PITFALLS IN EVALUATION OF PHOTOSENSITIZING AGENTS - AN EXAMPLE WITH Q-SWITCH II-DYE

Photosensitizing agents potentiating laser therapy should have limited toxicity, no mutagenicity, stable spectral characteristics, and accep table solubility when administered in vivo. Q-Switch II dye (QII) has been shown by others to be an effective chromophore for photodynamic t herapy at 1051 nm in fibroblast cell culture. The objective of this st udy was to determine the spectral stability of QII in biological media and then to localize QII after administration in vivo. Spectral evalu ation was performed between 250 and 1100 nm. QII dissolved in dimethyl sulfoxide (DMSO) rapidly lost its spectral characteristics, including its 1051-nm peak, when contacting water, minimal essential medium, hu man serum, organ surfaces, and intracellular fluid. One minute followi ng intramuscular (IM) injection of 0.1 mg QII in 0.2 mL of DMSO, the d ose precipitated as a discrete mass which was excised and reconstitute d in DMSO. A new spectral pattern was seen, with no absorption between 850 and 1100 nm. Following intravenous (IV), (IM), or intraperitoneal (IP) injection, QII was not detected in any organ. Q-Switch II dye is not a suitable chromophore for in vivo photodynamic therapy at 1051 n m. Previous cell culture reports to the contrary did not account for t he QII spectral change caused by biological media. Simple rapid assays are described to avoid this pitfall.