Synthesis of oxorhenium(V) and oxotechnetium(V) [SN(R)S/S] mixed ligand complexes containing a phenothiazine moiety on the tridentate SN(R)S ligand.

Two oxorhenium and two oxotechnetium [SN(R)S/S] mixed ligand complexes bearing the phenothiazine moiety on the tridentate ligand SN(R)S have been synthesized and characterized. The corresponding complexes at tracer level (99mTc) have also been prepared.     

Novel oxorhenium and oxotechnetium MO(NS)(S)2 complexes in the development of 5-HT1A receptor imaging agents

The [NS][S](2) mixed-ligand system was applied to synthesize oxorhenium and oxotechnetium complexes of the general formula MO(o-CH(3)OC(6)H(4)N(CH(2)CH(2))(2)NCH(2)CH(2)S)(p-CH(3)C(6)H(4)S)(2) (M=Re in 1, M=(99)Tc in 2, and M=(99m)Tc in 3). The bidentate [NS] ligand includes the 1-(2-methoxyphenyl)piperazine moiety which is a fragment of the true 5-HT(1A) antagonist WAY 100635. The oxorhenium complex 1 was prepared by a ligand exchange reaction using ReOCl(3)(PPh(3))(2) as precursor while [Bu(4)N][(99)TcOCl(4)] and (99)Tc-gluconate were used as precursors in the synthesis of the oxotechnetium-99 complex 2. Both complexes were characterized by elemental analysis and spectroscopic methods. Crystallographic analysis of 1 showed that the rhenium coordination geometry is trigonal bipyramidal. The basal plane of the trigonal bipyramid is defined by the oxo group and two sulphur atoms, one belonging to the [NS] ligand and the other to an aromatic thiol, while the apical positions are occupied by the nitrogen of the [NS] ligand and the sulphur of the second aromatic thiol. The oxotechnetium-99 complex 2 has almost identical unit cell parameters to those of the oxorhenium complex 1 indicating, in combination with the other analytical data, that the complexes are isostructural. The binding affinity of the oxorhenium complex 1 for the 5-HT(1A) receptor subtype was determined in rat brain hippocampal preparations (IC(50)=106 nM). The oxotechnetium-99m complex 3 was prepared by a ligand exchange reaction using (99m)Tc-glucoheptonate as the precursor. Its structure was established by comparative HPLC studies using the oxotechnetium-99 complex 2 as a reference. Complex 3 was administered by intravenous injection in rats. At 2 min post injection, 0.153% of the injected dose per gram of tissue was measured in rat brain.     

Synthesis,characterization, and biological evaluation of technetium(III) complexes with tridentate/bidentate S,E,S/P,S coordination (E = O,N(CH(3)), S): a novel approach to robust technetium chelates suitable for linking the metal to biomolecules

A novel type of mixed-ligand Tc(III) complexes, [Tc(SCH(2)CH(2)-E-CH(2)CH(2)S)(PR(2)S)] (E = S, N(CH(3)); PR(2)S = phosphinothiolate with R = aryl, alkyl) is described. These "3+2"-coordinated complexes can be prepared in a two-step reduction/substitution procedure via the appropriate chloro-containing oxotechnetium(V) complex [TcO(SES)Cl] [E=S, N(CH(3)]. Tc(III) compounds have been fully characterized both in solid and solution states and found to adopt the trigonal-bipyramidal coordination geometry. The equatorial trigonal plane is formed by three thiolate sulfur atoms, whereas the phosphorus of the bidentate P,S ligand and the neutral donor of the tridentate chelator occupy the apical positions. The (99)Tc(III) complexes have been proven to be identical with the (99m)Tc agents prepared at the no-carrier-added level by comparison of the corresponding UV/vis and radiometric HPLC profiles. Challenge experiments with glutathione clearly indicate that this tripeptide has no effect on the stability of the (99m)Tc complexes in solutions. Biodistribution studies have been carried out in rats at 5 and 120 min postinjection. The substituents at the bidentate P,S ligand significantly influence the biodistribution pattern. Remarkable differences are observed especially in brain, blood, lungs, and liver. All the complexes are able to penetrate the blood-brain barrier of rats and showed a relatively fast washout from the brain.     

Novel lipophilic amidate oxorhenium and oxotechnetium complexes as potential brain agents: synthesis, characterization and biological evaluation

Novel oxorhenium and oxotechnetium complexes based on the tetradentate 1-(2-hydroxybenzamido)-2-(pyridinecarboxamido)benzene, H3L, ligand have been synthesized and characterized herein. Thus, by reacting equimolar quantities of the triply deprotonated ligand L3- with the suitable MO3+ precursor, the following neutral MOL complexes could be easily produced following similar synthetic routes: M = Re (1), M = 99gTc (2), and M = 99mTc (3). Complexes 1 and 2, prepared in macroscopic amounts, were chemically characterized and their structure determined by single-crystal X-ray analysis. They are isostructural metal chelates, adopting a distorted square pyramidal geometry around the metal. The N3O donor atom set of the tetradentate ligand defines the basal plane and the oxygen atom of the M = O core occupies the apex of the pyramid. Complex 3 forms quantitatively at tracer level by mixing the H3L ligand with Na99mTcO4 generator eluate in aqueous alkaline media and using tin chloride as reductant in the presence of citrate. Its structure was established by chromatographic comparison with prototypic complexes 1 and 2 using high-performance liquid chromatographic techniques. When challenged with excess glutathione in vitro, complex 3 is rapidly converted to hydrophilic unidentified metal species. Tissue distribution data after administration of complex 3 in vivo revealed a significant uptake and retention of this compound in brain tissue.     

Trends in NMR chemical shifts and ligand mobility of TcO(V) and ReO(V) complexes with aminothiols

Detailed 1H and 13C NMR studies have been conducted in a series of oxotechnetium and oxorhenium complexes with aminothiol ligands ([SNS][S], [SNN][S], [SNNS]) designed as potential radiopharmaceuticals. The results of these studies in combination with others in the literature show that the oxometal core creates an anisotropic environment and affects the chemical shifts of the coordinated ligandbackbone in a consistent way. Protons oriented towards the oxygen appear deshielded relative to their geminals oriented away from the oxygen. In addition, the direction of a side chain (towards or away from the oxometal core) on the ligand backbone is shown to have a major effect on chemical shifts. The fluxional mobility of the ligand in complexes of the [SNS][S] type was also studied by NMR and the free energy of activation delta G(C)double dagger for the conformational inversion of the ligand was calculated from the temperature dependence of the carbon chemical shifts. delta G(C)double dagger was found to depend on the orientation of the side chain present on the coordinated nitrogen. The energy barrier for the inversion is larger for the oxorhenium complexes than for the analogous oxotechnetium complexes.     

Synthesis, characterization, and biological evaluation of neutral nitrido technetium(V) mixed ligand complexes containing dithiolates and aminodiphosphines. A novel system for linking technetium to biomolecules

A new biomolecule labeling method that utilizes the [(99m)Tc(N)(PNP)](2+) metal fragment is presented. Thus, a series of nitrido mixed-ligand M(V) complexes (M = (99m)Tc, (99g)Tc, Re), [M(N)(Ln)(PNP)], where Ln is the dianionic form of a dithiolate or substituted-dithiolate ligand and PNP is an aminodiphosphine, is described. (99m)Tc complexes can be prepared using either a two-step or a three-step procedure starting from generator-eluted pertechnetate through a prereduced mixture of [(99m)Tc(N)]-containing species, followed by sequential or contemporary addition of the relevant dithiolate and aminodiphosphine. The reactions of 2,3-dimercaptopropionic acid (H(2)L1) with [Tc(N)(PNP)](2+) were investigated in detail. It was found that this bidentate ligand coordinated the metal fragment through the [S(-),S(-)] donor atom pair, to yield neutral mixed-ligand complexes [(99m)Tc(N)(L1)(PNP)] in high specific activity. The additional carboxylic functional group was not involved in metal coordination, thus remaining available for conjugation to target-specific molecules. Dithiolates incorporating pendant functional group(s) gave rise to a 1:1 diastereoisomeric mixture of syn-[M(N)(Ln)(PNP)] and anti-[M(N)(Ln)(PNP)] derivatives, depending on the relative orientation of the dithiolate substituent(s) with respect to the terminal nitrido group, and no isomeric conversion was detected. (99m)Tc species had been proven to be identical with the (99g)Tc complexes prepared at the macroscopic level by comparison of the corresponding radiometric and UV/vis HPLC profiles. Challenge experiments with cysteine or glutathione indicated that these physiological agents had no effect on the stability of this class of mixed-ligand (99m)Tc-complexes. Biodistribution studies in rats of selected (99m)Tc-complexes showed a rapid clearance from the blood and tissues after 60 min pi.     

Preparation and biological characterization of isomeric 188Re(V) oxocomplexes with tetradentate S4 ligands derived from meso-dimercaptosuccinic acid for labeling of biomolecules

A new type of tetradentate S4 ligand has been synthesized by bridging two molecules of meso-2,3-dimercaptosuccinic acid for stable binding and easy conjugation of rhenium-188 to tumor targeting structures. The stereoisomeric tetrathiolato S4 ligands form very robust anionic five-coordinated oxorhenium(V) and oxotechnetium(V) complexes. Two routes for the preparation of the (188)Re(V) oxocomplexes with (iBu)2N(O)C-C(SH)C(SH)C(O)NH(CH2)3NH(CH2)3NHC(O)C(SH)C(SH)C(O)N(iBu)2 (ligand 1) and its hydrophilic crown ether derivative (ligand 2) were tested and optimized. Several isomers were separated by HPLC from the preparation solutions and characterized in vitro and in vivo. The identity of the species obtained was determined by comparison with the HPLC profiles of reference (185/187)Re analogues and (99/99m)Tc complexes which were characterized by ESI-MS. All of them were absolutely stable in rat and human plasma solutions. Challenge experiments with cysteine corroborated the high inertness of the isomers toward ligand exchange reactions. Various in vivo samples, taken off at different times from blood, intestine, and urine of rats, confirmed the high in vivo stability of the (188)Re-S4 complexes. Biodistribution studies using male Wistar rats were performed and exhibited a high uptake and fast clearance from the liver of the more lipophilic cis and trans isomers of complex I (log P(o/w) between 1.5 and 1.7), whereas the isomers of the hydrophilic complex II (log P(o/w) about -1.75) were rapidly excreted via the renal and the hepatobiliary pathway. The low level of activity in the stomach confirms good in vivo stability. Thus, these new (188)Re-S4 complexes fulfill the requirements for a stable and high specific activity labeling of biomolecules with rhenium-188.     

Synthesis and structural characterization of neutral “3 + 2” oxorhenium and oxotechnetium complexes of the 2-mercaptoethyl-N-glycine (SNO)/2,2′-bipyridine (NN) mixed ligand system

Neutral, hexacoordinated “3 + 2” mixed ligand oxorhenium (1) and oxotechnetium (2) complexes of the general formula MO[SNO][NN], where M = Re or ⁹⁹Tc, SNO is 2-mercaptoethyl-N-glycine and NN is 2,2′-bipyridine (bpy), were synthesized by simultaneous action of the tridentate SNO and the bidentate NN ligand on ReOCl₃(PPh₃)₂ or ⁹⁹TcO-gluconate precursors in a 1:1:1 molar ratio. Both complexes were characterized by elemental analysis, IR and NMR spectroscopy. X-ray structure determination of rhenium complex 1 revealed a distorted octahedral coordination geometry where the SNO donor atoms of the tridentate ligand and one bpy nitrogen atom occupy the equatorial positions of the octahedron, whereas the second bpy nitrogen atom and the oxo-group fill the apical positions.     

A novel approach to the high-specific-activity labeling of small peptides with the technetium-99m fragment [99mTc(N)(PXP)]2+ (PXP = diphosphine ligand).

A new labeling approach for incorporating bioactive peptides into a technetium-99m coordination complex is described. This method exploits the chemical properties of the novel metal-nitrido fragment [99mTc(N)(PXP)]2+, composed of a terminal Tc[triple bond] N multiple bond bound to an ancillary diphosphine ligand (PXP). It will be shown that this basic, molecular building block easily forms in solution as the dichloride derivative [99mTc(N)(PXP)Cl2], and that this latter complex selectively reacts with monoanionic and dianionic, bidentate ligands (YZ) having soft, pi-donor coordinating atoms to afford asymmetrical nitrido heterocomplexes of the type [99mTc(N)(PXP)(YZ)]0/+ without removal of the basic motif [99mTc(N)(PXP)]2+. The reactions of the amino acid cysteine was studied in detail. It was found that cysteine readily coordinates to the metal fragment [99mTc(N)(PXP)]2+ either through the [NH2, S-] pair of donor atoms or, alternatively, through the [O-, S-] pair, to yield the corresponding asymmetrical complexes in very high specific activity. Thus, these results were conveniently employed to devise a new, efficient procedure for labeling short peptide sequences having a terminal cysteine group available for coordination to the [99mTc(N)(PXP)]2+ fragment. Examples of the application of this novel approach to the labeling of the short peptide ligand H-Arg-Gly-Asp-Cys-OH (H(2)1) and of the peptidomimetic derivative H-Cys-Val-2-Nal-Met-OH (H2) will be discussed.     

Novel procedures for preparing 99mTc(III) complexes with tetradentate/monodentate coordination of varying lipophilicity and adaptation to 188Re analogues

Improved methods are presented for the preparation of 99mTc and 188Re mixed-ligand complexes with tetradentate and monodentate ligands of the general formula [MIII(Lm)(Ln)] (M = Tc, Re; Lm = NS3 or NS3COOH; Ln = isocyanide or phosphine). To avoid the undesired formation of reduced-hydrolyzed species of both metals, the preparation of complexes is performed in a two-step procedure. At first the Tc(III)- or Re(III)-EDTA complex is formed which reacts in a second step with the tripodal ligand 2,2',2' '-nitrilotris(ethanethiol) (NS3) or its carboxyl derivative NS3COOH (a) and the monodentate phosphine ligands (triphenylphosphine L1, dimethylphenylphosphine L2) or isocyanides (tert-butyl isonitrile L3, methoxyisobutyl isonitrile L4, 4-isocyanomethylbenzoic acid-L-arginine L5, 4-isocyanomethylbenzoic acid-L-arginyl-L-arginine L6, 4-isocyanomethylbenzoic acid-neurotensin(8-13) L7) to the so-called '4+1' complex. Copper(I) isocyanide complexes are used for preparing the '4+1' complexes. That facilitates storage stability and allows kit formulations, and, moreover, enables the formation of 188Re complexes in acidic solution. Only micromolar amounts of the monodentate ligand are needed, and that results in high specific activity labeling of interesting molecules. The lipophilicity of complexes can be controlled by introducing a carboxyl group into the tetradentate ligand and/or derivatization of the monodentate ligands. Furthermore, the carboxyl group enables the conjugation of biomolecules. As an example, the neurotensin derivative CN-NT(8-13) was prepared and labeled with 99mTc according to the '4+1' approach, and its behavior in vivo was studied.     

Extension of the single amino acid chelate concept (SAAC) to bifunctional biotin analogues for complexation of the M(CO)3(+1) Core (M = Tc and Re): syntheses, characterization, biotinidase stability, and avidin binding

Biotin and avidin form one of the most stable complexes known (K(D) = 10(-15) M(-1)) making this pairing attractive for a variety of biomedical applications including targeted radiotherapy. In this application, one of the pair is attached to a targeting molecule, while the other is subsequently used to deliver a radionuclide for imaging and/or therapeutic applications. Recently, we reported a new single amino acid chelate (SAAC) capable of forming stable complexes with Tc(CO)3 or Re(CO)3 cores. We describe here the application of SAAC analogues for the development of a series of novel radiolabeled biotin derivatives capable of forming robust complexes with both Tc and Re. Compounds were prepared through varying modification of the free carboxylic acid group of biotin. Each 99mTc complex of SAAC-biotin was studied for their ability to bind avidin, susceptibility to biotinidase, and specificity for avidin in an in vivo avidin-containing tumor model. The radiochemical stability of the 99mTc(CO)3 complexes was also investigated by challenging each 99mTc-complex with large molar excesses of cysteine and histidine at elevated temperature. All compounds were radiochemically stable for greater than 24 h at elevated temperature in the presence of histidine and cysteine. Both [99mTc(CO)3(L6)]+1 [TcL6; L6 = biotinylamidopropyl-N,N-(dipicolyl)amine] and [99mTc(CO)3(L12a)]+1 (TcL12; L12 = N,N-(dipicolyl)biotinamido-Boc-lysine; TcL12a; L12a = N,N-(dipicolyl)biotinamide-lysine) readily bound to avidin whereas [99mTc(CO)3(L9)]+1 [TcL9; L9 = N,N-(dipicolyl)biotinamine] demonstrated minimal specific binding. TcL6 and TcL9 were resistant to biotinidase cleavage, while TcL12a, which contains a lysine linkage, was rapidly cleaved. The highest uptake in an in vivo avidin tumor model was exhibited by TcL6, followed by TcL9 and TcL12a, respectively. This is likely the result of both intact binding to avidin and resistance to circulating biotinidase. Ligand L6 is the first SAAC analogue of biotin to demonstrate potential as a radiolabeled targeting vector of biotin capable of forming robust radiochemical complexes with both 99mTc and rhenium radionuclides. Computational simulations were performed to assess biotin-derivative accommodation within the binding site of the avidin. These calculations predict that deformation of the surface domain of the binding pocket can occur to accommodate the transition metal-biotin derivatives with negligible changes to the inner-beta-barrel, the region most responsible for binding and retaining biotin and its derivatives. The biological activity and biodistribution of the technetium complexes TcL6, TcL9, and TcL12a were examined in an avidin tumor model. In the avidin bead tumor localization model, TcL6 demonstrated the most favorable localization with a 7:1 ratio of avidin bead implanted muscle versus normal muscle, while TcL9 exhibited a 2:1 ratio. However, TcL9 displayed no specificity for avidin.     

Technetium-99m-labeled medium-chain fatty acid analogues metabolized by beta-oxidation: radiopharmaceutical for assessing liver function.

External imaging of energy production activity of living cells with 99mTc-labeled compounds is a challenging task requiring good design of 99mTc-radiopharmaceuticals. On the basis of our recent findings that 11C- and 123I-labeled medium-chain fatty acids are useful for measuring beta-oxidation activity of hepatocytes, we focused on development of 99mTc-labeled medium-chain fatty acid analogues that reflect beta-oxidation activity of the liver. In the present study, monoamine-monoamide dithiol (MAMA) ligand and triamido thiol (MAG) ligand were chosen as chelating groups because of the stability and size of their complexes with 99mTc and their ease of synthesis. Each ligand was attached to the omega-position of hexanoic acid (MAMA-HA and MAG-HA, respectively). In biodistribution studies, [99mTc]MAMA-HA showed high initial accumulation in the liver followed by clearance of the radioactivity in the urine. Analysis of the urine revealed [99mTc]MAMA-BA as the sole radiometabolite. Furthermore, when [99mTc]MAMA-HA was incubated with living liver slices, generation of [99mTc]MAMA-BA was observed. However, [99mTc]MAMA-HA remained intact when the compound was incubated with liver slices in the presence of 2-bromooctanoate, an inhibitor of beta-oxidation. The findings in this study indicated that [99mTc]MAMA-HA was metabolized by beta-oxidation after incorporation into the liver. On the other hand, poor hepatic accumulation was observed after administration of [99mTc]MAG-HA.     

N,N-bis(2-mercaptoethyl)methylamine: a new coligand for Tc-99m labeling of hydrazinonicotinamide peptides

Hydrazinonicotinamide (HYNIC) forms stable coordination complexes with Tc-99m when reacted with Tc(V)oxo species such as Tc-mannitol or other Tc-polyhydric complexes. However, radio-HPLC of [Tc-For-MLFK-HYNIC] labeled via Tc-polyhydric ligands demonstrated multiple radiochemical species each with unique biodistribution patterns. This is likely due to the fact that Tc can bind to the hydrazino moiety, as well as polyhydric ligands, in a variety of coordination geometries. Tridentate ligands, such as bis(mercaptoethyl)methylamine (NS2), may constrain the possible coordination geometries and improve overall stability. To investigate this, we synthesized NS2, converted the [Tc-mannitol-For-MLFK-HYNIC] to the corresponding NS2-containing complex [Tc-NS2-For-MLFK-HYNIC], and compared its infection imaging and biodistribution properties with [Tc-mannitol-For-MLFK-HYNIC]. Conversion to the NS2 complex was confirmed by HPLC which showed a single unique hydrophobic species with retention time greater than the [Tc-mannitol-For-MLFK-HYNIC] complex. Imaging experiments with both preparations were performed in rabbits with E. coli infections in the left thigh. Tissue radioactivity measurements demonstrated that compared to Tc-mannitol-peptide, accumulation of Tc-NS2-peptide was lower in blood, heart, and normal muscle and higher in spleen, infected muscle, and pus (p < 0.01). These results indicate that the Tc-NS2-peptide complex is chemically more homogeneous and exhibits improved infection localization and biodistribution properties. In an effort to model the interactions of the metal-HYNIC core with NS2 and related ligand types, the reactions of [ReCl3(NNC5H4NH)(NHNC5H4N)] and [99TcCl3(NNC5H4NH)(NHNC5H4N)], effective structural analogues for the [M(NNC5H4NH(x))2] core, with NS2, C5H3N-2,6-(CH2SH)2, O(CH2CH2SH)2, and S(CH2CH2SH)2 were investigated and the compounds [M[CH3N(CH2CH2S)2](NNC5H4N)(NHNC5H4N] (M = 99Tc (5a), Re (5b)), [Re[C5H3N-2,6-(CH2S)2](NNC5H4N)(NHNC5H4N)].CH2Cl2.0.5MeOH (7), [Re[SCH2CH2)2O] (NNC5H4N)(NHNC5H4N)] (8), and [Re[(SCH2CH2)2S](NNC5H4NH)(NHNC5H4N)]Cl (9) were isolated. Similarly, the reaction of [ReCl3(NNC5H4NH)(NHNC5H4N)] with the bidentate ligands pyridine-2-methanethiol and 3-(trimethlysilyl)pyridine-2-thiol led to the isolation of [ReCl(C5H4N-2-CH2S) (NNC5H4N)(NHNC5H4N)] (10) and [Re(2-SC5H3N-3-SiMe3)2 (NNC5H4N)(NHNC5H4N)] (11), respectively, while reaction with N-methylimidazole-2-thiol yielded the binuclear complex [Re(OH)Cl(SC3H2N2CH3)2(NNC5H4N)2 (NHNC5H4N)2] (12). The analogous metal-(HYNIC-OH) precursor, [ReCl3[NNC5H3NH(CO2R)] [NHNC5H3N(CO2R)]] (R = H, 13a; R = CH3, 13b) has been prepared and coupled to lysine to provide [RCl3[NNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)] [NHNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)]].2HCl (14.2HCl), while the reaction of the methyl ester 13b with 2-mercaptopyridine yields [Re(2-SC5H4N)2[NNC5H3N(CO2Me)][NHNC5H3N(CO2Me)]] (15). While the chemical studies confirm the robustness of the M-HYNIC core (M = Tc, Re) and its persistence in ligand substitution reactions at adjacent coordination sites of the metal, the isolation of oligomeric structures and the insolubility of the peptide conjugates of 13, 14, and 15 underscore the difficulty of characterizing these materials on the macroscopic scale, an observation relevant to the persistent concerns with reagent purity and identity on the tracer level.     

Preparation of no-carrier-added technetium-99m complexes via metal-assisted cleavage from a solid phase

A novel method for the preparation of no-carrier-added (nca) complexes [99mTc(CO)3L] (L = diethylenetriamine or picolylamine-N-acetic acid) is described. The ligands were covalently bound to a solid support of organic polymers via formation of a tertiary amine from the chelating unit. This C-N bond to the solid phase is selectively cleaved during the formation of the technetium complexes by intramolecular nucleophilic attack of a remaining hydroxy ligand to the alpha-carbon. The complex [99mTc(CO)3L] is released into solution while uncomplexed ligand and uncleaved complex remain solid-phase bound. High specific activity technetium complexes can then be isolated by simple filtration. Cleavage yield depends on temperature, pH, and ligand. Up to 50% release from the solid phase could be achieved under optimized conditions. Corresponding to the 99mTc concentration, free ligand is present in concentrations lower than 10(-7) M. If a targeting vector is conjugated to these ligands, no-carrier-added radiopharmaceuticals can be prepared in that way.     

99mTc-labeled MIBG derivatives: novel 99mTc complexes as myocardial imaging agents for sympathetic neurons

Radioactive-iodine-labeled meta-iodobenzylguanidine (MIBG) is currently being used as an in vivo imaging agent to evaluate neuroendocrine tumors as well as the myocardial sympathetic nervous system in patients with myocardial infarct and cardiomyopathy. It is generally accepted that MIBG is an analogue of norepinephrine and its uptake in the heart corresponds to the distribution of norepinephrine and the density of sympathetic neurons. A series of MIBG derivatives containing suitable chelating functional groups N2S2 for the formation of [TcvO]3+N2S2 complex was successfully synthesized, and the 99mTc-labeled complexes were prepared and tested in rats. One of the compounds, [99mTc]2, tested showed significant, albeit lower, heart uptakes post iv injection in rats (0.21% dose/g at 4 h) as compared to [125I]MIBG (1.7% dose/g at 4 h). The heart uptake of the 99mTc-labeled complex appears to be specific and can be reduced by co-injection with nonradioactive MIBG or by pretreatment with desipramine, a selective norepinephrine transporter inhibitor. Further evaluation of the in vitro uptake of [99mTc]2 in cultured neuroblastoma cells displayed consistently lower, but measurable uptake (approximately 10% of that for [125I]MIBG). These preliminary results suggested that the mechanisms of heart uptake of [99mTc]2 may be related to those for [125I]MIBG uptake. If suitable 99mTc-labeled MIBG derivatives can be further developed, the prevalent availability of 99mTc in nuclear medicine clinics will allow them to be readily available for widespread application.     

Novel six-coordinate oxorhenium(V) ‘3+2’ mixed-ligand complexes carrying the SNO/SN donor atom set

Ligand exchange reactions of oxorhenium(V) precursors with bidentate SN and tridentate Schiff bases derived from the condensation of ketones or aldehydes with dithiocarbazic acid methyl ester (H₂NNHC(S)SCH₃) produce novel ‘3+2’ mixed-ligand complexes carrying the SNO/SN donor atom set. Thus, reactions of either [NBu₄][ReOCl₄] or Na[ReO(Gluconate)₂] with SNO ligands (H₂Lⁿ) or a mixture of bidentate SN (HL^m) and tridentate SNO (H₂Lⁿ) in methanol solutions lead, respectively, to the six-coordinated mixed ligand oxorhenium(V) compounds of types [ReO(Lⁿ)(HLⁿ)] and [ReO(Lⁿ)(L^m)], combining one tridentate dianionic S⁻NO⁻ donor Schiff base (L) and one bidentate anionic S⁻N donor ligand (HL). Coordination geometry around rhenium is distorted octahedral with the two SN donor atom sets of each ligand defining the equatorial plane, while apical positions are occupied by the oxo group and the oxygen atom of the tridentate SNO ligand (L), as shown by single-crystal X-ray diffraction structure of [ReO(L¹)(HL¹)] 1.     

A novel ternary ligand system useful for preparation of cationic (99m)Tc-diazenido complexes and (99m)Tc-labeling of small biomolecules

This report describes a novel ternary ligand system composed of a phenylhydrazine, a crown ether-containing dithiocarbamate (DTC), and a PNP-type bisphosphine (PNP). The combination of three different ligands with (99m)Tc results in cationic (99m)Tc-diazenido complexes, [(99m)Tc(NNAr)(DTC)(PNP)]+, with potential radiopharmaceuticals for heart imaging. Synthesis of cationic (99m)Tc-diazenido complexes can be accomplished in two steps. For example, the reaction of phenylhydrazine with (99m)TcO4- at 100 degrees C in the presence of excess stannous chloride and 1,2-diaminopropane-N,N,N',N'-tetraacetic acid (PDTA) results in the [(99m)Tc(NNPh)(PDTA)n] intermediate, which then reacts with sodium N-(dithiocarbamato)-2-aminomethyl-15-Crown-5 (L4) and N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]ethoxyethylamine (PNP6) at 100 degrees C for 15 min to give the complex, [(99m)Tc(NNPh)(L4)(PNP6)]+ in high yield (>90%). Cationic complexes [(99m)Tc(NNPh)(DTC)(PNP)]+ are stable for > or = 6 h. Their composition was determined to be 1:1:1:1 for Tc:NNPh:DTC:PNP using the mixed-ligand experiments on the tracer ((99m)Tc) level and was further confirmed by the ESI-MS spectral data of a model compound [Re(NNPh)(L4)(L6)]+. It was found that both DTCs and bisphosphines have a significant impact on the lipophilicity of their cationic (99m)Tc-diazenido complexes. Results from a (99m)Tc-labeling efficiency experiment showed that 4-hydrazinobenzoic acid (HYBA) might be useful as a bifunctional coupling agent for (99m)Tc-labeling of small biomolecules. However, the (99m)Tc-labeling efficiency of HYBA is much lower than that of 6-hydrazinonicotinic acid (HYNIC) with tricine and trisodium triphenylphosphine-3,3',3'-trisulfonate (TPPTS) as coligands.     

Synthesis, metal complexation and biological evaluation of a novel semi-rigid bifunctional chelating agent for 99mTc labelling

A novel bifunctional chelating agent bearing an aromatic ring has been synthesised and characterised. This ligand formed well-defined oxorhenium complexes. The analogous 99mTcO-complex was obtained in an excellent yield with high radiochemical purity (>95%). The biodistribution of the 99mTc-complex after intravenous injection studied in normal rats showed that the activity was excreted mainly via renal-urinary pathway indicating its use for labelling peptides with 99mTc.     

99mTc-labeling of colchicine using [99mTc(CO)3(H2O)3]+ and [99mTc triple bond N]2+ core for the preparation of potential tumor-targeting agents

Multidrug resistance (MDR) mediated by over-expression of P-glycoprotein (Pgp) is one of the major causes of failure of chemotherapy in cancer treatment. Colchicine, a naturally occurring alkaloid, is a Pgp substrate and acts as an antimitotic agent by binding to microtubules. Hence, Colchicine and its analogues radiolabeled with 99mTc may have potential for visualization of MDR in tumors. Here we report 99mTc-labeling of colchicine derivatives using [99mTc(CO)3(H2O)3]+ and [99mTc triple bond N]2+ cores. Trimethylcolchicinic acid synthesized from colchicine was used as the precursor to prepare iminodiacetic acid and dithiocarbamate derivatives which were then radiolabeled with [99mTc(CO)3(H2O)3]+ and [99mTc triple bond N]2+ cores, respectively. Radiolabeling yield for both the complexes was > 98% as observed by HPLC and TLC patterns. In vitro studies in tumor cell lines showed significant uptake for 99mTc-carbonyl as well as for 99mTc-nitrido colchicine complexes. Biodistribution studies in Swiss mice bearing fibrosarcoma tumor showed 4.1 +/- 1.2% ID/g of uptake at 30 min pi for 99mTc(CO)3-complex as against 0.42 +/- 0.24% ID/g for the 99mTcN-complex. 99mTc(CO)3-colchicine complex exhibited better pharmacokinetics with lower liver accumulation as compared to the 99mTcN-complex. Thus, colchicine radiolabeled with [99mTc(CO)3(H2O)3]+ core is more promising with respect to in vivo distribution characteristics in tumor model.     

Chelate oxorhenium to assemble new integrin antagonists

Assembly of independent chemical modules through oxorhenium coordination by a NS₂ + S chelation motif was applied to the synthesis of RGD (Arg-Gly-Asp) analogs. Modules were assembled through oxorhenium chelation to give a series of 18 metal complexes in good yields and satisfactory purities. Screening of these oxorhenium coordinates as antagonists of integrins αVβ3, αIIbβ3 and αVβ5 led to the identification of 3 bioactive compounds that exhibit submicromolar affinities for the 3 integrins. Preliminary studies showed that the corresponding oxotechnetium complexes are stable in mice plasma and therefore could be proposed for the molecular imaging of pathologies that overexpress integrins αVβ3 and αVβ5.