Intense pulse light and 5‐ALA PDT: Phototoxic effects in vitro depend on the spectral overlap with protoporphyrine IX but do not match cut‐off filter notations

Abstract

Successful photodynamic therapy (PDT) requires a light source by which light is absorbed by the photosensitizer. Such absorption is achieved by adapting the emission spectrum of the lamp to the absorption-spectrum of the photosensitizer. Intense pulsed light sources (IPLs) are widely used in dermatology, but a standardized protocol for IPL-PDT is not available. Five different IPLs were chosen to evaluate their efficacy for PDT in vitro and the possibility for developing a standard protocol for PDT. Emission-spectra of IPLs were measured with an optical spectrograph and compared with the absorption spectrum of protoporphyrine IX (PpIX). Keratinocytes were incubated with 5-ALA and illuminated with the IPLs. Cell viability was determined for radiant exposures ranging from 0 to 504 J/cm(2) and pulse durations from 8 to 100 milliseconds. A standard LED light source was used as a reference. Cell viability is less effectively reduced by 5-ALA-PDT with IPLs than by a LED light source. Radiant exposures of the five IPLs ranged between 80 and 311 J/cm(2) to achieve the EC(50) value. This value correlated with the spectral overlap of the respective IPL and the absorption-spectrum of PpIX but not with the cut-off filter notations supplied by the manufacturer. All IPLs assessed emit different spectra because of different filter technologies. Different radiant exposures (J/cm(2) ) were necessary to achieve a photodynamic effect with 5-ALA in vitro depending on these spectra similar to the photodynamic effect of the standard LED light source. IPLs may be applicable in clinical PDT but radiant exposure protocols must be separately evaluated for each single IPL despite similar cut-off filter specifications. Such protocols are highly important for clinical practice to avoid a potential mismatch of excitation wavelengths and to prevent photothermal side effects when light intensities of up to hundreds of W/cm(2) are applied.