**1. Introduction**

Photodynamic therapy (PDT) constitutes an alternative therapy in the treatment of cancer and skin diseases. The photodynamic reaction comprises the interaction of a photosensitizer (PS), light (lasers, lamps, and LEDs), and oxygen present in the tissue. The photochemical process occurs when the PS absorbs light in one specific wavelength, interacts with subtracts and oxygen, and produces reactive oxygen species (ROS) and singlet oxygen (<sup>1</sup> O2 ), which are the main causes of PDT damage [1, 2]. Topical PDT using topical medication such as 5-aminolevulinic acid (ALA) and its methyl ester (methyl aminolevulinate [MAL]), has been widely employed to treat skin cancer, skin diseases, and aging skin [2–4]. When methyl, ethers, and other groups are added to ALA, its derivatives become more lipophilic, thereby increasing permeability through the skin [5].

[8, 10]. *In vitro* studies using porcine ear skin as a model for human skin have produced posi-

Research has shown that skin barriers vary among species as regards the amount of free fatty acids and triglycerides and density of hair follicles [8]. Stratum corneum (SC) lipid composition (ceramides, free fatty acids, cholesterol, and cholesterol esters) and organization in biological membranes differ from one species to another. According to Godin, the lack of correlation in transdermal drug permeation among species or different application sites in the same animal model is mainly due to variations in skin thickness, the composition of intercellular SC lipids, and a number of skin shafts [8]. Bearing this in mind, research has shown that porcine ear skin is anatomically similar to human skin regarding lipid composition, which confirms its suitability for use as a new animal model to study adnexal glands. In addition, its anatomic and physiologic characteristics with respect to cardiovascular, urinary, integumen-

Many authors claim that porcine skin models constitute the most relevant animal model for human skin because porcine skin and human skin have similar histological and biochemical properties [8, 9, 15]. Porcine skin is structurally similar to human skin regarding epidermal thickness and dermal–epidermal thickness ratio; their dermis thickness is approximately 3 mm and their SC and epidermis thicknesses are in the region of 21–26 and 70 μm, respectively [8, 13]. The collagen fiber arrangement in the dermis and the SC proteins (glycosphingolipids and ceramides) present in the porcine skin are also similar to those of human skin [8]. While the vascular anatomy of human skin is superior to that of porcine skin, neonatal porcine skin has the same structure, including sweat glands and hair follicles (730 follicles/cm2

Nowadays, many scientists consider porcine skin a suitable and readily available model for the human skin barrier and often employ it to test topical and transdermal pharmaceutical formulations both *in vivo* and *in vitro*. Indeed, its application in *in vitro* testing is increasing rapidly. Many studies using porcine skin models have compared its permeability with that of

Although several studies indicate similarities between porcine skin and human skin models, predictions about drug behavior in human skin based on results from tests using animal models are still under debate. Some authors believe that animal models constitute useful tools in biomedical research, but remark that effects obtained with animal models are not readily

The purpose of this work is to verify whether there is a robust correlation between porcine and human skin models and, if so, confirm that the porcine skin model is the best alternative

In the previous study [19], porcine skin was studied, and in this chapter we can evaluate the correlation between both models. Seven different samples (ALA, MAL, and mixtures from both) were applied to human and porcine skin and their PPIX production was monitored

to prediction studies with human skin volunteers using optical techniques.

using widefield fluorescence imaging and fluorescence spectroscopy techniques.

found in adult porcine skin [8, 17]. In this way, in this work we

Correlation between Porcine and Human Skin Models by Optical Methods

http://dx.doi.org/10.5772/intechopen.75788

),

163

tive results, suggesting a high similarity between both skin models [15].

tary, and digestive systems are similar to those of human skin [13, 16].

as opposed to 10 follicles/cm2

transferable to clinical settings [19].

performed the tests in animals of 3–4 months of age.

human skin and the results show high similarity [18].

ALA and MAL act as precursors of protoporphyrin IX (PPIX), an endogenous PS produced by mitochondria on cells [2]. While ALA and MAL application on PDT has the advantage of being localized and nonsystemic (transdermal application), it has some limitations as regards penetration through the skin [2, 6, 7].

ALA is a hydrophilic compound, making it difficult to cross the biological barriers of the skin, such as cell membranes. However, it has high efficiency in the production of PPIX. On the other hand, MAL has a lipophilic character allowing it to be transported by nonpolar amino acids via passive diffusion (does not require a driver) facilitating the ability to move across biological barriers reaching higher penetration in the desired tissue, and at a lesser cost than the production of PPIX [8, 9].

It is known that PPIX formation by ALA and MAL application in carcinomas is different to PPIX formation in healthy skin, thus there are few studies comparing ALA and MAL in healthy human skin. Lesar et al. compared the production efficiency of PPIX by the application of ALA and its precursors in various parts of the human body [10]. However, in our study we compared ALA, MAL, and mixtures from both on porcine and human skin models. ALA and MAL as precursors of PPIX were chosen in our study since they are used most in clinical topic PDT [11]. Many types of animal models have been suggested to replace human skin in research on transdermal permeation of molecules [8, 12], including primate, porcine, mouse, rat, guinea porcine, and snake models. Nowadays, the use of primates in research is highly constrained [8]. On the other hand, similarities between porcine skin and human skin models have been discussed in many papers [8, 13].

Animal skin differs morphologically from that of human skin with respect to epidermis and dermis thickness, hair follicles, and other characteristics. Despite their many similarities, porcine and human skin differ regarding structure, immunohistochemistry, and function. Notwithstanding, porcine appears the most suitable animal type to replace human skin in test models [14]. Indeed, porcine constitutes the nonrodent species of choice in the preclinical toxicological testing of pharmaceuticals [13].

The prospect of decreasing the number of human volunteers in studies using *in vitro* and *in vivo* methodologies is an advantage in the development of drugs at pharmaceutical companies [8, 10]. *In vitro* studies using porcine ear skin as a model for human skin have produced positive results, suggesting a high similarity between both skin models [15].

**1. Introduction**

permeability through the skin [5].

162 Human Skin Cancers - Pathways, Mechanisms, Targets and Treatments

penetration through the skin [2, 6, 7].

the production of PPIX [8, 9].

have been discussed in many papers [8, 13].

toxicological testing of pharmaceuticals [13].

Photodynamic therapy (PDT) constitutes an alternative therapy in the treatment of cancer and skin diseases. The photodynamic reaction comprises the interaction of a photosensitizer (PS), light (lasers, lamps, and LEDs), and oxygen present in the tissue. The photochemical process occurs when the PS absorbs light in one specific wavelength, interacts with subtracts

the main causes of PDT damage [1, 2]. Topical PDT using topical medication such as 5-aminolevulinic acid (ALA) and its methyl ester (methyl aminolevulinate [MAL]), has been widely employed to treat skin cancer, skin diseases, and aging skin [2–4]. When methyl, ethers, and other groups are added to ALA, its derivatives become more lipophilic, thereby increasing

ALA and MAL act as precursors of protoporphyrin IX (PPIX), an endogenous PS produced by mitochondria on cells [2]. While ALA and MAL application on PDT has the advantage of being localized and nonsystemic (transdermal application), it has some limitations as regards

ALA is a hydrophilic compound, making it difficult to cross the biological barriers of the skin, such as cell membranes. However, it has high efficiency in the production of PPIX. On the other hand, MAL has a lipophilic character allowing it to be transported by nonpolar amino acids via passive diffusion (does not require a driver) facilitating the ability to move across biological barriers reaching higher penetration in the desired tissue, and at a lesser cost than

It is known that PPIX formation by ALA and MAL application in carcinomas is different to PPIX formation in healthy skin, thus there are few studies comparing ALA and MAL in healthy human skin. Lesar et al. compared the production efficiency of PPIX by the application of ALA and its precursors in various parts of the human body [10]. However, in our study we compared ALA, MAL, and mixtures from both on porcine and human skin models. ALA and MAL as precursors of PPIX were chosen in our study since they are used most in clinical topic PDT [11]. Many types of animal models have been suggested to replace human skin in research on transdermal permeation of molecules [8, 12], including primate, porcine, mouse, rat, guinea porcine, and snake models. Nowadays, the use of primates in research is highly constrained [8]. On the other hand, similarities between porcine skin and human skin models

Animal skin differs morphologically from that of human skin with respect to epidermis and dermis thickness, hair follicles, and other characteristics. Despite their many similarities, porcine and human skin differ regarding structure, immunohistochemistry, and function. Notwithstanding, porcine appears the most suitable animal type to replace human skin in test models [14]. Indeed, porcine constitutes the nonrodent species of choice in the preclinical

The prospect of decreasing the number of human volunteers in studies using *in vitro* and *in vivo* methodologies is an advantage in the development of drugs at pharmaceutical companies

O2

), which are

and oxygen, and produces reactive oxygen species (ROS) and singlet oxygen (<sup>1</sup>

Research has shown that skin barriers vary among species as regards the amount of free fatty acids and triglycerides and density of hair follicles [8]. Stratum corneum (SC) lipid composition (ceramides, free fatty acids, cholesterol, and cholesterol esters) and organization in biological membranes differ from one species to another. According to Godin, the lack of correlation in transdermal drug permeation among species or different application sites in the same animal model is mainly due to variations in skin thickness, the composition of intercellular SC lipids, and a number of skin shafts [8]. Bearing this in mind, research has shown that porcine ear skin is anatomically similar to human skin regarding lipid composition, which confirms its suitability for use as a new animal model to study adnexal glands. In addition, its anatomic and physiologic characteristics with respect to cardiovascular, urinary, integumentary, and digestive systems are similar to those of human skin [13, 16].

Many authors claim that porcine skin models constitute the most relevant animal model for human skin because porcine skin and human skin have similar histological and biochemical properties [8, 9, 15]. Porcine skin is structurally similar to human skin regarding epidermal thickness and dermal–epidermal thickness ratio; their dermis thickness is approximately 3 mm and their SC and epidermis thicknesses are in the region of 21–26 and 70 μm, respectively [8, 13]. The collagen fiber arrangement in the dermis and the SC proteins (glycosphingolipids and ceramides) present in the porcine skin are also similar to those of human skin [8].

While the vascular anatomy of human skin is superior to that of porcine skin, neonatal porcine skin has the same structure, including sweat glands and hair follicles (730 follicles/cm2 ), as opposed to 10 follicles/cm2 found in adult porcine skin [8, 17]. In this way, in this work we performed the tests in animals of 3–4 months of age.

Nowadays, many scientists consider porcine skin a suitable and readily available model for the human skin barrier and often employ it to test topical and transdermal pharmaceutical formulations both *in vivo* and *in vitro*. Indeed, its application in *in vitro* testing is increasing rapidly. Many studies using porcine skin models have compared its permeability with that of human skin and the results show high similarity [18].

Although several studies indicate similarities between porcine skin and human skin models, predictions about drug behavior in human skin based on results from tests using animal models are still under debate. Some authors believe that animal models constitute useful tools in biomedical research, but remark that effects obtained with animal models are not readily transferable to clinical settings [19].

The purpose of this work is to verify whether there is a robust correlation between porcine and human skin models and, if so, confirm that the porcine skin model is the best alternative to prediction studies with human skin volunteers using optical techniques.

In the previous study [19], porcine skin was studied, and in this chapter we can evaluate the correlation between both models. Seven different samples (ALA, MAL, and mixtures from both) were applied to human and porcine skin and their PPIX production was monitored using widefield fluorescence imaging and fluorescence spectroscopy techniques.
