Chemistry with [ 18 F]Fluoride Ion

Synthesis and preliminary biological evaluations of [18F]-1-deoxy-1-fluoro-scyllo-inositol

A novel PET radiotracer, [18F]-1-deoxy-1-fluoro-scyllo-inositol, was synthesized via a one-pot reaction in 16 +/- 3% uncorrected radiochemical yield within 80 minutes; although this compound revealed low brain penetration it shows promise in rodent tumour models for breast cancer imaging.

Fluorinated isatin derivatives. Part 3. New side-chain fluoro-functionalized pyrrolidinyl sulfonyl isatins as potent caspase-3 and -7 inhibitors

Background: Dysregulation of type I programmed cell death (apoptosis) leads to a variety of diseases, among which cancer, cardiovascular and neurodegenerative disorders are the most prominent and widespread. Effector caspases such as caspases-3 and -7 get activated during the apoptotic signaling cascade and hence represent a biological target for the diagnosis and therapy of apoptosis-associated diseases. Methods: Synthesis of potent fluorinated analogs of the lead compound (S)-(+)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin facilitates the aim-oriented identification of precursor candidates for 18F-radiofluorination resulting in radiolabeled compounds that could be employed as tracers for the specific imaging of apoptosis in vivo, using positron-emission tomography. Conclusion: Within a series of new mono-, di- and trifluoromethylated pyrrolidine ring analogs of the lead compound, high inhibition potencies were found for caspases-3 and -7 with IC50 values up to 30 and 37 nM, respectively. A new oxidative desulfurization–fluorination protocol was shown to be a versatile technique for fluorine incorporation.

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.

Microreactors for radiopharmaceutical synthesis

Multiple advantages of microfluidics have been demonstrated in the area of organic synthesis. However, only a limited number of them have found applications in radiopharmaceutical synthesis, while that is an area where the need for improvements offered by microfluidics is very significant. The need is to create an environment where all reactions involving short-lived radioisotopes such as (18)F (110 min half-life) or (11)C (20 min half-life) are rapid and high-yielding while the devices are controlled remotely. Several groups have identified the potential of microfluidics in this area and have demonstrated that various steps of conventional radiosynthesis can be replaced by microfluidic devices. However, despite promising results that stir up the interest in the scientific community, none of these inventions has found commercial applications with broad use yet. This article will review the technologies reported to date and analyze the unmet needs that will have to be addressed before microfluidic technology has a chance of becoming a viable and truly advantageous method of preparation of commercial radiopharmaceuticals. The latter mostly center around Positron Emission Tomography (PET) biomarkers.

Synthesis and monoamine uptake inhibition of conformationally constrained 2beta-carbomethoxy-3beta-phenyl tropanes

A series of 2beta-carbomethoxy-3beta-phenyl tropanes with conformationally constrained nitrogen substituents were synthesized as potential selective dopamine transporter ligands. These novel compounds were examined for their monoamine uptake inhibition potency at the human dopamine transporter (hDAT), the human serotonin transporter (hSERT) and the human noradrenalin transporter (hNET), stably expressed in human embryonic kidney cells (HEK). A SAR-study was conducted to determine the contribution of extended, 4-fluorinated, conformationally constrained C4 chains at the tropane nitrogen to human monoamine transporter affinity and selectivity.

para-Functionalized aryl-di-tert-butylfluorosilanes as potential labeling synthons for (18)F radiopharmaceuticals

Broad spectrum: Novel para-functionalized aryl-di-tert-butylfluorosilanes, p-(tBu(2)FSi)C(6)H(4)X (X=functional group), have been made available and broaden the spectrum of silicon-based (18)F acceptors (SiFAs) for potential PET applications. For example, the [(18)F]maleimido derivative 1 has been employed for the synthesis of [(18)F]1- labeled rat serum albumin (RSA), the applicability of which for PET has been verified by in vivo experiments.The syntheses of the functionalized triorganofluorosilanes tBu(2)(p-XC(6)H(4))SiF (3 a, X=SH; 4 a, X=NCS; 4 b, X=NCO; 5, X=NC(4)H(2)O(2); 7, X=COOH; 8 a, X=COONC(4)H(4)O(2); 8 b, X=COOC(6)F(5)) are reported. These compounds display potential as silicon-based fluoride acceptors (SiFAs). The molecular structures of compounds 5, 7, and 8 a have been determined by single-crystal X-ray diffraction studies. With the exception of compounds 8 a and 8 b, all of the compounds could be (18)F-labeled by isotopic exchange in good to high radiochemical yields (RCY) with good to excellent specific activities. As proof of applicability, the maleimido-functionalized SiFA derivative 5, which is specific for thiol groups, has been used for the labeling of rat serum albumin (RSA) that had been derivatized with 2-iminothiolane. The incorporation of [(18)F]5 into the derivatized RSA reached a maximum yield after 30 min at ambient temperature. After purification, the [(18)F]RSA was evaluated in a healthy rat by means of muPET and displayed an expedient in vivo stability over 180 min.

Single-step high-yield radiosynthesis and evaluation of a sensitive 18F-labeled ligand for imaging brain peripheral benzodiazepine receptors with PET

Elevated levels of peripheral benzodiazepine receptors (PBR) are associated with activated microglia in their response to inflammation. Hence, PBR imaging in vivo is valuable for investigating brain inflammatory conditions. Sensitive, easily prepared, and readily available radioligands for imaging with positron emission tomography (PET) are desirable for this purpose. We describe a new 18F-labeled PBR radioligand, namely [18F]N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([18F]9). [18F]9 was produced easily through a single and highly efficient step, the reaction of [18F]fluoride ion with the corresponding bromo precursor, 8. Ligand 9 exhibited high affinity for PBR in vitro. PET showed that [18F]9 was avidly taken into monkey brain and gave a high ratio of PBR-specific to nonspecific binding. [18F]9 was devoid of defluorination in rat and monkey and gave predominantly polar radiometabolite(s). In rat, a low level radiometabolite of intermediate lipophilicity was identified as [18F]2-fluoro-N-(2-phenoxyphenyl)acetamide ([18F]11). [18F]9 is a promising radioligand for future imaging of PBR in living human brain.

Synthesis, radiosynthesis, and biological evaluation of fluorine-18-labeled 2beta-carbo(fluoroalkoxy)-3beta-(3'-((Z)-2-haloethenyl)phenyl)nortropanes: candidate radioligands for in vivo imaging of the serotonin transporter with positron emission tomography

The meta-vinylhalide fluoroalkyl ester nortropanes 1-4 were synthesized as ligands of the serotonin transporter (SERT) for use as positron emission tomography (PET) imaging agents. In vitro competition binding assays demonstrated that 1-4 have a high affinity for the SERT (K(i) values = 0.3-0.4 nM) and are selective for the SERT over the dopamine and norepinephrine transporters (DAT and NET). MicroPET imaging in anesthetized cynomolgus monkeys with [(18)F]1-[(18)F]4 demonstrated that all four tracers behave similarly with peak uptake in the SERT-rich brain regions achieved after 45-55 min, followed by a steady washout. An awake monkey study was performed with [(18)F]1, which demonstrated that the uptake of [(18)F]1 was not influenced by anesthesia. Chase studies with the SERT ligand 15 displaced [(18)F]1-[(18)F]4, but chase studies with the DAT ligand 16 did not displace [(18)F]1-[(18)F]4 thus indicating that the tracers were binding specifically to the SERT.

Synthesis and evaluation of indolinyl- and indolylphenylacetylenes as PET imaging agents for beta-amyloid plaques

Two new phenylacetylene derivatives, 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)indoline 8 and 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)-1H-indole 14, targeting beta-amyloid (Abeta) plaques have been prepared. In vitro binding carried out in tissue homogenates prepared from postmortem AD brains with [(125)I]IMPY (6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]pyridine) as the radioligand indicated good binding affinities (K(i)=4.0 and 1.5nM for 8 and 14, respectively). Brain penetration of the corresponding radiofluorinated ligands, evaluated in the normal mice, showed good initial brain penetration (4.50 and 2.43% ID/g (injected dose/gram) for [(18)F]8 and [(18)F]14 at 2min after injection) with moderate to low washout rates from the brain (1.71% ID/g at 2h and 2.10% ID/g at 3h, respectively). Autoradiography and homogenate binding studies demonstrated the high specific binding of [(18)F]14 to the Abeta plaques; however, [(18)F]8 showed low specific binding. These preliminary results identified that indolylphenylacetylene, 14, may be a good lead for further structural modification to develop a useful Abeta plaque imaging agent.