A New Strategy for Preparing Molecular Imaging and Therapy Agents Using Fluorine-Rich (Fluorous) Soluble Supports

Synthesis of Radioiodinated Compounds. Classical Approaches and Achievements of Recent Years

This review demonstrates the progress in the synthesis of radioiodinated compounds over the past decade. The possibilities and limitations of radiopharmaceuticals with different iodine isotopes, as well as the synthesis of low and high molecular weight compounds containing radioiodine, are discussed. An analysis of synthesis strategies, substrate frameworks, isolation methods, and metabolic stability, and the possibility of industrial production of radioiodinated organic derivatives which can find applications in the synthesis of drugs and diagnostics are presented.

Recent Advances in Synthetic Methods for Radioiodination

Organic compounds bearing radioisotopes of iodine are widely used for biological research, diagnostic imaging, and radiotherapy. Early reported synthetic methods for the incorporation of radioiodine have generally involved high temperature reactions or strongly oxidizing conditions. To overcome these limitations and to cope with the demand for novel radioiodinated probes, there has been a surge in the development of new synthetic methodology for radioiodination. This synopsis describes the key transformations developed recently.

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

Ionic liquid supported organotin reagents to prepare molecular imaging and therapy agents

Efficiency of ionic liquid supported organotin reagents in halodemetalation reaction has been investigated. High radiochemical yields of astatinated and iodinated compounds have been obtained using simple work-up procedure. This methodology represents a straightforward approach for the preparation of molecular imaging and therapy agents in nuclear medicine.

Solid-supported reagents composed of a copolymer possessing 2-O-sulfonyl mannosides and phase-transfer catalysts for the synthesis of 2-fluoroglucose

We described the synthesis of a solid-supported co-polymer possessing mannosides and phase-transfer catalysts and synthesis of 2-fluoroglucoside from it. We first prepared a soluble copolymer from two allene monomers possessing a precursor for the synthesis of 2-fluoroglycose and a crown ether. The copolymerization of the monomers via the π-ally nickel-catalyst smoothly proceeded at room temperature to provide a desired copolymer without decomposition of the sulfonate esters. The copolymer exhibited high reactivity towards fluorination in comparison with a conventional precursor. We next synthesized the solid-supported copolymer by using the solid-supported initiator attached with TentaGel® resins. TentaGel® enabled polymerization under stirring with stirring bar without decomposition. The solid-supported copolymer exhibited comparable reactivity towards fluorination in comparison with the soluble copolymer. In addition, it can be easily separated from the reaction vessel by filtration.

Development and Preliminary Evaluation of TFIB, a New Bimodal Prosthetic Group for Bioactive Molecule Labeling

The new readily available prosthetic group, tetrafluorophenyl 4-fluoro-3-iodobenzoate (TFIB), designed for both molecular imaging and targeted radionuclide therapy purposes was radiolabeled either with fluorine or iodine radionuclides with excellent radiochemical yields and purities. These radiolabeled tags were conjugated to N,N-diethylethylenediamine to give melanin-targeting radiotracers [ (125) I]9 and [ (18) F]9, which were successfully evaluated by PET and gamma scintigraphic imaging in B16F0 pigmented melanoma-bearing C57BL/6J mice. Then, radiolabeled [ (125) I]/[ (18) F]TFIB was used to tag tumor-targeting peptides (i.e., PEG3[c(RGDyK)]2 and NDP-MSH targeting αvβ3 integrin and MC1R receptors, respectively) in mild conditions and with good radiochemical yields (47-83% d.c.) and purities (>99%). The resulting radiolabeled peptides were assessed both in vitro and by PET imaging in animal models.

Resin-supported arylstannanes as precursors for radiolabeling with iodine: benzaldehydes, benzoic acids, benzamides, and NHS esters

A highly cross-linked polystyrene resin bearing a reactive chlorostannane moiety 1 has been used to generate a variety of arylstannane radiopharmaceutical precursors for no-carrier-added radioiodination. The resins were characterized for their solvent compatibility and sensitivity to acid cleavage. Resin-supported arylstannanes synthesized via their aryllithium analogues include 3- and 4-stannylbenzaldehydes, 3- and 4-stannylbenzoic acids, and 3- and 4-N-succinimidyl benzoates. A three-step route to the resin-supported stannylbenzoic acids 12a/b was developed through resin-supported benzaldehydes 11a/b. The aldehyde to acid conversion efficiency is >90%, and acid loading capacities of 0.66–0.94 mmol/g were obtained. Resin-supported N-succinimidyl benzoates 16a/b were prepared from the acid with 78%–84% conversion efficiency. Libraries of resin-supported benzamides 19a/b prepared from amine conjugation to corresponding benzoic acids or N-succinimidyl benzoates are described. A third approach describes the preparation of resin-supported benzamides via direct conjugation of the dilithio salt of the intact benzamide to the chlorostannane resin 1. Lastly, as proof-of-principle, a radiolabeling study with iodine-131 (131I) was performed with a resin-supported benzamide to afford the corresponding radioligand in moderate yields, and high radiochemical purity.

Isotope incorporation using organoboranes

Isotopes have played an important role in chemistry, biology, and medicine. For the last three decades, we have focused on the use of organoboron compounds as precursors to isotopically labeled physiologically active reagents. During that period, we have successfully developed methods for incorporating short- and long-lived isotopes of carbon, nitrogen, oxygen, and the halogens using a variety of reactive organoboron precursors. In addition, labeling strategies employing polymer-supported organoboron derivatives were developed. In this report, we present a short overview focused on the evolution of radiolabeling techniques based on boron chemistry.

Triazole Appending Agent (TAAG): A New Synthon for Preparing Iodine-Based Molecular Imaging and Radiotherapy Agents

A new prosthetic group referred to as the triazole appending agent (TAAG) was developed as a means to prepare targeted radioiodine-based molecular imaging and therapy agents. Tributyltin-TAAG and the fluorous analogue were synthesized in high yield using simple click chemistry and the products labeled in greater than 95% RCY with (123)I. A TAAG derivative of an inhibitor of prostate-specific membrane antigen was prepared and radiolabeled with (123)I in 85% yield where biodistribution studies in LNCap prostate cancer tumor models showed rapid clearance of the agent from nontarget tissues and tumor accumulation of 20% injected dose g(-1) at 1 h. The results presented demonstrate that the TAAG group promotes minimal nonspecific binding and that labeled conjugates can achieve high tumor uptake and exquisite target-to-nontarget ratios.

Polymer-supported organotin reagent for prosthetic group labeling of biological macromolecules with radioiodine

In this study, we investigated the use of poly-mer-bound precursor for generating a radiolabeled prosthetic group to be used for conjugate labeling of biological macromolecules. For the approach, a trialkyltin chloride in which the tin was bound to a hydrophilic PEG-based resin support via one of the alkyl groups was synthesized. This resin was then used to prepare a resin-bound trialkyltin benzoic acid, which in some cases was further derivatized on-resin by converting it to a succinimidyl ester. Exposure of the resin-bound compounds to electrophilic radioiodine (¹²⁵I) in either an aqueous or methanol solvent liberated either free radiolabeled [¹²⁵I]iodobenzoic acid or its succinimidyl ester without co-release of the resin-bound precursors. Radiochemical yield was between 35% and 75%, depending on the solvent system and precursor. As example applications for the released compounds, the amine-reactive N-succinimidyl-[¹²⁵I]iodobenzoate prosthetic group was used for conjugate radiolabeling of a peptide, tomato plant systemin, and two proteins, albumin and IgG antibody. These results demonstrate that resin-bound organotin precursors in which the compound to be labeled is tethered to the support via the tin group to be substituted can be used to produce radioiodine-labeled aromatic prosthetic groups in good specific activity without the need for HPLC purification. This solid-phase approach is potentially adaptable to kit-formulation for performing conjugate radiolabeling of biological macromolecules.

Synthesis and reactivity of [18F]-N-fluorobenzenesulfonimide

A novel [18F]NF reagent and two novel radiochemical transformations have been developed: [18F]NFSi has been prepared from sodium dibenzenesulfonimide and reacted in the presence of silyl enol ethers and allylsilanes to deliver labelled fluorinated ketones and allylic fluorides respectively; the radiosynthesis of the fluorinated A ring of vitamin D3 has also been completed with success.