Study on cellular uptake of a hydrophobic near-infrared-absorbing diradical-platinum(II) complex solubilized by albumin using hyperspectral imaging, spectrophotometry, and spectrofluorimetry.

Owing to its biopenetrability and minimal invasiveness, near-infrared (NIR) light in the region between 700-1100 nm has attracted attention in cancer diagnosis and therapy. Our group previously reported that the hydrophobic diradical-platinum(II) complex PtL is a promising agent for cancer photothermal therapy (L = 3,5-dibromo-1,2-diiminobenzosemiquinonate radical). Because PtL does not fluoresce, its intercellular uptake of PtL cannot be observed with a fluorescence microscope. In this study, we clarified the uptake and intracellular behavior of PtL solubilized by bovine serum albumin (BSA) using hyperspectral imaging enabling spectrophotometric analysis of the image. The spectral changes in the obtained images indicated that the internalization of PtL was followed by crystallization of the complex during the long incubation period (> 4 h). Additionally, the binding constant K = 5.91 × 10 M could be estimated upon fluorescence quenching analysis of BSA upon binding of PtL; K is two orders of magnitude smaller than that of albumin-common drugs. Considering the small K and low solubility of PtL in water, we ultimately proposed the internalization path and fate of PtL in the cell: release of PtL from BSA near cellular membranes and subsequent cellular uptake via membrane permeation followed by saturation, resulting in crystallization.