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

Reduction of radioactivity levels in nontarget tissues such as the liver and kidney constitutes a problem to be resolved in diagnostic and therapeutic applications of radiolabeled monoclonal antibodies (mAbs). A new radioiodination reagent with an ester bond to liberate m-iodohippuric acid from covalently conjugated proteins, maleimidoethyl 3-(tri-n-butylstannyl)hippurate (MIH), was recently developed. MIH liberated m-iodohippuric acid from galactosylneoglycoalbumin in murine liver, and the radiometabolite was rapidly eliminated from the liver into urine as an intact structure. In this study, intact IgG and Fab fragment of a mAb against osteogenic sarcoma were radioiodinated with MIH to further assess the applicability of MIH to radioimmunoimaging and therapy. For comparison, a mAb radioiodinated with N-succinimidyl iodobenzoate (SIB) and indium-111 (111In)-labeled mAbs with diethylenetriaminepentaacetic dianhydride (cDTPA) or 1-[4-[(5-maleimidopentyl)amino]benzyl]-ethylenediaminetetraacetic acid (EMCS-Bz-EDTA) were used. Size-exclusion HPLC analysis and cell binding assays indicated the preservation of both structure and antigen binding affinity of radioiodinated MIH-OST7 (IgG). In biodistribution studies in mice, [125I]MIH-OST7 (IgG) showed faster systemic clearance of radioactivity after 24 h postinjection than did [131I]SIB- and [111In]EMCS-Bz-EDTA-OST7 (IgG). [125I]MIH-OST7 (IgG) also exhibited much lower radioactivity levels in nontarget tissues such as the liver and kidney, with higher radioactivity levels in the blood up to 72 h postinjection when compared with [111In]cDTPA-OST7 (IgG). Radioactivity excreted from the mice was found in the urine as m-iodohippuric acid, following administration of [125I]MIH-OST7 (IgG). In athymic mice bearing osteogenic sarcoma, [131I]MIH-OST7 (IgG) indicated higher tumor-to-nontarget ratios of radioactivity at both 24 and 48 h postinjection than [125I]SIB-OST7 (IgG). Although both radioiodinated OST7s showed similar radioactivity levels in the target at 24 h postinjection, a small but significant decrease in the target radioactivity level was observed with [131I]MIH-OST7 (IgG) at 48 h postinjection. In addition, [131I]MIH-OST7 (Fab) showed very rapid cleavage of the ester bond both in vivo and in vitro. These findings indicated that while MIH may be a useful reagent for radioimmunoimaging using IgG, mAb, its application to smaller molecular weight mAbs and radioimmunotherapy would be hindered due to the labile characteristics of the ester bond in plasma. Thus, while the present study reinforced the usefulness of metabolizable linkages for reducing nontarget radioactivity levels, a development of plasma-stable metabolizable linkages is also warranted for radioimmunotherapy and for smaller molecular weight polypeptides.

Reassessment of diethylenetriaminepentaacetic acid (DTPA) as a chelating agent for indium-111 labeling of polypeptides using a newly synthesized monoreactive DTPA derivative

Previous studies on indium-111 (111In) labeling of polypeptides and peptides using cyclic diethylenetriaminepentaacetic dianhydride (cDTPA) as a bifunctional chelating agent (BCA) have indicated that DTPA might be a useful BCA for 111In labeling of polypeptides at high specific activities when DTPA can be incorporated without inducing intra- or intermolecular cross-linking. To investigate this hypothesis, a monoreactive DTPA derivative with a maleimide group as the peptide binding site (MDTPA) was designed and synthesized. A monoclonal antibody (OST7, IgG1) was used as a model polypeptide, and conjugation of MDTPA with OST7, 111In radiolabeling of MDTPA-OST7, and the stability of 111In-MDTPA-OST7 were investigated using cDTPA and benzyl-EDTA derivatives as references. SDS-PAGE analysis demonstrated that while cDTPA induced intramolecular cross-linking, no such undesirable side reactions were observed with MDTPA. MDTPA generated 111In-labeled OST7 with high radiochemical yields at higher specific activities than those produced using cDTPA and benzyl-EDTA derivatives as the BCAs. Incubation of each 111In-labeled OST7 in human serum indicated that MDTPA generated 111In-labeled OST7 of much higher and a little lower stability than those derived from cDTPA and benzyl-EDTA derivatives, respectively. These findings indicated that the low in vivo stability of cDTPA-conjugated antibody reported previously is not attributable to low stability of 111In-DTPA but to formation of intramolecular cross-linking during cDTPA conjugation reactions. The present study also indicated that MDTPA and its precursor, the tetra-tert-butyl derivative of DTPA, would be useful BCAs for 111In radiolabeling of polypeptides that have rapid blood clearance with high specific activities.

Stability of a metabolizable ester bond in radioimmunoconjugates

Ester bonds have been used as metabolizable linkages to reduce radioactivity levels in non-target tissues following the administration of antibodies labeled with metallic radionuclides. In this radiochemical design of antibodies, while the ester bonds should be cleaved rapidly in non-target tissues, high stability of the ester bonds in plasma is also required to preserve target radioactivity levels. To assess the structural requirements to stabilize the ester bond, a new benzyl-EDTA-derived bifunctional chelating agent with an ester bond, (1-[4-[4-(2- maleimidoethoxy)succinamido]benzyl]ethylenediamine-N,N,N',N' -tetraacetic acid; MESS-Bz-EDTA), was developed. MESS-Bz-EDTA was coupled with a thiolated monoclonal antibody (OST7, IgG1) prepared by reducing its disulfide bonds to introduce the ester bond close and proximal to the antibody molecule. For comparison, 1-[4-(5- maleimidopentyl)aminobenzyl]ethylenediamine-N,N,N',N'-tetraacetic acid (EMCS-Bz-EDTA) and meleimidoethyl 3-[131I]iodohippurate (MIH) was coupled to OST7 under the same conjunction chemistry. When incubated in 50% murine plasma or a buffered-solution of neutral pH, OST7-MESS-Bz-EDTA-111In rapidly released the radioactivity, and more than 95% of the initial radioactivity was liberated after a 24 h incubation in both solutions, due to a cleavage of the ester bond. On the other hand, only about 20% of the radioactivity was released from OST7-MIH-131I in both solutions during the same incubation period. In mice biodistribution studies, while a slightly faster radioactivity clearance from the blood with less radioactivity levels in the liver and kidneys was observed with OST7-MIH-131I than with OST7-EMCS-Bz-EDTA-111In, OST7-MESS-Bz-EDTA-111In indicated radioactivity clearance from the blood much faster than and almost comparable to that of OST7-MIH-131I and succinamidobenzyl-EDTA-111In, respectively. These findings as well as previous findings on radiolabeled antibodies with ester bonds suggested that while an introduction of an ester bond close to an antibody molecule stabilized the ester bond against esterase access, chemical structures of the linkages and radiolabels attached to the ester bonds play a significant role in the chemical stability of the ester bond. This may explain the different stability of the ester bonds in radioimmunoconjugates so far reported.

Radiolabeled metabolites of proteins play a critical role in radioactivity elimination from the liver

We have recently reported that the behavior of radiolabeled metabolites in the liver appears to be responsible for the hepatic radioactivity levels after administration of protein radiopharmaceuticals. To better understand the role played by radiolabeled metabolites in hepatic radioactivity levels, two benzyl-EDTA derivatives rendering different radiolabeled metabolites, 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-Bz-EDTA) and 1-[p-(5-maleimidopentyl)aminobenzyl]ethylenediaminetetraacetic acid (ECMS-Bz-EDTA), were selected as bifunctional chelating agents (BCAs), and 111In labeling of galactosyl-neoglycoalbumin (NGA) and mannosyl-neoglycoalbumin (NMA) was performed. Biodistribution of radioactivity in mice and subcellular distribution of radioactivity in hepatocytes were then compared. After accumulation in hepatic parenchymal cells, NGA-EMCS-Bz-EDTA-111In rendered a faster elimination rate of radioactivity from the liver than NGA-SCN-Bz-EDTA-111In. Although each 111In-NMA exhibited a delayed elimination rate of radioactivity from the liver compared to the 111In-NGA counterpart, NMA-EMCS-Bz-EDTA-111In showed faster elimination rate of radioactivity than NMA-SCN-Bz-EDTA-111In. Analyses of radioactivity excreted in feces and urine and remaining in the liver indicated that both BCAs rendered mono-amino acid adducts as the major radiolabeled metabolites (cysteine-EMCS-Bz-EDTA-111In and lysine-SCN-Bz-EDTA-111In), which were generated in both cell types of the liver within 1 h postinjection. Subcellular distribution of radioactivity indicated that the radioactivity was copurified with lysosomes. These results demonstrate that although in vivo stability of radiometal chelates is essential, the biological properties of the radiolabeled metabolites generated after lysosomal proteolysis in hepatocytes play a critical role in radioactivity elimination from the liver.

A biological method to evaluate bifunctional chelating agents to label antibodies with metallic radionuclides

Since the lysosome is a common organelle for protein digestion, pursuing the fate of radiolabeled metabolites after lysosomal proteolysis in liver cells is ideal to evaluate bifunctional chelating agents (BCAs).

METHODS

We used galactosyl-neoglycoalbumin (NGA) and mannosyl-neoglycoalbumin (NMA) as carrier proteins for hepatic parenchymal and nonparenchymal cells, respectively. These proteins were labeled with 111In using 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-Bz-EDTA) as a model.

RESULTS

NGA-SCN-Bz-EDTA-111In exhibited rapid accumulation in the hepatic parenchymal cells, followed by hepatobiliary excretion of the metabolites with an elimination rate that was faster and much slower than that of NGA-DTPA-111In and NGA-131I, respectively. This metabolite represented all the radioactivity registered in the liver at 1 hr postinjection. Subcellular distribution studies indicated the metabolites were located only in the lysosome fraction, and the difference in elimination rates of the metabolites from the lysosome fraction was responsible for the variations in radioactivity clearance from the cells.

CONCLUSION

The biological characteristics of radiolabeled metabolites play a critical role in eliminating the radiolabel from liver cells. The present method portrays a highly useful model to pursue the fate of radiolabeled metabolites in the liver.