Application of the Ugi four-component reaction to the synthesis of ditopic bifunctional chelating agents

Programming heterometallic 4f-4f' helicates under thermodynamic control: the circle is complete

Three non-symmetrical segmental ligand strands L4 can be wrapped around a linear sequence of one Zn2+ and two trivalent lanthanide cations Ln3+ to give quantitatively directional [ZnLn2(L4)3]8+ triple-stranded helicates in the solid state and in solution. NMR speciations in CD3CN show negligible decomplexation at a millimolar concentration and the latter helicate can be thus safely considered as a preorganized C3-symmetrical HHH-[(L43Zn)(LnA)(2-n)(LnB)n]8+ platform in which the thermodynamic properties of (i) lanthanide permutation between the central N9 and the terminal N6O3 binding sites and (ii) exchange processes between homo- and heterolanthanide helicates are easy to access (Ln = La, Eu, Lu). Deviations from statistical distributions could be programmed by exploiting specific site recognition and intermetallic pair interactions. Considering the challenging La3+ : Eu3+ ionic pair, for which the sizes of the two cations differ by only 8%, a remarkable excess (70%) of the heterolanthanide is produced, together with a preference for the formation of the isomer where the largest lanthanum cation lies in the central N9 site ([(La)(Eu)] : [(Eu)(La)] = 9 : 1). This rare design and its rational programming pave the way for the preparation of directional light-converters and/or molecular Q-bits at the (supra)molecular level.

Distributive Nd-to-Yb Energy Transfer within Pure [YbNdYb] Heterometallic Molecules

Facile access to site-selective hetero-lanthanide molecules will open new avenues in the search of novel photophysical phenomena based on Ln-to-Ln' energy transfer (ET). This challenge demands strategies to segregate efficiently different Ln metal ions among different positions in a molecule. We report here the one-step synthesis and structure of a pure [YbNdYb] (1) coordination complex featuring short Yb···Nd distances, ideal to investigate a potential distributive (i.e., from one donor to two acceptors) intramolecular ET from one Nd³⁺ ion to two Yb³⁺ centers within a well-characterized molecule. The difference in ionic radius is the mechanism allowing to allocate selectively both types of metal ion within the molecular structure, exploited with the simultaneous use of two β-diketone-type ligands. To assist the photophysical investigation of this heterometallic species, the analogues [YbLaYb] (2) and [LuNdLu] (3) have also been prepared. Sensitization of Yb³⁺ and Nd³⁺ in the last two complexes, respectively, was observed, with remarkably long decay times, facilitating the determination of the Nd-to-Yb ET within the [YbNdYb] composite. This ET was demonstrated by comparing the emission of iso-absorbant solutions of 1, 2, and 3 and through lifetime determinations in solution and solid state. The comparatively high efficiency of this process corroborates the facilitating effect of having two acceptors for the nonradiative decay of Nd³⁺ created within the [YbNdYb] molecule.

Versatile Para-Substituted Pyridine Lanthanide Coordination Complexes Allow Late Stage Tailoring of Complex Function

A series of cationic and neutral p-Br and p-NO2 pyridine substituted Eu(III) and Gd(III) coordination complexes serve as versatile synthetic intermediates. Nucleophilic aromatic substitution occurs readily at the para position under mild conditions, allowing C-N and C-C bond forming reactions to take place, permitting the introduction of azide, amino and alkynyl substituents. For Eu(III) complexes, this approach allows late stage tuning of absorption and emission spectral properties, exemplified by the lowering of the energy of an LMCT transition accompanied by a reduction in the Eu-Npy bond length. Additionally, these complexes provide direct access to the corresponding Eu(II) analogues. With the Gd(III) series, the nature of the p-substituent does not significantly change the EPR properties (linewidth, relaxation times), as required for their development as EPR spin probes that can be readily conjugated to biomolecules under mild conditions.

Rigid and Compact Binuclear Bis-hydrated Gd-complexes as High Relaxivity MRI Agents

The first binuclear Gd‐complex of the 12‐membered pyridine‐based polyaminocarboxylate macrocyclic ligand PCTA was synthesized by C−C connection of the pyridine units through two different synthetic procedures. A dimeric AAZTA‐ligand was also synthesized with the aim to compare the relaxometric results or the two ditopic Gd‐complexes. Thus, the ¹H relaxometric study on [Gd₂PCTA₂(H₂O)₄] and on [Gd₂AAZTA₂(H₂O)₄]²⁻ highlighted the remarkable rigidity and compactness of the two binuclear complexes, which results in molar relaxivities (per Gd), at 1.5 T and 298 K of ca. 12–12.6 mM⁻¹ s⁻¹ with an increase of ca. 80 % at 1.5 T and 298 K (+70 % at 310 K) with respect to the corresponding mononuclear complexes.

Towards Enhanced MRI Performance of Tumor-Specific Dimeric Phenylboronic Contrast Agents

It is known that phenylboronic acid (PBA) can target tumor tissues by binding to sialic acid, a substrate overexpressed by cancer cells. This capability has previously been explored in the design of targeting diagnostic probes such as Gd- and ⁶⁸Ga-DOTA-EN-PBA, two contrast agents for magnetic resonance imaging (MRI) and positron emission tomography (PET), respectively, whose potential has already been demonstrated through in vivo experiments. In addition to its high resolution, the intrinsic low sensitivity of MRI stimulates the search for more effective contrast agents, which, in the case of small-molecular probes, basically narrows down to either increased tumbling time of the entire molecule or elevated local concentration of the paramagnetic ions, both strategies resulting in enhanced relaxivity, and consequently, a higher MRI contrast. The latter strategy can be achieved by the design of multimeric Gd^III complexes. Based on the monomeric PBA-containing probes described recently, herein, we report the synthesis and characterization of the dimeric analogues (Gd^III-DOTA-EN)₂-PBA and (Gd^III-DOTA-EN)₂F₂PBA. The presence of two Gd ions in one molecule clearly contributes to the improved biological performance, as demonstrated by the relaxometric study and cell-binding investigations.

Design and Synthesis of Luminescent Lanthanide-Based Bimodal Nanoprobes for Dual Magnetic Resonance (MR) and Optical Imaging

Current biomedical imaging techniques are crucial for the diagnosis of various diseases. Each imaging technique uses specific probes that, although each one has its own merits, do not encompass all the functionalities required for comprehensive imaging (sensitivity, non-invasiveness, etc.). Bimodal imaging methods are therefore rapidly becoming an important topic in advanced healthcare. This bimodality can be achieved by successive image acquisitions involving different and independent probes, one for each mode, with the risk of artifacts. It can be also achieved simultaneously by using a single probe combining a complete set of physical and chemical characteristics, in order to record complementary views of the same biological object at the same time. In this scenario, and focusing on bimodal magnetic resonance imaging (MRI) and optical imaging (OI), probes can be engineered by the attachment, more or less covalently, of a contrast agent (CA) to an organic or inorganic dye, or by designing single objects containing both the optical emitter and MRI-active dipole. If in the first type of system, there is frequent concern that at some point the dye may dissociate from the magnetic dipole, it may not in the second type. This review aims to present a summary of current activity relating to this kind of dual probes, with a special emphasis on lanthanide-based luminescent nano-objects.

Two decades of recent advances of Ugi reactions: synthetic and pharmaceutical applications

Multicomponent reactions (MCRs) are powerful synthetic tools in which more than two starting materials couple with each other to form multi-functionalized compounds in a one-pot process, the so-called “tandem”, “domino” or “cascade” reaction, or utilizing an additional step without changing the solvent, the so-called a sequential-addition procedure, to limit the number of synthetic steps, while increasing the complexity and the molecular diversity, which are highly step-economical reactions. The Ugi reaction, one of the most common multicomponent reactions, has recently fascinated chemists with the high diversity brought by its four- or three-component-based isonitrile. The Ugi reaction has been introduced in organic synthesis as a novel, efficient and useful tool for the preparation of libraries of multifunctional peptides, natural products, and heterocyclic compounds with stereochemistry control. In this review, we highlight the recent advances of the Ugi reaction in the last two decades from 2000–2019, mainly in the synthesis of linear or cyclic peptides, heterocyclic compounds with versatile ring sizes, and natural products, as well as the enantioselective Ugi reactions. Meanwhile, the applications of these compounds in pharmaceutical trials are also discussed.

We highlight the recent advances of the Ugi reaction in the last two decades from 2000–2019, mainly in the synthesis of linear or cyclic peptides, heterocyclic compounds with versatile ring sizes, and natural products, as well as the enantioselective Ugi reactions.

Optimizing the Relaxivity of MRI Probes at High Magnetic Field Strengths With Binuclear GdIII Complexes

The key criteria to optimize the relaxivity of a Gd(III) contrast agent at high fields (defined as the region ≥ 1.5 T) can be summarized as follows: (i) the occurrence of a rotational correlation time τ_R in the range of ca. 0.2–0.5 ns; (ii) the rate of water exchange is not critical, but a τ_M < 100 ns is preferred; (iii) a relevant contribution from water molecules in the second sphere of hydration. In addition, the use of macrocycle-based systems ensures the formation of thermodynamically and kinetically stable Gd(III) complexes. Binuclear Gd(III) complexes could potentially meet these requirements. Their efficiency depends primarily on the degree of flexibility of the linker connecting the two monomeric units, the absence of local motions and the presence of contribution from the second sphere water molecules. With the aim to maximize relaxivity (per Gd) over a wide range of magnetic field strengths, two binuclear Gd(III) chelates derived from the well-known macrocyclic systems DOTA-monopropionamide and HPDO3A (Gd₂L1 and Gd₂L2, respectively) were synthesized through a multistep synthesis. Chemical Exchange Saturation Transfer (CEST) experiments carried out on Eu₂L2 at different pH showed the occurrence of a CEST effect at acidic pH that disappears at neutral pH, associated with the deprotonation of the hydroxyl groups. Then, a complete ¹H and ¹⁷O NMR relaxometric study was carried out in order to evaluate the parameters that govern the relaxivity associated with these complexes. The relaxivities of Gd₂L1 and Gd₂L2 (20 MHz, 298 K) are 8.7 and 9.5 mM⁻¹ s⁻¹, respectively, +77% and +106% higher than the relaxivity values of the corresponding mononuclear GdDOTAMAP-En and GdHPDO3A complexes. A significant contribution of second sphere water molecules was accounted for the strong relaxivity enhancement of Gd₂L2. MR phantom images of the dinuclear complexes compared to GdHPDO3A, recorded at 7 T, confirmed the superiority of Gd₂L2. Finally, ab initio (DFT) calculations were performed to obtain information about the solution structure of the dinuclear complexes.

Synthesis of an Amphiphilic Bis-Aqua Gd(OBETA) Complex for the Preparation of High-Relaxivity Supramolecular Magnetic Resonance Imaging Probes

Prompted by the favourable relaxometric, thermodynamic and kinetic properties of the bis-hydrated Gd(OBETA) (OBETA=2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid) complex, a novel derivative tailored with an n-hexadecyl chain was synthesised. The amphiphilic gadolinium complex was designed and prepared with the aim of obtaining high relaxivity supramolecular aggregates by self-assembly in micelles and liposomes. Thus, lipidic nanoparticles were prepared and characterised by dynamic light scattering and ¹ H NMR relaxometry. Relaxivity values of up to 48.3 mm^-1  s^-1 (20 MHz and 298 K) were registered in liposomal aggregates. The binding to human serum albumin (HSA), evaluated both in terms of affinity and relaxometric properties of the supramolecular adduct, yielded exceptionally high relaxivity values (71.4 mm^-1  s^-1 at 30 MHz and 298 K).

Utility of tris(4-bromopyridyl) europium complexes as versatile intermediates in the divergent synthesis of emissive chiral probes

The tribromopyridyl complex is a versatile late intermediate in the divergent synthesis of emissive triazole and aryl–alkynyl substituted complexes.

Towards polymetallic lanthanide complexes as dual contrast agents for magnetic resonance and optical imaging

In the spotlight: polymetallic complexes permitting efficient sensitization of lanthanide luminescence and exhibiting favorable relaxometric properties.

Enhanced tumor retention of a radiohalogen label for site-specific modification of antibodies

A known limitation of iodine radionuclides for labeling and biological tracking of receptor targeted proteins is the tendency of iodotyrosine to rapidly diffuse from cells following endocytosis and lysosomal degradation. In contrast, radiometal-chelate complexes such as indium-111-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (In-111-DOTA) accumulate within target cells due to the residualizing properties of the polar, charged metal-chelate-amino acid adduct. Iodine radionuclides boast a diversity of nuclear properties and chemical means for incorporation, prompting efforts to covalently link radioiodine with residualizing molecules. Herein, we describe the Ugi-assisted synthesis of [I-125]HIP-DOTA, a 4-hydroxy-3-iodophenyl (HIP) derivative of DOTA, and demonstration of its residualizing properties in a murine xenograft model. Overall, this study displays the power of multicomponent synthesis to yield a versatile radioactive probe for antibodies across multiple therapeutic areas with potential applications in both preclinical biodistribution studies and clinical radioimmunotherapies.

A new ditopic Gd(III) complex functionalized with an adamantyl moiety as a versatile building block for the preparation of supramolecular assemblies

A dimeric GdAAZTA-like complex (AAZTA is 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) bearing an adamantyl group (Gd2L1) able to form strong supramolecular adducts with specific hosts such as β-cyclodextrin (β-CD), poly-β-CD, and human serum albumin (HSA) is reported. The relaxometric properties of Gd2L1 were investigated in aqueous solution by measuring the (1)H relaxivity as a function of pH, temperature, and magnetic field strength. The relaxivity of Gd2L1 (per Gd atom) at 40 MHz and 298 K is 17.6 mM(-1) s(-1), a value that remains almost constant at higher fields owing to the great compactness and rigidity of the bimetallic chelate, resulting in an ideal value for the rotational correlation time for high-field MRI applications (1.5-3.0 T). The noncovalent interaction of Gd2L1 with β-CD, poly-β-CD, and HSA and the relaxometric properties of the resulting host-guest adducts were investigated using (1)H relaxometric methods. Relaxivity enhancements of 29 and 108 % were found for Gd2L1-β-CD and Gd2L1-poly-β-CD, respectively. Binding of Gd2L1 to HSA (KA = 1.2 × 10(4) M(-1)) results in a remarkable relaxivity of 41.4 mM(-1) s(-1) for the bound form (+248 %). The relaxivity is only limited by the local rotation of the complex within the binding site, which decreases on passing from Gd2L1-β-CD to Gd2L1-HSA. Finally, the applicability of Gd2L1 as tumor-targeting agent through passive accumulation of the HSA-bound adduct was evaluated via acquisition of magnetic resonance images at 1 T of B16-tumor-bearing mice. These experiments indicate a considerable signal enhancement (+160 %) in tumor after 60 min from the injection and a very low hepatic accumulation.

Target visualization by MRI using the avidin/biotin amplification route: synthesis and testing of a biotin-Gd-DOTA monoamide trimer

To design efficient targeting strategies in magnetic resonance (MR) molecular imaging applications, the formation of supramolecular adducts between (strept)avidin ((S)Av) and tribiotinylated Gd-DOTA-monoamide complexes (DOTA=1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) was explored. Two compounds based on the trivalent core of tris(2-aminoethyl)amine each containing three biotin molecules and one (L1) or three (L2) DOTA-monoamide (DOTAMA) ligands were synthesized. In these tribiotinylated derivatives the biotins are spaced far enough apart to allow the formation of the supramolecular adduct with the protein and to host the chelating units in between the (S)Av layers. Size exclusion HPLC analyses indicated complete formation of very high molecular weight polymers (>2 MDa) with (S)Av in solution. A (1)H NMR spectroscopy relaxometric study on the obtained polymeric adducts showed a marked increase of the relaxivity at 35-40 MHz as a consequence of the lengthening of the tumbling time due to the formation of Gd-chelates/(S)Av polymers. The most efficient Gd(3)L2/(S)Av polymeric system was used for a test in cell cultures. The target is represented by a neural cell adhesion molecule (NCAM), which is overexpressed in Kaposi's sarcoma cells and tumor endothelial cells (TEC) and that is efficiently recognized by a biotinylated tetrameric peptide (C3d-Bio). In vitro experiments showed that only cells incubated with both C3d-Bio and Gd(3)L2/SAv polymer were hyperintense with respect to the control. Relaxation rates of cell pellets incubated with Gd(3)L2/SAv alone were not significantly different from the untreated cells demonstrating the absence of a specific binding.