Aspartate-Based CXCR4 Chemokine Receptor Binding of Cross-Bridged Tetraazamacrocyclic Copper(II) and Zinc(II) Complexes.

The CXCR4 chemokine receptor is implicated in a number of diseases including HIV infection and cancer development and metastasis. Previous studies have demonstrated that configurationally restricted bis-tetraazamacrocyclic metal complexes are high-affinity CXCR4 antagonists. Here, we present the synthesis of Cu(2+) and Zn(2+) acetate complexes of six cross-bridged tetraazamacrocycles to mimic their coordination interaction with the aspartate side chains known to bind them to CXCR4. X-ray crystal structures for three new Cu(2+) acetate complexes and two new Zn(2+) acetate complexes demonstrate metal-ion-dependent differences in the mode of binding the acetate ligand concomitantly with the requisite cis-V-configured cross-bridged tetraazamacrocyle. Concurrent density functional theory molecular modelling studies produced an energetic rationale for the unexpected [Zn(OAc)(H2 O)](+) coordination motif present in all of the Zn(2+) cross-bridged tetraazamacrocycle crystal structures, which differs from the chelating acetate [Zn(OAc)](+) structures of known unbridged and side-bridged tetraazamacrocyclic Zn(2+) -containing CXCR4 antagonists.

Mono- and Bis-Alkylation of Glyoxal-Bridged Tetraazamacrocycles Using Mechanochemistry.

Glyoxal-bridged bisaminal tetraazamacrocyclic derivatives of 1,4,7,10-tetraazacyclododecane (cyclen) and 1,4,8,11-tetraazacyclotetradecane (cyclam) can be N-functionalized to incorporate coordinating groups or for conjugation to biomolecules. Herein, we present an improved N-functionalization methodology using mechanochemistry which reduces reaction times in comparison with conventional synthetic routes. A range of six alkyl halides were reacted with cyclen and cyclam bisaminal derivatives in various ratios to form mono- and bis-functionalized quaternary ammonium salts. Cross-bridged cyclam, a key intermediate for CB-TE2A, a commonly used chelator in positron emission tomography medical imaging with (64)Cu, has been synthesized using nonconventional synthetic methodologies (grinding and microwave heating) with intermediates characterized by 2D NMR and single crystal XRD. The overall synthesis time of CB-TE2A from cyclam could be shortened to 5 days from the 35 days required for the conventional synthesis.

Crystal structure of di-chlorido-(4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-deca-ne)iron(III) hexa-fluorido-phosphate.

The title compound, [FeCl2(C14H30N4)]PF6, contains Fe(3+) coordinated by the four nitro-gen atoms of an ethyl-ene cross-bridged cyclam macrocycle and two cis chloride ligands in a distorted octa-hedral environment. In contrast to other similar compounds this is a monomer. Inter-molecular C-H⋯Cl inter-actions exist in the structure between the complex ions. Comparison with the mononuclear Fe(2+) complex of the same ligand shows that the smaller Fe(3+) ion is more fully engulfed by the cavity of the bicyclic ligand. Comparison with the µ-oxido dinuclear complex of an unsubstituted ligand of the same size demonstrates that the methyl groups of 4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-decane prevent dimerization upon oxidation.

Crystal structures of two cross-bridged chromium(III) tetra-aza-macrocycles.

The crystal structure of di-chlorido-(4,10-dimethyl-1,4,7,10-tetra-aza-bicyclo-[5.5.2]tetra-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C12H26N4)]PF6, (I), has monoclinic symmetry (space group P21/n) at 150 K. The structure of the related di-chlorido-(4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C14H30N4)]PF6, (II), also displays monoclinic symmetry (space group P21/c) at 150 K. In each case, the Cr(III) ion is hexa-coordinate with two cis chloride ions and two non-adjacent N atoms bound cis equatorially and the other two non-adjacent N atoms bound trans axially in a cis-V conformation of the macrocycle. The extent of the distortion from the preferred octa-hedral coordination geometry of the Cr(III) ion is determined by the parent macrocycle ring size, with the larger cross-bridged cyclam ring in (II) better able to accommodate this preference and the smaller cross-bridged cyclen ring in (I) requiring more distortion away from octa-hedral geometry.

Challenges in chelating positron emitting copper isotopes: tailored synthesis of unsymmetric chelators to form ultra stable complexes.

The synthesis of chelators that form high stability complexes with copper(II) isotopes and do not suffer from transchelation in vivo has been a goal for many chemists. Such chelators will facilitate the exploitation of the (64)Cu isotope (t(1/2) = 12.7 h, ß(+) (19%); ß(-) (39%); EC (41%)) for positron emission tomography imaging studies, which has a longer half life relative to the more commonly used (18)F (t(1/2) = 109.8 min) and (11)C (t(1/2) = 20.4 min) isotopes. One option is the CBTE2A chelator, which has been championed by Weisman, Wong and Anderson, and, more recently, alternate bifunctional chelator (BFC) versions have been synthesised. Improved synthetic methods are required for unsymmetric derivatisation of these chelators to allow more selective biomolecule attachment. This work investigates synthetic routes to form new unsymmetric chelating ligands via stepwise reaction of the bisaminal precursor, determines their X-ray structures and demonstrates cold copper(II) isotope complex formation.

Binding optimization through coordination chemistry: CXCR4 chemokine receptor antagonists from ultrarigid metal complexes.

A new copper(II) containing bis-macrocyclic CXCR4 chemokine receptor antagonist is shown to have improved binding properties to the receptor protein in comparison to the drug AMD3100 (Plerixafor, Mozobil). The interaction of the metallodrug has been optimized by using ultrarigid chelator units that offer an equatorial site for coordination to the amino acid side chains of the protein. Binding competition assays with anti-CXCR4 antibodies show that the new compound stays bound longer and it has improved anti-HIV potency in vitro (EC(50) = 4.3 nM). X-ray structural studies using acetate as a model for carboxylate amino acid side chains indicate the nature of the coordination interaction.

Synthesis and Characterization of the Chromium(III) complexes of Ethylene Cross-Bridged Cyclam and Cyclen Ligands.

Dichloro(4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)chromium(III) chloride, Dichloro(4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane) chromium(III) chloride, and Dichloro(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2] hexadecane)chromium)(III) chloride have been prepared by the reaction of anhydrous chromium(III) chloride with the appropriate cross-bridged tetraazamacrocycle. Aquation of these complexes proved difficult, but Chlorohydroxo(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)chromium)(III) chloride was synthesized directly from chromium(II) chloride complexation followed by exposure or the reaction to air in the presence of water. The four complexes were characterized by X-ray crystal structure determination. All contain the chromium(III) ion in a distorted octahedral geometry and the macrocycle in the cis-V configuration, as dictated by the ethylene cross-bridge. Further characterization of the hydroxo complex reveals a magnetic moment of mu(eff) = 3.95 B.M. and electronic absorbtions in acetonitrile at lambda(max) = 583nm (epsilon = 65.8 L/cm.mol), 431nm (epsilon = 34.8 L/cm.mol) and 369nm (epsilon = 17 L/cm.mol).

Probing key coordination interactions: configurationally restricted metal activated CXCR4 antagonists.

The syntheses of configurationally restricted mono- and bis-macrocyclic copper(II) perchlorate complexes (copper(II) 5-benzyl-1,5,8,12-tetraazabicyclo[10.2.2]hexadecane and dicopper(II) 5,5'-[1,4-phenylenebis(methylene)]-bis(1,5,8,12-tetraazabicyclo[10.2.2]hexadecane)) are reported and the X-ray structure of the copper(II) mono-macrocyclic complex has been determined. EXAFS studies on the bis-macrocyclic species in aqueous solution show that the copper coordination spheres are essentially identical to the solid state structure, and do not vary in the presence of 20 equivalents of sodium acetate per metal centre. DFT calculations were carried out at the BP86/TZP level to determine the nature of potential binding interactions with CXCR4 aspartate residues. The alkylated single macrocyclic compound was modelled with an acetate included to represent the aspartate residue, demonstrating that the predicted macrocycle configuration has the lowest energy and the acetate interaction is effectively monodentate giving a distorted trigonal bipyramidal geometry at the copper centre. In vitro anti-HIV infection assays show that the configurationally restricted dicopper(II) complex is more active (average EC(50) = 0.026 microM against HIV-1) than the non-constrained dicopper(II) 1,1'-[1,4-phenylenebis(methylene)]-bis(1,4,8,11-tetraazacyclotetradecane) (average EC(50) = 0.047 microM against HIV-1) although it is an order of magnitude less active than the configurationally restricted dizinc(II) complex.

Copper(II) cyclam-based complexes for radiopharmaceutical applications: synthesis and structural analysis.

Two molecular structures of the copper(II) complex, Cu(H(2)TETA), have been determined by X-ray crystallography. The Jahn-Teller distortion differs between the two structures; occurring either along the axis of the pendant acetate arms or across the macrocyclic ring. An analysis of deposited data from over one hundred copper(II) cyclam X-ray structures in the Cambridge Structural Database (CSD) reveals that Jahn-Teller distortion across the ring is highly unusual for such compounds in the solid state. Novel chelators based on the piperazino/side-bridged cyclam have been prepared and copper(II) complexes formed. The single crystal X-ray structures of two copper(II) complexes, with either an ester or acid N-pendant arm, have been determined and in both cases the pendant arm is bound to the metal centre.

Fluorescent CXCR4 chemokine receptor antagonists: metal activated binding.

The copper(II) complex of a novel rhodamine-azamacrocycle conjugate binds to the CXCR4 chemokine receptor and competes effectively against anti-CXCR4 monoclonal antibodies. The copper macrocycle unit adopts a trans-II configuration in the solid state.

Dichloro(4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)iron(III) hexafluorophosphate.

The title compound, [FeCl2(C12H26N4)]PF6, is the first mononuclear Fe3+ complex of an ethylene cross-bridged tetraaza-macrocycle to be structurally characterized. Comparison with the mononuclear Fe2+ complex of the same ligand shows that the smaller Fe3+ ion is more fully encapsulated by the cavity of the bicyclic ligand. Comparison with the mu-oxo dinuclear complex of an unsubstituted ligand of the same size demonstrates that the methyl groups of 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane prevent dimerization upon oxidation of the metal centre. Nax -Fe3+ -Nax bond angles (ax is axial), and thus the degree of encapsulation by the ligand, are quite different between the mononuclear and dinuclear mu-oxo species, which is probably the consequence of steric considerations.