Fully automated synthesis of 4-[18F]fluorobenzylamine based on borohydride/NiCl2 reduction

Synthesis of a 2-nitroimidazole derivative N-(4-[18F]fluorobenzyl)-2-(2-nitro-1H-imidazol-1-yl)-acetamide ([18 F]FBNA) as PET radiotracer for imaging tumor hypoxia

BACKGROUND

Tissue hypoxia is a pathological condition characterized by reducing oxygen supply. Hypoxia is a hallmark of tumor environment and is commonly observed in many solid tumors. Non-invasive imaging techniques like positron emission tomography (PET) are at the forefront of detecting and monitoring tissue hypoxia changes in vivo.

RESULTS

We have developed a novel ¹⁸F-labeled radiotracer for hypoxia PET imaging based on cytotoxic agent benznidazole. Radiotracer N-(4-[¹⁸F]fluorobenzyl)-2-(2-nitro-1H-imidazol-1-yl)acetamide ([¹⁸F]FBNA) was synthesized through acylation chemistry with readily available 4-[¹⁸F]fluorobenzyl amine. Radiotracer [¹⁸F]FBNA was obtained in good radiochemical yields (47.4 ± 5.3%) and high radiochemical purity (> 95%). The total synthesis time was 100 min, including HPLC purification and the molar activity was greater than 40 GBq/µmol. Radiotracer [¹⁸F]FBNA was stable in saline and mouse serum for 6 h. [¹⁸F]FBNA partition coefficient (logP = 1.05) was found to be more lipophilic than [¹⁸F]EF-5 (logP = 0.75), [¹⁸F]FMISO (logP = 0.4) and [¹⁸F]FAZA (logP =  - 0.4). In vitro studies showed that [¹⁸F]FBNA accumulates in gastric cancer cell lines AGS and MKN45 under hypoxic conditions.

CONCLUSIONS

Hence, [¹⁸F]FBNA represents a novel and easy-to-prepare PET radioligand for imaging hypoxia.

Isotopic Radiolabeling of the Antiretroviral Drug [18F]Dolutegravir for Pharmacokinetic PET Imaging

Deciphering the drug/virus/host interactions at infected cell reservoirs is a key leading to HIV-1 remission for which positron emission tomography (PET) imaging using radiolabeled antiretroviral (ARV) drugs is a powerful asset. Dolutegravir (DTG) is one of the preferred therapeutic options to treat HIV and can be isotopically labeled with fluorine-18. [18F]DTG was synthesized via a three-step approach of radiofluorination/nitrile reduction/peptide coupling with optimization for each step. Radiofluorination was performed on 2-fluoro-4-nitrobenzonitrile in 90% conversion followed by nitrile reduction using sodium borohydride and aqueous nickel(II) chloride with 72% conversion. Final peptide coupling reaction followed by HPLC purification and formulation afforded ready-to-inject [18F]DTG in 5.1 ± 0.8% (n = 10) decay-corrected radiochemical yield within 95 min. The whole process was automatized using a TRACERlab® FX NPro module, and quality control performed by analytical HPLC showed that [18F]DTG was suitable for in vivo injection with >99% chemical and radiochemical purity and a molar activity of 83 ± 18 GBq/µmol (n = 10). Whole-body distribution of [18F]DTG was performed by PET imaging on a healthy macaque and highlighted the elimination routes of the tracer. This study demonstrated the feasibility of in vivo [18F]DTG PET imaging and paved the way to explore drug/virus/tissues interactions in animals and humans.

Convenient Entry to 18F-Labeled Amines through the Staudinger Reduction

Fluorine-18 possesses outstanding decay characteristics for positron emission tomography (PET) imaging. Therefore, it is ideally suited for clinical applications. As such, improved strategies to incorporate fluorine-18 into bioactive molecules are of utmost importance. Indirect 18F-labeling with amino-functionalized synthons is a convenient and versatile approach to synthesize a broad variety of PET tracers. Herein, we report a method to convert 18F-labeled azides to primary amines by means of the Staudinger reduction. Aliphatic and aromatic 18F-labeled azides were converted into the corresponding amines with high conversion yields. The method was easily automated. From a broader perspective, the applied strategy results in two useful synthons from a single precursor and thus increases the flexibility to label diverse chemical scaffolds with minimal synthetic effort.

Sulfo-click chemistry with 18F-labeled thio acids

The first application of sulfo-click chemistry with ¹⁸F-labeled thio acids is described.

Design and Synthesis of an Easily Obtainable Maleimide Reagent N-[2-(4-[18F]fluoro-N-methylbenzenesulfonamido)ethyl]maleimide ([18F]FBSEM) to Radiolabel Thiols in Proteins

An easily obtainable thiol-selective labeling reagent [¹⁸F]FBSEM (N-[2-(4-[¹⁸F]fluoro-N-methylbenzenesulfonamido)ethyl]maleimide) was developed. The advantage of the design is that the precursor and [¹⁸F]FBSEM have the same backbone and backbone construction is not required; in contrast, known thiol-specific labeling reagents do require backbone construction, and this is thought to be the cause of their complicated synthesis. [¹⁸F]FBSEM was successfully obtained in higher yield (25%) and in a simpler way (two fluorination and deprotection steps in 65 min) than the widely used [¹⁸F]FBEM (N-[2-(4-[¹⁸F]fluorobenzamide)ethyl]maleimide). The labeling efficacy of [¹⁸F]FBSEM was confirmed by conjugation with glutathione. [¹⁸F]FBSEM is a promising labeling agent for proteins.

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.

PET imaging of cyclooxygenase-2 (COX-2) in a pre-clinical colorectal cancer model

Background

Cyclooxygenase-2 (COX-2) is the inducible isoform of the cyclooxygenase enzyme family. COX-2 is involved in tumor development and progression, and frequent overexpression of COX-2 in a variety of human cancers has made COX-2 an important drug target for cancer treatment. Non-invasive imaging of COX-2 expression in cancer would be useful for assessing COX-2-mediated effects on chemoprevention and radiosensitization using COX-2 inhibitors as an emerging class of anti-cancer drugs, especially for colorectal cancer. Herein, we describe the radiopharmacological analysis of [¹⁸F]Pyricoxib, a novel radiolabeled COX-2 inhibitor, for specific PET imaging of COX-2 in colorectal cancer.

Methods

Uptake of [¹⁸F]Pyricoxib was assessed in human colorectal cancer cell lines HCA-7 (COX-2 positive) and HCT-116 (COX-2 negative). Standard COX-2 inhibitors were used to test for specificity of [¹⁸F]Pyricoxib for COX-2 binding in vitro and in vivo. PET imaging, biodistribution, and radiometabolite analyses were included into radiopharmacological evaluation of [¹⁸F]Pyricoxib.

Results

Radiotracer uptake in COX-2 positive HCA-7 cells was significantly higher than in COX-2 negative HCT-116 cells (P < 0.05). COX-2 inhibitors, celecoxib, rofecoxib, and SC58125, blocked uptake of [¹⁸F]Pyricoxib in HCA-7 cells in a concentration-dependent manner. The radiotracer was slowly metabolized in mice, with approximately 60 % of intact compound after 2 h post-injection. Selective COX-2-mediated tumor uptake of [¹⁸F]Pyricoxib in HCA-7 xenografts was confirmed in vivo. Celecoxib (100 mg/kg) selectively blocked tumor uptake by 16 % (PET image analysis; P < 0.05) and by 51 % (biodistribution studies; P < 0.01).

Conclusions

The novel PET radiotracer [¹⁸F]Pyricoxib displays a promising radiopharmacological profile to study COX-2 expression in cancer in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0192-9) contains supplementary material, which is available to authorized users.

Design, Synthesis, and Evaluation of an (18)F-Labeled Radiotracer Based on Celecoxib-NBD for Positron Emission Tomography (PET) Imaging of Cyclooxygenase-2 (COX-2)

A series of novel fluorine-containing cyclooxygenase-2 (COX-2) inhibitors was designed and synthesized based on the previously reported fluorescent COX-2 imaging agent celecoxib-NBD (3; NBD=7-nitrobenzofurazan). In vitro COX-1/COX-2 inhibitory data show that N-(4-fluorobenzyl)-4-(5-p-tolyl-3-trifluoromethylpyrazol-1-yl)benzenesulfonamide (5; IC50 =0.36 μM, SI>277) and N-fluoromethyl-4-(5-p-tolyl-3-trifluoromethylpyrazol-1-yl)benzenesulfonamide (6; IC50 =0.24 μM, SI>416) are potent and selective COX-2 inhibitors. Compound 5 was selected for radiolabeling with the short-lived positron emitter fluorine-18 ((18) F) and evaluated as a positron emission tomography (PET) imaging agent. Radiotracer [(18) F]5 was analyzed in vitro and in vivo using human colorectal cancer model HCA-7. Although radiotracer uptake into COX-2-expressing HCA-7 cells was high, no evidence for COX-2-specific binding was found. Radiotracer uptake into HCA-7 tumors in vivo was low and similar to that of muscle, used as reference tissue.

Radiolabeling of poly(lactic-co-glycolic acid) (PLGA) nanoparticles with biotinylated F-18 prosthetic groups and imaging of their delivery to the brain with positron emission tomography

The avidin–biotin interaction permits rapid and nearly irreversible noncovalent linkage between biotinylated molecules and avidin-modified substrates. We designed a biotinylated radioligand intended for use in the detection of avidin-modified polymer nanoparticles in tissue with positron emission tomography (PET). Using an F-18 labeled prosthetic group, [¹⁸F]4-fluorobenzylamine, and a commercially available biotin derivate, NHS-PEG₄-biotin, [¹⁸F]-fluorobenzylamide-poly(ethylene glycol)₄-biotin ([¹⁸F]NPB4) was prepared with high purity and specific activity. The attachment of the [¹⁸F]NPB4 radioligand to avidin-modified poly(lactic-co-glycolic acid) (PLGA) nanoparticles was tested by using PET imaging to measure the kinetics of convection-enhanced delivery (CED) of nanoparticles of varying size to the rat brain. PET imaging enabled the direct observation of nanoparticle delivery by measurement of the spatial volume of distribution of radiolabeled nanoparticles as a function of time, both during and after the infusion. This work thus validates new methods for radiolabeling PEG-biotin derivatives and also provides insight into the fate of nanoparticles that have been infused directly into the brain.

18F-glutathione conjugate as a PET tracer for imaging tumors that overexpress L-PGDS enzyme

Lipocalin-type prostaglandin D synthase (L-PGDS) has been correlated with the progression of neurological disorders. The present study aimed at evaluating the imaging potency of a glutathione conjugate of fluorine-18-labeled fluorobutyl ethacrynic amide ([¹⁸F]FBuEA-GS) for brain tumors. Preparation of [¹⁸F]FBuEA-GS has been modified from the -4-tosylate derivative via radiofluorination in 5% radiochemical yield. The mixture of nonradioactive FBuEA-GS derived from a parallel preparation has be resolved to two isomers in a ratio of 9∶1 using analytic chiral reversed phase high performance liquid chromatography (RP-HPLC). The two fluorine-18-labeled isomers purified through nonchiral semipreparative RP-HPLC as a mixture were studied by assessing the binding affinity toward L-PGDS through a gel filtration HPLC, by analyzing radiotracer accumulation in C6 glioma cells, and by evaluating the imaging of radiotracer in a C6 glioma rat with positron emission tomography. The inhibition percentage of the production of PGD₂ from PGH₂ at the presence of 200 µM of FBuEA-GS and 4-Dibenzo[a,d]cyclohepten-5-ylidene-1-[4-(2H-tetrazol-5-yl)butyl]piperidine (AT-56) were 74.1±4.8% and 97.6±16.0%, respectively. [¹⁸F]FBuEA-GS bound L-PGDS (16.3–21.7%) but not the isoform, microsomal prostaglandin E synthase 1. No binding to GST-alpha and GST-pi was observed. The binding strength between [¹⁸F]FBuEA-GS and L-PGDS has been evaluated using analytic gel filtration HPLC at the presence of various concentrations of the cold competitor FBuEA-GS. The contrasted images indicated that the radiotracer accumulation in tumor lesions is probably related to the overexpression of L-PGDS.

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.