Expression of vascular endothelial growth factor in mouse tumours subjected to photodynamic therapy

The aim of this study was to define the appropriate fractionation interval between photodynamic therapy (PDT) for the enhancement of its anti-tumour effects. Tumour reoxygenation and the kinetics of tumour vascular cells following PDT were evaluated in mice by means of immunohistochemical staining for the vascular endothelial growth factor (VEGF) and the proliferating cell nuclear antigen (PCNA), respectively. The VEGF labelling indices (LIs) of the tumour cells and the PCNA LIs of the tumour vascular cells were assessed at various time intervals after PDT. The tumour cell VEGF LIs of the experimental groups at time points from 0 to 6 h after PDT were significantly higher than those of the control groups, but subsequently returned to control levels at 24 h after PDT. The vascular cell PCNA LI of the experimental group at 24 h after PDT was significantly lower than that of the control group, but returned to the control level at 48 h. These results indicated that the tumour subjected to PDT might be reoxygenated, and that the maximum damage to the tumour vasculature emerged at 24 h after PDT. We propose here that the fractionation interval between PDTs should be 24 h.