Design and synthesis of a new [18F]fluoropyridine-based haloacetamide reagent for the labeling of oligonucleotides: 2-bromo-N-[3-(2-[18F]fluoropyridin-3-yloxy)propyl]acetamide

[18F]fluoride Activation and 18F-Labelling in Hydrous Conditions-Towards a Microfluidic Synthesis of PET Radiopharmaceuticals

18F-labelled radiopharmaceuticals are indispensable in positron emission tomography. The critical step in the preparation of 18F-labelled tracers is the anhydrous F-18 nucleophilic substitution reaction, which involves [18F]F- anions generated in aqueous media by the cyclotron. For this, azeotropic drying by distillation is widely used in standard synthesisers, but microfluidic systems are often not compatible with such a process. To avoid this step, several methods compatible with aqueous media have been developed. We summarised the existing approaches and two of them have been studied in detail. [18F]fluoride elution efficiencies have been investigated under different conditions showing high 18F-recovery. Finally, a large scope of precursors has been assessed for radiochemical conversion, and these hydrous labelling techniques have shown their potential for tracer production using a microfluidic approach, more particularly compatible with iMiDEV™ cassette volumes.

Deoxyfluorination of phenols for chemoselective 18F-labeling of peptides

The challenge of forming C-18F bonds is often a bottleneck in the development of new 18F-labeled tracer molecules for noninvasive functional imaging studies using positron emission tomography (PET). Nucleophilic aromatic substitution is the most widely employed reaction to functionalize aromatic substrates with the radioactive fluorine-18 but its scope is restricted to arenes containing electron-withdrawing substituents. Furthermore, many protic functional groups are incompatible with basic fluoride anions. Peptide substrates, which are highly desirable targets for PET molecular imaging, are particularly challenging to label with fluorine-18 because they are densely functionalized and sensitive to high temperatures and basic conditions. To expand the utility of nucleophilic aromatic substitution with fluorine-18, we describe two complementary procedures for the radiodeoxyfluorination of bench-stable and easy-to-access phenols that ensure rapid access to densely functionalized electron-rich and electron-poor 18F-aryl fluorides. The first procedure details the synthesis of an 18F-synthon and its subsequent ligation to the cysteine residue of Arg-Gly-Asp-Cys in 10.5 h from commercially available starting materials (189-min radiosynthesis). The second procedure describes the incorporation of commercially available CpRu(Fmoc-tyrosine)OTf into a fully protected peptide Lys-Met-Glu-(CpRu-Tyr)-Leu via solid-phase peptide synthesis and subsequent ruthenium-mediated uronium deoxyfluorination with fluorine-18 followed by deprotection, accomplished within 7 d (116-min radiosynthesis). Both radiolabeling methods are highly chemoselective and have conveniently been automated using commercially available radiosynthesis equipment so that the procedures described can be employed for the synthesis of peptide-based PET probes for in vivo imaging studies according to as low as reasonably achievable (ALARA) principles.

One-Step Synthesis of [18F]Fluoro-4-(vinylsulfonyl)benzene: A Thiol Reactive Synthon for Selective Radiofluorination of Peptides

Radiolabeled peptide-based molecular imaging probes exploit the advantages of large biologics and small molecules, providing both exquisite selectivity and favorable pharmacokinetic properties. Here, we report an operationally simple and broadly applicable approach for the ¹⁸F-fluorination of unprotected peptides via a new radiosynthon, [¹⁸F]fluoro-4-(vinylsulfonyl)benzene. This reagent demonstrates excellent chemoselectivity at the cysteine residue and rapid ¹⁸F-labeling of a diverse scope of peptides to generate stable thioether constructs.

[18 F]FPyZIDE: A versatile prosthetic reagent for the fluorine-18 radiolabeling of biologics via copper-catalyzed or strain-promoted alkyne-azide cycloadditions

Methods for the radiolabeling of biologics with fluorine-18 have been of interest for several decades. A common approach consists in the preparation of a prosthetic reagent, a small molecule bearing a fluorine-18 that is conjugated with the macromolecule to an appropriate function. Click chemistry, and more particularly cycloadditions, is an interesting approach to radiolabel molecules thanks to mild reaction conditions, high yields, low by-products formation, and strong orthogonality. Moreover, the chemical functions involved in the cycloaddition reaction are stable in the drastic radiofluorination conditions, thus allowing a simple radiosynthetic route to prepare the prosthetic reagent. We report herein the radiosynthesis of ¹⁸ F-FPyZIDE, a pyridine-based azide-bearing prosthetic reagent. We exemplified its conjugation via copper-catalyzed cycloaddition (CuAAC) and strain-promoted cycloaddition (SPAAC) with several terminal alkyne or strained alkyne model compounds.

Light-Triggered Radiochemical Synthesis: A Novel 18F-Labelling Strategy Using Photoinducible Click Reaction to Prepare PET Imaging Probes

Novel probe development for positron emission tomography (PET) is leading to expanding the scope of molecular imaging. To begin responding to challenges, several biomaterials such as natural products and small molecules, peptides, engineered proteins including affibodies, and antibodies have been used in the development of targeted molecular imaging probes. To prepare radiotracers, a few bioactive materials are unique challenges to radiolabelling because of their complex structure, poor stability, poor solubility in aqueous or chemical organic solutions, and sensitivity to temperature and nonphysiological pH. To overcome these challenges, we developed a new radiolabelling strategy based on photoactivated 1,3-dipolar cycloaddition between alkene dipolarophile and tetrazole moiety containing compounds. Herein, we describe a light-triggered radiochemical synthesis via photoactivated click reaction to prepare 18F-radiolabelled PET tracers using small molecular and RGD peptide.

Synthesis, in vitro and in vivo evaluation of 18F-fluoronorimatinib as radiotracer for Imatinib-sensitive gastrointestinal stromal tumors

INTRODUCTION

Gastrointestinal stromal tumors (GIST) have a wide range of mutations, but can mostly be treated with Imatinib, until eventually resistance towards this tyrosine kinase inhibitor is acquired. Early and non-invasive determination of the sensitivity of the tumor and its metastases towards Imatinib by positron emission tomography (PET) would be beneficial for therapy planning and monitoring.

METHODS

We developed a synthesis strategy towards the precursor molecule, performed the ¹⁸F-synthesis and in the following evaluated the radioligand in vitro regarding its lipophilicity, stability and biological activity (KIT binding properties) as well as its in vivo properties in GIST tumor-bearing mice.

RESULTS

[¹⁸F]fluoronorimatinib could be obtained in an overall radiochemical yield of 22.2±3.3% within 90min. The radioligand showed high GIST cell uptake and was able to distinguish between Imatinib-sensitive and resistant tumor cell lines (GIST-T1, GIST882, GIST430) in vitro. Further biological evaluations of the ligand towards 9 different GIST-relevant KIT mutations showed comparable binding affinities compared to the structural lead Norimatinib (65nM vs. 53nM for wt-KIT). The in vivo evaluation of the newly developed radioligand showed tumor-to-background-ratios comparable to previously described, similar radiotracers.

CONCLUSIONS

Thus, [¹⁸F]fluoronorimatinib is able to distinguish between Imatinib-resistant and sensitive KIT mutations. Although no improvement of in vivo tumor-to-background ratios could be achieved compared to formerly described radioligands, the hepatic uptake could be considerably reduced, being advantageous for the imaging of GIST. Advances in knowledge and implications for patient care: We were able to show that it is possible to significantly reduce the unfavorably high hepatic uptake of small-molecule radioligands applicable for GIST PET imaging. This work can thus be the basis for further work intending to develop a PET-radioligand for Imatinib-dependent GIST imaging.

Nucleic Acid Aptamers: Emerging Applications in Medical Imaging, Nanotechnology, Neurosciences, and Drug Delivery

Recent progresses in organic chemistry and molecular biology have allowed the emergence of numerous new applications of nucleic acids that markedly deviate from their natural functions. Particularly, DNA and RNA molecules-coined aptamers-can be brought to bind to specific targets with high affinity and selectivity. While aptamers are mainly applied as biosensors, diagnostic agents, tools in proteomics and biotechnology, and as targeted therapeutics, these chemical antibodies slowly begin to be used in other fields. Herein, we review recent progress on the use of aptamers in the construction of smart DNA origami objects and MRI and PET imaging agents. We also describe advances in the use of aptamers in the field of neurosciences (with a particular emphasis on the treatment of neurodegenerative diseases) and as drug delivery systems. Lastly, the use of chemical modifications, modified nucleoside triphosphate particularly, to enhance the binding and stability of aptamers is highlighted.

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

Alkylation of phosphorothioated thrombin binding aptamers improves the selectivity of inhibition of tumor cell proliferation upon anticoagulation

BACKGROUND

Recently, aptamers have been extensively researched for therapy and diagnostic applications. Thrombin-binding aptamer is a 15nt deoxyribonucleic acid screened by SELEX, it can specifically bind to thrombin and inhibit blood coagulation. Since it is also endowed with excellent antitumor activity, the intrinsic anticoagulation advantage converted to a main potential side effect for its further application in antiproliferative therapy.

METHODS

Site-specific alkylation was conducted through nucleophilic reaction of phosphorothioated TBAs using bromide reagents. Circular dichroism (CD) spectroscopy and surface plasmon resonance (SPR) measurements were used to evaluate anticoagulation activity, and a CCK-8 assay was used to determine cell proliferation activity.

RESULTS

The CD spectra of the modified TBAs were weakened, and their affinity for thrombin was dramatically reduced, as reflected by the K_D values. On the other hand, their inhibition of A549 cells was retained.

CONCLUSIONS

Incorporation of different alkyls apparently disrupted the binding of TBA to thrombin while maintaining the antitumor activity.

GENERAL SIGNIFICANCE

A new modification strategy was established for the use of TBA as a more selective antitumor agent.

[(18)F]-Organotrifluoroborates as Radioprosthetic Groups for PET Imaging: From Design Principles to Preclinical Applications

Positron emission tomography (PET) is revolutionizing our ability to visualize in vivo targets for target validation and personalized medicine. Of several classes of imaging agents, peptides afford high affinity and high specificity to distinguish pathologically distinct cell types by the presence of specific molecular targets. Of various available PET isotopes, [(18)F]-fluoride ion is preferred because of its excellent nuclear properties and on-demand production in hospitals at Curie levels. However, the short half-life of (18)F and its lack of reactivity in water continue to challenge peptide labeling. Hence, peptides are often conjugated to a metal chelator for late-stage, one-step labeling. Yet radiometals, while effective, are neither as desirable nor as available as [(18)F]-fluoride ion. Despite considerable past success in identifying semifeasible radiosyntheses, significant challenges continue to confound tracer development. These interrelated challenges relate to (1) isotope/prosthetic choice; (2) bioconjugation for high affinity; (3) high radiochemical yields, (4) specific activities of >1 Ci/μmol to meet FDA microdose requirements; and (5) rapid clearance and in vivo stability. These enduring challenges have been extensively highlighted, while a single-step, operationally simple, and generally applicable means of labeling a peptide with [(18)F]-fluoride ion in good yield and high specific activity has eluded radiochemists and nuclear medicine practitioners for decades. Radiosynthetic ease is of primordial importance since multistep labeling reactions challenge clinical tracer production. In the past decade, as we sought to meet this challenge, appreciation of reactions with aqueous fluoride led us to consider organotrifluoroborate (RBF3(-)) synthesis as a means of rapid aqueous peptide labeling. We have applied principles of mechanistic chemistry, knowledge of chemical reactivity, and synthetic chemistry to design stable RBF3(-)s. Over the past 10 years, we have developed several new [(18)F]-RBF3(-) radioprosthetic groups, all of which guarantee radiosynthetic ease while in most cases providing high tumor:nontumor (T:NT) ratios and moderate-to-high tumor uptake. Although others have developed methods for labeling of peptides with [(18)F]-silylfluorides or [(18)F]-Al-NOTA chelates, this Account focuses on the synthesis of [(18)F]-organotrifluoroborates. In this Account, I detail mechanistic, kinetic, thermodynamic, synthetic, and radiosynthetic approaches that enabled the translation of fundamental principles regarding the chemistry of RBF3(-)s into a tantalizingly close realization of a clinical application of an [(18)F]-organotrifluoroborate-peptide conjugate for imaging of neuroendocrine tumors and the generalization of this method for labeling of several other peptides.

Dual PET and Near-Infrared Fluorescence Imaging Probes as Tools for Imaging in Oncology

OBJECTIVE

The purpose of this article is to summarize advances in PET fluorescence resolution, agent design, and preclinical imaging that make a growing case for clinical PET fluorescence imaging.

CONCLUSION

Existing SPECT, PET, fluorescence, and MRI contrast imaging techniques are already deeply integrated into the management of cancer, from initial diagnosis to the observation and management of metastases. Combined positron-emitting fluorescent contrast agents can convey new or substantial benefits that improve on these proven clinical contrast agents.

Enabling [(18)F]-bicyclo[6.1.0]nonyne for oligonucleotide conjugation for positron emission tomography applications: [(18)F]-anti-microRNA-21 as an example

A bicyclononyne-based prosthetic group has been developed for (18)F-labeling of anti-microRNA-21, an oligonucleotide, in a near-stoichiometric manner.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

A novel 2-cyanobenzothiazole-based (18)F prosthetic group for conjugation to 1,2-aminothiol-bearing targeting vectors

In a bid to find an efficient means to radiolabel biomolecules under mild conditions for PET imaging, a bifunctional (18)F prosthetic molecule has been developed. The compound, dubbed [(18)F]FPyPEGCBT, consists of a 2-substituted pyridine moiety for [(18)F]F(-) incorporation and a 2-cyanobenzothiazole moiety for coupling to terminal cysteine residues. The two functionalities are separated by a mini-PEG chain. [(18)F]FPyPEGCBT could be prepared from its corresponding 2-trimethylammonium triflate precursor (100 °C, 15 min, MeCN) in preparative yields of 11% ± 2 (decay corrected, n = 3) after HPLC purification. However, because the primary radiochemical impurity of the fluorination reaction will not interact with 1,2-aminothiol functionalities, the (18)F prosthetic could be prepared for bioconjugation reactions by way of partial purification on a molecularly imprinted polymer solid-phase extraction cartridge. [(18)F]FPyPEGCBT was used to (18)F-label a cyclo-(RGDfK) analogue which was modified with a terminal cysteine residue (TCEP·HCl, DIPEA, 30 min, 43 °C, DMF). Final decay-corrected yields of (18)F peptide were 7% ± 1 (n = 9) from end-of-bombardment. This novel integrin-imaging agent is currently being studied in murine models of cancer. We argue that [(18)F]FPyPEGCBT holds significant promise owing to its straightforward preparation, 'click'-like ease of use, and hydrophilic character. Indeed, the water-tolerant radio-bioconjugation protocol reported herein requires only one HPLC step for (18)F peptide purification and can be carried out remotely using a single automated synthesis unit over 124-132 min.

Aptamers as radiopharmaceuticals for nuclear imaging and therapy

Today, radiopharmaceuticals belong to the standard instrumentation of nuclear medicine, both in the context of diagnosis and therapy. The majority of radiopharmaceuticals consist of targeting biomolecules which are designed to interact with a disease-related molecular target. A plethora of targeting biomolecules of radiopharmaceuticals exists, including antibodies, antibody fragments, proteins, peptides and nucleic acids. Nucleic acids have some significant advantages relative to proteinaceous biomolecules in terms of size, production, modifications, possible targets and immunogenicity. In particular, aptamers (non-coding, synthetic, single-stranded DNA or RNA oligonucleotides) are of interest because they can bind a molecular target with high affinity and specificity. At present, few aptamers have been investigated preclinically for imaging and therapeutic applications. In this review, we describe the use of aptamers as targeting biomolecules of radiopharmaceuticals. We also discuss the chemical modifications which are needed to turn aptamers into valuable (radio-)pharmaceuticals, as well as the different radiolabeling strategies that can be used to radiolabel oligonucleotides and, in particular, aptamers.

Fluorine-18 radiochemistry, labeling strategies and synthetic routes

Fluorine-18 is the most frequently used radioisotope in positron emission tomography (PET) radiopharmaceuticals in both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.7 min half-life, 635 keV positron energy), along with high specific activity and ease of large scale production, make it an attractive nuclide for radiochemical labeling and molecular imaging. Versatile chemistry including nucleophilic and electrophilic substitutions allows direct or indirect introduction of (18)F into molecules of interest. The significant increase in (18)F radiotracers for PET imaging accentuates the need for simple and efficient (18)F-labeling procedures. In this review, we will describe the current radiosynthesis routes and strategies for (18)F labeling of small molecules and biomolecules.

One-pot two-step radiosynthesis of a new (18)F-labeled thiol reactive prosthetic group and its conjugate for insulinoma imaging

N-(2-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethyl)-6-fluoronicotinamide ([¹⁸F]FNEM), a novel prosthetic agent that is thiol-specific, was synthesized using a one-pot two-step strategy: (1) ¹⁸F incorporation by a nucleophilic displacement of trimethylammonium substrate under mild conditions; (2) amidation of the resulting 6-[¹⁸F]fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester with N-(2-aminoethyl)maleimide trifluoroacetate salt. The radiosynthesis of the maleimide tracer was completed in 75 min from [¹⁸F]fluoride with 26 ± 5% decay uncorrected radiochemical yield, and specific activity of 19–88 GBq/μmol (decay uncorrected). The in vitro cell uptake, in vivo biodistribution, and positron emission tomography (PET) imaging properties of its conjugation product with [Cys⁴⁰]-exendin-4 were described. [¹⁸F]FNEM-Cys⁴⁰-exendin-4 showed specific targeting of glucagon-like peptide 1 receptor (GLP-1R) positive insulinomas and comparable imaging results to our recently reported [¹⁸F]FPenM-Cys⁴⁰-exendin-4.

18F labeling of arenes

Molecular imaging has witnessed an upsurge in growth, with positron emission tomography leading the way. This trend has encouraged numerous synthetic chemists to enter the field of (18) F-radiochemistry and provide generic solutions to address the well-recognized challenges of late-stage fluorination. This Minireview focuses on recent developments in the (18)F-labeling of aromatic substrates.

Synthesis of two new alkyne-bearing linkers used for the preparation of siRNA for labeling by click chemistry with fluorine-18

Oligonucleotides (ONs) and more particularly siRNAs are promising drugs but their pharmacokinetics and biodistribution are widely unknown. Positron Emission Tomography (PET) using fluorine-18 is a suitable technique to quantify these biological processes. Click chemistry (Huisgen cycloaddition) is the current method for labeling siRNA. In order to study the influence of a linker bearing by [(18)F] labeled ONs, on the in vivo pharmacokinetic and metabolism, we have developed two modified ONs by two new linkers. Here we report the synthesis of two alkyne-bearing linkers, the incorporation onto a ONs and the conjugation by click chemistry with a [(18)F] prosthetic group.

In vitro characterization of two novel biodegradable vectors for the delivery of radiolabeled antisense oligonucleotides

The development of antisense oligonucleotides suitable for tumor targeting applications is hindered by low stability and bioavailability of oligonucleotides in vivo and by the absence of efficient and safe vectors for oligonucleotide delivery. Stabilization in vivo has been achieved through chemical modification of oligonucleotides by various means, but effective approaches to enhance their intracellular delivery are lacking. This study reports on the characterization in vitro of a fully phosphorothioated 20-mer oligonucleotide, complementary to p21 mRNA, radiolabeled with fluorine-18 using a thiol reactive prosthetic group. The potential of two novel synthetic block copolymers containing grafted polyamines on their hydrophobic blocks for vector-assisted cell delivery was studied in vitro. Extensive cellular uptake studies were performed in human colon carcinoma cell lines with enhanced or deficient p21 expression to evaluate and compare the uptake mechanism of naked and vectorized radiolabeled formulations. Uptake studies with the two novel biodegradable vectors showed a moderate increase in cell uptake of the radiofluorinated antisense oligonucleotide. The two vectors show, however, promising advantages over conventional lipidic vectors regarding their biocompatibility and subcellular distribution.

Fast 18F labeling of a near-infrared fluorophore enables positron emission tomography and optical imaging of sentinel lymph nodes

We combine a novel boronate trap for F⁻ with a near-infrared fluorophore into a single molecule. Attachment to targeting ligands enables localization by positron emission tomography (PET) and near-infrared fluorescence (NIRF). Our first application of this generic tag is to label Lymphoseek (tilmanocept), an agent designed for receptor-specific sentinel lymph node (SLN) mapping. The new conjugate incorporates ¹⁸F⁻ in a single, aqueous step, targets mouse SLN rapidly (1 h) with reduced distal lymph node accumulation, permits PET or scintigraphic imaging of SLN, and enables NIRF-guided excision and histological verification even after ¹⁸F decay. This embodiment is superior to current SLN mapping agents such as nontargeted [^99mTc]sulfur colloids and Isosulfan Blue, as well as the phase III targeted ligand [^99mTc]SPECT Lymphoseek counterpart, species that are visible by SPECT or visible absorbance separately. Facile incorporation of ¹⁸F into a NIRF probe should promote many synergistic PET and NIRF combinations.

General method for labeling siRNA by click chemistry with fluorine-18 for the purpose of PET imaging

The alkyne-azide Cu(I)-catalyzed Huisgen cycloaddition, a click-type reaction, was used to label a double-stranded oligonucleotide (siRNA) with fluorine-18. An alkyne solid support CPG for the preparation of monostranded oligonucleotides functionalized with alkyne has been developed. Two complementary azide labeling agents (1-(azidomethyl)-4-[(18)F]fluorobenzene) and 1-azido-4-(3-[(18)F]fluoropropoxy)benzene have been produced with 41% and 35% radiochemical yields (decay-corrected), respectively. After annealing with the complementary strand, the siRNA was directly labeled by click chemistry with [(18)F]fluoroazide to produce the [(18)F]-radiolabeled siRNA with excellent radiochemical yield and purity.

Labeling of an antisense oligonucleotide with [(18)F]FPy5yne

Functional imaging of gene expression in vivo with short-lived positron emitter (18)F remains an unrealized goal, in part because the long reaction times and challenging protocols typically required to label nucleic acid-based molecular probes with this radionuclide (t(1/2) = 109.8 minutes). To this end, we synthesized prosthetic group 2-[(18)F]fluoro-3-(hex-5-ynyloxy)pyridine ([(18)F]FPy5yne), used previously to label peptides, and coupled it to an oligodeoxyribonucleotide with (18)F by way of a Cu(I)-mediated azide/alkyne cycloaddition reaction. HPLC-purified [(18)F]FPy5yne was ligated to a 5'-azide-modified DNA sequence antisense to mdr1 mRNA in the presence of Cu(I)-stabilizing ligand tris(benzyltriazolylmethyl)amine and 2,6-lutidine. Non-decay corrected, collected yields of the (18)F-labeled oligonucleotide from end-of-bombardment were 3.9% +/- 0.5% (n = 3; 24.6% +/- 0.5% decay corrected). Shortest preparation time was 276 minutes from start of synthesis.

Automated synthesis of an 18F-labelled pyridine-based alkylating agent for high yield oligonucleotide conjugation

Alkylating agents have been shown to be very promising for the radiolabelling of oligonucleotides with fluorine-18. In this report we describe the fully automated synthesis of 2-bromo-N-[3-(2-[(18)F]fluoropyridin-3-yloxy)propyl]acetamide ([(18)F]FPyBrA) utilizing a modular synthesis unit. Reaction conditions for the coupling of this pyridine-based alkylating agent at the 5' end of a fully phosphorothioated random 20-mer DNA sequence were optimized to achieve very high radiochemical yields (>90%) and a maximum specific activity of 5-6 GBq/micromoL. The potential for rapid purification by solid phase extraction without need of chromatographic isolation of the radiolabelled oligonucleotide presents an overall benefit for the application of oligonucleotides in preclinical studies and potential clinical applications.

Molecular imaging study on in vivo distribution and pharmacokinetics of modified small interfering RNAs (siRNAs)

Molecular imaging was used to study the biodistribution, pharmacokinetics, and activity of naked small interfering RNAs (siRNAs). siRNAs with riboses chemically modified in the 2' position were compared with unmodified siRNA. In vitro, replacement of the 2'-hydroxyl (2'OH) group of certain nucleotides in an siRNA sequence by a fluorine atom (2'F) on both antisense (AS) and sense (S) strands [2'F(AS/S)], or by a methoxy group (2'OMe) on the S strand [2'OH(AS)/2'OMe(S)], was compatible with RNA interference. Different siRNAs [2'F(AS/S), 2'OH(AS)/2'OMe(S), and 2'OH(AS/S)] were labeled with fluorine-18 (conjugation with [(18)F]FPyBrA), and comparative dynamic and quantitative imaging was performed with positron emission tomography. After intravenous injections of [(18)F]siRNAs in rodents, total radioactivity was rapidly eliminated by the kidneys and the liver. Tissue distribution of the different siRNAs were similar, and their bioavailability (as judged from blood persistence and stability) increased in the order 2'OH(AS/S) = 2'OH(AS)/2'OMe(S) < 2'F(AS/S). However, in our in vivo model, the 2'F(AS/S) siRNA, despite its higher bioavailability, was not able to induce a higher interference effect with respect to the 2'OH(AS/S) siRNA. Molecular imaging approaches, applied in the present work to both natural and chemically modified siRNAs, can contribute to the development of these macromolecules as therapeutic agents.

In vivo imaging of oligonucleotidic aptamers

In this chapter we present the methods developed in our laboratory for in vivo imaging of oligonucleotidic aptamers. These methods relate to (i) the labelling of aptamers with fluorine-18, a positron emitter, (ii) Positron Emission Tomography imaging of laboratory animals with [(18)F]aptamers and (iii) labelling with fluorescent dyes and optical imaging of aptamers in mice.

Toward [18F]-labeled aryltrifluoroborate radiotracers: in vivo positron emission tomography imaging of stable aryltrifluoroborate clearance in mice

The use of a boronic ester as a captor of aqueous [(18)F]-fluoride has been previously suggested as a means of labeling biomolecules in one step for positron emission tomography (PET) imaging. For this approach to be seriously considered, the [(18)F]-labeled trifluoroborate should be humorally stable such that it neither leaches free [(18)F]-fluoride to the bone nor accumulates therein. Herein, we have synthesized a biotinylated boronic ester that is converted to the corresponding trifluoroborate salt in the presence of aqueous [(18)F]-fluoride. In keeping with its in vitro aqueous kinetic stability at pH 7.5, the trifluoroborate appears to clear in vivo quite rapidly to the bladder as the stable trifluoroborate salt with no detectable leaching of free [(18)F]-fluoride to the bone. When this labeled biotin is preincubated with avidin, the pharmacokinetic clearance of the resulting complex is visibly altered. This work validates initial claims that boronic esters are potentially useful as readily labeled precursors to [(18)F]-PET reagents.

Synthesis and application of [18F]FDG-maleimidehexyloxime ([18F]FDG-MHO): a [18F]FDG-based prosthetic group for the chemoselective 18F-labeling of peptides and proteins

2-[(18)F]Fluoro-2-deoxy-D-glucose ([(18)F]FDG) as the most important PET radiotracer is available in almost every PET center. However, there are only very few examples using [(18)F]FDG as a building block for the synthesis of (18)F-labeled compounds. The present study describes the use of [(18)F]FDG as a building block for the synthesis of (18)F-labeled peptides and proteins. [(18)F]FDG was converted into [(18)F]FDG-maleimidehexyloxime ([(18)F]FDG-MHO), a novel [(18)F]FDG-based prosthetic group for the mild and thiol group-specific (18)F labeling of peptides and proteins. The reaction was performed at 100 degrees C for 15 min in a sealed vial containing [(18)F]FDG and N-(6-aminoxy-hexyl)maleimide in 80% ethanol. [(18)F]FDG-MHO was obtained in 45-69% radiochemical yield (based upon [(18)F]FDG) after HPLC purification in a total synthesis time of 45 min. Chemoselecetive conjugation of [(18)F]FDG-MHO to thiol groups was investigated by the reaction with the tripeptide glutathione (GSH) and the single cysteine containing protein annexin A5 (anxA5). Radiolabeled annexin A5 ([(18)F]FDG-MHO-anxA5) was obtained in 43-58% radiochemical yield (based upon [(18)F]FDG-MHO, n = 6), and [(18)F]FDG-MHO-anxA5 was used for a pilot small animal PET study to assess in vivo biodistribution and kinetics in a HT-29 murine xenograft model.

[18F]fluoropyridines: From conventional radiotracers to the labeling of macromolecules such as proteins and oligonucleotides

Molecular in vivo imaging with the high-resolution and sensitive positron emission tomography (PET) technique requires the preparation of a positron-emitting radiolabeled probe or radiotracer. For this purpose, fluorine-18 is becoming increasingly the radionuclide of choice due to its adequate physical and nuclear characteristics, and also because of the successful use in clinical oncology of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), which is currently the most widely used PET-radiopharmaceutical and probably the driving force behind the growing availability and interest for this positron-emitter in radiopharmaceutical chemistry. With a few exceptions, radiofluorinations involving fluorine-18 of high specific radioactivity (e.g. > 185 GBq/micromole) had, until recently, been limited to nucleophilic substitutions in homoaromatic and aliphatic series with [18F]fluoride. Considering chemical structures showing a fluoropyridinyl moiety, nucleophilic heteroaromatic substitution at the ortho-position with no-carrier-added [l8F]fluoride, as its K[18F]F-K222 complex, appears today as a highly efficient method for the radiosynthesis of radiotracers and radiopharmaceuticals. This chapter summarizes the recent applications of this methodology and highlights its potential in the design and preparation of, often drug-based, fluorine-18-labeled probes of high specific radioactivity for PET imaging, including macromolecules of biological interest such as peptides, proteins and oligonucleotides.

Labeling of low-density lipoproteins using the 18F-labeled thiol-reactive reagent N-[6-(4-[18F]fluorobenzylidene)aminooxyhexyl]maleimide

The novel thiol-group-selective bifunctional 18F-labeling agent N-[6-(4-[18F]fluoro-benzylidene)aminooxyhexyl]maleimide ([18F]FBAM) has been developed. The bifunctional labeling precursor N-(6-aminoxyhexyl)maleimide containing a thiol-reactive maleimide group and a carbonyl-group-reactive aminooxy group was prepared in only three steps in a total chemical yield of 59%. Subsequent radiolabeling with 4-[18F]fluorobenzaldehyde gave the bifunctional 18F-labeling agent [18F]FBAM in a radiochemical yield of 29%. In a typical experiment, 3.88 GBq of [18F]fluoride could be converted into 723 MBq of [18F]FBAM within 69 min. Conjugation of [18F]FBAM with thiol groups was exemplified with the cysteine-containing tripeptide glutathione and with various apolipoproteins of human low-density lipoprotein (LDL) subfractions. The latter was evaluated with respect to the uptake of [18F]FBAM-LDL subfractions in human hepatoma cells (HepG2) in vitro. In vivo biodistribution studies in rats revealed high stability for [18F]FBAM-LDL subfractions. Moreover, the metabolic fate of [18F]FBAM-LDL subfractions in vivo was delineated by dynamic positron emission tomography studies using a dedicated small animal tomograph. Data were compared to former studies that used the NH2-reactive 18F-labeling agent N-succinimidyl-4-[18F]fluorobenzoate. The compound [18F]FBAM can be considered as an excellent prosthetic group for the selective and mild 18F labeling of thiol-group-containing biomolecules suitable for subsequent investigations in vitro and in vivo.

Aspects of positron emission tomography radiochemistry as relevant for food chemistry

Positron emission tomography (PET) is a medical imaging technique using compounds labelled with short-lived positron emitting radioisotopes to obtain functional information of physiological, biochemical and pharmacological processes in vivo. The need to understand the potential link between the ingestion of individual dietary agents and the effect of health promotion or health risk requires the exact metabolic characterization of food ingredients in vivo. This exciting but rather new research field of PET would provide new insights and perspectives on food chemistry by assessing quantitative information on pharmocokinetics and pharmacodynamics of food ingredients and dietary agents. To fully exploit PET technology in food chemistry appropriately radiolabelled compounds as relevant for food sciences are needed. The most widely used short-lived positron emitters are (11)C (t(1/2) = 20.4 min) and (18)F (t(1/2) = 109.8 min). Longer-lived radioisotopes are available by using (76)Br (t(1/2) = 16.2 h) and (124)I (t(1/2) = 4.12 d). The present review article tries to discuss some aspects for the radiolabelling of food ingredients and dietary agents either by means of isotopic labelling with (11)C or via prosthetic group labelling approaches using the positron emitting halogens (18)F, (76)Br and (124)I.