Point fluorescence measurements of transformed tissues using 2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a (IIPPH)

Previous diagnostic studies by our group using a in situ non-imaging point fluorescence measurement technique have demonstrated the utility of using the fluorescence property of Photofrin to detect preinalignant lesions, tumor progression, and occult disease. This technique detects the 690 urn emission of the porphyrin after simultaneous excitation at 612 and 632 n.m. Recently, a new photosensitizer, 2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a (HPPH) was developed for photodynamic therapy (PDT) which possesses more rapid clearance from skin and greater cytotoxcity per drug dose than Photofrmn. The spectral characteristics of HPPH shows an absorption band at 665 nm (50,000 M^-1cm^-1) and peak emission at 680 nm (20 nm full width half maximum bandwidth). The aim of this study was to examine HPPH as a fluorescent diagnostic compound for the detection of transformed tissues using the in vivo fluorescence photodetector. The model of tissue transformation was the carcinogen (DMBA [9, 10-dimethyl-1, 2-benzanthracene])-induced premalignant and malignant lesions of the hamster buccal cheek pouch. The results demonstrated significant correlations between fluorescence levels and histological developments at all time points after injection. Time course studies of HPPH showed highest fluorescence readings at 48 hours after injection of 0.5 mg/kg HPPH (mi1dmoderate dysplasia, 0.35 ± 0.17 volts; papillary disease with severe dysplasia, 0.58 ± 0.33 volts; and squamous cell carcinoma, 1.04 ± 0.32 volts). Therefore, it appears that HPPH may be a promising fluorophore for the detection of transformed tissues. However, in parallel to the development of second generation photosensitizers, such as HPPH, it may be necessary to develop new detection systems adapted to the specific spectral characteristics of the photosensitizer to achieve optimal diagnostic capabilities.