Transient absorption changes in vivo during photodynamic therapy with pulsed-laser light

High intensity pulsed-laser light can be used to excite absorbing molecules to transient states in large proportions. The laser-induced spectral chang es can be characterized by transient changes in light propagation; through the tissue provided the excited states of these molecules have altered abso rption spectra. Characterization of these transient changes may then be use d to exploit new mechanisms in photosensitization and/or to optimize photob iological effects. In this study,transmittance and reflectance were measure d as a function df laser pulse energy, from tissue-simulating media as well as in rat muscle and liver slices, both with and without the photosensitiz er benzoporphyrin derivative monoacid (BPD-MA) present. There was a transie nt decrease in absorption from the photosensitizer at peak pulse irradiance in the range of 100-1000 W cm(-2). The depth of photodynamic treatment-ind uced tissue necrosis was measured in a subcutaneous prostate cancer model i n Copenhagen rats. A comparison between continuous wave irradiation and pul sed irradiation with the same average incident irradiance showed no statist ically significant difference in the depth of necrosis at 48 h after irradi ation. These results indicate that photosensitizer population-state changes are measurable in tissues and may provide a method for measuring triplet-s tate properties of photosensitizer in vivo, but for BPD-MA at clinically us ed concentrations these changes do not significantly affect the depth of ph otodynamically-induced tissue damage.