Fluorescence dynamics of ALA-induced PpIX in normal and malignant skin cells

We have applied a spectroscopic system capable of monitoring the fluorescence dynamics of photosensitiser at micron-scale locations within individual cells. This report shows that the accumulation of protoporphyrin IX (PpIX) within the nucleus of formalin-fixed keratinocytes, fibroblasts, and a metastatic squamous carcinoma cell line, following incubation with 5-aminolaevulinic acid (ALA), is dependent upon both incubation time and cell proliferation status. We demonstrate that the process of photobleaching can be monitored via the depletion in PpIX fluorescence emission during exposure to 532 nm laser light. All spectra show a progressive reduction of the 634 nm PpIX peak - following a bi-exponential decay which is consistent with a singlet oxygen mediated process. The rate of photobleaching, when plotted as a function of light dose, increases with reduced incident laser power. The generation of the hydroxyaldehyde-chlorin photoproduct, as monitored by the increase in fluorescence emission centred on 672 nm, is also greatest when the lowest laser power is applied. When light is delivered in two fractions, there is evidence of PpIX fluorescence recovery during the dark period, and an increase in bleaching rate at the onset of the second exposure. These results are in qualitative agreement with measurements performed in vivo which demonstrate that the photodynamic dose is dependent upon fluence-rate and oxygen status.