TROTEC-1: a new high-affinity ligand for labeling of the dopamine transporter

Mapping neuroreceptors with metal-labeled radiopharmaceuticals

The growing epidemiological and economic burden of neurological diseases on society is tremendous. A correct and timely diagnosis can help in lowering the burden and improving the life quality of both the diseased person and the caretaker. Imaging of the brain (neuroimaging) using CT, MRI, and nuclear imaging methods can provide anatomical and functional information. Neuroreceptors are central to neurotransmission and neuromodulation in the CNS. In vivo imaging of receptors in the brain provides powerful tools for the functional study of the central nervous system (CNS) in normal or diseased states. Presently, PET imaging using non-metallic radiotracers dominates the imaging of neuroreceptors. Metal-based probes for SPECT and PET can be economical and logistically easier to use without compromising the information. This review focuses on the development of metallic radiotracers for (99mTc) SPECT and (68Ga) PET along with future directions based on the metallic probes developed for other imaging modalities namely MRI.

EXAFS analyses of technetium(I) carbonyl complexes – stability studies in solutions

EXAFS analyses were successfully used to determine the structure of Re and Tc carbonyl thioether complexes in solid and liquid samples. In connection with chromatographic and mass spectrometric methods the behaviour of Tc carbonyl dithioether complexes in aqueous solution was studied. It was found that complexes containing a bidentate thioether ligand are able to react with water by exchange of the chloride ion, which results in the formation of a cationic complex. As expected, the exchange proceeds faster at the n.c.a. level because of concentration differences.

Evaluation of 99mTc-labeled tropanes with alkyl substituents on the 3β-phenyl ring as potential dopamine transporter tracers

Technetium(V)-oxo-3beta-(4-chlorophenyl)-8-methyl-8-azabicyclo[3.2.1]oct-2-yl[N-(2-mercaptoethyl), N-(N'-(2-mercaptoethyl)-2-aminoethyl)]-aminomethyl ((99m)Tc-TRODAT-1) and three derivatives with one or two substituents on the 3beta-phenyl ring (4-methylphenyl, 4-ethylphenyl and 2,4-dimethylphenyl) were prepared and evaluated as potential imaging agents for the central nervous dopamine transporter (DAT). Labeling of the ligands with (99m)Tc yielded for each of them a mixture of two radiolabeled species, which were purified and isolated using reversed-phase high-performance liquid chromatography. Employing radio-LC-MS, we found both species to have the same molecular mass suggesting diastereoisomers. After intravenous injection in mice and rats, the compounds were stable in vivo and no important metabolites were found in plasma or urine. Replacement of the 4-chloro atom on the 3beta-phenyl ring by a methyl group causes no loss of affinity for the DAT system. However, substitution of an ethyl group for the 4-chloro atom or introduction of a second methyl group in the 2-position of the phenyl ring results in a serious reduction of the affinity for the DAT transporter. Ex vivo autoradiography on mice brain slices and biodistribution studies in rats showed specific uptake of (99m)Tc-TRODAT-1 and the 4-methylphenyl derivative in striatum and putamen. Although the 4-ethylphenyl and 2,4-dimethylphenyl derivatives show brain uptake in rats and mice, no specific uptake in striatum was found. In addition, differences in biological behavior between the different diastereomers were observed. In conclusion, small changes to (99m)Tc-TRODAT-1 at the phenyl ring in the 3beta position of the tropane moiety significantly change the biological behavior of the studied compounds.

Synthesis and biological evaluation of a technetium-99m(I)-tricarbonyl-labelled phenyltropane derivative

A new tropane derivative was synthesized by combining a tridentate ligand, N-(2-picolylamine)-N-acetic acid (2-PAA), and a phenyltropane derivative. It was labelled with a [(99m)Tc(CO)(3)](+) moiety, resulting in the formation of two stable and neutral lipophilic isomers. Their identity was confirmed using radio-LC-MS. In normal mice, no brain uptake was observed for any of the isomers and in vitro autoradiography using mouse brain sections showed no specific uptake in the striatal area.

Lipase Active-Site-Directed Anchoring of Organometallics: Metallopincer/Protein Hybrids

The work described herein presents a strategy for the regioselective introduction of organometallic complexes into the active site of the lipase cutinase. Nitrophenol phosphonate esters, well known for their lipase inhibitory activity, are used as anchor functionalities and were found to be ideal tools to develop a single-site-directed immobilization method. A small series of phosphonate esters, covalently attached to ECE "pincer"-type d8-metal complexes through a propyl tether (ECE=[C6H3(CH2E)(2)-2,6]-; E=NR2 or SR), were designed and synthesized. Cutinase was treated with these organometallic phosphonate esters and the new metal-complex/protein hybrids were identified as containing exactly one organometallic unit per protein. The organometallic proteins were purified by membrane dialysis and analyzed by ESI-mass spectrometry. The major advantages of this strategy are: 1) one transition metal can be introduced regioselectively and, hence, the metal environment can potentially be fine-tuned; 2) purification procedures are facile due to the use of pre-synthesized metal complexes; and, most importantly, 3) the covalent attachment of robust organometallic pincer complexes to an enzyme is achieved, which will prevent metal leaching from these hybrids. The approach presented herein can be regarded as a tool in the development of regio- and enantioselective catalyst as well as analytical probes for studying enzyme properties (e.g., structure) and, hence, is a "proof-of-principle design" study in enzyme chemistry.

Characterisation and biodistribution of two neutral 99mTc(CO)3 complexes with a tridentate ligand

N-(2-Mercapto-propyl)-1,2-phenylenediamine (MPPDA) and N-beta-aminoethylglycine (AEG) were labelled with 99mTc(CO)3(+) to form the neutral complexes [99mTc(CO)3(MPPDA)] and [99mTc(CO)3(AEG)]. Both complexes were formed in excellent yields and their identity was confirmed by LC-MS. In mice, none of the new tracer agents showed brain uptake. [(99m)Tc(CO)3(MPPDA)] was trapped mainly in the liver and excreted via the hepatobiliary system, whereas [99mTc(CO)3(AEG)] was excreted rapidly via the kidneys to the urine.

Technetium-99m labelled integrated tropane–BAT as a potential dopamine transporter tracer

To reduce the molecular weight of 99mTc-labelled tropanes with the aim to enhance the passage over the blood-brain barrier, a so-called integrated tropane-BAT construct was developed. For this purpose a mercaptoethyl substituent was attached to the amine nitrogen atom of a nortropane precursor and the methyl carboxylate in 2beta-position was converted to a 2-mercaptoethylaminomethylene substituent. This integrated tropane-BAT construct could be labelled efficiently (85-90%) with technetium-99m. Results of LC-MS analysis of the tracer agent support the assumed structure. Biodistribution studies in normal rats (n=3) showed a slightly higher brain uptake for the new tracer agents as compared to 99mTc-TRODAT-1. These results indicate that further biological evaluation of the integrated 99mTc-tropane-BAT is warranted.

Syntheses and Characterization of Dicarbonyl−Nitrosyl Complexes of Technetium(I) and Rhenium(I) in Aqueous Media: Spectroscopic, Structural, and DFT Analyses

This work describes new synthetic routes to produce mixed carbonyl-nitrosyl complexes of technetium(I) and rhenium(I) in aqueous media. NaNO2, NOHSO4, and NO2(g) have been used to produce in situ nitrous acid as the primary source of NO+. Starting from the organometallic precursor fac-[MX3(CO)3]+, 1 (M = 99Tc, Re; X = Cl, Br), the formation of mixed dicarbonyl-mononitrosyl complexes was observed in aqueous hydrochloric and hydrobromic acid. Time-dependent analyses of the reactions by means of HATR-IR and 99Tc NMR spectroscopy in solution revealed the almost quantitative substitution of one CO ligand by NO+ and, thus, the formation of complexes with facial arrangement of the three pi-acceptor ligands. In the case of technetium, the monomeric complex (NEt4)[TcCl3(CO)2NO] (3a) and the dimeric, chloride-bridged, neutral complex [TcCl(mu-Cl)(CO)2NO]2 (4a) were produced. In the case of rhenium, the monomeric species (NEt4)[ReBr2X(CO)2NO] (X = Br (3b), NO3 (5)) was solely isolated. The X-ray structure of complexes 4a and 5 are discussed. The crystallographic analyses revealed the coordination of the NO+ group trans to the terminal chloride (4a) or the bromide (5), respectively. Crystal data: complex 4a (C4Cl4N2O(6)Tc2), monoclinic, Cc, a = 18.82(3) A, b = 6.103(6) A, c = 12.15(2) A, alpha = 90 degrees , beta = 105.8(2) degrees , gamma = 90 degrees , V = 1343(3) A(3), Z = 4; complex 5 (C10H20N3O(6)Br2Re), orthorhombic, P2(1)2(1)2(1), a = 10.2054(5) A, b = 12.5317(7) A, c = 13.9781(7) A, V = 1787.67(16) A(3), Z = 4. The isolated complexes and their potential facial isomers have been further investigated by density functional theory (DFT) calculations. The energy differences of the isomers are relatively small; however, the calculated energies are consistent with the formation of the observed and isolated compounds. The calculated bond lengths and angles of complex 5 are in good agreement with the data determined by X-ray diffraction. Experiments on the no-carrier-added level starting from fac-[99mTc(H2O)3(CO)3]+ revealed the formation of the complex fac-[99mTcCl(H2O)2(CO)2NO]+ in reasonable good yields. This aqueous-based, synthetic approach will enable the future evaluation of this novel, low-valent metal precursor for potential use in radiopharmacy.

Binding of the Oxo−Rhenium(V) Core to Methionine and to N-Terminal Histidine Dipeptides

The ReOX(2)(met) compounds (X = Cl, Br) adopt a distorted octahedral structure in which a carboxylato oxygen lies trans to the Re=O bond, whereas the equatorial plane is occupied by two cis halides, an NH(2), and an SCH(3) group. Coordination of the SCH(3) unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr(2)(d,l-met).1/2H(2)O and ReOBr(2)(d,l-met).1/2CH(3)OH contain only the syn isomer (S-CH(3) bond on the side of the Re=O bond), whereas ReOCl(2)(d-met) and ReOCl(2)(d,l-met) consist of the pure anti isomer. (1)H NMR spectroscopy shows that both isomers coexist in equilibrium in acetone (anti/syn ratio = 1:1 for X = Br, 3:1 for X = Cl). Exchange between these two isomers is fast above room temperature, but it slows down below 0 degrees C, and the sharp second-order spectra of both isomers at -20 degrees C were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX(2)(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr(2)(His-Ala)]Br reveals the presence of distorted octahedral cations containing the Re=O(3+) core and a dipeptide coordinated through the histidine residue via the imidazole nitrogen, the terminal amino group, and the amide oxygen, the site trans to the Re=O bond being occupied by the oxygen. The alanine residue is ended by a protonated carboxylic group that does not participate in the coordination. The constant pattern of the(1)H NMR signals for the protons in the histidine residue confirms that the various dipeptides adopt a similar binding mode, consistent with the solid-state structure being retained in CD(3)OD solution.

Synthesis and structural characterization of novel Re(i) tricarbonyl complexes anchored on a phosphinoarylbenzylamine and a phosphinoaryloxazoline generated in situ

Reduction of the amide or replacement of the hydroxyl by a bromide in 2-(diphenylphosphanyl)-N-(2-hydroxyethyl)benzamide (H2PNO) yielded the compounds 2-(diphenylphosphanyl)-N-(2-hydroxyethyl)benzylamine (H2CH2PNO, 1) and N-(2-bromoethyl)-2-(diphenylphosphanyl)benzamide (HPNBr, 2), respectively. Compound 2 is obtained in low yield and, depending on the reaction conditions, is mixed with starting material or with a product which has been identified as 2-(2-diphenylphosphinophenyl)oxazoline (PPh3oxaz, 3). Compounds 1 and 2 react with (NEt(4))(2)[ReBr(3)(CO)(3)], leading to the complexes [Re(CO)(3)(kappa(2)-H(2)CH(2)PNO)Br] (4) and [Re(CO)(3)(kappa(2)-PPh(3)oxazBr)] (5), fully characterized by (1)H and (31)P NMR spectroscopy and X-ray crystallographic analysis. Complex 5 is the first example of a Re(I) tricarbonyl anchored on a phosphorus-oxazoline ligand, which has been generated during the course of complex formation. In the unexpected and unusual complex 5, the Re atom is stabilized by a bidentate 2-(2-diphenylphosphinophenyl)oxazoline, by a bromide, and by three facially arranged carbonyl groups. In complex 4, the carbonyl groups are also facially coordinated to the metal center and the other three remaining coordination positions are occupied by a bromide and by the bidentate (P, N) ligand 2-(diphenylphosphanyl)-N-(2-hydroxyethyl)benzylamine.

A second-generation 99m technetium single photon emission computed tomography agent that provides in vivo images of the dopamine transporter in primate brain

The dopamine transporter (DAT), located presynaptically on dopamine neurons, provides a marker for Parkinson's disease (Pd) and attention deficit hyperactivity disorder (ADHD). In ADHD, DAT density levels are elevated, while in Pd these levels are depleted. The depletion of DAT levels also corresponds with the loss of dopamine. We now describe the design, synthesis, biology, and SPECT imaging in nonhuman primates of second-generation (99m)technetium-based tropane ligands that bind potently and selectively to the DAT. We demonstrate that improved selectivity and biological stability allows sufficient agent to enter the brain and label the DAT in vivo to provide a quantitative measure of DAT density in nonhuman primates. We introduce FLUORATEC (N-[(2-((3'-N'-propyl-(1"R)-3"alpha-(4-fluorophenyl)tropane-2"beta-1-propanoyl)(2-mercaptoethyl)amino)acetyl)-2-aminoethanethiolato]technetium(V) oxide), a DAT imaging agent that has emerged from these studies and is now in phase 1 clinical trials in the U.S.

Newer methods of labeling diagnostic agents with Tc-99m

The past several years have seen marked advances in technetium/rhenium chemistry applicable to the preparation of new 99mTc-labeled radiopharmaceuticals. This article focuses on recent developments in technetium chemistry, including the preparation of "3 + 1" complexes, the preparation and use of (99mTc[CO]3)+ complexes for labeling biomolecules, the preparation of rhenium steroid inclusion complexes, improvements in both hydrazinonicotinamide labeling chemistry and in the preformed 99mTc complex method of labeling biomolecules, and new solid-phase separation techniques that may allow the isolation of high specific-activity radiopharmaceuticals in a clinical setting.

Evolution of Tc-99m in diagnostic radiopharmaceuticals

The progress in diagnostic nuclear medicine over the years since the discovery of 99mTc is indeed phenomenal. Over 80% of the radiopharmaceuticals currently being used make use of this short-lived, metastable radionuclide, which has reigned as the workhorse of diagnostic nuclear medicine. The preeminence of 99mTc is attributable to its optimal nuclear properties of a short half-life and a gamma photon emission of 140 keV, which is suitable for high-efficiency detection and which results in low radiation exposure to the patient. 99mTcO4-, which is readily available as a column eluate from a 99Mo/99mTc generator, is reduced in the presence of chelating agents. The versatile chemistry of technetium emerging from the 8 possible oxidation states, along with a proper understanding of the structure-biologic activity relationship, has been exploited to yield a plethora of products meant for morphologic and functional imaging of different organs. This article reviews the evolution of 99mTc dating back to its discovery, the development of 99Mo/99mTc generators, and the efforts to exploit the diverse chemistry of the element to explore a spectrum of compounds for diagnostic imaging, planar, and single photon emission computed tomography. A brief outline of the 99mTc radiopharmaceuticals currently being used has been categorically presented according to the organs being imaged. Newer methods of labeling involving bifunctional chelating agents (which encompass the "3 + 1" ligand system, Tc(CO)3(+1)-containing chelates, hydrazinonicotinamide, water-soluble phosphines, and other Tc-carrying moieties) have added a new dimension for the preparation of novel technetium compounds. These developments in technetium chemistry have opened new avenues in the field of diagnostic imaging. These include fundamental aspects in the design and development of target-specific agents, including antibodies, peptides, steroids, and other small molecules that have specific receptor affinity.

Re tricarbonyl complexes with ligands containing P,N,N and P,N,O donor atom sets: synthesis and structural characterization

The coordination chemistry of the heterofunctionalized phosphines HPN2 and H2PNO and of an analogue containing a relevant biomolecule, HPN-Pip (Pip = 4-(3-aminopropyl)-1-(2-methoxyphenyl)piperazine), was studied toward the synthon (NE4)2[ReBr3(CO)3]. The complexes isolated, [Re(CO)3(kappa(3)-PN2)], 3, [Re(CO)3Br(kappa(2)-H2PNO)], 4, and [Re(CO)3Br(kappa(2)-HPN-Pip)], 5, are the first examples of Re(I) compounds stabilized by such a combination of donor atoms. All of the compounds are neutral, but the phosphines, depending on the combination of atoms, act as monoanionic and tridentate (3) or as neutral and bidentate (4, 5). The characterization of 3-5 included IR, 1H NMR, and 31P NMR spectroscopy and X-ray crystallographic analysis. Colorless crystals of compounds 3 and 4 were obtained by slow evaporation of a methanolic solution of 3 and from a boiling acetonitrile solution of 4. Compound 3 crystallizes with two molecules of MeOH per asymmetric unit in the monoclinic space group P2(1)/c, a = 10.1237(8) A, b = 9.4959(4) A, c = 28.365(2) A, beta = 98.707(9) degrees, V = 2695.4(3) A(3), Z = 4; 4 crystallizes in the triclinic space group Ponebar, a = 10.0241(9) A, b = 11.2060(10) A, c = 13.0656(12) A, alpha = 84.883(11) degrees, beta = 71.163(10) degrees, gamma = 63.650(9) degrees, V = 1241.19(19) A(3), Z = 2.

Development of Tc-99m labeled tropanes: TRODAT-1, as a dopamine transporter imaging agent

Advances in the technetium chemistry have significantly enhanced the development of Tc-99m radiopharmaceuticals for clinical use. Efforts in designing novel Tc-99m labeled receptor or site-specific imaging agents using [Tc(v)O](+3)N(2)S(2) core have produced useful imaging agents for single photo emission computed tomography (SPECT). The success in developing Tc-99m TRODAT-1 for CNS dopamine transporters (DAT) serves as the first example of Tc-99m site-specific imaging of human brain. New innovative Tc-99m labeled site-specific agents are particularly suitable to develop simple and useful routine diagnostic procedures.

Chemical and biological characterization of technetium(I) and Rhenium(I) tricarbonyl complexes with dithioether ligands serving as linkers for coupling the Tc(CO)(3) and Re(CO)(3) moieties to biologically active molecules

The organometallic precursor (NEt(4))(2)[ReBr(3)(CO)(3)] was reacted with bidendate dithioethers (L) of the general formula H(3)C-S-CH(2)CH(2)-S-R (R = -CH(2)CH(2)COOH, CH(2)-C&tbd1;CH) and R'-S-CH(2)CH(2)-S-R' (R' = CH(3)CH(2)-, CH(3)CH(2)-OH, and CH(2)COOH) in methanol to form stable rhenium(I) tricarbonyl complexes of the general composition [ReBr(CO)(3)L]. Under these conditions, the functional groups do not participate in the coordination. As a prototypic representative of this type of Re compounds, the propargylic group bearing complex [ReBr(CO(3))(H(3)C-S-CH(2)CH(2)-S-CH(2)C&tbd1;CH)] Re2 was studied by X-ray diffraction analysis. Its molecular structure exhibits a slightly distorted octahedron with facial coordination of the carbonyl ligands. The potentially tetradentate ligand HO-CH(2)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(2)-OH was reacted with the trinitrato precursor [Re(NO(3))(3)(CO)(3)](2-) to yield a cationic complex [Re(CO)(3)(HO-CH(2)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(2)-OH)]NO(3) Re8 which shows the coordination of one hydroxy group. Re8 has been characterized by correct elemental analysis, infrared spectroscopy, capillary electrophoresis, and X-ray diffraction analysis. Ligand exchange reaction of the carboxylic group bearing ligands H(3)C-S-CH(2)CH(2)-S-CH(2)CH(2)-COOH and HOOC-CH(2)-S-CH(2)CH(2)-S-CH(2)-COOH with (NEt(4))(2)[ReBr(3)(CO)(3)] in water and with equimolar amounts of NaOH led to complexes in which the bromide is replaced by the carboxylic group. The X-ray structure analysis of the complex [Re(CO)(3)(OOC-CH(2)-S-CH(2)CH(2)-S-CH(2)-COOH)] Re6 shows the second carboxylic group noncoordinated offering an ideal site for functionalization or coupling a biomolecule. The no-carrier-added preparation of the analogous (99m)Tc(I) carbonyl thioether complexes could be performed using the precursor fac-[(99m)Tc(H(2)O)(3)(CO)(3)](+), with yields up to 90%. The behavior of the chlorine containing (99m)Tc complex [(99m)TcCl(CO)(3)(CH(3)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(3))] Tc1 in aqueous solution at physiological pH value was investigated. In saline, the chromatographically separated compound was stable for at least 120 min. However, in chloride-free aqueous solution, a water-coordinated cationic species Tc1a of the proposed composition [(99m)Tc(H(2)O)(CO)(3)(CH(3)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(3))](+) occurred. The cationic charge of the conversion product was confirmed by capillary electrophoresis. By the introduction of a carboxylic group into the thioether ligand as a third donor group, the conversion could be suppressed and thus the neutrality of the complex preserved. Biodistribution studies in the rat demonstrated for the neutral complexes [(99m)TcCl(CO)(3)(CH(3)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(3))] Tc1 and [(99m)TcCl(CO)(3)(CH(2)-S-CH(2)CH(2)-S-CH(2)-C&tbd1;CH)] Tc2 a significant initial brain uptake (1.03 +/- 0.25% and 0.78 +/- 0.08% ID/organ at 5 min. p.i.). Challenge experiments with glutathione clearly indicated that no transchelation reaction occurs in vivo.

Synthesis and biological evaluation of two novel DAT-binding technetium complexes containing a piperidine based analogue of cocaine

Two new technetium complexes containing a piperidine template have been synthesized and evaluated as possible leads for the development of dopamine transporter (DAT) imaging agents. Binding data for the corresponding rhenium complexes containing either a monoaminomonoamide (MAMA') or a diaminodithiol (DADT) chelating unit exhibited significant affinity for the DAT. Initial biodistribution studies in rats revealed only a low brain uptake.

Dopaminergic agents for the treatment of cocaine abuse

Cocaine is a major drug of abuse whose devastating effects have captured the attention of health officials and policy makers. Based upon the alarming health and crime-related costs associated with the use of this powerful reinforcing drug, immediate therapies are needed for the treatment of cocaine addiction. In this review, some of the small-molecule-based approaches that have been pursued in the search for such medications are highlighted. Because the pharmacological actions of cocaine stem laargely from its ability to block the dopamine transporter, many intervention strategies have focused on the dopaminergic pathway.