Mitochondrial membrane potential (Δψ_m) alteration is an important target for cancer diagnosis. In this study, we designed a series of near‐infrared fluorescent cationic cyanine dyes with varying alkyl chain lengths (IC7‐1 derivatives) to provide diverse lipophilicities and serum albumin‐binding rates, and we evaluated the usefulness of these derivatives for in vivo Δψ_m imaging. IC7‐1 derivatives with side chains from methyl to hexyl (IC7‐1‐Me to IC7‐1‐He) were synthesized, and their optical properties were measured. Cellular uptake and intracellular distribution were investigated with depolarized HeLa cells from carbonyl cyanine m‐chlorophenylhydrazone (CCCP) treatment using a spectrofluorometer and a fluorescence microscope. Serum albumin‐binding rates were evaluated using albumin‐binding inhibitors. In vivo optical imaging was performed with HeLa cell xenograft mice following intravenous administration of IC7‐1 derivatives with or without warfarin and CCCP as in vivo blocking agents. IC7‐1 derivatives showing maximum excitation and emission wavelengths at 823 nm and ~845 nm, respectively, were synthesized. IC7‐1‐Me to ‐Bu showed fluorescence in mitochondria that decreased with CCCP treatment in a concentration‐dependent manner, which showed that IC7‐1‐Me to ‐Bu successfully indicated Δψ_m. Tumors were clearly visualized after IC7‐1‐Bu administration. Treatment with warfarin or CCCP significantly decreased IC7‐1‐Bu fluorescence in the tumor region. In summary, IC7‐1‐Bu exhibited fluorescence localized to mitochondria dependent on Δψ_m, which enabled clear in vivo tumor imaging via serum albumin as a drug carrier for effective tumor targeting. Our data suggest that IC7‐1‐Bu is a promising NIR probe for in vivo imaging of the altered Δψ_m of tumor cells.

Although 4-borono-l-phenylalanine (4-BPA) is currently the only marketed agent available for boron neutron capture therapy (BNCT), its low water solubility raises concerns. In this study, we synthesized 3-borono-l-phenylalanine (3-BPA), a positional isomer of 4-BPA, with improved water solubility. We further evaluated its physicochemical properties, tumor accumulation, and biodistribution. The water solubility of 3-BPA was 125 g/L, which is more than 100 times higher than that of 4-BPA. Due to the high water solubility, we prepared the administration solution of 3-BPA without a solubilizer sugar, which is inevitably added to 4-BPA preparation and has adverse effects. In in vitro and in vivo experiments, boron accumulation in cancers after administration was statistically equivalent in both sugar-complexed 3-BPA and 4-BPA. Furthermore, the biodistribution of 3-BPA was comparable with that of sugar-complexed 3-BPA. Since 3-BPA has high water solubility and tumor targetability equivalent to 4-BPA, 3-BPA can replace 4-BPA in future BNCT.