Experimental and density functional theory study of Raman and SERS spectra of 5-amino-2-mercaptobenzimidazole.

Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) simulations were employed to study 5-amino-2-mercaptobenzimidazole (5-A-2MBI) molecules. Ag colloids were used as SERS substrates which were prepared by using hydroxylamine hydrochloride as reducing agent. Raman vibration modes and SERS characteristic peaks of 5-A-2MBI were assigned with the aid of DFT calculations. The molecular electrostatic potential (MEP) of 5-A-2MBI was used to discuss the possible adsorption behavior of 5-A-2MBI on Ag colloids. The spectral analysis showed that 5-A-2MBI molecules were slightly titled via the sulfur atoms adhering to the surfaces of Ag substrates. The obtained SERS spectral intensity decreased when lowering the 5-A-2MBI concentrations. A final detection limit on the concentration of 5×10(-7) mol · L(-1) was gained. SERS proved to be a simple, fast and reliable method for the detection and characterization of 5-A-2MBI molecules.

[Density functional theory calculation of SERS spectra of trans-1,2-bis(4-pyridyl)-ethylene on silver].

It has been found that trans-1, 2-bis(4-pyridyl)-ethylene has the best SERS signal and can be employed as a kind of self-assembly ? Im for the exploration of the SERS-active on the silver foil substrate. The shifts in the experiment surface enhanced Raman scattering of trans-1,2-bis(4-pyridyl)-ethylene were simulated by density functional theory calculation with the BP86, BPw91, B3LYP method. The basis set of 6-31++G(d,p) and Lan12dz was used by H, C, N atoms and Ag atom for the t-BPE-Ag complex. The Raman spectra and surface enhanced Raman scattering of trans-1, 2-bis (4-pyridyl)-ethylene were assigned by the calculated results of potential energy distribution The density functional theory calculated results explain that the angles between pyridyl rings for t-BPE-Ag complex holding 0o. Thus, the calculated Raman spectra of trans-1, 2-bis(4-pyridyl)-ethylene and Ag complex accord with observed SERS results of t-BPE. The energy level space between the high occupied molecular orbital and lowest unoccupied molecular orbital is estimated to arise between 415 and 912 nm for trans-1,2-bis(4-pyridyl)-ethylene and Ag complex.

Florbetapir f-18: a histopathologically validated Beta-amyloid positron emission tomography imaging agent.

Florbetapir F-18 is a molecular imaging agent combining high affinity for ß-amyloid, pharmacokinetic properties that allow positron emission tomography (PET) imaging within a convenient time after dose administration, and the wide availability of the radionuclide fluorine-18. Florbetapir F-18 is prepared by nucleophilic radiofluorination in approximately 60 minutes with a decay-corrected yield of 20%-40% and with a specific activity typically exceeding 100 Ci/mmol. The florbetapir F-18 dissociation constant (K(d)) for binding to ß-amyloid in brain tissue from Alzheimer's disease (AD) patients was 3.7 ± 0.3 nmol/L, and the maximum binding capacity (B(max)) was 8800 ± 1600 fmol/mg protein. Autoradiography studies have shown that florbetapir F-18 selectively binds to ß-amyloid aggregates in AD patient brain tissue, and the binding intensity is correlated with the density of ß-amyloid quantified by standard neuropathologic techniques. Studies in animals revealed no safety concerns and rapid and transient normal brain uptake (6.8% injected dose/g at 2 minutes and 1.9% injected dose/g at 60 minutes in the mouse). Florbetapir F-18 has been well-tolerated in studies of more than 2000 human subjects. Biodistribution studies in humans revealed predominantly hepatobiliary excretion. The whole body effective dose was 7 mSv from a dose of 370 MBq. The pharmacokinetic of florbetapir F-18 make it possible to obtain a PET image with a brief (10 minutes) acquisition time within a convenient time window of 30-90 minutes after dose administration. Clinical studies have demonstrated a clear correlation between in vivo PET imaging with florbetapir F-18 and postmortem histopathologic quantitation of ß-amyloid in the brain.

Surface-enhanced Raman scattering of trans-1,2-bis (4-pyridyl)-ethylene on silver by theory calculations.

Surface-enhanced Raman spectra of trans-1,2-bis (4-pyridyl)-ethylene (t-BPE) on silver foil were detected at laser line of 514.5, 633, 785 and 1064 nm, respectively. The structure of Ag-t-BPE, Ag4-t-BPE, Ag6-t-BPE, Ag10-t-BPE and Ag20-t-BPE complexes has been calculated using a local version of the Amsterdam density functional program package. The Raman spectra and electronic polarizability of t-BPE-Ag at 514.5, 633, 785 and 1064 nm excitation lines were calculated. The Raman bands of t-BPE were assigned according to the calculation of potential energy distribution. The experimental and calculated Raman spectra of t-BPE-Ag at 514.5, 633, 785 and 1064 nm were compared. The relative Raman intensities change at different excitation lines were discussed based on the Raman enhanced mechanism and surface selection rules.

Preclinical properties of 18F-AV-45: a PET agent for Abeta plaques in the brain.

UNLABELLED: beta-amyloid plaques (Abeta plaques) in the brain, containing predominantly fibrillary Abeta peptide aggregates, represent a defining pathologic feature of Alzheimer disease (AD). Imaging agents targeting the Abeta plaques in the living human brain are potentially valuable as biomarkers of pathogenesis processes in AD. (E)-4-(2-(6-(2-(2-(2-(18)F-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-methyl benzenamine ((18)F-AV-45) is such as an agent currently in phase III clinical studies for PET of Abeta plaques in the brain. METHODS: In vitro binding of (18)F-AV-45 to Abeta plaques in the postmortem AD brain tissue was evaluated by in vitro binding assay and autoradiography. In vivo biodistribution of (18)F-AV-45 in mice and ex vivo autoradiography of AD transgenic mice (APPswe/PSEN1) with Abeta aggregates in the brain were performed. Small-animal PET of a monkey brain after an intravenous injection of (18)F-AV-45 was evaluated. RESULTS: (18)F-AV-45 displayed a high binding affinity and specificity to Abeta plaques (K(d), 3.72 +/- 0.30 nM). In vitro autoradiography of postmortem human brain sections showed substantial plaque labeling in AD brains and not in the control brains. Initial high brain uptake and rapid washout from the brain of healthy mice and monkey were observed. Metabolites produced in the blood of healthy mice after an intravenous injection were identified. (18)F-AV-45 displayed excellent binding affinity to Abeta plaques in the AD brain by ex vivo autoradiography in transgenic AD model mice. The results lend support that (18)F-AV-45 may be a useful PET agent for detecting Abeta plaques in the living human brain.

Density functional theory study on surface-enhanced Raman scattering of 4,4'-azopyridine on silver.

Surface-enhanced Raman scattering (SERS) of 4,4'-azopyridine (AZPY) on silver foil substrate was measured under 1064nm excitation lines. Density-functional theory (DFT) methods were used to calculate the structure and vibrational spectra of models such as Ag-AZPY, Ag(4)-AZPY and Ag(6)-AZPY complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of AZPY were identified on the ground of analog computation of potential energy distribution. The calculated spectra of Ag(4)-AZPY and Ag(6)-AZPY models were much approximated to the experimental results than that of Ag-AZPY model. The DFT results showed that the angles between two pyridyl rings keep 0 degrees from AZPY to Ag-AZPY, Ag(4)-AZPY and Ag(6)-AZPY model. The energy gaps between the HOMO and LUMO changed from 363 to 1140nm for AZPY-Ag complexes according to the DFT results. An conclusion was conceived that chemical enhancement mechanism may play an important role in the SERS of AZPY on silver substrate.

In vivo imaging of vesicular monoamine transporter 2 in pancreas using an (18)F epoxide derivative of tetrabenazine.

OBJECTIVES: Development of imaging agents for pancreatic beta cell mass may provide tools for studying insulin-secreting beta cells and their relationship with diabetes mellitus. In this paper, a new imaging agent, [(18)F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline [(18)F](+)4, which displays properties targeting vesicular monoamine transporter 2 (VMAT2) binding sites of beta cells in the pancreas, was evaluated as a positron emission tomography (PET) agent for estimating beta cell mass in vivo. The hydrolyzable epoxide group of (+)4 may provide a mechanism for shifting biodistribution from liver to kidney, thus reducing the background signal. METHODS: Both (18)F- and (19)F-labeled (+) and (-) isomers of 4 were synthesized and evaluated. Organ distribution was carried out in normal rats. Uptake of [(18)F](+)4 in pancreas of normal rats was measured and correlated with blocking studies using competing drugs, (+)dihydrotetrabenazine [(+)-DTBZ] or 9-fluoropropyl-(+)dihydro tetrabenazine [FP-(+)-DTBZ, (+)2]. RESULTS: In vitro binding study of VMAT2 using rat brain striatum showed a K(i) value of 0.08 and 0.15 nM for the (+)4 and (+/-)4, respectively. The in vivo biodistribution of [(18)F](+)4 in rats showed the highest uptake in the pancreas (2.68 %ID/g at 60 min postinjection). In vivo competition experiments with cold FP-(+)-DTBZ, (+)2, (3.5 mg/kg, 5 min iv pretreatment) led to a significant reduction of pancreas uptake (85% blockade at 60 min). The inactive isomer [(18)F](-)4 showed significantly lower pancreas uptake (0.22 %ID/g at 30 min postinjection). Animal PET imaging studies of [(18)F](+)4 in normal rats demonstrated an avid pancreatic uptake in rats. CONCLUSION: The preliminary results suggest that the epoxide, [(18)F](+)4, is highly selective in binding to VMAT2 and it has an excellent uptake in the pancreas of rats. The liver uptake was significantly reduced through the use of the epoxide group. Therefore, it may be potentially useful for imaging beta cell mass in the pancreas.

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.

Subcellular localization of sigma-2 receptors in breast cancer cells using two-photon and confocal microscopy.

Sigma-2 receptor agonists have been shown to induce cell death via caspase-dependent and caspase-independent pathways. Unfortunately, there is little information regarding the molecular function of sigma-2 receptors that can explain these results. In this study, two fluorescent probes, SW107 and K05-138, were used to study the subcellular localization of sigma-2 receptors by two-photon and confocal microscopy. The results indicate that sigma-2 receptors colocalize with fluorescent markers of mitochondria, lysosomes, endoplasmic reticulum, and the plasma membrane in both EMT-6 mouse and MDA-MB-435 human breast cancer cells. The fluorescent probe, K05-138, was internalized rapidly, reaching a plateau of fluorescent intensity at 5 min. The internalization of K05-138 was reduced approximately 40% by phenylarsine oxide, an inhibitor of endocytosis. These data suggest that sigma-2 ligands are internalized, in part, by an endocytotic pathway. The localization of sigma-2 receptors in several organelles known to have a role in both caspase-dependent and caspase-independent pathways of cell death supports the conclusions of previous studies suggesting that sigma-2 receptor ligands should be evaluated as potential cancer chemotherapeutic agents.

Fluoro-pegylated (FPEG) imaging agents targeting Abeta aggregates.

A novel approach of producing positron emission tomography (PET) imaging agents through the formation of bioconjugates based on a pegylation-fluorination strategy resulting in fluoro-pegylated (FPEG) molecules is reported. This approach offers a simple and easy method by which to incorporate 18F in the target molecule without an appreciable increase in the lipophilicity. After 18F labeling, this convenient approach leads to PET imaging probes binding to Abeta aggregates in the brain (an important factor associated with Alzheimer's disease) using the known core structures, such as [2-(4-dimethylaminophenyl)-vinyl]-benzoxazol (3') or 2-phenylbenzothiazole (4). This approach appears to be effective in some core structures, but it cannot be uniformly applied to all structures.

Surface-enhanced Raman spectroscopy and density functional theory study on 4,4'-bipyridine molecule.

The molecular geometry and vibrational frequencies of 4,4'-bipyridine (BPE) in the ground state were calculated using density functional theory (DFT) methods (B3LYP) with 6-31++G(d,p) basis set. The optimized geometric bond lengths and bond angles are obtained by DFT employing the hybrid of Beckes non-local three parameter exchange and correlation functional and Lee-Yang-Parr correlation functional. Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of BPE on the silver foil substrate have been recorded. All FT-IR, FT-Raman and NIR-SERS band were assigned on the basis of the B3LYP/6-31++G(d,p) method. The vibrational frequencies obtained by DFT(3LYP) are in good agreement with observed results. The NIR-SERS of BPE excited by 1064nm laser line is little difference with that excited by visible laser line. This phenomenon is result to the increase of the contribution of CHEM enhancement effect. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of BPE on the silver foil substrate surface. Some vibrational frequency which are sensitive to the planar or non-planar structure of BPE, and to the dihedral angle were concluded.

Synthesis of biphenyltrienes as probes for beta-amyloid plaques.

We report a series of p-hydroxy-, p-amino-, p-monomethylamino-, and p-monofluoroethylamino-substituted biphenyltrienes that displayed high binding affinities to beta-amyloid plaques. In an in vitro binding assay using postmortem brain homogenates of Alzheimer's patients and [(125)I]9, the triene compounds showed excellent binding affinities. When labeled with suitable radionuclides, they are useful as in vivo imaging agents for detecting Abeta plaques in the brains of Alzheimer's patients.

SPECT imaging of herpes simplex virus type1 thymidine kinase gene expression by [(123)I]FIAU(1).

RATIONALE AND OBJECTIVES: Introduction of suicide genes, such as herpes simplex virus type1 thymidine kinase (HSV1-tk), in tumor cells has provided a useful method for tumor gene therapy. Several L-nucleosides, such as Lamivudine (3TC) and Clevudine (L-FMAU), have been successfully tested as high-potency antiviral agents. To investigate the potential differences between D- and L-isomers of nucleosides, [(125/123)I]-2'-fluoro-2'-deoxy-1beta-D/L-arabino-furanosy-5-iodo-uracil (D/L-FIAU) have been synthesized and evaluated as potential SPECT agents for imaging HSV1-tk gene expression. MATERIALS AND METHODS: [(125/123)I]D- and L-FIAU were prepared by iododestannylation of the respective tin precursors with (125/123)I-sodium iodide. In vitro cell uptake studies were performed by incubation of [(125)I]D- and L-FIAU in RG2 cells expressing HSV1-tk (RG2TK+). In vivo studies including biodistribution and SPECT were performed in RG2TK+ and RG2TK- tumor-bearing nude mice using [(123)I]D- and L-FIAU. RESULTS: Cell uptake and biodistribution studies indicated that [(125/123)I]L-FIAU did not show any high accumulation (sensitivity) or uptake ratios (selectivity) in HSV1-TK-positive (RG2TK+) tumors as compared to control tumors. In contrast, [(125/123)I]D-FIAU displayed both sensitivity and selectivity to RG2TK+ tumors. The selective in vivo accumulation of [(123)I]D-FIAU increased with time and the tumor uptake ratios (RG2TK+/RG2TK-) for 2, 4, and 24 hours averaged 6.2, 22.7, and 58.8, respectively. High-resolution SPECT of four nude tumor-bearing mice demonstrated a very high uptake of [(123)I]D-FIAU in the RG2TK+ tumor, while no significant tracer accumulation was observed in the RG2TK- tumor and other organs. CONCLUSION: The data suggest that only the D-isomer of [(123)I]FIAU is useful for imaging HSV1-tk gene expression in mice by high-resolution SPECT imaging.

Biphenyls labeled with technetium 99m for imaging beta-amyloid plaques in the brain.

Formation and accumulation of excess aggregates of beta-amyloid (Abeta) plaques in the brain are critical factors contributing to the development and progression of Alzheimer's disease (AD). There is an urgent need for in vivo imaging agents that can specifically demonstrate the location and density of Abeta plaques in the brain. The aim of this study was to develop potential technetium 99m (99mTc)-labeled diagnostic imaging agents specific for the detection of Abeta plaques. Based on previously obtained Abeta plaque-specific biphenyls containing a p-N, N-dimethylaminophenyl group, a series of 99mTc and Re-N2S2-biphenyl derivatives was prepared. The stable neutral and lipophilic 99mTc complexes, [99mTc]19 and [99mTc]23, A+B, were successfully obtained. As surrogates for the 99mTc complexes, the corresponding surrogates, Re complexes of 23, were also prepared. Surprisingly, it was found that the Re complexes showed distinctively different retention profiles as compared with the corresponding 99mTc complexes. Biodistribution studies indicated that [99mTc]23A readily passed through the blood-brain barrier (1.18% dose/brain at 2 min) in contrast to the low brain penetration of [99mTc]19 (0.29% dose/brain at 2 min). Initial results suggested that [99mTc]23A showed selective binding to the Abeta plaque-like structures in the brain sections from transgenic mice but not in the postmortem human brain tissue of patients with confirmed AD. The results provide encouraging evidence that development of a 99mTc-labeled agent for imaging Abeta plaques in the brain may be feasible. Caution should be taken when comparing these 99mTc complexes with the corresponding surrogates--the Re complexes.

Binding of two potential imaging agents targeting amyloid plaques in postmortem brain tissues of patients with Alzheimer's disease.

In vivo imaging of amyloid plaques may be useful for evaluation and diagnosis of Alzheimer's disease (AD) patients. Towards that end, we have developed 6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2]pyridine (IMPY), and 4-N-methylamino-4'-hydroxystilbene (SB-13) as ligands for specifically targeting amyloid plaques. These ligands can be readily radiolabeled with I-123 or C-11, for in vivo imaging using single photon emission computerized tomography (SPECT) or positron emission tomography (PET), respectively. However, in order to be useful in vivo, probes must show selective high affinity binding to a sufficiently abundant binding site on amyloid plaques. Therefore, as a prelude to in vivo imaging studies, we evaluated the binding properties of these two potential imaging agents to amyloid plaques present in human brain tissues. In vitro binding studies were carried out with [(125)I]IMPY and [(3)H]SB-13 in homogenates prepared from postmortem samples of affected cortex and cerebellum of pathologically confirmed AD patients and age-matched controls. Binding parameters such as K(d) and B(max) were estimated. Competition study was designed to evaluate the amyloid plaque binding specificity using human brain tissues. Plaque binding was confirmed by thioflavin-S staining. Specific [(125)I]IMPY or [(3)H]SB-13 binding can be clearly measured in the cortical gray matter, but not in the white matter of AD cases. There was a very low specific binding in cortical tissue homogenates of control brains. Cerebellar homogenates prepared from either AD or control brains did not show any specific [(125)I]IMPY or [(3)H]SB-13 binding. The K(d) values of AD cortical homogenates were 5.3+/-1.0 and 2.4+/-0.2 nM for [(125)I]IMPY and [(3)H]SB-13, respectively. High binding capacity and comparable values were observed for both ligands (14-45 pmol/mg protein). The location and density of specific signal detected by [(125)I]IMPY or [(3)H]SB-13 correlated with the distribution of amyloid plaques in these brain specimens, as confirmed by thioflavin-S staining. Competition profiles of known ligands suggest that the binding is highly selective and comparable to that reported by using preformed Abeta peptide aggregates. [(125)I]IMPY and [(3)H]SB-13 show an abundant binding capacity with high binding affinities for amyloid plaques in affected cortical regions of AD brains. These properties suggest that when labeled with I-123 or C-11, these two ligands may be useful to quantitate amyloid plaque burdens in the living AD patients.

2-(2-(dimethylaminomethyl)phenoxy)-5-iodophenylamine: an improved serotonin transporter imaging agent.

Imaging serotonin transporters (SERT) is an emerging research tool potentially useful to cast light on the mechanisms of drug action as well as to monitor the treatment of depressed patients. We have prepared two new derivatives of 3, 2-(2-(dimethylaminomethyl)phenoxy)-5-iodophenylamine (4) and 2-(2-(dimethylaminomethyl)benzyl)-5-iodophenylamine (5) (K(i) for SERT = 0.37 and 48.6 nM, respectively). Both [(125)I]4 and [(125)I]5 displayed excellent brain uptakes in rats, and they showed a highest uptake in hypothalamus (between 60 and 240 min), a region populated with the highest density of SERT. The specific uptake of [(125)I]4 in the hypothalamus resulted in a target to nontarget ratio ([hypothalamus-cerebellum]/cerebellum) of 4.3 at 2 h. Autoradiography of rat brain sections (ex vivo at 2 h) of [(125)I]4 showed an excellent regional distribution pattern consistent with known SERT localization. These data suggest that [(123)I]4 may be useful for imaging SERT binding sites in the brain by single photon emission computed tomography (SPECT).

Development and evaluation of iodinated tracers targeting amyloid plaques for SPECT imaging.

Successful development of iodinated ligands for various neurotransmitter receptors prompted us to explore the feasability of having iodinated ligands to target amyloid plaques of Alzheimer's disease. Several potential iodinated tracers based on various chemical backbone structures have been successfully prepared and evaluated toward this purpose. High binding affinities for Abeta aggregates were consistently observed for those ligands. However, the desirable in vivo properties were generally missing in the majority of those iodinated ligands. Only ligands with the promising in vitro and in vivo characteristics such as IMPY will likely warrant their success to be potential imaging agents mapping amyloid plaques in living human brain.

Selective binding of 2-[125I]iodo-nisoxetine to norepinephrine transporters in the brain.

A radioiodinated ligand, (R)-N-methyl-(2-[(125)I]iodo-phenoxy)-3-phenylpropylamine, [(125)I]2-INXT, targeting norepinephrine transporters (NET), was successfully prepared. A no-carrier-added product, [(125)I]2-INXT, displayed a saturable binding with a high affinity (K(d)=0.06 nM) in the homogenates prepared from rat cortical tissues as well as from LLC-PK(1) cells expressing NET. A relatively low number of binding sties (B(max)=55 fmol/mg protein) measured with [(125)I]2-INXT in rat cortical homogenates is consistent with the value reported for a known NET ligand, [(3)H]nisoxetine. Competition studies with various compounds on [(125)I]2-INXT binding clearly confirmed the pharmacological specificity and selectivity for NET binding sites. Following a tail-vein injection of [(125)I]2-INXT in rats, a good initial brain uptake was observed (0.56% dose at 2 min) followed by a slow washout from the brain (0.2% remained at 3 hours post-injection). The hypothalamus (a NET-rich region) to striatum (a region devoid of NET) ratio was 1.5 at 3 hours post-i.v. injection. Pretreatment of rats with nisoxetine significantly inhibited the uptake of [(125)I]2-INXT (70-100% inhibition) in locus coeruleus, hypothalamus and raphe nuclei, regions known to have a high density of NET; whereas escitalopram, a serotonin transporter ligand, did not show a similar effect. Ex vivo autoradiography of rat brain sections of [(125)I]2-INXT (at 3 hours after an i.v. injection) displayed an excellent regional brain localization pattern corroborated to the specific NET distribution in the brain. The specific brain localization was significantly reduced by a dose of nisoxetine pretreatment. Taken together, the data suggest that [(123)I]2-INXT may be useful for mapping NET binding sites in the brain.

Characterization of IMPY as a potential imaging agent for beta-amyloid plaques in double transgenic PSAPP mice.

Deposition of beta-amyloid (Abeta) plaques in the brain is likely linked to the pathogenesis of Alzheimer's disease (AD). Developing specific Abeta aggregate-binding ligands as in vivo imaging agents may be useful for diagnosis and monitoring the progression of AD. We have prepared a thioflavin derivative, 6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]pyridine, IMPY, which is readily radiolabeled with 125I/123I for binding or single-photon emission computerized tomography (SPECT) imaging studies. Characterization of [125I]IMPY binding to plaque-like structures was evaluated in double transgenic PSAPP mice. [125I]IMPY labeled Abeta plaques in transgenic mouse brain sections, and the labeling was consistent with fluorescent staining and Abeta-specific antibody labeling. Significant amounts of Abeta plaques present in the cortical, hippocampal, and entorhinal regions of the transgenic mouse brain were clearly detected with [125I]IMPY via ex vivo autoradiography. In contrast, [125I]IMPY showed little labeling in the age-matched control mouse brain. Tissue homogenate binding further corroborated the Abeta plaque-specific distribution in various brain regions of transgenic mouse, and correlated well with the known density of Abeta deposition. Using a tissue dissection technique, [125I]IMPY showed a moderate increase in the cortical region of transgenic mice as compared to the age-matched controls. In vitro blocking of [125I]IMPY by "carrier" observed via autoradiography in mouse brain sections was not replicated by an in vivo blocking experiment in living TT mouse brain. The failure was most likely due to a significant carrier effect, which slows down the tracer in vivo metabolism, leading to an increased brain uptake. Taken together, these data indicate that [123I]IMPY is a potentially useful SPECT imaging agent for in vivo labeling of Abeta plaques in the living brain.

Characterization of radioiodinated ligand binding to amyloid beta plaques.

Several novel series of iodinated compounds based on the thioflavin backbone structure have been developed and characterized. These iodinated compounds showed high specific binding to amyloid beta (Abeta) aggregates with subnanomolar to nanomolar affinities. Probes like IMPY and MIPA display high brain uptakes and fast washout in normal mice, resulting in low background signals (presumably no amyloid plaques present in normal mouse brain), whereas TZDM shows long brain retention in normal mice suggesting high nonspecific in vivo binding. It is likely that tracers, that is, IMPY or MIPA, with desirable in vivo properties, will provide the highest target to non-target ratio; therefore, they are most likely to be successful as imaging agents targeting Abeta plaques in the brain.