Evaluation of Early Therapeutic Effects after Near-Infrared Photoimmunotherapy (NIR-PIT) Using Luciferase- Luciferin Photon-Counting and Fluorescence Imaging

Near-infrared photoimmunotherapy (NIRPIT) is a newly developed cancer treatment that induces highly selective immunogenic cell death. It is based on an antibody photoabsorber conjugate (APC) that is activated by NIR light. The purpose of this study was to investigate the effects of NIR-PIT as measured by luciferase luciferin photon counting and fluorescence imaging. Six days after subcutaneous injection of A431-luc-GFP cells tumors formed in a xenograft mouse model. The EGFR-targeting antibody, panitumumab, was conjugated to the photoabsorber, IRDye-700DX (pan-IR700), and was intravenously administered to tumor-bearing mice. Serial luciferase luciferin photon-counting images and both green fluorescent protein (GFP) and IR700 fluorescence images were obtained from the same mice before and after NIR-PIT treatment (0, 10, 20, 30 min (early phase), and 24, 48 h (late phase) after NIR light exposure). Optical signal intensities were compared for each modality. IR700 fluorescence and luciferase luciferin photon-counting images showed decreased intensities in both the early and late phases after NIR-PIT (p < 0.01). On the other hand, GFP fluorescence images showed decreased intensities only in the late phase (p < 0.01). In the early phase, GFP fluorescence images showed smaller intensity reductions compared to IR700 fluorescence and luciferase luciferin photon-counting (p < 0.01), while in the late phase, IR700 fluorescence showed smaller intensity reductions than luciferase luciferin photon-counting and GFP fluorescence (p < 0.05), due to redistribution of pan-IR700 within the tumor bed. In conclusion, luciferase luciferin photon-counting imaging is suitable to evaluate early phase NIR-PIT effects, while both luciferase luciferin photon-counting and GFP reflected later phase effects.