Influence of cell-internalization on relaxometric, optical and compositional properties of targeted paramagnetic quantum dot micelles

Quantum dot micelles (pQDs) with a paramagnetic coating are promising nanoparticles for bimodal molecular imaging. Their bright fluorescence allows for optical detection, while their Gd payload enables visualization with contrast-enhanced MRI. A popular approach in molecular MRI is the targeting of abundantly expressed cell surface receptors. Ligand-receptor binding often results in cell internalization of the targeted contrast agent. The interpretation of molecular imaging with pQDs therefore requires knowledge about the consequences of cellular internalization for their relaxometric, optical and compositional properties. To study these, Cd-containing core-shell-shell QDs coated with a monolayer of lipids, of which 50 mol% was a Gd-containing lipid, were incubated with human umbilical vein-derived endothelial cells (HUVECs) for up to 24 alpha(v)beta(3)-integrin targeted (RGD) and non-targeted (NT) pQDs were compared. pQDs uptake was monitored by fluorescence microscopy, FACS, ICP-MS, relaxometry and MRI. Cell-associated pQDs displayed longitudinal relaxation rates and fluorescent intensities which were linear with the cell-associated Gd and Cd concentrations, implying that the Gd and Cd uptake by HUVECs can be quantified using relaxometric and optical measurements, respectively. However, the Gd-to-Cd molar ratio in pellets of pQD-incubated cells was consistently higher than the Gd-to-Cd molar ratio of the pQDs as prepared. It is proposed that this increase in Gd-to-Cd molar ratio was due to non-specific lipid-transfer between the pQDs and the cellular membranes. These findings show that, in the case of contrast agents that are formed by non-covalent interactions, experimental procedures are needed with which representative components of the probes can be monitored. Copyright (C) 2010 John Wiley & Sons, Ltd.