Short- and Long-Term Tracking of Anionic Ultrasmall Nanoparticles in Kidney

While biodistribution of nanoparticles (NPs) has been widely studied at the organ level, relatively little is known about their disposition in organs at the cellular level, especially after long-term exposure. The kidney is regarded as the key organ for the clearance of ultrasmall NPs (<5.5 nm). However, recent studies indicate that NPs in this size range could accumulate in the kidney for extended times without urinary excretion. Using negatively charged quantum dots (QDs) (∼3.7 nm) as a model system, we examined the suborgan disposition of anionic ultrasmall NPs in the kidney at the cellular level after intravenous injection by multiphoton microscopy coupled with fluorescence lifetime imaging. Most of the NPs were initially distributed in the peritubular capillaries or glomerular arterioles after injection, whereas they passed through the fenestrated glomerular endothelium and were gradually taken up by mesangial cells up to 30 days after injection. Only trace amounts of anionic QDs could be detected in the urine, which could be attributed to the barrier of the anionic glomerular basement membrane preventing filtration of anionic QDs. In contrast, cationic QDs of similar size (∼5.67 nm) were found to be readily excreted into urine. This study thus highlights the importance of surface charge in determining renal clearance of ultrasmall NPs. It provides a framework for characterizing and predicting the subcellular disposition in organs and long-term targeting of other NPs, with a physiologically based kinetic model being subsequently developed to describe the suborgan kinetics of anionic ultrasmall NPs.