N-succinimidyl 3-[211At]astato-4-guanidinomethylbenzoate: an acylation agent for labeling internalizing antibodies with alpha-particle emitting 211At

Ortho-functionalization of a 211At-labeled aryl compound provides stabilization of the C-At bond against oxidative dehalogenation

Targeted alpha therapy appears to be a promising approach in nuclear medicine for the treatment of cancers. Thanks to its appropriate physical properties, 211At is an ideal candidate compared with other alpha emitters. Because of its enigmatic nature, the chemical element astatine is the subject of growing interest to better understand its radiochemistry. The application of 211At in the clinic, which has shown good therapeutic results, is however still hampered. Stability issues of 211At-radiolabeling were quickly encountered in early preclinical trials and later confirmed in the clinic that mainly studied 211At-radiopharmaceuticals labeled by formation of an astatobenzamide derivative. Recent studies have shed light on the deastatination mechanisms encountered in vivo, in particular potential oxidative mechanisms that may weaken the carbon-astatine bond formed during the radiolabeling. In this work, we show that ortho-functionalization of astatoaryl compounds with benzyl alcohols protects radiolabeling from deastatination in a strongly oxidizing and acidic medium, as well as in liver microsomal media reproducing in vivo deastatination via cytochrome P450 (CYP450) mediated mechanisms. Our results open the way to the rational design of new 211At-aryl-based compounds with improved stability.

Astatine-211 radiolabelling chemistry: from basics to advanced biological applications

BACKGROUND

211At-radiopharmaceuticals are currently the subject of growing studies for targeted alpha therapy of cancers, which leads to the widening of the scope of the targeting vectors, from small molecules to peptides and proteins. This has prompted, during the past decade, to a renewed interest in developing novel 211At-labelling approaches and novel prosthetic groups to address the diverse scenarios and to reach improved efficiency and robustness of procedures as well as an appropriate in vivo stability of the label.

MAIN BODY

Translated from the well-known (radio)iodine chemistry, the long preferred electrophilic astatodemetallation using trialkylaryltin precursors is now complemented by new approaches using electrophilic or nucleophilic At. Alternatives to the astatoaryl moiety have been proposed to improve labelling stability, and the range of prosthetic groups available to label proteins has expanded.

CONCLUSION

In this report, we cover the evolution of radiolabelling chemistry, from the initial strategies developed in the late 1970's to the most recent findings.

Development of a Neopentyl 211At-Labeled Activated Ester Providing In Vivo Stable 211At-Labeled Antibodies for Targeted Alpha Therapy

In this study we developed a neopentyl 211At-labeled activated ester that incorporates a triazole spacer and applied it to the synthesis of an 211At-labeled cetuximab. The activated ester was synthesized via the nucleophilic 211At-astatination of a neopentyl sulfonate carrying two long alkyl chains that serve as a lipid tag, which was followed by the hydrolysis of an acetal. Additionally, we developed a novel Resin-Assisted Purification and Deprotection (RAPD) protocol involving a solid-phase extraction of the protected 211At-labeled compound from the mixture of the labeling reaction, hydrolysis of the acetal on the resin, and finally an elution of the 211At-labeled activator from the resin. This method allows the synthesis of an 211At-labeled activated ester with high purity through a simplified procedure that circumvents the need for HPLC purification. Using this 211At-labeled activated ester, we efficiently synthesized 211At-labeled cetuximab in 27±1 % radiochemical yield with 95 % radiochemical purity. This 211At-activated ester demonstrated high reactivity, and enabled the completion of the reaction with the antibody within 10 min. In comparative biodistribution studies between 211At-labeled cetuximab and the corresponding 125I-labeled cetuximab in normal mice, both the thyroid and stomach showed radioactivity levels that were less than 1.0 % of the injected dose.

In vivo stability of 211At-radiopharmaceuticals: on the impact of halogen bond formation

211At, when coupled to a targeting agent, is one of the most promising radionuclides for therapeutic applications. The main labelling approach consists in the formation of astatoaryl compounds, which often show a lack of in vivo stability. The hypothesis that halogen bond (XB) interactions with protein functional groups initiate a deastatination mechanism is investigated through radiochemical experiments and DFT modelling. Several descriptors agree on the known mechanism of iodoaryl substrates dehalogenation by iodothyronine deiodinases, supporting the higher in vivo dehalogenation of N-succinimidyl 3-[211At]astatobenzoate (SAB) conjugates in comparison with their iodinated counterparts. The guanidinium group in 3-[211At]astato-4-guanidinomethylbenzoate (SAGMB) prevents the formation of At-mediated XBs with the selenocysteine active site in iodothyronine deiodinases. The initial step of At-aryl bond dissociation is inhibited, elucidating the better in vivo stability of SAGMB conjugates compared with those of SAB. The impact of astatine's ability to form XB interactions on radiopharmaceutical degradation may not be limited to the case of aryl radiolabeling.

Alpha-Emitting Radionuclides: Current Status and Future Perspectives

The use of radionuclides for targeted endoradiotherapy is a rapidly growing field in oncology. In particular, the focus on the biological effects of different radiation qualities is an important factor in understanding and implementing new therapies. Together with the combined approach of imaging and therapy, therapeutic nuclear medicine has recently made great progress. A particular area of research is the use of alpha-emitting radionuclides, which have unique physical properties associated with outstanding advantages, e.g., for single tumor cell targeting. Here, recent results and open questions regarding the production of alpha-emitting isotopes as well as their chemical combination with carrier molecules and clinical experience from compassionate use reports and clinical trials are discussed.

Production, purification and availability of 211At: Near term steps towards global access

The promising characteristics of the 7.2-h radiohalogen 211At have long been recognized; including having chemical properties suitable for labeling targeting vectors ranging from small organic molecules to proteins, and the emission of only one α-particle per decay, providing greater control over off-target effects. Unfortunately, the impact of 211At within the targeted α-particle therapy domain has been constrained by its limited availability. Paradoxically, the most commonly used production method - via the 209Bi(α,2n)211At reaction - utilizes a widely available natural material (bismuth) as the target and straightforward cyclotron irradiation methodology. On the other hand, the most significant impediment to widespread 211At availability is the need for an accelerator capable of generating ≥28 MeV α-particles with sufficient beam intensities to make clinically relevant levels of 211At. In this review, current methodologies for the production and purification of 211At - both by the direct production route noted above and via a 211Rn generator system - will be discussed. The capabilities of cyclotrons that currently produce 211At will be summarized and the characteristics of other accelerators that could be utilized for this purpose will be described. Finally, the logistics of networks, both academic and commercial, for facilitating 211At distribution to locations remote from production sites will be addressed.

Recent Achievements about Targeted Alpha Therapy-Based Targeting Vectors and Chelating Agents

One of the most rapidly growing options in the management of cancer therapy is Targeted Alpha Therapy (TAT) through which lethal α-emitting radionuclides conjugated to tumor-targeting vectors selectively deliver high amount of radiation to cancer cells.²²⁵Ac, ²¹²Bi, ²¹¹At, ²¹³Bi, and ²²³Ra have been investigated by plenty of clinical trials and preclinical researches for the treatment of smaller tumor burdens, micro-metastatic disease, and post-surgery residual disease. In order to send maximum radiation to tumor cells while minimizing toxicity in normal cells, a high affinity of targeting vectors to cancer tissue is essential. Besides that, the stable and specific complex between chelating agent and α-emitters was found as a crucial parameter. The present review was planned to highlight recent achievements about TAT-based targeting vectors and chelating agents and provide further insight for future researches.

Synthesis and preliminary evaluation of 211At-labeled inhibitors of prostate-specific membrane antigen for targeted alpha particle therapy of prostate cancer

INTRODUCTION

The high potency and short tissue range of α-particles are attractive features for targeted radionuclide therapy, particularly for cancers with micro-metastases. In the current study, we describe the synthesis of a series of 211At-labeled prostate-specific membrane antigen (PSMA) inhibitors and their preliminary evaluation as potential agents for metastatic prostate cancer treatment.

METHODS

Four novel Glu-urea based PSMA ligands containing a trialkyl stannyl group were synthesized and labeled with 211At, and for comparative purposes, 131I, via halodestannylation reactions with N-chlorosuccinimide as the oxidant. A PSMA inhibitory assay was performed to evaluate PSMA binding of the unlabeled, iodinated compounds. A series of paired-label biodistribution experiments were performed to compare each 211At-labeled PSMA ligand to its 131I-labeled counterpart in mice bearing subcutaneous PC3 PSMA+ PIP xenografts.

RESULTS

Radiochemical yields ranged from 32% to 65% for the 211At-labeled PSMA inhibitors and were consistently lower than those obtained with the corresponding 131I-labeled analogue. Good localization in PC3 PSMA+ PIP but not control xenografts was observed for all labeled molecules studied, which exhibited a variable degree of in vivo dehalogenation as reflected by thyroid and stomach activity levels. Normal tissue uptake and in vivo stability for several of the compounds was markedly improved compared with the previously evaluated compounds, [211At]DCABzL and [*I]DCIBzL.

CONCLUSIONS AND IMPLICATIONS FOR PATIENT CARE

Compared with the first generation compound [211At]DCABzL, several of the novel 211At-labeled PSMA ligands exhibited markedly improved stability in vivo and higher tumor-to-normal tissue ratios. [211At]GV-620 has the most promising characteristics and warrants further evaluation as a targeted radiotherapeutic for prostate cancer.

Preliminary Evaluation of Astatine-211-Labeled Bombesin Derivatives for Targeted Alpha Therapy

There are various diagnostic and therapeutic agents for prostate cancer using bombesin (BBN) derivatives, but astatine-211 (²¹¹At)-labeled BBN derivatives have yet to be studied. This study presented a preliminary evaluation of ²¹¹At-labeled BBN derivative. Several nonradioactive iodine-introduced BBN derivatives (IB-BBNs) with different linkers were synthesized and their binding affinities measured. Because IB-3 exhibited a comparable affinity to native BBN, [²¹¹At]AB-3 was synthesized and the radiochemical yields of [²¹¹At]AB-3 was 28.2 ± 2.4%, with a radiochemical purity of >90%. The stability studies and cell internalization/externalization experiments were performed. [²¹¹At]AB-3 was taken up by cells and internalized; however, radioactivity effluxed from cells over time. In addition, the biodistribution of [²¹¹At]AB-3, with and without excess amounts of BBN, were evaluated in PC-3 tumor-bearing mice. Despite poor stability in murine plasma, [²¹¹At]AB-3 accumulated in tumor tissue (4.05 ± 0.73%ID/g) in PC-3 tumor-bearing mice, which was inhibited by excess native BBN (2.56 ± 0.24%ID/g). Accumulated radioactivity in various organs is probably due to free ²¹¹At. Peptide degradation in murine plasma and radioactivity efflux from cells are areas of improvement. The development of ²¹¹At-labeled BBN derivatives requires modifying the BBN sequence and preventing deastatination.

Realizing Clinical Trials with Astatine-211: The Chemistry Infrastructure

Despite the consensus around the clinical potential of the α-emitting radionuclide astatine-211 (211At), there are only a limited number of research facilities that work with this nuclide. There are three main reasons for this: (1) Scarce availability of the nuclide. Despite a relatively large number of globally existing cyclotrons capable of producing 211At, few cyclotron facilities produce the nuclide on a regular basis. (2) Lack of a chemical infrastructure, that is, isolation of 211At from irradiated targets and the subsequent synthesis of an astatinated product. At present, the research groups that work with 211At depend on custom systems for recovering 211At from the irradiated targets. Setting up and implementing such custom units require long lead times to provide a proper working system. (3) The chemistry of 211At. Compared with radiometals there are no well-established and generally accepted synthesis methods for forming sufficiently stable bonds between 211At and the tumor-specific vector to allow for systemic applications. Herein we present an overview of the infrastructure of producing 211At radiopharmaceuticals, from target to radiolabeled product including chemical strategies to overcome hurdles for advancement into clinical trials with 211At.

Astatine-211 labeled anti-HER2 5F7 single domain antibody fragment conjugates: radiolabeling and preliminary evaluation

INTRODUCTION

Derived from heavy chain only camelid antibodies, ~15-kDa single-domain antibody fragments (sdAbs) are an attractive platform for developing molecularly specific imaging probes and targeted radiotherapeutics. The rapid tumor accumulation and normal tissue clearance of sdAbs might be ideal for use with 211At, a 7.2-h half-life α-emitter, if appropriate labeling chemistry can be devised to trap 211At in cancer cells after sdAb binding. This study evaluated two reagents, [211At]SAGMB and iso-[211At]SAGMB, for this purpose.

METHODS

[211At]SAGMB and iso-[211At]SAGMB, and their radioiodinated analogues [131I]SGMIB and iso-[131I]SGMIB, were synthesized by halodestannylation and reacted with the anti-HER2 sdAb 5F7. Radiochemical purity, immunoreactivity and binding affinity were determined. Paired-label internalization assays on HER2-expressing BT474M1 breast carcinoma cells directly compared [131I]SGMIB-5F7/[211At]SAGMB-5F7 and iso-[131I]SGMIB-5F7/iso-[211At]SAGMB-5F7 tandems. The biodistribution of the two tandems was evaluated in SCID mice with subcutaneous BT474M1 xenografts.

RESULTS

Radiochemical yields for Boc2-iso-[211At]SAGMB and Boc2-[211At]SAGMB synthesis, and efficiencies for coupling of iso-[211At]SAGMB and [211At]SAGMB to 5F7 were similar, with radiochemical purities of [211At]SAGMB-5F7 and iso-[211At]SAGMB-5F7 >98%. iso-[211At]SAGMB-5F7 and [211At]SAGMB-5F7 had immunoreactive fractions >80% and HER2 binding affinities of less than 5 nM. Internalization assays demonstrated high intracellular trapping of radioactivity, with little difference observed between corresponding 211At- and 131I-labeled 5F7 conjugates. Higher BT474M1 intracellular retention was observed from 1-6 h for the iso-conjugates (iso-[211At]SAGMB-5F7, 74.3 ± 2.8%, vs. [211At]SAGMB-5F7, 63.7 ± 0.4% at 2 h) with the opposite behavior observed at 24 h. Peak tumor uptake for iso-[211At]SAGMB-5F7 was 23.4 ± 2.2% ID/g at 4 h, slightly lower than its radioiodinated counterpart, but significantly higher than observed with [211At]SAGMB-5F7. Except in kidneys and lungs, tumor-to-normal organ ratios for iso-[211At]SAGMB-5F7 were greater than 10:1 by 2 h, and significantly higher than those for [211At]SAGMB-5F7.

CONCLUSION

These 211At-labeled sdAb conjugates, particularly iso-[211At]SAGMB-5F7, warrant further evaluation for targeted α-particle radiotherapy of HER2-expressing cancers.

N-Succinimidyl guanidinomethyl iodobenzoate protein radiohalogenation agents: influence of isomeric substitution on radiolabeling and target cell residualization

INTRODUCTION

N-succinimidyl 4-guanidinomethyl-3-[(*)I]iodobenzoate ([(*)I]SGMIB) has shown promise for the radioiodination of monoclonal antibodies (mAbs) and other proteins that undergo extensive internalization after receptor binding, enhancing tumor targeting compared to direct electrophilic radioiodination. However, radiochemical yields for [(131)I]SGMIB synthesis are low, which we hypothesize is due to steric hindrance from the Boc-protected guanidinomethyl group ortho to the tin moiety. To overcome this, we developed the isomeric compound, N-succinimidyl 3-guanidinomethyl-5-[(131)I]iodobenzoate (iso-[(131)I]SGMIB) wherein this bulky group was moved from ortho to meta position.

METHODS

Boc2-iso-SGMIB standard and its tin precursor, N-succinimidyl 3-((1,2-bis(tert-butoxycarbonyl)guanidino)methyl)-5-(trimethylstannyl)benzoate (Boc2-iso-SGMTB), were synthesized using two disparate routes, and iso-[*I]SGMIB synthesized from the tin precursor. Two HER2-targeted vectors - trastuzumab (Tras) and a nanobody 5F7 (Nb) - were labeled using iso-[(*)I]SGMIB and [(*)I]SGMIB. Paired-label internalization assays in vitro with both proteins, and biodistribution in vivo with trastuzumab, labeled using the two isomeric prosthetic agents were performed.

RESULTS

When the reactions were performed under identical conditions, radioiodination yields for the synthesis of Boc2-iso-[(131)I]SGMIB were significantly higher than those for Boc2-[(131)I]SGMIB (70.7±2.0% vs 56.5±5.5%). With both Nb and trastuzumab, conjugation efficiency also was higher with iso-[(131)I]SGMIB than with [(131)I]SGMIB (Nb, 33.1±7.1% vs 28.9±13.0%; Tras, 45.1±4.5% vs 34.8±10.3%); however, the differences were not statistically significant. Internalization assays performed on BT474 cells with 5F7 Nb indicated similar residualizing capacity over 6h; however, at 24h, radioactivity retained intracellularly for iso-[(131)I]SGMIB-Nb was lower than for [(125)I]SGMIB-Nb (46.4±1.3% vs 56.5±2.5%); similar results were obtained using Tras. Likewise, a paired-label biodistribution of Tras labeled using iso-[(125)I]SGMIB and [(131)I]SGMIB indicated an up to 22% tumor uptake advantage at later time points for [(131)I]SGMIB-Tras.

CONCLUSION

Given the higher labeling efficiency obtained with iso-SGMIB, this residualizing agent might be of value for use with shorter half-life radiohalogens.

Uptake, internalization and nuclear translocation of radioimmunotherapeutic agents

Radioimmunotherapy (RIT) agents that incorporate short-range particle-emitting radionuclides exploit the high linear energy transfer of α-particles and Auger electrons. Both are densely ionizing, generate complex DNA double-strand breaks and so are profoundly cytotoxic. Internalizing RIT agents enter tumor cells through receptor-mediated endocytosis and by incorporation of cell-penetrating peptides. Once internalized, some RIT agents mediate escape from endosomes and/or translocate to the nucleus. In the classical nuclear import pathway, α/β-importins recognize nuclear localization sequences in RIT agents. Translocation through nuclear pores enables RIT agents to bind to nuclear targets induced by, for example, cellular stress, growth factors or anticancer therapy, such as γH2AX or p27(KIP-1). This review discusses RIT agents designed to exploit the mechanisms underlying these complex processes and compares them with noninternalizing RIT agents.

Improved tumor targeting of anti-HER2 nanobody through N-succinimidyl 4-guanidinomethyl-3-iodobenzoate radiolabeling

Nanobodies are approximately 15-kDa proteins based on the smallest functional fragments of naturally occurring heavy chain-only antibodies and represent an attractive platform for the development of molecularly targeted agents for cancer diagnosis and therapy. Because the human epidermal growth factor receptor type 2 (HER2) is overexpressed in breast and ovarian carcinoma, as well as in other malignancies, HER2-specific Nanobodies may be valuable radiodiagnostics and therapeutics for these diseases. The aim of the present study was to evaluate the tumor-targeting potential of anti-HER2 5F7GGC Nanobody after radioiodination with the residualizing agent N-succinimidyl 4-guanidinomethyl 3-(125/131)I-iodobenzoate (*I-SGMIB).

METHODS

The 5F7GGC Nanobody was radiolabeled using *I-SGMIB and, for comparison, with N(ε)-(3-*I-iodobenzoyl)-Lys(5)-N(α)-maleimido-Gly(1)-GEEEK (*I-IB-Mal-d-GEEEK), another residualizing agent, and by direct radioiodination using IODO-GEN ((125)I-Nanobody). The 3 labeled Nanobodies were evaluated in affinity measurements, and paired-label internalization assays were performed on HER2-expressing BT474M1 breast carcinoma cells and in paired-label tissue distribution measurements in mice bearing subcutaneous BT474M1 xenografts.

RESULTS

*I-SGMIB-Nanobody was produced in 50.4% ± 3.6% radiochemical yield and exhibited a dissociation constant of 1.5 ± 0.5 nM. Internalization assays demonstrated that intracellular retention of radioactivity was up to 1.5-fold higher for *I-SGMIB-Nanobody than for coincubated (125)I-Nanobody or *I-IB-Mal-d-GEEEK-Nanobody. Peak tumor uptake for *I-SGMIB-Nanobody was 24.50% ± 9.89% injected dose/g at 2 h, 2- to 4-fold higher than observed with other labeling methods, and was reduced by 90% with trastuzumab blocking, confirming the HER2 specificity of localization. Moreover, normal-organ clearance was fastest for *I-SGMIB-Nanobody, such that tumor-to-normal-organ ratios greater than 50:1 were reached by 24 h in all tissues except lungs and kidneys, for which the values were 10.4 ± 4.5 and 5.2 ± 1.5, respectively.

CONCLUSION

Labeling anti-HER2 Nanobody 5F7GGC with *I-SGMIB yields a promising new conjugate for targeting HER2-expressing malignancies. Further research is needed to determine the potential utility of *I-SGMIB-5F7GGC labeled with (124)I, (123)I, and (131)I for PET and SPECT imaging and for targeted radiotherapy, respectively.

Radioiodinated and astatinated NHC rhodium complexes: synthesis

INTRODUCTION

The clinical development of radioimmunotherapy with astatine-211 is limited by the lack of a stable radiolabeling method for antibody fragments. An astatinated N-heterocyclic carbene (NHC) Rhodium complex was assessed for the improvement of radiolabeling methodologies with astatine.

METHODS

Wet harvested astatine-211 in diisopropyl ether was used. Astatine was first reduced with cysteine then was reacted with a chlorinated Rh-NHC precursor to allow the formation of the astatinated analogue. Reaction conditions have been optimized. Astatine and iodine reactivity were also compared. Serum stability of the astatinated complex has been evaluated.

RESULTS

Quantitative formation of astatide was observed when cysteine amounts higher than 46.2 nmol/μl of astatine solution were added. Nucleophilic substitution kinetics showed that high radiolabeling yields were obtained within 15 min at 60°C (88%) or within 5 min at 100°C (95%). Chromatographic characteristics of this new astatinated compound have been correlated with the cold iodinated analog ones. The radioiodinated complex was also synthesized from the same precursor (5 min. at 100°C, up to 85%) using [(125)I]NaI as a radiotracer. In vitro stability of the astatinated complex was controlled after 15 h incubation in human serum at 4°C and 37°C. No degradation was observed, indicating the good chemical and enzymatic stability.

CONCLUSION

The astatinated complex was obtained in good yield and exhibited good chemical and enzymatic stability. These preliminary results demonstrate the interest of this new radiolabeling methodology, and further functionalizations should open new possibilities in astatine chemistry.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Although there are many steps and pitfalls before clinical use for a new prosthetic group from the family of NHC complexes, this work may open a new path for astatine-211 targeting.

Production of [(211)At]-astatinated radiopharmaceuticals and applications in targeted α-particle therapy

(211)At is a promising radionuclide for α-particle therapy of cancers. Its physical characteristics make this radionuclide particularly interesting to consider when bound to cancer-targeting biomolecules for the treatment of microscopic tumors. (211)At is produced by cyclotron irradiation of (209)Bi with α-particles accelerated at ~28 MeV and can be obtained in high radionuclidic purity after isolation from the target. Its chemistry resembles iodine, but there is also a tendency to behave as a metalloid. However, the chemical behavior of astatine has not yet been clearly established, primarily due to the lack of any stable isotopes of this element, which precludes the use of conventional analytical techniques for its characterization. There are also only a limited number of research centers that have been able to produce this element in sufficient amounts to carry out extensive investigations. Despite these difficulties, chemical reactions typically used with iodine can be performed, and a number of biomolecules of interest have been labeled with (211)At. However, most of these compounds exhibit unacceptable instability in vivo due to the weakness of the astatine-biomolecule bond. Nonetheless, several compounds have shown high potential for the treatment of cancers in vitro and in several animal models, thus providing a promising basis that has allowed initiation of the first two clinical studies.

Evaluation of anti-podoplanin rat monoclonal antibody NZ-1 for targeting malignant gliomas

INTRODUCTION

Podoplanin/aggrus is a mucin-like sialoglycoprotein that is highly expressed in malignant gliomas. Podoplanin has been reported to be a novel marker to enrich tumor-initiating cells, which are thought to resist conventional therapies and to be responsible for cancer relapse. The purpose of this study was to determine whether an anti-podoplanin antibody is suitable to target radionuclides to malignant gliomas.

METHODS

The binding affinity of an anti-podoplanin antibody, NZ-1 (rat IgG(2a)), was determined by surface plasmon resonance and Scatchard analysis. NZ-1 was radioiodinated with (125)I using Iodogen [(125)I-NZ-1(Iodogen)] or N-succinimidyl 4-guanidinomethyl 3-[(131)I]iodobenzoate ([(131)I]SGMIB-NZ-1), and paired-label internalization assays of NZ-1 were performed. The tissue distribution of (125)I-NZ-1(Iodogen) and that of [(131)I]SGMIB-NZ-1 were then compared in athymic mice bearing glioblastoma xenografts.

RESULTS

The dissociation constant (K(D)) of NZ-1 was determined to be 1.2 × 10(-10) M by surface plasmon resonance and 9.8 × 10(-10) M for D397MG glioblastoma cells by Scatchard analysis. Paired-label internalization assays in LN319 glioblastoma cells indicated that [(131)I]SGMIB-NZ-1 resulted in higher intracellular retention of radioactivity (26.3 ± 0.8% of initially bound radioactivity at 8 h) compared to that from the (125)I-NZ-1(Iodogen) (10.0 ± 0.1% of initially bound radioactivity at 8 h). Likewise, tumor uptake of [(131)I]SGMIB-NZ-1 (39.9 ± 8.8 %ID/g at 24 h) in athymic mice bearing D2159MG xenografts in vivo was significantly higher than that of (125)I-NZ-1(Iodogen) (29.7 ± 6.1 %ID/g at 24 h).

CONCLUSIONS

The overall results suggest that an anti-podoplanin antibody NZ-1 warrants further evaluation for antibody-based therapy against glioblastoma.

The alpha-camera: a quantitative digital autoradiography technique using a charge-coupled device for ex vivo high-resolution bioimaging of alpha-particles

Bioconjugates used in internal radiotherapy exhibit heterogeneous distributions in organs and tumors, implying a risk of nonuniform dose distribution in therapeutic applications using α-particle emitters. Tools are required that provide data on the activity distribution for estimation of absorbed dose on a suborgan level. The α-camera is a quantitative imaging technique developed to detect α-particles in tissues ex vivo. The aim of this study was to evaluate the characteristics of this imaging system and to exemplify its potential use in the development of α-radioimmunotherapy.

METHODS

The α-camera combines autoradiography with a scintillating technique and optical registration by a charge-coupled device (CCD). The imaging system characteristics were evaluated by measurements of linearity, uniformity, and spatial resolution. The technique was applied for quantitative imaging of (211)At activity distribution in cryosections of tumors, kidney, and whole body. Intratumoral activity distributions of tumor-specific (211)At-MX35-F(ab')(2) were studied at various times after injection. The postinjection activity distributions in the renal cortex and whole kidneys were compared for (211)At-F(ab')(2) and (211)At-IgG trastuzumab.

RESULTS

Quantitative analysis of α-camera images demonstrated that the pixel intensity increased linearly with activity in the imaged specimen. The spatial resolution was 35 ± 11 μm (mean ± SD) and the uniformity better than 2%. Kidney cryosections revealed a higher cortex-to-whole kidney ratio for (211)At-F(ab')(2) than for (211)At-IgG (1.38 ± 0.03 and 0.77 ± 0.04, respectively) at 2 h after injection. Nonuniform intratumoral activity distributions were found for tumor-specific (211)At-MX35-F(ab')(2) at 10 min and 7 h after injection; after 21 h, the distribution was more uniform.

CONCLUSION

The characteristics of the α-camera are promising, suggesting that this bioimaging system can assist the development, evaluation, and refinement of future targeted radiotherapy approaches using α-emitters. The α-camera provides quantitative data on the activity distribution in tissues on a near-cellular scale and can therefore be used for small-scale dosimetry, improving the prediction of biologic outcomes with α-particles with short path length and high linear energy transfer.

Modular nanotransporters of anticancer drugs conferring cell specificity and higher efficiency

This review deals with artificial modular nanotransporters (MNT) of polypeptide nature for drug delivery into target cells and then into a specified cell compartment like the nucleus. The developed approach is based on the use of intracellular transport processes characteristic of practically all cells, including cancer cells. The first MNT module ligand carries out a double function: specific recognition of a cancer target cell and penetration into the cell via receptor-mediated endocytosis. The movement of the MNT within the cell along this path specifies the need to supply the MNT with an endosomolytic module making it possible to leave the endocytotic pathway before getting into lysosomes in order to have time for interaction with importins. For this purpose, a polypeptide fragment able to make defects in membranes only at the pH of endosomes is used as the second module. Delivery into the cell nucleus is provided by the third module containing an amino acid sequence of nuclear localization, "recognized" by importins located in the hyaloplasm. And finally, the fourth module, a carrier for joining the transported drug, is incorporated into the MNT. Depending on the type of ligand module, MNT for different target cell types have been produced. Each module retains its activity within the MNT, ligand modules bind target receptors with high affinity, while the module with the nuclear localization sequence binds importins. The endosomolytic module forms pores in lipid membranes through which MNT are able to leave acidifying cell compartments (endosomes). Modules within MNT can be replaced or transposed, which makes it possible to use them for delivery of different drugs into different target cells and their compartments. It was shown that photosensitizers and radionuclides used for cancer therapy acquire pronounced cell specificity as well as the 10-1000-fold higher efficiency resulting from their delivery into the most vulnerable compartment--the cell nucleus.

Evaluation of an anti-p185(HER2) (scFv-C(H)2-C(H)3)2 fragment following radioiodination using two different residualizing labels: SGMIB and IB-Mal-D-GEEEK

INTRODUCTION

A 105-kDa double mutant single-chain Fv-Fc fragment (scFv-Fc DM) derived from the anti-p185(HER2) hu4D5v8 antibody (trastuzumab; Herceptin) has been described recently. The goal of this study was to investigate whether improved tumor targeting could be achieved with this fragment through the use of residualizing radioiodination methods.

METHODS

The scFv-Fc DM fragment was radioiodinated using N-succinimidyl 4-guanidinomethyl 3-[(131)I]iodobenzoate ([(131)I]SGMIB) and N(epsilon)-(3-[(131)I]iodobenzoyl)-Lys(5)-N(alpha)- maleimido-Gly(1)-GEEEK ([(131)I]IB-Mal-D-GEEEK), two residualizing radioiodination agents that have been used successfully with intact antibodies. Paired-label internalization assays of the labeled fragments were performed in vitro using MCF7 human breast cancer cells transfected to express HER2 (MCF7-HER2); comparisons were made to scFv-Fc DM directly radioiodinated using Iodogen. The tissue distribution of the scFv-Fc DM labeled with [(125)I]IB-Mal-d-GEEEK and [(131)I]SGMIB was compared in athymic mice bearing MCF7-HER2 xenografts.

RESULTS

The scFv-Fc DM fragment was labeled with [(131)I]SGMIB and [(131)I]IB-Mal-d-GEEEK in conjugation yields of 53% and 25%, respectively, with preservation of immunoreactivity for HER2. Internalization assays indicated that labeling via SGMIB resulted in a 1.6- to 3.5-fold higher (P<.05) retention of radioactivity, compared to that from the directly labeled fragment, in HER2-expressing cells during a 24-h observation period. Likewise, the amount of radioactivity retained in cells from the IB-Mal-d-GEEEK-labeled fragment was 1.4- to 3.3-fold higher (P<.05). Tumor uptake of radioiodine activity in athymic mice bearing MCF7-HER2 xenografts in vivo was significantly higher for the [(125)I]IB-Mal-d-GEEEK-labeled scFv-Fc DM fragment compared with that of the [(131)I]SGMIB-labeled fragment, particularly at later time points. The uptake of (125)I was threefold (3.6+/-1.1 %ID/g vs. 1.2+/-0.4 %ID/g) and fourfold (3.1+/-1.7 %ID/g vs. 0.8+/-0.4 %ID/g) higher than that for (131)I at 24 and 48 h, respectively. However, the [(125)I]IB-Mal-d-GEEEK-labeled scFv-Fc DM fragment also exhibited considerably higher levels of radioiodine activity in liver, spleen and kidney.

CONCLUSIONS

The overall results further demonstrate the potential utility of these two prosthetic groups for the radiohalogenation of internalizing monoclonal antibodies and their fragments. Specifically, the trastuzumab-derived double mutant fragment in combination with these residualizing agents warrants further evaluation for imaging and possibly treatment of HER2 expressing malignancies.

Treatment of HER2-positive breast carcinomatous meningitis with intrathecal administration of alpha-particle-emitting (211)At-labeled trastuzumab

INTRODUCTION

Carcinomatous meningitis (CM) is a devastating disease characterized by the dissemination of malignant tumor cells into the subarachnoid space along the brain and spine. Systemic treatment with monoclonal antibody (mAb) trastuzumab can be effective against HER2-positive systemic breast carcinoma but, like other therapies, is ineffective against CM. The goal of this study was to evaluate the therapeutic effect of alpha-particle emitting (211)At-labeled trastuzumab following intrathecal administration in a rat model of breast carcinoma CM.

METHODS

Athymic rats were injected intrathecally with MCF-7/HER2-18 breast carcinoma cells through a surgically implanted indwelling intrathecal catheter. In Experiment 1, animals received 33 or 66 muCi (211)At-labeled trastuzumab, cold trastuzumab or saline. In Experiment 2, animals were inoculated with a lower tumor burden and received 46 or 92 muCi (211)At-labeled trastuzumab or saline. In Experiment 3, animals received 28 muCi (211)At-labeled trastuzumab, 30 muCi (211)At-labeled TPS3.2 control mAb or saline. Histopathological analysis of the neuroaxis was performed at the end of the study.

RESULTS

In Experiment 1, median survival increased from 21 days for the saline and cold trastuzumab groups to 45 and 48 days for 33 and 66 muCi (211)At-labeled trastuzumab, respectively. In Experiment 2, median survival increased from 23 days for saline controls to 68 and 92 days for 46 and 92 muCi (211)At-labeled trastuzumab, respectively. In Experiment 3, median survival increased from 20 days to 29 and 36 days for animals treated with (211)At-labeled TPS3.2 and (211)At-labeled trastuzumab, respectively. Long-term survivors were observed exclusively in the (211)At-trastuzumab-treated groups.

CONCLUSION

Intrathecal (211)At-labeled trastuzumab shows promise as a treatment for patients with HER2-positive breast CM.

Reagents for astatination of biomolecules. 3. Comparison of closo-decaborate(2-) and closo-dodecaborate(2-) moieties as reactive groups for labeling with astatine-211

In vivo deastatination has been a major problem in the development of reagents for therapeutic applications of the alpha-particle emitting radionuclide (211)At. Our prior studies demonstrated that the use of a closo-decaborate(2-) ([closo-B(10)H(9)R](2-)) moiety for (211)At labeling of biomolecules provides conjugates that are stable to in vivo deastatination. In this investigation, the closo-decaborate(2-) moiety was compared with the structurally similar closo-dodecaborate(2-) ([closo-B(12)H(11)R](2-)) to determine if one has more favorable properties than the other for use in pendant groups as (211)At labeling molecules. To determine the differences, two sets of structurally identical molecules, with the exception that they contained either a closo-decaborate(2-) or a closo-dodecaborate(2-) moiety, were compared with regard to their synthesis, radiohalogenation, stability to in vivo deastatination and tissue distribution. Quite different rates of reaction were noted in the synthetic steps for the two closo-borate(2-) moieties, but ultimately the yields were similar, making these differences of little importance. Differences in radiohalogenation rates were also noted between the two closo-borate(2-) moieties, with the more electrophilic closo-decaborate(2-) reacting more rapidly. This resulted in somewhat higher yields of astatinated closo-decaborate(2-) derivatives (84% vs 53%), but both cage moieties gave good radioiodination yields (e.g., 79-96%). Importantly, both closo-borate(2-) cage moieties were shown to have high stability to in vivo deastatination. The largest differences between pairs of compounds containing the structurally similar boron cage moieties were in their in vivo tissue distributions. For example, [Et(3)NH](2)B(12)H(10)I-CONHpropyl, [(125)I]2b had high concentrations in kidney (1 h, 19.8%ID/g; 4 h, 26.5%ID/g), whereas [Et(3)NH](2)B(10)H(8)I-CONHpropyl, [(125)I]1e had much lower concentrations in kidney (1 h, 6.6%ID/g; 4 h, 0.27%ID/g). Interestingly, when another salt of the closo-decaborate(2-), [nBu(4)N](2)B(10)H(8)I-CONHpropyl, [(125)I]1b, was evaluated, the route of excretion appeared to be hepatobiliary rather than renal. Identical biotin derivatives containing the two closo-borate(2-) cage moieties had similar tissue distributions, except the closo-decaborate(2-) derivative had lower concentrations in kidney (1 h, 19.9%ID/g; 4 h, 24.4%ID/g vs 1 h, 38.9%ID/g; 4 h, 40.6%ID/g). In summary, the higher reactivity, faster tissue clearance, and lower kidney concentrations make the closo-decaborate(2-) more favorable for further studies using them in reactive groups for (211)At labeling of biomolecules.

Engineered modular recombinant transporters: application of new platform for targeted radiotherapeutic agents to alpha-particle emitting 211 At

PURPOSE

To generate and evaluate a modular recombinant transporter (MRT) for targeting 211 At to cancer cells overexpressing the epidermal growth factor receptor (EGFR).

METHODS AND MATERIALS

The MRT was produced with four functional modules: (1) human epidermal growth factor as the internalizable ligand, (2) the optimized nuclear localization sequence of simian vacuolating virus 40 (SV40) large T-antigen, (3) a translocation domain of diphtheria toxin as an endosomolytic module, and (4) the Escherichia coli hemoglobin-like protein (HMP) as a carrier module. MRT was labeled using N-succinimidyl 3-[211 At]astato-5-guanidinomethylbenzoate (SAGMB), its 125 I analogue SGMIB, or with 131 I using Iodogen. Binding, internalization, and clonogenic assays were performed with EGFR-expressing A431, D247 MG, and U87MG.wtEGFR human cancer cell lines.

RESULTS

The affinity of SGMIB-MRT binding to A431 cells, determined by Scatchard analysis, was 22 nM, comparable to that measured before labeling. The binding of SGMIB-MRT and its internalization by A431 cancer cells was 96% and 99% EGFR specific, respectively. Paired label assays demonstrated that compared with Iodogen-labeled MRT, SGMIB-MRT and SAGMB-MRT exhibited more than threefold greater peak levels and durations of intracellular retention of activity. SAGMB-MRT was 10-20 times more cytotoxic than [211 At]astatide for all three cell lines.

CONCLUSION

The results of this study have demonstrated the initial proof of principle for the MRT approach for designing targeted alpha-particle emitting radiotherapeutic agents. The high cytotoxicity of SAGMB-MRT for cancer cells overexpressing EGFR suggests that this 211 At-labeled conjugate has promise for the treatment of malignancies, such as glioma, which overexpress this receptor.

Astatine Radiopharmaceuticals: Prospects and Problems

For the treatment of minimum residual diseases such micrometastases and residual tumor margins that remain after debulking of the primary tumor, targeted radiotherapy using radiopharmaceuticals tagged with alpha-particle-emitting radionuclides is very attractive. In addition to the their short range in tissue, which helps minimize harmful effects on adjacent normal tissues, alpha-particles, being high LET radiation, have several radiobiological advantages. The heavy halogen, astatine-211 is one of the prominent alpha-particle-emitting radionuclides in practice. Being a halogen, it can often be incorporated into biomolecules of interest by adapting radioiodination chemistry. A wide spectrum of compounds from the simple [(211)At]astatide ion to small organic molecules, peptides, and large proteins labeled with (211)At have been investigated with at least two reaching the stage of clinical evaluation. The chemistry, cytotoxic advantages, biodistribution studies, and microdosimetry/pharmacokinetic modeling of some of these agents will be reviewed. In addition, potential problems such as the harmful effect of radiolysis on the synthesis, lack of sufficient in vivo stability of astatinated compounds, and possible adverse effects when they are systemically administered will be discussed.

Targeted alpha-particle radiotherapy with 211At-labeled monoclonal antibodies

An attractive feature of targeted radionuclide therapy is the ability to select radionuclides and targeting vehicles with characteristics that are best suited for a particular clinical application. One combination that has been receiving increasing attention is the use of monoclonal antibodies (mAbs) specifically reactive to receptors and antigens that are expressed in tumor cells to selectively deliver the alpha-particle-emitting radiohalogen astatine-211 (211At) to malignant cell populations. Promising results have been obtained in preclinical models with multiple 211At-labeled mAbs; however, translation of the concept to the clinic has been slow. Impediments to this process include limited radionuclide availability, the need for suitable radiochemistry methods operant at high activity levels and lack of data concerning the toxicity of alpha-particle emitters in humans. Nonetheless, two clinical trials have been initiated to date with 211At-labeled mAbs, and others are planned for the near future.

Synthesis of N-succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate: a radio-iodination agent for labeling internalizing proteins and peptides

This protocol describes a detailed procedure for the synthesis of N-succinimidyl 4-guanidinomethyl-3-[*I]iodobenzoate ([*I]SGMIB), an agent useful in the radio-iodination of proteins, including monoclonal Abs, and peptides that undergo internalization after receptor or antigen binding. In this procedure, the tin precursor N-succinimidyl 4-[N1,N2-bis(tert-butyloxycarbonyl)guanidinomethyl]-3-(trimethylstannyl)benzoate (Boc-SGMTB, 3) was first radio-iodinated to [*I]Boc-SGMIB, a derivative of [*I]SGMIB with the guanidine function protected with Boc groups. Treatment of [*I]Boc-SGMIB with trifluoroacetic acid delivered the final product. The total time for the synthesis and purification of [*I]Boc-SGMIB and its subsequent de-protection is approximately 140 min.

Radiobromination of humanized anti-HER2 monoclonal antibody trastuzumab using N-succinimidyl 5-bromo-3-pyridinecarboxylate, a potential label for immunoPET

Combining the specificity of radioimmunoscintigraphy and the high sensitivity of PET in an in vivo detection technique could improve the quality of nuclear diagnostics. Positron-emitting nuclide (76)Br (T(1/2)=16.2 h) might be a possible candidate for labeling monoclonal antibodies (mAbs) and their fragments, provided that the appropriate labeling chemistry has been established. For internalizing antibodies, such as the humanized anti-HER2 monoclonal antibody, trastuzumab, radiobromine label should be residualizing, i.e., ensuring that radiocatabolites are trapped intracellularly after the proteolytic degradation of antibody. This study evaluated the chemistry of indirect radiobromination of trastuzumab using N-succinimidyl 5-(tributylstannyl)-3-pyridinecarboxylate. Literature data indicated that the use of this method provided residualizing properties for iodine and astatine labels on some antibodies. An optimized "one-pot" procedure produced an overall labeling efficiency of 45.5+/-1.2% over 15 min. The bromine label was stable under physiological and denaturing conditions. The labeled trastuzumab retained its capacity to bind specifically to HER2-expressing SKOV-3 ovarian carcinoma cells in vitro (immunoreactivity more than 75%). However, in vitro cell test did not demonstrate that the radiobromination of trastuzumab using N-succinimidyl 5-bromo-3-pyridinecarboxylate improves cellular retention of radioactivity in comparison with the use of N-succinimidyl 4-bromobenzoate.

Adsorption and volatility of free 211At and 125I-

The present study was undertaken to extend our knowledge of the behaviour of 211At in the laboratory environment. An unexpectedly high volatility of free 211At was found, up to 85% during an hour. Free 211At also adsorbed more onto the plastic material studied than 125I-. The results of this study show that it is of great importance to pay careful attention to radiation protection procedures during the practical handling of free 211At.

Monoclonal antibodies for brain tumour treatment

Conventional treatment of brain tumours includes surgical, radiotherapeutic and chemotherapeutic modalities. Nonetheless, the outcome of patients with brain tumours, in particular glioblastoma, remains poor. Immunotherapy with armed or unarmed monoclonal antibodies targeting tumour-specific antigens has emerged in the last two decades as a novel potential adjuvant treatment for all types of neoplasia. Many challenges to its implementation as a safe and viable therapy for brain tumours still need to be addressed; nevertheless, results from ongoing Phase I/II clinical trials are encouraging, as disease stabilisation and patient survival prolongation have been observed. Advances in preclinical and clinical research indicate that treatment of brain tumours with monoclonal antibodies can be increasingly adjusted to the characteristics of the targeted tumour and its environment. This aspect relies on the careful selection of the target antigen and corresponding specific monoclonal antibody, and antibody format (size, class, affinity), conjugation to the appropriate toxin or radioactive isotope (half-life, range), and proper compartmental administration.

Radioiodine and 211At-labeled guanidinomethyl halobenzoyl octreotate conjugates: potential peptide radiotherapeutics for somatostatin receptor-positive cancers

Derivatives of the somatostatin analogues octreotide and octreotate labeled with radioiosotopes are used in the diagnosis and therapy of somatostatin receptor (SSTR)-positive tumors. A method has been devised to synthesize {N-(4-guanidinomethyl-3-iodobenzoyl)-Phe1-octreotate (GMIBO). Receptor binding assay and scatchard analysis yielded a Kd of 4.83 +/- 0.19 nM for this peptide. Derivatives of this peptide labeled with radioiodine ([*I]GMIBO) and the alpha-particle-emitting radiohalogen 211At N-(3-[211At]astato-4-guanidinomethylbenzoyl)-Phe1-octreotate; [211At]AGMBO} were prepared in a single step from a tin precursor in radiochemical yields of 30-35% and 15-20%, respectively. Paired-label internalization assays performed with the SSTR-positive D341 Med human medulloblastoma cell line demonstrated that [125I]GMIBO and [211At]AGMBO were specifically internalized 20-40% more than Nalpha-(1-deoxy-D-fructosyl)-[131I]I-Tyr3-octreotate ([131I]I-Glu-TOCA), the radioiodinated octreotide derivative previously shown to exhibit maximum internalization in this cell line. Uptake of [131I]GMIBO in D341 Med subcutaneous xenografts in a murine model (8.34 +/- 1.82 versus 8.10 +/- 2.23% ID/g at 1h) and SSTR-expressing normal tissues was comparable to that of [125I]I-Glu-TOCA and was shown to be specific. However, the uptake of [131I]GMIBO also was substantially higher in liver (16.9 +/- 3.15 versus 1.39 +/- 0.45% ID/g at 1 h) and in kidneys (44.33 +/- 6.47 versus 3.44 +/- 0.68% ID/g at 1h) compared to that of [125I]I-Glu-TOCA. These data suggest that these novel peptide conjugates retain their specificity for SSTR both in vitro and in vivo; however, because of their higher accumulation in normal tissues they would be best applied in settings amenable to loco-regional administration such as medulloblastoma neoplastic meningitis.