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

Synthesis and Biological Evaluation of 99mTc-Labeled Phenylpiperazine Derivatives as Selective Serotonin-7 Receptor Ligands for Brain Tumor Imaging

With a poor prognosis, glioblastoma multiforme is the most aggressive tumor of the central nervous system in humans. The aim of this study was to develop novel tracers for the tumor targeting and imaging of overexpressed serotonin-7 receptors (5-HT₇Rs) in U-87 MG glioma xenografted nude mice. Two phenylpiperazine derivatives named as PHH and MPHH were designed, and the corresponding radiotracers ^99mTc-PHH and ^99mTc-MPHH were synthesized in high radiochemical purity (>95%). ^99mTc-MPHH showed a higher affinity to 5-HT₇Rs on U-87 MG cells compared to ^99mTc-PHH. In biodistribution studies, the radiocomplexes showed good brain uptake at 15 min combined with good radioactivity retention in the brain for 240 min. Regional rabbit brain studies indicated a higher radioactivity concentration in the hippocampus and diencephalon than in the cerebellum. Compared to ^99mTc-MPHH, the ^99mTc-PHH exhibited a significantly increased tumor uptake at 15 and 60 min, but the rapid blood clearance of ^99mTc-MPHH led to enhanced tumor-to-muscle ratios at 240 min. A significant reduction in tumor uptake 60 min after an injection of pimozide (5-HT₇ receptor antagonist) confirms the tumor uptake was receptor-mediated specifically. The tumor-to-contralateral muscle tissue ratio of ^99mTc-PHH and ^99mTc-MPHH in nude mice with U-87 MG xenograft was measured (5.25 and 4.65) at 60 min as well as (6.25 and 6.76) at 240 min, respectively.

Synthesis and characterization of fac-Re(CO)3-aspartic-N-monoacetic acid, a structural analogue of a potential new renal tracer, fac-99mTc(CO)3(ASMA)

The reaction of an aminopolycarboxylate ligand, aspartic-N-monoacetic acid (ASMA), with [Re(CO)3(H2O)3]+ was examined. The tridentate coordination of ASMA to this ReI tricarbonyl precursor yielded fac-Re(CO)3(ASMA) as a mixture of diastereomers. The chemistry is analogous to that of the TcI tricarbonyl complex, which yields fac-99mTc(CO)3(ASMA) under similar conditions. The formation, structure, and isomerization of fac-Re(CO)3(ASMA) products were characterized by HPLC, 1H NMR spectroscopy, and X-ray crystallography. The two major fac-Re(CO)3(ASMA) diastereomeric products each have a linear ONO coordination mode with two adjacent five-membered chelate rings, but they differ in the endo or exo orientation of the uncoordinated acetate group, in agreement with expectations based on previous studies. Conditions have been identified for the expedient isomerization of fac-Re(CO)3(ASMA) to a mixture consisting primarily of one major product. Because different isomeric species typically have different pharmacokinetic characteristics, these conditions may provide for the practical isolation of a single 99mTc(CO)3(ASMA) species, thus allowing the isolation of the isomer that has optimal imaging and pharmacokinetic characteristics. This information will aid in the design of future 99mTc radiopharmaceuticals.

fac-Bromidotricarbonyl[2-(diisopropylphosphanyl)benzaldehyde-κ(2)O,P]rhenium(I)

The structure of the title complex, [ReBr(C(13)H(19)OP)(CO)(3)], displays a facial coordination of the three CO ligands and a κ(2)O,P coordination mode of the 2-diisopropyl-phosphino-benzaldehyde ligands. The Re-C bond distance for the CO ligand trans to the P atom is, due to its trans influence, elongated to 1.943 (3) Å, showing that this CO ligand is more weakly bound to the Re centre than the other two.

fac-[Re(CO)3(dmso-O)3](CF3SO3): a new versatile and efficient Re(i) precursor for the preparation of mono and polynuclear compounds containing fac-[Re(CO)3]+ fragments

We show here that the new complex fac-[Re(CO)3(dmso-O)3](CF3SO3) (1), efficiently prepared in one step from [ReBr(CO)5] and featuring a broad range of solubility, is, in general, a better precursor for the one-step synthesis of mono- and polynuclear inorganic compounds containing fac-[Re(CO)3]+ fragments compared to the commonly used (NEt4)2fac-[ReBr3(CO)3] and fac-[Re(CO)3(CH3CN)3](Y) (Y = PF6, BF4, ClO4) species. Compound 1 is the first example of a Re(I)-dmso complex structurally characterized and confirms the rule that dmso is always O-bonded when trans to CO. The reactivity of 1 was tested in the one-step preparation of several new and known complexes. The O-bonded sulfoxides of 1 are replaced under mild conditions by tri- (L3) and bidentate ligands (L2) to produce fac-[Re(CO)3(L3)]+ and fac-[Re(CO)3(L2)(dmso-O)]+ compounds, respectively. An excess of monodentate ligands (L) and more forcing conditions are needed to prepare fac-[Re(CO)3(L)3]+ compounds. The new compounds include fac-[Re(CO)3(bipy)(dmso-O)](CF3SO3) (4), that turned out to be an excellent precursor for binding the luminescent fac-[Re(CO)3(bipy)]+ fragment to polytopic ligands for the construction of more elaborate assemblies. One example reported here is the two-step preparation of fac-[{Re(CO)3(bipy)}(mu-4,4'-bipy){Ru(TPP)(CO)}](CF3SO3) (8) (TPP = tetraphenylporphyrin). The X-ray structures of the new compounds 1, 4, of the bis-porphyrin complex fac-[Re(CO)3Cl(4'MPyP)2] (13) (4'MPyP = 5-(4'pyridyl)-10,15,20-triphenylporphyrin), and of the rhenium-cyclophane [{(CO)3Re(mu-OH)2Re(CO)3}2(micro-4,4'-bipy)2] (15), among others, are described. Compound 1 might find useful applications in supramolecular chemistry (metal-mediated assembly of large architectures), in the in situ preparation of stable Re compounds to be used in nuclear medicine, and for the labeling of biomolecules.

Synthesis and biological evaluation of silylated mixed-ligand 99mTc complexes with the [PNS/S] donor atom set

New oxotechnetium complexes of general formula [99mTc(O)(PNS)(S(CH2)nOSiR3)] (4-6) were synthesized by direct reduction of [99mTcO4]- with stannous chloride, in the presence of the tridentate heterofunctionalized phosphine H2PNS and of the monodentate silylated thiols [HS(CH2)nOSiR3] (n = 2, R = Ph (1); n = 3, R = Ph (2); n = 3, R = Et (3)). The mixed-ligand rhenium and technetium complexes of general formula [M(O)(PNS)(S(CH2)nOH)] (n = 2: M = 99mTc, (7), M = Re, (7a); n = 3: M = 99mTc, (8), M = Re, (8a)) were also prepared. All the 99mTc complexes were obtained with high radiochemical purity (> 95%), after purification by HPLC, and were characterized by comparison of their HPLC profiles with the ones obtained for the corresponding Re compounds. The silylated compounds 4-6 are stable in phosphate saline buffer (PBS) pH 7.4, rat plasma, human serum and whole blood, and do not bind to plasmatic proteins, and also do not challenge with glutathione. The biological behavior of [99mTc(O)(PNS)(S(CH2)nOH)] (7, 8) and [99mTc(O)(PNS)(S(CH2)nOSiR3)] (4-6) was studied. The effect of the pH on the cleavage of the O-Si bond in complexes 4-6 was also evaluated.

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.

Bifunctional single amino acid chelates for labeling of biomolecules with the [Tc(CO)(3)](+) and [Re(CO)(3)](+) cores. Crystal and molecular structures of [ReBr(CO)(3)(H(2)NCH(2)C(5)H(4)N)], [Re(CO)(3)[(C(5)H(4)NCH(2))(2)NH]]Br, [Re(CO)(3)[(C(5)H(4)NCH(2))(2)NCH(2)CO(2)H]]Br, [Re(CO)(3)[X(Y)NCH(2)CO(2)CH(2)CH(3)]]Br (X = Y = 2-pyridylmethyl; X = 2-pyridylmethyl, Y = 2-(1-methylimidazolyl)methyl; X = Y = 2-(1-methylimidazolyl)methyl), [ReBr(CO)(3)[(C(5)H(4)NCH(2))NH(CH(2)C(4)H(3)S)]], and [Re(CO)(3)[(C(5)H(4)NCH(2))N(CH(2)C(4)H(3)S)(CH(2)CO(2))]]

The reactions of a series of potentially tridentate ligands, derived from single amino acids or amino acid analogues, with [NEt(4)](2)[ReBr(3)(CO)(3)] have been investigated. The model compounds [Re(CO)(3)Br[(2-pyridylmethyl)NH(2)]] (1) and [Re(CO)(3)[(2-pyridylmethyl)(2)NH]]Br (2) were also prepared and structurally characterized. With ligands possessing two pyridyl appendages, (2-pyridylmethyl)(2)NX (X = -CH(2)CO(2)H, -CH(2)CO(2)Et, -CH(2)CH(2)CO(2)H, -CH(2)CH(2)CO(2)Et, -CH(2)CH(2)CH(2)CH(2)CH(NHCO(2)(t)Bu)CO(2)H), complexes of the type [Re(CO)(3)(ligand)]Br (3-6) were isolated. All possess the fac-[Re(CO)(3)N(3)] coordination geometry in the cationic molecular unit. Similarly, the ligands with the imidazolyl arms (2-pyridylmethyl)[2-(1-methylimidazolyl)methyl]NCH(2)CO(2)Et and [2-(1-methylimidazolyl)methyl](2)NCH(2)CO(2)Et, complexes 7 and 8 of the same [Re(CO)(3)(ligand)]Br type, were prepared. Replacement of one pyridyl arm with a thiophene group yielded the complex [Re(CO)(3)[(2-pyridylmethyl)N(CH(2)CO(2))(2-thiophenemethyl)]] (9), while additional substitution of X = -H for -CH(2)CO(2)H yielded [Re(CO)(3)Br[(2-pyridylmethyl)NH(2-thiophenemethyl)]] (10). In both 9 and 10, the thiophene is uncoordinated and pendant, and the derivatives display fac-[Re(CO)(3)N(2)O] and fac-[Re(CO)(3)N(2)Br] coordination geometries, respectively. Crystal data: C(9)H(8)BrN(2)O(3)Re (1), triclinic P1, a = 8.156(1) A, b = 12.077(1) A, c = 12.945(2) A, alpha = 92.183(3) degrees, beta = 107.848(3) degrees, gamma = 100.955(7) degrees, V = 1185.1(3) A, Z = 4; C(15)H(13)BrN(3)O(3)Re (2), tetragonal P4(1), a = 8.6095(3) A, c = 22.228(1) A, V = 1646.9(1) A(3), Z = 4; C(17)H(14)BrN(3)O(5)Re.CH(3)OH (3), monoclinic P2(1)/m, a = 7.4425(3) A, b = 9.7596(4) A, c = 14.0646(6) A, beta = 97.753(1) degrees, V = 1012.26(7) A(3), Z = 2; C(19)H(19)BrN(3)O(5)Re (4), tetragonal P42(1)c, a = 16.895(3) A, c = 15.042(3) A, V = 4293.7(13) A(3), Z = 8; C(18)H(20)BrN(4)O(5)Re.CH(3)OH.H(2)O (7), monoclinic P2(1)/c, a = 10.2816(4) A, b = 30.386(1) A, c = 14.5810(6) A, beta = 99.868(1) degrees, V = 4488.03(3) A(3), Z = 8; C(17)H(21)BrN(5)O(5)Re.0.5CH(2)Cl(2).0.5H(2)O (8), triclinic P1, a = 11.5363(6) A, b = 13.1898(6) A, c = 16.4933(8) A, alpha = 89.356(1) degrees, beta = 74.907(1) degrees, gamma = 76.216(1) degrees, V = 2349.8(2) A(3), Z = 4; C(16)H(13)N(2)O(5)ReS (9), monoclinic P2(1)/c, a = 17.2072(7) A, b = 8.5853(4) A, c = 11.5607(5) A, beta = 101.73(1) degrees, V = 1672.2(1) A(3), Z = 4; and C(14)H(12)N(2)O(3)BrReS (10), triclinic P1, a = 7.5585(3) A, b = 9.7713(4) A, c = 11.7103(4) A, alpha = 109.566(1) degrees, beta = 98.298(1) degrees, gamma = 100.925(1) degrees, V = 779.73(5) A(3), Z = 2.