Design, synthesis, and evaluation of [Re]Organorhenium-labeled antibody fragments with renal enzyme-cleavable linkage for low renal radioactivity levels

Uehara, Tomoya*; Koike, Miho*; Nakata, Hideo*; Hanaoka, Hirofumi*; Iida, Yasuhiko*; Hashimoto, Kazuyuki; Akizawa, Hiromichi*; Endo, Keigo*; Arano, Yasushi*

Renal localization of radiolabeled antibody fragments constitutes a problem in targeted imaging and radiotherapy. To estimate the applicability of the molecular design to metallic radionuclides, [Re]tricarbonyl(cyclopentadienylcarbonate)rhenium ([Re]CpTR-COOH) was conjugated with maleoyl-glycyl-lysine to prepare [Re]CpTR-GK. The cleavage of the glycyl-lysine linkage of the compound generates a glycine conjugate of [Re]CpTR-Gly. [Re]CpTR-GK was conjugated to thiolated Fab fragments to prepare [Re]CpTR-GK-Fab. The biodistribution of radioactivity after injection of [Re]CpTR-GK-Fab was compared with that of [Re]CpTR-Fab. [Re]CpTR-GK-Fab exhibited significantly lower renal radioactivity levels than did [Re]CpTR-Fab. The analysis of urine samples collected for 6 h postinjection of [Re]CpTR-GK-Fab showed that [Re]CpTR-Gly was the major radiometabolite. In tumor-bearing mice, [Re]CpTR-GK-Fab significantly reduced renal radioactivity levels without impairing the radioactivity levels in tumor. These findings indicate that the molecular design of radioparmaceuticals labeled with metallic radionuclides can be useful by using a radiometal chelate of high inertness and by designing a radiometabolite of high urinary excretion when released from antibody fragments following cleavage of a glycyl-lysine linkage. This study also indicates that a change in chemical structure of a radiolabel attached to a glycyl-lysine linkage significantly affected enzymes involved in the hydrolysis reaction.