Gerard M. J. Beijersbergen van Henegouwen

Prodrugs of 5-Aminolevullinic Acid for Photodynamic Therapy

Abstract— The use of 5‐aminolevulinic acid (ALA) as a protoporphyrin IX (PpIX) precursor for photodynamic therapy (PDT) became very popular in a short time. However, despite its advantages, ALA also has a drawback; it shows a poor ability to diffuse through biological membranes because of its low Iipophilicity. As a consequence, a high dose of ALA must be administered in order to increase PpIX in the afflicted tissue at a level sufficient for PDT. A possible solution to this problem is the use of derivatives of ALA. ALA prodrugs are expected to have better diffusing properties as a result of their enhanced Jipophilicity and are converted into the parent ALA after enzymatic hydrolysis. In this report, results are presented of the synthesis of a number of ALA derivatives. The ALA prodmgs were investigated regarding the optimum conditions for cell penetration and PPIX formation in an in vitro cellular test system. It is shown that several prod· rugs do indeed enhance the amount of accumulated PPIX considerably as compared to ALA. Finally, the most promising prodrugs were tested in an animal model and showed increased PPIX formation under these conditionsas well.

The use of endogenous antioxidants to improve photoprotection.

The skin possesses an elaborate antioxidant defence system to deal with UV-induced oxidative stress. However, excessive exposure to UV can overwhelm the cutaneous antioxidant capacity, leading to oxidative damage and ultimately to skin cancer, immunosuppression and premature skin aging. Therefore, an interesting strategy for photoprotection is the support of the endogenous antioxidant system. This can be accomplished by induction or transdermal delivery of the various antioxidant enzymes, such as glutathione peroxidase, catalase, or superoxide dismutase. Supplementation of non-enzymatic antioxidants such as glutathione, alpha-tocopherol, ascorbate and beta-carotene was also found to be very effective in photoprotection. Although treatments with single components of the antioxidant system were successful against a wide variety of photodamage, the balance between the different antioxidants in the skin is very important. In some studies, it was found that too much of a single component could even have deleterious effects. The most promising results were obtained in studies combining several compounds, often resulting in synergism of the protective effects.

Derivatives of 5-Aminolevulinic Acid for Photodynamic Therapy: Enzymatic Conversion into Protoporphyrin

In recent years, 5‐aminolevulinic acid (ALA) has become a widespread agent for photodynamic therapy (PDT). In nucleated cells, ALA is converted into the endogenous photosensitizer protoporphyrin IX (PpIX). A major drawback of ALA is its low bioavailability. As a result, high doses of ALA must be administered in order to reach clinically relevant levels of PpIX. Moreover, only superficially located lesions can be treated as a result of the poor penetration of ALA into tissues. A possible solution for this problem may be provided by the prod rug concept. In the present study, prodrugs of ALA have been synthesized. These ALA prodrugs are shown to result in higher PpIX levels in cells than does ALA itself. Of a range of ester prodrugs of ALA, the ALA‐pentyl ester elicits the highest fluorescence. Further‐more, the enzymatic conversion of the derivatives into ALA and PpIX has been studied in lysed cells. Under these circumstances, the esters with the shorter alkyl chains induce the highest fluorescence. The alcohols that arise as side products from enzymatic conversion of the prodrugs are shown to have no influence on the experiments.

Hydrolysis of RRR-α-tocopheryl acetate (vitamin E acetate) in the skin and its UV protecting activity (an in vivo study with the rat)

Vitamin E acetate is often used rather than vitamin E as an ingredient of skin care products and dermatological preparations, because it lacks the free phenolic OH group. However, because of this the acetate as such is biologically inactive. In spite of this intrinsic inactivity, the skin is protected against the harmful effects of sunlight after topical application of vitamin E acetate. Therefore it is supposed that hydrolysis takes place in the skin and that the reaction product, the radical scavenger vitamin E, is responsible for the protection observed. In this in vivo study with the rat, we have investigated the hydrolysis of RRR-alpha-tocopheryl acetate (vitamin E acetate) in the epidermis in relation to UV radiation protection. (As a measure of protection, we used the UV-induced binding of 8-methoxypsoralen to epidermal biomacromolecules.) After a period of 5 h from a single application of vitamin E acetate, hydrolysis into free vitamin E was not observed. No protection was found at this time point, corresponding with the absence of vitamin E. After treatment for 5 days, consisting of one topical application daily, the percentage of acetate present in the stratum corneum which was hydrolysed into free vitamin E was less than 1%, whereas the corresponding value for the viable layer of the epidermis was about 5%. The hydrolysis of vitamin E acetate in the epidermis proceeded very slowly. As a result, the absolute amount of free vitamin E, found in the total epidermis after treatment for 5 days with the acetate, was only a few times higher than the normal level. Yet, this very small amount of free vitamin E proved to be sufficient for maximal protection in this animal model. The results show that vitamin E acetate acts as a prodrug, which very slowly releases minute amounts of active vitamin E.

Treatment with 8-MOP and UVA enhances MHC class I synthesis in RMA cells: preliminary results.

The response of psoriasis and cutaneous T-cell lymphoma to treatment with 8-methoxypsoralen (8-MOP) and long wavelength ultraviolet light (UVA) is only partly understood. Psoralens form photoadducts within the DNA after activation by UVA and this damage leads to the inhibition of DNA synthesis. Additionally, it has been shown that different forms of DNA damage can induce a stress response, leading to upregulation of selected products. Among these are the major histocompatibility complex (MHC) class I genes. Thus the aim of the present study was to assess the rate of synthesis of MHC class I proteins in murine T-cell lymphoma cells (RMA) after treatment with 8-MOP and UVA. RMA cells were treated with 8-MOP (50-200 ng ml-1) and UVA (1 J.cm-2) and metabolically labelled with 35S-methionine 4 and 24 h after treatment. MHC class I synthesis was determined by immunoprecipitation of the cell lysates with an anti-Kb monoclonal antibody, Y3. After 4 h, treated and untreated cells demonstrated no differences in the rate of MHC class I synthesis. However, after 24 h a dose-dependent increase in MHC class I synthesis was observed. This increase in MHC class I expression could be responsible, at least partly, for the responses observed in patients treated with photopheresis.

In Vitro Fluorescence, Toxicity and Phototoxicity Induced by δ-Aminolevulinic Acid (ALA) or ALA-Esters

Abstract Synthesis of δ-aminolevulinic acid (ALA) derivatives is a promising way to improve the therapeutic properties of ALA, particularly cell uptake or homogeneity of protoporphyrin IX (PpIX) synthesis. The fluorescence emission kinetics and phototoxic properties of ALA-n-pentyl ester (E1) and R,S-ALA-2-(hydroxymethyl) tetrahydrofuranyl ester (E2) were compared with those of ALA and assessed on C6 glioma cells. ALA (100 μg/mL), E1 and E2 (10 μg/mL) induced similar PpIX-fluorescence kinetics (maximum between 5 and 7 h incubation), fluorescence being limited to the cytoplasm. The 50% lethal dose occurred after 6 h with 45, 4 and 8 μg/mL of ALA, E1 and E2, respectively. ALA, E1 and E2 induced no dark toxicity when drugs were removed after 5 min of incubation. However, light (25 J/cm2) applied 6 h after 5 min incubation with 168 μg/mL of each compound induced 85% survival with ALA, 27% with E1 and 41% with E2. Increasing the incubation time with ALA, E1 and E2 before washing increased the phototoxicity, but E1 and E2 remained more efficient than ALA, regardless of incubation time. ALA-esters were more efficient than ALA in inducing phototoxicity after short incubation times, probably through an increase of the amount of PpIX synthesized by C6 cells.

Topically applied N-acetylcysteine as a protector against UVB-induced systemic immunosuppression

The potential protective efficacy of N-acetylcysteine against systemic immunosuppression in mice, as a result of UVB exposure, was investigated. The contact hypersensitivity response to trinitrochlorobenzene applied at a distant, non-irradiated site, was used to assess the systemic immunosuppression. Topical application of N-acetylcysteine (0.4-3.2 mumol cm-2), 30 min prior to irradiation (15 kJ m-2), markedly inhibited the UVB-induced immunosuppression. Because N-acetylcysteine does not absorb UVA or UVB radiation, the mechanism of protection must be different from that of sunscreens. The results of this study may have important practical implications in protecting human beings against the deleterious effects of UVB radiation.

Protoporphyrin IX Fluorescence Kinetics and Localization after Topical Application of ALA Pentyl Ester and ALA on Hairless Mouse Skin with UVB‐Induced Early Skin Cancer

Abstract In order to improve the efficacy of 5-aminolevulinic acid-based (ALA) photodynamic therapy (PDT), different ALA derivatives are presently being investigated. ALA esters are more lipophilic and therefore may have better skin penetration properties than ALA, possibly resulting in enhanced protoporphyrin IX (PpIX) production. In previous studies it was shown that ALA pentyl ester (ALAPE) does considerably enhance the PpIX production in cells in vitro compared with ALA. We investigated the in vivo PpIX fluorescence kinetics after application of ALA and ALAPE to hairless mice with and without UVB-induced early skin cancer. ALA and ALAPE (20% wt/wt) were applied topically to the mouse skin and after 30 min, the solvent was wiped off and PpIX fluorescence was followed in time with in vivo fluorescence spectroscopy and imaging. At 6 and 12 h after the 30 min application, skin samples of visible lesions and adjacent altered skin (UVB-exposed mouse skin) and normal mouse skin were collected for fluorescence microscopy. From each sample, frozen sections were made and phase contrast images and fluorescence images were recorded. The in vivo fluorescence kinetics showed that ALAPE induced more PpIX in visible lesions and altered skin of the UVB-exposed mouse skin, but not in the normal mouse skin. In the microscopic fluorescence images, higher ALAPE-induced PpIX levels were measured in the stratum corneum, but not in the dysplastic layer of the epidermis. In deeper layers of the skin, PpIX levels were the same after ALA and ALAPE application. In conclusion, ALAPE does induce higher PpIX fluorescence levels in vivo in our early skin cancer model, but these higher PpIX levels are not located in the dysplastic layer of the epidermis.

Systemic) phototoxicity of drugs and other xenobiotics.

Abstract Xenobiotics extensively used in drugs, cosmetics, food and agricultural chemicals can produce adverse biological effects. These toxic effects are separated into classes, e.g. hepatotoxicity, genotoxicity and neurotoxicity. Skin allergy, part of immunotoxicity, is also a subdivision of toxicology. When light is an essential condition for toxicity, the xenobiotic is called phototoxic. Thus it fits into the logic of toxicology that photoallergic compounds are a subdivision of phototoxic compounds. Phototoxicons as a group do not differ from the group of phototherapeutics with regard to their eventual biological effects. The primary photoreactions, secondary molecular processes, biomolecules involved and cellular and tissue damage are similar. The difference between the two groups is in the appreciation of the photobiological effects: adverse vs. desired. The aim of research is to determine the part of the molecular structure which makes a given compound phototoxic. With that knowledge the structure of the phototoxicon can be changed. This can result in a derivative which still has the desired properties of the parent compound, but is no longer phototoxic. This aim can be reached by combining data from both in vitro and in vivo research. The variety and number of phototoxic compounds is large. This, together with the limited research effort devoted to this subject so far, means that for most phototoxic xenobiotics a relationship between structure and in vivo photoreactivity is not available. In this review, emphasis is placed on xenobiotics whose in vitro and in vivo photochemistry have been studied. Furthermore, possible phototoxic effects which do not concern the skin but involve inner organs (systemic effects) are considered. References in this review mostly concern investigations over the last 10 years. For older literature or for additional information, references to other reviews are given. Important groups of phototoxic xenobiotics not dealt with in this article were already sufficiently covered in the reviews referred to.

Photoprotection by Antioxidants against UVB-Radiation-Induced Damage in Pig Skin Organ Culture

Abstract Rijnkels, J. M., Moison, R. M. W., Podda, E. and Beijersbergen van Henegouwen, G. M. J. Photoprotection by Antioxidants against UVB-Radiation-Induced Damage in Pig Skin Organ Culture. Radiat. Res. 159, 210–217 (2003). Topically applied antioxidants constitute an important group of protective agents against skin damage induced by ultraviolet radiation. The current study was performed to investigate whether a recently developed ex vivo pig skin model was suitable for short-term studies of the mechanism(s) of UVB-radiation-induced skin damage; the protective effect of topical application of α-tocopherol, l-ascorbic acid, α-lipoic acid, glutathione ethylester and N-acetylcysteine was tested. Increasing doses of the antioxidants were applied topically on ex vivo pig skin explants and allowed to penetrate for 60 min. Epidermal antioxidant bioavailability was measured before and 60 min after exposure to an ultraviolet B (UVB) radiation of 7.5 kJ/m2. Cell viability (trypan blue dye exclusion) and apoptosis were measured 48 h later in isolated keratinocytes. UVB-radiation-induced epidermal lipid peroxidation was determined immediately after exposure of the skin to a UVB dose of 28 kJ/m2. All antioxidants tested became bioavailable in pig skin epidermis, and none of them were depleted after UVB-radiation exposure. Increasing doses of the antioxidants tested decreased UVB-radiation-induced cell death and apoptosis. The highest doses of antioxidants prevented UVB-radiation-induced lipid peroxidation; α-lipoic acid only tended to decrease lipid peroxidation. In conclusion, a single topical dose of the above antioxidants on ex vivo pig skin can reduce UVB-radiation-induced oxidative stress and lipid peroxidation and thereby reduce apoptotic stimuli and cell death. Furthermore, the ex vivo pig skin model was a useful tool for testing compounds for their antioxidant activity.