Chiral DOTA chelators as an improved platform for biomedical imaging and therapy applications

Development of Ln(III) Derivatives as 19F Parashift Probes

19F parashift probes with paramagnetically shifted reporter nuclei provide attractive platforms to develop molecular imaging probes. These probes enable ratiometric detection of molecular disease markers using a direct detection technique. Here, we describe a series of trivalent lanthanide (Ln(III)) complexes that are structural analogues of the clinically approved MR contrast agent (CA) ProHance to obtain LnL 19F parashift probes. We evaluated trans-gadolinium paramagnetic lanthanides compared to diamagnetic YL for 19F chemical shift and relaxation rate enhancement. The paramagnetic contribution to chemical shift (δPCS) for paramagnetic LnL exhibited either shifts to lower frequency (δPCS < 0 for TbL, DyL, and HoL) or shifts to higher frequency (δPCS > 0 for ErL, TmL, and YbL) compared to YL 19F spectroscopic signal. Zero-echo time pulse sequences achieved 56-fold sensitivity enhancement for DyL over YL, while developing probe-specific pulse sequences with fast delay times and acquisition times achieved 0.6-fold enhancement in limit of detection for DyL. DyL provides an attractive platform to develop 19F parashift probes for ratiometric detection of enzymatic activity.

Chiral Pyclen-Based Heptadentate Chelates as Highly Stable MRI Contrast Agents

In recent years, pyclen-based complexes have attracted a great deal of interest as magnetic resonance imaging (MRI) contrast agents (CAs) and luminescent materials, as well as radiopharmaceuticals. Remarkably, gadopiclenol, a Gd(III) bishydrated complex featuring a pyclen-based heptadentate ligand, received approval as a novel contrast agent for clinical MRI application in 2022. To maximize stability and efficiency, two novel chiral pyclen-based chelators and their complexes were developed in this study. Gd-X-PCTA-2 showed significant enhancements in both thermodynamic and kinetic stabilities compared to those of the achiral parent derivative Gd-PCTA. 1H NMRD profiles reveal that both chiral gadolinium complexes (Gd-X-PCTA-1 and Gd-X-PCTA-2) have a higher relaxivity than Gd-PCTA, while variable-temperature 17O NMR studies show that the two inner-sphere water molecules have distinct residence times τMa and τMb. Furthermore, in vivo imaging demonstrates that Gd-X-PCTA-2 enhances the signal in the heart and kidneys of the mice, and the chiral Gd complexes exhibit the ability to distinguish between tumors and normal tissues in a 4T1 mouse model more efficiently than that of the clinical agent gadobutrol. Biodistribution studies show that Gd-PCTA and Gd-X-PCTA-2 are primarily cleared by a renal pathway, with 24 h residues of Gd-X-PCTA-2 in the liver and kidney being lower than those of Gd-PCTA.

Controlling the broadband enhanced light chirality with L-shaped dielectric metamaterials

The inherently weak chiroptical responses of natural materials limit their usage for controlling and enhancing chiral light-matter interactions. Recently, several nanostructures with subwavelength scale dimensions were demonstrated, mainly due to the advent of nanofabrication technologies, as a potential alternative to efficiently enhance chirality. However, the intrinsic lossy nature of metals and the inherent narrowband response of dielectric planar thin films or metasurface structures pose severe limitations toward the practical realization of broadband and tailorable chiral systems. Here, we tackle these problems by designing all-dielectric silicon-based L-shaped optical metamaterials based on tilted nanopillars that exhibit broadband and enhanced chiroptical response in transmission operation. We use an emerging bottom-up fabrication approach, named glancing angle deposition, to assemble these dielectric metamaterials on a wafer scale. The reported strong chirality and optical anisotropic properties are controllable in terms of both amplitude and operating frequency by simply varying the shape and dimensions of the nanopillars. The presented nanostructures can be used in a plethora of emerging nanophotonic applications, such as chiral sensors, polarization filters, and spin-locked nanowaveguides.

Using Chiral Auxiliaries to Mimic the Effect of Chiral Media on the Structure of Lanthanide(III) Complexes Common in Bioimaging and Diagnostic MRI

[Ln·DOTA]- complexes and systems derived therefrom are commonly used in MRI and optical bioimaging. These lanthanide(III) complexes are chiral, and, in solution, they are present in four forms, with two sets of enantiomers, with the ligand donors arranged in either a square antiprismatic, SAP, or twisted square antiprismatic geometry, TSAP. This complicated speciation is found in laboratory samples. To investigate speciation in biological media, when Ln·DOTA-like complexes interact with chiral biomolecules, six Eu·DOTA-monoamide complexes were prepared and investigated by using 1D and 2D 1H NMR. To emulate the chirality of biological media, the amide pendant arm was modified with one or two chiral centers. It is known that a chiral center on the DOTA scaffold significantly influences the properties of the system. Here, it was found that chirality much further away from the metal center changes the available conformational space and that both chiral centers and amide cis/trans isomerism may need to be considered─a fact that, for the optically enriched materials, led to the conclusion that eight chemically different forms may need to be considered, instead of the four forms necessary for DOTA. The results reported here clearly demonstrate the diverse speciation that must be considered when correlating an observation to a structure of a lanthanide(III) complex.

Decoration of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) with N-oxides increases the T1 relaxivity of Gd-complexes

High complex stability and longitudinal relaxivity of Gd-based contrast agents are important requirements for magnetic resonance imaging (MRI) because they ensure patient safety and contribute to measurement sensitivity. Charged and zwitterionic Gd3+ -complexes of the well-known chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) provide an excellent basis for the development of safe and sensitive contrast agents. In this report, we describe the synthesis of DOTA-NOx, a DOTA derivative with four N-oxide functionalities via "click" functionalization of the tetraazide DOTAZA. The resulting complexes Gd-DOTA-NOx and Eu-DOTA-NOx are stable compounds in aqueous solution. NMR-spectroscopic characterization revealed a high excess of the twisted square antiprismatic (TSAP) coordination geometry over square antiprismatic (SAP). The longitudinal relaxivity of Gd-DOTA-NOx was found to be r1 =7.7 mm-1  s-1 (1.41 T, 37 °C), an unusually high value for DOTA complexes of comparable weight. We attribute this high relaxivity to the steric influence and an ordering effect on outer sphere water molecules surrounding the complex generated by the strongly hydrated N-oxide groups. Moreover, Gd-DOTA-NOx was found to be stable against transchelation with high excess of EDTA (200 eq) over a period of 36 h, and it has a similar in vitro cell toxicity as clinically used DOTA-based GBCAs.

Design and synthesis of chiral DOTA-based MRI contrast agents with remarkable relaxivities

Due to the adverse effects of de-metallation in past concerning FDA-approved gadolinium-based contrast agents (GBCAs), researchers have been focusing on developing safer and more efficient alternatives that could avoid toxicity caused by free gadolinium ions. Herein, two chiral GBCAs, Gd-LS with sulfonate groups and Gd-T with hydroxyl groups, are reported as potential candidates for magnetic reasonance imaging (MRI). The r1 relaxivities of TSAP, SAP isomers of Gd-LS and SAP isomer of Gd-T at 1.4 T, 37 °C in water are 7.4 mM-1s-1, 14.5 mM-1s-1 and 5.2 mM-1s-1, respectively. Results show that the hydrophilic functional groups introduced to the chiral macrocyclic scaffold of Gd-T and Gd-LS both give constructive influences on the second-sphere relaxivity and enhance the overall r1 value. Both cases indicate that the design of GBCAs should also focus on the optimal window in Solomon-Bloembergen-Morgan (SBM) theory and the effects caused by the second-sphere and outer-sphere relaxivity.

Chiral Gd-DOTA as a Versatile Platform for Hepatobiliary and Tumor Targeting MRI Contrast Agents

The leakage of gadolinium ions (Gd3+) from commercial Gd3+-based contrast agents (GBCAs) in patients is currently the major safety concern in clinical magnetic resonance imaging (MRI) scans, and the lack of task-specific GBCAs limits its usage in the early detection of disease and imaging of specific biological regions. Herein, ultrastable GBCAs were constructed via decorating chiral Gd-DOTA with a phenylic analogue to one of the pendent arms, and the stability constant was determined as high as 27.08, accompanied by negligible decomplexation in 1 M of HCl over 2 years. A hepatic-specific chiral Gd-DOTA was screened out as a potential alternative to commercial Gd-EOB-DTPA, while combination with functional molecules favored chiral Gd-DOTA as tumor targeting probes. Therefore, the novel chiral Gd-DOTA is believed to be an ideal platform for designing the next generation of GBCAs for various clinical purposes due to its outstanding inert nature.

Circularly polarized luminescence in the one-dimensional assembly of binaphtyl-based Yb(iii) single-molecule magnets

Lanthanide ions have attracted great interest owing to their optical and magnetic properties. Single-molecule magnet (SMM) behavior has been a fascinating science for thirty years. Moreover, chiral lanthanide complexes allow the observation of remarkable circularly polarized luminescence (CPL). However, the combination of both SMM and CPL behaviors in a single molecular system is very rare and deserves attention in the design of multifunctional materials. Four chiral one-dimensional coordination compounds involving 1,1'-Bi-2-naphtol (BINOL)-derived bisphosphate ligands and the Yb(iii) centre were synthesized and characterized by powder and single-crystal X-ray diffraction. All the Yb(iii)-based polymers displayed field-induced SMM behavior with magnetic relaxation occurring by applying Raman processes and near infrared CPL in the solid state.

A Manganese Porphyrin Platform for the Design and Synthesis of Molecular and Targeted MRI Contrast Agents

Magnetic resonance imaging (MRI) contrast agents, in contrast to the plethora of fluorescent agents available to target disease biomarkers or exogenous implants, have remained predominantly non-specific. That is, they do not preferentially accumulate in specific locations in vivo because doing so necessitates longer contrast retention, which is contraindicated for current gadolinium (Gd) agents. This double-edge sword implies that Gd agents can offer either rapid elimination (but lack specificity) or targeted accumulation (but with toxicity risks). For this reason, MRI contrast agent innovation has been severely constrained. Gd-free alternatives based on manganese (Mn) chelates have been largely ineffective, as they are inherently unstable. In this study, we present a Mn(III) porphyrin (MnP) platform for bioconjugation, offering the highest stability and chemical versatility compared to any other T₁ contrast agent. We exploit the inherent metal stability conferred by porphyrins and the absence of pendant bases (found in Gd or Mn chelates) that limit versatile functionalization. As proof-of-principle, we demonstrate labeling of human serum albumin, a model protein, and collagen hydrogels for applications in in-vivo targeted imaging and material tracking, respectively. In-vitro and in-vivo results confirm unprecedented metal stability, ease of functionalization, and high T₁ relaxivity. This new platform opens the door to ex-vivo validation by fluorescent imaging and multipurpose molecular imaging in vivo.

Rigidified and Hydrophilic DOTA-like Lanthanoid Ligands: Design, Synthesis, and Dynamic Properties

Limiting the dynamics of paramagnetic tags is crucial for the accuracy of the structural information derived from paramagnetic nuclear magnetic resonance (NMR) experiments. A hydrophilic rigid 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA)-like lanthanoid complex was designed and synthesized following a strategy that allows the incorporation of two sets of two adjacent substituents. This resulted in a C₂ symmetric hydrophilic and rigid macrocyclic ring, featuring four chiral hydroxyl-methylene substituents. NMR spectroscopy was used to investigate the conformational dynamics of the novel macrocycle upon complexation with europium and compared to DOTA and its derivatives. The twisted square antiprismatic and square antiprismatic conformers coexist, but the former is favored, which is different from DOTA. Two-dimensional ¹H exchange spectroscopy shows that ring flipping of the cyclen-ring is suppressed due to the presence of the four chiral equatorial hydroxyl-methylene substituents at proximate positions. The reorientation of the pendant arms causes conformational exchange between two conformers. The reorientation of the coordination arms is slower when the ring flipping is suppressed. This indicates that these complexes are suitable scaffolds to develop rigid probes for paramagnetic NMR of proteins. Due to their hydrophilic nature, it is anticipated that they are less likely to cause protein precipitation than their more hydrophobic counterparts.

Gadolinium-hybridized mesoporous organosilica nanoparticles with high magnetic resonance imaging performance for targeted drug delivery

Magnetic resonance (MR) imaging techniques, which can provide images with excellent anatomical detail, are widely used in clinical diagnosis. However, the current clinical small molecule gadolinium (Gd) contrast agents have the defects of relatively low sensitivity and poor tumor-target specificity, preventing their adoption in biology and medicine. Herein, a facile synthetic strategy to fabricate gadolinium-hybridized mesoporous organosilica nanoparticles (MOSG) through a nanoprecipitation reaction, with the surface of nanoparticles grafted with the fluorescent dye isothiocyanate (FITC) and arginine-glycine-aspartic acid (RGD) for delivery of the antitumour drug doxorubicin hydrochloride (DOX), resulting in a high-performance nanotheranostic (RGD-MOSG-FITC/DOX) for targeted magnetic resonance imaging and chemotherapy of tumors. The prepared MOSG had a particle size of 60-80 nm and gadolinium elements were distributed in clusters that exhibited boosted longitudinal relaxivity. Routine blood tests and histopathology indicated good biocompatibility of MOSG. Furthermore, after being decorated with Arg-Gly-Asp peptide (RGD), RGD-MOSG-FITC demonstrated more preferable cellular uptake by HeLa cells (high expression of α_Ⅴβ₃) than MOSG without RGD grafting. Additionally, the tumor growth inhibition effect of RGD-MOSG-FITC/DOX was substantially more effective than that of the other groups. Therefore, this new delivery platform has good application potential in the field of tumor diagnosis and treatment.

Preparation of Patient Doses of [177Lu]Lu-DOTATATE and [177Lu]Lu-PSMA-617 with Carrier Added (CA) and No Carrier Added (NCA) 177Lu

Purpose

[177Lu]Lu-DOTATATE and [177Lu]Lu-PSMA-617 used for targeted radionuclide therapy are very often prepared in the hospital radiopharmacy. The preparation parameters vary depending upon the specific activity of the 177Lu used. The aim of this study was to develop optimized protocols to be used in the nuclear medicine department for the preparation of patient doses of the above radiopharmaceuticals.

Method

177Lu (CA and NCA) were used for radiolabeling DOTATATE and PSMA-617. Parameters studied are 177Lu of different specific activity and different peptide concentrations and two different buffer systems. Paper and thin layer chromatography systems were used for estimating the radiochemical yield as well as radiochemical purity. Solid-phase extraction was used for the purification of the labeled tracers.

Results

[177Lu]Lu-DOTATATE was prepared with CA 177Lu (n = 13) and NCA177Lu (n = 6). Four batches each of [177Lu]Lu-PSMA-617 were prepared using CA and NCA 177Lu. Radiochemical yields > 80% and final product with less than < 1% radiochemical impurity could be obtained in all batches which were used for therapy.

Conclusion

Robust protocols for the preparation of clinical doses of [177Lu]Lu-DOTATATE and [177Lu]Lu-PSMA-617 were developed and used for the preparation of clinical doses. The quality of the SPECT images of both the tracers are consistent with the expected uptake in respective diseases.

Long story short: donor set symmetry in [Eu(DOTA)(H2O)]- crystals determines the electronic structure

Lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid DOTA have been studied in great detail due to their use as MRI contrast agents. Since the first report from Desreux in 1980, the Ln[DOTA]^- complexes of gadolinium(III) in particular have been thoroughly investigated. The forms of the nine-coordinated [Ln(DOTA)(H₂O)]^- complexes are well known, and the ligand backbone has been used extensively to create functional MRI contrast agents, luminescent probes, and as a model system for studying the properties of lanthanide(III) ions. In solution, the photophysical properties have been mapped, but as the structures are not known, direct structure-property relationships have not been created. Here, the electronic properties of two Eu[DOTA] compounds (1 and 2) and a Eu[DOTA]-like compound (3) were studied using single-crystal luminescence spectroscopy. The donor set in the three compounds is identical (4N 4O 1O), and using the symmetry deviation value σ_ideal it was shown that the coordination geometry is close to identical. Nevertheless, the electronic properties evaluated using the luminescence spectrum were found to differ significantly between the three compounds. The magnitude of the crystal field splitting was found not to scale with the symmetry of the coordination geometry. It was concluded that the donor set dictates the splitting, yet the structure-property relationships governing the electronic properties of europium(III) ions still elude us.

China’s radiopharmaceuticals on expressway: 2014–2021

This review provides an essential overview on the progress of rapidly-developing China’s radiopharmaceuticals in recent years (2014–2021). Our discussion reflects on efforts to develop potential, preclinical, and in-clinical radiopharmaceuticals including the following areas: (1) brain imaging agents, (2) cardiovascular imaging agents, (3) infection and inflammation imaging agents, (4) tumor radiopharmaceuticals, and (5) boron delivery agents (a class of radiopharmaceutical prodrug) for neutron capture therapy. Especially, the progress in basic research, including new radiolabeling methodology, is highlighted from a standpoint of radiopharmaceutical chemistry. Meanwhile, we briefly reflect on the recent major events related to radiopharmaceuticals along with the distribution of major R&D forces (universities, institutions, facilities, and companies), clinical study status, and national regulatory supports. We conclude with a brief commentary on remaining limitations and emerging opportunities for China’s radiopharmaceuticals.

Versatile Macrocyclic Platform for the Complexation of [natY/90Y]Yttrium and Lanthanide Ions

We report a macrocyclic ligand (H₃L⁶) based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing three acetate pendant arms and a benzyl group attached to the fourth nitrogen atom of the macrocycle. The X-ray structures of the YL⁶ and TbL⁶ complexes reveal nine coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and three oxygen atoms of the carboxylate pendants. A combination of NMR spectroscopic studies (¹H, ¹³C, and ⁸⁹Y) and DFT calculations indicated that the structure of the YL⁶ complex in the solid state is maintained in an aqueous solution. The detailed study of the emission spectra of the EuL⁶ and TbL⁶ complexes revealed Ln³⁺-centered emission with quantum yields of 7.0 and 60%, respectively. Emission lifetime measurements indicate that the ligand offers good protection of the metal ions from surrounding water molecules, preventing the coordination of water molecules. The YL⁶ complex is remarkably inert with respect to complex dissociation, with a lifetime of 1.7 h in 1 M HCl. On the other hand, complex formation is fast (∼1 min at pH 5.4, 2 × 10^–5 M). Studies using the ⁹⁰Y-nuclide confirmed fast radiolabeling since [⁹⁰Y]YL⁶ is nearly quantitatively formed (radiochemical yield (RCY) > 95) in a short time over a broad range of pH values from ca. 2.4 to 9.0. Challenging experiments in the presence of excess ethylenediaminetetraacetic acid (EDTA) and in human serum revealed good stability of the [⁹⁰Y]YL⁶ complex. All of these experiments combined suggest the potential application of H₃L⁶ derivatives as Y-based radiopharmaceuticals.

A new versatile asymmetric ligand based on an 18-membered macrocycle possessing three acetate pendant arms and a benzyl moiety provides fast complexation with both ^natY(III) and ⁹⁰Y(III), as well as slow dissociation kinetics. A detailed structural study in the solid state and in solution evidences nona coordination of the metal ion by the ligand, offering good protection from solvent water molecules. These favorable properties make this ligand an attractive candidate to develop yttrium-based radiopharmaceuticals.

Strong Circularly Polarized Luminescence at 1550 nm from Enantiopure Molecular Erbium Complexes

Circularly polarized luminescence (CPL) in two subregions of the near-infrared (NIR) has been achieved. By leveraging the rigidity and diminishing detrimental vibrations of the heterobimetallic binolate complexes of erbium [(Binol)₃ErNa₃], species exhibiting an exceptionally high dissymmetry factor (|g_lum |) of 0.47 at 1550 nm were obtained. These erbium complexes are the first reported examples of CPL observed beyond 1200 nm. Analogous complexes of ytterbium and neodymium also exhibited strong CPL (|g_lum| = 0.17, 0.05, respectively) in a higher energy NIR window (800-1200 nm). All complexes exhibit high quantum yields (Er: 0.58%, Yb: 17%, Nd: 9.3%) and high B_CPL values (Er: 57 M^-1 cm^-1, Yb: 379 M^-1 cm^-1, Nd: 29 M^-1 cm^-1). Because of their strong CPL emission in the telecom band (1550 nm), biologically relevant NIR emission window (800-1100 nm), and synthetic versatility, the complexes reported here could permit further promising developments in quantum communication technologies and biologically relevant sensors.

A New Generation of Ultrasmall Nanoparticles Inducing Sensitization to Irradiation and Copper Depletion to Overcome Radioresistant and Invasive Cancers

An emerging target to overcome cancer resistance to treatments is copper, which is upregulated in a wide variety of tumors and may be associated with cancer progression and metastases. The aim of this study was to develop a multimodal ultrasmall nanoparticle, CuPRiX, based on the clinical AGuIX nanoparticle made of the polysiloxane matrix on which gadolinium chelates are grafted. Such hybrid nanoparticles allow: (i) a localized depletion of copper in tumors to prevent tumor cell dissemination and metastasis formation and (ii) an increased sensitivity of the tumor to radiotherapy (RT) due to the presence of high Z gadolinium (Gd) atoms. CuPRiX nanoparticles are obtained by controlled acidification of AGuIX nanoparticles. They were evaluated in vitro on two cancer cell lines (lung and head and neck) using the scratch-wound assay and clonogenic cell survival assay. They were able to reduce cell migration and invasion and displayed radiosensitizing properties.

Albumin-Based Therapeutics Capable of Glutathione Consumption and Hydrogen Peroxide Generation for Synergetic Chemodynamic and Chemotherapy of Cancer

A nanoscale therapeutic system with good biocompatibility was facilely fabricated by the coassembly of human serum albumin and glucose oxidase (GOD), where the former was pretreated with metal ions through a chelating agent or the chemotherapeutic prodrug oxaliplatin (Oxa(IV)). Among different chelating metal ions used, Mn²⁺ ion was selected to produce hydroxyl radical (•OH) efficiently through Fenton-like reaction, while GOD loaded in the system was able to generate a large amount of hydrogen peroxide for promoting efficient conversion into highly toxic •OH. In the meanwhile, the conversion of the Oxa(IV) prodrug into chemotherapeutic Oxa(II) was beneficial for the consumption of glutathione, thereby enhancing the chemodynamic therapy (CDT) efficacy. Based on the combined chemotherapy and CDT, the treatment with this system leads to superior antitumor outcome.

How the Chemical Properties of GBCAs Influence Their Safety Profiles In Vivo

The extracellular class of gadolinium-based contrast agents (GBCAs) is an essential tool for clinical diagnosis and disease management. In order to better understand the issues associated with GBCA administration and gadolinium retention and deposition in the human brain, the chemical properties of GBCAs such as relative thermodynamic and kinetic stabilities and their likelihood of forming gadolinium deposits in vivo will be reviewed. The chemical form of gadolinium causing the hyperintensity is an open question. On the basis of estimates of total gadolinium concentration present, it is highly unlikely that the intact chelate is causing the T1 hyperintensities observed in the human brain. Although it is possible that there is a water-soluble form of gadolinium that has high relaxitvity present, our experience indicates that the insoluble gadolinium-based agents/salts could have high relaxivities on the surface of the solid due to higher water access. This review assesses the safety of GBCAs from a chemical point of view based on their thermodynamic and kinetic properties, discusses how these properties influence in vivo behavior, and highlights some clinical implications regarding the development of future imaging agents.

Structural dynamism of chiral sodium peraza-macrocycle complexes derived from cyclic peptoids

A variety of cyclen and hexacyclen derivatives decorated with (S)-1-phenylethyl side chains or (S)-pyrrolidine units have been prepared via a reductive approach from the corresponding cyclic peptoids containing N-(S)-(1-phenylethyl)glycine and l-proline residues. Spectroscopic and DFT studies on their Na⁺ complexes show that point chirality and ring size play a crucial role in controlling the structural dynamism of 1,2-diaminoethylene units and pendant arms. The detection of highly symmetric C₄- and C₃-symmetric metalated species demonstrates that a full understanding of the relationship between the structure and conformational properties of peraza-macrocyclic metal complexes is possible.

Design of Functional Chiral Cyclen-Based Radiometal Chelators for Theranostics

A series of water-soluble chiral cyclen-based chelators with chemical handles for selective targeting have been synthesized (cyclen = 1,4,7,10-Tetraazacyclododecane). Optical studies, relaxivity measurements, and competitive titrations were performed to show the versatility of these chiral chelators. The complexations of L3, L4, and L5 with Lu³⁺, Y³⁺, Sc^3+, and Cu²⁺ were successfully demonstrated in around 90% to 100% yields. Efficient and rapid radiolabeling of L5 with ¹⁷⁷Lu was achieved under mild conditions with 96% yield. The chelators exhibit near quantitative labeling efficiencies with a wide range of radiometal ions, which are promising for the development of targeting specific radiopharmaceutical and molecular magnetic resonance imaging contrast agents.

Origin of the Isomer Stability of Polymethylated DOTA Chelates Complexed with Ln3+ ions

DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-based chelates that give only a single isomer in solution when complexed with lanthanide (Ln³⁺) ions is of value for studying protein dynamics and interactions via NMR. Herein, we have investigated the geometries, energetics, and electrostatic potentials of Lu complexed with DOTA (1), ring methylated M4DOTA (2), and arm methylated R-DOTMA (3) and S-DOTMA (4), as well as, both ring and arm methylated 4S-4S-M4DOTMA (5) and 4S-4R-M4DOTMA (6) at the level of M06-L/6-31+G(d)-SDD, to elucidate the origin of the isomer stability. These analyses indicate that the electrostatic repulsion between the arm methyl and the neighboring carboxylate significantly destabilizes the square antiprism (SAP) isomer of Lu-5 and the twisted square antiprism (TSAP) isomer of Lu-6, while the steric repulsion between the ring and arm methyl groups attenuates the stability of both TSAP of Lu-5 and SAP of Lu-6. To rationalize the variable temperature proton NMR spectra, the energy barriers for the inter-conversion in Lu-5 and Lu-6 via arm rotation were also calculated. The modulation of the stability and rigidity of Ln complexes via a modification of DOTA is also discussed. Our investigation will aid to design better chelates for the Ln³⁺ ions for its use in molecular medicine.

Unusual Magnetic Field Responsive Circularly Polarized Luminescence Probes with Highly Emissive Chiral Europium(III) Complexes

Chirality is ubiquitous within biological systems where many of the roles and functions are still undetermined. Given this, there is a clear need to design and develop sensitive chiral optical probes that can function within a biological setting. Here we report the design and synthesis of magnetically responsive Circularly Polarized Luminescence (CPL) complexes displaying exceptional photophysical properties (quantum yield up to 31 % and |glum | up to 0.240) by introducing chiral substituents onto the macrocyclic scaffolds. Magnetic CPL responses are observed in these chiral EuIII complexes, promoting an exciting development to the field of magneto-optics. The |glum | of the 5 D0 → 7 F1 transition increases by 20 % from 0.222 (0 T) to 0.266 (1.4 T) displaying a linear relationship between the Δglum and the magnetic field strength. These EuIII complexes with magnetic CPL responses, provides potential development to be used in CPL imaging applications due to improved sensitivity and resolution.

The emerging value of 64Cu for molecular imaging and therapy

Along with other novel metallic radionuclides, copper-64 (⁶⁴Cu) is currently being investigated as an alternative option to the gallium-68 (⁶⁸Ga) and lutetium-177 (¹⁷⁷Lu) radiopharmaceuticals widely used for targeting somatostatin receptors, expressed by neuroendocrine tumors (NETs), and recently prostate specific membrane antigen (PSMA), expressed by prostate cancer cells. This interest is mostly driven by the peculiar nuclear properties of ⁶⁴Cu that make it an almost ideal example of theranostic radionuclide. In fact, ⁶⁴Cu emits both low-energy positrons, β^- particles and a swarm of Auger electrons. This combination of different emissions may allow to collect high-resolution PET images, but also to use the same radiopharmaceutical for eliciting a therapeutic effect. Another unique behavior of ⁶⁴Cu originates from the fundamental biological role played in organisms by the ionic forms of the copper element, which is naturally involved in a multitude of cellular processes including cell replication. These intrinsic biological characteristics has led to the discovery that ⁶⁴Cu, under its simplest dicationic form Cu²⁺, is able to specifically target a variety of cancerous cells and to detect the onset of a metastatic process in its initial stage. This short review reports an outline of the status of ⁶⁴Cu radiopharmaceuticals and of the most relevant results that are constantly disclosed by preclinical and investigational clinical studies.

Chiral Separation of rac-Propylene Oxide on Penicillamine Coated Gold NPs

The surfaces of chemically synthesized spherical gold NPs (Au-NPs) have been modified using chiral L- or D-penicillamine (Pen) in order to impart enantioselective adsorption properties. These chiral Au-NPs have been used to demonstrate enantioselective adsorption of racemic propylene oxide (PO) from aqueous solution. In the past we have studied enantioselective adsorption of racemic PO on L- or D-cysteine (Cys)-coated Au-NPs. This prior work suggested that adsorption of PO on Cys-coated Au-NPs equilibrates within an hour. In this work, we have studied the effect of time on the enantioselective adsorption of racemic PO from solution onto chiral Pen/Au-NPs. Enantioselective adsorption of PO on chiral Pen/Au-NPs is time-dependent but reaches a steady state after ~18 h at room temperature. More importantly, L- or D-Pen/Au-NPs are shown to adsorb R- or S-PO enantiospecifically and to separate the two PO enantiomers from racemic mixtures of RS-PO.

Highly Stable Silver(I) Complexes with Cyclen-Based Ligands Bearing Sulfide Arms: A Step Toward Silver-111 Labeled Radiopharmaceuticals

With a half-life of 7.45 days, silver-111 (β_max 1.04 MeV, E_γ 245.4 keV [I_γ 1.24%], E_γ 342.1 keV [I_γ 6.7%]) is a promising candidate for targeted cancer therapy with β^- emitters as well as for associated SPECT imaging. For its clinical use, the development of suitable ligands that form sufficiently stable Ag⁺-complexes in vivo is required. In this work, the following sulfur-containing derivatives of tetraazacyclododecane (cyclen) have been considered as potential chelators for silver-111: 1,4,7,10-tetrakis(2-(methylsulfanyl)ethyl)-1,4,7,10-tetraazacyclododecane (DO4S), (2S,5S,8S,11S)-2,5,8,11-tetramethyl-1,4,7,10-tetrakis(2-(methylsulfanyl)ethyl)-1,4,7,10-tetraazacyclododecane (DO4S4Me), 1,4,7-tris(2-(methylsulfanyl)ethyl)-1,4,7,10-tetraazacyclododecane (DO3S), 1,4,7-tris(2-(methylsulfanyl)ethyl)-10-acetamido-1,4,7,10-tetraazacyclododecane (DO3SAm), and 1,7-bis(2-(methylsulfanyl)ethyl)-4,10,diacetic acid-1,4,7,10-tetraazacyclododecane (DO2A2S). Natural Ag⁺ was used in pH/Ag-potentiometric and UV-vis spectrophotometric studies to determine the metal speciation existing in aqueous NaNO₃ 0.15 M at 25 °C and the equilibrium constants of the complexes, whereas NMR and DFT calculations gave structural insights. Overall results indicated that sulfide pendant arms coordinate Ag⁺ allowing the formation of very stable complexes, both at acidic and physiological pH. Furthermore, radiolabeling, stability in saline phosphate buffer, and metal-competition experiments using the two ligands forming the strongest complexes, DO4S and DO4S4Me, were carried out with [¹¹¹Ag]Ag⁺ and promising results were obtained.

Analysis of the Relaxometric Properties of Extremely Rapidly Exchanging Gd3+ Chelates: Lessons from a Comparison of Four Isomeric Chelates

Relaxometric analyses and in particular the use of fast-field cycling techniques have become routine in the study of paramagnetic metal complexes. The field dependence of the solvent proton relaxation properties (nuclear magnetic relaxation dispersion, NMRD) can provide unparalleled insights into the chemistry of these complexes. However, analyzing NMRD data is a multiparametric problem, and some sets of variables are mutually compensatory. Specifically, when fitting NMRD profiles, the metal-proton distance and the rotational correlation time constant have a push-pull relationship in which a change to one causes a predictable compensation in the other. A relaxometric analysis of four isomeric chelates highlights the pitfalls that await when fitting the NMRD profiles of chelates for which dissociative water exchange is extremely rapid. In the absence of independently verified values for one of these parameters, NMRD profiles can be fitted to multiple parameter sets. This means that NMRD fitting can inadvertently be used to buttress a preconceived notion of how the complex should behave when a different parameter set may more accurately describe the actual behavior. These findings explain why the effect of very rapid dissociative exchange on the hydration state of Gd³⁺ has remained obscured until only recently.

Relaxometric Studies of Gd-Chelate Conjugated on the Surface of Differently Shaped Gold Nanoparticles

Nowadays, magnetic resonance imaging (MRI) is one of the key, noninvasive modalities to detect and stage cancer which benefits from contrast agents (CA) to differentiate healthy from tumor tissue. An innovative class of MRI CAs is represented by Gd-loaded gold nanoparticles. The size, shape and chemical functionalization of Gd-loaded gold nanoparticles appear to affect the observed relaxation enhancement of water protons in their suspensions. The herein reported results shed more light on the determinants of the relaxation enhancement brought by Gd-loaded concave cube gold nanoparticles (CCGNPs). It has been found that, in the case of nanoparticles endowed with concave surfaces, the relaxivity is remarkably higher compared to the corresponding spherical (i.e., convex) gold nanoparticles (SPhGNPs). The main determinant for the observed relaxation enhancement is represented by the occurrence of a large contribution from second sphere water molecules which can be exploited in the design of high-efficiency MRI CA.

Gadolinium Complexes as Contrast Agent for Cellular NMR Spectroscopy

Aqua Gd³⁺ and Gd-DOTA (gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacete) complexes were studied as a contrast agent in cellular NMR (nuclear magnetic resonance) spectroscopy for distinguishing between intracellular and extracellular spaces. The contrast agents for this purpose should provide strong paramagnetic relaxation enhancement and localize in the extracellular space without disturbing biological functions. Cell membrane permeability to Gd complexes was evaluated from the concentrations of gadolinium complexes in the inside and outside of E. coli cells measured by the ¹H-NMR relaxation. The site-specific binding of the complexes to E. coli cells was also analyzed by high-resolution solid-state ¹³C-NMR. The aqua Gd³⁺ complex did not enhance T₁ relaxation in proportion to the amount of added Gd³⁺. This Gd³⁺ concentration dependence and the ¹³C-NMR indicated that its strong cytotoxicity should be due to the binding of the paramagnetic ions to cellular components especially at the lipid membranes. In contrast, Gd-DOTA stayed in the solution states and enhanced relaxation in proportion to the added amount. This agent exhibited strong T₁ contrast between the intra- and extracellular spaces by a factor of ten at high concentrations under which the cells were viable over a long experimental time of days. These properties make Gd-DOTA suitable for selectively contrasting the living cellular space in NMR spectroscopy primarily owing to its weak interaction with cellular components.

Harnessing chemical exchange: 19F magnetic resonance OFF/ON zinc sensing with a Tm(iii) complex

A fluorinated, thulium(iii) complex (Tm-PFZ-1) serves as an off-on ¹⁹F magnetic resonance probe for Zn(ii). Rapid exchange among different conformations combined with paramagnetic relaxation and chemical shift effects of Tm(iii) effectively eliminate the ¹⁹F NMR/MRI signal in Tm-PFZ-1. Chelation of Zn(ii) induces increased structural rigidity and reduces exchange rate, affording a robust ¹⁹F NMR/MRI signal. Tm-PFZ-1 represents a first-in-class paramagnetic ¹⁹F MR agent that exploits a novel sensing mechanism for Zn(ii) and is the first ¹⁹F MR-based scaffold to provide an "off-on" response to Zn(ii) in aqueous solution.

Towards an Improved Design of MRI Contrast Agents: Synthesis and Relaxometric Characterisation of Gd-HPDO3A Analogues

The properties of Ln^III -HPDO3A complexes as relaxation enhancers and paraCEST agents are essentially related to the hydroxylpropyl moiety. A series of three HPDO3A derivatives, with small modifications to the hydroxyl arm, were herein investigated to understand how heightened control can be gained over the parameters involved in the design of these agents. A full ¹ H and ¹⁷ O-NMR relaxometric analysis was conducted and demonstrated that increasing the length of the OH group from the lanthanide centre significantly enhanced the water exchange rate of the gadolinium complex, but with a subsequent reduction in kinetic stability. Alternatively, the introduction of an additional methyl group, which increased the steric bulk around the OH moiety, resulted in the formation of almost exclusively the TSAP isomer (95 %) as identified by ¹ H-NMR of the europium complex. The gadolinium analogue of this complex also exhibited a very fast water exchange rate, but with no detectable loss of kinetic stability. This complex therefore demonstrates a notable improvement over Gd-HPDO3A.

FN3 Protein Conjugates for Cancer Diagnosis and Imaging Studies

Bioconjugation of biologically useful proteins is in great demand (e.g., conjugation to biotins, metal chelators, and drug carriers to target specific tissues for both in vitro and in vivo use). These conjugates provide widespread opportunities for various biological and biomedical applications. Evolving state-of-the-art protein conjugation strategies have led to the development of many affinity ligands, including for cancer imaging and diagnosis. However, to achieve the desirable protein conjugates, there are many challenges that remain to be addressed in order to obtain a reproducible procedure for all proteins and ligands. These include a control over the protein modification and the efficiency of the conjugation while retaining the original biological protein affinity postmodification. Here we present detailed conjugation methods for the human fibronectin tenth type III domain (FN3) protein scaffold for use in preclinical PET imaging. More specifically, this chapter provides detailed methods to produce a FN3 and a FN3-chelator-conjugate, its labeling with the radionuclide 64-Cu, and its use for noninvasive PET imaging in mice.

Stereospecific interactions between chiral inorganic nanomaterials and biological systems

Chirality is ubiquitous in nature and plays mysterious and essential roles in maintaining key biological and physiological processes.

DOTA-Branched Organic Frameworks as Giant and Potent Metal Chelators

Multinuclear complexes as metallo-agents for clinical use have caught extensive attention. In this paper, using 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) as both a functioning unit and a constructing junction, we build a series of DOTA-branched organic frameworks with multiple chelating holes by organizing DOTA layer by layer. These giant chelators are well characterized, which reveals their nanosized and soft structures. Further experiments demonstrate that they could efficiently hold abundant metal ions with much higher kinetic stabilities than the conventional small DOTA chelator. Their corresponding polynuclear complexes containing Gd³⁺, Tb³⁺, or both show superior imaging properties, excellent feasibility for peripheral modification, and unusual kinetic stability. This work can be easily extended to the fabrication of diverse homomultinuclear complexes and core/shell heteromultinuclear complexes with multifunctional properties. We expect that this new type of giant molecules and the ligand-branching strategy would open up a new avenue for the design and construction of next-generation polymetallic agents with high performance and stabilities for biomedical applications.

A simple time-resolved fluorescence assay for quantitative determination of DOTA chelator

DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) is one of the preeminent metal chelator applied for diagnostic and therapeutic purposes, however to date there is no versatile and reliable nonradioisotopic method for its precise determination. In this technical note, we present a novel and sensitive fluorimetric assay for quantitative determination of DOTA based on the luminescence quenching of the highly luminescent europium ions complex with trioctyl phosphine oxide and naphthoyl trifluoroacetone sensitizing activators. The assay is carried out in two simple steps and enables the determination of DOTA in the nanomolar range providing a superior tool compared to commonly applied spectrophotometric assay with Arsenazo-III reagent.

Synthesis of Water-Soluble Chiral DOTA Lanthanide Complexes with Predominantly Twisted Square Antiprism Isomers and Circularly Polarized Luminescence

One-step cyclization of a tetraazamacrocycle 5 with 70% yield in a 25-g scale was performed. Its chiral DOTA derivatives, L4, has ∼93% of TSAP coordination isomer in its Eu(III) and Yb(III) complexes in aqueous solution. [GdL4]^5- exhibits a high relaxivity, making it a promising and efficient MRI contrast agent. High luminescence dissymmetry factor (g_lum) values of 0.285 (ΔJ = 1) for [TbL3]^- and 0.241 (ΔJ = 1) for [TbL4]^5- in buffer solutions were recorded.

Ligand design strategies to increase stability of gadolinium-based magnetic resonance imaging contrast agents

Gadolinium(III) complexes have been widely utilised as magnetic resonance imaging (MRI) contrast agents for decades. In recent years however, concerns have developed about their toxicity, believed to derive from demetallation of the complexes in vivo, and the relatively large quantities of compound required for a successful scan. Recent efforts have sought to enhance the relaxivity of trivalent gadolinium complexes without sacrificing their stability. This review aims to examine the strategic design of ligands synthesised for this purpose, provide an overview of recent successes in gadolinium-based contrast agent development and assess the requirements for clinical translation.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Synthesis of chiral nine and twelve-membered cyclic polyamines from natural building blocks

A rational strategy for the facile and efficient cyclization of amino acid-based linear precursors forming nine and twelve-membered cyclic peptidomimetics is reported.