Synthesis and structural characterization of new oxorhenium and oxotechnetium complexes with XN2S-tetradentate semi-rigid ligands (X = O, S, N)

Rhenium and Technetium-oxo Complexes with Thioamide Derivatives of Pyridylhydrazine Bifunctional Chelators Conjugated to the Tumour Targeting Peptides Octreotate and Cyclic-RGDfK

This research aimed to develop new tumor targeted theranostic agents taking advantage of the similarities in coordination chemistry between technetium and rhenium. A γ-emitting radioactive isotope of technetium is commonly used in diagnostic imaging, and there are two β- emitting radioactive isotopes of rhenium that have the potential to be of use in radiotherapy. Variants of the 6-hydrazinonicotinamide (HYNIC) bifunctional ligands have been prepared by appending thioamide functional groups to 6-hydrazinonicotinamide to form pyridylthiosemicarbazide ligands (SHYNIC). The new bidentate ligands were conjugated to the tumor targeting peptides Tyr3-octreotate and cyclic-RGD. The new ligands and conjugates were used to prepare well-defined {M═O}3+ complexes (where M = 99mTc or natRe or 188Re) that feature two targeting peptides attached to the single metal ion. These new SHYNIC ligands are capable of forming well-defined rhenium and technetium complexes and offer the possibility of using the 99mTc imaging and 188/186Re therapeutic matched pairs.

Synthesis of Oxorhenium(V) and Oxotechnetium(V) Complexes That Bind to Amyloid-β Plaques

Alzheimer's disease is characterized by the presence of amyloid plaques in the brain. The primary constituents of the plaques are aggregated forms of the amyloid-β (Aβ) peptide. With the goal of preparing technetium-99(m) complexes that bind to Aβ plaques with the potential to be diagnostic imaging agents for Alzheimer's disease, new tetradentate ligands capable of forming neutral and lipophilic complexes with oxotechentium(V) and oxorhenium(V) were prepared. Nonradioactive isotopes of technetium are not available so rhenium was used as a surrogate for exploratory chemistry. Two planar tetradentate N3O ligands were prepared that form charge-neutral complexes with oxorhenium(v) as well as a ligand featuring a styrylpyridyl functional group designed to bind to Aβ plaques. All three ligands formed complexes with oxorhenium(V), and each complex was characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The oxorhenium(V) complex with a styrylpyridyl functional group binds to Aβ plaques present in post-mortem human brain tissue. The chemistry was extrapolated to technetium-99(m) at the tracer level for two of the ligands. The resulting oxotechnetium(V) complexes were sufficiently lipophilic and charge-neutral to suggest that they have the potential to cross the blood-brain barrier but exhibited modest stability with respect to exchange with histidine. The chemistry presented here identifies a strategy to integrate styrylpyridyl functional groups into tetradentate ligands capable of forming complexes with [M═O](3+) cores (M = Re or Tc).

Synthesis and preliminary biological evaluation of the first (99m)Tc(I)-specific semi-rigid tridentate ligand based on a click chemistry strategy

A novel bifunctional chelating agent based on a click chemistry strategy has been synthesized and characterized on the basis of spectroscopic techniques. The metal chelating part of this new class of tridentate N2O ligand combined a triazole unit and an aromatic ring. This latter semi-rigid framework induced a pre-organization of the chelating cavity, improving the stability of the corresponding metallic complexes (M = (99m) Tc, Re). Thus, the (99m) Tc(CO)3 complex, obtained with good yield and excellent radiochemical purity (>90%), exhibited a high in vitro serum stability. Tissue biodistribution in normal mice showed a rapid clearance, no long-term retention in organs and no in vivo reoxidation of technetium-99m, making this compound a promising (99m)Tc-chelating system.

Design, synthesis and structure-activity relationship of rhenium 2-arylbenzothiazoles as β-amyloid plaque binding agents

To continue our efforts toward the development of (99m)Tc PiB analogs, we have synthesized 24 neutral and lipophilic Re (as a surrogate of (99m)Tc) 2-arylbenzothiazoles, and explored their structure-activity relationship for binding to Aβ1-40 fibrils. These Re complexes were designed and synthesized via the integrated approach, so their (99m)Tc analogs would have a greater chance of crossing the blood-brain barrier. While the lipophilicities (logPC18=1.59-3.53) of these Re 2-arylbenzothiazoles were all within suitable range, their binding affinities (Ki=30-617nM) to Aβ1-40 fibrils varied widely depending on the selection and integration of the tetradentate chelator into the 2-phenylbenzothiazole pharmacophore. For potential clinical applications, further refinement to obtain Re 2-arylbenzothiazoles with better binding affinities (<10nM) will likely be needed. The integrated approach reported here to generate compact, neutral and lipophilic Re 2-arylbenzothiazoles could be applied to other potent pharmacophores as well to convert other current Aβ PET tracers to their (99m)Tc analogs for more widespread application via the use of SPECT scanners.

Synthesis and beta-amyloid binding properties of rhenium 2-phenylbenzothiazoles

As a first step toward the development of (99m)Tc PiB analogs, we have synthesized six neutral Re 2-phenylbenzothiazoles via pendant or integrated approach. These Re compounds bind to Abeta(1-40) fibrils with fairly good affinities (K(i)=10.0-88.6nM) and have moderate lipophilicities (logP(C18)=1.21-3.26). The Re compounds prepared via the integrated approach are smaller in size, and therefore their corresponding (99m)Tc analogs would have a greater chance of crossing the blood-brain barrier well. For potential clinical applications, further optimization on the structure-activity relationship to obtain Re 2-phenylbenzothiazoles with higher binding affinities (<10nM) might be needed. The integrated approach reported here to obtain neutral, compact and lipophilic Re 2-phenylbenzothiazoles could to be applied to other high affinity pharmacophores as well as to generate (99m)Tc analogs that could hold promise for extending the use of Abeta imaging in living human brain to many more clinical settings because they could be used with SPECT.

Rhenium(v) oxocomplexes with novel pyrazolyl-based N4- and N3S-donor chelators

The novel pyrazolyl-based ligands 3,5-Me2pz(CH2)2NH(CH2)2NH(CH2)2NH2 and pz*(CH2)2NH-Gly-CH2STrit (pz*=pz, 3,5-Me2pz, 4-(EtOOC)CH(2)-3,5-Me2pz) were synthesized, and their suitability to stabilize Re(V) oxocomplexes was evaluated using different starting materials, namely (NBu4)[ReOCl4], [ReOCl3(PPh3)2] and trans-[ReO2(py)4]Cl. Compound reacts with trans-[ReO2(py)4]Cl yielding the cationic compound [ReO(OMe){3,5-Me2pz(CH2)2N(CH2)2NH(CH2)2NH2}](BPh4) in a low isolated yield. In contrast, the neutral complexes [ReO{pz*(CH2)2NH-Gly-CH2S}] (pz*=pz, 3,5-Me2pz, 4-(EtOOCCH2)-3,5-Me2pz) were synthesized almost quantitatively by reacting [ReOCl3(PPh3)2] or (NBu4)[ReOCl4] with the trityl-protected chelators. The X-ray diffraction analysis of and confirmed the tetradentate coordination mode of the respective ancillary ligands. In the monoanionic chelator coordinates to the metal through four nitrogen atoms, while in the chelator is trianionic, coordinating to the metal through three nitrogens and one sulfur atom. Solution NMR studies of , including two-dimensional NMR techniques (1H COSY and 1H/13C HSQC), confirmed that the N3S coordination mode of the chelators is retained in solution. Unlike , complexes may be considered relevant in the development of radiopharmaceuticals, as further corroborated by the synthesis of the congener [99mTcO{pz(CH2)2-NH-Gly-CH2S}]. This radioactive compound was obtained from 99mTcO4- in aqueous medium, in almost quantitative yield and with high specific activity and radiochemical purity.