[Gd(HB-DO3A)]: Equilibrium, Dissociation Kinetic and Structural Differences in a Simple Homolog of [Gd(HP-DO3A)] (Prohance®).

[Gd(HP-DO3A)] (gadoteridol) as an active compound of ProHance® is a widely employed contrast agent in clinical MRI scans in the last 30 years. Recent concerns about the long-term retention of gadolinium-based contrast agents (GBCAs) led to a deeper investigation of the structural features underlying the integrity of the paramagnetic metal complex. Several human and nonclinical studies have noted marked differences among the macrocyclic GBCAs, with the least retention of Gd traces and most rapid elimination consistently being reported for [Gd(HP-DO3A)]. It was deemed of interest to assess how minor structural/electronic changes associated to the ligand structure may affect basic properties of the metal complex with several [Gd(HP-DO3A)] analogues synthesized and characterized in the last years. We recently reported that the closest homolog of [Gd(HP-DO3A)], i. e.: [Gd(HB-DO3A)], in which a (±)-2-hydroxy-1-propyl pendant arm is replaced by a (±)-2-hydroxy-1-butyl moiety, showed a significantly different retention behaviour in the model interaction with collagen, despite the apparently very minor structural difference. In this paper we report a comprehensive study of the structural, thermodynamic, kinetic and relaxation properties of [Gd(HB-DO3A)], compared to the parent [Gd(HP-DO3A)] and to other closely related macrocyclic GBCAs to assess whether very minor structural changes can modulate the physico-chemical properties of Gd3+ complexes.

TRASUTA: The Effect of the Structural Rigidity of a Mesocyclic AAZTA-like Chelating Agent on the Thermodynamic, Kinetic, and Structural Properties of Some Divalent Metal and Ga3+ Complexes.

Mesocyclic chelating agents such as AAZTA and its derivatives have been recently reported to overcome the relatively low thermodynamic stability of metal complexes of acyclic chelating agents and the slow complexation kinetics of macrocyclic chelating agents. This work reports the preparation of a spirobicyclic hexadentate AAZTA-like chelating agent (TRASUTA) and the investigation of the thermodynamic, kinetic, and structural properties of the corresponding chelates with the PET-relevant Ga3+ and selected metal ions. A combination of analytical techniques allowed identification of a coordination isomerization process, involving the coordinating side arms and the inversion of a nitrogen atom and leading to lower thermodynamic and kinetic inertness with respect to mononuclear mesocyclic analogues. The bicyclic system of TRASUTA retains significant dynamics despite the conformational constraint imposed by the spiro-fusion, resulting in a lower stability of the corresponding metal chelates.

Improved synthesis of DA364, an NIR fluorescence RGD probe targeting αvß3 integrin.

Optical imaging (OI) is gaining increasing attention in medicine as a non-invasive diagnostic imaging technology and as a useful tool for image-guided surgery. OI exploits the light emitted in the near-infrared region by fluorescent molecules able to penetrate living tissues. Cyanines are an important class of fluorescent molecules and by their conjugation to peptides it is possible to achieve optical imaging of tumours by selective targeting. We report here the improvements obtained in the synthesis of DA364, a small fluorescent probe (1.5 kDa) prepared by conjugation of pentamethine cyanine Cy5.5 to an RGD peptidomimetic, which can target tumour cells overexpressing integrin αvß3 receptors.

Boosting Bismuth(III) Complexation for Targeted α-Therapy (TAT) Applications with the Mesocyclic Chelating Agent AAZTA.

Targeted α therapy (TAT) is a promising tool in the therapy of cancer. The radionuclide 213 BiIII shows favourable physical properties for this application, but the fast and stable chelation of this metal ion remains challenging. Herein, we demonstrate that the mesocyclic chelator AAZTA quickly coordinates BiIII at room temperature, leading to a robust complex. A comprehensive study of the structural, thermodynamic and kinetic properties of [Bi(AAZTA)]- is reported, along with bifunctional [Bi(AAZTA-C4-COO- )]2- and the targeted agent [Bi(AAZTA-C4-TATE)]- , which incorporates the SSR agonist Tyr3 -octreotate. An unexpected increase in the stability and kinetic inertness of the metal chelate was observed for the bifunctional derivative and was maintained for the peptide conjugate. A cyclotron-produced 205/206 Bi mixture was used as a model of 213 Bi in labelling, stability, and biodistribution experiments, allowing the efficiency of [213 Bi(AAZTA-C4-TATE)]- to be estimated. High accumulation in AR42J tumours and reduced kidney uptake were observed with respect to the macrocyclic chelate [213 Bi(DOTA-TATE)]- .

PIDAZTA: Structurally Constrained Chelators for the Efficient Formation of Stable Gallium-68 Complexes at Physiological pH.

Two structurally constrained chelators based on a fused bicyclic scaffold, 4-amino-4-methylperhydro-pyrido[1,2-a][1,4]diazepin-N,N',N'-triacetic acids [(4R*,10aS*)-PIDAZTA (L1) and (4R*,10aR*)-PIDAZTA (L2)], were designed for the preparation of GaIII -based radiopharmaceuticals. The stereochemistry of the ligand scaffold has a deep impact on the properties of the complexes, with unexpected [Ga(L2)OH] species being superior in terms of both thermodynamic stability and inertness. This peculiar behavior was rationalized on the basis of molecular modeling and appears to be related to a better fit in size of GaIII into the cavity of L2. Fast and efficient formation of the GaIII chelates at room temperature was observed at pH values between 7 and 8, which enables 68 Ga radiolabeling under truly physiological conditions (pH 7.4).

Influence of Silicodactyly in the Preparation of Hybrid Materials.

The organic⁻inorganic hybrid materials have attracted great attention due to their improved or unusual properties that open promising applications in different areas such as optics, electronics, energy, environment, biology, medicine and heterogeneous catalysis. Different types of silicodactyl platforms grafted on silica inorganic supports can be used to synthesize hybrid materials. A careful evaluation of the dactyly of the organic precursors, normally alkoxysilanes, and of the type of interaction with the inorganic supports is presented. In fact, depending on the hydrophilicity of the silica surface (e.g., number and density of surface silanols) as well as on the grafting conditions, the hydrolysis and condensation reaction of the silylated moieties can involve only one or two out of three alkoxysilane groups. The influence of silicodactyly in the preparation of organic-inorganic silica-based hybrids is studied by TGA, 29Si, ¹H and 13C solid-state NMR and FTIR spectroscopies, with the support of Molecular Dynamics calculations. Computational studies are used to forecast the influence of the different grafting configurations on the tendency of the silane to stick on the inorganic surface.

AAZTA: An Ideal Chelating Agent for the Development of 44 Sc PET Imaging Agents.

Unprecedented fast and efficient complexation of ScIII was demonstrated with the chelating agent AAZTA (AAZTA=1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-methylperhydro-1,4-diazepine) under mild experimental conditions. The robustness of the 44 Sc(AAZTA)- chelate and conjugated biomolecules thereof is further shown by in vivo PET imaging in healthy and tumor mice models. The new results pave the way towards development of efficient Sc-based radiopharmaceuticals using the AAZTA chelator.

Comprehensive evaluation of the physicochemical properties of Ln(III) complexes of aminoethyl-DO3A as pH-responsive T(1) -MRI contrast agents.

N-Substituted aminoethyl groups were attached to 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) with the aim to design pH-responsive Ln(III) complexes based on the pH-dependent on/off ligation of the amine nitrogen to the metal ion. The following ligands were synthesized: AE-DO3A (aminoethyl-DO3A), MAE-DO3A (N-methylaminoethyl-DO3A), DMAE-DO3A (N,N-dimethylaminoethyl-DO3A) and MEM-AE-DO3A (N-methoxyethyl-N-methylaminoethyl-DO3A). The physicochemical properties of the Ln(III) complexes were investigated for the evaluation of their potential applicability as magnetic resonance imaging (MRI) contrast agents. In particular, a (1) H and (17) O NMR relaxometric study was carried out for these Gd(III) complexes at two different pH values: at basic pH (pendant amino group coordinated to the metal centre) and at acidic pH (protonated amine, not interacting with the metal ion). Eu(III) complexes allow one to estimate the number of inner-sphere water molecules through luminescence lifetime measurements and obtain some structural information through variable-temperature (VT) high-resolution (1) H NMR studies. Equilibria between differently hydrated species were found for most of the complexes at both acidic and basic pH. The thermodynamic stability of Ca(II) , Zn(II) , Cu(II) and Ln(III) complexes and kinetics of formation and dissociation reactions of Ln(III) complexes of AE-DO3A and DMAE-DO3A were investigated showing stabilities comparable to currently approved Gd(III) -based CAs. In detail, higher total basicity (Σlog Ki (H) ) and higher stability constants of Ln(III) complexes were found for AE-DO3A with respect to DMAE-DO3A (i.e., log KGd-AE-DO3A =22.40 and log KGd-DMAE-DO3A =20.56). The transmetallation reactions of Gd(III) complexes are very slow (Gd-AE-DO3A: t1/2 =2.7×10(4)  h; Gd-DMAE-DO3A: 1.1×10(5)  h at pH 7.4 and 298 K) and occur through proton-assisted dissociation.

Lower denticity leading to higher stability: structural and solution studies of Ln(III)-OBETA complexes.

The heptadentate ligand OBETA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid) was reported to form complexes with Ln(3+) ions more stable than those formed by the octadentate and more popular congener EGTA (ethylene glycol O,O'-bis(ethylamine)-N,N,N',N'-tetraacetic acid). The structural features leading to this puzzling coordination paradox were investigated by X-ray diffraction, solution state NMR, molecular modeling, and relaxometric studies. The stability constant of Gd(OBETA) (log KGdL = 19.37, 0.1 M KCl) is 2 orders of magnitude higher than that of the higher denticity analogue Gd(EGTA) (log KGdL = 17.66, 0.1 M KCl). The half-lives (t1/2) for the dissociation reactions of Gd(OBETA) and Gd(EGTA) ([Cu(2+)]tot = 0.2 mM, [Cit(3-)]tot = 0.5 mM, [PO4(3-)]tot = 1.0 mM, and [CO3(2-)]tot = 25 mM at pH = 7.4 and 25 °C in 0.1 M KCl solution) are 6.8 and 0.63 h, respectively, reflecting the much higher inertness of Gd(OBETA) near physiological conditions. NMR studies and DFT calculations using the B3LYP functional and a large-core ECP indicate that the [Gd(OBETA)(H2O)2](-) complex most likely exists in solution as the Δ(λλ)(δδδδ)A/Λ(δδ)(λλλλ)A enantiomeric pair, with an activation free energy for the enantiomerization process of ∼40 kJ·mol(-1). The metal ion is nine-coordinate by seven donor atoms of the ligand and two inner-sphere water molecules. The X-ray crystal structure of [C(NH2)3]3[Lu(OBETA)(CO3)]·2H2O is in agreement with the predictions of DFT calculations, the two coordinated water molecules being replaced by a bidentate carbonate anion. The (1)H NMRD and (17)O NMR study revealed that the two inner-sphere water molecules in Gd(OBETA) are endowed with a relatively fast water exchange rate (kex(298) = 13 × 10(6) s(-1)). The higher thermodynamic stability and inertness of Ln(OBETA) complexes, peaking in the center of the 4f series, combined with the presence of two coordinated water molecules suggests that Gd(OBETA) is a promising paramagnetic probe for MRI applications.

An enzymatic approach to bifunctional chelating agents.

Bifunctional chelating agents (BFCAs) combine the complexing properties of a multidentate ligand with the presence of a free reactive functional group, mainly devoted to conjugation purposes. Indeed, products obtained by conjugation of a BFCA to a biomolecule and coordination of a suitable metal ion are widely applied in medicine nowadays as diagnostic and therapeutic agents. BFCAs are generally prepared through multi-step syntheses and with extensive application of protection-deprotection strategies, due to the large number of functional groups involved. Hydrolytic enzymes, with their unique chemoselectivity, provided the best results in the preparation of three different BFCAs based on very useful and well known ligand platforms.

N-Polybenzylated alicyclic 1,2-diamines: cytotoxicity and G1 phase arrest in cancer cell line.

Cytotoxicity in the µM range was observed in cancer cell lines treated with N,N,N',N'-tetrabenzyl-4,5-diamino-2-cyclopentenone. Cell cycle analysis on HeLa cells showed a clear G1 phase arrest. A preliminary SAR on structural analogs was performed in order to identify the pharmacophores.

A serendipitous one-pot synthesis of the octahydro-2H-pyrazino[1,2-a]pyrazine core.

An unexpected nitro group displacement during a nitro-Mannich reaction led to the one-pot formation of the octahydro-2H-pyrazino[1,2-a]pyrazine core, representing the shortest access to date to this pharmacologically relevant heterobicyclic system. A mechanistic hypothesis is suggested and supported by specific experiments and HRMS analysis of reaction mixtures.

EHDTA: a green approach to efficient Ln3+-chelators.

The rich coordination chemistry of lanthanoid ions (Ln3+) is currently exploited in a vast and continuously expanding array of applications. Chelating agents are central in the development of Ln3+-complexes and in tuning their physical and chemical properties. Most chelators for Ln3+-complexation are derived from the macrocyclic DOTA or from linear DTPA platforms, both of which arise from fossil-based starting materials. Herein, we report a green and efficient approach to a chelating agent (EHDTA), derived from cheap and largely available furfurylamine. The oxygenated heterocycle of the latter is converted to a stereochemically defined and rigid heptadentate chelator, which shows good affinity towards Ln3+ ions. A combination of NMR, relaxometric, potentiometric and spectrophotometric techniques allows us to shed light on the interesting coordination chemistry of Ln3+-EHDTA complexes, unveiling a promising ligand for the chelation of this important family of metal ions.

N-arylbenzamides: extremely simple scaffolds for the development of novel estrogen receptor agonists.

The research of estrogen receptor (ER) ligands has benefited in the last decade from the implementation of combinatorial chemistry. The general pharmacophore has been identified and subsequently a multitude of compounds have been synthesized. Surprisingly, up to now simple amides have not been taken into consideration. Here we show that amides resulting from the condensation of hydroxybenzoic acids with aminophenols result in compounds retaining the pharmacophore structure of an ER ligand with a clear estrogenic activity.

Lower ligand denticity leading to improved thermodynamic and kinetic stability of the Gd3+ complex: the strange case of OBETA.

OBETA, OBETA, you bet: Thermodynamic and kinetic measurements show an apparent paradox. The stability of complexes of lanthanide trivalent ions is higher with the heptadentate ligand OBETA (ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid) than with its octadentate homologue EGTA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid). The unusual properties of Gd(OBETA)(-) (see structure), combined with the presence of two fast exchanging coordinated water molecules, candidates this complex as an MRI contrast agent.

Equilibrium and NMR relaxometric studies on the s-triazine-based heptadentate ligand PTDITA showing high selectivity for Gd3+ ions.

A complete potentiometric and NMR relaxometric solution study on the heptadentate 2,2',2″,2'″-[(6-piperidinyl-1,3,5-triazine-2,4-diyl)dihydrazin-2-yl-1-ylidene]tetraacetic acid (PTDITA) ligand has been carried out. This ligand is based on the 1,3,5-triazine ring with two hydrazine-N,N-diacetate groups in positions 2 and 4 and a piperidine moiety in position 6. The introduction of the triazine ring into the ligand backbone is expected to modify its flexibility and then to affect the stability of the corresponding complexes with transition-metal and lanthanide ions. Thermodynamic stabilities have been determined by pH potentiometry, UV spectrophotometry, and (1)H NMR spectroscopy for formation of the complexes with Mg(2+), Ca(2+), Cu(2+), Zn(2+), La(3+), Gd(3+), and Lu(3+) ions. PTDITA shows a good binding affinity for Gd(3+) (logK = 18.49, pGd = 18.6) and an optimal selectivity for Gd(3+) over the endogenous Ca(2+), Zn(2+), and Cu(2+) (K(sel) = 6.78 × 10(7)), which is 3 orders of magnitude higher that that reported for Gd(DTPA) (K(sel) = 2.85 × 10(4)). This is mainly due to the lower stability of the Cu(II)- and Zn(II)(PTDITA) complexes compared to the corresponding DTPA complexes, which suggests an important role of the triazine ring on the selectivity for the Gd(3+) ion. The relaxometric properties of Gd(PTDITA) have been investigated in aqueous solution by measuring the (1)H relaxivity as a function of the pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion profile). Variable-temperature (17)O NMR data have provided direct information on the kinetic parameters for exchange of the coordinated water molecules. A simultaneous fit of the data suggests that the high relaxivity value (r(1) = 10.2 mM(-1) s(-1)) is a result of the presence of two inner-sphere water molecules along with the occurrence of relatively slow rotation and electronic relaxation. The water residence lifetime, (298)τ(M) = 299 ns, is quite comparable to that of clinically approved magnetic resonance imaging contrast agents. The displacement of the inner-sphere water molecules by bidentate endogeneous anions (citrate, phosphate, and carbonate) has also been evaluated by (1)H relaxometry. In general, the binding interaction is markedly weak, and only in the case of citrate, a ca. 35% decrease in relaxivity was observed in the presence of 60 equiv of the anion. Phosphate and carbonate also interact with the paramagnetic ion, likely as monodentate ligands, but formation of the ternary complex is accompanied by a modest increase of r(1) due to the contribution of second-sphere water molecules.

Stevens rearrangement as a tool for the structural modification of polyaminopolycarboxylic ligands.

Polyaminopolycarboxylic acids are a well known class of ligands employed for metal ion complexation. Despite the large commercial availability, reports of their use as substrates for direct structural modifications are rare. Herein we report a simple and efficient protocol for the preparation of substituted polyaminopolycarboxylic ligands relying on a one-pot N-alkylation-Stevens rearrangement cascade.

Reductive amination with zinc powder in aqueous media.

Zinc powder in aqueous alkaline media was employed to perform reductive amination of aldehydes with primary amines. The corresponding secondary amines were obtained in good yields along with minor amounts of hydrodimerization byproducts. The protocol is a green alternative to the use of complex hydrides in chlorinated or highly flammable solvents.

Dramatic increase of selectivity for heavy lanthanide(III) cations by tuning the flexibility of polydentate chelators.

Two novel octadentate ligands have been synthesized by attaching two terminal iminodiacetic groups to either 1,4-diazepane (BCAED) or piperazine (BCAEP) as central scaffold. The introduction of the seven- or six-membered ring into the ligand backbone is expected to modify their overall flexibility and then to affect the stability of the corresponding lanthanide(III) complexes. In this work, thermodynamic stability data are determined for the formation of the complexes of BCAED and BCAEP with La(3+), Nd(3+), Eu(3+), Gd(3+), Ho(3+), and Lu(3+). The ligand BCAED shows a strong binding affinity for Lu(3+) (logK = 20.99), moderate for Gd(3+) (logK = 17.15) and rather weak for La(3+) (logK = 12.77). Thus, the variation of logK across the Ln series assumes the remarkable value of 8.22, the largest so far reported. This points to a predominant role of a suitable size match between the metal ion and the ligand cavity, determined by its structure and flexibility. The ligand BCAEP forms less stable complexes with lanthanide(III) cations although it retains a good selectivity (DeltalogK(La-Lu) = 5.66). The Gd(III) complexes have been investigated in aqueous solution by measuring their relaxivity as a function of pH, at 20 MHz and 25 degrees C. The results can be interpreted very well in terms of the species distribution curves calculated from the thermodynamic data and indicate that in these complexes Gd(3+) is octacoordinated, without any bound water molecule. This coordination geometry is maintained in the solid state as shown by the X-ray crystal structure of [Na(H(2)O)(2)][Gd(BCAED)] where the metal ion is at the center of a bicapped-trigonal prism. Finally, the (13)C NMR spectra (9.4 T, 25 degrees C) of the diamagnetic La(3+), Y(3+), and Lu(3+) complexes show that a pronounced stereochemical rigidity is associated with the thermodynamically more stable complexes.

Interaction of macrocyclic gadolinium-based MR contrast agents with Type I collagen. Equilibrium and kinetic studies.

The interactions of gadoterate meglumine, gadobutrol, gadoteridol and Gd(HB-DO3A) with bovine Type I collagen were investigated by ultrafiltration and dialysis. The affinity of the four agents to collagen is similar. However, the maximum adsorbed amount of GdIII-complexes decreases in the following order: gadoterate meglumine > gadobutrol > gadoteridol > Gd(HB-DO3A). Calculations with the open three-compartment model reveal that the structural homologs gadoteridol and Gd(HB-DO3A) have a lower adsorption onto collagen, which may explain the less prolonged in vivo retention of gadoteridol observed in soft tissues of rats.