Labeling Moiety Research Articles

ETFC-01: a dual-labeled chelate-bridged tracer for SSTR2-positive tumors.

Integrating radioactive and optical imaging techniques can facilitate the prognosis and surgical guidance for cancer patients. Using a single dual-labeled tracer ensures consistency in both imaging modalities. However, developing such molecule is challenging due to the need to preserve the biochemical properties of the tracer while introducing bulky labeling moieties. In our study, we designed a trifunctional chelate that facilitates the coupling of the targeting vector and fluorescent dye at opposite sites to avoid undesired steric hindrance effects. The synthesis of the trifunctional chelate N3-Py-DOTAGA-(tBu)3 (7) involved a five-step synthetic route, followed by conjugation to the linear peptidyl-resin 8 through solid-phase synthesis. After deprotection and cyclization, the near-infrared fluorescent dye sulfo-Cy.5 was introduced using copper free click chemistry, resulting in eTFC-01. Subsequently, eTFC-01 was labeled with [111In]InCl3. In vitro assessments of eTFC-01 binding, uptake, and internalization were conducted in SSTR2-transfected U2OS cells. Ex-vivo biodistribution and fluorescence imaging were performed in H69-tumor bearing mice. eTFC-01 demonstrated a two-fold higher IC50 value for SSTR2 compared to the gold standard DOTA-TATE. Labeling of eTFC-01 with [111In]InCl3 gave a high radiochemical yield and purity. The uptake of [111In]In-eTFC-01 in U2OS.SSTR2 cells was two-fold lower than the uptake of [111In]In-DOTA-TATE, consistent with the binding affinity. Tumor uptake in H69-xenografted mice was lower for [111In]In-eTFC-01 at all-time points compared to [111In]In-DOTA-TATE. Prolonged blood circulation led to increased accumulation of [111In]In-eTFC-01 in highly vascularized tissues, such as lungs, skin, and heart. Fluorescence measurements in different organs correlated with the radioactive signal distribution. The successful synthesis and coupling of the trifunctional chelate to the peptide and fluorescent dye support the potential of this synthetic approach to generate dual labeled tracers. While promising in vitro, the in vivo results obtained with [111In]In-eTFC-01 suggest the need for adjustments to enhance tracer distribution.

Synthesis and Detection of BODIPY‐, Biotin‐, and 19F‐ Labeled Single‐Entity Dendritic Heparan Sulfate Mimetics

AbstractHeparin and heparan sulfate (HS) are naturally occurring mammalian glycosaminoglycans, and their synthetic and semi‐synthetic mimetics have attracted significant interest as potential therapeutics. However, understanding the mechanism of action by which HS, heparin, and HS mimetics have a biological effect is difficult due to their highly charged nature, broad protein interactomes, and variable structures. To address this, a library of novel single‐entity dendritic mimetics conjugated to BODIPY, Fluorine‐19 (19F), and biotin was synthesized for imaging and localization studies. The novel dendritic scaffold allowed for the conjugation of labeling moieties without reducing the number of sulfated capping groups, thereby better mimicking the multivalent nature of HS‐protein interactions. The 19F labeled mimetics were assessed in phantom studies and were detected at concentrations as low as 5 mM. Flow cytometric studies using a fluorescently labeled mimetic showed that the compound associated with immune cells from tumors more readily than splenic counterparts and was directed to endosomal‐lysosomal compartments within immune cells and cancer cells. Furthermore, the fluorescently labeled mimetic entered the central nervous system and was detectable in brain‐infiltrating immune cells 24 hours after treatment. Here, we report the enabling methodology for rapidly preparing various labeled HS mimetics and molecular probes with diverse potential therapeutic applications.

Synthesis and Detection of BODIPY-, Biotin-, and 19 F- Labeled Single-Entity Dendritic Heparan Sulfate Mimetics.

Heparin and heparan sulfate (HS) are naturally occurring mammalian glycosaminoglycans, and their synthetic and semi-synthetic mimetics have attracted significant interest as potential therapeutics. However, understanding the mechanism of action by which HS, heparin, and HS mimetics have a biological effect is difficult due to their highly charged nature, broad protein interactomes, and variable structures. To address this, a library of novel single-entity dendritic mimetics conjugated to BODIPY, Fluorine-19 (19 F), and biotin was synthesized for imaging and localization studies. The novel dendritic scaffold allowed for the conjugation of labeling moieties without reducing the number of sulfated capping groups, thereby better mimicking the multivalent nature of HS-protein interactions. The 19 F labeled mimetics were assessed in phantom studies and were detected at concentrations as low as 5 mM. Flow cytometric studies using a fluorescently labeled mimetic showed that the compound associated with immune cells from tumors more readily than splenic counterparts and was directed to endosomal-lysosomal compartments within immune cells and cancer cells. Furthermore, the fluorescently labeled mimetic entered the central nervous system and was detectable in brain-infiltrating immune cells 24 hours after treatment. Here, we report the enabling methodology for rapidly preparing various labeled HS mimetics and molecular probes with diverse potential therapeutic applications.

Recognition Moieties of Small Molecular Fluorescent Probes for Bioimaging of Enzymes.

Enzymes are a class of important substances for life, and their abnormal levels are associated with many diseases. Thus, great progress has been made in the past decade in detecting and imaging enzymes in living biosystems, and in this respect fluorescent probes combined with confocal microscopy have attracted much attention because of their high sensitivity and unrivaled spatiotemporal resolution. Fluorescent probes are usually composed of three moieties: a signal or fluorophore moiety, a recognition or labeling moiety, and an appropriate linker to connect the two aforementioned moieties. At present, however, research and reviews on enzymatic probes mostly focus on fluorophores and/or linkers, whereas those on the recognition moiety are relatively few. Moreover, current enzymatic probes with some recognition moieties have drawbacks such as poor selectivity, high background fluorescence, or/and low sensitivity and are unsatisfactory for practical applications. Thus, developing new recognition moieties with higher specificity or/and sensitivity to the enzyme of interest is very desirable but still challenging. In this Account, we introduce the recognition moieties of fluorescent probes for several enzymes, including tyrosinase, monoamine oxidase A (MAO-A), nitroreductase (NTR), and aminopeptidases. Highlights are given on how new specific recognition moieties of tyrosinase and MAO-A were designed to eliminate the interference by reactive oxygen species (ROS) and MAO-B, respectively. Here we present four recent examples in which designed fluorescent probes are employed to image enzymes in living biosystems. The first example shows that 3-hydroxyphenyl can serve as a new and more specific recognition moiety than the traditional 4-hydroxyphenyl group for tyrosinase, enabling the development of a highly selective fluorescent probe for imaging of tyrosinase without interference by ROS. The second presents a general design strategy for fluorescent probes specific for an enzyme, which involves combining the characteristic structure of an inhibitor of the target enzyme along with its traditional reactive group as a new recognition moiety, and successfully demonstrates it by selective detection of MAO-A in the presence of its isomeric MAO-B. The third mainly illustrates that 5-nitrothiophen-2-yl alcohol with a stronger electron-donating S atom is a better fluorescence quenching and recognition moiety than 5-nitrofuran-2-yl alcohol for NTR, leading to the development of a highly sensitive method for NTR assay. Lastly, on the basis of known observations, we show that besides the specific interaction with the target, another function of some recognition moieties may be responsible for tuning the fluorescence signal, which is exemplified by the linking of several aminopeptidases' recognition moieties to the free hydroxyl or amino group of different fluorophores. It is our wish that this Account will promote the appearance of more specific recognition moieties and fluorescent probes with excellent properties and that new biofunctions of the enzymes will be uncovered.

One-Step 18F-Labeling and Preclinical Evaluation of Prostate-Specific Membrane Antigen Trifluoroborate Probes for Cancer Imaging.

After the identification of the high-affinity glutamate-ureido scaffold, the design of several potent 18F- and 68Ga-labeled tracers has allowed spectacular progress in imaging recurrent prostate cancer by targeting the prostate-specific membrane antigen (PSMA). We evaluated a series of PSMA-targeting probes that are 18F-labeled in a single step for PET imaging of prostate cancer. Methods: We prepared 8 trifluoroborate constructs for prostate cancer imaging, to study the influence of the linker and the trifluoroborate prosthetic on pharmacokinetics and image quality. After 1-step labeling by 19F–18F isotopic exchange, the radiotracers were injected in mice bearing LNCaP xenografts, with or without blocking controls, to assess specific uptake. PET/CT images and biodistribution data were acquired at 1 h after injection and compared with 18F-DCFPyL on the same mouse strain and tumor model. Results: All tracers exhibited nanomolar affinities, were labeled in good radiochemical yields at high molar activities, and exhibited high tumor uptake in LNCaP xenografts with clearance from nontarget organs. Most derivatives with a naphthylalanine linker showed significant gastrointestinal excretion. A radiotracer incorporating this linker with a dual trifluoroborate-glutamate labeling moiety showed high tumor uptake, low background activity, and no liver or gastrointestinal track accumulation. Conclusion: PSMA-targeting probes with trifluoroborate prosthetic groups represent promising candidates for prostate cancer imaging because of facile labeling while affording high tumor uptake values and contrast ratios that are similar to those obtained with 18F-DCFPyL.

Synthesis and Characterization of Furoxan Incorporating Photoaffinity Labeling Probes

Event Abstract Back to Event Synthesis and Characterization of Furoxan Incorporating Photoaffinity Labeling Probes Ethel N. Tackie-Yarboi1*, Anvitha R. Madhavaram1* and Isaac Schiefer1* 1 University of Toledo, United States Nitric oxide (NO) plays a critical role in neuronal function and synaptic plasticity. Molecules capable of enhancing NO signaling have shown promise as potential therapeutics for neurodegenerative disorders. We are engaged in the development of a novel class of NO mimetics known as furoxans (1,2,5 oxadiazole-N-oxides). Furoxans are commonly described as thiol-dependent NO mimetics, which must react with biological thiols to exhibit NO bioactivity. We have created a series of attenuated furoxans, which do not readily react with generic cellular thiols, such as glutathione or L-cysteine. Nevertheless, our furoxans display neuroprotection in cell culture and the ability to reverse scopolamine induced cognition deficits in a model of contextual fear memory. We have demonstrated that furoxan neuroprotection is reversed upon blockade of NO/sGC (soluble guanylyl cyclase) signaling. We hypothesize our attenuated furoxans react with a subset of yet to be identified relatively reactive protein thiols. The work presented focuses on the synthesis and characterization of novel furoxan probes containing, 1) a photoaffinity labeling group, either a diazirine or perfluoroarylazide, for transient covalent modification of interacting proteins; and 2) a terminal alkyne for conjugation with a fluorophore and subsequent visualization of protein modification by SDS-PAGE, to elucidate this hypothesis. We have observed distinct advantages and disadvantages of each photoaffinity labeling moiety with regards to stability and rate of photolysis. Efforts are ongoing to identify furoxan target proteins in primary cell culture. Keywords: Furoxan, Neuroprotection, Nitric Oxide, Photoaffimty labelling, PROBES Conference: National Organization for the Professional Advancement of Black Chemists and Chemical Engineers (NOBCChE) 45th Annual Conference , Orlando, Florida, United States, 17 Sep - 20 Sep, 2018. Presentation Type: Oral Presentation Topic: Medicinal Chemistry Citation: Tackie-Yarboi EN, Madhavaram AR and Schiefer I (2019). Synthesis and Characterization of Furoxan Incorporating Photoaffinity Labeling Probes. Front. Chem. Conference Abstract: National Organization for the Professional Advancement of Black Chemists and Chemical Engineers (NOBCChE) 45th Annual Conference . doi: 10.3389/conf.fchem.2018.01.00013 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 08 Oct 2018; Published Online: 17 Jan 2019. * Correspondence: Ms. Ethel N Tackie-Yarboi, University of Toledo, Toledo, United States, Ethel.Tackie-Yarboi@rockets.utoledo.edu Ms. Anvitha R Madhavaram, University of Toledo, Toledo, United States, anvitha.madhavaram@rockets.utoledo.edu Dr. Isaac Schiefer, University of Toledo, Toledo, United States, isaac.schiefer@utoledo.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Ethel N Tackie-Yarboi Anvitha R Madhavaram Isaac Schiefer Google Ethel N Tackie-Yarboi Anvitha R Madhavaram Isaac Schiefer Google Scholar Ethel N Tackie-Yarboi Anvitha R Madhavaram Isaac Schiefer PubMed Ethel N Tackie-Yarboi Anvitha R Madhavaram Isaac Schiefer Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Electrochemical behavior of anthraquinone- and nitrophenyl-labeled deoxynucleoside triphosphates: a contribution to development of multipotential redox labeling of DNA

Electrochemical properties of base-modified cytosine or 7-deazaadenine nucleoside triphosphates (dNTPs) bearing electrochemically active anthraquinone or 3-nitrophenyl moieties were studied using cyclic voltammetry with the hanging mercury drop electrode. The anthraquinone moiety in the dNTPs gives well-pronounced reversible quinone/hydroquinone redox signals around −0.40 V (against Ag|AgCl|3M KCl reference electrode), while the nitro group in 3-nitrophenyl exhibits irreversible reduction to hydroxylamine around −0.45 V that can be reversibly oxidized to corresponding nitroso compound close to 0.0 V. Both anthraquinone and hydroxylamine redox groups can be selectively switched off by further electrochemical transformation, depending on negative potential applied and composition of the background electrolyte. Results of this study suggest that both nucleobase and the conjugate label moiety influence remarkably the adsorbability and/or intermolecular interactions taking part at the electrode surface. The potential analytical utilization of these phenomena is discussed.

Switchable lanthanide luminescent binary probes in efficient single nucleotide mismatch discrimination

Switchable lanthanide luminescence is a novel proximity-based binary probe technology wherein two oligonucleotide probes are labelled either with lanthanide ion carrier chelate or light-absorbing antenna ligand. A highly luminescent complex is formed when the two label moieties are brought in close contact by adjacent hybridization of the labelled probes to a target sequence. Herein we report how hybridization distance between the labelled probes and single nucleotide mismatches in the target sequence affect to the luminescence intensity of the complex formed from the label moieties. The highest luminescence intensity was observed when the probes were hybridized to full matched target at a distance of 4–10 nucleotides, a fact that indicates the lanthanide complex requires a certain space to be formed. At optimal hybridization distance between the labelled probes the maximal discrimination efficiency between a full matched and a single mismatched target was 1400-fold. Furthermore, computer-based modelling of the luminescent complex formation was in good agreement with the experimental results.

Bonding in nitroxide spin labels from 14N electric-quadrupole interactions.

Nitrogen nuclear electric quadrupole couplings from FT-EPR of nitroxide spin labels can be used to deduce the covalent transfer πc in the N–O π-bond, and ionicities iσ(NO) and iσ(NC) of the N–O and N–C σ-bonds, if they are combined with the unpaired spin density on the nitrogen ρπ(N) obtained from the dipolar hyperfine couplings. Application to EPR data from an analogue of the MTSSL nitroxide that is used in site-directed spin-labeling demonstrates how environmental polarity and hydrogen bonding are reflected in the bonding parameters of the C–NO–C spin label moiety. Several recent publications erroneously claim to have deduced three independent bonding parameters from nitrogen quadrupole couplings alone.

Rapid homogeneous immunoassay for cardiac troponin I using switchable lanthanide luminescence

Homogeneous assays are advantageous because of their simplicity and rapid kinetics but typically their performance is severely compromised compared to heterogeneous assay formats. Here, we report a homogeneous immunoassay utilizing switchable lanthanide luminescence for detection and site-specifically labeled recombinant antibody fragments as binders to improve the assay performance. Switchable lanthanide luminescence enabled elimination of assay background due to division of the luminescent lanthanide chelate into two non-luminescent label moieties. Simultaneous biomolecular recognition of model analyte cardiac troponin I by two antibody fragments brought the label moieties together and resulted in self-assembly of luminescent mixed chelate complex. The assay was very rapid as maximal signal-to-background ratios were achieved already after 6min of incubation. Additionally, the limit of detection was 0.38ng/mL (16pM), which was comparable to the limit of detection for the heterogeneous reference assay based on the same binders (0.26ng/mL or 11pM). This is the first study to apply switchable lanthanide luminescence in immunoassays and demonstrates the versatile potential of the technology for rapid and sensitive homogeneous assays.

Analyzing receptor assemblies in the cell membrane using fluorescence anisotropy imaging with TIRF microscopy.

Signaling within and between animal cells is controlled by the many receptor proteins in their membrane. They variously operate as trans-membrane monomers and homo- or hetero-dimers, and may assemble with ion-channels: analyses thereof are needed in studies of receptor actions in tissue physiology and pathology. Interactions between membrane proteins are detectable when pre-labeled with fluorophores, but a much fuller analysis is achievable via advanced optical techniques on living cells. In this context, the measurement of polarization anisotropy in the emitted fluorescence has been the least exploited. Here we demonstrate its methodology and particular advantages in the study of receptor protein assembly. Through excitation in both TIRF and EPI fluorescence illumination modes we are able to quantify and suppress contributions to the signal from extraneous intra-cellular fluorescence, and we show that the loss of fluorescence-polarization measured in membrane proteins reports on receptor protein assembly in real time. Receptor monomers and homo-dimers in the cell membrane can be analyzed quantitatively and for homo-dimers only a single fluorescent marker is needed, thus suppressing ambiguities that arise in alternative assays, which require multiple label moieties and which are thus subject to stoichiometric uncertainty.

Quantification of Nanosized Extracellular Membrane Vesicles with Scanning Ion Occlusion Sensing

Cells secrete different types of membrane vesicles (MVs), which may act as important entities in normal human physiology and in various pathological processes. The established methods for quantification of MVs require purification or preanalytical handling of samples with labeling moieties. The authors' aim was to develop a method for high-throughput, labeling-free quantification of nonpurified MVs. Scanning ion occlusion sensing technology, which relies on the detection of particles upon their movement through a nanopore, was investigated for the ability to quantify nanosized MVs (<400 nm) in bodily fluids and cell culture supernatants. Scanning ion occlusion sensing allowed for rapid and easy measurement of the concentration of MVs in all biological fluids tested. Scanning ion occlusion sensing technology enables the quantification of MVs in biological samples without the requirement of MV isolation and/or labeling. This offers a highly valuable addition to the currently used repertoire of MV quantification methods.

Aptamer-directed lanthanide chelate self-assembly for rapid thrombin detection

We report a sensitive assay method for homogeneous thrombin detection. The method is based on lanthanide chelate complementation, where the luminescent complex is split into two separate label moieties, which are intrinsically non-luminescent. A luminescent mixed chelate complex is formed only when the label moieties are brought into close proximity directed by two separate binding events of aptamers to the analyte. This results in high specificity in signal generation while time-resolved fluorescence detection eliminates the short lifetime autofluorescence, which is inherent to many homogeneous assays and limits their applicability. The developed method is also very rapid as the maximum signal is obtained in just five minutes. Lanthanide chelate complementation can be applied for the detection of other proteins when two binders recognizing separate epitopes of the analyte are available.

Squaric Acid Mediated Chemoselective PEGylation of Proteins: Reactivity of Single‐Step‐Activated α‐Amino Poly(ethylene glycol)s

The covalent attachment of poly(ethylene glycol) (PEG) to therapeutically active proteins (PEGylation) has become an important method to deal with the pharmacological difficulties of these polypeptides, such as short body-residence times and immunogenicity. However, the derivatives of PEG used for PEGylation lack further functional groups that would allow the addition of targeting or labeling moieties. Squaric acid diethyl ester was used for the chemoselective single-step activation of poly(ethylene glycol)s into the respective ester amides. The resultant selective protein-reactive poly(ethylene glycol)s were investigated with respect to their selectivity towards amino acid residues in bovine serum albumin (as a model protein). The presented procedure relies on a robust two-step protocol and was found to be selective towards lysine residues; the activated polyethers are efficient and stoichiometric PEGylation agents with a remarkable hydrolytic stability over a period of several days. By adjusting the pD value of the conjugation mixture, the chemoselectivity of the activated PEGs towards the α- and ε-amino groups of lysine methyl ester was effectively changed.

Homogeneous Detection of Avidin Based on Switchable Lanthanide Luminescence

We have developed switchable lanthanide luminescence-based binary probe technology for homogeneous detection of avidin, which is a tetrameric protein. Two different nonluminescent label moieties--a light-absorbing antenna ligand and a lanthanide ion carrier chelate--were conjugated to separate biotins, which is known as avidin's natural ligand. The assay was based on binding of the two differently labeled biotins on separate binding sites on the target protein and consequent self-assembly of a luminescent complex from the two label moieties. Specific luminescence signal was observed only at the presence of the target protein. The characteristics of the switchable lanthanide luminescence assay were compared to the reference assay, based on lanthanide resonance energy transfer. Both assays had a limit of detection in the low-picomolar concentration range; however, the lanthanide chelate complementation-based assay had wider dynamic range and its optimization was more straightforward. The switchable lanthanide luminescence technology could be further applied to generic protein detection, using reagents that are analogous to the proximity ligation assay principle.

Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation

We studied the adsorption kinetics of supported ultrathin films of dye-labeled polystyrene (l-PS) by combining dielectric spectroscopy (DS) and the interface-specific nonlinear optical second harmonic generation (SHG) technique. While DS is sensitive to the fraction of mobile dye moieties (chromophores), the SHG signal probes their anisotropic orientation. Time-resolved measurements were performed above the glass transition temperature on two different sample geometries. In one configuration, the l-PS layer is placed in contact with the aluminum surface, while in the other one, the deposition is done on a strongly adsorbed layer of neat PS. From the time dependence of the dielectric strength and SHG signal of the l-PS layer in contact with the metal, we detected two different kinetics regimes. We interpret these regimes in terms of the interplay between adsorption and orientation of the adsorbing labeling moieties. At early times, dye moieties get adsorbed adopting an orientation parallel to the surface. When adsorption proceeds to completeness, the kinetics slows down and the dye moieties progressively orient normal to the surface. Conversely, when the layer of l-PS layer is deposited on the strongly adsorbed layer of neat PS, both the dielectric strength and the SHG signal do not show any variation with time. This means that no adsorption takes place.

Luminescence Switching by Hybridization-Directed Mixed Lanthanide Complex Formation

We have developed a homogeneous assay method in which the lanthanide ion carrier and light absorbing components of a luminescent lanthanide chelate are separated in two distinct molecules that can together form a luminescent mixed chelate complex. The separated label moieties were conjugated to oligonucleotides which were used as probes to detect a complementary target DNA. The background signal of the assay was very low, indicating the signal was highly dependent on the hybridization of the two probes on adjacent positions on the target oligonucleotide.

Biophysical Characterization of a New Phospholipid Analogue with a Spin-Labeled Unsaturated Fatty Acyl Chain

Spin-labeled analogs of phospholipids have been used widely to characterize the biophysical properties of membranes. We describe synthesis and application of a new spin-labeled phospholipid analog, SL-POPC. The advantage of this molecule is that the EPR active doxyl group is linked to an unsaturated fatty acyl chain different to saturated phospholipid analogs used so far. The need for those analogs arises from the fact that biological membranes contain unsaturated phospholipids to a large extent. The biophysical properties of SL-POPC in membranes were characterized using EPR and NMR spectroscopy and compared with those of the saturated spin-labeled phospholipid, SL-PSPC. To this end, POPC membranes were labeled with either analog to assess whether the spin-labeled counterpart SL-POPC mimics the membrane properties better than the often used SL-PSPC. The results show that SL-POPC and SL-PSPC explore different molecular environments of the bilayer, and that the type and degree of perturbation of bilayer caused by the label moiety also differs between both analogs. We found that SL-POPC is more appropriate to assess the versatile dynamics of POPC membranes than SL-PSPC.

Synthesis, 18F-Labeling, and in Vitro and in Vivo Studies of Bombesin Peptides Modified with Silicon-Based Building Blocks

The gastrin-releasing peptide receptor (GRPr) is overexpressed on various human tumors. The goal of our study was the synthesis of new 18F-labeled bombesin analogues for the PET imaging of GRPr expression in prostate tumor using a silicon-based one-step n. c. a. radiolabeling method. The silicon-containing building blocks were efficiently coupled to the N-terminus of the peptides via solid-phase synthesis. Radiolabeling of the obtained peptide precursors proceeded smoothly under acidic conditions (34-85% conversion). Using the di-tert-butyl silyl building block as labeling moiety, products containing a hydrolytically stable 18F-label were obtained. In in vitro receptor binding experiments 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-NMeGly-His-Sta-Leu-NH 2 ( 4b, IC50 = 22.9 nM) displayed a 12-fold higher binding affinity than 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-Gly-His(3Me)-Sta-Leu-NH2 ( 3b, IC50 = 276.6 nM), and 4b was therefore chosen for further evaluation. In vitro and ex vivo metabolite studies of [18F]4b showed no significant degradation. In biodistribution experiments, tumor uptake of [18F]4b was low and unspecific, whereas the GRPr-rich pancreas revealed a high and specific accumulation of the radiotracer. This study demonstrates the applicability of our silicon-based one-step n. c. a. radiolabeling method for the synthesis of new 18F-labeled bombesin derivatives. This innovative approach represents a general, straightforward access to radiolabeled peptides as PET imaging probes.

Water Soluble Metallo-Phthalocyanines: The Role of the Functional Groups on the Spectral and Photophysical Properties

Strategies are reported that produce symmetrical metal-free and metallo-phthalocyanine dyes, Pc and MPc, respectively, containing various numbers of water solubilizing carboxylic acid groups on their periphery that provide a dual role by also serving as functional groups to covalently link primary amine-containing targets to these dyes. In order to induce water compatibility and to minimize the degree of aggregation, the periphery of the macrocycle was decorated with various numbers of water-solubilizing groups and/or altering the identity of the metal center. The influence of the number of solubilizing groups and metal center on the spectral and photophysical properties were evaluated. MPc dyes containing 4, 8, or 16 carboxylic acid groups exhibited similar absorption and emission maxima (677 and 686 nm, respectively) with the molar absorptivity of the Q-band approximately 10(5) M(-1) cm(-1). Results indicated that the fluorescence lifetimes and quantum yields varied as a function of the metal center; the degree of carboxylation did not significantly alter these properties in DMSO, but did mediate the solubility and aggregation states when placed in aqueous solvents. The water solubilizing groups could also serve as labeling moieties for targets bearing primary amines. Results showed that the conjugate, produced by covalently linking an MPc to streptavidin through one of its carboxylate groups, generated a red-shift in the emission maximum with a fluorescence lifetime shorter than that of the native MPc dye.