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In clinical 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT) of skin tumors it is desirable to develop vehicles that minimize the penetration of ALA through normal stratum corneum and maximize it through the compromised stratum corneum of the tumors to improve tumor selectivity. We have designed a bioadhesive patch, which may be able to achieve this aim. It induces levels of protoporphyrin IX (PpIX) in skin overlying tumors similar to those induced by the proprietary cream (Porphin®) but at the same time induces less PpIX to form in normal skin and at distant sites. The mechanisms of action of the patch, as compared with that of the cream, were studied by means of Cuprophan® barriers that mimic compromised tumor stratum corneum and in a mouse model with transplanted tumors.

INTRODUCTION

5-Aminolevulinic acid (ALA), a naturally occurring precursor in the biosynthesis of heme, is a commonly used agent in clinical photodynamic therapy (PDT) (1). Administration of exogenous ALA, systemically or topically, is reported to cause rapid and highly selective accumulation of the potent photosensitizer protoporphyrin IX (PpIX) in neoplastic cells. The reasons cited for the high selectivity of PpIX accumulation include enhanced porphobilinogen deaminase activity and reduced ferrochelatase activity in neoplastic cells (2,3). A compromised stratum corneum barrier to drug diffusion is reported to further enhance the selectivity of PpIX accumulation in neoplastic skin lesions by allowing increased ALA penetration following topical application (4). ALA is most commonly administered to neoplastic skin lesions in a topical semisolid vehicle in Europe and in a hydroalcoholic solution in the United States.

Because of the inability of semisolid or liquid vehicles to persist at moist or irregularly shaped regions of the body, we have previously described a bioadhesive patch system containing ALA, which allowed efficient delivery of the drug to the vulva (5-7) and mouth (8). In this paper, we compare the selectivity of PpIX accumulation in normal skin and skin overlying tumors in vivo following topical application of patch and semisolid cream systems containing ALA.

MATERIALS AND METHODS

Chemicals. Gantrez® AN-139, a copolymer of methylvinylether and maleic anhydride (PMVE/MA), was provided by ISP Co. Ltd. Guildford, UK. Tripropyleneglycol methyl ether (TPM; Dowanol(TM) TPM) was purchased from Sigma Aldrich, Dorset, UK. Plastisol® medical grade polylvinyl chloride) emulsion, containing diethylphthalate as plasticizer, was provided by BASF Coatings Ltd., Clwyd, UK. 5-Aminolevulinic acid, hydrochloride salt and Porphin® cream (20% w/w ALA in Unguentum Merck®) were purchased from Crawford Pharmaceuticals, Milton Keynes, UK. Unguentum Merck® was obtained from Merck, Darmstadt, Germany.

Patch and cream manufacture. Bioadhesive patches evaluated in this study were prepared by a conventional casting technique (9) using a 20% w/w PMVE/MA and 10% wt/wt TPM gel. PMVE/MA was added to ice-cooled water (reagent grade 1), stirred vigorously and heated to 95°C until a clear solution was formed. Upon cooling, the required amount of TPM was added and the casting blend adjusted to a final weight with water. Appropriate amounts of ALA were dissolved directly into defined volumes of this aqueous blend immediately prior to casting.

Determination of approximate loadings of ALA to be included in bioadhesive patches was by consultation with a clinician experienced in the use of topical creams in PDT. It was decided to load each square centimeter of the patch with a dose of ALA equivalent to that contained in the amounts of proprietary creams typically applied per square centimeter to neoplastic lesions.

A cream (Unguentum Merck®) was applied in the thickness used clinically to each of 25 square centimeters, ruled out on the back of a gloved hand. Each square centimeter was individually cleared of cream using a microspatula and each aliquot of cream was weighed. Because Porphin® cream, the most commonly used ALA product in the UK, contains 20% wt/wl ALA, the mean ALA dose per square centimeter was determined by calculation. This estimation of drug loading was then used as a starting point in the patch design process. Patches containing ALA loadings of 38 mg cm^sup -2^ were prepared as a result of this process.