A kit method for the high level synthesis of [211At]MABG

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Abstract

meta-[211At]Astatobenzylguanidine ([211At]MABG), an analogue of meta-iodobenzylguanidine (MIBG) labeled with the α-emitter 211At, targets the norepinephrine transporter. Because MABG has been shown to have excellent characteristics in preclinical studies, it has been considered to be a promising targeted radiotherapeutic for the treatment of tumors such as micrometastatic neuroblastoma that overexpress the norepinephrine transporter. To facilitate clinical evaluation of this agent, a convenient method for the high level synthesis of [211At]MABG that is adaptable for kit formulation has been developed. A tin precursor anchored to a solid-support was treated with a methanolic solution of 211At in the presence of a mixture of H2O2/HOAc as the oxidant; [211At]MABG was isolated by simple solid-phase extraction. By using C-18 solid-phase extraction, the radiochemical yield from 25 batches was 63 ± 13%; however, loss of radioactivity during evaporation of the methanolic solution was a problem. This difficulty was avoided by use of a cation exchange resin cartridge for isolation of [211At]MABG, which resulted in radiochemical yields of 63 ± 9% in a shorter duration of synthesis. The radiochemical purity was more than 90% and no chemical impurity has been detected. The final doses were sterile and apyrogenic. These results demonstrate that [211At]MABG can be prepared via a kit method at radioactivity levels anticipated for initiation of clinical studies.

Introduction

The norepinephrine transporter (NET) represents an attractive target for the delivery of therapeutic radiopharmaceuticals to malignancies that arise from the neuroendocrine system. NET is highly over-expressed in neuroendocrine tumors such as neuroblastoma, pheochromocytoma/paraganglioma, and carcinoid. meta-Iodobenzylguanidine (MIBG or Iobenguane) is an avid substrate for NET1, 2 and radioiodinated MIBG has been shown to accumulate in these tumors to such an extent as to be highly useful for both disease detection using nuclear imaging and targeted radiopharmaceutical therapy in humans.3, 4

[131I]MIBG is an active agent in the monotherapy of heavily pretreated, refractory high-risk neuroblastoma, with 37% responding per International Neuroblastoma Response Criteria (INRC), 27% alive and without disease progression 6 months after [131I]MIBG monotherapy, and with 30% alive at 12 months, including those with progressive disease.5 [131I]MIBG has been used in combination with radiosensitizing agents,6 surgery,7 myeloablative chemotherapy,8 and/or external beam radiotherapy9 in attempts to improve therapeutic response. Similarly, [131I]MIBG is an effective agent in the treatment of malignant pheochromocytoma patients, with 30% showing tumor response and 45% showing hormonal response.10

As [131I]MIBG has demonstrated activity in neuroendocrine cancers, significant data also have demonstrated the ability of other radiohalogenated analogues of MIBG such as 77/76Bromo-, 18Fluoro-, and 211Astato-labeled benzylguanidines to accumulate in neuroendocrine tumors.11, 12, 13 The ability of this transporter (NET) to recognize and transport these various halogenated benzylguanidines offers several opportunities for medical imaging and radiotherapy applications.

Most intriguing may be the use of the α-particle emitting analogue meta-[211At]astatobenzylguanidine ([211At]MABG)13 for the treatment of neuroendocrine cancers in conjunction with [131I]MIBG, particularly in the treatment of micrometastatic lesions that are often associated with neuroblastoma. β-Particles of 131I have a range in tissue that is longer than the dimensions of neuroblastoma metastases. Because of this, the fraction of radiation dose from β-particles that is deposited in these metastatic tumors will be considerably less.14 α-Particles, on the other hand, have a range in tissue of only a few cell diameters and are more appropriate for the targeted radiotherapy of smaller tumors. Also, because of their high energy and short path length, α-particles are radiations of high linear energy transfer (LET). The LET of 211At α-particles is about 100 keV/μm, a value at which the relative biological effectiveness of ionizing radiation is the highest.15 Astatine-211 has a half-life of 7.2 h and decays by a double-branched pathway, producing one α-particle as a consequence of each decay.16 Its physical half-life is well matched with the biological half-life of small molecular weight pharmaceuticals such as MIBG. Furthermore, being a heavy halogen, it is often facile to label organic compounds with 211At using aromatic tin intermediates.

For these reasons, we have developed a synthesis of [211At]MABG13 and it has been demonstrated that the mechanism for cell uptake and the tissue distribution of [211At]MABG are quite similar to [131I]MIBG but the α-particle emitting analogue is considerably more cytotoxic to human tumor cells that over-express NET.13, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26

One of the major impediments to the clinical evaluation of [211At]MABG is that the labeling method developed for this compound is not ideal for large-scale production, in part, because it involves an HPLC purification step. Although the method provided [211At]MABG in high yield, only <1 mCi had been synthesized at a time. Because this method would not be practical for the synthesis of [211At]MABG at higher radioactivity levels on a routine basis, a simpler method was sought before getting regulatory approval to initiate clinical evaluation of [211At]MABG as a cancer therapeutic. Our goal was to develop a simple, high yielding procedure that could be adapted to kit formulation in a hospital radiopharmacy.

Solid-phase organic synthesis has become a powerful tool for the syntheses of a variety of chemicals and drug molecules.27 Originally developed by Merrifield for the synthesis of peptides,28 this methodology has several advantages over solution-phase techniques including ease of product isolation via simple filtration, the ability to use large excess of reagents to drive the reaction to completion, and amenability to automation. The speed and simplicity of solid-phase organic synthesis is appealing for radiopharmaceutical applications, especially for those involving high levels of radioactivity.

A very few examples of solid-phase synthesis of radiopharmaceuticals exist. Of particular interest to [211At]MABG synthesis is a work reported for the synthesis of no-carrier-added [I]MIBG in high yields from poly-(3-{dibutyl[2-(3-and-4-vinylphenyl)ethyl]stannyl}benzylguanidinium acetate)-co-divinylbenzene (tin precursor; Fig. 1).29 Herein, we have adapted this strategy for the synthesis of [211At]MABG utilizing a procedure that is amenable to kit formulation. To date, up to 9 mCi of [211At]MABG in a form suitable for patient administration has been produced using this approach.

Section snippets

Results and discussion

The availability of a practical procedure for radiopharmaceutical preparation, especially for molecules tagged with short-lived radionuclides such as 211At at high radioactivity levels, via a kit method should greatly facilitate their clinical evaluation. With this goal in mind, we have evaluated the synthesis of [211At]MABG from a solid-supported tin precursor (Fig. 1) that was originally developed for the synthesis of no-carrier-added [I]MIBG.29

The conditions used by Hunter and Zhu for the

Conclusions

In summary, we have developed a method for the synthesis of relatively high amounts of [211At]MABG in good radiochemical yields from a tin precursor that was anchored to a solid support. This method is adaptable to a kit formulation and the quality control characteristics of the final dose are consistent with those appropriate for clinical studies.

General

All chemicals were purchased from Sigma–Aldrich unless otherwise noted. A tin precursor anchored to a polystyrene matrix (1; Fig. 1) was synthesized as reported before.29 High-pressure liquid chromatography was performed using a Beckman System Gold HPLC equipped with a Model 126 programmable solvent module, a Model 166 NM variable wavelength detector (a wavelength of 254 nm was used for all runs), a Model 170 radioisotope detector, and a Beckman System Gold remote interface module SS420X; data

Acknowledgments

This work was supported by Grants CA42324 and CA 93371 from the National Institutes of Health, a Grant DE-FG05-05ER63963 from the Department of Energy, and a Grant from the Pediatric Brain Tumor Foundation.

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