Determining an in vitro dose-response relationship of photodynamic therapy with first and second-generation photosensitisers for high grade tumours

Photodynamic Therapy (PDT) is used with photosensitizing agents to enhance the extent of tumour destruction intra-operatively, thereby enhancing survival. However, high-grade tumour types demonstrate significant heterogeneity and their susceptibility to PDT may vary. This study aimed to determine the response of different invasive tumour types to different photosensitizers and differing light irradiation doses. Immortalized neuroblastoma (SHSY-5Y), proliferative human keratinocytes (HaCaT) and Glioblastoma Multiforme (U87MG) cell lines were exposed in vitro to progressively increasing 630nm laser irradiation, with the use of a first (5-Aminolevulinic Acid) and second-generation photosensitizer (Photofrin). Cell kill was assessed using a viability assay, quantified by a 96-well plate reader. Individually, neither irradiation, 5-aminolevulinic acid (ALA) or Photofrin caused death of U87MG, SHSY-5Y or HaCaT cells. However, when combining light irradiation with photosensitizers, both 5-ALA and Photofrin caused a dose-dependent reduction in the viability of U87MG glioblastoma cells, but the potency of light was higher in cells treated with Photofrin. SHSY-5Y neuroblastoma cells exhibited higher sensitivity to PDT (using 5-ALA) than U87MG and HaCat keratinocytes. SHSY-5Y and U87MG exhibited similar sensitivities to irradiation with Photofrin as the photosensitizer. Highly invasive tumours can demonstrate different behaviours when exposed to similar PDT doses with certain photosensitizers, while others produce a uniformity of response. This has strong implications for ongoing research, suggesting a mechanism beyond simple free radical generation, as well as the potential for those tumours exhibiting a poor response to PDT in prior work requiring investigation with other photosensitizers.