Pretargeted radioimmunotherapy of tumor using a novel radiorhenium-labeled biotin derivative and streptavidin conjugated anti-cytokeratin 19 monoclonal antibody

Objectives In radioimmunotherapy (RIT), slow blood clearance of the antibodies results in continuous radiation exposure to nontargeted tissues including radiosensitive bone marrow. To overcome this problem, multistep RIT with pretargeting (PRIT) has been developed. In this study, we selected cytokeratin 19 (CK19) as a target molecule and then constructed a PRIT system using a novel radiorhenium-labeled biotin (Bt) derivative and streptavidin (SAv) conjugated antibody based on Bt-SAv interaction.

Methods Bt derivative with radiorhenium-tricarbonyl complex (^186/188Re(CO)₃-His-Bt) and SAv conjugated anti-CK19 antibody (SAv-Ab) were newly synthesized. In vitro pretargeting binding study was performed using human lung cancer cell line SK MES 1. SAv-Ab, biotinylated galactosyl BSA (clearing agent) and ^186/188Re(CO)₃-His-Bt were sequentially injected i.v. into BALB/c nude mice bearing SK MES 1, then in vivo biodistribution of the radioactivity was measured by scintigraphy. To evaluate the therapeutic effect and adverse effect, tumor size and peripheral blood cell counts were determined.

Results In in vitro pretargeting study, when SAv-Ab was first added to the culture medium, the cellular accumulation of radioactivity derived from ¹⁸⁶Re(CO)₃-His-Bt was significantly higher than that of negative control IgG-treated group as well as nontreated group, demonstrating the feasibility of the new pretargeting method. In biodistribution study, the subcutaneously grafted tumor was visualized at 48 hr postinjection of ¹⁸⁶Re(CO)₃-His-Bt. Moreover, in therapeutic study using ¹⁸⁸Re(CO)₃-His-Bt, tumor growth-inhibitory effect was dose-dependently observed in pretargeted group. Transient myelosuppression was also induced, however, this adverse effect disappeared in 3 weeks.

Conclusions The constructed system using a newly synthesized radiorhenium-labeled Bt derivative and SAv conjugated antibody would be available for PRIT