| Photobleaching and photoproduct formation occur simultaneously both in vitro and in vivo during photodynamic therapy (PDT) using aminolevulinic acid (ALA)-generated protoporphyrin IX as the photosensitizer. Although photobleaching and photoproduct formation, which is possibly a consequence of photobleaching, have been widely reported in the literature, their impact on ALA-PDT is not known. This project started with studying the impact, formation of photoproducts may have on ALA-PDT and whether they have the potential to act as photosensitizers themselves. Dual wavelength studies showed that photoproducts absorbing at 645nm and 670nm could act as photosensitizers when Colo-26 tumor-bearing mice were treated using ALA-PDT. When the dual wavelength was split 50/50, there was a significant increase in tumor response (635nm + 645nm and 635nm + 670nm vs. 635nm alone p = 0.0063, p = 0.015 respectively). The 645nm photoproduct, not previously characterized, generated in vitro had a chlorin-type spectrum and a molecular weight of 592.; Studies were done to investigate the relationship between the rate and extent of PpIX photobleaching and therapeutic efficiency and efficacy, which may allow photobleaching to be used as a metric for ALA-PDT and allow for real-time treatment management. Light doses that produced 50%, 90% photobleaching and twice the 90% photobleaching dose at 10mW/cm2, 25mW/cm 2 and 75mW/cm2 dose rates were used in a Colo-26 tumor regrowth assay. We found that 10 mW/cm2 was the most efficient fluence rate for photobleaching, with a 90% photobleaching light dose 37.5 J/cm2. At 25 and 75 mW/cm2, the 90% photobleaching light dose increased to 52.5 and ∼80J/cm2, respectively. With treatment, tumor responses correlated with photobleaching rates; the greatest responses occurring at 10 mW/cm2. Tumor responses were better at the 90% than the 50% photobleaching light dose at each fluence rate, but were not improved by increasing the light to twice the 90% photobleaching dose. At the 90% photobleaching doses, the residual tumor volume at 5 days post-PDT were 36%, 46% and 78% for 10, 25 and 75mW/cm 2 respectively. |
