery vehicle for cancer therapy.