Can topically applied optical clearing agents increase the epidermal damage threshold and enhance therapeutic efficacy?

Lasers in Surgery and Medicine 35:93–95 (2004) Letter to the Editor Can Topically Applied Optical Clearing Agents Increase the Epidermal Damage Threshold and Enhance Therapeutic Efficacy? Human skin is a complex and highly scattering medium at visible and near-infrared wavelengths due to variations in refractive indices of different constituents. In light- based therapeutics, epidermal and dermal scattering diminishes the depth of light penetration and attenuates the effective fluence that reaches the targeted chromo- phores [1]. As a result, higher radiant exposures or fluences may be required to produce the desired therapeutic effect, particularly for more deeply located chromophores. Higher fluences can be beneficial but also increase the risk of epidermal and dermal injury, which can result in dyspig- mentation and scarring. Epidermal injury can be reduced by topically applying certain hyperosmotic solutions prior to laser therapy, such as sucrose, glycerol, and water soluble gels (e.g., surgilube) that have a refractive index matching closely to that of stratum corneum, i.e., 1.4. Thus, surface scattering from incident light is thereby reduced and the overall intensity of backscattered light is also reduced in the superficial epidermis. However, such solutions are highly hydro- philic and penetrate intact skin very poorly when applied topically [2–4]. Our group has recently reported the use of topically applied hyperosmotic, optical clearing agents composed of a pre-polymer mixture of polypropylene glycol and polyethy- lene glycol, PPG:PEG that can penetrate intact human skin [5] and temporarily alter local optical properties in vivo. Epidermal and papillary dermal scattering events are reduced significantly and more light penetrates deeper into the skin. Moreover, reduced epidermal and dermal back scattering increases the threshold for epidermal injury, which allows safe use of higher fluences. In conclusion, by changing the optical properties of human skin in vivo and reducing epidermal and dermal scattering, more photons will reach deeply located chromophores such as hair follicles, tattoo pigments, sebaceous glands, and blood vessels more effectively. In this letter, we present the use of a topically applied clearing agent (PPG:PEG) prior to Q-switched 532 and 694 nm laser treatment of human skin ex vivo and in vivo, respectively. Freshly excised skin samples (skin phototype, SPT III) were obtained and randomly divided into three groups. Group I served as control and no topical agents were applied prior to laser exposure. For group II, an index matching lotion (surgilube, refractive index 1.38) was s 2004 Wiley-Liss, Inc. topically applied on skin samples prior to laser exposure. For group III, a clearing agent (PPG:PEG, refractive index ¼ 1.47) was topically applied prior to laser exposure. Frequency doubled, Q-switched Nd:YAG laser, (l ¼ 532 nm) was used at a fluence of 2.8 J/cm 2 with a 2 mm diameter spot size to create a single pass of exposures on skin samples. Punch biopsies were obtained immediately after laser exposure and processed for fresh frozen sectioning and hematoxylin and eosin staining (HE area (1) control, no agent or laser treatment; (2) laser alone; (3) skin pre-treated with optical clearing agent (2 hours under occlusion) followed by laser treatment (Fig. 2A). No post-operative wound care was provided for any areas. One day post-treatment, the patient developed several hemor- rhagic blisters on area 2 while area 3 showed only minimal Contract grant sponsor: Arnold and Mabel Beckman Fellows Program (to BC); Contract grant sponsor: National Institutes of Health (to JSN); Contract grant numbers: 47551, 48458; Contract grant sponsor: National Institutes of Health (to JSN); Contract grant number: 002495; Contract grant sponsor: Dermatology Foundation (to KMK). *Correspondence to: Misbah H. Khan, MD, Beckman Laser Institute and Medical Clinic, University of California, Irvine, California 92612. E-mail: mhkhan@uci.edu Accepted 13 May 2004 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/lsm.20078

Human skin is a complex and highly scattering medium at visible and near-infrared wavelengths due to variations in refractive indices of different constituents. In lightbased therapeutics, epidermal and dermal scattering diminishes the depth of light penetration and attenuates the effective fluence that reaches the targeted chromophores [1]. As a result, higher radiant exposures or fluences may be required to produce the desired therapeutic effect, particularly for more deeply located chromophores. Higher fluences can be beneficial but also increase the risk of epidermal and dermal injury, which can result in dyspigmentation and scarring.
Epidermal injury can be reduced by topically applying certain hyperosmotic solutions prior to laser therapy, such as sucrose, glycerol, and water soluble gels (e.g., surgilube) that have a refractive index matching closely to that of stratum corneum, i.e., $1.4. Thus, surface scattering from incident light is thereby reduced and the overall intensity of backscattered light is also reduced in the superficial epidermis. However, such solutions are highly hydrophilic and penetrate intact skin very poorly when applied topically [2][3][4].
Our group has recently reported the use of topically applied hyperosmotic, optical clearing agents composed of a pre-polymer mixture of polypropylene glycol and polyethylene glycol, PPG:PEG that can penetrate intact human skin [5] and temporarily alter local optical properties in vivo. Epidermal and papillary dermal scattering events are reduced significantly and more light penetrates deeper into the skin. Moreover, reduced epidermal and dermal back scattering increases the threshold for epidermal injury, which allows safe use of higher fluences. In conclusion, by changing the optical properties of human skin in vivo and reducing epidermal and dermal scattering, more photons will reach deeply located chromophores such as hair follicles, tattoo pigments, sebaceous glands, and blood vessels more effectively.
In this letter, we present the use of a topically applied clearing agent (PPG:PEG) prior to Q-switched 532 and 694 nm laser treatment of human skin ex vivo and in vivo, respectively. Freshly excised skin samples (skin phototype, SPT III) were obtained and randomly divided into three groups. Group I served as control and no topical agents were applied prior to laser exposure. For group II, an index matching lotion (surgilube, refractive index $1.38) was topically applied on skin samples prior to laser exposure. For group III, a clearing agent (PPG:PEG, refractive index ¼ 1.47) was topically applied prior to laser exposure. Frequency doubled, Q-switched Nd:YAG laser, (l ¼ 532 nm) was used at a fluence of 2.8 J/cm 2 with a 2 mm diameter spot size to create a single pass of exposures on skin samples. Punch biopsies were obtained immediately after laser exposure and processed for fresh frozen sectioning and hematoxylin and eosin staining (H&E) (Fig. 1). Control skin samples, without prior topical application, showed significant basal cell vacuolization and separation at the dermoepidermal junction (DEJ) at various points (Fig. 1A). Group II skin samples, with prior topical application of an index matching lotion, showed less basal cell vacuolization as compared to the control (Fig. 1B). However, clefting and separation of the DEJ was still seen. Group III skin samples, with prior application of a clearing agent (PPG:PEG) showed no basal cell vacuolization or clefting or separation of the DEJ (Fig. 1C).
The effects of topically applied clearing agents prior to laser treatment of dermal chromophores (such as tattoos) were also studied in vivo. A 40-year-old man (SPT IV) seeking laser tattoo removal for a 15-year-old India ink arm tattoo was recruited in an Institutional Review Board (IRB) approved study. Q-switched ruby laser (l ¼ 694 nm) was used at a fluence of 6 J/cm 2 with a 4 mm diameter spot size. The region of interest was divided into three areas; area (1) control, no agent or laser treatment; (2) laser alone; (3) skin pre-treated with optical clearing agent (2 hours under occlusion) followed by laser treatment (Fig. 2A). No post-operative wound care was provided for any areas. One day post-treatment, the patient developed several hemorrhagic blisters on area 2 while area 3 showed only minimal erythema (Fig. 2B). At 1 month follow-up, significant clearing of tattoo in area 3 was observed as opposed to only minimal clearing in area 2 (Fig. 2C).
Although, it is currently not possible to model precisely the effective light distribution in human skin due to local changes in the scattering coefficient, it is instructive to investigate the effects of gross changes in epidermal scattering. We used the freeware Monte-Carlo multilayered (MCML) code [6] to perform first order calculations of the heat source term (J/cm 3 ) in tattooed skin, assuming a slab geometry. Optical properties were derived from data presented in various sources [1,7,8]. Epidermal and superficial papillary dermal (upper 200-300 mm thickness) scattering coefficients were varied from 0.4 mm À1 (assumed to be native skin at 694 nm) to decreased values of 0.2 and 0.1 mm À1 after the application of clearing agents. The resulting heat source term profiles (Fig. 3) show $40% decrease at the DEJ due to reduced epidermal scattering,  thus serving as preliminary support for our current working hypothesis.
Our in vivo results (Fig. 2) suggest that a reduction in optical scattering in human skin improves the outcome of light-based therapeutic procedures. Topically applied clearing agents (PPG:PEG) allow more photons to reach the target as well as reduce the back scattered light. Higher fluences for deeper dermal targets can be used much more safely and effectively without damaging the epidermis, especially when treating patients with darker skin types.
PPG:PEG is safe, effective, and well tolerated for optical clearing of human skin. Prospective, comparative, and controlled clinical studies are being performed at our Institute so that the role of topically applied optical clearing agents in light-based therapeutics can be fully understood. Fig. 3. Normalized heat source term in the upper 0.3 mm of human skin, calculated with a Monte-Carlo simulation of laser tattoo removal. Note that the tattoo is modeled at a depth of 1 mm and thus not included here. The numbers in parentheses in the figure legend represent assumed epidermal scattering coefficients before (black solid line) and at two stages after clearing agent application. The heat source term in the epidermis, and in particular at the DEJ, decreases as epidermal scattering coefficient is decreased.