EGFR-Targeted Metal Chelating Polymers (MCPs) Harboring Multiple Pendant PEG2K Chains for MicroPET/CT Imaging of Patient-Derived Pancreatic Cancer Xenografts

In this study, we developed a new generation of metal chelating polymer (MCP) reagents that carry multiple polyethylene glycol (PEG) pendant groups to provide stealth to MCP-based radioimmunoconjugates (RICs). We describe the MCP synthesis for covalent attachment to panitumumab F(ab')(2) fragments (pmabF(ab')(2)) in which different numbers of pendant methoxy-PEG chains [M = 2000, similar to 45 ethylene glycol (EG) repeat units, referred to as PEG(2K)] are incorporated into the polymer backbone. The pendant PEG(2K) chains were designed to provide a protein-repellant corona so that metal chelators attached closer to the polymer backbone will be less apparent to the physiological environment. DOTA (1,4,7,10-tetraazac-yclododecane-1,4,7,10-tetraacetic acid) groups to chelate Cu-64 were installed on these conjugates to be employed for PET imaging. The conjugation of MCPs to pmabF(ab')(2) was based on a UV quantifiable bis-aromatic hydrazone formation under mild conditions (pH 5-6) between an aromatic aldehyde introduced on epsilon-NH2 groups of lysines in the F(ab')(2) fragments and a hydrazinonicotinamide (HyNic) group installed on the initiating end of the MCP. Three MCPs with 17 polyglutamide (PGlu) repeat units, DOTA chelators and with an average of 2, 4, and 8 pendant PEG(2K) chains were studied to examine their in vitro and in vivo characteristics, as well as their potential for PET/CT imaging. A pmabF(ab')(2)-MCP conjugate carrying 2 PEG(2K) and one carrying 8 PEG(2K) pendant chains in the polymer were selected for microPET/CT imaging and biodistribution studies in tumor-bearing mice. Orthotopic pancreatic patient-derived xenografts tumors were visualized by PET/CT imaging. These RICs showed low levels of liver and spleen uptake along with even lower levels of kidney uptake. These encouraging results confirm the stealth properties of the MCPs with pendant PEG(2K) chains.