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

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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 (211)At, a 7.2-h half-life α-emitter, if appropriate labeling chemistry can be devised to trap (211)At in cancer cells after sdAb binding. This study evaluated two reagents, [(211)At]SAGMB and iso-[(211)At]SAGMB, for this purpose. METHODS: [(211)At]SAGMB and iso-[(211)At]SAGMB, and their radioiodinated analogues [(131)I]SGMIB and iso-[(131)I]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 [(131)I]SGMIB-5F7/[(211)At]SAGMB-5F7 and iso-[(131)I]SGMIB-5F7/iso-[(211)At]SAGMB-5F7 tandems. The biodistribution of the two tandems was evaluated in SCID mice with subcutaneous BT474M1 xenografts. RESULTS: Radiochemical yields for Boc2-iso-[(211)At]SAGMB and Boc2-[(211)At]SAGMB synthesis, and efficiencies for coupling of iso-[(211)At]SAGMB and [(211)At]SAGMB to 5F7 were similar, with radiochemical purities of [(211)At]SAGMB-5F7 and iso-[(211)At]SAGMB-5F7 >98%. iso-[(211)At]SAGMB-5F7 and [(211)At]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 (211)At- and (131)I-labeled 5F7 conjugates. Higher BT474M1 intracellular retention was observed from 1-6 h for the iso-conjugates (iso-[(211)At]SAGMB-5F7, 74.3 ± 2.8%, vs. [(211)At]SAGMB-5F7, 63.7 ± 0.4% at 2 h) with the opposite behavior observed at 24 h. Peak tumor uptake for iso-[(211)At]SAGMB-5F7 was 23.4 ± 2.2% ID/g at 4 h, slightly lower than its radioiodinated counterpart, but significantly higher than observed with [(211)At]SAGMB-5F7. Except in kidneys and lungs, tumor-to-normal organ ratios for iso-[(211)At]SAGMB-5F7 were greater than 10:1 by 2 h, and significantly higher than those for [(211)At]SAGMB-5F7. CONCLUSION: These (211)At-labeled sdAb conjugates, particularly iso-[(211)At]SAGMB-5F7, warrant further evaluation for targeted α-particle radiotherapy of HER2-expressing cancers.





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Choi, Jaeyeon, Ganesan Vaidyanathan, Eftychia Koumarianou, Choong Mo Kang and Michael R Zalutsky (2017). Astatine-211 labeled anti-HER2 5F7 single domain antibody fragment conjugates: radiolabeling and preliminary evaluation. Nucl Med Biol, 56. pp. 10–20. 10.1016/j.nucmedbio.2017.09.003 Retrieved from

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Ganesan Vaidyanathan

Professor Emeritus in Radiology

Dr. Vaidyanathan is a professor in the Department of Radiology.  He is a member of the Nuclear Medicine track of the Medical Physics Graduate Program.  His research involves development of radiopharmaceuticals especially for oncologic applications.  Some of the projects he is involved in are given below.

I.          New methods of radiohalogenating antibodies and its variants 

a) Development of newer residualizing agents for the radiohalogenation of internalizing monoclonal antibodies.

b)  Development of fluorine-18 labeled residualizing agents for labeling nanobodies.

c) Pre-targeting approach via bioorthogonal chemistry for in vivo labeling of antibodies and nanobodies with 18F and 211At.

d)  Methods to label antibodies pre-conjugated with a prosthetic group of the tin precursor of residualizing agents.

e) Multimodal prosthetic groups for labeling antibodies and peptides with multiple radioisotopes.

II.         MIBG Analogs for PET imaging

Radioiodinated MIBG is used in the diagnosis of the pathophysiology of the heart as well as neuroendocrine tumors such as neuroblastoma (NB).  Design and development of newer fluorine-18 labeled MIBG analogues useful in the PET imaging of NB as well as that of myocardial diseases.

III. Noninvasive Imaging of Alkylguanine-DNA alkyltransferase (AGT) 

AGT is a DNA repair protein and is primarily responsible for drug resistance in alkylator chemotherapy. An inverse correlation has been established between the tumor AGT content and the therapeutic outcome. The amount of AGT varies from tumor to tumor and within a group of patients of similar cancer. Thus, it is important to quantify tumor AGT of individual patients before administering alkylator chemotherapy. Our goal is to develop radiolabeled agents with which AGT can be quantified in a noninvasive manner by PET or SPECT imaging. 

IV. PSMA targeting for prostate cancer therapy 

Development of At-211 labeled urea-based inhibitor of Prostate-specific membrane antigen.


Michael Rod Zalutsky

Jonathan Spicehandler, M.D. Distinguished Professor of Neuro Oncology, in the School of Medicine

The overall objective of our laboratory is the development of novel radioactive compounds for improving the diagnosis and treatment of cancer. This work primarily involves radiohalo-genation of biomolecules via site-specific approaches, generally via demetallation reactions. Radionuclides utilized for imaging include I-123, I-124 and F-18, the later two being of particular interest because they can be used for the quantification of biochemical and physiological processes in the living human through positron emission tomography. For therapy, astatine-211 decays by the emission of alpha-particles, a type of radiation considerably more cytotoxic that the beta-particles used in conventional endoradiotherapy. The range of At-211 alpha particles is only a few cell diameters, offering the possibility of extremely focal irradiation of malignant cells while leaving neighboring cells intact. Highlights of recent work include: a)
development of reagents for protein and peptide radioiodination that decrease deiodination in vivo by up to 100-fold, b) demonstration that At-211 labeled monoclonal antibodies are effective in the treatment of a rat model of neoplastic meningitis, c) synthesis of a thymidine analogue labeled with At-211 and the demonstration that this molecule is taken up in cellular DNA with highly cytotoxicity even at levels of only one atom bound per cell and d) development of
radiohalobenzylguanidines which are specifically cytotoxic for human neuroblastoma cells.

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