In 2015, the Japanese health insurance approved the use of a second-generation photodynamic therapy (PDT) using talaporfin sodium (TS); however, its cancer cell selectivity and antitumor effects of TS PDT are not comprehensive. The Warburg effect describes the elevated rate of glycolysis in cancer cells, despite the presence of sufficient oxygen. Because cancer cells absorb considerable amounts of glucose, they are visible using positron emission tomography (PET). We developed a third-generation PDT based on the Warburg effect by synthesizing novel photosensitizers (PSs) in the form of sugar-conjugated chlorins. Glucose-conjugated (tetrafluorophenyl) chlorin (G-chlorin) PDT revealed significantly stronger antitumor effects than TS PDT and induced immunogenic cell death (ICD). ICD induced by PDT enhances cancer immunity, and a combination therapy of PDT and immune checkpoint blockers is expected to synergize antitumor effects. Mannose-conjugated (tetrafluorophenyl) chlorin (M-chlorin) PDT, which targets cancer cells and tumor-associated macrophages (TAMs), also shows strong antitumor effects. Finally, we synthesized a glucose-conjugated chlorin e6 (SC-N003HP) that showed 10,000–50,000 times stronger antitumor effects than TS (IC₅₀) in vitro, and it was rapidly metabolized and excreted. In this review, we discuss the potential and the future of next-generation cancer cell-selective PDT and describe three types of sugar-conjugated PSs expected to be clinically developed in the future.

Membrane-permeable fluorescent dyes that stain DNA are useful reagents for microscopic imaging, as they can be introduced into living cells to label DNA. However, the number of these dyes, such as Hoechst 33342, is limited. Here, we show that the icosahedral dodecaborate B₁₂Br₁₂^2-, a superchaotropic carrier that delivers different types of molecules into cells, functions as an excellent carrier for membrane-impermeable fluorescent dyes. Propidium iodide (PI) and 4',6-diamidino-2-phenylindole (DAPI), dicationic membrane-impermeable fluorescent dyes that stain DNA, can permeate cell membranes in the presence of boron clusters. Methyl green (MG), a dicationic dye used in the histological and fluorescent staining of DNA, permeated cell membranes in the presence of boron clusters. In contrast, monocationic membrane-permeable fluorescent dyes, such as acridine orange and pyronin Y, exhibited reduced fluorescence in cells in the presence of boron clusters. Boron clusters do not quench dicationic fluorescent dyes in water in vitro but have quenching effects on monocationic fluorescent dyes. We have demonstrated that the addition of B₁₂Br₁₂^2- to impermeable dicationic fluorescent DNA-staining dyes, such as DAPI, PI, and MG, which have been widely used for numerous years, imparts membrane permeability to introduce these dyes into living cells.

By using the Warburg effect—a phenomenon where tumors consume higher glucose levels than normal cells—on cancer cells to enhance the effect of photodynamic therapy (PDT), we developed a new photosensitizer, glucose-conjugated chlorin e6 (G-Ce6). We analyzed the efficacy of PDT with G-Ce6 against canine mammary carcinoma (CMC) in vitro and in vivo. The pharmacokinetics of G-Ce6 at 2, 5, and 20 mg/kg was examined in normal dogs, whereas its intracellular localization, concentration, and photodynamic effects were investigated in vitro using CMC cells (SNP cells). G-Ce6 (10 mg/kg) was administered in vivo at 5 min or 3 h before laser irradiation to SNP tumor-bearing murine models. The in vitro study revealed that G-Ce6 was mainly localized to the lysosomes. Cell viability decreased in a G-Ce6 concentration- and light intensity-dependent manner in the PDT group. Cell death induced by PDT with G-Ce6 was not inhibited by an apoptosis inhibitor. In the in vivo study, 5-min-interval PDT exhibited greater effects than 3-h-interval PDT. The mean maximum blood concentration and half-life of G-Ce6 (2 mg/kg) were 15.19 ± 4.44 μg/mL and 3.02 ± 0.58 h, respectively. Thus, 5-min-interval PDT with G-Ce6 was considered effective against CMC.