In vivo action spectra, absorption and fluorescence excitation spectra of photosensitizers for photodynamic therapy.

Recently the importance of measuring in vivo action spectra for photodynamic therapy (PDT) with various photosensitizers was pointed out in this journal [1]. It was shown that the effect of PDT with zinc phthalocyanine tetrasulphonic acid (ZnPcS4) increases considerably when the wavelength of the activating light is increased from 680 to 692 nm. In this regard a further comparison with haematoporphyrin derivative/photofrin and other recently presented information about phthalocyanines may be useful. In particular we wish to point out in this note the possible relevance of the fluorescence excitation spectrum and the possibility to measure the in vivo absorption spectrum of a photosensitizer by reflection spectroscopy. Preclinical and clinical studies on PDT with haematoporphyrin derivative (HpD) and photofrin have mostly been performed with light of 630 nm wavelength. The use of this wavelength is not supported by the in vivo action spectrum. The action spectrum for in vitro cell killing by HpD-PDT has a peak close to 630 nm [2]. However, the in vivo action spectrum for HpD-PDT (determined from damage to rat ears) has peaks close to 500 and 625 nm [3]. These wavelengths are the same as the wavelengths where the fluorescence excitation spectrum has a maximum [4]. Farrell et al. [5] have studied the wavelength dependence of the threshold for PDT-induced damage to rat liver and also found a maximum at 625 nm. Recently Potter et al. [6] (and personal communication) measured the absorption spectrum of photofrin in vivo in a mouse tumor by reflection spectroscopy and observed a maximum at 625 nm. Thus it appears that the in vivo PDT action spectrum, absorption spectrum and fluorescence excitation spectrum using HpD/photofrin all have a maximum at 625 nm, which differs somewhat from the in vitro PDT action spectrum. The difference between 625 and 630 nm is