Photodynamic therapy (
PDT) utilizes the localized delivery of light to activate a photosensitizing
drug (such as
Photofrin) which is selectively retained by the tumour tissues. The intrinsic in vitro sensitivity of tumour cells to
PDT is thought to be an important determinant of clinical tumour response to
PDT. In this work we show the feasibility of using a viral capacity assay for adenovirus (Ad)
DNA synthesis as an
indicator of cellular sensitivity to and recovery from
Photofrin-mediated
PDT. Rif-1 mouse
fibrosarcoma cells and a
PDT resistant derivative, Rif-8A, as well as Chinese hamster ovary (CHO) cells and CHO-MDR multi-
drug resistant mutant cells were studied. Consistent with the clonogenic survival of these cells, the capacity of
PDT-treated cells for Ad
DNA synthesis was greater for Rif-8A compared to Rif-1 cells and for CHO-MDR compared to CHO-N cells. Delaying
infection of the Rif cells from immediately after, to 6 hours after
PDT, resulted in an increased capacity for Ad
DNA synthesis, which was greater for Rif-8A compared to Rif-1 cells, suggesting that the increased resistance of Rif-8A cells to
PDT results from an elevated recovery and/or repair of
PDT damage. The capacity of UV-irradiated cells for Ad
DNA synthesis was also greater for Rif-8A compared to Rif-1 cells indicating a cross-resistance of Rif-8A cells to UV. These results suggest some overlap in the types of cellular damage induced by UV and
PDT and/or overlap in the pathways for the repair of UV and
PDT damage in Rif cells.