COMPARISON OF 4 BIFUNCTIONAL CHELATES FOR RADIOLABELING MONOCLONAL-ANTIBODIES WITH COPPER RADIOISOTOPES - BIODISTRIBUTION AND METABOLISM

The bifunctional chelating agents (BFCs), 8,11-tetraazacyclotetradecan e-1,4,8,11-tetraacetic acid (BAT), 8,11-tetraazacyclotetradecane-1,4,8 ,11-tetraacetic acid (SCN-TETA), 1,4,8,11-tetraazacyclotetradec-1-yl)m ethyl]benzoic acid (CPTA), and 1-[(1,4,7,10, 13-pentaazacyclopentadec- 1-yl)methyl]benzoic acid (PCBA), were synthesized and conjugated to th e anti-colorectal monoclonal antibody (mAb), 1A3, and antibody fragmen ts, 1A3-F(ab')(2), for radiolabeling with Cu-64,Cu-67 and comparison i n 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 b iodistribution studies were performed in Golden Syrian hamsters bearin g 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 betwe en 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-la beled BAT and SCN-TETA 1A3-F(ab')a conjugates, while rats injected wit h Cu-64-CPTA-1A3F(ab')(2) and Cu-64-PCBA-1A3-F(ab')(2) had nearly twic e 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 fr om 1-5 days post-injection of the radiolabeled conjugates. Liver and k idney homogenates were analyzed by size exclusion chromatography and t hin 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 unmetaboli zed. By day 5, a 35 kDa peak appeared in the liver of rats injected wi th the Cu-67-labeled 1A3 conjugates, possibly due to transchelation of the Cu-67 to proteins. Superoxide dismutase chromatographically elute s at the same retention time as this Cu-67-labeled metabolite. The TLC data indicate that the low molecular weight metabolite (< 5 kDa) of b oth Cu-67-CPTA-1A3 and Cu-67-CPTA-1A3-F(ab')(2) conjugates co-chromato graphed with a Cu-67-CPTA-epsilon-lysine standard. Our data suggest th at chelate charge and lipophilicity play a large role in kidney retent ion of Cu-64/67-labeled BFC-1A3-F(ab')(2) conjugates, while transchela tion of the copper label appears to be the major factor for liver accu mulation of (64)/Cu-67-labeled BFC-1A3 conjugates.