Synthesis of Organosilicon Ligands for Europium (III) and Gadolinium (III) as Potential Imaging Agents.

The relaxivity of MRI contrast agents can be increased by increasing the size of the contrast agent and by increasing concentration of the bound gadolinium. Large multi-site ligands able to coordinate several metal centres show increased relaxivity as a result. In this paper, an "aza-type Michael" reaction is used to prepare cyclen derivatives that can be attached to organosilicon frameworks via hydrosilylation reactions. A range of organosilicon frameworks were tested including silsesquioxane cages and dimethylsilylbenzene derivatives. Michael donors with strong electron withdrawing groups could be used to alkylate cyclen on three amine centres in a single step. Hydrosilylation successfully attached these to mono-, di-, and tri-dimethylsilyl-substituted benzene derivatives. The europium and gadolinium complexes were formed and studied using luminescence spectroscopy and relaxometry. This showed the complexes to contain two bound water moles per lanthanide centre and T1 relaxation time measurements demonstrated an increase in relaxivity had been achieved, in particular for the trisubstituted scaffold 1,3,5-tris((pentane-sDO3A)dimethylsilyl)benzene-Gd3. This showed a marked increase in the relaxivity (13.1 r1p/mM-1s-1).

Influence of the initial chemical conditions on the rational design of silica particles.

The influence of the water content in the initial composition on the size of silica particles produced using the Stöber process is well known. We have shown that there are three morphological regimes defined by compositional boundaries. At low water levels (below stoichiometric ratio of water:tetraethoxysilane), very high surface area and aggregated structures are formed; at high water content (>40 wt%) similar structures are also seen. Between these two boundary conditions, discrete particles are formed whose size are dictated by the water content. Within the compositional regime that enables the classical Stöber silica, the structural evolution shows a more rapid attainment of final particle size than the rate of formation of silica supporting the monomer addition hypothesis. The clearer understanding of the role of the initial composition on the output of this synthesis method will be of considerable use for the establishment of reliable reproducible silica production for future industrial adoption.

Fully protected glycosylated zinc (II) phthalocyanine shows high uptake and photodynamic cytotoxicity in MCF-7 cancer cells.

Phthalocyanine photosensitizers are effective in anticancer photodynamic therapy (PDT) but suffer from limited solubility, limited cellular uptake and limited selectivity for cancer cells. To improve these characteristics, we synthesized isopropylidene-protected and partially deprotected tetra ß-glycosylated zinc (II) phthalocyanines and compared their uptake and accumulation kinetics, subcellular localization, in vitro photocytotoxicity and reactive oxygen species generation with those of disulfonated aluminum phthalocyanine. In MCF-7 cancer cells, one of the compounds, zinc phthalocyanine {4}, demonstrated 10-fold higher uptake, 5-fold greater PDT-induced cellular reactive oxygen species concentration and 2-fold greater phototoxicity than equimolar (9 µm) disulfonated aluminum phthalocyanine. Thus, isopropylidene-protected ß-glycosylation of phthalocyanines provides a simple method of improving the efficacy of PDT.

Antioxidant inhibitors potentiate the cytotoxicity of photodynamic therapy.

Photodynamic therapy (PDT) is an increasingly popular anticancer treatment that uses photosensitizer, light and tissue oxygen to generate cytotoxic reactive oxygen species (ROS) within illuminated cells. Acting to counteract ROS-mediated damage are various cellular antioxidant pathways. In this study, we combined PDT with specific antioxidant inhibitors to potentiate PDT cytotoxicity in MCF-7 cancer cells. We used disulphonated aluminium phthalocyanine photosensitizer plus various combinations of the antioxidant inhibitors: diethyl-dithiocarbamate (DDC, a Cu/Zn-SOD inhibitor), 2-methoxyestradiol (2-ME, a Mn-SOD inhibitor), l-buthionine sulfoximine (BSO, a glutathione synthesis inhibitor) and 3-amino-1,2,4-triazole (3-AT, a catalase inhibitor). BSO, singly or in combination with other antioxidant inhibitors, significantly potentiated PDT cytotoxicity, corresponding with increased ROS levels and apoptosis. The greatest potentiation of cell death over PDT alone was seen when cells were preincubated for 24 h with 300 µM BSO plus 10 mM 3-AT (1.62-fold potentiation) or 300 µM BSO plus 1 µM 2-ME (1.52-fold), or with a combination of all four inhibitors (300 µM BSO, 10 mM 3-AT, 1 µM 2-ME and 10 µM DDC: 1.4-fold). As many of these inhibitors have already been clinically tested, this work facilitates future in vivo studies.

Near IR-emitting DNA-probes exploiting stepwise energy transfer processes.

The synthesis and characterisation of two new cyclen-based near IR-emitting lanthanide complexes is reported; the lanthanides are sensitised by rhodamine, which in turn is excited by energy transfer from a coumarin 2 moiety. The three lumophores function as an energy transfer cascade spanning the UV-visible-near IR region of the spectrum, resulting in large Stokes shifts. Double stranded DNA selectively switches one of the two energy transfer processes off, enabling luminescent DNA-sensing in the near IR region. The regioselective di-alkylation of the cyclen scaffold is explained with the help of DFT calculations.

Synthesis of asymmetrically substituted cyclen-based ligands for the controlled sensitisation of lanthanides.

A series of unsymmetrical cyclen-based ligands incorporating an antenna and a quencher have been prepared for the complexation of the visible- (Eu, Tb) and near IR-emitting (Nd, Yb) lanthanides. Eu and Tb were sensitised with coumarin 2, and Nd and Yb with rhodamine. Both antennae were paired with nucleoside (uridine and adenosine) quenchers. The interaction between the quencher and the antenna can be regulated by the addition of the complementary base or DNA to the complexes, resulting in changes in the lanthanide luminescence intensity and lifetime.

Design and synthesis of mono- and multimeric targeted radiopharmaceuticals based on novel cyclen ligands coupled to anti-MUC1 aptamers for the diagnostic imaging and targeted radiotherapy of cancer.

Targeted radiopharmaceuticals offer the possibility of improved tumor imaging and radiotherapy, with reduced side effects. A variety of monoclonal antibodies and antibody fragments have previously been successfully radiolabeled and used in diagnostic imaging and targeted radiotherapy of cancer. Many such antibodies have been shown to recognize the well-characterized MUC1 tumor marker and have recently been in clinical trials. Furthermore, a number of chelators have been synthesized and are currently used as radiopharmaceuticals for imaging and therapy. We now report the synthesis of a novel, cyclen-based ligand with a sulfur-containing arm that offers increased stability of the ligand-metal complex. We have coupled this ligand with previously selected aptamers to the MUC1 tumor marker to generate a novel targeted radiopharmaceutical with improved properties. We have tested the complex against known, commercially available chelators such as MAG3 in model breast cancer systems. To improve the pharmacokinetic properties of the aptamer-based targeted radiopharmaceutical, we have generated multi-aptamer complexes around a central chelator. Such multi-aptamer complexes have increased retention of the complex in circulation, without affecting the lack of immunogenicity of the complex or altering its superior tumor penetration properties.

Synthesis of asymmetrically substituted 1,4,7,10-tetraazacyclododecanes for the triggered near infrared emission from lanthanide complexes.

A synthetic strategy to prepare asymmetrically substituted 1,4,7,10-tetrazadodecane derivatives was developed to prepare a novel series of photoactive donor-acceptor quencher triads based on Yb and Nd complexes; a nucleoside quencher is used to regulate the extent of energy transfer between the donor and the acceptor.

Structural, luminescence, and NMR studies of the reversible binding of acetate, lactate, citrate, and selected amino acids to chiral diaqua ytterbium, gadolinium, and europium complexes.

The nature of the ternary complexes formed in aqueous media at ambient pH on reversible binding of acetate, lactate, citrate, and selected amino acids and peptides to chiral diaqua europium, gadolinium, or ytterbium cationic complexes has been examined. Crystal structures of the chelated ytterbium acetate and lactate complexes have been defined in which the carboxylate oxygen occupies an "equatorial" site in the nine-coordinate adduct. The zwitterionic adduct of the citrate anion with [EuL1] was similar to the chelated lactate structure, with a 5-ring chelate involving the apical 3-hydroxy group and the alpha-carboxylate. Analysis of Eu and Yb emission CD spectra and lifetimes (H2O and D2O) for each ternary complex, in conjunction with 1H NMR analyses of Eu/Yb systems and 17O NMR and relaxometric studies of the Gd analogues, suggests that carbonate, oxalate, and malonate each form a chelated (q = 0) square-antiprismatic complex in which the dipolar NMR paramagnetic shift (Yb, Eu) and the emission circular polarization (gem for Eu) are primarily determined by the polarizability of the axial ligand. The ternary complexes with hydrogen phosphate, with fluoride, and with Phe, His, and Ser at pH 6 are suggested to be monoaqua systems with Eu/Gd with an apical bound water molecule. However, for the ternary complexes of simple amino acids with [YbL1]3+, the enhanced charge demand favors a chelate structure with the amine N in an apical position. Crystal structures of the Gly and Ser adducts confirm this. In peptides and proteins (e.g. albumin) containing Glu or Asp residues, the more basic side chain carboxylate may chelate to the Ln ion, displacing both waters.

Survey of factors determining the circularly polarised luminescence of macrocyclic lanthanide complexes in solution.

The development of emissive lanthanide complexes as structural or reactive probes to signal changes in their local chiral or ionic environment has been inhibited by the lack of understanding of correlating structural and electronic spectral information. The definition of relatively rigid enantiopure macrocyclic lanthanide complexes, whose inter- and intramolecular exchange dynamics have been defined, offers scope for remedying this situation. Chiral axially symmetric lanthanide complexes in solution give rise to large emission dissymmetry values (g(em)) in CPL spectra. The sign and magnitude of g(em) are determined by the degree of twist about the principal axis, which is predicted to be a maximum at +/-22.5 degrees, and by the site symmetry and local ligand field. In particular, the polarisability of the ligand donor atoms, especially for any axial donor, is very important. Examples of each case are discussed for structurally related cationic Eu(III) complexes.

Spin-Orbit Mixing and Nephelauxetic Effects in the Electronic Spectra of Nickel(II)-Encapsulating Complexes Involving Nitrogen and Sulfur Donors.

A study of spin-orbit mixing and nephelauxetic effects in the electronic spectra of nickel(II)-encapsulating complexes involving mixed nitrogen and sulfur donors is reported. As the number of sulfur donors is systematically varied through the series [Ni(N(6)(-)(x)()S(x)())](2+) (x = 0-6), the spin-forbidden (3)A(2g) --> (1)E(g) and (3)A(2g) --> (1)A(1g) transitions undergo a considerable reduction in energy whereas the spin-allowed transitions are relatively unchanged. The [Ni(diAMN(6)sar)](2+) and [Ni(AMN(5)Ssar)](2+) complexes exhibit an unusual band shape for the (3)A(2g) --> (3)T(2g) transition which is shown to arise from spin-orbit mixing of the E spin-orbit levels associated with the (1)E(g) and (3)T(2g) states. A significant differential nephelauxetic effect also arises from the covalency differences between the t(2g) and e(g) orbitals with the result that no single set of Racah B and C interelectron repulsion parameters adequately fit the observed spectra. Using a differential covalency ligand-field model, the spectral transitions are successfully reproduced with three independent variables corresponding to 10Dq and the covalency parameters f(t) and f(e), associated with the t(2g) and e(g) orbitals, respectively. The small decrease in f(t) from unity is largely attributed to central-field covalency effects whereas the dramatic reduction in f(e) with increasing number of sulfur donors is a direct consequence of the increased metal-ligand covalency associated with the sulfur donors. Covalency differences between the t(2g) and e(g) orbitals also result in larger 10Dq values than those obtained simply from the energy of the (3)A(2g) --> (3)T(2g) spin-allowed transition.