Development of a silicon phthalocyanine analogue for near-infrared photoimmunotherapy and its application to HTLV-1-infected leukemic cells

Near-infrared photoimmunotherapy (NIR-PIT) employing an antibody labeled with a silicon phthalocyanine dye, IR700, was approved as a minimally invasive treatment for unresectable recurrent head and neck cancer in Japan in 2020. However, further derivatization of IR700 is needed to increase the efficiency of cancer treatment. Here, we developed SiPc-1 as an IR700 analog, in which the linker was constructed using click chemistry to simplify the synthetic scheme and its position was switched from α to β on the benzene ring of phthalocyanine to eliminate intramolecular steric repulsion. We evaluated the cleavage rate of the water-soluble axial moieties of SiPc-1 upon photoirradiation, the cytotoxicity, and the morphological change (blebbing) of treated cells upon photoirradiation. We performed gene expression and protein expression analyses to find a target antigen selectively expressed on cells infected with human T-cell lymphotropic virus type 1 (HTLV-1), the causative virus of adult T-cell leukemia/lymphoma (ATL), and identified CD25 as a suitable target antigen. An anti-CD25 antibody, basiliximab, labeled with SiPc-1 (bas-SiPc-1) showed selective toxicity towards HTLV-1-infected cultured cells and ATL patients' peripheral blood mononuclear cells upon photoirradiation.