Comparison of four bifunctional chelates for radiolabeling monoclonal antibodies with copper radioisotopes: Biodistribution and metabolism

The bifunctional chelating agents (BFCs), 6-[p-(bromoacetamido)benzyl]-1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (BAT), 6-[p-(isothiocyanato)benzyl]-1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (SCN-TETA), 4-[(1,4,8,11-tetraazacyclotetradec-1-yl)methyl]benzoic acid (CPTA), and 1-[(1,4,7,10, 13-pentaazacyclopentadec-1-yl)methyl]benzoic acid (PCBA), were synthesized and conjugated to the anti-colorectal monoclonal antibody (mAb), 1A3, and antibody fragments, 1A3-F(ab')(2), for radiolabeling with Cu-64,Cu-67 and comparison in animal models. In vivo metabolism studies were carried out in liver and kidneys in order to correlate the nature of the metabolites formed to the uptake and retention of the radiolabel in each organ. Animal biodistribution studies were performed in Golden Syrian hamsters bearing the GW39 human colon cancer tumors and in normal Sprague-Dawley rats. All conjugates showed good tumor uptake in hamsters. Biodistribution in rats showed that Cu-64-BAT-2IT-1A3 had the lowest liver and kidney uptake of the intact 1A3 conjugates (p < 0.03), whereas in hamsters, there were no significant differences in liver and kidney uptake between the four intact BFC-1A3 conjugates. Tumor-bearing hamsters injected with Cu-64-CPTA-1A3-F(ab')(2) and Cu-64-PCBA-1A3-F(ab')(2) had from 3 to 7 times greater uptake in the kidneys than hamsters given Cu-64-labeled BAT and SCN-TETA 1A3-F(ab')a conjugates, while rats injected with Cu-64-CPTA-1A3F(ab')(2) and Cu-64-PCBA-1A3-F(ab')(2) had nearly twice the uptake. The in vivo metabolism of the mAbs 1A3 and 1A3-F(ab')(2) radiolabeled with (CU)-C-67 through the SCN-TETA, CPTA, and PCBA BFCs was investigated by excising the livers and kidneys of normal rats from 1-5 days post-injection of the radiolabeled conjugates. Liver and kidney homogenates were analyzed by size exclusion chromatography and thin layer chromatography (TLC). The size exclusion chromatography data showed that all of the Cu-67-labeled 1A3-F(ab')(2) conjugates were > 85% degraded in the kidneys to small molecular weight metabolites by 1 day post-injection. In contrast, in the liver at 1 day post-injection, greater than 70% of the Cu-67-labeled 1A3 conjugates were unmetabolized. By day 5, a 35 kDa peak appeared in the liver of rats injected with the Cu-67-labeled 1A3 conjugates, possibly due to transchelation of the Cu-67 to proteins. Superoxide dismutase chromatographically elutes at the same retention time as this Cu-67-labeled metabolite. The TLC data indicate that the low molecular weight metabolite (< 5 kDa) of both Cu-67-CPTA-1A3 and Cu-67-CPTA-1A3-F(ab')(2) conjugates co-chromatographed with a Cu-67-CPTA-epsilon-lysine standard. Our data suggest that chelate charge and lipophilicity play a large role in kidney retention of Cu-64/67-labeled BFC-1A3-F(ab')(2) conjugates, while transchelation of the copper label appears to be the major factor for liver accumulation of (64)/Cu-67-labeled BFC-1A3 conjugates.