Stability and Biodistribution of Thiol-Functionalized and 177Lu-Labeled Metal Chelating Polymers Bound to Gold Nanoparticles

We are studying a novel radiation nanomedicine approach to treatment of breast cancer using 30 nm gold nanoparticles (AuNP) modified with polyethylene glycol (PEG) metal-chelating polymers (MCP) that incorporate 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelators for complexing the β-particle emitter, 177Lu. Our objective was to compare the stability of AuNP conjugated to MCP via a single thiol [DOTA-PEG-ortho-pyridyl disulfide (OPSS)], a dithiol [DOTA-PEG-lipoic acid (LA)] or multithiol end-group [PEG-pGlu(DOTA)8-LA4] and determine the elimination and biodistribution of these 177Lu-labeled MCP-AuNP in mice. Stability to aggregation in the presence of thiol-containing dithiothreitol (DTT), l-cysteine or glutathione was assessed and dissociation of 177Lu-MCP from AuNP in human plasma measured. Elimination of radioactivity from the body of athymic mice and excretion into the urine and feces was measured up to 168 h post-intravenous (i.v.) injection of 177Lu-MCP-AuNP and normal tissue uptake was determined. ICP-AES was used to quantify Au in the liver and spleen and these were compared to 177Lu. Our results showed that PEG-pGlu(DOTA)8-LA4-AuNP were more stable to aggregation in vitro than DOTA-PEG-LA-AuNP and both forms of AuNP were more stable to thiol challenge than DOTA-PEG-OPSS-AuNP. PEG-pGlu(177Lu-DOTA)8-LA4 was the most stable in plasma. Whole body elimination of 177Lu was most rapid for mice injected with 177Lu-DOTA-PEG-OPSS-AuNP. Urinary excretion accounted for >90% of eliminated 177Lu. All 177Lu-MCP-AuNP accumulated in the liver and spleen. Liver uptake was lowest for PEG-pGlu(177Lu-DOTA)8-LA4-AuNP but these AuNP exhibited the greatest spleen uptake. There were differences in Au and 177Lu in the liver for PEG-pGlu(177Lu-DOTA)8-LA4-AuNP. These differences were not correlated with in vitro stability of the 177Lu-MCP-AuNP. We conclude that conjugation of AuNP with PEG-pGlu(177Lu-DOTA)8-LA4 via a multithiol functional group provided the greatest stability in vitro and lowest liver uptake in vivo and is, therefore, the most promising for constructing 177Lu-MCP-AuNP for radiation treatment of breast cancer.