António Paulo

Rhenium(I) organometallic complexes with novel bis(mercaptoimidazolyl)borates and with hydrotris(mercaptoimidazolyl)borate: chemical and structural studies

Lithium salts of novel poly(mercaptoimidazolyl)borate anions [H(R)B(tim Me ) 2 ] − have been synthesised under mild conditions by reaction of the corresponding lithium organoborohydrides with 2-mercapto-1-methylimidazole. Treatment of the Re(I) starting materials [Re(CO) 5 Br] or (NEt 4 ) 2 [Re(CO) 3 Br 3 ] with the stoichiometric amount of Li[H(R)B(tim Me ) 2 ] (R=Me ( 1 ), Ph ( 2 )) or Na[HB(tim Me ) 3 ] gave the tricarbonyl complexes [Re{κ 3 -R(μ-H)B(tim Me ) 2 }(CO) 3 ] (R=Me ( 3 ), Ph ( 4 )) and [Re{κ 3 -HB(tim Me ) 3 }(CO) 3 ] ( 5 ). These complexes were also prepared under aqueous and aerobic conditions, in almost quantitative yield, using (NEt 4 ) 2 [ReBr 3 (CO) 3 ] as starting material. Compounds 1 – 5 have been characterised by the usual analytical techniques and by X-ray crystallographic analysis in the case of 3 – 5 . The X-ray diffraction analysis of 3 – 5 showed that the rhenium atom adopts a slightly distorted octahedral coordination with a facial arrangement of the carbonyl ligands. In complex 5 the three remaining coordination positions are occupied by the three thione sulphur atoms from the tripodal hydrotris(2-mercapto-1-methylimidazolyl)borate, and in 3 and 4 these positions are occupied by the two thione sulphur atoms and by one hydrogen atom, which is involved in a strong agostic BH⋯Re interaction.

Copper(II) complexes with tridentate pyrazole-based ligands: synthesis, characterization, DNA cleavage activity and cytotoxicity.

Tridentate pyrazole-containing ligands of the Schiff base type, SalPz - HL(1), Cl(2)SalPz - HL(2) and I(2)SalPz - HL(3), were used to prepare a series of new Cu(II) complexes (CuSalPz - 1, CuCl(2)SalPz - 2 and CuI(2)SalPz - 3). These new complexes have been studied by different analytical techniques (electrospray ionization mass spectrometry (ESI-MS), elemental analysis, FT-IR and EPR). The spectroscopic properties of 1-3 are consistent with the formation of Cu(II) complexes coordinated by monoanionic and tridentate (N,N,O)-chelators, behaving as monomeric species in aqueous solution, as shown by EPR studies. Crystals of 2 and 3, obtained by slow concentration of methanolic solutions of the compounds, were also analyzed by X-ray diffraction analysis. The X-ray structural study has shown that 2 crystallized as a dinuclear compound, [Cu(2)(μ-Cl)(2)(Cl(2)SalPz)(2)], while the solid state structure determined for 3 is best described by monomeric units of [CuCl(I(2)SalPz)] displaying short Cu···Cl intermolecular contacts. The in vitro evaluation of 1-3 comprised the study of their DNA-cleaving ability using plasmid DNA and the assessment of their cytotoxic activity against several human cancer cell lines (PC-3 prostate, MCF-7 breast and A2780 and A2780cisR-ovary). The studies with plasmid DNA have shown that 2 and 3 induce extensive DNA cleavage in the presence of different additives. The cytotoxic activity of 2 and 3 is comparable to the one presented by cisplatin, with the exception of the A2780 cell line where cisplatin is more active. It has been found that the introduction of halogen substituents in the phenolate rings of the chelators enhanced the cytotoxicity of the respective Cu(II) complexes.

Pyrazolyl-diamine ligands that bear anthracenyl moieties and their rhenium(I) tricarbonyl complexes: synthesis, characterisation and DNA-binding properties.

Two novel families of pyrazolyl–diamine ligands that bear an anthracen‐9‐yl group as a DNA‐binding fragment, pz*(CH2)2NH(CH2)2NHCH2‐9‐anthryl (pz*=pz (L1), 3,5‐Me2pz (L2)) and pz*(CH2)2NH(CH2)2NH2 (pz*=4‐(9‐anthrylmethyl)pz (L3), 3,5‐Me2‐4‐(9‐anthrylmethyl)pz (L4)), have been prepared and fully characterised. In the case of L2–L4, the evaluation of their coordination capability towards the fac‐[Re(CO)3]+ core led to the synthesis of the organometallic complexes fac‐[Re(CO)3{3,5‐Me2pz(CH2)2NH(CH2)2NHCH2‐9‐anthryl}]Br (7) and fac‐[Re(CO)3{4‐(9‐anthrylmethyl)pz*(CH2)2NH(CH2)2NH2}]Br (pz*=pz (8), 3,5‐Me2pz (9)). The interaction of the novel pyrazole–diamine ligands and the rhenium(I) complexes with calf thymus (CT) DNA has been investigated with a variety of spectroscopic techniques (UV‐visible, fluorescence, circular dichroism (CD) and linear dichroism (LD)). All of the evaluated compounds have a moderate affinity to CT DNA (3.46×103

Tris(pyrazolyl)methane 99mTc tricarbonyl complexes for myocardial imaging

Novel ether-containing tris(pyrazolyl)methane chelators stabilize thefac-[99mTc(CO)3]+ moiety providing complexes with significant, fast and stable heart uptake, together with a extremely fast liver clearance.

99mTc-Tricarbonyl Complexes Functionalized with Anthracenyl Fragments: Synthesis, Characterization, and Evaluation of Their Radiotoxic Effects in Murine Melanoma Cells

Different pyrazolyl-diamine ligands bearing anthracenyl or anthrapyrazole functionalities as DNA-binding groups, at different positions of the chelator framework, were labeled with the fac-[(99m)Tc(CO)(3)](+) core. The resulting complexes, 1-4, are highly stable in vitro under physiologic conditions; all of them have been identified by high-performance liquid chromatography comparison with the Re congeners, with the exception of 3, that is anchored by an anthrapyrazole diamine ligand. Aiming to assess the ability of these complexes to target the cell nucleus and to induce enhanced cell death by effect of the Auger electrons emitted by (99m)Tc, the intracellular distribution and radiotoxicity of 1-4 were evaluated by using B16F1 murine melanoma cells. The radiotoxic effects depend very much on the position used to introduce the DNA-binding group and are well correlated with the nuclear uptake of the compounds. Complex 2, having the anthracenyl substituent at the 4-position of the pyrazolyl ring, rapidly entered the cells and accumulated inside the nucleus, exhibiting the highest radiotoxic effects. This compound induced an apoptotic cellular outcome, and its enhanced radiotoxic effects were certainly due to the Auger electrons emitted by the radiometal in close proximity to DNA.

Pt(II) complexes with bidentate and tridentate pyrazolyl-containing chelators: synthesis, structural characterization and biological studies

A series of four Pt(II) complexes anchored by bidentate or tridentate pyrazolyl-alkylamine chelators bearing different substituents at the azolyl rings has been prepared with the aim to assess their interest in the design of novel anticancer drugs. All complexes have been fully characterized by classical analytical methods and three of them were characterized also by X-ray diffraction analysis. Their solution behavior, together with lipophilicity measurements, cell uptake, antiproliferative properties, DNA interaction have been evaluated. Albeit all the complexes were less active than cisplatin on ovarian carcinoma A2780 cell line, greatly retained their activity in the cisplatin-resistant A2780cisR cell line and presented a lower resistance factor compared to cisplatin. Moreover, the Pt(II) complexes under investigation were less prone to undergo deactivation by glutathione, believed to be the major cellular target of cisplatin that inactivates the drug by binding to it irreversibly.

Synthesis, characterization and cytotoxic activity of gallium(III) complexes anchored by tridentate pyrazole-based ligands

Reactions of GaCl(3) with pyrazole-containing ligands of the pyrazole-imine-phenol (HL(1)-HL(3)) or pyrazole-amine-phenol (HL(4)-HL(6)) types led to the synthesis of well-defined [GaL(2)](+) homoleptic complexes (1-6). Complexes 1-6 were characterized by elemental analysis, ESI-MS (electrospray ionization-mass spectrometry), IR and NMR spectroscopies, and in the case of Complex 1 also by X-ray diffraction analysis. In complexes 1-3, the pyrazole-imine-phenolate ligands act as monoanionic chelators that coordinate to the metal in a meridional fashion, while 4-6 contain monoanionic and facially coordinated pyrazole-amine-phenolate ligands. Complexes 1-3 have a greater stability in solution compared to 4-6, which have shown a more pronounced tendency to release the respective ancillary ligands. The cytotoxicity of 1-6 and of the respective ligands (HL(1)-HL(6)) was evaluated against human prostate cancer cells PC-3 and human breast cancer cells MCF-7. The substituents of the phenolate rings strongly influenced the cytotoxicity of the compounds. Complexes 3 and 6 that contain chloride substituents at the phenolate rings have shown the highest cytotoxicity, including in the cisplatin-resistant PC-3 cell line. The cytotoxic profile of 3 and 6 is very similar to the one displayed by the respective anchor ligands, respectively HL(1) and HL(6). The cytotoxic activity of 3 and 6 is slightly increased by the presence of transferrin, and both complexes provoke cell death mainly by induction of apoptotic pathways.

Synthesis, characterization and biological evaluation of tricarbonyl M(I) (M = Re, 99mTc) complexes functionalized with melanin-binding pharmacophores

Aiming to evaluate their potential as radioactive probes for in vivo targeting of melanotic melanoma and its metastases, we have synthesized 99mTc(I) tricarbonyl complexes (Tc1–Tc8) anchored by pyrazolyl-containing chelators with (N3) or (N2O) donor atom sets and functionalized with 2-aminoethyldiethylamine and 4-amino-N-(2-diethylaminoethyl)benzamide groups as melanin-binding pharmacophores. The chemical identification of the several 99mTc complexes has been accomplished by HPLC comparison with the Re congeners (Re1–Re8), which were synthesized at the macroscopic level and fully characterized by common analytical techniques. The biological evaluation of the 99mTc(I) complexes comprised the determination of their in vitro binding to synthetic melanin, measurement of cellular uptake in B16F1 murine melanoma cells, as well as biodistribution studies in B16F1 melanoma-bearing mice. All the tested complexes have shown a moderate to high in vitro affinity to melanin, with percentages of binding spanning between 60 and 94%. In agreement with the poor cellular uptake measured in vitro, the in vivo tumor uptake of the complexes was in general relatively low, ranging between 0.12 and 1.69% ID g−1 at 4 h p.i. However, some complexes have shown favorable tumor-to-organ ratios with values as high as 28 and 5.3 for tumor–muscle and tumor–blood ratios, respectively. This seems to indicate that some selectivity towards melanoma tissue was conserved, and encourages further optimization of the in vitro/in vivo biological properties of this type of complexes aimed at finding novel radioactive probes for non-invasive imaging of melanoma.

Re and Tc Complexes with Pyrazolyl-Containing Chelators: from Coordination Chemistry to Target-Specific Delivery of Radioactivity

The design of novel target-specific imaging agents based on 99mTc requires a considerable development of its coordination chemistry. Among all oxidation states available for technetium (-I to VII), the V oxidation state has been the most extensively studied in radiopharmaceutical chemistry, and the majority of the 99mTc-radiopharmaceuticals in clinical use contain the core [99mTc(O)]3+. More recently, the remarkable features of the organometallic precursor fac- [M(CO)3(H2O)3]+ (M = Re, Tc), introduced by Alberto et al., brought renewed interest in the design of innovative lowoxidation 99mTc-based radiopharmaceuticals. Owing to our interest on the design of innovative target-specific radioactive probes, we have been recently involved in the study of the chemistry of [M(O)]3+ and fac-[M(CO)3]+ (M = Re, Tc) with chelators combining a pyrazolyl unit with aliphatic amines and/or carboxylic acids or thioethers. Such research efforts are reviewed herein, where we present an overview of the chemistry, radiochemistry and biological properties of Re and 99mTc complexes anchored by those pyrazolyl-containing chelators with relevance in radiopharmaceutical research. The revised work focuses mainly on tricarbonyl M(I) complexes but M(V) oxocomplexes are also covered. This contribution intends to highlight the potential of pyrazolyl-containing chelators for the labeling of biologically active molecules with 99mTc(I), being presented a variety of examples which include peptides, peptide nucleic acids, inhibitors/substrates of enzymes and DNA-binders.

Synthesis, characterization and biological evaluation of 99m Tc/Re–tricarbonyl quinolone complexes

New rhenium(I) tricarbonyl complexes with the quinolone antimicrobial agents oxolinic acid (Hoxo) and enrofloxacin (Herx) and containing methanol, triphenylphosphine (PPh3) or imidazole (im) as unidentate co-ligands, were synthesized and characterized. The crystal structure of complex [Re(CO)3(oxo)(PPh3)]∙0.5MeOH was determined by X-ray crystallography. The deprotonated quinolone ligands are bound bidentately to rhenium(I) ion through the pyridone oxygen and a carboxylate oxygen. The binding of the rhenium complexes to calf-thymus DNA (CT DNA) was monitored by UV spectroscopy, viscosity measurements and competitive studies with ethidium bromide; intercalation was suggested as the most possible mode and the DNA-binding constants of the complexes were calculated. The rhenium complex [Re(CO)3(erx)(im)] was assayed for its topoisomerase IIα inhibition activity and was found to be active at 100μM concentration. The interaction of the rhenium complexes with human or bovine serum albumin was investigated by fluorescence emission spectroscopy (through the tryptophan quenching) and the corresponding binding constants were determined. The tracer complex [(99m)Tc(CO)3(erx)(im)] was synthesized and identified by comparative HPLC analysis with the rhenium analog. The (99m)Tc complex was found to be stable in solution. Upon injection in healthy mice, fast tissue clearance of the (99m)Tc complex was observed, while both renal and hepatobiliary excretion took place. Preliminary studies in human K-562 erythroleukemia cells showed cellular uptake of the (99m)Tc tracer with distribution primarily in the cytoplasm and the mitochondria and less in the nucleus. These preliminary results indicate that the quinolone (99m)Tc/Re complexes show promise to be further evaluated as imaging or therapeutic agents.