Revisiting the Radiosynthesis of [18F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease.

[18F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [18F]FPEB has been an ongoing challenge. Herein, five metal-free precursors for the radiofluorination of [18F]FPEB were compared, namely, a chloro-, nitro-, sulfonium salt, and two spirocyclic iodonium ylide (SCIDY) precursors bearing a cyclopentyl (SPI5) and a new adamantyl (SPIAd) auxiliary. The chloro- and nitro-precursors resulted in a low radiochemical yield (<10% RCY), whereas both SCIDY precursors and the sulfonium salt precursor produced [18F]FPEB in the highest RCYs of 25% and 36%, respectively. Preliminary PET/CT imaging studies with [18F]FPEB were conducted in a transgenic model of Alzheimer's Disease (AD) using B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J (APP/PS1) mice, and data were compared with age-matched wild-type (WT) B6C3F1/J control mice. In APP/PS1 mice, whole brain distribution at 5 min post-injection showed a slightly higher uptake (SUV = 4.8 ± 0.4) than in age-matched controls (SUV = 4.0 ± 0.2). Further studies to explore mGluR5 as an early biomarker for AD are underway.

Selected PET Radioligands for Ion Channel Linked Neuroreceptor Imaging: Focus on GABA, NMDA and nACh Receptors.

Positron emission tomography (PET) neuroimaging of ion channel linked receptors is a developing area of preclinical and clinical research. The present review focuses on recent advances with radiochemistry, preclinical and clinical PET imaging studies of three receptors that are actively pursued in neuropsychiatric drug discovery: namely the γ-aminobutyric acid-benzodiazapine (GABA) receptor, nicotinic acetylcholine receptor (nAChR), and N-methyl-D-aspartate (NMDA) receptor. Recent efforts to develop new PET radioligands for these targets with improved brain uptake, selectivity, stability and pharmacokinetics are highlighted.

Identification of a Potent Inhibitor of CREB-Mediated Gene Transcription with Efficacious in Vivo Anticancer Activity.

Recent studies have shown that nuclear transcription factor cyclic adenosine monophosphate response element binding protein (CREB) is overexpressed in many different types of cancers. Therefore, CREB has been pursued as a novel cancer therapeutic target. Naphthol AS-E and its closely related derivatives have been shown to inhibit CREB-mediated gene transcription and cancer cell growth. Previously, we identified naphthamide 3a as a different chemotype to inhibit CREB's transcription activity. In a continuing effort to discover more potent CREB inhibitors, a series of structural congeners of 3a was designed and synthesized. Biological evaluations of these compounds uncovered compound 3i (666-15) as a potent and selective inhibitor of CREB-mediated gene transcription (IC50 = 0.081 ± 0.04 µM). 666-15 also potently inhibited cancer cell growth without harming normal cells. In an in vivo MDA-MB-468 xenograft model, 666-15 completely suppressed the tumor growth without overt toxicity. These results further support the potential of CREB as a valuable cancer drug target.

PET neuroimaging studies of [(18)F]CABS13 in a double transgenic mouse model of Alzheimer's disease and nonhuman primates.

Fluorine-18 labeled 2-fluoro-8-hydroxyquinoline ([(18)F]CABS13) is a promising positron emission tomography (PET) radiopharmaceutical based on a metal chelator developed to probe the "metal hypothesis of Alzheimer's disease". Herein, a practical radiosynthesis of [(18)F]CABS13 was achieved by radiofluorination followed by deprotection of an O-benzyloxymethyl group. Automated production and formulation of [(18)F]CABS13 resulted in 19 ± 5% uncorrected radiochemical yield, relative to starting [(18)F]fluoride, with ≥95% chemical and radiochemical purities, and high specific activity (>2.5 Ci/µmol) within 80 min. Temporal PET neuroimaging studies were carried out in female transgenic B6C3-Tg(APPswe,PSEN 1dE9)85Dbo/J (APP/PS1) and age-matched wild-type (WT) B6C3F1/J control mice at 3, 7, and 10 months of age. [(18)F]CABS13 showed an overall higher uptake and retention of radioactivity in the central nervous system of APP/PS1 mice versus WT mice with increasing age. However, PET/magnetic resonance imaging in normal nonhuman primates revealed that the tracer had low uptake in the brain and rapid formation of a hydrophilic radiometabolite. Identification of more metabolically stable (18)F-hydroxyquinolines that can be readily accessed by the radiochemical strategy presented herein is underway.

Iodonium ylide-mediated radiofluorination of 18F-FPEB and validation for human use.

UNLABELLED: Translation of new methodologies for labeling nonactivated aromatic molecules with (18)F remains a challenge. Here, we report a one-step, regioselective, metal-free (18)F-labeling method that uses a hypervalent iodonium(III) ylide precursor, to prepare the radiopharmaceutical (18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB). METHODS: Automated radiosynthesis of (18)F-FPEB was achieved by reaction of the ylide precursor (4 mg) with (18)F-Et4NF in dimethylformamide at 80°C for 5 min and formulated for injection within 1 h. RESULTS: (18)F-FPEB was synthesized in 20% ± 5% (n = 3) uncorrected radiochemical yields relative to (18)F-fluoride, with specific activities of 666 ± 51.8 GBq (18 ± 1.4 Ci)/µmol at the end of synthesis and was validated for human use. CONCLUSION: Radiofluorination of iodonium (III) ylides proved to be an efficient radiosynthetic strategy for synthesis of (18)F-labeled radiopharmaceuticals.

Discovery of a Potent Anti-tumor Agent through Regioselective Mono-N-acylation of 7H-Pyrrolo[3,2-f]quinazoline-1,3-diamine.

7H-Pyrrolo[3,2-f]quinazoline-1,3-diamine (1) is a privileged chemical scaffold with significant biological activities. However, the currently accessible chemical space derived from 1 is rather limited. Here we expanded the chemical space related to 1 by developing efficient methods for regioselective monoacylation at N1 , N3 and N7 , respectively. With this novel methodology, a focused library of mono-N-acylated pyrroloquinazoline-1,3-diamines were prepared and screened for anti-breast cancer activity. The structure-activity relationship (SAR) results showed that N3 -acylated compounds were in general more potent than N1 -acylated compounds while N7 -acylation significantly reduced their solubility. Among the compounds evaluated, 7f possessed 8-fold more potent activity than 1 in MDA-MB-468 cells. More importantly, 7f was not toxic to normal human cells. These results suggest that 7f is a novel compound as a potential anti-breast cancer agent without harming normal cells.

Developmental onset of reproductive barriers and associated proteome changes in stigma/styles of Solanum pennellii.

Although self-incompatibility (SI) in plants has been studied extensively, far less is known about interspecific reproductive barriers. One interspecific barrier, known as unilateral incongruity or incompatibility (UI), occurs when species display unidirectional compatibility in interspecific crosses. In the wild tomato species Solanum pennellii, both SI and self-compatible (SC) populations express UI when crossed with domesticated tomato, offering a useful model system to dissect the molecular mechanisms involved in reproductive barriers. In this study, the timing of reproductive barrier establishment during pistil development was determined in SI and SC accessions of S. pennellii using a semi-in vivo system to track pollen-tube growth in developing styles. Both SI and UI barriers were absent in styles 5 days prior to flower opening, but were established by 2 days before flower opening, with partial barriers detected during a transition period 3-4 days before flower opening. The developmental expression dynamics of known SI factors, S-RNases and HT proteins, was also examined. The accumulation of HT-A protein coincided temporally and spatially with UI barriers in developing pistils. Proteomic analysis of stigma/styles from key developmental stages showed a switch in protein profiles from cell-division-associated proteins in immature stigma/styles to a set of proteins in mature stigma/styles that included S-RNases, HT-A protein and proteins associated with cell-wall loosening and defense responses, which could be involved in pollen-pistil interactions. Other prominent proteins in mature stigma/styles were those involved in lipid metabolism, consistent with the accumulation of lipid-rich material during pistil maturation.