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The novel tetradentate N3O bifunctional agent 3-hydroxy-4-[2-(2′-pyridinecarboxamido)acetylamino]benzoic acid (1), reacts with [Bu4N][ReOCl4] of [Bu4N][ 99gTcOCl4] in EtOH resulting in neutral and stable oxorhenium 2 or oxotechnetium-99 g 3 complexes. Both complexes were characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy. X-ray analysis of 2 showed that the rhenium atom is pentacoordinate presenting a distorted square-pyramidal coordination geometry. The analogous oxotechnetium-99m complex 4 was also prepared quantitatively by the reaction of ligand 1 with Na99mTcO4 in isotonic saline using tin chloride as reducing agent. Its identity was established by chromatographic comparison to the oxorhenium 2 and oxotechnetium 3 complexes. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

A series of (TcO)-Tc-99m[SN(R)S][S] complexes carrying the 1-(2-methoxyphenyl)piperazine moiety on the tridentate ligand [SN(R)S] was synthesized. For structural characterization and for in vitro binding assays the analogous oxorhenium complexes were prepared. As demonstrated by appropriate competition binding tests in rat hippocampal preparations, all oxorhenium analogues showed affinity for the 5-HT1A receptor binding sites with IC50 values at the nanomolar range (IC50 = 5.8-103 nM). All (TcO)-Tc-99m[SN(R)S]/[S] complexes showed significant brain uptake in rats at 2 min p.i. (0.24-1.31% ID). However, a clear correlation between distribution of radioactivity in the brain and distribution of 5-HT1A receptors could not be established. (C) 2002 Elsevier Science Inc. All rights reserved.

Two novel five-coordinate cis-dioxorhenium(V) complexes of the general formula ReO2[R2NCH2CH2S][PPh3] (R2N=Et2N, complex 1 and R2N=(o-CH3O-C6H4N(CH2CH2)2N, complex 2) have been synthesized by reacting ReOCl3(PPh3)2 with the respective bidentate ligands R2NCH2CH2SH. The complexes have been characterized by elemental analysis, IR, NMR spectroscopies and X-ray crystallography. X-ray crystallographic studies showed that the coordination geometry around rhenium is distorted trigonal bipyramidal. the basal plane is defined by the two doubly bonded oxygen atoms and the sulfur of the bidentate ligand, while the nitrogen of the ligand and the phosphorus occupy the apical positions.

PURPOSE: To check the influence of structural characteristics on their permeation through the blood-Brain barrier (BBB), a set of radioactive [99mTc]chelates bearing amine groups was synthesized and tested in vitro as well as in vivo. METHODS: Compounds with different log P and pKa values were obtained by complex forming reactions of [99mTc]pertechnetate with varying substituents. Transport was studied in rats and mice, as well as in an ECV304 cell culture model. RESULTS: In vitro higher permeation was found for compounds with electron attracting substituents in beta-position to the amine group (pKa values 7.4 to 8.3) than for those with more basic amine groups (pKa values > 8.9) even for similar logDph 7.4. In vivo brain uptake between 0.8 and 4.8% of the injected dose (ID) per organ was found for the former, whereas < 0.4% ID were present for the latter. CONCLUSIONS: Three structurally diverse classes of [99mTc]chelates showed distinct patterns with regard to brain uptake in vivo and BBB permeability in vitro which could not be predicted by their lipophilicity alone. The close correlation between the data from rats and mice and those obtained with cell cultures render the ECV304 cells an attractive model for the screening of new compounds.

A series of oxorhenium(V) SNS/S mixed ligand complexes [ReO(Ln/L)] carrying different types of tridentate ligand [L1: C2H5N(CH2CH2S)2, L2: (C2H5)2NCH2CH2N(CH2CH2S)2, L3: C2H5SCH2CH2N(CH2CH2S)2 and L4: 2,6-(SCH2)2NC5H3] and the same monodentate coligand (L: C6H5S) have been synthesized and characterized by spectroscopic methods and elemental analyses. X-ray structure determination was performed for complexes 3 and 4. Complex 3 adopts the expected distorted trigonal bipyramidal geometry around the metal in a syn configuration, while complex 4, due to the aromatic character of the nitrogen of the SNS donor-atom set, exhibits a distorted square pyramidal geometry. The interaction of complexes 1 - 4 with glutathione (GSH) was studied by high performance liquid chromatography (HPLC), revealing the rapid formation of the respective daughter complexes 5 - 8, wherein the L coligand has been substituted by GS. The daughter complexes 5 - 8 have been characterized by ES-MS and a battery of NMR measurements involving HETCOR, COSY and NOESY procedures. Kinetic aspects of the interaction of complexes 1 ¾ 3 with GSH have been studied by isothermal titration microcalorimetry providing direct measurements of the interaction rate constants as well as the total enthalpy change. The reaction of complex 1 is characterized by the slowest and that of complex 2 by the fastest rate. This is in agreement with previously reported trends for analogous 99mTc complexes.

A set of technetium(V) mixed-ligand complexes was synthesized with different protonation constants pKa, brought about by substitution. They were characterized regarding their lipophilicity and compared to the corresponding rhenium(V) complexes. Partition coefficients P, determined by reversed phase HPLC and the octanol shake flask method, were higher for the technetium than for the analogous rhenium species. Investigations were performed to assess the influence of the physicochemical molecule parameters on the biological behaviour of these compounds. The state of ionisation of the molecules had a significant influence on brain uptake in vivo. Less uptake was found for compounds with pKa values around 9.5, which are fully protonated at physiological pH, than for those with pKa values around 7.5, which are ionised to only about 50% at pH 7.4. transport through cells was studied with an in vitro blood-brain barrier (BBB) model, based on the human umbilical cord cell line ECV304. The resulting in vitro permeation curves agree with the in vivo results obtained in mice and rats with short incubation times (up to 30 min).

Theoretically, the reaction of the tridentate aminedithiol ligand EtN(CH2CH2SH)2, L1H2, and the monodentate thiol 4-ClC6H4SH, L2H, with the ReVOCl3(PPh3)2 may lead to the formation of the following complexes: (i) neutral mixed ligand complexes ReOL1L2 (the syn isomer, complex 1 and the anti isomer, complex 2, (ii) binuclear complex of the tridentate ligand, (MO)2(L1)3, complex 3, and (iii) anionic complex of the monothiol [MO(L2)4]-, complex 4. When a mixture of L1H2/L2H, 1/1 ratio, is applied, the major product of the reaction is the syn isomer 1. A small amount of the anti isomer 2 is also isolated (yield < 2 %) while none of the complexes 3 and 4 form during the reaction under the above conditions. The oxorhenium complexes 3 and 4 have been synthesized by the reaction of L1H2 or L2H with the precursor ReOCl3(PPh3)2. The crystal structure of 1, 3, and 4 is determined by X-ray crystallography. The corresponding Tc-99m complexes have been prepared by exchange reaction using Tc-99m glucoheptonate as precursor. Similarly the major reaction product is the syn isomer, complex 1´, while none of the other complexes form during the reaction at tracer level. The above studies demonstrate that simultaneous action of a tridentate SNS ligand and a monodentate thiol in equimolar quantities on ReVOCl3(PPh3)2 or Tc-99m glucoheptonate leads to a single rhenium or technetium-99m species, the syn MOL1L2.

Our work over the past few years has been focused on the design and evaluation of novel chelating systems suitable for oxorhenium and/or oxotechnetium that may lead to useful radioagents. Several 99mTc mixed ligand complexes carrying the SNS/S or the SNN/S donor atom set have shown promising biological properties for future application. The in vivo stability and thereby biological properties of these complexes is greatly dependent on the electronic and lipophilic character of the monothiol. We report here on the comparative resistance to cysteine substitution of two parallel series of 99mTcO{[Et2NCH2CH2N(CH2CH2S)2](p-RPhS)} and 99mTcO-[(Et2NCH2CH2NCH2CH2S) (p-RPhS)] complexes wherein the phenyl monothiolate coligand is substituted in the p-position by R: -NH2 (electron donating group), -H and NO2 (electron withdrawing group). The electronic influence on cysteine substitution rate can thus be investigated. All above complexes are prepared at tracer level using glucoheptonate as transfer ligand and are purified by organic extraction and HPLC. Their structure has been previously resolved by comparison with authentic samples. The 99mTc complexes are stable in solution in the absence of thiol for long periods of time. Competition experiments with cysteine are performed in aqueous medium at pH 7.4 and using two different cysteine concentrations, 10 mM and 1 mM, by incubation at 37°C. Aliquots of the incubate withdrawn at 0, 15, 30, 60, 120 min and 24 h time intervals are subjected to HPLC analysis. A RP C18 column is eluted by TEAP buffer pH 7.4/EtOH gradient system, whereby all radioactive ingredients are well separated. All complexes are attacked by cysteine under the above conditions forming mixtures of intermediates, that with time lead to total loss of the original 99mTc compounds. Relative resistance to cysteine substitution is directly proportional to monothiol nucleophilicity in both series of complexes: -NH2 > -H> -NO2. Since the free thiol content - mainly cysteine and its derivatives - varries in vivo, mixed ligand systems as the above can be used for directing and trapping 99mTc in tissues of choice by finely monitoring stability via the monothiol.