A novel immunoscintigraphy technique using metabolizable linker with angiotensin II treatment

Reagents for astatination of biomolecules. 5. Evaluation of hydrazone linkers in (211)At- and (125)I-labeled closo-decaborate(2-) conjugates of Fab' as a means of decreasing kidney retention

Evaluation of monoclonal antibody (mAb) fragments (e.g., Fab', Fab, or engineered fragments) as cancer-targeting reagents for therapy with the α-particle emitting radionuclide astatine-211 ((211)At) has been hampered by low in vivo stability of the label and a propensity of these proteins localize to kidneys. Fortunately, our group has shown that the low stability of the (211)At label, generally a meta- or para-[(211)At]astatobenzoyl conjugate, on mAb Fab' fragments can be dramatically improved by the use of closo-decaborate(2-) conjugates. However, the higher stability of radiolabeled mAb Fab' conjugates appears to result in retention of radioactivity in the kidneys. This investigation was conducted to evaluate whether the retention of radioactivity in kidney might be decreased by the use of an acid-cleavable hydrazone between the Fab' and the radiolabeled closo-decaborate(2-) moiety. Five conjugation reagents containing sulfhydryl-reactive maleimide groups, a hydrazone functionality, and a closo-decaborate(2-) moiety were prepared. In four of the five conjugation reagents, a discrete poly(ethylene glycol) (PEG) linker was used, and one substituent adjacent to the hydrazone was varied (phenyl, benzoate, anisole, or methyl) to provide varying acid sensitivity. In the initial studies, the five maleimido-closo-decaborate(2-) conjugation reagents were radioiodinated ((125)I or (131)I), then conjugated with an anti-PSMA Fab' (107-1A4 Fab'). Biodistributions of the five radioiodinated Fab' conjugates were obtained in nude mice at 1, 4, and 24 h post injection (pi). In contrast to closo-decaborate(2-) conjugated to 107-1A4 Fab' through a noncleavable linker, two conjugates containing either a benzoate or a methyl substituent on the hydrazone functionality displayed clearance rates from kidney, liver, and spleen that were similar to those obtained with directly radioiodinated Fab' (i.e., no conjugate). The maleimido-closo-decaborate(2-) conjugation reagent containing a benzoate substituent on the hydrazone was chosen for study with (211)At. That reagent was conjugated with 107-1A4 Fab', then labeled (separately) with (125)I and (211)At. The radiolabeled Fab' conjugates were coinjected into nude mice bearing LNCaP human tumor xenografts, and biodistribution data were obtained at 1, 4, and 24 h pi. Tumor targeting was achieved with both (125)I- and (211)At-labeled Fab', but the (211)At-labeled Fab' reached a higher concentration (25.56 ± 11.20 vs 11.97 ± 1.31%ID/g). Surprisingly, while the (125)I-labeled Fab' was cleared from kidney similar to earlier studies, the (211)At-labeled Fab'was not (i.e., kidney conc. for (125)I vs (211)At; 4 h, 13.14 ± 2.03 ID/g vs 42.28 ± 16.38%D/g; 24 h, 4.23 ± 1.57 ID/g vs 39.52 ± 15.87%ID/g). Since the Fab' conjugate is identical in both cases except for the radionuclide, it seems likely that the difference in tissue clearance seen is due to an effect that (211)At has on either the hydrazone cleavage or on the retention of a metabolite. Results from other studies in our laboratory suggest that the latter case is most likely. The hydrazone linkers tested do not provide the tissue clearance sought for (211)At, so additional hydrazones linkers will be evaluated. However, the results support the use of hydrazone linkers when Fab' conjugated with closo-decaborate(2-) reagents are radioiodinated.

Design, Synthesis, and Evaluation of [188Re]Organorhenium-Labeled Antibody Fragments with Renal Enzyme-Cleavable Linkage for Low Renal Radioactivity Levels

Renal localization of radiolabeled antibody fragments constitutes a problem in targeted imaging and radiotherapy. We have reported that Fab fragments labeled with 3'-[131I]iodohippuryl Nepsilon-maleoyl-lysine (HML) showed markedly low renal radioactivity levels even shortly after injection, due to a rapid and selective release of m-[131I]iodohippuric acid by the action of brush border enzymes. To estimate the applicability of the molecular design to metallic radionuclides, [188Re]tricarbonyl(cyclopentadienylcarbonate)rhenium ([188Re]CpTR-COOH) was conjugated with Nepsilon-tert-butoxycarbonyl-glycyl-lysine or Nepsilon-maleoyl-glycyl-lysine to prepare [188Re]CpTR-GK-Boc or [188Re]CpTR-GK. The cleavage of the glycyl-lysine linkage of the two compounds generates a glycine conjugate of [188Re]CpTR-COOH ([188Re]CpTR-Gly), which possesses in vivo behaviors similar to those of m-iodohippuric acid. The hydrolysis rate of the peptide bond in [188Re]CpTR-GK-Boc was compared with that in 3'-[125I]iodohippuryl Nepsilon-Boc-lysine ([125I]HL-Boc) using brush border membrane vesicles (BBMVs) prepared from rat kidneys. [188Re]CpTR-GK was conjugated to thiolated Fab fragments to prepare [188Re]CpTR-GK-Fab. The biodistribution of radioactivity after injection of [188Re]CpTR-GK-Fab was compared with that of [125I]HML-Fab and [188Re]CpTR-Fab prepared by conjugating N-hydroxysuccinimidyl ester of [188Re]CpTR-COOH with antibody fragments. While [188Re]CpTR-GK-Boc liberated [188Re]CpTR-Gly in BBMVs, [125I]HL-Boc liberated m-[125I]iodohippuric acid at a much faster rate. In addition, although [125I]HL-Boc was hydrolyzed by both metalloenzymes and nonmetalloenzymes, metalloenzymes were responsible for the cleavage of the peptide linkage in [188Re]CpTR-GK-Boc. In biodistribution studies, [188Re]CpTR-GK-Fab exhibited significantly lower renal radioactivity levels than did [188Re]CpTR-Fab. However, the renal radioactivity levels of [188Re]CpTR-GK-Fab were slightly higher than those of [125I]HML-Fab. The analysis of urine samples collected for 6 h postinjection of [188Re]CpTR-GK-Fab showed that [188Re]CpTR-Gly was the major radiometabolite. In tumor-bearing mice, [188Re]CpTR-GK-Fab significantly reduced renal radioactivity levels without impairing the radioactivity levels in tumor. These findings indicate that the molecular design of HML can be applied to metallic radionuclides 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. Since there are many kinds of enzymes that cleave a variety of peptide linkages on the renal brush border membrane, selection of a peptide linkage optimal to a radiometal chelate of interest may provide radiolabeled antibody fragments that exhibit renal radioactivity levels similar to those of [131I]HML-labeled ones. The in vitro system using BBMVs might be useful for selecting an appropriate peptide linkage.

In Vitro System To Estimate Renal Brush Border Enzyme-Mediated Cleavage of Peptide Linkages for Designing Radiolabeled Antibody Fragments of Low Renal Radioactivity Levels

Renal localization of radiolabeled antibody fragments presents a problem in targeted imaging and radiotherapy. We recently reported that Fab fragments labeled with 3'-[(131)I]iodohippuryl N(epsilon)-maleoyl-l-lysine (HML) demonstrated markedly low renal radioactivity levels from early postinjection in mice. Previous studies suggested that low renal radioactivity levels were attributable to cleavage of the glycyl-lysine sequence in HML by the action of renal brush border enzymes, followed by urinary excretion of the resulting m-iodohippuric acid. In this study, an in vitro system using brush border membrane vesicles (BBMVs) isolated from the rat kidney cortex was developed to estimate renal brush border enzyme(s)-mediated cleavage of the peptide linkage. Low molecular weight HML derivatives, 3'-[(125)I]iodohippuryl l-lysine (HL), 3'-[(125)I]iodohippuryl N(epsilon)-tert-butoxycarbonyl-l-lysine (HBL), and their d-amino acid counterparts, were synthesized and incubated in BBMVs. Both [(125)I]HL and [(125)I]HBL generated m-[(125)I]iodohippuric acid after incubation in BBMVs at 37 degrees C while the latter liberated significantly higher amounts of the metabolite. [(125)I]d-HL and [(125)I]d-HBL failed to release the metabolite under similar conditions. The liberation of m-[(125)I]iodohippric acid from [(125)I]HL was significantly facilitated or completely inhibited by the addition of an activator or an inhibitor for carboxypeptidase M. The release of m-[(125)I]iodohippuric acid from [(125)I]HBL increased by the addition of the activator, whereas the inhibitor partially inhibited the release of the metabolite from [(125)I]HBL. The BBMV-mediated release of m-[(125)I]iodohippuric acid from [(125)I]HBL was not impaired by the addition of inhibitors for neutral endopeptidase or renal dipeptidase. These findings showed that the glycyl-l-lysine sequence in HML would be recognized and cleaved by metalloenzymes and nonmetalloenzymes on the renal brush border even when iodine was incorporated into a benzene ring and the N(epsilon)-amine residue of lysine was chemically modified, which supported the hypothesis that low renal radioactivity levels of HML-conjugated Fab fragments would be attributed to the release of m-iodohippuric acid by renal brush border enzymes. This study suggested that this in vitro system using BBMVs would be useful to estimate radiolabeling reagents of antibody fragments or peptides designed to reduce renal radioactivity with a variety of radionuclides.

Renal metabolism of 3'-iodohippuryl N(epsilon)-maleoyl-L-lysine (HML)-conjugated Fab fragments

Renal localization of radiolabeled antibody fragments constitutes a problem in targeted imaging and radiotherapy. Recently, we reported use of a novel radioiodination reagent, 3'-[131I]iodohippuryl N(epsilon)-maleoyl-L-lysine (HML), that liberates m-iodohippuric acid before antibody fragments are incorporated into renal cells. In mice, HML-conjugated Fab demonstrated low renal radioactivity levels from early postinjection times. In this study, renal metabolism of HML-conjugated Fab fragments prepared by different thiolation chemistries and by direct radioiodination were investigated to determine the mechanisms responsible for the low renal radioactivity levels. Fab fragments were thiolated by 2-iminothiolane modification or by reduction of disulfide bonds in the Fab fragments, followed by conjugation with radioiodinated HML to prepare [131I]HML-IT-Fab and [125I]HML-Fab, respectively. In biodistribution studies in mice, both [131I]HML-IT-Fab and [125I]HML-Fab demonstrated significantly lower renal radioactivity levels than those of [125I]Fab. In subcellular distribution studies, [125I]Fab showed migration of radioactivity from the membrane to the lysosomal fraction of the renal cells from 10 to 30 min postinjection. On the other hand, the majority of the radioactivity was detected only in the membrane fraction at the same time points after injection of both [131I]HML-IT-Fab and [125I]HML-Fab. In metabolic studies, while [125I]Fab remained intact at 10 min postinjection, both HML-conjugated Fab fragments generated m-iodohippuric acid as a radiometabolite at the same postinjection time. [131I]HML-IT-Fab registered two radiometabolites (intact [131I]HML-IT-Fab and m-iodohippuric acid), whereas additional radiometabolites were observed with [125I]HML-Fab. This suggested that metabolism of both HML-conjugated Fab fragments would occur in the membrane fractions of the renal cells. The findings of this study reinforced our previous hypothesis that radiochemical design of antibody fragments that liberate radiometabolites that are excreted into the urine by the action of brush border enzymes would constitute a useful strategy to reduce renal radioactivity levels from early postinjection times.