Antony D. Gee

Synthesis of 11C, 18F, 15O, and 13N Radiolabels for Positron Emission Tomography

Positron emission tomography (PET) is a powerful and rapidly developing area of molecular imaging that is used to study and visualize human physiology by the detection of positron-emitting radiopharmaceuticals. Information about metabolism, receptor/enzyme function, and biochemical mechanisms in living tissue can be obtained directly from PET experiments. Unlike magnetic resonance imaging (MRI) or computerized tomography (CT), which mainly provide detailed anatomical images, PET can measure chemical changes that occur before macroscopic anatomical signs of a disease are observed. PET is emerging as a revolutionary method for measuring body function and tailoring disease treatment in living subjects. The development of synthetic strategies for the synthesis of new positron-emitting molecules is, however, not trivial. This Review highlights key aspects of the synthesis of PET radiotracers with the short-lived positron-emitting radionuclides (11)C, (18)F, (15)O, and (13)N, with emphasis on the most recent strategies.

AuI/AuIII Catalysis: An Alternative Approach for CC Oxidative Coupling

When reacted in the presence of external oxidants, gold complexes are capable of catalyzing oxidative homo- and cross-coupling reactions involving the formation of new C-C bonds. Over the last few years, several cascade processes have been reported in which coupling is preceded by a gold-mediated aryl C-H functionalization or nucleophilic addition. These reactions combine the unique reactivity of gold with oxidative coupling, enabling the construction of C-C bonds between coupling partners that are not easily accessed using alternative catalysts. In this Concept paper, the development of gold-catalyzed oxidative coupling reactions is discussed focusing on C-C bond-forming reactions of broad synthetic appeal.

Palladium‐Catalyzed Allylic Fluorination

The title reaction is presented and its applicability to 18F radiolabeling is demonstrated (see scheme; TBAF=tetra‐n‐butylammonium fluoride, THF=tetrahydrofuran, dba=dibenzylideneacetone). The use of p‐nitrobenzoate as the leaving group is significant to the success of this catalytic organometallic fluorination process. A range of allylic fluorides were synthesized by this method.

Drug interactions with lipid membranes

The field of drug-membrane interactions is one that spans a wide range of scientific disciplines, from synthetic chemistry, through biophysics to pharmacology. Cell membranes are complex dynamic systems whose structures can be affected by drug molecules and in turn can affect the pharmacological properties of the drugs being administered. In this tutorial review we aim to provide a guide for those new to the area of drug-membrane interactions and present an introduction to areas of this topic which need to be considered. We address the lipid composition and structure of the cell membrane and comment on the physical forces present in the membrane which may impact on drug interactions. We outline methods by which drugs may cross or bind to this membrane, including the well understood passive and active transport pathways. We present a range of techniques which may be used to study the interactions of drugs with membranes both in vitro and in vivo and discuss the advantages and disadvantages of these techniques and highlight new methods being developed to further this field.

Delineation of positron emission tomography imaging agent binding sites on β-amyloid peptide fibrils

A range of imaging agents for use in the positron emission tomography of Alzheimers disease is currently under development. Each of the main compound classes, derived from thioflavin T (PIB), Congo Red (BSB), and aminonaphthalene (FDDNP) are believed to bind to mutually exclusive sites on the β-amyloid (Aβ) peptide fibrils. We recently reported the presence of three classes of binding sites (BS1, BS2, BS3) on the Aβ fibrils for thioflavin T derivatives and now extend these findings to demonstrate that these sites are also able to accommodate ligands from the other chemotype classes. The results from competition assays using [3H]Me-BTA-1 (BS3 probe) indicated that both PIB and FDDNP were able to displace the radioligand with Ki values of 25 and 42 nm, respectively. BSB was unable to displace the radioligand tracer from the Aβ fibrils. In contrast, each of the compounds examined were able to displace thioflavin T (BS1 probe) from the Aβ fibrils when evaluated in a fluorescence competition assay with Ki values for PIB, FDDNP, and BSB of 1865, 335, and 600 nm, respectively. Finally, the Kd values for FDDNP and BSB binding to Aβ fibrils were directly determined by monitoring the increases in the ligand intrinsic fluorescence, which were 290 and 104 nm, respectively. The results from these assays indicate that (i) the three classes of thioflavin T binding sites are able to accommodate a wide range of chemotype structures, (ii) BSB binds to two sites on the Aβ fibrils, one of which is BS2, and the other is distinct from the thioflavin T derivative binding sites, and (iii) there is no independent binding site on the fibrils for FDDNP, and the ligand binds to both the BS1 and BS3 sites with significantly lower affinities than previously reported.

Gold-catalyzed intramolecular oxidative cross-coupling of nonactivated arenes

Homogeneous catalysis by gold has received considerable interest in recent years. The isolation of a gold(I) fluoride complex stabilized by an N-heterocyclic carbene (NHC) ligand led to the first Au-catalyzed hydrofluorination of alkynes, a transformation carried out in the presence of Et3N·3HF, a mildly acidic fluoride source. [2] In 2008, our group explored the chemistry of Au complexes with electrophilic fluorinating reagents. Trifluorodihydropyranones were obtained upon Au-catalyzed cyclization–fluorodeauration of b-hydroxy-a,a-difluoroynones, a reaction performed with Selectfluor. In more recent studies by Zhang and co-workers, Selectfluor was found to be a suitable oxidant for the gold-catalyzed oxidative dimerization of propargylic acetates and for oxidative cross-coupling reactions with preactivated arylboronic acids. The development of gold-catalyzed oxidative cross-coupling reactions that do not require preactivated arenes is more challenging and, to date, is restricted to homocoupling reactions with PhIACHTUNGTRENNUNG(OAc)2.[5] We recently sought to investigate whether the gold-catalyzed oxidative cross-coupling of nonactivated arenes might be accomplished with electrophilic N F reagents, such as Selectfluor. We herein report the successful gold-catalyzed oxidative intramolecular C H arylation using allenoate esters as model substrates (Scheme 1). The benzyl-substituted tert-butyl allenoate ester 1 a was treated with Selectfluor (2.5 equiv) and AuCl (5 mol%) in acetonitrile. Three products were formed, none of which were fluorinated (Scheme 2). The major product obtained in 36 % yield was unambiguously identified by X-ray analysis as the tricyclic 8,8a-dihydro-2H-indenoACHTUNGTRENNUNG[2,1-b]furan-2-one (2 a). The separable dimeric butenolides ( )-(2S,2’S)-3 a and (2R,2’S)-3 a were also formed and isolated in 20 and 15 % yield, respectively. X-Ray diffraction analysis of analytically pure samples confirmed that the dimeric butenolides were, indeed, diastereomers. The butenolide 4 a resulting from cyclization–protodeauration was not detected in the crude reaction mixture. In a control experiment performed with AuCl (5 mol %) in acetonitrile, but omitting Selectfluor, compound 4 a was formed as the only product after 6 days (Scheme 2). The formation of 2 a from 1 a, a new addition to the list of gold-catalyzed oxidative cross-coupling reactions reported to date, is a challenging operation. Under the same reaction conditions, but with PtCl2, CuOAc, H2SO4, or AgOTf as the catalyst, 2 a and 3 a were not observed and with Pd ACHTUNGTRENNUNG(OAc)2 (5 mol %) the dimers 3 a were formed as the only products with no trace of 2 a. Following extensive optimization studies, Ph3PAuNTf2 [11] (10 mol%) was identified as the catalyst of choice when used in acetonitrile and H2O (10 equiv), a suitable solvent system for Selectfluor (2.5 equiv) (Table 1, entry 5). Under these conditions, compound 2 a was isolated in 62 % yield. Although Selectfluor was the best oxidant to access dihydroindenofuranones from allenoates, N-fluorobenzenesulfonimide (NFSI) was also suitable, but less efficient (Table 1, entry 6). With the latter fluorinating reagent, the [a] M. N. Hopkinson, Dr. A. Tessier, A. Salisbury, G. T. Giuffredi, L. E. Combettes, Prof. V. Gouverneur Chemistry Research Laboratory (CRL), University of Oxford 12 Mansfield Road, Oxford, OX1 3TA (UK) Fax: (+44) 1865-275-644 E-mail : veronique.gouverneur@chem.ox.ac.uk [b] Prof. A. D. Gee GSK Clinical Imaging Centre, Imperial College London Hammersmith Hospital, Du Cane Road, London, W12 0NN (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000322. Scheme 1. Reactivity of vinylgold(I) species with “F”.

Gold-catalyzed cascade cyclization-oxidative alkynylation of allenoates.

A gold(I)-catalyzed cascade cyclization-oxidative cross-coupling process has been applied to prepare β-alkynyl-γ-butenolides directly from allenoates and various terminal alkynes. Following an initial gold-catalyzed C-O bond forming allenoate cyclization, a mechanism based on a Au(I)/Au(III) redox cycle has been proposed with Selectfluor acting as the external oxidant.

Measuring drug-related receptor occupancy with positron emission tomography

Several techniques can be used to measure indirectly the effect of drugs (e.g., EEG, fMRI) in healthy volunteers and in patients. Although each technique has its merits, a direct link between drug efficacy and site of action in vivo usually cannot be established. In addition, when the specific mode of action of a drug has been determined from preclinical studies, it is often not known whether the administered dose is optimal for humans. Both industry and academia are becoming more and more interested in determining the dose-related occupancy of specific targets caused by administration of drugs under test. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are noninvasive imaging techniques that can give insight into the relationship between target occupancy and drug efficacy, provided a suitable radioligand is available. Although SPECT has certain advantages (e.g., a long half-life of the radionuclides), the spatial and temporal resolution as well as the labeling possibilities of this technique are limited. This review focuses on PET methodology for conducting drug occupancy studies in humans.

The peripheral benzodiazepine receptor ligand PK11195 binds with high affinity to the acute phase reactant alpha1-acid glycoprotein: implications for the use of the ligand as a CNS inflammatory marker.

The peripheral benzodiazepine receptor ligand PK11195 has been used as an in vivo marker of neuroinflammation in positron emission tomography studies in man. One of the methodological issues surrounding the use of the ligand in these studies is the highly variable kinetic behavior of [(11)C]PK11195 in plasma. We therefore undertook a study to measure the binding of [(3)H]PK11195 to whole human blood and found a low level of binding to blood cells but extensive binding to plasma proteins. Binding assays using [(3)H]PK11195 and purified human plasma proteins demonstrated a strong binding to alpha1-acid glycoprotein (AGP) and a much weaker interaction with albumin. Immunodepletion of AGP from plasma resulted in the loss of plasma [(3)H]PK11195 binding demonstrating: (i) the specificity of the interaction and (ii) that AGP is the major plasma protein to which PK11195 binds with high affinity. PK11195 was able to displace fluorescein-dexamethasone from AGP with IC(50) of <1.2 microM, consistent with a high affinity interaction. These findings are important for understanding the behavior of the ligand in positron emission tomography studies for three reasons. Firstly, AGP is an acute phase protein and its levels will vary during infection and pathological inflammatory diseases such as multiple sclerosis. This could significantly alter the free plasma concentrations of the ligand and contribute to its variable kinetic behavior. Secondly, AGP and AGP-bound ligand may contribute to the access of [(11)C]PK11195 to the brain parenchyma in diseases with blood brain barrier breakdown. Finally, local synthesis of AGP at the site of brain injury may contribute the pattern of [(11)C]PK11195 binding observed in neuroinflammatory diseases.

A microfluidic approach for high-throughput droplet interface bilayer (DIB) formation

We present a simple, automated method for high-throughput formation of droplet interface bilayers (DIBs) in a microfluidic device. We can form complex DIB networks that are able to fill predefined three dimensional architectures. Moreover, we demonstrate the flexibility of the system by using a variety of lipids including 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC).