Applications of Positron Emission Tomography in Neuropsychiatric Pharmaceutical Drug Development

Palladium-mediated11C-carbonylations using aryl halides and cyanamide

A rapid, efficient and high-yielding synthesis of¹¹C-cyanobenzamides, including novel analogs of various drug molecules, is described.

Radiofluorination of diaryliodonium tosylates under aqueous-organic and cryptand-free conditions

Positron emission tomography (PET) has growing importance as a molecular imaging technique in clinical research and drug development. Methods for producing PET radiotracers utilizing cyclotron-produced [(18)F]fluoride ion (t1/2 = 109.7 min) without the need for complete removal of irradiated target [(18)O]water and addition of cryptand are keenly sought for practical convenience and efficiency. Several structurally diverse diaryliodonium tosylates, XArI(+)Ar'Y TsO(-) (X = H or p-MeO), were investigated in a microfluidic apparatus for their reactivity towards radiofluorination with high specific activity (no-carrier-added) [(18)F]fluoride ion in mixtures of DMF and irradiated target [(18)O]water in the absence of cryptand. Salts bearing a para or ortho electron-withdrawing group Y (e.g., Y = p-CN) reacted rapidly (∼3 min) to give the expected major [(18)F]fluoroarene product, [(18)F]FArY, in useful moderate radiochemical yields even when the solvent had an [(18)O]water content up to 28%. Salts bearing electron-withdrawing groups in meta position (e.g., Y = m-NO2), or an electron-donating substituent (Y = p-OMe), gave low radiochemical yields under the same conditions.

Analytical and biological methods for probing the blood-brain barrier

The blood-brain barrier (BBB) is an important interface between the peripheral and central nervous systems. It protects the brain against the infiltration of harmful substances and regulates the permeation of beneficial endogenous substances from the blood into the extracellular fluid of the brain. It can also present a major obstacle in the development of drugs that are targeted for the central nervous system. Several methods have been developed to investigate the transport and metabolism of drugs, peptides, and endogenous compounds at the BBB. In vivo methods include intravenous injection, brain perfusion, positron emission tomography, and microdialysis sampling. Researchers have also developed in vitro cell-culture models that can be employed to investigate transport and metabolism at the BBB without the complication of systemic involvement. All these methods require sensitive and selective analytical methods to monitor the transport and metabolism of the compounds of interest at the BBB.

Synthesis and evaluation of potent and selective human V1a receptor antagonists as potential ligands for PET or SPECT imaging

SRX246 is a potent, highly selective human vasopressin V1a antagonist that crosses the blood-brain barrier in rats. CNS penetration makes SRX246 an ideal candidate for potential radiolabeling and use in visualization and characterization of the role of the V1a receptor in multiple stress-related disorders. Before radiolabeling studies, cold reference analogs of SRX246 were prepared. This study describes the synthesis and in vitro screening for human V1a receptor binding and permeability of fluoro, iodo, and methyl reference compounds for SRX246 and the preparation of a tin precursor. For each compound, the potential utility of corresponding radiolabeled analogs for PET and SPECT imaging is discussed.

Rapid and Efficient Radiosyntheses of Meta-substituted [F]Fluoroarenes from [F]Fluoride Ion and Diaryliodonium Tosylates within a Microreactor

Effective methods for the introduction of the short-lived positron-emitter fluorine-18 (t(1/2) = 109.7 min) at high specific radioactivity into fluoroarenes are valuable for the development of radiotracers for molecular imaging with positron emission tomography. Here we have explored the scope of the radiofluorination of diaryliodonium salts with no-carrier-added [(18)F]fluoride ion for the preparation of otherwise difficult to access meta-substituted [(18)F]fluoroarenes. A microfluidic reaction platform was used to establish optimal radiochemical yields. Rapid, high yielding and selective radiofluorinations were achieved in unsymmetrical diaryliodonium tosylates (ArI(+)Ar'TsO(-)) in which Ar carried either a meta electron-withdrawing (CN, NO(2), CF(3)) or a meta electron-donating (Me or MeO) group, and in which the partner aryl group (Ar') was relatively electron-rich, such as Ph, 3-Me-C(6)H(4), 4-MeO-C(6)H(4), 2-thienyl or 5-Me-2-thienyl. The radiofluorination of appropriate diaryliodonium tosylates is therefore a generally useful method for the preparation of simple [(18)F]m-fluoroarenes ([(18)F]ArF).

Synthesis of [F]Xenon Difluoride as a Radiolabeling Reagent from [F]Fluoride Ion in a Micro-reactor and at Production Scale

[(18)F]Xenon difluoride ([(18)F]XeF(2)), was produced by treating xenon difluoride with cyclotron-produced [(18)F]fluoride ion to provide a potentially useful agent for labeling novel radiotracers with fluorine-18 (t(1/2) = 109.7 min) for imaging applications with positron emission tomography, Firstly, the effects of various reaction parameters, for example, vessel material, solvent, cation and base on this process were studied at room temperature. Glass vials facilitated the reaction more readily than polypropylene vials. The reaction was less efficient in acetonitrile than in dichloromethane. Cs(+) or K(+) with or without the cryptand, K 2.2.2, was acceptable as counter cation. The production of [(18)F]XeF(2) was retarded by K(2)CO(3), suggesting that generation of hydrogen fluoride in the reaction milieu promoted the incorporation of fluorine-18 into xenon difluoride. Secondly, the effect of temperature was studied using a microfluidic platform in which [(18)F]XeF(2) was produced in acetonitrile at elevated temperature (≥ 85 °C) over 94 s. These results enabled us to develop a method for obtaining [(18)F]XeF(2) on a production scale (up to 25 mCi) through reaction of [(18)F]fluoride ion with xenon difluoride in acetonitrile at 90 °C for 10 min. [(18)F]XeF(2) was separated from the reaction mixture by distillation at 110 °C. Furthermore, [(18)F]XeF(2) was shown to be reactive towards substrates, such as 1-((trimethylsilyl)oxy)cyclohexene and fluorene.