4-Fluorobenzylamine and phenylalanine methyl ester conjugates of 2-nitroimidazole: evaluation as hypoxic cell radiosensitizers

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.

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.

Positron emission tomography radiotracers for imaging hypoxia

Localized hypoxia, the physiological hallmark of many clinical pathologies, is the consequence of acute or chronic ischemia in the affected region or tissue. The versatility, sensitivity, quantitative nature, and increasing availability of positron emission tomography (PET) make it the preclinical and clinical method of choice for functional imaging of tissue hypoxia at the molecular level. The progress and current status of radiotracers for hypoxia-specific PET imaging are reviewed in this article including references mainly focused on radiochemistry and also relevant to molecular imaging of hypoxia in preclinical and clinical studies.

Synthesis of a F-18 labeled analog of antitumor prostaglandin delta 7-PGA1 methyl ester using p-[18F]fluorobenzylamine

A fluorine-18 labeled analog of an antitumor prostaglandin delta 7-PGA1 methyl ester, 15-deoxy-13,14-dihydro-delta 7-PGA1 4-[18F]fluorobenzyl amide ([18F]3), was synthesized as a tracer candidate for detecting tumors with positron emission tomography. p-[18F]Fluorobenzylamine (p-[18F]FBnA) used as a labeled precursor for the synthesis of [18F]3 was prepared by fluorination of a 4-N, N, N-trimethylammonium-benzonitrile triflate with [18F]fluoride and subsequent reduction with borane-dimethylsulfide. Radiochemical yield and purity of p-[18F]FBnA obtained were 39-49% (decay uncorrected) and 91-96%, respectively, after C18 Sep-Pak purification. Treatment of p-[18F]FBnA with a 15-deoxy-13,14-dihydro-delta 7-PGA1 N-succinimidyl ester in acetonitrile and subsequent HPLC purification gave radiochemically pure (> 99%) [18F]3 with a 58% decay uncorrected yield. The total synthesis time was 70 min from the start of the radiosynthesis of p-[18F]FBnA.

Synthesis and radioiodination of a nido-1,2-carboranyl derivative of 2-nitroimidazole

The synthesis of a nido-carboranyl congener of misonidazole, 1-(3'-nido-carboranyl-2'-hydroxy)propyl-2-nitroimidazole, has been carried out. Alternative methods of preparations were conducted to optimize the chemical yield, with a five step synthesis giving an overall yield (from 1,2-carborane) of 36%. A diastereomeric pair of nido-carboranyl compounds was obtained. The diastereomeric nido-carboranyl misonidazole congeners were (radio)iodinated to yield (> 90%) a mixture of diastereomeric compounds in which the iodine had bonded to a boron atom on the nido-carborane moiety. These compounds will be investigated for their application to boron neutron capture therapy (BNCT) and hypoxia imaging of cancer.

Strategies for the design and synthesis of boronated nucleic acid and protein components as potential delivery agents for neutron capture therapy

PURPOSE

Strategies for the design and synthesis of boronated nucleosides, amino acids, and peptides as potential delivery agents for boron neutron capture therapy (BNCT) are described.

METHODS AND MATERIALS

For BNCT to be a useful treatment modality, there is a need to design and synthesize nontoxic boron compounds that selectively target tumor cells, accumulate in sufficient amounts (20-30 micrograms 10B/g of tumor) and persist at therapeutic levels for a sufficient time prior to neutron irradiation. Boronated nucleosides, amino acids and peptides are such promising target compounds. Such structures may be selectively used by proliferating neoplastic cells compared with mitotically less active normal cells and therefore achieve the tissue differentials necessary for BNCT.

RESULTS

The rationale for synthesis of boronated nucleic acid and protein components is discussed. Results of biological and clinical studies of some boronated nucleosides, nucleotides, amino acids and peptides are presented.

CONCLUSION

Boronated nucleosides, amino acids and peptides can be considered as potential targeting agents for BNCT.