In vitro and in vivo characterization of ¹⁷⁷Lu-huA33. A radioimmunoconjugate against colorectal cancer

Introduction: The humanized monoclonal antibody A33 (huA33) is a potential targeting agent against colorectal carcinoma since the A33 antigen is highly and homogenously expressed in >95% of all colorectal cancers, both primary tumors and metastases. The aim of this study was to determine the biodistribution and tumor-targeting ability of ¹⁷⁷Lu-labeled huA33. Methods: huA33 was labeled with the β-emitting therapeutic nuclide ¹⁷⁷Lu using the chelator CHX-A″-DTPA, and the properties of the ¹⁷⁷Lu-CHX-A″-huA33 (¹⁷⁷Lu-huA33) conjugate was determined both in vitro and in vivo in a biodistribution study in nude mice xenografted with colorectal SW1222 tumor cells. Results: The ¹⁷⁷Lu-huA33 conjugate bound specifically to colorectal cancer cells in vitro (with a K_D value of 2.3±0.3 nM, determined by a saturation assay) and in vivo. The tumor uptake of ¹⁷⁷Lu-huA33 was very high, peaking at 134±21%ID/g 72 h postinjection (pi). Normal tissue uptake was low; radioactivity concentration in blood (which had the second highest radioactivity concentration) was lower than in tumor at all time points studied (8 h to 10 days). The tumor-to-blood ratio increased with time, reaching 70±30, 10 days pi. Throughout the study, the uptake of ¹⁷⁷Lu in bone (known to accumulate free ¹⁷⁷Lu) was low, and the fraction of protein-bound ¹⁷⁷Lu in plasma samples was high (95% to 99%). This indicates high stability of the ¹⁷⁷Lu-huA33 conjugate in vivo. Conclusion: The ¹⁷⁷Lu-huA33 conjugate shows a very favorable biodistribution, with an impressively high tumor uptake and high tumor-to-organ ratios, indicating that the conjugate may be suitable for radioimmunotherapy of colorectal cancer.