Panitumumab Modified with Metal-Chelating Polymers (MCP) Complexed to 111In and 177Lu-An EGFR-Targeted Theranostic for Pancreatic Cancer

A metal-chelating polymer (MCP) with a poly-glutamide (PGlu) backbone presenting on average 13 DOTA (tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelators for complexing In-111 or Lu-177 and 10 polyethylene glycol (PEG) chains to minimize liver and spleen uptake was conjugated to antiepidermal growth factor receptor (EGFR) monoclonal antibody (mAb), panitumumab. Because panitumumab-MCP may be dual-labeled with In-111 and Lu-177 for SPECT, or radioimmunotherapy (RIT) exploiting the Auger electrons or beta-particle emissions, respectively, we propose that panitumumab-MCP could be a useful theranostic agent for EGFR-positive PnCa. Bioconjugation was achieved by reaction of a hydrazine nicotinamide (HyNIC) group on the MCP with an aryl aromatic aldehyde introduced into panitumumab by reaction with succinimidyl-4-formylbenzamide (S-4FB). The conjugation reaction was monitored by measurement of the chromophoric bis-aryl hydrazone bond formed (epsilon(350 nm) = 24 500 M-1 cm(-1)) to achieve two MCPs/panitumumab. Labeling of panitumumab-MCP with In-111 or Lu-177 demonstrated that masses as small as 0.1 mu g were labeled to >90% labeling efficiency (L.E.) and a specific activity (SA) of >70 MBq/g. Panitumumab-DOTA incorporating two DOTA per mAb was labeled with In-111 or Lu-177 to a maximum SA of 65 MBq/mu g and 46 MBq/mu g, respectively. Panitumumab-MCP-Lu-177 exhibited saturable binding to EGFR-overexpressing MDA-MB-468 human breast cancer cells. The K-d for binding of panitumumab-MCP-Lu-177 to EGFR (2.2 +/- 0.6 nmol/L) was not significantly different than panitumumab-DOTA-Lu-177 (1.0 +/- 0.4 nmol/L). In-111 and Lu-177 were stably complexed to panitumumab-MCP. Panitumumab-MCP-In-111 exhibited similar whole body retention (55-60%) as panitumumab-DOTA-In-111 in NOD-scid mice up to 72 h postinjection (p.i.) and equivalent excretion of radioactivity into the urine and feces. The uptake of panitumumab-MCP-In-111 in most normal tissues in NOD-scid mice with EGFR-positive PANC-1 human pancreatic cancer (PnCa) xenografts at 72 h p.i. was not significantly different than panitumumab-DOTA-In-111, except for the liver which was 3-fold greater for panitumumab-MCP-In-111. Tumor uptake of panitumumab-MCP-In-111 (6.9 +/- 1.3% ID/g) was not significantly different than panitumumab-DOTA-In-111 (6.6 +/- 3.3%ID/g). Tumor uptake of panitumumab-MCP-In-111 and panitumumab-DOTA-In-111 were reduced by preadministration of excess panitumumab, suggesting EGFR-mediated uptake. Tumor uptake of nonspecific IgG-MCP (5.4 +/- 0.3%ID/g) was unexpectedly similar to panitumumab-MCP-In-111. An increased hydrodynamic radius of IgG when conjugated to an MCP may encourage tumor uptake via the enhanced permeability and retention (EPR) effect. Tumor uptake of panitumumab-DOTA-In-111 was 3.5-fold significantly higher than IgG-DOTA-In-111. PANC-1 tumors were imaged by microSPECT/CT at 72 h p.i. of panitumumab-MCP-In-111 or panitumumab-DOTA-In-111. Tumors were not visualized with preadministration of excess panitumumab to block EGFR, or with nonspecific IgG radioimmunoconjugates. We conclude that linking panitumumab to an MCP enabled higher SA labeling with In-111 and Lu-177 than DOTA-conjugated panitumumab, with preserved EGFR binding in vitro and comparable tumor localization in vivo in mice with s.c. PANC-1 human PnCa xenografts. Normal tissue distribution was similar except for the liver which was higher for the polymer radioimmunoconjugates.