Reduction of the hepatic radioactivity levels of [111In]In-DOTA–labeled antibodies via cleavage of a linkage metabolized in lysosomes

Introduction

Radiolabeled antibodies are promising tools for cancer diagnosis using nuclear medicine. A DOTA-chelating system is useful for preparing immuno-positron emission tomography and immuno-single-photon emission computed tomography probes with various radiometals. Radiolabeled antibodies are generally metabolized in the reticuloendothelial system, producing radiometabolites after proteolysis in hepatic lysosomes. Because of the bulkiness and extremely high hydrophilicity of DOTA, radiometabolites containing a radiometal–DOTA complex typically exhibit high and persistent localization in hepatic lysosomes. Radioactivity in the liver impairs the accurate diagnosis of cancer surrounding the liver and liver metastasis, and a high tumor/liver ratio is desirable. In this study, we reduced the hepatic radioactivity of radiometal-labeled antibodies containing a DOTA-chelating system. A cleavable linkage was inserted to liberate the radiometabolite, which exhibited a short residence time in hepatocytes.

Methods

Using indium-111 (¹¹¹In)-labeled antibodies, we prepared ¹¹¹In-labeled galactosyl-neoglycoalbumins (NGAs) because they are useful for evaluating the residence time of radiometabolites in the liver. An ¹¹¹In-labeled NGA with a cleavable linkage ([¹¹¹In]In-DO3AiBu-Bn-FGK-NGA) was administered to normal mice, and biodistribution studies and metabolic analyses of urinary and fecal samples were performed with comparison to an ¹¹¹In-labeled NGA prepared by a conventional method ([¹¹¹In]In-DOTA-Bn-SCN-NGA). Then, ¹¹¹In-labeled antibodies ([¹¹¹In]In-DO3AiBu-Bn-FGK-IgG and [¹¹¹In]In-DOTA-Bn-SCN-IgG) were prepared using a procedure similar to that for ¹¹¹In-labeled NGAs. In vitro plasma stability and biodistribution were investigated for both ¹¹¹In-labeled antibodies in U87MG tumor-bearing mice.

Results

Through the liberation of radiometabolites including [¹¹¹In]In-DO3AiBu-Bn-F, [¹¹¹In]In-DO3AiBu-Bn-FGK-NGA was cleared more rapidly from the liver than [¹¹¹In]In-DOTA-Bn-SCN-NGA (4.07 ± 1.54%ID VS 71.68 ± 3.03%ID at 6 h postinjection). [¹¹¹In]In-DO3AiBu-Bn-FGK-IgG exhibited lower tumor accumulation (8.83 ± 1.48%ID/g) but a significantly higher tumor/liver ratio (2.21 ± 0.53) than [¹¹¹In]In-DOTA-Bn-SCN-IgG (11.65 ± 2.17%ID/g in the tumor and a tumor/liver ratio of 0.85 ± 0.18) at 72 h after injection.

Conclusion

A molecular design that reduces the high and persistent hepatic radioactivity of radiolabeled antibodies by liberating radiometabolites with a short hepatic residence time in lysosomes would be applicable for radiometal-labeled antibodies using a DOTA-chelating system.