IN-VIVO KINETICS AND SPECTRA OF 5-AMINOLEVULINIC ACID-INDUCED FLUORESCENCE IN AN AMELANOTIC MELANOMA OF THE HAMSTER

For successful photodynamic diagnosis (PDD) and effective photodynamic therapy (PDT) with the clinically used 'photosensitiser' 5-aminolaevulinic acid (ALA), knowledge of the maximal fluorescence intensity and of the maximal tumour-host tissue fluorescence ratio following systemic or local application is required. Therefore, time course and type of porphyrin accumulation were investigated in neoplastic and surrounding host tissue by measuring the kinetics and spectra of ALA-induced fluorescence in vivo. Experiments were performed in the amelanotic melanoma A-Mel-3 grown in the dorsal skinfold chamber preparation of Syrian golden hamsters, The kinetics of fluorescent porphyrins was quantified up to 24h after i.v. injection of 100mgkg(-1), 500 mgkg(-1) or 1,000mgkg(-1) body weight ALA by intravital fluorescence microscopy and digital image analysis (n = 18). In separate experiments fluorescence spectra were obtained for each dose by a simultaneous optical multichannel analysing device (n = 3). A three-compartment model was developed to simulate fluorescence kinetics in tumours. Maximal fluorescence intensity (per cent of reference standard; mean +/- s.e.) in the tumour arose 150 min post injection p.i.) (1,000mgkg(-1), 109 +/- 34%; 500mgkg(-1), 148 +/- 36%) and 120 min p.i. (100mgkg(-1), 16 +/- 8%). The fluorescence in the surrounding host tissue was far less and reached its maximum at 240 min (100mgkg(-1), 6 +/- 3%) and 360 min p.i, (500mgkg(-1), 50 +/- 8%) and (1,000mgkg(-1), 6 +/- 19%). Maximal tumour-host tissue ratio (90:1) was encountered at 90 min after injection of 500mgkg(-1). The spectra of tissue fluorescence showed maxima at 637 nm and 704 nm respectively. After 300 min (host tissue) and 360 min (tumour tissue) additional emission bands al 618 nm and 678 nm were detected. These bands indicate the presence of protoporphyrin IX (PPIX) and of another porphyrin species in the tumour not identified yet. Tumour selectivity of ALA-induced PPIX accumulation occurs only during a distinct interval depending on the administered dose. Based on the presented data the optimal time for PDD and PDT in this model following intravenous administration of 500mgkg(-1) ALA would be around 90 min and 150 min respectively. The transient selectivity is probably caused by an earlier and higher uptake of ALA in the neoplastic tissue most likely as a result of increased vascular permeability of tumours as supported by the mathematical model.