Preparation, biodistribution and dosimetry of copper-64-labeled anti- colorectal carcinoma monoclonal antibody fragments 1A3-F(ab')₂

Antibody fragments labeled with a radiometal using bifunctional chelates generally undergo renal clearance followed by trapping of the metabolites, leading to high radiation doses to the kidneys. Copper-64-labeled BAT-2IT- 1A3-F(ab')₂ was recently reported to accumulate in colorectal tumors in an animal model, however, kidney uptake was also high. In this study, the preparation of ⁶⁴Cu-BAT-2IT-1A3-F(ab')₂ was optimized to reduce the renal uptake. Methods: The bifunctional chelate 6-bromoacetamidobenzyl-1,4,8,11- tetraazacyclotetradecane-N,N',N',N''-tetraacetic acid (BAT) was conjugated to 1A3-F(ab')₂ using the linking agent 2-iminothiolane (2IT). The conjugation reaction produced 20% of a lower molecular weight (molecular weight) impurity found to be TETA-1A3-Fab'. The conjugation procedure was optimized to include FPLC purification of the BAT-2IT-1A3-F(ab')₂ from TETA-1A3-Fab' after conjugation prior to labeling with ⁶⁴Cu. The biodistribution of ⁶⁴Cu- labeled FPLC-purified and unpurified conjugates was determined in normal Sprague-Dawley rats and tumor-bearing Golden Syrian hamsters. Human absorbed doses were calculated from rat biodistribution data and PET imaging of a baboon. Results: Upon FPLC purification of the BAT-2IT-1A3-F(ab')₂, the immunoreactivity of ⁶⁴Cu-labeled 1A3-F(ab')₂ was significantly improved over that of non-FPLC-purified ⁶⁴Cu-BAT-2IT-1A3-F(ab')₂, and the kidney uptake was decreased in normal rats. The biodistribution in hamsters showed some improvement in both tumor uptake and kidney clearance with FPLC-purified ⁶⁴Cu-BAT-2IT-1A3-F(ab')₂. Conclusion: The improved dosimetry of ⁶⁴Cu- labeled FPLC purified BAT-2IT-1A3-F(ab')₂ should more readily allow this agent to be investigated clinically to image colorectal cancer using PET.