A novel method for stereospecific fluorination at the 2′-arabino-position of pyrimidine nucleoside: synthesis of [18F]-FMAU

Ring opening of epoxides with [18F]FeF species to produce [18F]fluorohydrin PET imaging agents

A simple technique for the preparation of [18F]HF has been developed and applied to the generation of an [18F]FeF species for opening sterically hindered epoxides. This method has been successfully employed to prepare four drug-like molecules, including 5-[18F]fluoro-6-hydroxy-cholesterol, a potential adrenal/endocrine PET imaging agent. This easily automated one-pot procedure produces sterically hindered fluorohydrin PET imaging agents in good yields and high molar activities.

Synthesis and in vitro growth inhibitory activity of novel silyl- and trityl-modified nucleosides

Seventeen silyl- and trityl-modified (5'-O- and 3',5'-di-O-) nucleosides were synthesized with the aim of investigating the in vitro antiproliferative activities of these nucleoside derivatives. A subset of the compounds was evaluated at a fixed concentration of 100μM against a small panel of tumor cell lines (HL-60, K-562, Jurkat, Caco-2 and HT-29). The entire set was also tested at varying concentrations against two human glioma lines (U373 and Hs683) to obtain GI50 values, with the best results being values of ∼25μM.

Radiosynthesis of 6'-Deoxy-6'[18F]Fluorosucrose via Automated Synthesis and Its Utility to Study In Vivo Sucrose Transport in Maize (Zea mays) Leaves

Sugars produced from photosynthesis in leaves are transported through the phloem tissues within veins and delivered to non-photosynthetic organs, such as roots, stems, flowers, and seeds, to support their growth and/or storage of carbohydrates. However, because the phloem is located internally within the veins, it is difficult to access and to study the dynamics of sugar transport. Radioactive tracers have been extensively used to study vascular transport in plants and have provided great insights into transport dynamics. To better study sucrose partitioning in vivo, a novel radioactive analog of sucrose was synthesized through a completely chemical synthesis route by substituting fluorine-18 (half-life 110 min) at the 6' position to generate 6'-deoxy-6'[(18)F]fluorosucrose ((18)FS). This radiotracer was then used to compare sucrose transport between wild-type maize plants and mutant plants lacking the Sucrose transporter1 (Sut1) gene, which has been shown to function in sucrose phloem loading. Our results demonstrate that (18)FS is transported in vivo, with the wild-type plants showing a greater rate of transport down the leaf blade than the sut1 mutant plants. A similar transport pattern was also observed for universally labeled [U-(14)C]sucrose ([U-(14)C]suc). Our findings support the proposed sucrose phloem loading function of the Sut1 gene in maize, and additionally demonstrate that the (18)FS analog is a valuable, new tool that offers imaging advantages over [U-(14)C]suc for studying phloem transport in plants.

A novel radiochemical approach to 1-(2'-deoxy-2'-[(18) F]fluoro-β-d-arabinofuranosyl)cytosine ((18) F-FAC)

(18) F-FAC (1-(2'-deoxy-2'-[(18) F]fluoro-β-D-arabinofuranosyl)-cytosine) is an important 2'-fluoro-nucleoside-based positron emission tomography (PET) tracer that has been used for in vivo prediction of response to the widely used cancer chemotherapy drug gemcitabine. Previously reported synthetic routes to (18) F-FAC have relied on early introduction of the (18) F radiolabel prior to attachment to protected cytosine base. Considering the (18) F radiochemical half-life (110 min) and the technical challenges of multi-step syntheses on PET radiochemistry modular systems, late-stage radiofluorination is preferred for reproducible and reliable radiosynthesis with in vivo applications. Herein, we report the first late-stage radiosynthesis of (18) F-FAC. Cytidine derivatives with leaving groups at the 2'-position are particularly prone to undergo anhydro side-product formation upon heating because of their electron density at the 2-carbonyl pyrimidone oxygen. Our rationally developed fluorination precursor showed an improved reactivity-to-stability ratio at elevated temperatures. (18) F-FAC was obtained in radiochemical yields of 4.3-5.5% (n = 8, decay-corrected from end of bombardment), with purities ≥98% and specific activities ≥63 GBq/µmol. The synthesis time was 168 min.

Radiochemical synthesis of 2'-[18F]-labelled and 3'-[18F]-labelled nucleosides for positron emission tomography imaging

This review article considers 2'-labelled and 3'-labelled nucleosides, which are of great importance as positron emission tomography (PET) probes in clinical diagnostics and PET research. Although the radiochemical preparation of several [(18)F]-labelled nucleosides such as [(18)F]fluorothymidine or [(18)F](fluoroarabinofuranosyl)cytosine has been accomplished within the last two decades, a number of potentially interesting nucleoside-based biomarkers are not yet available for automated good manufacturing practice production due to the lack of fast and efficient synthetic methods for late-stage [(18)F]-introduction. In order to meet recent demands for new PET-based biomarkers in various clinical applications, appropriate precursors that can easily be fluorinated and deprotected need to be developed.

Abstract B32: Synthetic routes to 18F-labeled gemcitabine and related 2'-fluoronucleosides

Gemcitabine (2,2′-difluoro-2′-deoxycytidine, dFdC) is an established chemotherapeutic agent used in the treatment of various carcinomas such as lung, breast, bladder and especially pancreatic cancer (1)]. However, its general application and bioavailability is compromised due to both poor cell uptake and rapid metabolism by gut and liver cytidine deaminase (CD)(2). An 18F-gemcitabine positron emission tomography (PET) probe would allow tracking of this important drug on its biomolecular pathways in vivo. Treatment improvement and a deeper understanding of cancer biochemistry are some possible beneficial outcomes. The half-life of 18F (110 minutes) and GMP regulations restrict synthetic and purification procedures to a short period of time. Thus, the main focus of this project is the development of new powerful strategies to introduce fluorine at the latest stage of the chemical synthesis into appropriate nucleoside precursors. In order to approach the synthetic target molecule dFdC, proof-of-principle studies on more straightforward synthetic targets including 1-(2′-deoxy-2′-[18F]fluoro-beta-D-arabinofuranosyl)uracil (18F-FAU) and 1-(2′-deoxy-2′-[18F]fluoro-beta-D-arabinofuranosyl)cytidine (18F-FAC) are carried out first. An appropriate precursor synthesis for 2′-stereoselective radiofluorination is followed by the introduction of the 18F-isotope under recently developed conditions (3,4). Radiochemical syntheses are carried out using an Eckert & Ziegler module system. The obtained radio-nucleosides are confirmed by radio-HPLC and subsequent radio-ESI-MS at the Wales Research & Diagnostic Positron Emission Tomography Imaging Centre (PETIC) located at Heath Hospital in Cardiff. Several fluorination methods and nucleoside precursors were tested under different conditions. The 2′-fluorinated arabinonucleoside 2′-deoxy-2′-fluorouracil (FAU), which was considered as a rapidly accessible 2′-fluorinated uracil-based dFdC analogue was successfully synthesized in our laboratory. Here, a protected mesylate precursor was treated with tetrabutylammonium fluoride (TBAF) under anhydrous conditions at reflux temperatures in acetonitrile to give the fluorinated nucleoside FAU in acceptable overall yields after subsequent deprotection. This procedure was successfully used as a template method to obtain 18F-FAU and 18F-FIAU in a new and efficient way in good radiochemical yields (14-17%, decay-corrected). This knowledge is now being transferred to the late-stage synthesis of the more challenging and biologically relevant cytidine-based radiotracer 18F-FAC at PETIC which has potential application in the imaging of neurogenesis. Additionally, the successful radiochemical synthesis of the PET probe FIAU will enable its potential applications in reporter-gene imaging developments. Considering the final target, the development of an appropriate fluorination precursor for the radiosynthesis of 18F-Gemcitabine will be investigated mainly based on the results obtained from the mono-fluoro model studies.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B32.

Citation Format: Jan-Philip Meyer. Synthetic routes to 18F-labeled gemcitabine and related 2'-fluoronucleosides. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B32.

Nucleoside-based probes for imaging tumor proliferation using positron emission tomography

Cancer is one of the leading causes of human death, and early detection can be beneficial for its timely therapy and management. For the early detection of cancer, positron emission tomography (PET) is more accurate and sensitive than other imaging modalities, such as computed tomography and magnetic resonance imaging. [(18) F]-Labeled fluorodeoxyglucose is the most useful PET probe in early detection of cancer; however, its nonspecific accumulation and consequent false-positive findings warrant the identification of other PET probes. Thymidine (TdR) and its analogs have been radiolabeled for PET imaging of cellular proliferation and DNA synthesis. Because of its in vivo instability, radiolabeled TdR has not been successful in PET imaging. However, some of its radiolabeled analogs have been developed for PET imaging of cellular proliferation and DNA synthesis. In this review, the radiochemistry and production of (11) C-TdR and (11) C/(18) F-labeled TdR analogs published to date are presented.

An investigation on stereospecific fluorination at the 2'-arabino-position of a pyrimidine nucleoside: radiosynthesis of 2'-deoxy-2'-[(18)F]fluoro-5-methyl-1-β-D-arabinofuranosyluracil

Direct fluorination at the 2'-arabino-position of a pyrimidine nucleoside has been a long-standing challenge, yet we recently reported such a stereospecific fluorination for the first time in the synthesis of [(18)F]FMAU, albeit in low yields. Herein we report the results of an investigation on stereospecific fluorination on a variety of precursors for synthesis of [(18)F]FMAU. Several precursors were synthesized in multiple steps and fluorination was performed at the 2'-arabino position using K[(18)F]/kryptofix 2.2.2. All precursors produced [(18)F]FMAU in low yields.

An improved strategy for the synthesis of [¹⁸F]-labeled arabinofuranosyl nucleosides

The expression of the herpes simplex virus type-1 thymidine kinase (HSV1-tk) gene can be imaged efficaciously using a variety of 2'-[(18)F]fluoro-2'-deoxy-1-b-D-arabinofuranosyl-uracil derivatives [[(18)F]-FXAU, X=I(iodo), E(ethyl), and M(methyl)]. However, the application of these derivatives in clinical and translational studies has been impeded by their complicated and long syntheses (3-5h). To remedy these issues, in the study at hand we have investigated whether microwave or combined catalysts could facilitate the coupling reaction between sugar and nucleobase and, further, have probed the feasibility of establishing a novel approach for [(18)F]-FXAU synthesis. We have demonstrated that the rate of the trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed coupling reaction between the 2-deoxy-sugar and uracil derivatives at 90 °C can be significantly accelerated by microwave-driven heating or by the addition of Lewis acid catalyst (SnCl(4)). Further, we have observed that the stability of the α- and β-anomers of [(18)F]-FXAU derivatives differs during the hydrolysis step. Using the microwave-driven heating approach, overall decay-corrected radiochemical yields of 19%-27% were achieved for [(18)F]-FXAU in 120min at a specific activity of >22MBq/nmol (595Ci/mmol). Ultimately, we believe that these high yielding syntheses of [(18)F]-FIAU, [(18)F]-FMAU and [(18)F]-FEAU will facilitate routine production for clinical applications.