In Vitro Characterization of 211 At-Labeled Antibody A33—a Potential Therapeutic Agent Against Metastatic Colorectal Carcinoma

The humanized antibody A33 binds to the A33 antigen, expressed in 95% of primary and metastatic colorectal carcinomas. The restricted pattern of expression in normal tissue makes this antigen a possible target for radioimmunotherapy of colorectal micrometastases. In this study, the A33 antibody was labeled with the therapeutic nuclide ²¹¹At using N-succinimidyl para-(tri-methylstannyl)benzoate (SPMB). The in vitro characteristics of the ²¹¹At-benzoate-A33 conjugate (²¹¹At-A33) were investigated and found to be similar to those of ¹²⁵I-benzoate-A33 (¹²⁵I-A33) in different assays. Both conjugates bound with high affinity to SW1222 cells (K_d = 1.7 ± 0.2 nM, and 1.8 ± 0.1 nM for ²¹¹At-A33 and ¹²⁵I-A33, respectively), and both showed good intracellular retention (70% of the radioactivity was still cell associated after 20 hours). The cytotoxic effect of ²¹¹At-A33 was also confirmed. After incubation with ²¹¹At-A33, SW1222 cells had a survival of approximately 0.3% when exposed to some 150 decays per cell (DPC). The cytotoxic effect was found to be dose-dependent, as cells exposed to only 56 DPC had a survival of approximately 5%. The ²¹¹At-A33 conjugate shows promise as a potential radioimmunotherapy agent for treatment of micrometastases originating from colorectal carcinoma.