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

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

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.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Preclinical validations of [18F]FPyPEGCBT-c(RGDfK): a 18F-labelled RGD peptide prepared by ligation of 2-cyanobenzothiazole and 1,2-aminothiol to image angiogenesis

Background

α_Vβ₃, α_Vβ₅ and α₅β₁ integrins are known to be involved in carcinogenesis and are overexpressed in many types of tumours compared to healthy tissues; thereby they have been selected as promising therapeutic targets. Positron emission tomography (PET) is providing a unique non-invasive screening assay to discriminate which patient is more prone to benefit from antiangiogenic therapies, and extensive research has been carried out to develop a clinical radiopharmaceutical that binds specifically to integrin receptors. We recently reported the synthesis of a new ¹⁸F-labelled RGD peptide prepared by 2-cyanobenzothiazole (CBT)/1,2-aminothiol conjugation. This study aims at characterising the preclinical biologic properties of this new tumour-targeting ligand, named [¹⁸F]FPyPEGCBT-c(RGDfK).

The in vitro binding properties of [¹⁸F]FPyPEGCBT-c(RGDfK) were analysed by standard binding assay in U-87 MG and SKOV-3 cancer models and its selectivity towards integrins by siRNA depletions. Its preclinical potential was studied in mice bearing subcutaneous tumours by ex vivo biodistribution studies and in vivo microPET/CT imaging.

Results

In vitro, FPyPEGCBT-c(RGDfK) efficiently bound RGD-recognising integrins as compared to a control c(RGDfV) peptide (IC₅₀ = 30.8 × 10⁻⁷ M vs. 6.0 × 10⁻⁷ M). [¹⁸F]FPyPEGCBT-c(RGDfK) cell uptake was mediated by an active transport through binding to α_V, β₃ and β₅ but not to β₁ subunits. In vivo, this new tracer demonstrated specific tumour uptake with %ID/g of 2.9 and 2.4 in U-87 MG and SKOV-3 tumours 1 h post injection. Tumour-to-muscle ratios of 4 were obtained 1 h after intravenous administration of the tracer allowing good visualisation of the tumours. However, unfavourable background accumulation and high hepatobiliary excretion were observed.

Conclusion

[¹⁸F]FPyPEGCBT-c(RGDfK) specifically detects tumours expressing RGD-recognising integrin receptors in preclinical studies. Further optimisation of this radioligand may yield candidates with improved imaging properties and would warrant the further use of this efficient labelling technique for alternative targeting vectors.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-016-0019-z) contains supplementary material, which is available to authorized users.

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.

Click Reaction: An Applicable Radiolabeling Method for Molecular Imaging

In recent years, the click reaction has found rapidly growing applications in the field of radiochemistry, ranging from a practical labeling method to molecular imaging of biomacromolecules. This present review details the development of highly reliable, powerful and selective click chemistry reactions for the rapid synthesis of new radiotracers for molecular imaging.

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.

Dual isotope labeling: conjugation of 32P-oligonucleotides with 18F-aryltrifluoroborate via copper(I) catalyzed cycloaddition

A one-pot-two-step labeling of an oligonucleotide with an (18)F-ArBF3(-)(aryltrifluoroborate) radioprosthetic is reported herein. In order to characterize labeling in terms of radiochemistry, phosphorus-32 was also introduced to the 5'-terminus of the oligonucleotide via enzymatic phosphorylation. A pendant azide group was subsequently conjugated to the 5'-phosphate of the oligonucleotide. Copper(I) catalyzed [2+3] cycloaddition was undertaken to conjugate an alkyne-bearing(18)F-ArBF3(-) to the oligonucleotide. Following polyacrylamide gel electrophoresis, this doubly-labeled bioconjugate exhibited decay properties of both the phosphorus-32 and fluorine-18, that were confirmed by autoradiography at selected lengths of time, which in turn provided concrete evidence of successful conjugation. These results are corroborated by HPLC analysis of the labeled material. Taken together this work demonstrates viable use of (18)F-ArBF3(-) prosthetics for labeling oligonucleotides for use in PET imaging.

Recent trends in bioorthogonal click-radiolabeling reactions using fluorine-18

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile bioorthogonal conjugation techniques especially for the radiolabeling of biologically active high molecular weight compounds like peptides, proteins or antibodies. Taking into consideration that the introduction of fluorine-18 (t(1/2) = 109.8 min) proceeds under harsh conditions, radiolabeling of these biologically active molecules represents an outstanding challenge and is of enormous interest. Special attention has to be paid to the method of 18F-introduction. It should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. For these reasons and due to the high number of functional groups found in these compounds, a specific labeling procedure has to be developed for every bioactive macromolecule. Bioorthogonal strategies including the Cu-assisted Huisgen cycloaddition and its copper-free click variant, both Staudinger Ligations or the tetrazine-click reaction have been successfully applied and represent valuable alternatives for the selective introduction of fluorine-18 to overcome the afore mentioned obstacles. This comprehensive review deals with the progress and illustrates the latest developments in the field of bioorthogonal labeling with the focus on the preparation of radiofluorinated building blocks and tracers for molecular imaging.

2-Fluoropyridine prosthetic compounds for the 18F labeling of bombesin analogues

Acetylene-bearing 2-[(18)F]fluoropyridines [(18)F]FPy5yne and PEG-[(18)F]FPyKYNE were prepared via efficient nucleophilic heteroaromatic [(18)F]fluorination of their corresponding 2-trimethylammoniumpyrdinyl precursors. The prosthetic groups were conjugated to azide- and PEG3-modified bombesin(6-14) analogues via copper-catalyzed azide-alkyne cycloaddition couplings to yield mono- and di-mini-PEGylated ligands for PET imaging of the gastrin- releasing peptide receptor. The PEG3- and PEG2/PEG3-bearing (18)F peptides showed decreased lipophilicity relative to an analogous non-mini-PEGylated (18)F peptide. Assessment of water-soluble peptide pharmacokinetics and tumour-targeting capabilities in a mouse model of prostate cancer is currently underway.

4-[18F]Fluoro-N-methyl-N-(propyl-2-yn-1-yl)benzenesulfonamide ([18F]F-SA): a versatile building block for labeling of peptides, proteins and oligonucleotides with fluorine-18 via Cu(I)-mediated click chemistry

Cu(I)-mediated [3+2]cycloaddition between azides and alkynes has evolved into a valuable bioconjugation tool in radiopharmaceutical chemistry. We have developed a simple, convenient and reliable radiosynthesis of 4-[18F]fluoro-N-methyl-N-(propyl-2-yn-1-yl)benzenesulfonamide ([18F]F-SA) as a novel aromatic sulfonamide-based click chemistry building block. [18F]F-SA could be prepared in a remotely controlled synthesis unit in 32 ± 5% decay-corrected radiochemical yield in a total synthesis time of 80 min. The determined lipophilicity of [18F]F-SA (logP = 1.7) allows handling of the radiotracer in aqueous solutions. The versatility of [18F]F-SA as click chemistry building block was demonstrated by the labeling of a model peptide (phosphopeptide), protein (HSA), and oligonucleotide (L-RNA). The obtained radiochemical yields were 77% (phosphopeptide), 55-60% (HSA), and 25% (L-RNA), respectively. Despite the recent emergence of a multitude of highly innovative novel bioconjugation methods for 18F labeling of biopolymers, Cu(I)-mediated click chemistry with [18F]F-SA represents a reliable, robust and efficient radiolabeling technique for peptides, proteins, and oligonucleotides with the short-lived positron emitter 18F.

Chemical architecture and applications of nucleic acid derivatives containing 1,2,3-triazole functionalities synthesized via click chemistry

There is considerable attention directed at chemically modifying nucleic acids with robust functional groups in order to alter their properties. Since the breakthrough of copper-assisted azide-alkyne cycloadditions (CuAAC), there have been several reports describing the synthesis and properties of novel triazole-modified nucleic acid derivatives for potential downstream DNA- and RNA-based applications. This review will focus on highlighting representative novel nucleic acid molecular structures that have been synthesized via the “click” azide-alkyne cycloaddition. Many of these derivatives show compatibility for various applications that involve enzymatic transformation, nucleic acid hybridization, molecular tagging and purification, and gene silencing. The details of these applications are discussed. In conclusion, the future of nucleic acid analogues functionalized with triazoles is promising.

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.

New F-18 prosthetic group via oxime coupling

A novel fluorine-18 prosthetic ligand, 5-(1,3-dioxolan-2-yl)-2-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)pyridine [(18)F]2, has been synthesized. The prosthetic ligand is formed in high radiochemical yield (rcy = 71 ± 2%, n = 3) with excellent radiochemical purity (rcp = 99 ± 1%, n = 3) in a short reaction time (10 min). [(18)F]2 is a small, neutral, organic complex, easily synthesized in four steps from a readily available starting material. It can be anchored onto a target molecule containing an aminooxy functional group under acidic conditions by way of an oxime bond. We report herein two examples [(18)F]23 and [(18)F]24, potential imaging agents for β-amyloid plaques, which were labeled with this prosthetic group. This approach could be used for labeling proteins and peptides containing an aminooxy group. Biodistribution in male ICR mice for both oxime labeled complexes [(18)F]23 and [(18)F]24 were compared to that of the known β-amyloid plaque indicator, [(18)F]-AV-45, florbetapir 1. Oximes [(18)F]23 and [(18)F]24 are larger in size and therefore should reduce the blood-brain barrier (BBB) penetration. The brain uptake for oxime [(18)F]23 appeared to be reduced, but still retained some capability to cross the BBB. Oxime [(18)F]24 showed promising results after 2 min post injection (0.48% dose/gram); however, the uptake increased after 30 min post injection (0.92% dose/gram) suggesting an in vivo decomposition/metabolism of compound [(18)F]24. We have demonstrated a general protocol for the fluoride-18 labeling with a new prosthetic ligand [(18)F]2 that is tolerant toward several functional groups and is formed via chemoselective oxime coupling.

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).

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.

Stoichiometry-focused (18)F-labeling of alkyne-substituted oligodeoxynucleotides using azido([(18)F]fluoromethyl)benzenes by Cu-catalyzed Huisgen reaction

A novel method for (18)F-radiolabeling of oligodeoxynucleotides (ODNs) by a Cu-catalyzed Huisgen reaction has been developed by using the lowest possible amount of the precursor biomolecule for the realization of stoichiometry-oriented PET (positron emission tomography) chemistry. Under the optimized cyclization conditions of p- or m-azido([(18)F]fluoromethyl)benzene and alkyne-substituted ODN (20nmol) at 40°C for 15min in the presence of CuSO(4), TBTA [tris((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amine], and sodium ascorbate (2:1:2), the synthesis of (18)F-labeled ODNs with sufficiently high radioactivities of 2.1-2.5GBq and specific radioactivities of 1800-2400GBq/μmol have been accomplished for use in animal and human PET studies.