Maleimidoethyl 3-(Tri-n-butylstannyl)hippurate: A Useful Radioiodination Reagent for Protein Radiopharmaceuticals To Enhance Target Selective Radioactivity Localization

Abstract

In pursuit of radiolabeled monoclonal antibodies (mAbs) with rapid urinary excretion of radioactivity from nontarget tissues, radioiodinated mAbs releasing a m-iodohippuric acid from the mAbs in nontarget tissues were designed. A novel reagent, maleimidoethyl 3-(tri-n-butylstannyl)hippurate (MIH), was synthesized by reacting N-(hydroxyethyl)maleimide with N-Boc-glycine before coupling with N-succinimidyl 3-(tri-n-butylstannyl)benzoate (ATE). MIH possessed a maleimide group for mAb conjugation and a butylstannyl moiety for high-yield and site-specific radioiodination, and the two functional groups were linked via an ester bond to release m-iodohippuric acid. To investigate the fate of radiolabels after lysosomal proteolysis, hepatic parenchymal cells were used as a model nontarget tissue and 131I-labeled MIH was conjugated with galactosyl-neoglycoalbumin (NGA). Further conjugation of [131I]MIH with a mAb against osteogenic sarcoma (OST7) after reduction of its disulfide bonds was followed up. In murine biodistribution studies, [131I]MIH-NGA exhibited rapid accumulation in the liver followed by radioactivity elimination from the liver at a rate that was identical to and faster than those of 131I-labeled NGA via direct iodination ([131I]NGA) and [131I]ATE-labeled NGA, respectively. While [131I]NGA indicated high radioactivity levels in the murine neck, stomach, and blood, such increases in the radioactivity count were not detectable by the administration of either [131I]MIH-NGA or [131I]ATE-NGA. At 6 h postinjection of [131I]MIH-NGA, 80% of the injected radioactivity was recovered in the urine. Analyses of urine samples indicated that m-iodohippuric acid was the sole radiolabeled metabolite. In biodistribution studies using [131I]-MIH-OST7 and [131I]ATE-OST7, while both 131I-labeled OST7s registered almost identical radioactivity levels in the blood up to 6 h postinjection, the former demonstrated a lower radioactivity level than [131I]ATE-OST7 in nontarget tissues throughout the experiment. Such chemical and biological characteristics of MIH would enable high target/nontarget ratios in diagnostic and therapeutic nuclear medicine using mAbs and other polypeptides.

Assessment of Radiochemical Design of Antibodies Using an Ester Bond as the Metabolizable Linkage: Evaluation of Maleimidoethyl 3-(Tri-n-butylstannyl)hippurate as a Radioiodination Reagent of Antibodies for Diagnostic and Therapeutic Applications

“…No significant decrease in the radioactivity level in the tumor was observed with [ 125 I]-SIB-OST7 (IgG) between the postinjection intervals. In our previous study using NGA, although [ 131 I]SIB-NGA was eliminated from murine liver more slowly than [ 131 I]-MIH-NGA, the former registered residual radioactivity levels in the liver similar to the latter at 24 h postinjection (19). In addition, radiolabeled OST7 (IgG) via direct radioiodination (26,42), via [ 67 Ga]succinyldeferoxamine with an ester bond (8), and via [ 111 In]-cDTPA (43) indicated unchanged radioactivity levels in the same tumor model (KT005) during the same postinjection intervals.…”

“…Recently, we developed a novel radioiodination reagent with an ester bond to liberate m-iodohippuric acid, maleimidoethyl 3-(tri-n-butylstannyl)hippurate (MIH) (19). Preliminary studies using galactosylneoglycoalbumin (NGA) as a model protein indicated that MIH liberates m-iodohippuric acid as the major radiometabolite after lysosomal proteolysis in hepatic parenchymal cells, and the metabolite was excreted rapidly from the hepatocytes into the urine intact.…”

In vivo evaluation of 188Re‐labeled alpha‐melanocyte stimulating hormone peptide analogs for melanoma therapy

“…Meta-iodohippuric acid was selected to conjugate Fab fragment because it was excreted through the urinary system rapidly and it resisted in vivo deiodination. 41,42 Their findings indicated that HML was able to reduce renal activity levels without sacrificing the target activity accumulation capacity. 43 The radiolabeled peptide, 111 In-DTPA-D-Phe 1 -Octreotide, has been used clinically to visualize somatostatin receptor-positive tumors.…”

Comparison of Radioiodine- or Radiobromine-Labeled RGD Peptides between Direct and Indirect Labeling Methods

“…125 I]SIB-c(RGDfK) (6) N-Succinimidyl ester of 3-(tri-n-butylstannyl) benzoate (ATE) (0.5 mg), which was synthesized using a method reported previously with a slight modification from 3-broobenzoic acid via a fourstep, 18,29) was dissolved in 79 µL of ethanol. Fifteen microliter of chloramine-T (1 mg/mL) in ethanol, 21 µL of 1% acetic acid in ethanol, and […”