Organofluorosilanes as Model Compounds for18F-Labeled Silicon-Based PET Tracers and their Hydrolytic Stability: Experimental Data and Theoretical Calculations (PET=Positron Emission Tomography)

CycloSiFA: The Next Generation of Silicon-Based Fluoride Acceptors for Positron Emission Tomography (PET)

The ring-opening Si-fluorination of a variety of azasilole derivatives cyclo-1-(iPr2 Si)-4-X-C6 H3 -2-CH2 NR (4: R=2,6-iPr2 C6 H3 , X=H; 4 a: R=2,4,6-Me3 C6 H2 , X=H; 9: R=2,6-iPr2 C6 H3 , X=tBuMe2 SiO; 10: R=2,6-iPr2 C6 H3 , X=OH; 13: R=2,6-iPr2 C6 H3 , X=HCCCH2 O; 22: R=2,6-iPr2 C6 H3 , X=tBuMe2 SiCH2 O) with different 19 F-fluoride sources was studied, optimized and the experience gained was used in a translational approach to create a straightforward 18 F-labelling protocol for the azasilole derivatives [18 F]6 and [18 F]14. The latter constitutes a potential clickable CycloSiFA prosthetic group which might be used in PET tracer development using Cu-catalysed triazole formation. Based on our findings, CycloSiFA has the potential to become a new entry into non-canonical labelling methodologies for radioactive PET tracer development.

An Investigation into the In Vitro Metabolic Stability of Aryl Sulfonyl Fluorides for their Application in Medicinal Chemistry and Radiochemistry

Molecules that feature a sulfonyl fluoride (SO₂F) moiety have been gaining increasing interest due to their unique reactivity and potential applications in synthetic chemistry, medicinal chemistry, and other biological uses. A particular interest is towards ¹⁸F-radiochemistry where sulfonyl fluorides can be used as a method to radiolabel biomolecules or can be used as radiofluoride relay reagents that facilitate radiolabeling of other molecules. The low metabolic stability of sulfonyl fluoride S-F bonds, however, presents an issue and limits the applicability of sulfonyl fluorides. The aim of this work was to increase understanding of what features contribute to the metabolic instability of the S-F bond in model aryl sulfonyl fluorides and identify approaches to increasing sulfonyl fluoride stability for ¹⁸F-radiochemistry and other medicinal, synthetic chemistry and biological applications. To undertake this, 14 model aryl sulfonyl fluorides compounds with varying functional groups and substitution patterns were investigated, and their stabilities were examined in various media, including phosphate-buffered saline and rat serum as a model for biological conditions. The results indicate that both electronic and steric factors affect the stability of the S-F bond, with the 2,4,6-trisubstituted model aryl sulfonyl fluorides examined displaying the highest in vitro metabolic stability.

Reaction of [18F]Fluoride at Heteroatoms and Metals for Imaging of Peptides and Proteins by Positron Emission Tomography

The ability to radiolabel proteins with [18F]fluoride enables the use of positron emission tomography (PET) for the early detection, staging and diagnosis of disease. The direct fluorination of native proteins through C-F bond formation is, however, a difficult task. The aqueous environments required by proteins severely hampers fluorination yields while the dry, organic solvents that promote nucleophilic fluorination can denature proteins. To circumvent these issues, indirect fluorination methods making use of prosthetic groups that are first fluorinated and then conjugated to a protein have become commonplace. But, when it comes to the radiofluorination of proteins, these indirect methods are not always suited to the short half-life of the fluorine-18 radionuclide (110 min). This review explores radiofluorination through bond formation with fluoride at boron, metal complexes, silicon, phosphorus and sulfur. The potential for these techniques to be used for the direct, aqueous radiolabeling of proteins with [18F]fluoride is discussed.

Metabolism of a Bioorthogonal PET Tracer Candidate [19F/18F]SiFA-Tetrazine in Mouse Liver Microsomes: Biotransformation Pathways and Defluorination Investigated by UHPLC-HRMS

Organofluorosilicon based ¹⁸F-radiolabeling is an efficient method for incorporating fluorine-18 into ¹⁸F-radiopharmaceuticals for positron emission tomography (PET) by ¹⁹F/¹⁸F isotopic exchange (IE). The first PET radiopharmaceutical, ¹⁸F-SiFAlin-TATE, radiolabeled with a silicon-based [¹⁸F]fluoride acceptor (SiFA), namely, a para-substituted di-tert-butyl[¹⁸F]fluorosilylbenzene, has entered clinical trials, and is paving the way for other potential [¹⁸F]SiFA-labeled radiopharmaceuticals for diagnostic use. In this study, we report the in vitro metabolism of an oxime-linked SiFA tetrazine (SiFA–Tz), a new PET-radiotracer candidate, recently evaluated for pretargeted PET imaging and macromolecule labeling. Metabolism of SiFA–Tz was studied in mouse liver microsomes (MLM) for elucidating its major biotransformation pathways. Nontargeted screening by ultrahigh performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) was utilized for detection of unknown metabolites. The oxime bond between the SiFA and Tz groups forms two geometric (E/Z) isomers, which underwent the same biotransformations, but unexpectedly with different kinetics. In total, nine proposed metabolites of SiFA–Tz from phase I and II reactions were detected, five of which were defluorinated in MLMs, elucidating the metabolic pathway leading to previously reported defluorination of [¹⁸F]SiFA–Tz in vivo. Based on the HRMS studies a biotransformation pathway is proposed: hydroxylation (+O) to tert-butyl group adjacent to the silicon, followed by oxidative defluorination (+OH/-F) cleaving the fluorine off the silicon. Interestingly, eight proposed metabolites of a reduced dihydrotetrazine analogue, SiFA–H₂Tz, from phase I and II reactions were additionally detected. To the best of our knowledge, this is the first reported comprehensive investigation of enzyme mediated metabolic pathway of tetrazines and para-substituted di-tert-butylfluorosilylbenzene fluoride acceptors, providing novel structural information on the biotransformation and fragmentation patterns of radiotracers bearing these structural motifs. By investigating the metabolism preceding defluorination, structurally optimized new SiFA compounds can be designed for expanding the portfolio of efficient ¹⁹F/¹⁸F isotopic exchange labeling probes for PET imaging.

Copper-Catalyzed Oxidative Acetalization of Boronic Esters: An Umpolung Strategy for Cyclic Acetal Synthesis

A protocol for the acetalization of boronic esters is described. The reaction is catalyzed by copper, and the conditions proved to be mild and were amenable to a variety of functional groups. We expanded the Chan-Lam coupling to include C(sp³) nucleophiles and converted them into corresponding acetals. This method allows for the orthogonal acetalization of substrates with reactive, acid-sensitive functional groups.

Radiohybrid Ligands: A Novel Tracer Concept Exemplified by 18F- or 68Ga-Labeled rhPSMA Inhibitors

When we critically assess the reason for the current dominance of 68Ga-labeled peptides and peptide-like ligands in radiopharmacy and nuclear medicine, we have to conclude that the major advantage of such radiopharmaceuticals is the apparent lack of suitable 18F-labeling technologies with proven clinical relevance. To prepare and to subsequently perform a clinical proof-of-concept study on the general suitability of silicon-fluoride-acceptor (SiFA)-conjugated radiopharmaceuticals, we developed inhibitors of the prostate-specific membrane antigen (PSMA) that are labeled by isotopic exchange (IE). To compensate for the pronounced lipophilicity of the SiFA unit, we used metal chelates, conjugated in close proximity to SiFA. Six different radiohybrid PSMA ligands (rhPSMA ligands) were evaluated and compared with the commonly used 18F-PSMA inhibitors 18F-DCFPyL and 18F-PSMA-1007. Methods: All inhibitors were synthesized by solid-phase peptide synthesis. Human serum albumin binding was measured by affinity high-performance liquid chromatography, whereas the lipophilicity of each tracer was determined by the n-octanol/buffer method. In vitro studies (IC50, internalization) were performed on LNCaP cells. Biodistribution studies were conducted on LNCaP tumor-bearing male CB-17 SCID mice. Results: On the laboratory scale (starting activities, 0.2-9.0 GBq), labeling of 18F-rhPSMA-5 to -10 by IE was completed in < 20 min (radiochemical yields, 58% ± 9%; radiochemical purity, >97%) with molar activities of 12-60 GBq/μmol. All rhPSMAs showed low nanomolar affinity and high internalization by PSMA-expressing cells when compared with the reference radiopharmaceuticals, medium-to-low lipophilicity, and high human serum albumin binding. Biodistribution studies in LNCaP tumor-bearing mice revealed high tumor uptake, sufficiently fast clearance kinetics from blood, low hepatobiliary excretion, fast renal excretion, and very low uptake of 18F activity in bone. Conclusion: The novel 18F-rhPSMA radiopharmaceuticals developed under the radiohybrid concept show equal or better targeting characteristics than the established 18F-PSMA tracers 18F-DCFPyL and 18F-PSMA-1007. The unparalleled simplicity of production, the possibility to produce the identical 68Ga-labeled 19F-68Ga-rhPSMA tracers, and the possibility to extend this concept to true theranostic radiohybrid radiopharmaceuticals, such as F-Lu-rhPSMA, are unique features of these radiopharmaceuticals.

Highly hindered 2-(aryl-di-tert-butylsilyl)-N-methyl-imidazoles: a new tool for the aqueous 19F- and 18F-fluorination of biomolecule-based structures

A new class of silicon-based fluoride acceptors with a C-linked heterocycle as the leaving group was synthesized in one step from commercial chemicals, and linked to biomolecules. The resulting conjugates were efficiently 19F-fluorinated in aqueous mixtures, and switching to 18F-labelling provided nucleoside- and peptide-based bioconjugates with excellent molar activities suitable for biological applications.

Crown Ether Nucleophilic Catalysts (CENCs): Agents for Enhanced Silicon Radiofluorination

New bifunctional phase transfer agents were synthesized and investigated for their abilities to promote rapid fluorination at silicon. These agents, dubbed crown ether nucleophilic catalysts (CENCs), are 18-crown-6 derivatives containing a side-arm and a potentially nucleophilic hydroxyl group. These CENCs proved efficacious in the fluorination of hindered silicon substrates, with fluorination yields dependent on the length of linker connecting the metal chelating unit to the hydroxyl group. The efficacy of these CENCs was also demonstrated for rapid radiofluorination under mild conditions for eventual application in molecular imaging with positron emission tomography (PET). The hydrolysis-resistant aryl silicon fragment is promising as a convenient synthon for labeling potential PET radiotracers.

Exclusively Gas-Phase Passivation of Native Oxide-Free Silicon(100) and Silicon(111) Surfaces

Reactions in the gas phase are of primary technological importance for applications in nano- and microfabrication technology and in the semiconductor industry. We present exclusively gas-phase protocols to chemically passivate oxide-free Si(111) and Si(100) surfaces with short-chain alkynes. The resulting surfaces showed equal or better oxidation resistance than most existing liquid-phase-derived surfaces and rivaled the outstanding stability of a full-coverage Si(111)-propenyl surface.1,2 The most stable surface (Si(111)-ethenyl) grew one-fifth of a monolayer of oxide (0.04 nm) after 1 month of air exposure. We monitored the regrowth of oxides on passivated Si(111) and Si(100) surfaces by X-ray photoelectron spectroscopy (XPS) and observed a significant crystal-orientation dependence of initial rates when total oxide thickness was below approximately one monolayer (0.2 nm). This difference was correlated with the desorption kinetics of residual surface Si-F bonds formed during HF treatment. We discuss applications of the technology and suggest future directions for process optimization.

From Unorthodox to Established: The Current Status of (18)F-Trifluoroborate- and (18)F-SiFA-Based Radiopharmaceuticals in PET Nuclear Imaging

Unorthodox (18)F-labeling strategies not employing the formation of a carbon-(18)F bond are seldom found in radiochemistry. Historically, the formation of a boron- or silicon-(18)F bond has been introduced very early on into the repertoire of labeling chemistries, but is without translation into any clinical radiotracer besides inorganic B[(18)F]F4(-) for brain tumor diagnosis. For many decades these labeling methodologies were forgotten and have just recently been revived by a handful of researchers thinking outside the box. When breaking with established paradigms such as the inability to obtain labeled compounds of high specific activity via isotopic exchange or performing radiofluorination in aqueous media, the research community often reacts skeptically. In 2005 and 2006, two novel labeling methodologies were introduced into radiochemistry for positron emission tomography (PET) tracer development: RBF3(-) labeling reported by Perrin et al. and the SiFA methodology by Schirrmacher, Jurkschat, and Waengler et al. which is based on isotopic exchange (IE). Both labeling methodologies have been complemented by other noncanonical strategies to introduce (18)F into biomolecules of diagnostic importance, thus profoundly enriching the landscape of (18)F radiolabeling. B- and Si-based labeling strategies finally revealed that IE is a viable alternative to established and traditional radiochemistry with the advantage of simplifying both the labeling effort as well as the necessary purification of the radiotracer. Hence IE will be the focus of this contribution over other noncanonical labeling methods. Peptides for tumor imaging especially lend themselves favorably toward one-step labeling via IE, but small molecules have been described as well, taking advantage of these new approaches, and have been used successfully for brain imaging. This Review gives an account of both radiochemistries centered on boron and silicon, describing the very beginnings of their basic research, the path that led to optimization of their chemistries, and the first encouraging preclinical results paving the way to their clinical use. This side by side approach will give the reader the opportunity to follow the development of a new basic discovery into a clinically applicable radiotracer including all the hurdles that have had to be overcome.

[(18)F]-Group 13 fluoride derivatives as radiotracers for positron emission tomography

The field of (18)F chemistry is rapidly expanding because of the use of this radionuclide in radiotracers for positron emission tomography (PET). Until recently, most [(18)F]-radiotracers were generated by the direct attachment of (18)F to a carbon in the organic backbone of the radiotracer. The past decade has witnessed the emergence of a new strategy based on the formation of an (18)F-group 13 element bond. This approach, which is rooted in the field of fluoride anion complexation/coordination chemistry, has led to the development of a remarkable family of boron, aluminium and gallium [(18)F]-fluoride anion complexing agents which can be conjugated with peptides and small molecules to generate disease specific PET radiotracers. This review is dedicated to the chemistry of these group 13 [(18)F]-fluorides anion complexing agents and their use in PET. Some of the key fluoride-binding motifs covered in this review include the trifluoroborate unit bound to neutral or cationic electron deficient backbones, the BF2 unit of BODIPY dyes, and AlF or GaF3 units coordinated to multidentate Lewis basic ligands. In addition to describing how these moieties can be converted into their [(18)F]-analogs, this review also dicusses their incorporation into bioconjugates for application in PET.

Methods to Increase the Metabolic Stability of (18)F-Radiotracers

The majority of pharmaceuticals and other organic compounds incorporating radiotracers that are considered foreign to the body undergo metabolic changes in vivo. Metabolic degradation of these drugs is commonly caused by a system of enzymes of low substrate specificity requirement, which is present mainly in the liver, but drug metabolism may also take place in the kidneys or other organs. Thus, radiotracers and all other pharmaceuticals are faced with enormous challenges to maintain their stability in vivo highlighting the importance of their structure. Often in practice, such biologically active molecules exhibit these properties in vitro, but fail during in vivo studies due to obtaining an increased metabolism within minutes. Many pharmacologically and biologically interesting compounds never see application due to their lack of stability. One of the most important issues of radiotracers development based on fluorine-18 is the stability in vitro and in vivo. Sometimes, the metabolism of ¹⁸F-radiotracers goes along with the cleavage of the C-F bond and with the rejection of [¹⁸F]fluoride mostly combined with high background and accumulation in the skeleton. This review deals with the impact of radiodefluorination and with approaches to stabilize the C-F bond to avoid the cleavage between fluorine and carbon.

18F-Labeled Peptides: The Future Is Bright

Radiolabeled peptides have been the subject of intense research efforts for targeted diagnostic imaging and radiotherapy over the last 20 years. Peptides offer several advantages for receptor imaging and targeted radiotherapy. The low molecular weight of peptides allows for rapid clearance from the blood and non-target tissue, which results in favorable target-to-non-target ratios. Moreover, peptides usually display good tissue penetration and they are generally non-immunogenic. A major drawback is their potential low metabolic stability. The majority of currently used radiolabeled peptides for targeted molecular imaging and therapy of cancer is labeled with various radiometals like 99mTc, 68Ga, and 177Lu. However, over the last decade an increasing number of 18F-labeled peptides have been reported. Despite of obvious advantages of 18F like its ease of production in large quantities at high specific activity, the low β+ energy (0.64 MeV) and the favorable half-life (109.8 min), 18F-labeling of peptides remains a special challenge. The first part of this review will provide a brief overview on chemical strategies for peptide labeling with 18F. A second part will discuss recent technological advances for 18F-labeling of peptides with special focus on microfluidic technology, automation, and kit-like preparation of 18F-labeled peptides.

¹⁸F-labeled silicon-based fluoride acceptors: potential opportunities for novel positron emitting radiopharmaceuticals

BACKGROUND

Over the recent years, radiopharmaceutical chemistry has experienced a wide variety of innovative pushes towards finding both novel and unconventional radiochemical methods to introduce fluorine-18 into radiotracers for positron emission tomography (PET). These "nonclassical" labeling methodologies based on silicon-, boron-, and aluminium-(18)F chemistry deviate from commonplace bonding of an [(18)F]fluorine atom ((18)F) to either an aliphatic or aromatic carbon atom. One method in particular, the silicon-fluoride-acceptor isotopic exchange (SiFA-IE) approach, invalidates a dogma in radiochemistry that has been widely accepted for many years: the inability to obtain radiopharmaceuticals of high specific activity (SA) via simple IE.

METHODOLOGY

The most advantageous feature of IE labeling in general is that labeling precursor and labeled radiotracer are chemically identical, eliminating the need to separate the radiotracer from its precursor. SiFA-IE chemistry proceeds in dipolar aprotic solvents at room temperature and below, entirely avoiding the formation of radioactive side products during the IE.

SCOPE OF REVIEW

A great plethora of different SiFA species have been reported in the literature ranging from small prosthetic groups and other compounds of low molecular weight to labeled peptides and most recently affibody molecules.

CONCLUSIONS

The literature over the last years (from 2006 to 2014) shows unambiguously that SiFA-IE and other silicon-based fluoride acceptor strategies relying on (18)F(-) leaving group substitutions have the potential to become a valuable addition to radiochemistry.

Automated radiosynthesis of N-succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB) for peptides and proteins radiolabeling for positron emission tomography

Recently, silicon fluoride building blocks (SiFA) have emerged as valuable and promising tools to overcome challenges in the labeling of peptides and proteins for positron emission tomography (PET). Herein, we report a fully automated synthesis of N-succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB) by a commercially available Scintomics Hot Box 3 synthesis module, to be used as a prosthetic group for peptide and protein labeling. The drying of K2.2.2./K (18)F complex was performed according to the Munich method modified by our group (avoiding azeotropic drying) using oxalic acid to neutralize the base from the (18)F(-) containing QMA eluent. This K2.2.2./K (18)F complex was then used for SiFA (18)F-(19)F isotopic exchange followed by a fast purification by a solid-phase-extraction (SPE) to afford [(18)F]SiFB with an average preparative radiochemical yield (RCY) of 24±1% (non-decay corrected (NDC)) within a synthesis time of 30 min. The [(18)F]SiFB produced by automated synthesis was then used for the (18)F-labeling of rat serum albumin (RSA) as a proof of applicability.

Enhanced Nucleophilic Fluorination and Radiofluorination of Organosilanes Appended with Potassium-Chelating Leaving Groups

Here we aimed to explore the feasibility of enhancing the fluorination of organosilanes by appending potassium-chelating groups to the substrates. For this purpose, eight organosilanes were prepared in which a linear or cyclic leaving group, with putative potassium-chelating ability, was attached covalently to a congested silicon atom via an ether linkage to serve as a potential nucleophilic assisting leaving group (NALG). Organosilicon-NALGs with expected strong potassium-chelating capability enhanced reactions with potassium fluoride in acetonitrile to produce organofluorosilanes without any need to separately add phase transfer reagent. Similar rate enhancements were also observed with cyclotron-produced [¹⁸F]fluoride ion (t_1/2 = 109.7 min, β⁺ = 97%) in the presence of potassium carbonate in MeCN-0.5% H₂O. This study found that metal-chelating NALG units can accelerate fluorination and radiofluorination reactions at sterically crowded silicon atoms.

Studies toward the development of new silicon-containing building blocks for the direct (18)F-labeling of peptides

Silicon-containing prosthetic groups have been conjugated to peptides to allow for a single-step labeling with (18)F radioisotope. The fairly lipophilic di-tert-butylphenylsilane building block contributes unfavorably to the pharmacokinetic profile of bombesin conjugates. In this article, theoretical and experimental studies toward the development of more hydrophilic silicon-based building blocks are presented. Density functional theory calculations were used to predict the hydrolytic stability of di-tert-butylfluorosilanes 2-23 with the aim to improve the in vivo properties of (18)F-labeled silicon-containing biomolecules. As a further step toward improving the pharmacokinetic profile, hydrophilic linkers were introduced between the lipophilic di-tert-butylphenylsilane building block and the bombesin congeners. Increased tumor uptake was shown with two of these peptides in xenograft-bearing mice using positron emission tomography and biodistribution studies. The introduction of a hydrophilic linker is thus a viable approach to improve the tumor uptake of (18)F-labeled silicon-bombesin conjugates.

Oxalic acid supported Si-18F-radiofluorination: one-step radiosynthesis of N-succinimidyl 3-(di-tert-butyl[18F]fluorosilyl)benzoate ([18F]SiFB) for protein labeling

N-Succinimidyl 3-(di-tert-butyl[(18)F]fluorosilyl)benzoate ([(18)F]SiFB), a novel synthon for one-step labeling of proteins, was synthesized via a simple (18)F-(19)F isotopic exchange. A new labeling technique that circumvents the cleavage of the highly reactive active ester moiety under regular basic (18)F-labeling conditions was established. In order to synthesize high radioactivity amounts of [(18)F]SiFB, it was crucial to partially neutralize the potassium oxalate/hydroxide that was used to elute (18)F(-) from the QMA cartridge with oxalic acid to prevent decomposition of the active ester moiety. Purification of [(18)F]SiFB was performed by simple solid-phase extraction, which avoided time-consuming HPLC and yielded high specific activities of at least 525 Ci/mmol and radiochemical yields of 40-56%. In addition to conventional azeotropic drying of (18)F(-) in the presence of [K(+)⊂2.2.2.]C(2)O(4), a strong anion-exchange (SAX) cartridge was used to prepare anhydrous (18)F(-) for nucleophilic radio-fluorination omitting the vacuum assisted drying of (18)F(-). Using a lyophilized mixture of [K(+)⊂2.2.2.]OH resolubilized in acetonitrile, the (18)F(-) was eluted from the SAX cartridge and used directly for the [(18)F]SiFB synthesis. [(18)F]SiFB was applied to the labeling of various proteins in likeness to the most commonly used labeling synthon in protein labeling, N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB). Rat serum albumin (RSA), apo-transferrin, a β-cell-specific single chain antibody, and erythropoietin were successfully labeled with [(18)F]SiFB in good radiochemical yields between 19% and 36%. [(18)F]SiFB- and [(18)F]SFB-derivatized RSA were directly compared as blood pool imaging agents in healthy rats using small animal positron emission tomography. Both compounds demonstrated identical biodistributions in healthy rats, accurately visualizing the blood pool with PET.

t-Bu2SiF-derivatized D2-receptor ligands: the first SiFA-containing small molecule radiotracers for target-specific PET-imaging

The synthesis, radiolabeling and in vitro evaluation of new silicon-fluoride acceptor (SiFA) derivatized D(2)-receptor ligands is reported. The SiFA-technology simplifies the introduction of fluorine-18 into target specific biomolecules for Positron-Emission-Tomography (PET). However, one of the remaining challenges, especially for small molecules such as receptor-ligands, is the bulkiness of the SiFA-moiety. We therefore synthesized four Fallypride SiFA-conjugates derivatized either directly at the benzoic acid ring system (SiFA-DMFP, SiFA-FP, SiFA-DDMFP) or at the butyl-side chain (SiFA-M-FP) and tested their receptor affinities. We found D(2)-receptor affinities for all compounds in the nanomolar range (K(i(SiFA-DMFP)) = 13.6 nM, K(i(SiFA-FP)) = 33.0 nM, K(i(SiFA-DDMFP)) = 62.7 nM and K(i(SiFA-M-FP)) = 4.21 nM). The radiofluorination showed highest yields when 10 nmol of the precursors were reacted with [(18)F]fluoride/TBAHCO(3) in acetonitrile. After a reversed phased cartridge purification the desired products could be isolated as an injectable solution after only 10 min synthesis time with radiochemical yields (RCY) of more than 40% in the case of SiFA-DMFP resulting in specific activities >41 GBq/µmol (>1,100 Ci/mmol). Furthermore, the radiolabeled products were shown to be stable in the injectable solutions, as well as in human plasma, for at least 90 min.

One-step radiosynthesis of ¹⁸F-AlF-NOTA-RGD₂ for tumor angiogenesis PET imaging

PURPOSE

One of the major obstacles of the clinical translation of (18)F-labeled arginine-glycine-aspartic acid (RGD) peptides has been the laborious multistep radiosynthesis. In order to facilitate the application of RGD-based positron emission tomography (PET) probes in the clinical setting we investigated in this study the feasibility of using the chelation reaction between Al(18)F and a macrocyclic chelator-conjugated dimeric RGD peptide as a simple one-step (18)F labeling strategy for development of a PET probe for tumor angiogenesis imaging.

METHODS

Dimeric cyclic peptide E[c(RGDyK)](2) (RGD(2)) was first conjugated with a macrocyclic chelator, 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), and the resulting bioconjugate NOTA-RGD(2) was then radiofluorinated via Al(18)F intermediate to synthesize (18)F-AlF-NOTA-RGD(2). Integrin binding affinities of the peptides were assessed by a U87MG cell-based receptor binding assay using (125)I-echistatin as the radioligand. The tumor targeting efficacy and in vivo profile of (18)F-AlF-NOTA-RGD(2) were further evaluated in a subcutaneous U87MG glioblastoma xenograft model by microPET and biodistribution.

RESULTS

NOTA-RGD(2) was successfully (18)F-fluorinated with good yield within 40 min using the Al(18)F intermediate. The IC(50) of (19)F-AlF-NOTA-RGD(2) was determined to be 46 ± 4.4 nM. Quantitative microPET studies demonstrated that (18)F-AlF-NOTA-RGD(2) showed high tumor uptake, fast clearance from the body, and good tumor to normal organ ratios.

CONCLUSION

NOTA-RGD(2) bioconjugate has been successfully prepared and labeled with Al(18)F in one single step of radiosynthesis. The favorable in vivo performance and the short radiosynthetic route of (18)F-AlF-NOTA-RGD(2) warrant further optimization of the probe and the radiofluorination strategy to accelerate the clinical translation of (18)F-labeled RGD peptides.

Inorganic approaches for radiolabelling biomolecules with fluorine-18 for imaging with positron emission tomography

Conventional methods for radiolabelling biomolecules such as proteins and peptides with fluorine-18 for PET imaging rely on carbon-fluorine bond formation and are complex and inefficient. Several non-carbon elements form strong bonds (i.e. with high bond enthalpy) with fluorine, but with lower activation energy for their formation compared to carbon-fluorine bonds, whilst preserving a relatively high kinetic stability. In particular, by incorporating boron-, aluminium- and silicon-containing prosthetic groups into biomolecules, promising results have recently been achieved in the radiolabelling with F-18-fluoride under mild aqueous conditions, affording a level of convenience, efficiency and specific activity potentially superior to those offered by conventional C-F bond formation methods. The promise already shown by these early studies heralds a new branch of bioconjugate radiochemistry involving a wider range of "fluoridephilic" elements for synthesis of PET molecular imaging agents.

Silicon-based chemistry: an original and efficient one-step approach to [18F]-nucleosides and [18F]-oligonucleotides for PET imaging

Take it eaSi! Nucleosides, dinucleotides, and one oligonucleotide, all modified by click chemistry, have for the first time been directly and very efficiently labeled with (18)F by using a silicon-based, one-step approach that opens the way for the development of a new class of positron emission tomography (PET) tracers (see graphic).

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t₁(/)₂=20.4min), ¹³N (t₁(/)₂=9.9min), ¹⁵O (t₁(/)₂=2min), ⁶⁸Ga (t₁(/)₂=68min), ¹⁸F (t₁(/)₂=109.8min), ⁶⁴Cu (t₁(/)₂=12.7h), and ¹²⁴I (t₁(/)₂=4.12d).

PET designated flouride-18 production and chemistry

Positron emission tomography (PET) is a nuclear medicine imaging technology which allows for four-dimensional, quantitative determination of the distribution of labeled biological compounds within the human body. PET is becoming an increasingly important tool for the measurement of physiological, biochemical and pharmacological functions at the molecular level in healthy and pathological conditions. This review will focus on Flouride-18, one of the common isotopes used for PET imaging, which has a half life of 109.8 minutes. This isotope can be produced with an efficient yield in a cyclotron as a nucleophile or as an electrophile. Flouride-18 can be thereafter introduced into small molecules or biomolecules using various chemical synthetic routes, to give the desired imaging agent.