Sweetening pharmaceutical radiochemistry by (18)f-fluoroglycosylation: a short review

Synthetic 18F labeled biomolecules that are selective and promising for PET imaging: major advances and applications

The concept of positron emission tomography (PET) based imaging was developed more than 40 years ago. It has been a widely adopted technique for detecting and staging numerous diseases in clinical settings, particularly cancer, neuro- and cardio-diseases. Here, we reviewed the evolution of PET and its advantages over other imaging modalities in clinical settings. Primarily, this review discusses recent advances in the synthesis of 18F radiolabeled biomolecules in light of the widely accepted performance for effective PET. The discussion particularly emphasizes the 18F-labeling chemistry of carbohydrates, lipids, amino acids, oligonucleotides, peptides, and protein molecules, which have shown promise for PET imaging in recent decades. In addition, we have deliberated on how 18F-labeled biomolecules enable the detection of metabolic changes at the cellular level and the selective imaging of gross anatomical localization via PET imaging. In the end, the review discusses the future perspective of PET imaging to control disease in clinical settings. We firmly believe that collaborative multidisciplinary research will further widen the comprehensive applications of PET approaches in the clinical management of cancer and other pathological outcomes.

Fluorinated carbohydrates for 18F-positron emission tomography (PET)

Carbohydrate diversity is foundational in the molecular literacy that regulates cellular function and communication. Consequently, delineating and leveraging this structure-function interplay continues to be a core research objective in the development of candidates for biomedical diagnostics. A totemic example is the ubiquity of 2-deoxy-2-[18F]-fluoro-D-glucose (2-[18F]-FDG) as a radiotracer for positron emission tomography (PET), in which metabolic trapping is harnessed. Building on this clinical success, more complex sugars with unique selectivities are gaining momentum in molecular recognition and personalised medicine: this reflects the opportunities that carbohydrate-specific targeting affords in a broader sense. In this Tutorial Review, key milestones in the development of 2-[18F]-FDG and related glycan-based radiotracers for PET are described, with their diagnostic functions, to assist in navigating this rapidly expanding field of interdisciplinary research.

Sweetening Pharmaceutical Radiochemistry by 18F-Fluoroglycosylation: Recent Progress and Future Prospects

In the field of ¹⁸F-chemistry for the development of radiopharmaceuticals for positron emission tomography (PET), various labeling strategies by the use of prosthetic groups have been implemented, including chemoselective ¹⁸F-labeling of biomolecules. Among those, chemoselective ¹⁸F-fluoroglycosylation methods focus on the sweetening of pharmaceutical radiochemistry by offering a highly valuable tool for the synthesis of ¹⁸F-glycoconjugates with suitable in vivo properties for PET imaging studies. A previous review covered the various ¹⁸F-fluoroglycosylation methods that were developed and applied as of 2014 (Maschauer and Prante, BioMed. Res. Int. 2014, 214748). This paper is an updated review, providing the recent progress in ¹⁸F-fluoroglycosylation reactions and the preclinical application of ¹⁸F-glycoconjugates, including small molecules, peptides, and high-molecular-weight proteins.

Targeting Fibroblast Activation Protein: Radiosynthesis and Preclinical Evaluation of an 18F-Labeled FAP Inhibitor

Fibroblast activation protein (FAP) has emerged as an interesting molecular target used in the imaging and therapy of various types of cancers. ⁶⁸Ga-labeled chelator-linked FAP inhibitors (FAPIs) have been successfully applied to PET imaging of various tumor types. To broaden the spectrum of applicable PET tracers for extended imaging studies of FAP-dependent diseases, we herein report the radiosynthesis and preclinical evaluation of an ¹⁸F-labeled glycosylated FAPI ([¹⁸F]FGlc-FAPI). Methods: An alkyne-bearing precursor was synthesized and subjected to click chemistry-based radiosynthesis of [¹⁸F]FGlc-FAPI by 2-step ¹⁸F-fluoroglycosylation. FAP-expressing HT1080hFAP cells were used to study competitive binding to FAP, cellular uptake, internalization, and efflux of [¹⁸F]FGlc-FAPI in vitro. Biodistribution studies and in vivo small-animal PET studies of [¹⁸F]FGlc-FAPI compared with [⁶⁸Ga]Ga-FAPI-04 were conducted in nude mice bearing HT1080hFAP tumors or U87MG xenografts. Results: [¹⁸F]FGlc-FAPI was synthesized with a 15% radioactivity yield and a high radiochemical purity of more than 99%. In HT1080hFAP cells, [¹⁸F]FGlc-FAPI showed specific uptake, a high internalized fraction, and low cellular efflux. Compared with FAPI-04 (half maximal inhibitory concentration [IC₅₀] = 32 nM), the glycoconjugate, FGlc-FAPI (IC₅₀ = 167 nM), showed slightly lower affinity for FAP in vitro, whereas plasma protein binding was higher for [¹⁸F]FGlc-FAPI. Biodistribution studies revealed significant hepatobiliary excretion of [¹⁸F]FGlc-FAPI; however, small-animal PET studies in HT1080hFAP xenografts showed higher specific tumor uptake of [¹⁸F]FGlc-FAPI (4.5 percentage injected dose per gram of tissue [%ID/g]) than of [⁶⁸Ga]Ga-FAPI-04 (2 %ID/g). In U87MG tumor-bearing mice, both tracers showed similar tumor uptake, but [¹⁸F]FGlc-FAPI showed a higher tumor retention. Interestingly, [¹⁸F]FGlc-FAPI demonstrated high specific uptake in bone structures and joints. Conclusion: [¹⁸F]FGlc-FAPI is an interesting candidate for translation to the clinic, taking advantage of the longer half-life and physical imaging properties of ¹⁸F. The availability of [¹⁸F]FGlc-FAPI may allow extended PET studies of FAP-related diseases, such as cancer, but also arthritis, heart diseases, or pulmonary fibrosis.

Efficient cartridge purification for producing high molar activity [18F]fluoro-glycoconjugates via oxime formation

INTRODUCTION

¹⁸F-fluoroglycosylation via oxime formation is a chemoselective and mild radiolabeling method for sensitive molecules. Glycosylation can also improve the bioavailability, in vivo kinetics, and stability of the compound in blood, as well as accelerate clearance of biomolecules. A typical synthesis procedure for ¹⁸F-fluoroglycosylation with [¹⁸F]FDG (2-deoxy-2-[¹⁸F]fluoro-d-glucose) and [¹⁸F]FDR (5-deoxy-5-[¹⁸F]fluoro-d-ribose) involves two HPLC (high performance liquid chromatography) purifications: one after ¹⁸F-fluorination of the carbohydrate to remove its labeling precursor, and a second one after the oxime formation step to remove the aminooxy precursor. The two HPLC purifications can be time consuming and complicate the adaptation of the synthetic strategy in nuclear medicine applications and automated synthesis. We have developed a procedure in which SPE (solid phase extraction) and resin purification methods replace both of the needed HPLC purification steps.

METHODS

We used [¹⁸F]FDR and [¹⁸F]FDG as prosthetic groups to radiolabel two aminooxy-modified model molecules, a tetrazine and a PSMA (prostate specific membrane antigen) inhibitor. After fluorination, the excess carbohydrate precursor was removed by derivatizing it with 4,4'-dimethoxytrityl chloride (DMT-Cl). The DMT moiety increases the hydrophobicity of the unreacted precursor making the separation from the fluorinated precursor possible with simple C18 Sep-Pak cartridge. For removal of the aminooxy precursor, we used a commercially available aldehyde resin (AminoLink, Thermo Fisher Scientific). C18 Sep-Pak SPE cartridge was used to separate [¹⁸F]FDR and [¹⁸F]FDG from the ¹⁸F-fluoroglycoconjugate end product.

RESULTS

[¹⁸F]FDR and [¹⁸F]FDG were efficiently purified from their precursors, free fluorine-18, and other impurities. The aldehyde resin quantitatively removed the unreacted aminooxy precursors after the oxime formation. The fluorine-18 labeled oxime end products were obtained with high radiochemical purity (>99%) and molar activity (>600 GBq μmol^-1).

CONCLUSIONS

We have developed an efficient cartridge purification method for producing high molar activity ¹⁸F-glycoconjugates synthesized via oxime formation.

Synthesis and evaluation of an N-[18F]fluorodeoxyglycosyl amino acid for PET imaging of tumor metabolism

INTRODUCTION

The limitations of [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG), including producing false-positive or -negative results, low image contrast in brain tumor diagnosis and poor differentiation of tumor and inflammatory, necessitate the development of new radiopharmaceuticals. In the present study, a novel [¹⁸F]fluoroglycoconjugate tracer, [¹⁸F]FDGly-NH-Phe, for tumor metabolism imaging was prepared and evaluated.

METHODS

[¹⁸F]FDGly-NH-Phe was prepared by condensing [¹⁸F]FDG with L-4-aminophenylalanine in an acidic condition, and purified with semi-preparative-high performance liquid chromatography (HPLC). The in vitro stability study was conducted in phosphate-buffered saline (PBS, pH 4.0-9.18) at room temperature (RT) and in fetal bovine serum (FBS) at 37 °C. The preliminary cellular uptake studies were performed using Hep-2 cell. The bio-distribution studies, PET/CT imaging and metabolism studies were performed and compared with [¹⁸F]FDG on ICR or BALB/c nude model mice.

RESULTS

[¹⁸F]FDGly-NH-Phe was derived from a direct condensation of [¹⁸F]FDG with L-4-aminophenylalanine with high stability in FBS and PBS (pH of 6.5-9.18). In vitro cell experiments showed that [¹⁸F]FDGly-NH-Phe uptake in Hep-2 cells was primarily transported through amino acid transporters including Na⁺-dependent A system, ASC system, and system B^0,+ system. The bio-distribution of [¹⁸F]FDGly-NH-Phe in normal ICR mice showed faster blood radioactivity clearance, and lower uptake in brain and heart than [¹⁸F]FDG. The performance of PET/CT imaging for [¹⁸F]FDGly-NH-Phe in the mice model manifested excellent tumor visualization, high tumor-to-background ratios, and low accumulation in inflammatory lesions. Metabolism studies for [¹⁸F]FDGly-NH-Phe indicated high in vivo stability in plasma and urine and decomposition into [¹⁸F]FDG in the tumor microenvironment.

CONCLUSION

The results demonstrated that [¹⁸F]FDGly-NH-Phe as a novel amino acid PET tracer showed the capability to differentiate tumor from inflammation, and the potentials for future clinical applications.

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.

β-Configured clickable [18F]FDGs as novel 18F-fluoroglycosylation tools for PET

We have developed a chemoselective ¹⁸F-fluoroglycosylation method for PET imaging that employ β-configured [¹⁸F]FDGs as prosthetic groups for ¹⁸F-labeling using CuAAC click chemistry.

Radiosynthesis and Preclinical Evaluation of 18F-Fluoroglycosylated Octreotate for Somatostatin Receptor Imaging

Short synthetic octapeptide analogs derived from the native somatostatin peptides SST-14 and SST-28, namely, octreotate (TATE) or octreotide (TOC), bind with high affinity to somatostatin receptors (sstr), mainly to subtypes 2 and 5, which are expressed in high density on neuroendocrine tumors (NET). Therefore, radiolabeled TATE or TOC derivatives represent highly valuable imaging probes for NET diagnosis by positron emission tomography (PET). The aim of our study was the development of an ¹⁸F-labeled octreotate analog as an alternative radiotracer for the clinically established ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE. We applied our previously developed method based on copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) to the radiosynthesis of ¹⁸F-fluoroglycosylated TATE ([¹⁸F]FGlc-TATE). [¹⁸F]FGlc-TATE was obtained in high yields of 19-22% (non-decay-corrected, referred to [¹⁸F]fluoride) and in high specific activities of 32-106 GBq/μmol. [¹⁸F]FGlc-TATE showed high affinity to sstr expressed on AR42J cells (IC₅₀ = 4.2 nM) with fast and high internalization, and a beneficial logD_7.4 of -1.8. In AR42J tumor bearing nude mice, [¹⁸F]FGlc-TATE showed high and specific tumor uptake of 5.6%ID/g at 60 min post-injection, as determined by blocking experiments using octreotide, and fast clearance from other organs, resulting in excellent tumor-to-blood ratios increasing from 9 to 17 from 30 to 60 min post-injection. Small animal PET studies revealed high uptake of [¹⁸F]FGlc-TATE in the tumor which could be blocked with octreotide by >99%. Overall, [¹⁸F]FGlc-TATE revealed excellent in vitro and in vivo properties and is therefore a viable alternative ¹⁸F-labeled radiopeptide for imaging somatostatin receptor-positive tumors by PET.

18F-labelled intermediates for radiosynthesis by modular build-up reactions: newer developments

This brief review gives an overview of newer developments in (18)F-chemistry with the focus on small (18)F-labelled molecules as intermediates for modular build-up syntheses. The short half-life (<2 h) of the radionuclide requires efficient syntheses of these intermediates considering that multistep syntheses are often time consuming and characterized by a loss of yield in each reaction step. Recent examples of improved synthesis of (18)F-labelled intermediates show new possibilities for no-carrier-added ring-fluorinated arenes, novel intermediates for tri[(18)F]fluoromethylation reactions, and (18)F-fluorovinylation methods.

PET radiopharmaceuticals for imaging integrin expression: tracers in clinical studies and recent developments

Noninvasive determination of integrin expression has become an interesting approach in nuclear medicine. Since the discovery of the first 18F-labeled cyclic RGD peptide as radiotracer for imaging integrin αvβ3 expression in vivo, there have been carried out enormous efforts to develop RGD peptides for PET imaging. Moreover, in recent years, additional integrins, including α5β1 and αvβ6 came into the focus of pharmaceutical radiochemistry. This review will discuss the tracers already evaluated in clinical trials and summarize the preliminary outcome. It will also give an overview on recent developments to further optimize the first-generation compounds such as [18F]Galacto-RGD. This includes recently developed 18F-labeling strategies and also new approaches in 68Ga-complex chemistry. Furthermore, the approaches to develop radiopharmaceuticals targeting integrin α5β1 and αvβ6 will be summarized and discussed.

18F-labeling using click cycloadditions

Due to expanding applications of positron emission tomography (PET) there is a demand for developing new techniques to introduce fluorine-18 (t_1/2 = 109.8 min). Considering that most novel PET tracers are sensitive biomolecules and that direct introduction of fluorine-18 often needs harsh conditions, the insertion of ¹⁸F in those molecules poses an exceeding challenge. Two major challenges during ¹⁸F-labeling are a regioselective introduction and a fast and high yielding way under mild conditions. Furthermore, attention has to be paid to functionalities, which are usually present in complex structures of the target molecule. The Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and several copper-free click reactions represent such methods for radiolabeling of sensitive molecules under the above-mentioned criteria. This minireview will provide a quick overview about the development of novel ¹⁸F-labeled prosthetic groups for click cycloadditions and will summarize recent trends in copper-catalyzed and copper-free click ¹⁸F-cycloadditions.