Investigation of cyanine dyes for in vivo optical imaging of altered mitochondrial membrane potential in tumors

Mitochondrial membrane potential (Δψm) 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 Δψm 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 Δψm. 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 Δψm, 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 Δψm of tumor cells. Through compound screening, we developed a novel cyanine dye (IC7‐1‐Bu) which enabled in vivo imaging of tumor mitochondrial hyperpolarization. We found that the length of the dye alkyl side chains greatly affected biodistribution as well as membrane potential sensitivity. This is the first report of a NIR dye that successfully images tumor mitochondrial hyperpolarization in vivo.