There is an ongoing effort to develop better methods for noninvasive detection and characterization of thrombus. Here we describe the synthesis and evaluation of three new fibrin-targeted PET probes (FBP1, FBP2, FBP3). Three fibrin-specific peptides were conjugated as DOTA-monoamides at the C- and N- termini, and chelated with 64CuCl2. Probes were prepared with a specific activity ranging from 10 – 130 µCi/nmol. Both the peptides and the probes exhibited nanomolar dissociation constants (Kd) for the soluble fibrin fragment DD(E), although the Cu-DOTA derivatization resulted in a 2–3 fold loss in affinity relative to the parent peptide. Biodistribution and imaging studies were performed in a rat model of carotid artery thrombosis. For FBP1 and FBP2 at 120 min post injection, the vessel containing thrombus showed the highest concentration of radioactivity after the excretory organs, i.e. the liver and kidneys. This was confirmed ex vivo by autoradiography which showed > 4-fold activity in the thrombus containing artery compared to the contralateral artery. FBP3 showed much lower thrombus uptake and the difference was traced to greater metabolism of this probe. Hybrid MR-PET imaging with FBP1 or FBP2 confirmed that these probes were effective for detection of arterial thrombus in this rat model. Thrombus was visible on PET images as a region of high activity that corresponded to a region of arterial occlusion identified by simultaneous MR angiography. FBP1 and FBP2 represent promising new probes for the molecular imaging of thrombus.

A photosensitive caged copper complex has been prepared from a tetradentate ligand (H2cage) composed of two pyridyl-amide arms connected by a photoreactive nitrophenyl group. H2cage binds Cu2+ in aqueous solution with a stability constant (log beta) of 10.8, which corresponds to a KD of 16 pM at pH 7.4. The neutral Cu2+ complex, [Cu(OH2)(cage)], crystallizes as a distorted trigonal bipyramid coordinated by two amide and two pyridyl N atoms, with a water molecule bound in the trigonal plane. Photolysis with 350 nm UV light cleaves the ligand backbone to release photoproducts with significantly diminished affinity for Cu2+, thereby uncaging the metal ion. When coordinated as the caged complex, copper has diminished reactivity to produce hydroxyl radicals from Fenton-like reaction mixtures containing hydrogen peroxide and ascorbic acid. Postphotolysis, uncaged copper promotes hydroxyl radical formation under the same conditions. The strategy of caging copper is promising for applications where light could be used to trigger release of copper as a pro-oxidant to increase oxidative stress or as a tool to release copper intracellularly to study mechanisms of copper trafficking.

Photolabile metal-containing cages are metal complexes that undergo a change in coordination environment upon exposure to light of an appropriate wavelength. The light-responsive functionality can either be a component of the encapsulating ligand or a property of the metal complex itself. The altered coordination properties of light-responsive complexes can result in release of the coordinated metal ion into its surroundings, a differential reactivity of the metal center, or the liberation of a reactive molecule that had been passivated by binding to the metal center. These triggerable agents can be useful tools for manipulating the bioavailability of metals or their coordinating ligands in order to study biological pathways or for potential therapeutic purposes.

A novel platinum(II) photocaged complex called [Pt(cage)] has been prepared and characterized by X-ray crystallography. The complex contains a photolabile nitrophenyl group incorporated into the backbone of a tetradentate ligand that contains two pyridyl and two amide nitrogen donor sites. The intact complex is unreactive toward ligand-exchange reactions until activation with UV light cleaves the ligand backbone, releasing a Pt II complex that more readily exchanges

This review focuses on recent approaches in using targeted MRI probes for noninvasive molecular imaging of thrombosis. Probe design strategies are discussed: choice of molecular target; nanoparticle versus small-molecule probe; and gadolinium versus iron oxide imaging reporter. Examples of these different design strategies are chosen from the recent literature. Novel contrast agents used to image direct and indirect binding to fibrin have been described as well as direct binding to activated platelets. Emphasis is placed on probes where utility has been demonstrated in animal models or in human clinical trials.

Seven new nitrogen-donor ligands that contain a photoactive nitrophenyl group within the ligand backbone have been prepared and evaluated for their binding affinity for copper(ii) and zinc(ii). Among this series, the ligand 3Gcage (pyridine-2-carboxylic acid {1-(2-nitro-phenyl)-3-[(pyridin-2-ylmethyl)-amino]-propyl}-amide) has the best affinity for copper(ii), with an apparent dissociation constant at pH 7.4 of 0.18 fM. Exposure of buffered aqueous solutions of 3Gcage or Cu(ii)-bound 3Gcage to UV light induces bond cleavage in the ligand backbone, which reduces the denticity of the ligands. The quantum yields of photolysis for 3Gcage in the absence and presence of Cu(ii) are 0.66 and 0.43, respectively. Prior to photolysis, the 3Gcage ligand inhibits copper from generating hydroxyl radicals in the presence of hydrogen peroxide and ascorbic acid; however, hydroxyl radical formation increases by more than 300% following light activation, showing that the reactivity of the copper center can be triggered by light.

A new type of Cu 2+ fluorescent sensor, coucage, has been prepared with a photosensitive nitrophenyl group incorporated into the backbone of a coumarin-tagged tetradentate ligand. Coucage provides a selective fluorescence response for Cu 2+ over other biologically relevant metal ions. Coordination of Cu 2+ dims the fluorescence output until irradiation with UV light cleaves the ligand backbone, which relieves the copper-induced quenching to provide a turn-on response. Experiments in live MCF-7 cells show that coucage can be used for detecting changes in intracellular Cu 2+ upon the addition of excess exogenous copper. If improvements can be made to increase its affinity for copper, this new type of turn-on sensor could be used as a tool for visualizing the cellular distribution of labile copper to gain insight into the mechanisms of copper trafficking.Copper, the third most abundant transition metal in the human body, plays a critical role in many fundamental physiological processes; however, it also catalyzes the production of highly reactive oxygen species that damage biomolecules.1 Due to copper's dual nature, cells have developed strict regulatory processes to control its cellular distribution.1 Alterations in copper homeostasis are linked to neurodegenerative diseases such as Menkes and Wilson diseases, Alzheimer's, familial amyotrophic lateral sclerosis, and prion diseases. 2 Being able to visualize the cellular distribution of copper in both its physiological oxidation states, Cu + and Cu 2+ , would offer insight into how cells acquire, maintain, and utilize copper while suppressing its toxicity. Whereas reliable fluorescence sensors exist for Cu + , there are fewer options for detecting Cu 2+ in living cells.3 A common strategy in designing fluorescent probes for metal ions is to link a ligand to a fluorophore such that metal binding causes an increase in fluorescence only in response to the target ion. Cell permeable fluorescent sensors have proven useful for investigating intracellular metal ion distribution, particularly for Ca 2+ ,4 Zn 2+, 5 and Cu + .6 The development of this type of "turn-on" sensor for Cu 2+ , however, is hampered by the fluorescence quenching effect of this paramagnetic metal ion. As a consequence, many Cu 2+ sensors have a "turn-off" mechanism,7 which is generally less sensitive, gives false-positive results, and offers limited spatial resolution. Several examples of turn-on sensors have appeared recently,3 , 8 but limitations include sensing mechanisms that operate only in organic solvent or at non-physiological pH,8a-d low quantum yields in aqueous solution,8e or potential off-target responses.8f-i Therefore, there is a need to develop new strategies that provide a fluorescent turn-on response in order to investigate intracellular Cu 2+ . We present here coucage, a new type of fluorescent sensor that uses UV light to uncage a Cu 2+ -dependent fluorescence response.Katherine.franz@duke.edu. Supporting Information Available: Full experimental details, including synthesis...

Photolabile metallhaltige Käfige sind Metallkomplexe, die bei Bestrahlung mit Licht geeigneter Wellenlänge eine Veränderung der Koordinationsumgebung erfahren. Dabei kann entweder der verkapselnde Ligand allein oder der gesamte Metallkomplex photoresponsiv sein. Die veränderten Koordinationseigenschaften photoresponsiver Komplexe können mehrere Auswirkungen haben: eine Freisetzung des koordinierten Metallions in seine Umgebung, eine veränderte Reaktivität des Metallzentrums oder die Abspaltung eines reaktiven Moleküls, das zuvor durch Bindung an das Metallzentrum passiviert war. Diese photoaktivierbaren Systeme können von Nutzen sein, um die Bioverfügbarkeit von Metallen oder ihren koordinierenden Liganden für das Studium biologischer Reaktionspfade oder für mögliche therapeutische Zwecke zu manipulieren.