Radiolabeling of dextran with rhenium-188

This study describes a method for the radiolabeling of dextran with rhenium-188 (188Re). In nuclear oncology 188Re is very useful for therapeutic applications. Its nuclear characteristics allow radiotherapy and in situ monitoring of tumor uptake as well as dosimetry calculations. Consequently new compounds with this radiolabel are of general interest. Dextran was oxidized with sodium periodate yielding reactive aldehyde groups and subsequently reacted with cysteine. The linkage was stabilized by reducing the Schiff bases with sodium cyanoborohydride. The conjugate was then radiolabeled with 188Re by using 188Re-gluconate as the transchelator, labeling the free thiols. Synthesis and radiolabeling were done in the absence of oxygen. The labeling efficiency was 60-70% and the radiochemical purity > 95%. The in vitro stability study, using "cysteine challenge" demonstrated that 50% of the radiolabel was transcomplexed to the 100 mM cysteine solution (after 1 h incubation at 37 degrees C). However, at physiologic conditions and presence of an antioxidant good stability was achieved. The 188Re labeled dextran presented in this study provides a template with therapeutic and diagnostic potential in nuclear oncology, either alone for local treatment or as a backbone in a tumor specific conjugate for systemic treatment.