Radiohalogenation of proteins: An overview of radionuclides, labeling methods and reagents for conjugate labeling

Abstract

Direct labeling of proteins with radionuclides of iodine will continue to be the method of choice to answer questions addressed in many future studies. However, it seems likely that a increasing number of applications of radiohalogenated proteins will require, or benefit from, conjugate labeling. While many radiohalogen conjugates have been studied, a large proportion of them have only undergone preliminary studies to date, leaving a question of their overall utility. Phenolic conjugates give good radioiodination labeling yields, but mixtures of radiohalogenated products and problems with in vivo stability can be expected. This fact, along with the fact that phenolic compounds do not have a general application to radiohalogens, makes them less attractive than other alternatives. Radiohalogen labeling through the use of organometallic intermediates has proven to be facile, resulting in high yields of high specific activity labeled small-molecule conjugates. Although the choice of which organometallic intermediate to use may depend somewhat on the radionuclide employed, arylstannanes appear to have the most general applicability. Fluorine-18 labeling of small-molecule conjugates has been best accomplished by ipso aromatic nucleophilic substitution (exchange) reactions. Radiohalogenated small molecules have been prepared which can be conjugated with specific functional groups (e.g. amines, sulfhydryl groups, and carbohydrates) or conjugated nonspecifically with groups in the proximity of the conjugate when it is photolyzed. On the basis of previous studies, good conjugation yields (i.e. 60-90%) can be expected for reactions with specific groups, whereas low yields (i.e. 1-5%) can be expected for conjugations with reactive nitrenes and carbenes. However, recent developments in the chemistry of conjugates that produce nitrenes and carbenes will likely improve the radiolabeling yields. There have been too few comparative studies to readily assess which is the best approach to take when beginning a study involving radiohalogenation of a protein or peptide. However, it is clear that radiohalogenated conjugates of proteins can offer an advantage over direct labeling in that conjugates may be designed which provide some control over in vivo stability and secondary distribution of metabolites. Conjugates can be prepared which are designed to utilize in vivo biochemical processes to release a radiohalogenated small molecule from a tissue (i.e. kidney or liver) or retain the radioactivity at the target tissue (e.g. tumor). Aside from the designing of conjugates with linking molecules for desired biological effects, the ultimate future goal for the radiolabeling chemical should be to prepare protein conjugates which can be radiohalogenated in a single one-step procedure.

18F-labeled mini-PEG spacered RGD dimer (18F-FPRGD2): synthesis and microPET imaging of αvβ3 integrin expression

“…For 18 F labeling through the amino group at the N terminus or the lysine side chain, oxime formation and reductive amination using 4-18 F-fluorobenzaldehyde ( 18 F-FBA) [33,34], imidation reaction using 3-18 F-fluoro-5-nitrobenzimidate ( 18 F-FNB) [35], photochemical conjugation using 4-azidophenacyl 18 F-fluoride ( 18 F-APF) [36], and alkylation reactions using 4-18 Ffluorophenacyl bromide ( 18 F-FPB) [35] have been reported previously. 18 F labeling of peptide or protein via the carboxylic acid group at the C terminus or glutamic/ aspartic acid side chain is less common and only a few reports exist [37]. We have previously reported the thiol-reactive synthon for thiolated RGD peptide labeling [13].…”

Tissue Distribution Studies of Protein Therapeutics Using Molecular Probes: Molecular Imaging

“…One of the main attractions of molecular imaging with radioiodine labels is the already widespread use of iodinated proteins, including antibodies (45,46), minimizing the need to develop and characterize novel reagents. Iodinated proteins are commonly used for measuring biodistribution through autoradiography (32,47) and in biochemical experiments such as ELISA (48) and competitive binding assays (49).…”

Cancer Therapy With Radiolabeled Antibodies An Overview

“…the most important factor is the chemical stability of the conjugate in vivo. Recently, a comprehensive review of procedures for radiohalogenation (42) and the radiochemistry of therapeutic radionuclides has been published (43).…”