Synthesis and Hypoxia Selective Radiosensitization Potential of β-2-FAZA and β-3-FAZL: Fluorinated Azomycin β-Nucleosides

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.

Influence of 2-Nitroimidazoles in the Response of FaDu Cells to Ionizing Radiation and Hypoxia/Reoxygenation Stress

Cellular adaptations to hypoxia promote resistance to ionizing radiation (IR). This presents a challenge for treatment of head and neck cancer (HNC) that relies heavily on radiotherapy. Standard radiosensitizers often fail to reach diffusion-restricted hypoxic cells, whereas nitroimidazoles (NIs) [such as iodoazomycin arabinofuranoside (IAZA) and fluoroazomycin arabinofuranoside (FAZA)] can preferentially accumulate in hypoxic tumours. Here, we explored if the hypoxia-selective uptake of IAZA and FAZA could be harnessed to make HNC cells (FaDu) susceptible to radiation therapy. Cellular response to treatment was assessed through clonogenic survival assays and by monitoring DNA damage (immunofluorescence staining of DNA damage markers, γ-H2AX and p-53BP1, and by alkaline comet assay). The effects of reoxygenation were studied using the following assays: estimation of nucleoside incorporation to assess DNA synthesis rates, immunofluorescent imaging of chromatin-associated replication protein A as a marker of replication stress, and quantification of reactive oxygen species (ROS). Both IAZA and FAZA sensitized hypoxic HNC cells to IR, albeit the former is a better radiosensitizer. Radiosensitization by these compounds was restricted only to hypoxic cells, with no visible effects under normoxia. IAZA and FAZA impaired cellular adaptation to reoxygenation; high levels of ROS, reduced DNA synthesis capacity, and signs of replication stress were observed in reoxygenated cells. Overall, our data highlight the therapeutic potentials of IAZA and FAZA for targeting hypoxic HNC cells and provide rationale for future preclinical studies.

Cellular mechanism of action of 2-nitroimidzoles as hypoxia-selective therapeutic agents

Solid tumours are often poorly oxygenated, which confers resistance to standard treatment modalities. Targeting hypoxic tumours requires compounds, such as nitroimidazoles (NIs), equipped with the ability to reach and become activated within diffusion limited tumour niches. NIs become selectively entrapped in hypoxic cells through bioreductive activation, and have shown promise as hypoxia directed therapeutics. However, little is known about their mechanism of action, hindering the broader clinical usage of NIs. Iodoazomycin arabinofuranoside (IAZA) and fluoroazomycin arabinofuranoside (FAZA) are clinically validated 2-NI hypoxic radiotracers with excellent tumour uptake properties. Hypoxic cancer cells have also shown preferential susceptibility to IAZA and FAZA treatment, making them ideal candidates for an in-depth study in a therapeutic setting. Using a head and neck cancer model, we show that hypoxic cells display higher sensitivity to IAZA and FAZA, where the drugs alter cell morphology, compromise DNA replication, slow down cell cycle progression and induce replication stress, ultimately leading to cytostasis. Effects of IAZA and FAZA on target cellular macromolecules (DNA, proteins and glutathione) were characterized to uncover potential mechanism(s) of action. Covalent binding of these NIs was only observed to cellular proteins, but not to DNA, under hypoxia. While protein levels remained unaffected, catalytic activities of NI target proteins, such as the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the detoxification enzyme glutathione S-transferase (GST) were significantly curtailed in response to drug treatment under hypoxia. Intraperitoneal administration of IAZA was well-tolerated in mice and produced early (but transient) growth inhibition of subcutaneous mouse tumours.

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Hypoxic cells display preferential sensitivity to IAZA and FAZA.

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They alter cell morphology and induce cytostasis.

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IAZA and FAZA generate covalent adducts of proteins but not DNA.

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GAPDH and GST activities, but not protein levels, are significantly reduced.

Efficient preparation of 2-nitroimidazole nucleosides as precursors for hypoxia PET tracers

Abstract

2-Deoxy-D-ribose was converted to α/β-mixtures of methyl 3-O-acetyl- and methyl 3-O-benzoyl-2-deoxy-5-(p-toluenesulfonyl)-D-ribofuranosides. These were reacted with boron trichloride to generate ribofuranosyl chlorides, which afforded precursors for tracers to image tumor hypoxia on substitution with salts of 2-nitroimidazole. The anomeric ratio of the nucleosides was delicately influenced by the reaction conditions.

Graphical abstract

Synthesis and preclinical characterization of 1-(6'-deoxy-6'-[18F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-6'-[18F]FAZAL) as a positron emission tomography radiotracer to assess tumor hypoxia

Positron emission tomography (PET) using fluorine-18 (¹⁸F)-labeled 2-nitroimidazole radiotracers has proven useful for assessment of tumor oxygenation. However, the passive diffusion-driven cellular uptake of currently available radiotracers results in slow kinetics and low tumor-to-background ratios. With the aim to develop a compound that is actively transported into cells, 1-(6'-deoxy-6'-[¹⁸F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-[¹⁸F]1), a putative nucleoside transporter substrate, was synthetized by nucleophilic [¹⁸F]fluoride substitution of an acetyl protected labeling precursor with a tosylate leaving group (β-6) in a final radiochemical yield of 12±8% (n=10, based on [¹⁸F]fluoride starting activity) in a total synthesis time of 60min with a specific activity at end of synthesis of 218±58GBq/μmol (n=10). Both radiolabeling precursor β-6 and unlabeled reference compound β-1 were prepared in multistep syntheses starting from 1,2:5,6-di-O-isopropylidene-α-d-allofuranose. In vitro experiments demonstrated an interaction of β-1 with SLC29A1 and SLC28A1/2/3 nucleoside transporter as well as hypoxia specific retention of β-[¹⁸F]1 in tumor cell lines. In biodistribution studies in healthy mice β-[¹⁸F]1 showed homogenous tissue distribution and excellent metabolic stability, which was unaffected by tissue oxygenation. PET studies in tumor bearing mice showed tumor-to-muscle ratios of 2.13±0.22 (n=4) at 2h after administration of β-[¹⁸F]1. In ex vivo autoradiography experiments β-[¹⁸F]1 distribution closely matched staining with the hypoxia marker pimonidazole. In conclusion, β-[¹⁸F]1 shows potential as PET hypoxia radiotracer which merits further investigation.

[18F]Fluoro-azomycin-2´-deoxy-β-d-ribofuranoside - A new imaging agent for tumor hypoxia in comparison with [18F]FAZA

INTRODUCTION

Radiolabeled 2-nitroimidazoles (azomycins) are a prominent class of biomarkers for PET imaging of hypoxia. [¹⁸F]Fluoro-azomycin-α-arabinoside ([¹⁸F]FAZA) - already in clinical use - may be seen as α-configuration nucleoside, but enters cells only via diffusion and is not transported by cellular nucleoside transporters. To enhance image contrast in comparison to [¹⁸F]FAZA our objective was to ¹⁸F-radiolabel an azomycin-2´-deoxyriboside with β-configuration ([¹⁸F]FAZDR, [¹⁸F]-β-8) to mimic nucleosides more closely and comparatively evaluate it versus [¹⁸F]FAZA.

METHODS

Precursor and cold standards for [¹⁸F]FAZDR were synthesized from methyl 2-deoxy-d-ribofuranosides α- and β-1 in 6 steps yielding precursors α- and β-5. β-5 was radiolabeled in a GE TRACERlab FX_F-N synthesizer in DMSO and deprotected with NH₄OH to give [¹⁸F]FAZDR ([¹⁸F]-β-8). [¹⁸F]FAZA or [¹⁸F]FAZDR was injected in BALB/c mice bearing CT26 colon carcinoma xenografts, PET scans (10min) were performed after 1, 2 and 3h post injection (p.i.). On a subset of mice injected with [¹⁸F]FAZDR, we analyzed biodistribution.

RESULTS

[¹⁸F]FAZDR was obtained in non-corrected yields of 10.9±2.4% (9.1±2.2GBq, n=4) 60min EOB, with radiochemical purity >98% and specific activity >50GBq/μmol. Small animal PET imaging showed a decrease in uptake over time for both [¹⁸F]FAZDR (1h p.i.: 0.56±0.22% ID/cc, 3h: 0.17±0.08% ID/cc, n=9) and [¹⁸F]FAZA (1h: 1.95±0.59% ID/cc, 3h: 0.87±0.55% ID/cc), whereas T/M ratios were significantly higher for [¹⁸F]FAZDR at 1h (2.76) compared to [¹⁸F]FAZA (1.69, P<0.001), 3h p.i. ratios showed no significant difference. Moreover, [¹⁸F]FAZDR showed an inverse correlation between tracer uptake in carcinomas and oxygen breathing, while muscle tissue uptake was not affected by switching from air to oxygen.

CONCLUSIONS

First PET imaging results with [¹⁸F]FAZDR showed advantages over [¹⁸F]FAZA regarding higher tumor contrast at earlier time points p.i. Availability of precursor and cold fluoro standard together with high output radiosynthesis will allow for a more detailed quantitative evaluation of [¹⁸F]FAZDR, especially with regard to mechanistic studies whether active transport processes are involved, compared to passive diffusion as observed for [¹⁸F]FAZA.

Measurement of Tumor Hypoxia in Patients with Advanced Pancreatic Cancer Based on 18F-Fluoroazomyin Arabinoside Uptake

Pancreatic cancers are thought to be unusually hypoxic, which might sensitize them to drugs that are activated under hypoxic conditions. In order to develop this idea in the clinic, a minimally invasive technique for measuring the oxygenation status of pancreatic cancers is needed.

METHODS

We tested the potential for minimally invasive imaging of hypoxia in pancreatic cancer patients, using the 2-nitroimidazole PET tracer (18)F-fluoroazomycin arabinoside (or (18)F-1-α-D-[5-fluoro-5-deoxyarabinofuranosyl]-2-nitroimidazole [(18)F-FAZA]). Dynamic and static scans were obtained in 21 patients with either locally advanced or metastatic disease. The hypoxic fraction was determined in the 2-h static scans as the percentage of voxels with SUVs more than 3 SDs from the mean values obtained for skeletal muscle.

RESULTS

Hypoxia was detected in 15 of 20 evaluable patients, with the hypoxic fraction ranging from less than 5% to greater than 50%. Compartmental analysis of the dynamic scans allowed us to approximate the tumor perfusion as mL/min/g of tissue, a value that is independent of the extent of hypoxia derived from tracer uptake in the 2-h static scan. There was no significant correlation between tumor perfusion and hypoxia; nor did we see an association between tumor volume and hypoxia.

CONCLUSION

Although pancreatic cancers can be highly hypoxic, a substantial proportion appears to be well oxygenated. Therefore, we suggest that a minimally invasive technique such as the one described in this study be used for patient stratification in future clinical trials of hypoxia-targeting agents.

Synthesis of 68Ga-labeled DOTA-nitroimidazole derivatives and their feasibilities as hypoxia imaging PET tracers

The imaging of hypoxia is important for therapeutic decision making in various diseases. (68)Ga is an important radionuclide for positron emission tomography (PET), and its usage is increasing, due to the development of the (68)Ge/(68)Ga-generator. In the present study, the authors synthesized two nitroimidazole derivatives by conjugating nitroimidazole and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) via an amide bond (4) and a thiourea bond (5). Both derivatives were labeled with (68)Ga with high labeling efficiency and were stable after labeling. The low partition coefficients (logP) of (68)Ga-4 (-4.6) and (68)Ga-5 (-4.5) demonstrated the hydrophilic natures of the derivatives, and both showed higher uptake in cancer cell lines cultured under hypoxic condition than under normoxic condition. However, (68)Ga-5 showed higher liver uptake than (68)Ga-4 in a biodistribution study due to higher lipophilicity. In an animal PET study, (68)Ga-4 showed higher standard uptake values (SUV) in tumors than (68)Ga-5 in mice xenografted with CT-26 mouse colon cancer cells.