Ex vivo Application of δ‐Aminolevulinic Acid Induces High and Specific Porphyrin Levels in Human Skin Tumors: Possible Basis for Selective Photodynamic Therapy

Abstract

In photodynamic therapy with topically applied delta-aminolevulinic acid porphyrins are acting as photosensitizers. The profile of porphyrin metabolites in normal or in neoplastic skin after administration of delta-aminolevulinic acid has not been determined in detail yet. Thus, to study porphyrin biosynthesis in human skin an organ culture model was developed. Explant pieces of normal skin, keratoacanthoma, and basal cell carcinoma were incubated with 1 mM delta-aminolevulinic acid for 36 h. Levels of delta-aminolevulinic acid, porphyrins and porphyrin metabolites were measured in tissues and supernatants. After incubation with delta-aminolevulinic acid, higher porphyrin levels were demonstrated in tumors as compared to normal skin. In supernatants, most of formed porphyrins, preferentially highly carboxylated porphyrin metabolites, were measured. The pattern of synthesized porphyrins differed between normal and neoplastic skin explants. In tissues of basal cell carcinomas protoporphyrin was preferentially shown and tissues of keratoacanthomas were characterized by a predominance of coproporphyrin as compared to normal skin. The results show that explant cultures offer an easy approach to examine the porphyrin biosynthesis of various tissues. The tumor-specific delta-aminolevulinic acid metabolism indicates additional porphyrin metabolites such as coproporphyrin apart from protoporphyrin as effective photosensitizers and may offer a novel approach to tumor-selective photodynamic damage.

Effects of light fractionation and different fluence rates on photodynamic therapy with 5-aminolaevulinic acid in vivo

“…The increase of tissue pO 2 in tumour and surrounding tissue during the irradiation break can be explained by a reduced cellular respiration break by functioning oxygen supply. This assumption is supported by the fact that ALA is primarily metabolised to porphyrins in the tumour parenchyma (Fritsch et al, 1997). Therefore, direct tumour cell toxicity by PDT with ALA is expected.…”

“…The only experimental difference is the use of an argon-pumped dye laser tuned to 635 nm for irradiation, while in the present investigation an incoherent light source (l ¼ 580 -740 nm) was employed. Since significant amounts of additional porphyrins, besides PpIX, are formed in this model following systemic application of ALA (Fritsch et al, 1997) absorbing beyond 635 nm; the wavelength range of the incoherent light source covers their absorption spectrum, thus leading to a more pronounced photodynamic effect.…”

Comparative effect of ALA derivatives on protoporphyrin IX production in human and rat skin organ cultures

“…It is worthy of note that the explant culture system has been a useful and simple tool for testing ex-vivo the ability of organs to synthesize PpIX from ALA (Fukuda et al, 1989;Fritsch et al, 1997). Using floating explant cultures, the drugs are absorbed through the dermis to reach the epidermis, resembling the systemic pathway of drug delivery.…”

“…The mechanism of ALA transport in human cells is not yet clear, but it has been well characterized in yeast (Bermudez Moretti et al, 1995). It would therefore be of interest to know whether ALA derivatives diffuse passively across the plasma membrane or have other active transport mechanisms similar or different from ALA.At present it is not clear which enzymes might be involved in the liberation of ALA although in the case of compounds 3 and 4 and the methyl and hexyl esters it is likely that non-specific esterases are implicated and, in the case of compounds 5, 6 and 7 it may be that proteases are responsible for the release.It is worthy of note that the explant culture system has been a useful and simple tool for testing ex-vivo the ability of organs to synthesize PpIX from ALA (Fukuda et al, 1989;Fritsch et al, 1997). Using floating explant cultures, the drugs are absorbed through the dermis to reach the epidermis, resembling the systemic pathway of drug delivery.…”

Optimum porphyrin accumulation in epithelial skin tumours and psoriatic lesions after topical application of δ-aminolaevulinic acid

“…According to our clinical experience, BCC and SCC reveal a strong, homogeneous, ALA-induced porphyrin fluorescence under Wood's light ( Figure 1A) (Fritsch et al, 1996a(Fritsch et al, , 1997b, whereas about 20-40% of psoriatic lesions show areas with inhomogeneous or absent fluorescence ( Figure 1B). For better comparison of intralesional porphyrin formation in PS and tumours, we selected only uniform fluorescing psoriatic areas for our study.…”