Fluence rate differences in photodynamic therapy efficacy and activation of immune checkpoints of murine colorectal cancer

Photodynamic therapy (PDT) is a promising treatment for colorectal cancer owing to its high selectivity and limited systemic side effects. However, the true potential of PDT for therapeutic applications against tumors hasn't been realized partly due to the complexity of PDT regimen. In the present study, we examine the efficacy of different fluence rates of hematoporphyrin derivative (HpD)-mediated PDT to predict long-term control rates of murine CT26 colorectal cancer. We further show how variation in the expression of immune checkpoints in the response of HpD-PDT at different fluence rates. Tumor-bearing mice were injected with 5 mg/kg HpD and subjected 48 h later to an 80 J/cm2 red light dose administered at fluence rates of 10, 50, and 100 mW/cm2. The expression of immune checkpoints (PD-1, LAG-3, and TIM-3) on tumor-infiltrating lymphocytes was measured 10 days after PDT. Mice treated with fluence rates of 10 and 50 mW/cm2 exhibited significantly longer survival than those treated at 100mW/cm2. Immune checkpoints on tumor-infiltrating CD8+ T cells were upregulated following PDT. Tumors treated with fluence rate of 10 mW/cm2 showed a significant increase of immune checkpoints on CD8+ T cells than those treated at 50 and 100 mW/cm2. Low light fluence rate results in significant tumor control and immune checkpoints upregulation. Improved tumor control could be expected by reducing the rate and combining checkpoint inhibitors with PDT using low fluence rate. Our data establish a correlation between activation of immune checkpoint and fluence rate and show the potential to combine PDT with checkpoint inhibitors.