Radiometal-labeled agents (non-technetium) for diagnostic imaging

H3trica: Versatile Macrocyclic Chelator for [225Ac]Ac3+ and [155/161Tb]Tb3+ Theranostics

H3trica is a nonadentate chelating ligand intended for coordinating large radiometal ions, such as those used in nuclear medicine. This chelator, featuring a triaza-18-crown-6 macrocycle with three pendant carboxylic acid functional groups, was synthesized and characterized. Complementary nuclear magnetic resonance (NMR) spectroscopy and high-resolution electrospray-ionization mass spectroscopy (HR-ESI-MS) studies were used to explore the coordination of H3trica with metal ions such as La3+, Y3+ (as a model for Tb3+), and Lu3+ at the bulk scale. Thermodynamic solution studies provided valuable insights, highlighting robust metal complexation of H3trica with La3+, Tb3+, and Lu3+, with the most noteworthy log KML value observed for Tb3+ (log KTbL = 17.08), followed by La3+ (log KLaL = 16.64) and Lu3+ (log KLuL = 16.25). Concentration-dependent radiolabeling studies with [225Ac]Ac3+, [155Tb]Tb3+, and [161Tb]Tb3+ demonstrated rapid complexation (5-30 min) under mild conditions (pH 6-7, 25 °C). Importantly, the radiolabeled complexes exhibited stability during incubation in human serum for one-half-life of the corresponding radiometal. Thus, H3trica emerges as a valuable chelator, demonstrating its potential to coordinate the theranostic couple [225Ac]Ac3+/[155Tb]Tb3+ as well as the powerful terbium quartet ([149/152/155/161Tb]Tb3+) with efficiency and stability.

MIB Guides: Measuring the Immunoreactivity of Radioimmunoconjugates

Immunoglobulins, both full-length antibodies and smaller antibody fragments, have long been regarded as effective platforms for diagnostic and therapeutic radiopharmaceuticals. The construction of radiolabeled immunoglobulins (i.e., radioimmunoconjugates) requires the manipulation of the biomolecule through the attachment of a radiohalogen or the bioconjugation of a chelator that is subsequently used to coordinate a radiometal. Both synthetic approaches have historically relied upon the stochastic modification of amino acids within the immunoglobulin, a process which poses a risk to the structural and functional integrity of the biomolecule itself. Not surprisingly, radioimmunoconjugates with impaired antigen binding capacity will inevitably exhibit suboptimal in vivo performance. As a result, the biological characterization of any newly synthesized radioimmunoconjugate must include an assessment of whether it has retained its ability to bind its antigen. Herein, we provide straightforward and concise protocols for three assays that can be used to determine the immunoreactivity of a radioimmunoconjugate: (1) a cell-based linear extrapolation assay; (2) a cell-based antigen saturation assay; and (3) a resin- or bead-based assay. In addition, we will provide a critical analysis of the relative merits of each assay, an examination of the inherent limitations of immunoreactivity assays in general, and a discussion of other approaches that may be used to interrogate the biological behavior of radioimmunoconjugates.

Transition and Post-Transition Radiometals for PET Imaging and Radiotherapy

Radiometals are an exciting class of radionuclides because of the large number of metallic elements available that have medically useful isotopes. To properly harness radiometals, they must be securely bound by chelators, which must be carefully matched to the radiometal ion to maximize radiolabeling performance and the stability of the resulting complex. This chapter focuses on practical aspects of radiometallation chemistry including chelator selection, radiolabeling procedures and conditions, radiolysis prevention, purification, quality control, requisite equipment and reagents, and useful tips.

Radiolabeling and in vivo evaluation of lanmodulin with biomedically relevant lanthanide isotopes

Short-lived, radioactive lanthanides comprise an emerging class of radioisotopes attractive for biomedical imaging and therapy applications. To deliver such isotopes to target tissues, they must be appended to entities that target antigens overexpressed on the target cell's surface. However, the thermally sensitive nature of biomolecule-derived targeting vectors requires the incorporation of these isotopes without the use of denaturing temperatures or extreme pH conditions; chelating systems that can capture large radioisotopes under mild conditions are therefore highly desirable. Herein, we demonstrate the successful radiolabeling of the lanthanide-binding protein, lanmodulin (LanM), with medicinally relevant radioisotopes: ¹⁷⁷Lu, ^132/135La and ⁸⁹Zr. Radiolabeling of the endogenous metal-binding sites of LanM, as well exogenous labeling of a protein-appended chelator, was successfully conducted at 25 °C and pH 7 with radiochemical yields ranging from 20-82%. The corresponding radiolabeled constructs possess good formulation stability in pH 7 MOPS buffer over 24 hours (>98%) in the presence of 2 equivalents of ^natLa carrier. In vivo experiments with [¹⁷⁷Lu]-LanM, [^132/135La]-LanM, and a prostate cancer targeting-vector linked conjugate, [^132/135La]-LanM-PSMA, reveal that endogenously labeled constructs produce bone uptake in vivo. Exogenous, chelator-tag mediated radiolabeling to produce [⁸⁹Zr]-DFO-LanM enables further study of the protein's in vivo behavior, demonstrating low bone and liver uptake, and renal clearance of the protein itself. While these results indicate that additional stabilization of LanM is required, this study establishes precedence for the radiochemical labeling of LanM with medically relevant lanthanide radioisotopes.

Challenges in predicting ΔrxnG in solution: The chelate effect

Gibbs energies for reactions involving aqueous ions are challenging to predict due to the large solvation energies of such ions. A stringent test would be the ab initio reproduction of the aqueous-phase chelate effect, an entropic effect in reactions of very small enthalpy changes. This paper examines what is required to achieve such a reproduction for the paradigmatic reaction M(NH3)42+ + 2 en → M( en)22+ + 4 NH3 ( en = 1,2-ethylenediamine), for which ΔrxnG* and ΔrxnH* are −2.3 and +1.6 kcal mol−1, respectively, if M = Zn. Explicit solvation via simulation was avoided in order to allow sufficiently accurate electronic structure models; this required the use of continuum solvation models (CSMs), and a great deal of effort was made in attempting to lower the relative errors of ΔsolvG*[M(NH3)42+] vs ΔsolvG*[M( en)22+] from the CSMs available in Gaussian software. CSMs in ADF and JDFTx software were also tested. A uniform 2.2 kcal mol−1 accuracy in ΔrxnG* for all three metal-atom choices M = {Zn, Cd, Hg} was eventually achieved, but not from any of the known CSMs tested, nor from cavity size reoptimization, nor from semicontinuum modeling: post facto solvation energy corrections [one per solute type, NH3, en, M(NH3)42+, M( en)22+] were needed. It is hoped that this study will aid (and encourage) further CSM development for coordination-complex ions.

Nano-Theranostics for the Sensing, Imaging and Therapy of Prostate Cancers

We highlight hereby recent developments in the emerging field of theranostics, which encompasses the combination of therapeutics and diagnostics in a single entity aimed for an early-stage diagnosis, image-guided therapy as well as evaluation of therapeutic outcomes of relevance to prostate cancer (PCa). Prostate cancer is one of the most common malignancies in men and a frequent cause of male cancer death. As such, this overview is concerned with recent developments in imaging and sensing of relevance to prostate cancer diagnosis and therapeutic monitoring. A major advantage for the effective treatment of PCa is an early diagnosis that would provide information for an appropriate treatment. Several imaging techniques are being developed to diagnose and monitor different stages of cancer in general, and patient stratification is particularly relevant for PCa. Hybrid imaging techniques applicable for diagnosis combine complementary structural and morphological information to enhance resolution and sensitivity of imaging. The focus of this review is to sum up some of the most recent advances in the nanotechnological approaches to the sensing and treatment of prostate cancer (PCa). Targeted imaging using nanoparticles, radiotracers and biomarkers could result to a more specialised and personalised diagnosis and treatment of PCa. A myriad of reports has been published literature proposing methods to detect and treat PCa using nanoparticles but the number of techniques approved for clinical use is relatively small. Another facet of this report is on reviewing aspects of the role of functional nanoparticles in multimodality imaging therapy considering recent developments in simultaneous PET-MRI (Positron Emission Tomography-Magnetic Resonance Imaging) coupled with optical imaging in vitro and in vivo, whilst highlighting feasible case studies that hold promise for the next generation of dual modality medical imaging of PCa. It is envisaged that progress in the field of imaging and sensing domains, taken together, could benefit from the biomedical implementation of new synthetic platforms such as metal complexes and functional materials supported on organic molecular species, which can be conjugated to targeting biomolecules and encompass adaptable and versatile molecular architectures. Furthermore, we include hereby an overview of aspects of biosensing methods aimed to tackle PCa: prostate biomarkers such as Prostate Specific Antigen (PSA) have been incorporated into synthetic platforms and explored in the context of sensing and imaging applications in preclinical investigations for the early detection of PCa. Finally, some of the societal concerns around nanotechnology being used for the detection of PCa are considered and addressed together with the concerns about the toxicity of nanoparticles–these were aspects of recent lively debates that currently hamper the clinical advancements of nano-theranostics. The publications survey conducted for this review includes, to the best of our knowledge, some of the most recent relevant literature examples from the state-of-the-art. Highlighting these advances would be of interest to the biomedical research community aiming to advance the application of theranostics particularly in PCa diagnosis and treatment, but also to those interested in the development of new probes and methodologies for the simultaneous imaging and therapy monitoring employed for PCa targeting.

Chelators for Treatment of Iron and Copper Overload: Shift from Low-Molecular-Weight Compounds to Polymers

Iron and copper are essential micronutrients needed for the proper function of every cell. However, in excessive amounts, these elements are toxic, as they may cause oxidative stress, resulting in damage to the liver and other organs. This may happen due to poisoning, as a side effect of thalassemia infusion therapy or due to hereditary diseases hemochromatosis or Wilson's disease. The current golden standard of therapy of iron and copper overload is the use of low-molecular-weight chelators of these elements. However, these agents suffer from severe side effects, are often expensive and possess unfavorable pharmacokinetics, thus limiting the usability of such therapy. The emerging concepts are polymer-supported iron- and copper-chelating therapeutics, either for parenteral or oral use, which shows vivid potential to keep the therapeutic efficacy of low-molecular-weight agents, while avoiding their drawbacks, especially their side effects. Critical evaluation of this new perspective polymer approach is the purpose of this review article.

Bis-Argentivorous Molecules Bridged by Phenyl and 4,4'-Biphenyl Groups: Structural and Dynamic Behavior of Silver Complexes

Bis-argentivorous molecules (L^a and L^b), which have phenyl and 4,4'-biphenyl groups as linkers, have been prepared. The structures of Ag⁺ complexes with the new ligands (L^a and L^b) were investigated in solution and the solid state. The CSI-MS and ¹H NMR titration of L^a and L^b with Ag⁺ show 1:1 and 1:2 complexes depending on the [Ag⁺]:[L] ratios. In the solid-state structures, single crystals of L^a and L^b with 2 equiv of Ag⁺ were prepared. X-ray crystallography of the silver(I) complexes with L^a and L^b showed that an intramolecular racemic structure (Δ(δδδδ)Λ(λλλλ) form) and a racemic mixture of Δ(δδδδ)Δ(δδδδ) and Λ(λλλλ)Λ(λλλλ) forms were formed, respectively. The dynamic ¹H NMR studies suggest the following: (i) the activation entropies (ΔS^⧧) of the side arm rotations in the Ag⁺ complex with L^a were all negative, indicating restricted rotation of the side arms due to their shortness, and (ii) the ΔS^⧧ values of the Ag⁺ complexes with L^b were negative only when the side arms of both cyclens rotated simultaneously, and the ΔS^⧧ values for the 1:1 and 1:2 complexes were positive when one cyclen side arm was rotated. These values of ΔS^⧧ indicate that the biphenyl side arms between the two cyclens are not long enough to rotate the ring freely.

Copper Coordination Chemistry of Sulfur Pendant Cyclen Derivatives: An Attempt to Hinder the Reductive-Induced Demetalation in 64/67Cu Radiopharmaceuticals

The Cu²⁺ complexes formed by a series of cyclen derivatives bearing sulfur pendant arms, 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S), 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), were studied in aqueous solution at 25 °C from thermodynamic and structural points of view to evaluate their potential as chelators for copper radioisotopes. UV-vis spectrophotometric out-of-cell titrations under strongly acidic conditions, direct in-cell UV-vis titrations, potentiometric measurements at pH >4, and spectrophotometric Ag⁺-Cu²⁺ competition experiments were performed to evaluate the stoichiometry and stability constants of the Cu²⁺ complexes. A highly stable 1:1 metal-to-ligand complex (CuL) was found in solution at all pH values for all chelators, and for DO2A2S, protonated species were also detected under acidic conditions. The structures of the Cu²⁺ complexes in aqueous solution were investigated by UV-vis and electron paramagnetic resonance (EPR), and the results were supported by relativistic density functional theory (DFT) calculations. Isomers were detected that differed from their coordination modes. Crystals of [Cu(DO4S)(NO₃)]·NO₃ and [Cu(DO2A2S)] suitable for X-ray diffraction were obtained. Cyclic voltammetry (CV) experiments highlighted the remarkable stability of the copper complexes with reference to dissociation upon reduction from Cu²⁺ to Cu⁺ on the CV time scale. The Cu⁺ complexes were generated in situ by electrolysis and examined by NMR spectroscopy. DFT calculations gave further structural insights. These results demonstrate that the investigated sulfur-containing chelators are promising candidates for application in copper-based radiopharmaceuticals. In this connection, the high stability of both Cu²⁺ and Cu⁺ complexes can represent a key parameter for avoiding in vivo demetalation after bioinduced reduction to Cu⁺, often observed for other well-known chelators that can stabilize only Cu²⁺.

Recent Advances in Radiometals for Combined Imaging and Therapy in Cancer

Nuclear medicine is defined as the use of radionuclides for diagnostic and therapeutic applications. The imaging modalities positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are based on γ-emissions of specific energies. The therapeutic technologies are based on β^- -particle-, α-particle-, and Auger electron emitters. In oncology, PET and SPECT are used to detect cancer lesions, to determine dosimetry, and to monitor therapy effectiveness. In contrast, radiotherapy is designed to irreparably damage tumor cells in order to eradicate or control the disease's progression. Radiometals are being explored for the development of diagnostic and therapeutic radiopharmaceuticals. Strategies that combine both modalities (diagnostic and therapeutic), referred to as theranostics, are promising candidates for clinical applications. This review provides an overview of the basic concepts behind therapeutic and diagnostic radiopharmaceuticals and their significance in contemporary oncology. Select radiometals that significantly impact current and upcoming cancer treatment strategies are grouped as clinically suitable theranostics pairs. The most important physical and chemical properties are discussed. Standard production methods and current radionuclide availability are provided to indicate whether a cost-efficient use in a clinical routine is feasible. Recent preclinical and clinical developments and outline perspectives for the radiometals are highlighted in each section.

Zinc and copper complexes with azacrown ethers and their comparative stability in vitro and in vivo

Copper-based radiopharmaceuticals are of high interest these days owing to the decay properties of copper radioisotopes. In contrast, labeled zinc compounds have been less studied for applications in nuclear medicine. In this study, the stability of labeled zinc and copper complexes with two azacrown ether ligands was investigated and compared. Then, the in vitro and in vivo stability of the studied zinc complexes was demonstrated, with the complexes showing promise for biomedical applications. In contrast, analogous copper complexes quickly dissociated in the presence of serum proteins. Furthermore, a simple method for the production of radiochemically pure 65Zn was proposed, and the opportunity for its use as a surrogate radionuclide for research into potential zinc-containing radiopharmaceuticals was demonstrated.

InCl3: A Versatile Catalyst for Synthesizing a Broad Spectrum of Heterocycles

This review deals with the recent applications of the indium trichloride (InCl₃) catalyst in the synthesis of a broad spectrum of heterocyclic compounds. Over the years, a number of reviews on the applications of InCl₃-catalyzed organic synthesis have appeared in the literature. It is evident that InCl₃ has emerged as a valuable catalyst for a wide range of organic transformations due to its stability when exposed to moisture and also in an aqueous medium. The most attractive feature of this review is the application of the InCl₃ catalyst for synthesizing bioactive heterocyclic compounds. The study of InCl₃-catalyzed organic reactions has high potential and better intriguing aspects, which are anticipated to originate from this field of research.

SC, Schrage BR, Ziegler CJ, Gao X, Wheeler SE. Topomeric aza/thia cryptands: synthesis and theoretical aspects of in/out isomerism using n-alkyl bridging

A series of heterobicyclic aza/thia-lactams and cryptands incorporating changes in n-alkyl bridging length have been synthesized, characterized, chelated to heavy metals and computationally assessed.

A novel multi-purpose Zn-MOF fluorescent sensor for 2,4-dinitrophenylhydrazine, picric acid, La3+ and Ca2+: Synthesis, structure, selectivity, sensitivity and recyclability

A new three-dimensional luminescence Zn-MOF sensor with the molecular formula [Zn₄(μ₃-OH)₂(ptptc)_1.5(DMA)(H₂O)₂]·2DMA (complex 1) for the selective sensing of 2,4-dinitrophenylhydrazine (2,4-DNPH), picric acid (PA), La³⁺ and Ca²⁺ has been synthesized from terphenyl-3,3',5,5'-tetracarboxylic acid (H₄ptptc) and zinc nitrate under solvothermal conditions. XRD analysis reveals that complex 1 crystallizes in monoclinic system P2₁/n space group and consists of a three-dimensional network with one-dimensional channels, which are expected to facilitate the diffusion, concentration and detection processes. Real-time fluorescence quenching responses and good reversibility were observed in the fluorescence titration experiments with nano-molar scale detection limits for 2,4-dinitrophenylhydrazine (2,4-DNPH, 100 nM) and picric acid (PA, 500 nM). Noticeable emission band shift from 365 nm to 420 nm was observed when treated complex 1 with La³⁺ and a new emission band centered at 475 nm appeared when treated complex 1 with Ca²⁺ in the metal ions sensing experiments. In virtue of its high selectively, good sensitively and recyclability complex 1 could be a promising fluorescent sensor for explosives and metal ions.

A positron-emission tomography (PET)/magnetic resonance imaging (MRI) platform to track in vivo small extracellular vesicles

Here we report a two-step surface modification methodology to radiolabel small extracellular vesicles (SEVs) with 64CuCl2 for PET/MRI imaging. The modification did not change or damage the morphology, surface receptor proteins and internal RNA content. Radiolabeled SEVs could be detected in organs with low accumulation such as the brain (0.4-0.5% ID g-1) and their brain location determined by MRI.

Nuclear Imaging of Glucose Metabolism: Beyond 18F-FDG

Glucose homeostasis plays a key role in numerous fundamental aspects of life, and its dysregulation is associated with many important diseases such as cancer. The atypical glucose metabolic phenomenon, known as the Warburg effect, has been recognized as a hallmark of cancer and serves as a promising target for tumor specific imaging. At present, 2-deoxy-2-[¹⁸F]fluoro-glucose (¹⁸F-FDG)-based positron emission tomography/computed tomography (PET/CT) represented the state-of-the-art radionuclide imaging technique for this purpose. The powerful impact of ¹⁸F-FDG has prompted intensive research efforts into other glucose-based radiopharmaceuticals for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging. Currently, glucose and its analogues have been labeled with various radionuclides such as ^99mTc, ¹¹¹In, ¹⁸F, ⁶⁸Ga, and ⁶⁴Cu and have been successfully investigated for tumor metabolic imaging in many preclinical studies. Moreover, ^99mTc-ECDG has advanced into its early clinical trials and brings a new era of tumor imaging beyond ¹⁸F-FDG. In this review, preclinical and early clinical development of glucose-based radiopharmaceuticals for tumor metabolic imaging will be summarized.

New Cyclams and Their Copper(II) and Iron(III) Complexes: Synthesis and Potential Application as Anticancer Agents

New cyclam derivatives (HOCH₂ CH₂ CH₂ )₂ (PhCH₂ )₂ Cyclam and (HOCH₂ CH₂ CH₂ )₂ ( 4 - CF 3 PhCH₂ )₂ Cyclam, as well as their Cu^II and Fe^III complexes, were synthesized and characterized and their stability in cellular media was assessed. The cytotoxic effect of all compounds was examined on human cervical cancer (HeLa) cells, revealing strong anticancer activity. After 24 h, only complexes with the (HOCH₂ CH₂ CH₂ )₂ ( 4 - CF 3 PhCH₂ )₂ Cyclam ligand are cytotoxic, whereas after incubation for 72 h all compounds show significant antiproliferative effects. Notably, compounds containing 4 - CF 3 PhCH₂ pendant arms on the cyclam ring revealed the most activity, with cytotoxicity values up to 12 times higher than those of cisplatin. All metal complexes seem to induce cell death through the formation of reactive oxygen species.

Mobility and retention of indium and gallium in saturated porous media

Transport of indium and gallium is reported in laboratory column experiments using quartz sand as a model porous medium representative of a groundwater system. With increased use of indium and gallium in recent years, mainly in the semiconductor industry, concerns arise regarding their environmental effects. The transport and retention behavior of these two metals were quantified via batch and column experiments, and numerical modeling. The effect of natural organic matter on indium and gallium mobility was studied by addition of humic acid (HA). Measured breakthrough curves from column experiments demonstrated different binding capacities between indium and gallium, stronger for indium, with the presence of HA affecting retention dynamics. For indium, the binding capacity on quartz decreases significantly in the presence of HA, leading to enhanced mobility. In contrast, gallium exhibits slightly higher retention and lower mobility in the presence of HA. In all cases, the binding capacity of gallium to quartz is much weaker than that of indium. These results are consistent with the assumption that indium and gallium form different types of complexes with organic ligands, with gallium complexes appearing more stable than indium complexes. Quantitative modeling confirmed that metal retention is controlled by complex stability.

Synthesis of Camphor-Derived Bis(pyrazolylpyridine) Rhodium(III) Complexes: Structure-Reactivity Relationships and Biological Activity

Two novel rhodium(III) complexes, namely, [Rh^III(X)Cl₃] (X = 2 2,6-bis((4 S,7 R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-1 H-4,7-methanoindazol-3-yl)pyridine or 2,6-bis((4 S,7 R)-1,7,8,8-tetramethyl-4,5,6,7-tetrahydro-1 H-4,7-methanoindazol-3-yl)pyridine), were synthesized from camphor derivatives of a bis(pyrazolylpyridine), tridentate nitrogen-donor chelate system, giving [Rh^III(H₂L*)Cl₃] (1a) and [Rh^III(Me₂L*)Cl₃] (1b). A rhodium(III) terpyridine (terpy) ligand complex, [Rh^III(terpy)Cl₃] (1c), was also synthesized. By single-crystal X-ray analysis, 1b crystallizes in an orthorhombic P2₁2₁2₁ system, with two molecules in the asymmetric unit. Tridentate coordination by the N,N,N-donor localizes the central nitrogen atom close to the rhodium(III) center. Compounds 1a and 1b were reactive toward l-methionine (l-Met), guanosine-5'-monophosphate (5'-GMP), and glutathione (GSH), with an order of reactivity of 5'-GMP > GSH > l-Met. The order of reactivity of the Rh^III complexes was: 1b> 1a > 1c. The Rh^III complexes showed affinity for calf thymus DNA and bovine serum albumin by UV-vis and emission spectral studies. Furthermore, 1b showed significant in vitro cytotoxicity against human epithelial colorectal carcinoma cells. Since the Rh^III complexes have similar coordination modes, stability differences were evaluated by density functional theory (DFT) calculations (B3LYP(CPCM)/LANL2DZp). With (H₂L*) and (terpy) as model ligands, DFT calculations suggest that both tridentate ligand systems have similar stability. In addition, molecular docking suggests that all test compounds have affinity for the minor groove of DNA, while 1b and 1c have potential for DNA intercalation.

Recent Advances in Zirconium-89 Chelator Development

The interest in zirconium-89 (⁸⁹Zr) as a positron-emitting radionuclide has grown considerably over the last decade due to its standardized production, long half-life of 78.2 h, favorable decay characteristics for positron emission tomography (PET) imaging and its successful use in a variety of clinical and preclinical applications. However, to be utilized effectively in PET applications it must be stably bound to a targeting ligand, and the most successfully used ⁸⁹Zr chelator is desferrioxamine B (DFO), which is commercially available as the iron chelator Desferal^®. Despite the prevalence of DFO in ⁸⁹Zr-immuno-PET applications, the development of new ligands for this radiometal is an active area of research. This review focuses on recent advances in zirconium-89 chelation chemistry and will highlight the rapidly expanding ligand classes that are under investigation as DFO alternatives.

Radiometallic Complexes of DO3A-Benzothiazole Aniline for Nuclear Medicine Theranostics

To develop a radioactive metal complex platform for tumor theranostics, we introduced three radiopharmaceutical derivatives of 1,4,7,10-tetraazacyclododecane-1,4,7-trisacetic acid-benzothiazole aniline (DO3A-BTA, L1) labeled with medical radioisotopes for diagnosis (⁶⁸Ga/⁶⁴Cu) and therapy (¹⁷⁷Lu). The tumor-targeting ability of these complexes was demonstrated in a cellular uptake experiment, in which ¹⁷⁷Lu-L1 exhibited markedly higher uptake in HeLa cells than the ¹⁷⁷Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complex. According to in vivo positron emission tomography imaging, high accumulation of ⁶⁸Ga-L1 and ⁶⁴Cu-L1 was clearly visualized in the tumor site, while ¹⁷⁷Lu-L1 showed therapeutic efficacy in therapy experiments. Consequently, this molecular platform represents a useful approach in nuclear medicine toward tumor-theranostic radiopharmaceuticals when ⁶⁸Ga-L1 or ⁶⁴Cu-L1 is used for diagnosis, ¹⁷⁷Lu-L1 is used for therapy, or two of the compounds are used in conjunction with each other.

Assessing the intracellular fate of rhodium(ii) complexes

Rhodium(ii)-fluorophore conjugates have strong rhodium-based fluorescence quenching that can be harnessed to report on a conjugate's cellular uptake and the intracellular decomposition rate. Information gleened from this study allowed the design of an improved STAT3 metalloinhibitor.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

VPAC1 Targeted 64Cu-TP3805 kit preparation and its evaluation

INTRODUCTION

Previously, our laboratory has shown that ⁶⁴Cu-TP3805 can specifically target VPAC1 receptors and be used for positron emission tomography (PET) imaging of breast (BC) and prostate cancer (PC) in humans. Present work is aimed at the formulation of a freeze-dried diaminedithiol-peptide (N₂S₂-TP3805) kit and it's evaluation for the preparation of ⁶⁴Cu labeled TP3805. Parameters such as pH, temperature and incubation time were examined that influenced the radiolabeling efficiency and stability of the product.

METHODS

Kits were prepared under different conditions and radiolabeling efficiency of TP3805 kit was evaluated for a range of pH3.5-8.5, after addition of ⁶⁴Cu in 30μl, 0.1M HCl. Incubation temperature (37-90°C) and time (30-120min.) were also investigated. Kits were stored at -10°C and their long term stability was determined as a function of their radiolabeling efficiency. Further, stability of ⁶⁴Cu-TP3805 complex was evaluated in presence of fetal bovine serum and bovine serum albumin by using SDS polyacrylamide gel electrophoresis. Kits were then used for PET imaging of BC and PC following eIND (101550) and institutional approvals. Specificity of ⁶⁴Cu-TP3805 for VPAC1 was examined with digital autoradiography (DAR) of prostate tissues obtained after prostatectomy, benign prostatic hyperplasia (BPH) tissue, and benign and malignant lymph nodes. Results were compared with corresponding tissue histology.

RESULTS

Radiolabeling efficiency was ≥95% at final pH ~7.2 when incubated at 50°C for 90min. Kits were stable up to 18months when stored at -10°C, and ⁶⁴Cu-TP3805 complex exhibited excellent stability for up to 4h at room temperature. ⁶⁴Cu-TP3805 complex did not show any transchelation even after 2h incubation at 37°C in 10% FBS as well as in BSA as determined by SDS PAGE analysis. DAR identified ≥95% of malignant lesions 11 new PC lesions, 20 high grade prostatic intraepithelial neoplasia, 2/2 ejaculatory ducts and 5/5 urethra verumontanum not previously identified The malignant lymph nodes were correctly identified by DAR and for 3/3 BPH patients, and 5/5 cysts, DAR was negative. In human BC (n=19) and PC (n=26) were imaged with 100% sensitivity.

CONCLUSION

Availability of ready to use N₂S₂-peptide kits for ⁶⁴Cu labeling is convenient and eliminates possible day to day variation during its routine preparation for clinical use.

Tuning the copper(ii) coordination properties of cyclam by subtle chemical modifications

A copper(ii) coordination investigation of modified cyclams bearing “oxo” and/or “N-benzyl” and/or “C-hydroxyethyl” units was performed by potentiometry, ESI-MS, UV-Vis, electrochemistry and DFT.

Synthesis and characterization of novel rhenium(I) complexes towards potential biological imaging applications

BACKGROUND

Re(I) tricarbonyl complexes exhibit immense potential as fluorescence imaging agents. However, only a handful of rhenium complexes have been utilized in biological imaging. The present study describes the synthesis of four novel rhenium complexes, their characterization and preliminary biological studies to assess their potential as biological imaging agents.

RESULTS

Four facial rhenium tricarbonyl complexes containing a pyridyl triazine core, (L1 = 5,5'(3-(2-pyridyl)-1,2,4-triazine-5,6-diyl)-bis-2-furansulfonic acid disodium salt and L2 = (3-(2- pyridyl)-5,6-diphenyl-1,2,4-triazine-4',4''-disulfonic acid sodium salt) have been synthesized by utililzing two different Re metal precursors, Re(CO)5Br and [Re(CO)3(H2O)3]OTf in an organic solvent mixture and water, respectively. The rhenium complexes [Re(CO)3(H2O)L1]+ (1), Re(CO)3L1Br (2), [Re(CO)3(H2O)L2]+ (3), and Re(CO)3L2Br (4), were obtained in 70-85% yield and characterized by 1H NMR, IR, UV, and luminescence spectroscopy. In both H2O and acetonitrile, complexes display a weak absorption band in the visible region which can be assigned to a metal to ligand charge transfer excitation and fluorescent emission lying in the 650-710 nm range. Cytotoxicity assays of complexes 1, 3, and 4 were carried out for rat peritoneal cells. Both plant cells (Allium cepa bulb cells) and rat peritoneal cells were stained using the maximum non-toxic concentration levels of the compounds, 20.00 mg ml-1 for 1 and 3 and 5.00 mg ml-1 for 4 to observe under the epifluorescence microscope. In both cell lines, compound concentrated specifically in the nuclei region. Hence, nuclei showed red fluorescence upon excitation at 550 nm.

CONCLUSIONS

Four novel rhenium complexes have been synthesized and characterized. Remarkable enhancement of fluorescence upon binding with cells and visible range excitability demonstrates the possibility of using the new complexes in biological applications.Graphical abstractMicrograph of rat peritoneal cells incubated with novel rhenium complex under epifluorescence microscope.

Synthetic nanoparticles for delivery of radioisotopes and radiosensitizers in cancer therapy

Radiotherapy has been, and will continue to be, a critical modality to treat cancer. Since the discovery of radiation-induced cytotoxicity in the late 19th century, both external and internal radiation sources have provided tremendous benefits to extend the life of cancer patients. Despite the dramatic improvement of radiation techniques, however, one challenge persists to limit the anti-tumor efficacy of radiotherapy, which is to maximize the deposited dose in tumor while sparing the rest of the healthy vital organs. Nanomedicine has stepped into the spotlight of cancer diagnosis and therapy during the past decades. Nanoparticles can potentiate radiotherapy by specifically delivering radionuclides or radiosensitizers into tumors, therefore enhancing the efficacy while alleviating the toxicity of radiotherapy. This paper reviews recent advances in synthetic nanoparticles for radiotherapy and radiosensitization, with a focus on the enhancement of in vivo anti-tumor activities. We also provide a brief discussion on radiation-associated toxicities as this is an area that, up to date, has been largely missing in the literature and should be closely examined in future studies involving nanoparticle-mediated radiosensitization.

Spot-welding solid targets for high current cyclotron irradiation

Zirconium-89 finds broad application for use in positron emission tomography. Its cyclotron production has been limited by the heat transfer from yttrium targets at high beam currents. A spot welding technique allows a three-fold increase in beam current, without affecting ⁸⁹Zr quality. An yttrium foil, welded to a jet-cooled tantalum support base accommodates a 50µA proton beam degraded to 14MeV. The resulting activity yield of 48±4 MBq/(μA∙hr) now extends the outreach of ⁸⁹Zr for a broader distribution.

Copper-64 Radiopharmaceuticals for Oncologic Imaging

The positron emitting radionuclide (64)Cu has a radioactive half-life of 12.7 hours. The decay characteristics of (64)Cu allow for PET images that are comparable in quality to those obtained using (18)F. Given the longer radioactive half-life of (64)Cu compared with (18)F and the versatility of copper chemistry, copper is an attractive alternative to the shorter-lived nuclides for PET imaging of peptides, antibodies, and small molecules that may require longer circulation times. This article discusses a number of copper radiopharmaceuticals, such as Cu-ATSM, that have been translated to the clinic and new developments in copper-based radiopharmaceuticals.

Applications and Challenges for Use of Cell-Penetrating Peptides as Delivery Vectors for Peptide and Protein Cargos

The hydrophilic nature of peptides and proteins renders them impermeable to cell membranes. Thus, in order to successfully deliver peptide and protein-based therapeutics across the plasma membrane or epithelial and endothelial barriers, a permeation enhancing strategy must be employed. Cell-penetrating peptides (CPPs) constitute a promising tool and have shown applications for peptide and protein delivery into cells as well as across various epithelia and the blood-brain barrier (BBB). CPP-mediated delivery of peptides and proteins may be pursued via covalent conjugation of the CPP to the cargo peptide or protein or via physical complexation obtained by simple bulk-mixing of the CPP with its cargo. Both approaches have their pros and cons, and which is the better choice likely relates to the physicochemical properties of the CPP and its cargo as well as the route of administration, the specific barrier and the target cell. Besides the physical barrier, a metabolic barrier must be taken into consideration when applying peptide-based delivery vectors, such as the CPPs, and stability-enhancing strategies are commonly employed to prolong the CPP half-life. The mechanisms by which CPPs translocate cell membranes are believed to involve both endocytosis and direct translocation, but are still widely investigated and discussed. The fact that multiple factors influence the mechanisms responsible for cellular CPP internalization and the lack of sensitive methods for detection of the CPP, and in some cases the cargo, further complicates the design and conduction of conclusive mechanistic studies.

Evaluation of H2CHXdedpa, H2dedpa- and H2CHXdedpa-N,N′-propyl-2-NI ligands for 64Cu(ii) radiopharmaceuticals

Hexadentate acyclic chelate H₂CHXdedpa and related N,N′-alkylated ligands are radiolabeled with radioactive ⁶⁴Cu(ii) under mild conditions forming kinetically inert complexes.

Synthesis, preclinical evaluation and molecular modelling of macrocyclic appended 1-(2-methoxyphenyl)piperazine for 5-HT1A neuroreceptor imaging

An efficient approach in the design and synthesis of a multi-functional chelating agent based on 1-(2-methoxyphenyl)piperazine for targeting 5-HT_1A receptors in brain was envisaged.

Standardisation and half-life measurements of (111)In

The standardisation of (111)In by 4π(LS)-γ coincidence and anticoincidence counting is presented. Absolute measurements were performed for samples with different concentrations of carrier solution and for different window settings in the gamma channel. The radioactive concentration of the master solution determined on the same reference date was consistent for all measurements performed. The evaluated typical uncertainty was 0.43%. The half-life of (111)In was determined using a time series of measurements performed with an ionisation chamber. A least squares fit of the measured data resulted in a half-life of 2.8067 (34) days consistent with Decay Data Evaluation Project recommended value (0.064% higher than the DDEP value).

[(18)F]-Group 13 fluoride derivatives as radiotracers for positron emission tomography

The field of (18)F chemistry is rapidly expanding because of the use of this radionuclide in radiotracers for positron emission tomography (PET). Until recently, most [(18)F]-radiotracers were generated by the direct attachment of (18)F to a carbon in the organic backbone of the radiotracer. The past decade has witnessed the emergence of a new strategy based on the formation of an (18)F-group 13 element bond. This approach, which is rooted in the field of fluoride anion complexation/coordination chemistry, has led to the development of a remarkable family of boron, aluminium and gallium [(18)F]-fluoride anion complexing agents which can be conjugated with peptides and small molecules to generate disease specific PET radiotracers. This review is dedicated to the chemistry of these group 13 [(18)F]-fluorides anion complexing agents and their use in PET. Some of the key fluoride-binding motifs covered in this review include the trifluoroborate unit bound to neutral or cationic electron deficient backbones, the BF2 unit of BODIPY dyes, and AlF or GaF3 units coordinated to multidentate Lewis basic ligands. In addition to describing how these moieties can be converted into their [(18)F]-analogs, this review also dicusses their incorporation into bioconjugates for application in PET.

Biodistribution and PET imaging of a novel [68Ga]-anti-CD163-antibody conjugate in rats with collagen-induced arthritis and in controls

PURPOSE

The hemoglobin scavenger receptor CD163 is exclusively expressed on monocytes and tissue macrophages, also at sites of inflammation. We examined whether gallium-68 (Ga-68)-labeled anti-CD163-antibody can detect the receptor in vivo.

PROCEDURES

We radiolabeled anti-CD163 antibody with Ga-68 and evaluated stability and binding specificity of the conjugate ([(68)Ga]ED2) in vitro. Furthermore, tracer biodistribution was assessed in vivo in healthy rats and rats with acute collagen-induced arthritis (CIA) by MicroPET and tissue analysis.

RESULTS

Radiosynthesis of [(68)Ga]ED2 antibody yielded a tracer with high-specific activity and radiochemical purity. [(68)Ga]ED2 bound specifically to CD163 in vitro. In rats, [(68)Ga]ED2 rapidly accumulated in macrophage-rich tissues. The arthritic paws exhibited a low but significant [(68)Ga]ED2 uptake. Interestingly, the systemic distribution was also changed in the sense that a significantly higher liver uptake and lower spleen uptake of [(68)Ga]ED2 was measured in CIA rats that accordingly showed a corresponding change in level of CD163 expression.

CONCLUSIONS

[(68)Ga]ED2 specifically binds CD163 in vitro and in vivo. Biodistribution studies in CIA rats suggest that this novel tool may have applications in studies of inflammatory diseases.