Photosensitizers as radiosensitizers for metastatic and deep-seated tumors: in vitro and ex-ovo preclinical studies

Photodynamic therapy (PDT) is a well-described technique that is primarily used for thin and superficial tumors. Despite the advantages of immune response upregulation, high complete response rates and excellent cosmetics and preservation of tissue structure, the limited light penetration into tissue complicates achieving full-thickness/large-volume treatment of pigmented, thicker, deep-seated or diffuse metastatic tumors1. Reported in vitro studies in melanoma and breast cancer cells show increased expression of major histocompatibility complex I after the combination of psoralen and X-radiation, which is representative of a potential immune response upregulation2. The aim here is to investigate the clinical photosensitizer Verteporfin (VP) as a radiosensitizer to achieve deep tissue penetration while retaining immune upregulation, and thereby enhance tumor-specific radiation responses in pancreatic cancer3 and melanoma4, which are aggressive tumors with high metastatic potential and low survival rates. In vitro studies were carried out with bioluminescent human pancreatic cancer (Panc-1-Luc-2) and murine melanotic melanoma (B16F10) cell lines. The cells were irradiated using 225 kVp (XRAD) or 6 MeV (Linac) X-ray beams, and the treatment response was measured by clonogenicity. In all cases, the presence of VP in the cells reduced survival compared with X-ray only controls: e.g. the group irradiated in the Linac, which includes a contribution from VP activation by X-ray induced Cherenkov light, reached up to 5 Logs of cell death after a single fraction of 10 Gy. In the ex-ovo model, duck eggs at 8 days post fertilization were engrafted with 15 µL (∼5 × 107) of Panc-1-Luc-2 or B16F10-Luc cells. Treatment was given 3 days later using topical VP administration with irradiation 1.5 h later. Combining a single fraction of 5 Gy with 5 µg/mL VP gave >2-fold reduction of the bioluminescence compared to <1 fold for radiation alone. After 3 daily fractions, the in ovo tumors treated with VP + radiation showed undetectable bioluminescence signal until sacrificed at 2 d after the last fraction. These studies are intended primarily to generate baseline response data for radiodynamic therapy with different nanoparticle formulations, including both lipid-porphyrin porphysomes and metal nanoclusters.