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 PtL₂ is a promising agent for cancer photothermal therapy (L = 3,5-dibromo-1,2-diiminobenzosemiquinonate radical). Because PtL₂ does not fluoresce, its intercellular uptake of PtL₂ cannot be observed with a fluorescence microscope. In this study, we clarified the uptake and intracellular behavior of PtL₂ 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 PtL₂ was followed by crystallization of the complex during the long incubation period (> 4 h). Additionally, the binding constant K_b = 5.91 × 10⁴ M^-1 could be estimated upon fluorescence quenching analysis of BSA upon binding of PtL₂; K_b is two orders of magnitude smaller than that of albumin-common drugs. Considering the small K_b and low solubility of PtL₂ in water, we ultimately proposed the internalization path and fate of PtL₂ in the cell: release of PtL₂ from BSA near cellular membranes and subsequent cellular uptake via membrane permeation followed by saturation, resulting in crystallization.