Enhancement of Anti-Robo1 Immunotoxin Cytotoxicity to Head and Neck Squamous Cell Carcinoma via Photochemical Internalization

Background: Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide. The long-term complications arising from standard therapy with surgery, chemotherapy and radiotherapy lower the quality of life (QOL) due to eating disorders, speech problems and cosmetic issues. Recently the importance of antibody therapy has increasingly come to be recognized for its capacity to enhance QOL. Robo1, an axon guidance receptor, has been received considerable attention as a monoclonal antibody target in various cancers. Methods and findings: We checked the expression of Robo1 and the cytotoxic effect of saporin-conjugated anti- Robo1 immunotoxin (IT) against several HNSCC cell lines. The Robo1 cell surface expression level estimated by flow cytometry using the antibody B5209B was approximately 220,000 copies in Robo1 over-expressed CHO (Robo1/ CHO) cells, and in HNSCCs, 22,000 copies in HSQ-89 cells, 3,000 copies in Sa3 cells, and few copies inSAS cells. Conventional IT treatment exhibited an inadequate cytotoxic effect even in HSQ-89 cells. When we added a photosensitizer and LED light illumination (650 nm), the cytotoxic effect was remarkably improved in HSQ89. With longer exposure, significant cytotoxicity was observed, even in Sa3 cells. Conclusion: These results suggest that the photochemical internalization (PCI) is a promising method for augmenting the cytotoxicity of IT against tumors. The drug delivery system shown in this study should be applicable to other targets with low level expression on cancer cells, thus widening the therapeutic window in rare cancers.