New F-18 prosthetic group via oxime coupling

A closer look at the synthesis of 2-[18F]fluoroethyl tosylate to minimize the formation of volatile side-products

BACKGROUND

2-[¹⁸F]Fluoroethyltosylate ([¹⁸F]FEtOTs) is a well-known ¹⁸F-fluoroalkylating agent widely used to synthesize radiotracers for positron emission tomography. The widespread use of [¹⁸F]FEtOTs is due in part to its low volatility when compared to other halide and sulfonate building blocks. In this work, the radioactive volatile side-products formed during the synthesis of [¹⁸F]FEtOTs were identified and characterized for the first time, and an optimization of the reaction conditions to minimize their formation was proposed.

RESULTS

In order to characterize the volatiles produced during [¹⁸F]FEtOTs synthesis, the reaction mixtures of both cold FEtOTs and [¹⁸F]FEtOTs were co-injected onto the HPLC system. The radioactive peaks corresponding to the volatile compounds were collected, analyzed through headspace gas chromatography mass spectrometry sampler (HS-GC-MS) and identified as vinyl fluoride ([¹⁹F]VF) and 2-fluoroethanol ([¹⁹F]FEOH). By using a rotatable central composite design with a two-level full factorial core of two factors (2²), it was determined that temperature and time are independent variables which affect the generation of [¹⁸F]VF and [¹⁸F]FEOH during the radiosynthesis of [¹⁸F]FEtOTs. In addition, in order to reduce the formation of the volatiles ([¹⁸F]VF and [¹⁸F]FEOH) and increase the yield of [¹⁸F]FEtOTs, it was demonstrated that the molar ratio of base to precursor must also be considered.

CONCLUSION

[¹⁸F]VF and [¹⁸F]FEOH are volatile side-products formed during the radiosynthesis of [¹⁸F]FEtOTs, whose yields depend on the reaction time, temperature, and the molar ratio of base to precursor. Therefore, special care should be taken during the radiosynthesis and subsequent reactions using [¹⁸F]FEOTs in order to avoid environmental contamination and to improve the yield of the desired products.

Fluorine-18 labeled aldehydes as prosthetic groups for oxime coupling with a FVIIa protein

New 18 F-labeled nonvolatile aldehyde prosthetic groups derived from [18 F]F-Py-TFP and spirocyclic iodonium (III)ylide precursors for late stage 18 F-labeling were developed. These precursors were characterized, 18 F-labeled, and compared in reactivity for oxime coupling. Oxime coupling was performed on an amino-oxy modified inhibited factor VII (FVIIai-ONH2 ) in low concentration to prove the applicability of the proposed method.

Oxime Coupling of Active Site Inhibited Factor Seven with a Nonvolatile, Water-Soluble Fluorine-18 Labeled Aldehyde

A nonvolatile fluorine-18 aldehyde prosthetic group was developed from [¹⁸F]SFB, and used for site-specific labeling of active site inhibited factor VII (FVIIai). FVIIai has a high affinity for tissue factor (TF), a transmembrane protein involved in angiogenesis, proliferation, cell migration, and survival of cancer cells. A hydroxylamine N-glycan modified FVIIai (FVIIai-ONH₂) was used for oxime coupling with the aldehyde [¹⁸F]2 under mild and optimized conditions in an isolated RCY of 4.7 ± 0.9%, and a synthesis time of 267 ± 5 min (from EOB). Retained binding and specificity of the resulting [¹⁸F]FVIIai to TF was shown in vitro. TF-expression imaging capability was evaluated by in vivo PET/CT imaging in a pancreatic human xenograft cancer mouse model. The conjugate showed exceptional stability in plasma (>95% at 4 h) and a binding fraction of 90%. In vivo PET/CT imaging showed a mean tumor uptake of 3.8 ± 0.2% ID/g at 4 h post-injection, a comparable uptake in liver and kidneys, and low uptake in normal tissues. In conclusion, FVIIai was labeled with fluorine-18 at the N-glycan chain without affecting TF binding. In vitro specificity and a good in vivo imaging contrast at 4 h postinjection was demonstrated.

Borylated oximes: versatile building blocks for organic synthesis

Herein, we demonstrate the synthesis and functionalization of α-boryl aldoximes from α-boryl aldehydes, with no sign of C-to-N boryl migration. Selective modification of the oxime functionality enables access to a wide range of borylated compounds, such as borylated heterocycles and N-acetoxyamides. By reducing the α-boryl aldoximes, MIDA deprotection yields the corresponding β-boryl hydroxylamines. As part of this study, we also demonstrate the utility of the boryl aldoxime motif in peptide conjugation.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

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.

Single step 18F-labeling of dimeric cycloRGD for functional PET imaging of tumors in mice

INTRODUCTION

Arylboronates afford rapid aqueous (18)F-labeling via the creation of a highly polar (18)F-aryltrifluoroborate anion ((18)F-ArBF3(-)).

HYPOTHESIS

Radiosynthesis of an (18)F-ArBF3(-) can be successfully applied to a clinically relevant peptide. To test this hypothesis, we labeled dimeric-cylcoRGD, [c(RGDfK)]2E because a) it is molecularly complex and provides a challenging substrate to test the application of this technique, and b) [c(RGDfK)]2E has already been labeled via several (18)F-labeling methods which provide for a preliminary comparison.

GOAL

To validate this labeling method in the context of a complex and clinically relevant tracer to show tumor-specific uptake ex vivo with representative PET images in vivo.

METHODS

An arylborimidine was conjugated to [c(RGDfK)]2E to give the precursor [c(RGDfK)]2E-ArB(dan), which was aliquoted and stored at -20 °C. Aliquots of 10 or 25 nmol, containing only micrograms of precursor, were labeled using relatively low levels of (18)F-activity. Following purification eight mice (pre-blocked/unblocked) with U87M xenograft tumors were injected with [c(RGDfK)]2E-(18)F-ArBF3(-) (n = 4) for ex vivo tissue dissection. Two sets of mice (pre-blocked/unblocked) were also imaged with PET-CT (n = 2).

RESULTS

The [c(RGDfK)]2E-ArB(dan) is converted within 15 min to [c(RGDfK)]2E-(18)F-ArBF3(-) in isolated radiochemical yields of ~10% (n = 3) at a minimum effective specific activity of 0.3 Ci/μmol. Biodistribution shows rapid clearance to the bladder via the kidney resulting in high tumor-to-blood and tumor-to-muscle ratios of >9 and >6 respectively while pre-blocking with [c(RGDfK)]2E showed high tumor specificity. PET imaging showed good contrast between tumor and non-target tissues confirming the biodistribution data.

CONCLUSION

An arylborimidine-RGD peptide is rapidly (18)F-labeled in one step, in good yield, at useful specific activity. Biodistribution studies with blocking controls show tumor specificity, which is corroborated by PET images. Advances in Knowledge and Implications for patient Care: Despite many antecedent examples of labeled RGD tracers, this work is the first to show direct aqueous labeling of bisRGD with an (18)F-ArBF3(-). Labeling occurs in near record rapidity (45 min) at useful effective specific activities and competitive yields for high contrast tumor specific images. As bisRGD has been imaged in humans with several prosthetics, this work suggests potential clinical applications of tracers appended with an (18)F-ArBF3(-). More generally, the ability to label a molecularly complex tracer suggests that this method could be useful to label many other peptides. Furthermore, these results portend the development of kits that use only microgram quantities of lyophilized precursor for on demand labeling. The ability to perform one-step aqueous labeling in under an hour to provide tracers with high T:NT ratios has important implications for developing radiotracers for use in fundamental research and in preclinical tracer studies.

Kit-like 18F-labeling of RGD-19F-arytrifluroborate in high yield and at extraordinarily high specific activity with preliminary in vivo tumor imaging

INTRODUCTION

Positron Emission Tomography (PET) is a rapidly expanding, cutting edge technology for preclinical evaluation, cancer diagnosis and staging, and patient management. A one-step aqueous (18)F-labeling method, which can be applied to peptides to provide functional in vivo images, has been a long-standing challenge in PET imaging. Over the past few years, we have sought a rapid and mild radiolabeling method based on the aqueous radiosynthesis of in vivo stable aryltrifluoroborate (ArBF(3)(-)) conjugates. Recent access to production levels of (18)F-Fluoride led to a fluorescent-(18)F-ArBF(3)(-) at unprecedentedly high specific activities of 15Ci/μmol. However, extending this method to labeling peptides as imaging agents has not been explored.

METHODS

In order to extend these results to a peptide of clinical interest in the context of production-level radiosynthesis, we applied this new technology for labeling RGD, measured its specific activity by standard curve analysis, and carried out a preliminary evaluation of its imaging properties.

RESULTS

RGD was labeled in excellent radiochemical yields at exceptionally high specific activity (~14Ci/μmol) (n = 3). Preliminary tumor-specific images corroborated by ex vivo biodistribution data with blocking controls show statistically significant albeit relatively low tumor uptake along with reasonably high tumor:blood ratios (n = 3).

CONCLUSIONS

Isotope exchange on a clinically useful (18)F-ArBF(3)(-) radiotracer leads to excellent radiochemical yields and exceptionally high specific activities while the anionic nature of the aryltrifluoroborate prosthetic results in very rapid clearance. Since rapid clearance of the radioactive tracer is generally desirable for tracer development, these results suggest new directions for varying linker arm composition to slightly retard clearance rather than enhancing it.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This work is the first to use production levels of (18)F-activity to directly label RGD at specific activities that are an order of magnitude higher than most reports and thereby increases the distribution window for radiotracer production and delivery.

Fluoride-18 radiolabeling of peptides bearing an aminooxy functional group to a prosthetic ligand via an oxime bond

We have developed a novel F-18 prosthetic ligand named fluoro-PEG-benzaldehyde (FPBA) 1. [(18)F]-FPBA 1 is formed in situ from its radiolabeled precursor [(18)F]6. Compound 6 is efficiently synthesized in four steps starting from commercially available 6-bromo-3-pyridine carbaldehyde 2. [(18)F]-FPBA was evaluated as a prosthetic ligand to radiolabel three cyclic peptides bearing an aminooxy functional group at the N-terminus position. Acetal [(18)F]6 is purified by either solid-phase extraction (SPE) or reverse-phase HPLC with the overall radiochemical yields (RCY) and radiochemical purity (RCP) in very close agreement. The SPE purification process has the advantage of shorter reaction times (71-87 min for entire reaction sequence), while the use of the reverse-phase HPLC purification process allows the use of up to fifty times less of the expensive synthetic peptides (~ 50 nmol) in the oxime coupling reaction. We have demonstrated an efficient methodology in the production of [(18)F]-FPBA 1 and demonstrated its use as a prosthetic ligand for the labeling of peptides possessing an aminooxy functional group.