**5. Conclusion**

The fitting results shown in **Figure 5** indicate the best correlation between porcine and human skin models by widefield fluorescence imaging measurements. The possibility of predicting drug behavior on transdermal skin application promotes the success of clinical topical PDT treatment.

The aim of the study was to evaluate PPIX formation due to ALA, MAL, and cream sample mixtures from both (M2, M3, M4, M5, and M6) application on normal skin models (porcine and human) and then to show that there is a narrow correlation between both models. In this work we use ALA and MAL on topical application as the precursor of PPIX, since these are the most common drugs applied to clinical topical PDT. The fluorescence measurements were collected after 3 h of cream incubation time since this time is also applied to clinical PDT [19, 22].

In our group [22] the clinical PDT studies on skin cancer are done using 20% ALA and MAL cream application in 3 h of incubation time before light irradiation. During this time, PPIX

The fluorescence measurements were done using two techniques: fluorescence spectroscopy and widefield fluorescence imaging. With fluorescence spectroscopy using a 532 nm laser (green light) it is possible to evaluate the skin at greater depths (reaching the dermal papillae) when compared with widefield fluorescence imaging using a 405 nm LED (violet light) bring-

In the study the choice of animal age had great influence; in agreement with the literature the thickness of porcine skin is similar to human skin at around 2 months after birth [8, 13].

PPIX formation on normal skin is not homogeneous and depends on ALA, MAL, and mixtures from both (M3, M4, M5, and M6) penetration through the skin; evaluations using images by widefield fluorescence imaging can be useful and decrease the variability on experiments. Fluorescence spectroscopy evaluation, despite being collected punctually, which can lead to erroneous measurements and high variability, reveals information about PPIX formation on the deeper skin [2, 19] and is important to understand the replacement mechanism of PPIX

As shown by Valentine et al. [23], there was no difference after increasing the amount of PPIX using ALA and MAL when analyzed by fluorescence spectroscopy using a laser emission at 405 nm (violet light). Fluorescence emission due to 405 nm illumination allows us to measure the output of PPIX on the superficial skin (stratum corneous and superior epidermis). In our work, this superficial skin analysis was performed using widefield fluorescence imaging.

There are few studies concerning the comparison of ALA and MAL in healthy human skin, but Lesar et al. [10] compared the formation efficiency of PPIX from these precursors in various parts of the human body (arm, forearm, back, and legs) with fluorescence (4–29 h) after topical application. They then observed that there were differences in PPIX production, which applied regardless of where the ALA accumulated more PPIX, but the location (back) where

production is elevated since the previous preparation was performed (curettage).

ing images from PPIX on the superficial skin [2, 19].

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

from deeper layers up to superficial skin layers.

they applied the tape striping difference was only after 24 h.

**4. Discussion**

The correlation found between human and porcine skin models measured by widefield fluorescence imaging confirms that porcine skin can be used for establishing human protocols in clinical topical PDT using ALA, MAL, and mixtures from both. The capacity of porcine skin models to predict PDT results in humans can be beneficial to clinical studies optimizing PDT treatment on patients.
