Thiolations, Tc-99m labelings, and animal in vivo biodistributions of divalent monoclonal antibody fragments

A number of divalent monoclonal antibody fragments have been derivatized with a bifunctional reagent containing a latent thiol group, and the conjugates were thiol-deprotected by exposure to aqueous hydroxylamine. This two-step procedure enabled convenient Tc-99m labeling of thiolated MAb divalent fragments, using either preformed Tc-99m-glucoheptonate or [Tc-99m]pertechnetate and single-vial lyophilized formulations of the conjugate and tin. Nonspecific incorporation of radiolabel was negligible, if any, when using nonthiolated antibodies. In animal biodistributions, in nude mice bearing LS174T human colon carcinoma xenografts, the Tc-99m-labeled MAb divalent fragments studied were found to result in a 3-fold reduction in the kidney uptake of radioactivity at 24 h, compared to a Tc-99m-Fab, and also to lead to improvements in other tumor:nontumor ratios.