Cellular uptake, intracellular behavior, and acute/sub-acute cytotoxicity of a PEG-modified quantum dot with promising in-vivo biomedical applications

Quantum Dots (QDs) modified with branched Polyethylene Glycol-amine (6- or 8-arm PEG-amine) coupled with methoxy PEG (mPEG) hold great promise for in vivo biomedical applications due to a long half-life in blood and negligible toxicity. However, the potential risks regarding their concomitant prolonged co-incubation with cardiovascular and blood cells remains inconclusive. In the present study, the feasible, effective and convenient proliferating-restricted cell line models representing the circulatory system were established to investigate the cellular internalization followed by intracellular outcomes and resulting acute/sub-acute cytotoxicity of the 6-arm PEG-amine/mPEG QDs. We found a dose-, time- and cell type-dependent cellular uptake of the 6-arm PEG-amine/mPEG QDs, which was ten-fold lower compared to the traditional linear PEG-modified counterpart. The QDs entered cells via multiple endocytic pathways and were mostly preserved in Golgi apparatus for at least one week instead of degradation in lysosomes, resulting in a minimal acute cytotoxicity, which is much lower than other types of PEG-modified QDs previously reported. However, a sub-acute cytotoxicity of QDs were observed several days post exposure using the concentrations eliciting no-significant acute cytotoxic effects, which was associated with elevated ROS generation caused by QDs remained inside cells. Finally, a non-cytotoxic concentration of the QDs was identified at the sub-acute cytotoxic level. Our study provided important information for clinical translation of branched PEG-amine/mPEG QDs by elucidating the QDs-cell interactions and toxicity mechanism using the proliferation-restricted cell models representing circulatory system. What's more, we emphasized the indispensability of sub-acute cytotoxic effects in the whole biosafety evaluation process of nanomaterials like QDs.