Fluorinase-coupled base swaps: synthesis of [18F]-5'-deoxy-5'-fluorouridines

State of the art procedures towards reactive [18F]fluoride in PET tracer synthesis

BACKGROUND

Positron emission tomography (PET) is a powerful, non-invasive preclinical and clinical nuclear imaging technique used in disease diagnosis and therapy assessment. Fluorine-18 is the predominant radionuclide used for PET tracer synthesis. An impressive variety of new 'late-stage' radiolabeling methodologies for the preparation of 18F-labeled tracers has appeared in order to improve the efficiency of the labeling reaction.

MAIN BODY

Despite these developments, one outstanding challenge into the early key steps of the process remains: the preparation of reactive [18F]fluoride from oxygen-18 enriched water ([18O]H2O). In the last decade, significant changes into the trapping, elution and drying stages have been introduced. This review provides an overview of the strategies and recent developments in the production of reactive [18F]fluoride and its use for radiolabeling.

CONCLUSION

Improved, modified or even completely new fluorine-18 work-up procedures have been developed in the last decade with widespread use in base-sensitive nucleophilic 18F-fluorination reactions. The many promising developments may lead to a few standardized drying methodologies for the routine production of a broad scale of PET tracers.

Chemoenzymatic Syntheses of Fluorine-18-Labeled Disaccharides from [18F] FDG Yield Potent Sensors of Living Bacteria In Vivo

Chemoenzymatic techniques have been applied extensively to pharmaceutical development, most effectively when routine synthetic methods fail. The regioselective and stereoselective construction of structurally complex glycans is an elegant application of this approach that is seldom applied to positron emission tomography (PET) tracers. We sought a method to dimerize 2-deoxy-[18F]-fluoro-d-glucose ([18F]FDG), the most common tracer used in clinical imaging, to form [18F]-labeled disaccharides for detecting microorganisms in vivo based on their bacteria-specific glycan incorporation. When [18F]FDG was reacted with β-d-glucose-1-phosphate in the presence of maltose phosphorylase, the α-1,4- and α-1,3-linked products 2-deoxy-[18F]-fluoro-maltose ([18F]FDM) and 2-deoxy-2-[18F]-fluoro-sakebiose ([18F]FSK) were obtained. This method was further extended with the use of trehalose (α,α-1,1), laminaribiose (β-1,3), and cellobiose (β-1,4) phosphorylases to synthesize 2-deoxy-2-[18F]fluoro-trehalose ([18F]FDT), 2-deoxy-2-[18F]fluoro-laminaribiose ([18F]FDL), and 2-deoxy-2-[18F]fluoro-cellobiose ([18F]FDC). We subsequently tested [18F]FDM and [18F]FSK in vitro, showing accumulation by several clinically relevant pathogens including Staphylococcus aureus and Acinetobacter baumannii, and demonstrated their specific uptake in vivo. Both [18F]FDM and [18F]FSK were stable in human serum with high accumulation in preclinical infection models. The synthetic ease and high sensitivity of [18F]FDM and [18F]FSK to S. aureus including methicillin-resistant (MRSA) strains strongly justify clinical translation of these tracers to infected patients. Furthermore, this work suggests that chemoenzymatic radiosyntheses of complex [18F]FDG-derived oligomers will afford a wide array of PET radiotracers for infectious and oncologic applications.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

General Principles for Yield Optimization of Nucleoside Phosphorylase-Catalyzed Transglycosylations

The biocatalytic synthesis of natural and modified nucleosides with nucleoside phosphorylases offers the protecting-group-free direct glycosylation of free nucleobases in transglycosylation reactions. This contribution presents guiding principles for nucleoside phosphorylase-mediated transglycosylations alongside mathematical tools for straightforward yield optimization. We illustrate how product yields in these reactions can easily be estimated and optimized using the equilibrium constants of phosphorolysis of the nucleosides involved. Furthermore, the varying negative effects of phosphate on transglycosylation yields are demonstrated theoretically and experimentally with several examples. Practical considerations for these reactions from a synthetic perspective are presented, as well as freely available tools that serve to facilitate a reliable choice of reaction conditions to achieve maximum product yields in nucleoside transglycosylation reactions.

Self-assembled nano-aggregates of fluorinases demonstrate enhanced enzymatic activity, thermostability and reusability

Three SAP (self-assembling peptide)-tagged fluorinases (FLAs), namely, FLA-ELK16, FLA-L6KD and FLA-18A (named after the SAP used for tagging FLA) were successfully engineered. All three SAP-tagged FLAs could be highly over-expressed using engineered E. coli host cells despite being in the form of aggregates (inclusion bodies). It was noted that all three SAP-tagged FLAs exhibited enzymatic activity. It was also observed that all three SAP-tagged FLAs were capable of self-assembly to form nano-sized particles with different dimensions in aqueous solutions. Strikingly, one of the SAP-tagged FLA (FLA-L6KD) displayed improved enzyme activity, thermostability and reusability, which is potentially ideal for bio-transformation. FLA is an exotic enzyme that is capable of catalysing the formation of C-F bonds using inorganic fluorine ions as substrates. This significant feature enables it to incorporate [18F]-fluoride into different small molecules to generate radiopharmaceuticals in PET (positron emission tomography) labeling. In addition, fluorinase is greatly valuable in synthetic biology for incorporating the fluorine element into building blocks to produce non-natural organofluorines or as a biocatalyst for transforming non-native substrates. Our method would be a further step in making FLA-based biocatalysis even 'greener' by enhancing the enzymatic activity, thermostability and reusability of FLA through the introduction of nano-sized aggregates. Enzymes are such nontrivial biomaterials, which can be manifested in different scenarios. Our research expands their reach and tunes their properties by tagging SAP partners. Thus, this methodology can be put into the 'toolbox' of enzymologists, which can be further explored and generalised for others.

Enzyme-catalyzed C–F bond formation and cleavage

Organofluorines are widely used in a variety of applications, ranging from pharmaceuticals to pesticides and advanced materials. The widespread use of organofluorines also leads to its accumulation in the environment, and two major questions arise: how to synthesize and how to degrade this type of compound effectively? In contrast to a considerable number of easy-access chemical methods, milder and more effective enzymatic methods remain to be developed. In this review, we present recent progress on enzyme-catalyzed C–F bond formation and cleavage, focused on describing C–F bond formation enabled by fluorinase and C–F bond cleavage catalyzed by oxidase, reductase, deaminase, and dehalogenase.

A coupled chlorinase-fluorinase system with a high efficiency of trans-halogenation and a shared substrate tolerance

Enzymatic trans-halogenation enables radiolabeling under mild and aqueous conditions, but rapid reactions are desired. We developed a coupled chlorinase-fluorinase system for rapid trans-halogenation. Notably, the chlorinase shares a substrate tolerance with the fluorinase, enabling these two enzymes to cooperatively produce 5'-fluorodeoxy-2-ethynyladenosine (5'-FDEA) in up to 91.6% yield in 1 h.

Probing the molecular determinants of fluorinase specificity

Molecular determinants of FlA1 fluorinase specificity were probed using 5'-chloro-5'-deoxyadenosine (5'-ClDA) analogs as substrates and FlA1 active site mutants. Modifications at F213 or A279 residues are beneficial towards these modified substrates, including 5'-chloro-5'-deoxy-2-ethynyladenosine, ClDEA (>10-fold activity improvement), and conferred novel activity towards substrates not readily accepted by wild-type FlA1.

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.

Development of Halogenase Enzymes for Use in Synthesis

Nature has evolved halogenase enzymes to regioselectively halogenate a diverse range of biosynthetic precursors, with the halogens introduced often having a profound effect on the biological activity of the resulting natural products. Synthetic endeavors to create non-natural bioactive small molecules for pharmaceutical and agrochemical applications have also arrived at a similar conclusion: halogens can dramatically improve the properties of organic molecules for selective modulation of biological targets in vivo. Consequently, a high proportion of pharmaceuticals and agrochemicals on the market today possess halogens. Halogenated organic compounds are also common intermediates in synthesis and are particularly valuable in metal-catalyzed cross-coupling reactions. Despite the potential utility of organohalogens, traditional nonenzymatic halogenation chemistry utilizes deleterious reagents and often lacks regiocontrol. Reliable, facile, and cleaner methods for the regioselective halogenation of organic compounds are therefore essential in the development of economical and environmentally friendly industrial processes. A potential avenue toward such methods is the use of halogenase enzymes, responsible for the biosynthesis of halogenated natural products, as biocatalysts. This Review will discuss advances in developing halogenases for biocatalysis, potential untapped sources of such biocatalysts and how further optimization of these enzymes is required to achieve the goal of industrial scale biohalogenation.

Natural production of fluorinated compounds and biotechnological prospects of the fluorinase enzyme

Fluorinated compounds are finding increasing uses in several applications. They are employed in almost all areas of modern society. These compounds are all produced by chemical synthesis and their abundance highly contrasts with fluorinated molecules of natural origin. To date, only some plants and a handful of actinomycetes species are known to produce a small number of fluorinated compounds that include fluoroacetate (FA), some ω-fluorinated fatty acids, nucleocidin, 4-fluorothreonine (4-FT), and the more recently identified (2R3S4S)-5-fluoro-2,3,4-trihydroxypentanoic acid. This largely differs from other naturally produced halogenated compounds, which totals more than 5000. The mechanisms underlying biological fluorination have been uncovered after discovering the first actinomycete species, Streptomyces cattleya, that is capable of producing FA and 4-FT, and a fluorinase has been identified as the enzyme responsible for the formation of the C-F bond. The discovery of this enzyme has opened new perspectives for the biotechnological production of fluorinated compounds and many advancements have been achieved in its application mainly as a biocatalyst for the synthesis of [¹⁸F]-labeled radiotracers for medical imaging. Natural fluorinated compounds may also be derived from abiogenic sources, such as volcanoes and rocks, though their concentrations and production mechanisms are not well known. This review provides an outlook of what is currently known about fluorinated compounds with natural origin. The paucity of these compounds and the biological mechanisms responsible for their production are addressed. Due to its relevance, special emphasis is given to the discovery, characterization and biotechnological potential of the unique fluorinase enzyme.

Directed Evolution of a Fluorinase for Improved Fluorination Efficiency with a Non-native Substrate

Fluorinases offer an environmentally friendly alternative for selective fluorination under mild conditions. However, their diversity is limited in nature and they have yet to be engineered through directed evolution. Herein, we report the directed evolution of the fluorinase FlA1 for improved conversion of the non-native substrate 5'-chloro-5'-deoxyadenosine (5'-ClDA) into 5'-fluoro-5'-deoxyadenosine (5'-FDA). The evolved variants, fah2081 (A279Y) and fah2114 (F213Y, A279L), were successfully applied in the radiosynthesis of 5'-[¹⁸ F]FDA, with overall radiochemical conversion (RCC) more than 3-fold higher than wild-type FlA1. Kinetic studies of the two-step reaction revealed that the variants show a significantly improved k_cat value in the conversion of 5'-ClDA into S-adenosyl-l-methionine (SAM) but a reduced k_cat value in the conversion of SAM into 5'-FDA.

A localized tolerance in the substrate specificity of the fluorinase enzyme enables "last-step" 18F fluorination of a RGD peptide under ambient aqueous conditions

A strategy for last-step (18)F fluorination of bioconjugated peptides is reported that exploits an "Achilles heel" in the substrate specificity of the fluorinase enzyme. An acetylene functionality at the C-2 position of the adenosine substrate projects from the active site into the solvent. The fluorinase catalyzes a transhalogenation of 5'-chlorodeoxy-2-ethynyladenosine (ClDEA) to 5'-fluorodeoxy-2-ethynyladenosine (FDEA). Extending a polyethylene glycol linker from the terminus of the acetylene allows the presentation of bioconjugation cargo to the enzyme for (18)F labelling. The method uses an aqueous solution (H2(18)O) of [(18)F]fluoride generated by the cyclotron and has the capacity to isotopically label peptides of choice for positron emission tomography (PET).

Characterization of a SAM-dependent fluorinase from a latent biosynthetic pathway for fluoroacetate and 4-fluorothreonine formation in Nocardia brasiliensis

Fluorination has been widely used in chemical synthesis, but is rare in nature. The only known biological fluorination scope is represented by the fl pathway from Streptomyces cattleya that produces fluoroacetate (FAc) and 4-fluorothreonine (4-FT). Here we report the identification of a novel pathway for FAc and 4-FT biosynthesis from the actinomycetoma-causing pathogen Nocardia brasiliensis ATCC 700358. The new pathway shares overall conservation with the fl pathway in S. cattleya. Biochemical characterization of the conserved domains revealed a novel fluorinase NobA that can biosynthesize 5’-fluoro-5’-deoxyadenosine (5’-FDA) from inorganic fluoride and S-adenosyl-l-methionine (SAM). The NobA shows similar halide specificity and characteristics to the fluorination enzyme FlA of the fl pathway. Kinetic parameters for fluoride ( K _m 4153 μM, k _cat 0.073 min ^-1) and SAM ( K _m 416 μM, k _cat 0.139 min ^-1) have been determined, revealing that NobA is slightly (2.3 fold) slower than FlA. Upon sequence comparison, we finally identified a distinct loop region in the fluorinases that probably accounts for the disparity of fluorination activity.

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.

Fluorinase mediated chemoenzymatic synthesis of [(18)F]-fluoroacetate

A novel preparation of sodium [(18)F]-fluoroacetate is described where 5'-[(18)F]-fluoro-5'-deoxyadenosine is generated by a fluorinase catalysed reaction of S-adenosyl-l-methionine (SAM) with no carrier added [(18)F]-fluoride and then oxidation to [(18)F]-fluoroacetate by a Kuhn-Roth oxidative degradation.

Recent Trends in the Nucleophilic [(18)F]-radiolabeling Method with No-carrier-added [(18)F]fluoride

Noninvasive imaging in living subjects with positron emission tomography (PET) provides early detection of diseases in humans. For this application, it is necessary to prepare specific molecular imaging probes labeled with positron-emitting radioisotopes such as fluorine-18 for obtaining high-quality PET imaging. In this review, we describe recent trends in the F-18 radiolabeling method for the introduction of no-carrier-added fluorine-18, which was obtained from an (18)O(p,n)(18)F reaction, into a specific molecular site, which in turn is intended to serve as an imaging agent for PET study. These labeling protocols are based on ionic liquid media (18)F radiofluorination in the presence of some water, enzymatic (18)F fluorination using fluorinase in water solution, non-polar protic alcohol media (18)F radiofluorination and its mechanism, and nucleophilic (18)F fluorination of an aromatic iodonium salt precursor.

Modular strategies for PET imaging agents

In recent years, modular and simplified chemical and biological strategies have been developed for the synthesis and implementation of positron emission tomography (PET) radiotracers. New developments in bioconjugation and synthetic methodologies, in combination with advances in macromolecular delivery systems and gene-expression imaging, reflect a need to reduce radiosynthesis burden in order to accelerate imaging agent development. These new approaches, which are often mindful of existing infrastructure and available resources, are anticipated to provide a more approachable entry point for researchers interested in using PET to translate in vitro research to in vivo imaging.

Fluorinase: a tool for the synthesis of 18F-labeled sugars and nucleosides for PET

There is an increasing interest in the preparation of 18F-labeled radiopharmaceuticals with potential applications in PET for medicinal imaging. Appropriate synthetic methods require a quick and efficient route in which to incorporate the 18F into a ligand, due to the relatively short half-life of the 18F isotope. Enzymatic methods are rare in this area; however, the discovery of a fluorinating enzyme from Streptomyces cattleya (EC 2.5.1.63) has opened up the possibility of the enzymatic synthesis and formation of C-18F bonds from the [18F]fluoride ion. In this article, the development of enzymatic preparations of 18F-labeled sugars and nucleosides as potential radiotracers using the fluorinase from S. cattleya for PET applications is reviewed. Enzymatic reactions are not traditional in PET synthesis, but this enzyme has some attractive features. The enzyme is available in an overexpressed form from Escherichia coli and it is relatively stable and can be easily purified and manipulated. Most notably, it utilizes [18F] fluoride, the form of the isotope normally generated by the cyclotron and usually in very high specific radioactivity. The disadvantage with the enzyme is that it is substrate specific; however, when the fluorinase is used in combination biotransformations with a second or third enzyme, then a range of radiolabeled nucleosides and ribose sugars can be prepared. The fluorinase enzyme has emerged as a curiosity from biosynthesis studies, but it now has some potential as a new catalyst for 18F incorporation for PET syntheses. The focus is now on delivering a user-friendly catalyst to the PET synthesis community and establishing a clinical role for some of the 18F-labeled molecules available using this technology.