Tumour targeting with radiometals for diagnosis and therapy

Incorporation of Carboxylate Pendant Arms into 18-Membered Macrocycles: Effects on [nat/203Pb]Pb(II) Complexation

We present a detailed investigation on the coordination chemistry of [nat/203Pb]Pb(II) with chelators H4PYTA and H4CHX-PYTA. These chelators belong to the family of ligands derived from the 18-membered macrocyclic backbone PYAN and present varying degrees of rigidity due to the presence of either ethyl or cyclohexyl spacers. A complete study of the stable Pb(II) complexes is carried out via NMR, X-Ray crystallography, stability constant determination and computational studies. While these studies indicated that Pb(II) complexation is achieved, and the thermodynamic stability of the resulting complexes is very high, a certain degree of fluxionality does exist in both cases. Nevertheless, radiolabeling studies were carried out using SPECT (single photon emission computed tomography) compatible isotope lead-203 (203Pb, t1/2=51.9 h), and while both chelators complex the radioisotope, the incorporation of carboxylate pendant arms appears to be detrimental towards the stability of the complexes when compared to the previously described amide analogues. Additionally, incorporation of a cyclohexyl spacer does not improve the kinetic inertness of the system.

Exploring the Limits of Ligand Rigidification in Transition Metal Complexes with Mono-N-Functionalized Pyclen Derivatives

We report the synthesis of a new family of side-bridged pyclen ligands. The incorporation of an ethylene bridge between two adjacent nitrogen atoms was reached from the pyclen-oxalate precursor described previously. Three new side-bridged pyclen macrocycles, Hsb-3-pc1a, sb-3-pc1py, and Hsb-3-pc1pa, were obtained with the aim to assess their coordination properties toward Cu2+ and Zn2+ ions. We also prepared their nonreinforced analogues H3-pc1a, 3-pc1py, and H3-pc1pa as comparative benchmarks. The two series of ligands were characterized and their coordination properties were investigated in detail. The Zn2+ and Cu2+ complexes with the nonside-bridged series H3-pc1a, 3-pc1py, and H3-pc1pa were successfully isolated and their structures were assessed by X-ray diffraction studies. In the case of the side-bridged family, the synthesis of the complexes was far more difficult and, in some cases, unsuccessful. The results of our studies demonstrate that this difficulty is related to the extreme stiffening and basicity of such side-bridged pyclens.

Tuning the Softness of the Pendant Arms and the Polyazamacrocyclic Backbone to Chelate the 203Pb/212Pb Theranostic Pair

A series of macrocyclic ligands were considered for the chelation of Pb2+: 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), 1,7-bis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane-4,10-diacetic acid (DO2A2S), 1,5,9-tris[2-(methylsulfanyl)ethyl]-1,5,9-triazacyclododecane (TACD3S), 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetrazacyclotridecane (TRI4S), and 1,4,8,11-tetrakis[2-(methylsulfanyl)ethyl]-1,4,8,11-tetrazacyclotetradecane (TE4S). The equilibrium, the acid-mediated dissociation kinetics, and the structural properties of the Pb2+ complexes formed by these chelators were examined by UV-Visible and nuclear magnetic resonance (NMR) spectroscopies, combined with potentiometry and density functional theory (DFT) calculations. The obtained results indicated that DO4S, DO3S, DO3SAm, and DO2A2S were able to efficiently chelate Pb2+ and that the most suitable macrocyclic scaffold for Pb2+ is 1,4,7,10-tetrazacyclododecane. NMR spectroscopy gave insights into the solution structures of the Pb2+ complexes, and 1H-207Pb interactions confirmed the involvement of S and/or O donors in the metal coordination sphere. Highly fluxional solution behavior was discovered when Pb2+ was coordinated to symmetric ligands (i.e., DO4S and DO2A2S) while the introduction of structural asymmetry in DO3S and DO3SAm slowed down the intramolecular dynamics. The ligand ability to chelate [203Pb]Pb2+ under highly dilute reaction conditions was explored through radiolabeling experiments. While DO4S and DO3S possessed modest performance, DO3SAm and DO2A2S demonstrated high complexation efficiency under mild reaction conditions (pH = 7, 5 min reaction time). The [203Pb]Pb2+ complexes' integrity in human serum over 24 h was appreciably good for [203Pb][Pb(DO4S)]2+ (80 ± 5%) and excellent for [203Pb][Pb(DO3SAm)]2+ (93 ± 1%) and [203Pb][Pb(DO2A2S)] (94 ± 1%). These results reveal the promise of DO2A2S and DO3SAm as chelators in cutting-edge theranostic [203/212Pb]Pb2+ radiopharmaceuticals.

Synthesis of new acyclic chelators H4aPyta and H6aPyha and their complexes with Cu2+, Ga3+, Y3+, and Bi3

In this article, we present the synthesis and characterization of new acyclic pyridine-containing polyaminocarboxylate ligands H4aPyta and H6aPyha, which differ in structural rigidity and the number of chelating groups. Their abilities to form complexes with Cu2+, Ga3+, Y3+, and Bi3+ cations, as well as the stability of the complexes, were evaluated by potentiometric titration method, radiolabeling with the corresponding radionuclides, in vitro studies, mass spectrometry, and HPLC. The structures of the resulting complexes were determined using NMR spectroscopy and DFT calculations. The results obtained made it possible to evaluate the influence of the structural features of the complexes on their stability. The developed chelators H4aPyta and H6aPyha were proved to be promising for further research in the field of radiopharmaceuticals.

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.

Exploring the Potential of Nanogels: From Drug Carriers to Radiopharmaceutical Agents

Nanogels open up access to a wide range of applications and offer among others hopeful approaches for use in the field of biomedicine. This review provides a brief overview of current developments of nanogels in general, particularly in the fields of drug delivery, therapeutic applications, tissue engineering, and sensor systems. Specifically, cyclodextrin (CD)-based nanogels are important because they have exceptional complexation properties and are highly biocompatible. Nanogels as a whole and CD-based nanogels in particular can be customized in a wide range of sizes and equipped with a desired surface charge as well as containing additional molecules inside and outside, such as dyes, solubility-mediating groups or even biological vector molecules for pharmaceutical targeting. Currently, biological investigations are mainly carried out in vitro, but more and more in vivo applications are gaining importance. Modern molecular imaging methods are increasingly being used for the latter. Due to an extremely high sensitivity and the possibility of obtaining quantitative data on pharmacokinetic and pharmacodynamic properties, nuclear methods such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) using radiolabeled compounds are particularly suitable here. The use of radiolabeled nanogels for imaging, but also for therapy, is being discussed.

Role of Pure Technetium Chemistry: Are There Still Links to Applications in Imaging?

The discovery and development of new 99mTc-based radiopharmaceuticals or labeled drugs in general is based on innovative, pure chemistry and subsequent, application-targeted research. This was the case for all currently clinically applied imaging agents. Most of them were market-introduced some 20 years ago, and the few more recent ones are based on even older chemistry, albeit technetium chemistry has made substantial progress over the last 20 years. This progress though is not mirrored by new molecular imaging agents and is even accompanied by a steady decrease in the number of groups active in pure and applied technetium chemistry, a contrast to the trends in most other fields in which d-elements play a central role. The decrease in research with technetium has been partly counterbalanced by a strong increase of research activities with homologous, cold rhenium compounds for therapy, disclosing in the future eventually a quite unique opportunity for theranostics. This Viewpoint analyzes the pathways that led to radiopharmaceuticals in the past and their underlying fundamental contributions. It attempts to tackle the question of why new chemistry still does not lead to new imaging agents, i.e., the question of whether pure technetium chemistry is still needed at all.

Bispidine Chelators for Radiopharmaceutical Applications with Lanthanide, Actinide, and Main Group Metal Ions

Octadentate and specifically nonadentate ligands with a bispidine scaffold (3,7-diazabicyclo[3.3.1]nonane) are known to be efficiently coordinated to a range of metal ions of interest in radiopharmaceutical chemistry and lead to exceedingly stable and inert complexes. Nonadentate bispidine L2 (with a tridentate bipyridine acetate appended to N3 and a picolinate at N7) has been shown before to be an ideal chelator for 111In3+, 177Lu3+, and 225Ac3+, nuclides of interest for diagnosis and therapy, and a proof-of-principle study with an SSTR2-specific octreotate has shown potential for theranostic applications. We now have extended these studies in two directions. First, we present ligand derivative L3, in which the bipyridine acetate is substituted with terpyridine, a softer donor for metal ions with a preference for more covalency. L3 did not fulfill the hopes because complexation is much less efficient. While for Bi3+ and Pb2+ the ligand is an excellent chelator with properties similar to those of L2, Lu3+ and La3+ show very slow and inefficient complexation with L3 in contrast to L2, and 225Ac3+ is not fully coordinated, even at an increased temperature (92% radiochemical yield at 80 °C, 60 min, [L3] = 10-4 M). These observations have led to a hypothesis for the complexation pathway that is in line with all of the experimental data and supported by a preliminary density functional theory analysis, which is important for the design of further optimized bispidine chelators. Second, the coordination chemistry of L2 has been extended to Bi3+, La3+, and Pb2+, including solid state and solution structural work, complex stabilities, radiolabeling, and radiostability studies. All complexes of this ligand (La3+, Ac3+, Lu3+, Bi3+, In3+, and Pb2+), including nuclides for targeted α therapy (TAT), single-photon emission computed tomography, and positron emission tomography, are formed efficiently under physiological conditions, i.e., suitable for the labeling of delicate biological vectors such as antibodies, and the complexes are very stable and inert. Importantly, for TAT with 225Ac, the daughter nuclides 213Bi and 209Pb also form stable complexes, and this is important for reducing damage to healthy tissue.

Toward 68Ga and 64Cu Positron Emission Tomography Probes: Is H2dedpa-N,N'-pram the Missing Link for dedpa Conjugation?

H2dedpa-N,N'-pram (H2L1), a new chelator derived from the hexadentate ligand 1,2-bis[[(6-carboxypyridin-2-yl)methyl]amino]ethane (H2dedpa), which incorporates 3-propylamine chains anchored to the secondary amines of the ethylenediamine core of the latter, has emerged as a very promising scaffold for preparing 68Ga- and 64Cu-based positron emission tomography probes. This new platform is cost-effective and easy to prepare, and the two pendant primary amines make it versatile for the preparation of bifunctional chelators by conjugation and/or click chemistry. Reported herein, we have also included the related H2dedpa-N,N'-prpta (H2L2) platform as a simple structural model for its conjugated systems. X-ray crystallography confirmed that the N4O2 coordination sphere provided by the dedpa2- core is maintained at both Ga(III) and Cu(II). The complex formation equilibria were deeply investigated by a thorough multitechnique approach with potentiometric, NMR spectrometric, and UV-vis spectrophotometric titrations, revealing effective chelation. The thermodynamic stability of the Ga(III) complexes at physiological relevant conditions is slightly higher than that of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), the common and clinically approved chelator used in the clinic [pGa = 19.5 (dedpa-N,N'-pram) and 20.8 (dedpa-N,N'-prpta) versus 18.5 (DOTA) at identical conditions], and significantly higher for the Cu(II) complexes [pCu = 21.96 (dedpa-N,N'-pram) and 22.8 (dedpa-N,N'-prpta) versus 16.2 (DOTA)], which are even more stable than that of the parent ligand dedpa2- (pCu = 18.5) and that of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) (pCu = 18.5). This high stability found for Cu(II) complexes is related to the conversion of the secondary amines of the ethylenediamine core of dedpa2- into tertiary amines, whereby the architecture of the new H2L1 chelator is doubly optimal in the case of this metal ion: high accessibility of the primary amine groups and their incorporation via the secondary amines, which contributes to a significant increase in the stability of the metal complex. Quantitative labeling of both chelators with both radionuclides ([68Ga]Ga3+ and [64Cu]Cu2+) was observed within 15 min at room temperature with concentrations as low as 10-5 M. Furthermore, serum stability studies confirmed a high radiochemical in vitro stability of all systems and therefore confirmed H2L1 as a promising and versatile chelator for further radiopharmaceutical in vivo studies.

Tuning the Framework of Thioether-Functionalized Polyazamacrocycles: Searching for a Chelator for Theranostic Silver Radioisotopes

Silver-111 is an attractive unconventional candidate for targeted cancer therapy as well as for single photon emission computed tomography and can be complemented by silver-103 for positron emission tomography noninvasive diagnostic procedures. However, the shortage of chelating agents capable of forming stable complexes tethered to tumor-seeking vectors has hindered their in vivo application so far. In this study, a comparative investigation of a series of sulfur-containing structural homologues, namely, 1,4,7-tris[2-(methylsulfanyl)ethyl)]-1,4,7-triazacyclononane (NO3S), 1,5,9-tris[2-(methylsulfanyl)ethyl]-1,5,9-triazacyclododecane (TACD3S), 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclotridecane (TRI4S), and 1,4,8,11-tetrakis[2-(methylsulfanyl)ethyl]-1,4,8,11-tetraazacyclotetradecane (TE4S) was conducted to appraise the influence of different polyazamacrocyclic backbones on Ag+ complexation. The performances of these macrocycles were also compared with those of the previously reported Ag+/[111Ag]Ag+-chelator 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S). Nuclear magnetic resonance data supported by density functional theory calculations and X-ray crystallographic results gave insights into the coordination environment of these complexes, suggesting that all of the donor atoms are generally involved in the metal coordination. However, the modifications of the macrocycle topology alter the dynamic binding of the pendant arms or the conformation of the ring around the metal center. Combined pH/pAg-potentiometric and spectroscopic experiments revealed that the 12-member N4 backbone of DO4S forms the most stable Ag+ complex while both the enlargement and the shrinkage of the macrocyclic frame dwindle the stability of the complexes. Radiolabeling experiments, conducted with reactor-produced [111Ag]Ag+, evidenced that the thermodynamic stability trend is reflected in the ligand's ability to incorporate the radioactive ion at high molar activity, even in the presence of a competing cation (Pd2+), as well as in the integrity of the corresponding complexes in human serum. As a consequence, DO4S proved to be the most favorable candidate for future in vivo applications.

Separation of 44mSc/44gSc Nuclear Isomers Based on After-Effects

44gSc presents a particular interest for application in nuclear medicine for positron emission tomography (PET) due to its favorable nuclear decay properties (t1/2 = 3.97 h, Emax = 1.47 MeV, branching ratio 94.3% β+). Its nuclear isomer 44mSc (t1/2 = 58.61 h) decays by isomeric transition (IT) into 44gSc, accompanied by ≈12% of conversion electron emission, which can cause a partial release of the daughter 44gSc from the chelate complex. A 13 MeV cyclotron at TRIUMF was used to produce both 44mSc and 44gSc via the natCa(p,n)44m,gSc reaction. A 44mSc/44gSc generator was designed by using a Strata C-18E cartridge. After several tested systems, a successful separation method was developed using DOTATOC as a chelator, a Strata C-18E cartridge as a generator column, and an elution solution of 0.1 M NH4-α-HIB. The yield of the generator with the daughter 44gSc release was equal to 9.8 ± 1.0% (or ≈80% per portion of conversion). This result shows the important role of after-effects in the design of radionuclide generators. Nuclear cross-section calculations were applied using the TALYS code to allow for the determination of the most promising alternative routes for 44mSc production, which will enable the development of a full-scale 44mSc/44gSc radionuclide generator based on after-effects.

H3 Derivatives Possessing Picolyl and Picolinate Pendants for Ga3+ Coordination and 67Ga3+ Radiolabeling

We synthesized, thanks to the regiospecific N-functionalization using an orthoamide intermediate, two 1,4,7-triazacyclononane derivatives containing an acetate arm and either a methylpyridine or a picolinic acid group, respectively, Hnoapy and H2noapa, as new Ga3+ chelators for potential use in nuclear medicine. The corresponding Ga3+ complexes were synthesized and structurally characterized in solution by 1H and 13C NMR. The [Ga(noapy)]2+ complex appears to exist in solution as two diasteroisomeric pairs of enantiomers, as confirmed by density functional theory (DFT) calculations, while for [Ga(noapa)]+, a single species is present in solution. Solid-state investigations were possible for the [Ga(noapa)]+ complex, which crystallized from water as a pair of enantiomers. The average length of the N-Ga bonds of 2.090 Å is identical with that found for the [Ga(nota)] complex, showing that the presence of the picolinate arm does not hinder the coordination of the ligand to the metal ion. Protonation constants of noapy- and noapa2- were determined by potentiometric titrations, providing an overall basicity ∑log KiH (i = 1-4) that increases in the order noapy- < noapa2- < nota3- with increases in the negative charge of the ligand. Stability constants determined by pH-potentiometric titrations supplemented with 71Ga NMR data show that the stabilities of [Ga(noapy)]2+ and [Ga(noapa)]+ are lower compared to that of [Ga(nota)] but higher than those of other standards such as [Ga(aazta)]-. 67Ga radiolabeling studies were performed in order to demonstrate the potential of these chelators for 67/68Ga-based radiopharmaceuticals. The labelings of Hnoapy and H2noapa were nearly identical, outperforming H3nota. Stability studies were conducted in phosphate-buffered saline and in the presence of human serum transferrin, revealing no significant decomplexation of [67Ga][Ga(noapy)]2+ and [67Ga][Ga(noapa)]+ compared to [67Ga][Ga(nota)]. Finally, all complexes were found to be highly hydrophilic, with calculated log D7.4 values of -3.42 ± 0.05, -3.34 ± 0.04, and -3.00 ± 0.23 for Hnoapy, H2noapa, and H3nota, respectively, correlating with the charge of each complex and the electrostatic potentials obtained with DFT.

Is Smaller Better? Cu2+/Cu+ Coordination Chemistry and Copper-64 Radiochemical Investigation of a 1,4,7-Triazacyclononane-Based Sulfur-Rich Chelator

The biologically triggered reduction of Cu2+ to Cu+ has been postulated as a possible in vivo decomplexation pathway in 64/67Cu-based radiopharmaceuticals. In an attempt to hinder this phenomenon, we have previously developed a family of S-containing polyazamacrocycles based on 12-, 13-, or 14-membered tetraaza rings able to stabilize both oxidation states. However, despite the high thermodynamic stability of the resulting Cu2+/+ complexes, a marked [64Cu]Cu2+ release was detected in human serum, likely as a result of the partially saturated coordination sphere around the copper center. In the present work, a new hexadentate macrocyclic ligand, 1,4,7-tris[2-(methylsulfanyl)ethyl)]-1,4,7-triazacyclononane (NO3S), was synthesized by hypothesizing that a smaller macrocyclic backbone could thwart the observed demetalation by fully encapsulating the copper ion. To unveil the role of the S donors in the metal binding, the corresponding alkyl analogue 1,4,7-tris-n-butyl-1,4,7-triazacyclononane (TACN-n-Bu) was considered as comparison. The acid-base properties of the free ligands and the kinetic, thermodynamic, and structural properties of their Cu2+ and Cu+ complexes were investigated in solution and solid (crystal) states through a combination of spectroscopic and electrochemical techniques. The formation of two stable mononuclear species was detected in aqueous solution for both ligands. The pCu2+ value for NO3S at physiological pH was 6 orders of magnitude higher than that computed for TACN-n-Bu, pointing out the significant stabilizing contribution arising from the Cu2+-S interactions. In both the solid state and solution, Cu2+ was fully embedded in the ligand cleft in a hexacoordinated N3S3 environment. Furthermore, NO3S exhibited a remarkable ability to form a stable complex with Cu+ through the involvement of all of the donors in the coordination sphere. Radiolabeling studies evidenced an excellent affinity of NO3S toward [64Cu]Cu2+, as quantitative incorporation was achieved at high apparent molar activity (∼10 MBq/nmol) and under mild conditions (ambient temperature, neutral pH, 10 min reaction time). Human serum stability assays revealed an increased stability of [64Cu][Cu(NO3S)]2+ when compared to the corresponding complexes formed by 12-, 13-, or 14-membered tetraaza rings.

Comparative Study of a Decadentate Acyclic Chelate, HOPO-O10, and Its Octadentate Analogue, HOPO-O8, for Radiopharmaceutical Applications

Radiolanthanides and actinides are aptly suited for the diagnosis and treatment of cancer via nuclear medicine because they possess unique chemical and physical properties (e.g., radioactive decay emissions). These rare radiometals have recently shown the potential to selectively deliver a radiation payload to cancer cells. However, their clinical success is highly dependent on finding a suitable ligand for stable chelation and conjugation to a disease-targeting vector. Currently, the commercially available chelates exploited in the radiopharmaceutical design do not fulfill all of the requirements for nuclear medicine applications, and there is a need to further explore their chemistry to rationally design highly specific chelates. Herein, we describe the rational design and chemical development of a novel decadentate acyclic chelate containing five 1,2-hydroxypyridinones, 3,4,3,3-(LI-1,2-HOPO), referred to herein as HOPO-O10, based on the well-known octadentate ligand 3,4,3-(LI-1,2-HOPO), referred to herein as HOPO-O8, a highly efficient chelator for 89Zr[Zr4+]. Analysis by 1H NMR spectroscopy and mass spectrometry of the La3+ and Tb3+ complexes revealed that HOPO-O10 forms bimetallic complexes compared to HOPO-O8, which only forms monometallic species. The radiolabeling properties of both chelates were screened with [135La]La3+, [155/161Tb]Tb3+, [225Ac]Ac3+ and, [227Th]Th4+. Comparable high specific activity was observed for the [155/161Tb]Tb3+ complexes, outperforming the gold-standard 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, yet HOPO-O10 surpassed HOPO-O8 with higher [227Th]Th4+ affinity and improved complex stability in a human serum challenge assay. A comprehensive analysis of the decadentate and octadentate chelates was performed with density functional theory for the La3+, Ac3+, Eu3+, Tb3+, Lu3+, and Th4+ complexes. The computational simulations demonstrated the enhanced stability of Th4+-HOPO-O10 over Th4+-HOPO-O8. This investigation reveals the potential of HOPO-O10 for the stable chelation of large tetravalent radioactinides for nuclear medicine applications and provides insight for further chelate development.

The Curies' element: state of the art and perspectives on the use of radium in nuclear medicine

BACKGROUND

The alpha-emitter radium-223 (223Ra) is presently used in nuclear medicine for the palliative treatment of bone metastases from castration-resistant prostate cancer. This application arises from its advantageous decay properties and its intrinsic ability to accumulate in regions of high bone turnover when injected as a simple chloride salt. The commercial availability of [223Ra]RaCl2 as a registered drug (Xofigo®) is a further additional asset.

MAIN BODY

The prospect of extending the utility of 223Ra to targeted α-therapy of non-osseous cancers has garnered significant interest. Different methods, such as the use of bifunctional chelators and nanoparticles, have been explored to incorporate 223Ra in proper carriers designed to precisely target tumor sites. Nevertheless, the search for a suitable scaffold remains an ongoing challenge, impeding the diffusion of 223Ra-based radiopharmaceuticals.

CONCLUSION

This review offers a comprehensive overview of the current role of radium radioisotopes in nuclear medicine, with a specific focus on 223Ra. It also critically examines the endeavors conducted so far to develop constructs capable of incorporating 223Ra into cancer-targeting drugs. Particular emphasis is given to the chemical aspects aimed at providing molecular scaffolds for the bifunctional chelator approach.

A review of recent advancements in Actinium-225 labeled compounds and biomolecules for therapeutic purposes

In nuclear medicine, cancers that cannot be cured or can only be treated partially by traditional techniques like surgery or chemotherapy are killed by ionizing radiation as a form of therapeutic treatment. Actinium-225 is an alpha-emitting radionuclide that is highly encouraging as a therapeutic approach and more promising for targeted alpha therapy (TAT). Actinium-225 is the best candidate for tumor cells treatment and has physical characteristics such as high (LET) linear energy transfer (150 keV per μm), half-life (t1/2  = 9.92d), and short ranges (400-100 μm) which prevent the damage of normal healthy tissues. The introduction of various new radiopharmaceuticals and radioisotopes has significantly assisted the advancement of nuclear medicine. Ac-225 radiopharmaceuticals continuously demonstrate their potential as targeted alpha therapeutics. 225 Ac-labeled radiopharmaceuticals have confirmed their importance in medical and clinical areas by introducing [225 Ac]Ac-PSMA-617, [225 Ac]Ac-DOTATOC, [225 Ac]Ac-DOTA-substance-P, reported significantly improved response in patients with prostate cancer, neuroendocrine, and glioma, respectively. The development of these radiopharmaceuticals required a suitable buffer, incubation time, optimal pH, and reaction temperature. There is a growing need to standardize quality control (QC) testing techniques such as radiochemical purity (RCP). This review aims to summarize the development of the Ac-225 labeled compounds and biomolecules. The current state of their reported resulting clinical applications is also summarized as well.

Targeted Alpha Therapy (TAT) with Single-Domain Antibodies (Nanobodies)

The persistent threat of cancer necessitates the development of improved and more efficient therapeutic strategies that limit damage to healthy tissues. Targeted alpha therapy (TαT), a novel form of radioimmuno-therapy (RIT), utilizes a targeting vehicle, commonly antibodies, to deliver high-energy, but short-range, alpha-emitting particles specifically to cancer cells, thereby reducing toxicity to surrounding normal tissues. Although full-length antibodies are often employed as targeting vehicles for TαT, their high molecular weight and the presence of an Fc-region lead to a long blood half-life, increased bone marrow toxicity, and accumulation in other tissues such as the kidney, liver, and spleen. The discovery of single-domain antibodies (sdAbs), or nanobodies, naturally occurring in camelids and sharks, has introduced a novel antigen-specific vehicle for molecular imaging and TαT. Given that nanobodies are the smallest naturally occurring antigen-binding fragments, they exhibit shorter relative blood half-lives, enhanced tumor uptake, and equivalent or superior binding affinity and specificity. Nanobody technology could provide a viable solution for the off-target toxicity observed with full-length antibody-based TαT. Notably, the pharmacokinetic properties of nanobodies align better with the decay characteristics of many short-lived α-emitting radionuclides. This review aims to encapsulate recent advancements in the use of nanobodies as a vehicle for TαT.

Production and radiochemistry of antimony-120m: Efforts toward Auger electron therapy with 119Sb

Targeted Meitner-Auger Therapy (TMAT) has potential for personalized treatment thanks to its subcellular dosimetric selectivity, which is distinct from the dosimetry of β- and α particle emission based Targeted Radionuclide Therapy (TRT). To date, most clinical and preclinical TMAT studies have used commercially available radionuclides. These studies showed promising results despite using radionuclides with theoretically suboptimal photon to electron ratios, decay kinetics, and electron emission spectra. Studies using radionuclides whose decay characteristics are considered more optimal are therefore important for evaluation of the full potential of Meitner-Auger therapy; 119Sb is among the best such candidates. In the present work, we develop radiochemical purification of 120Sb from irradiated natural tin targets for TMAT studies with 119Sb.

Vitamin-based radiopharmaceuticals for tumor imaging

Advances in nuclear medicine, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), are among the most sensitive methods that can be used in diagnostic imaging and radionuclide therapy with higher efficiency and reduced toxicity benefits. In order to improve the success of treatment, it is also important to develop methods that can be used when lesions can be detected at the earliest stages. Vitamins are macromolecules that play a crucial role in numerous biological processes in both animals and humans. Escalating development of vitamin-based radiopharmaceuticals for application in the diagnosis and treatment of cancers. In this review, we aimed to discuss about recent research utilizing radio-labeled vitamins for targeted tumor imaging.

Radiolabeled nanomaterial for cancer diagnostics and therapeutics: principles and concepts

In the last three decades, radiopharmaceuticals have proven their effectiveness for cancer diagnosis and therapy. In parallel, the advances in nanotechnology have fueled a plethora of applications in biology and medicine. A convergence of these disciplines has emerged more recently with the advent of nanotechnology-aided radiopharmaceuticals. Capitalizing on the unique physical and functional properties of nanoparticles, radiolabeled nanomaterials or nano-radiopharmaceuticals have the potential to enhance imaging and therapy of human diseases. This article provides an overview of various radionuclides used in diagnostic, therapeutic, and theranostic applications, radionuclide production through different techniques, conventional radionuclide delivery systems, and advancements in the delivery systems for nanomaterials. The review also provides insights into fundamental concepts necessary to improve currently available radionuclide agents and formulate new nano-radiopharmaceuticals.

Postmortem reference concentrations of 68 elements in blood and urine

INTRODUCTION

Fatal intoxications, both accidental and intentional, are a global issue. In the Western world, intoxications with pharmaceuticals dominate, but in other parts of the world, other substances are more common. In a forensic setting, elemental intoxications are of great importance when investigating both accidental, suicidal, and homicidal deaths. The current study presents normal postmortem reference concentrations of 68 elements in femoral blood and urine. In addition, possible sources of error such as contamination from sample tubes, preservative potassium fluoride (KF) solution, and storage time are evaluated.

METHODS

Paired femoral blood and urine samples from 120 cases of death by suicidal hanging in Sweden were collected. Additionally, multiple batches of sample tubes and multiple batches of KF solution were also analyzed. Concentrations of elements were determined by double focusing sector field ICP-MS.

RESULTS

Key descriptive statistics for 68 elements are provided in blood and urine. Contamination from sample tubes was minor compared to the overall mean elemental concentrations in both blood and urine. KF solution contained a large assortment of elements, but the overall contribution is relatively minor for most elements given the small amounts of solution added to samples. There were significant differences for 22 elements in blood and 17 elements in urine between samples with short and long storage time.

CONCLUSION

The present study provides an important tool when evaluating postmortem elemental concentrations. It fills a needed gap between large antemortem population studies and postmortem case reports or small case series of elemental intoxications.

Selective Chelation of the Exotic Meitner-Auger Emitter Mercury-197 m/g with Sulfur-Rich Macrocyclic Ligands: Towards the Future of Theranostic Radiopharmaceuticals

Mercury-197 m/g are a promising pair of radioactive isomers for incorporation into a theranostic as they can be used as a diagnostic agent using SPECT imaging and a therapeutic via Meitner-Auger electron emissions. However, the current absence of ligands able to stably coordinate ^197m/g Hg to a tumour-targeting vector precludes their use in vivo. To address this, we report herein a series of sulfur-rich chelators capable of incorporating ^197m/g Hg into a radiopharmaceutical. 1,4,7,10-Tetrathia-13-azacyclopentadecane (NS₄ ) and its derivatives, (2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetic acid (NS₄ -CA) and N-benzyl-2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetamide (NS₄ -BA), were designed, synthesized and analyzed for their ability to coordinate Hg²⁺ through a combination of theoretical (DFT) and experimental coordination chemistry studies (NMR and mass spectrometry) as well as ^197m/g Hg radiolabeling studies and in vitro stability assays. The development of stable ligands for ^197m/g Hg reported herein is extremely impactful as it would enable their use for in vivo imaging and therapy, leading to personalized treatments for cancer.

H3TPAN-Triazole-Bn-NH2: Tripicolinate Clicked-Bifunctional Chelate for [225Ac]/[111In] Theranostics

A new, high-denticity, bifunctional ligand─H₃TPAN-triazole-Bn-NH₂─has been synthesized and studied in complexation with [²²⁵Ac]Ac³⁺ and [¹¹¹In]In³⁺ for radiopharmaceutical applications. The bifunctional chelator is readily synthesized, using a high-yielding four-step prep, which is highly adaptable and allows for straightforward incorporation of different covalent linkers using Cu^I-catalyzed alkyne-azide cycloaddition (click) chemistry. Nuclear magnetic resonance (NMR) studies of H₃TPAN-triazole-Bn-NH₂ with La³⁺ and In³⁺ metal ions show the formation of a single, asymmetric complex with each ion in solution, corroborated by density functional theory (DFT) calculations. Radiolabeling studies with [²²⁵Ac]Ac³⁺ and [¹¹¹In]In³⁺ showed highly effective complexation, achieving quantitative radiochemical conversions at low ligand concentrations (<10^-6 M) under mild conditions (RT, 10 min), which is further accompanied by high stability in human serum. The bioconjugate─H₃TPAN-triazole-Bn-Aoc-Pip-Nle-CycMSH_hex─was prepared for targeting of MC1R-positive tumors, and the corresponding ¹¹¹In-radiolabeled tracer was studied in vivo. SPECT/CT and biodistribution studies in C57BL/6J mice bearing B16-F10 tumors were performed, with the radiotracer showing good in vivo stability; tumor uptake was achieved. This work highlights a new promising and versatile bifunctional chelator, easily prepared and encouraging for ²²⁵Ac/¹¹¹In theranostics.

On the dissociation pathways of copper complexes relevant as PET imaging agents

Several bifunctional chelators have been synthesized in the last years for the development of new ⁶⁴Cu-based PET agents for in vivo imaging. When designing a metal-based PET probe, it is important to achieve high stability and kinetic inertness once the radioisotope is coordinated. Different competitive assays are commonly used to evaluate the possible dissociation mechanisms that may induce Cu(II) release in the body. Among them, acid-assisted dissociation tests or transchelation challenges employing EDTA or SOD are frequently used to evaluate both solution thermodynamics and the kinetic behavior of potential metal-based systems. Despite of this, the Cu(II)/Cu(I) bioreduction pathway that could be promoted by the presence of bioreductants still remains little explored. To fill this gap we present here a detailed spectroscopic study of the kinetic behavior of different macrocyclic Cu(II) complexes. The complexes investigated include the cross-bridge cyclam derivative [Cu(CB-TE1A)]⁺, whose structure was determined using single-crystal X-ray diffraction. The acid-assisted dissociation mechanism was investigated using HClO₄ and HCl to analyse the effect of the counterion on the rate constants. The complexes were selected so that the effects of complex charge and coordination polyhedron could be assessed. Cyclic voltammetry experiments were conducted to investigate whether the reduction to Cu(I) falls within the window of common bioreducing agents. The most striking behavior concerns the [Cu(NO2Th)]²⁺ complex, a 1,4,7-triazacyclononane derivative containing two methylthiazolyl pendant arms. This complex is extremely inert with respect to dissociation following the acid-catalyzed mechanism, but dissociates rather quickly in the presence of a bioreductant like ascorbic acid.

Bismuth chelation for targeted alpha therapy: Current state of the art

Current interest in the α-emitting bismuth radionuclides, bismuth-212 (²¹²Bi) and bismuth-213 (²¹³Bi), stems from their great potential for targeted alpha therapy (TAT), an expanding and promising approach for the treatment of micrometastatic disease and the eradication of single malignant cells. To selectively deliver their emission to the cancer cells, these radiometals must be firmly coordinated by a bifunctional chelator (BFC) attached to a tumour-seeking vector. This review provides a comprehensive overview of the current state-of-the-art chelating agents for bismuth radioisotopes. Several aspects are reported, from their 'cold' chelation chemistry (thermodynamic, kinetic, and structural properties) and radiolabelling investigations to the preclinical and clinical studies performed with a variety of bioconjugates. The aim of this review is to provide both a guide for the rational design of novel optimal platforms for the chelation of these attractive α-emitters and emphasize the prospects of the most encouraging chelating agents proposed so far.

Cutting edge rare earth radiometals: prospects for cancer theranostics

Background

With recent advances in novel approaches to cancer therapy and imaging, the application of theranostic techniques in personalised medicine has emerged as a very promising avenue of research inquiry in recent years. Interest has been directed towards the theranostic potential of Rare Earth radiometals due to their closely related chemical properties which allow for their facile and interchangeable incorporation into identical bifunctional chelators or targeting biomolecules for use in a diverse range of cancer imaging and therapeutic applications without additional modification, i.e. a “one-size-fits-all” approach. This review will focus on recent progress and innovations in the area of Rare Earth radionuclides for theranostic applications by providing a detailed snapshot of their current state of production by means of nuclear reactions, subsequent promising theranostic capabilities in the clinic, as well as a discussion of factors that have impacted upon their progress through the theranostic drug development pipeline.

Main body

In light of this interest, a great deal of research has also been focussed towards certain under-utilised Rare Earth radionuclides with diverse and favourable decay characteristics which span the broad spectrum of most cancer imaging and therapeutic applications, with potential nuclides suitable for α-therapy (¹⁴⁹Tb), β⁻-therapy (⁴⁷Sc, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁶⁹Er, ¹⁴⁹Pm, ¹⁴³Pr, ¹⁷⁰Tm), Auger electron (AE) therapy (¹⁶¹Tb, ¹³⁵La, ¹⁶⁵Er), positron emission tomography (⁴³Sc, ⁴⁴Sc, ¹⁴⁹Tb, ¹⁵²Tb, ¹³²La, ¹³³La), and single photon emission computed tomography (⁴⁷Sc, ¹⁵⁵Tb, ¹⁵²Tb, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁴⁹Pm, ¹⁷⁰Tm). For a number of the aforementioned radionuclides, their progression from ‘bench to bedside’ has been hamstrung by lack of availability due to production and purification methods requiring further optimisation.

Conclusions

In order to exploit the potential of these radionuclides, reliable and economical production and purification methods that provide the desired radionuclides in high yield and purity are required. With more reactors around the world being decommissioned in future, solutions to radionuclide production issues will likely be found in a greater focus on linear accelerator and cyclotron infrastructure and production methods, as well as mass separation methods. Recent progress towards the optimisation of these and other radionuclide production and purification methods has increased the feasibility of utilising Rare Earth radiometals in both preclinical and clinical settings, thereby placing them at the forefront of radiometals research for cancer theranostics.

Trastuzumab-conjugated oxine-based ligand for [89Zr]Zr4+ immunoPET

A new, bifunctional chelating ligand for immuno-Positron Emission Tomography (PET) was designed, synthesized, and conjugated to Trastuzumab for a proof-of-concept study with ⁸⁹Zr. H₄neunox was synthesized from the tris(2-aminoethyl)amine backbone, decorated with 8-hydroxyquinoline moieties, and utilizes a primary amine for functionalization. A maleimide moiety extends the chelator to create H₄neunox-mal for antibody conjugation via maleimide-thiol click chemistry. Preliminary ⁸⁹Zr radiolabeling of H₄neunox indicated quantitative radiolabeling at 1 × 10^-5 M, but improved inertness towards human serum (96% intact at 7 d) and Fe³⁺ (92% intact at 24 h) compared to the previously synthesized H₅decaox. The chelator was successfully conjugated to the monoclonal antibody, Trastuzumab, and used in preliminary radiolabeling reactions (37 °C, 2 h) with ⁸⁹Zr. Radiochemical assessments of the new H₄neunox-Trastuzumab conjugate include ⁸⁹Zr radiolabeling, spin filter purification, cell-binding immunoreactivity, and in vivo PET imaging and biodistribution in SKOV-3 tumour bearing nude mice, performed in comparison with the desferrioxamine B analog, DFO-Trastuzumab. The [⁸⁹Zr]Zr(neunox-Trastuzumab) showed lowered inertness towards serum (76% intact at 24 h) as well as demetallation in vivo through bone uptake (21% ID/g) in PET imaging and biodistribution studies when compared to [⁸⁹Zr]Zr(DFO-Trastuzumab). Although the combination of the chelator and antibody had detrimental effects on their intended purposes, nonetheless, the primary amine platform of H₄neunox developed here provides an oxine-based bifunctional ligand for further derivatizations with other targeting vectors.

Chelation of Theranostic Copper Radioisotopes with S-Rich Macrocycles: From Radiolabelling of Copper-64 to In Vivo Investigation

Copper radioisotopes are generally employed for cancer imaging and therapy when firmly coordinated via a chelating agent coupled to a tumor-seeking vector. However, the biologically triggered Cu²⁺-Cu⁺ redox switching may constrain the in vivo integrity of the resulting complex, leading to demetallation processes. This unsought pathway is expected to be hindered by chelators bearing N, O, and S donors which appropriately complements the borderline-hard and soft nature of Cu²⁺ and Cu⁺. In this work, the labelling performances of a series of S-rich polyazamacrocyclic chelators with [⁶⁴Cu]Cu²⁺ and the stability of the [⁶⁴Cu]Cu-complexes thereof were evaluated. Among the chelators considered, the best results were obtained with 1,7-bis [2-(methylsulfanyl)ethyl]-4,10,diacetic acid-1,4,7,10-tetraazacyclododecane (DO2A2S). DO2A2S was labelled at high molar activities in mild reaction conditions, and its [⁶⁴Cu]Cu²⁺ complex showed excellent integrity in human serum over 24 h. Biodistribution studies in BALB/c nude mice performed with [⁶⁴Cu][Cu(DO2A2S)] revealed a behavior similar to other [⁶⁴Cu]Cu-labelled cyclen derivatives characterized by high liver and kidney uptake, which could either be ascribed to transchelation phenomena or metabolic processing of the intact complex.

H2ampa─Versatile Chelator for [203Pb]Pb2+, [213Bi]Bi3+, and [225Ac]Ac3

A new decadentate chelator, H₂ampa, was designed to be a potential radiopharmaceutical chelator component. The chelator involves both amide and picolinate functional groups on a large non-macrocyclic, ether-bridged backbone. With its large scaffold, H₂ampa was paired with [^nat/203Pb]Pb²⁺, [^nat/213Bi]Bi³⁺, and ^natLa³⁺/[²²⁵Ac]Ac³⁺ ions. Nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry were used to study the non-radioactive metal complexes. A single crystal of [Bi(ampa)](NO₃) was obtained; its asymmetric, 10-coordinate complex structure was revealed by X-ray diffraction. Optimal conformations of the metal complexes were assessed by density functional theory studies to provide further structural information. Solution studies providing thermodynamic insights into metal complex formation revealed H₂ampa coordinated Bi³⁺, Pb²⁺, and La³⁺ ions to obtain pM values of 26, 14.8, and 15.1, respectively. Preliminary concentration-dependent radiolabeling experiments were carried out between H₂ampa and three different radiometals to evaluate their compatibility for radiopharmaceutical applications. The chelator radiolabeled [²⁰³Pb]Pb²⁺, [²¹³Bi]Bi³⁺, and [²²⁵Ac]Ac³⁺ in short reaction times (7-30 min), at dilute concentrations, and under mild conditions. Thus, H₂ampa was proven to be a versatile chelator able to well coordinate a small range of radiometals frequently considered to be alpha therapeutic candidates.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

Revisiting Lead(II)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic Acid Coordination Chemistry in Aqueous Solutions: Evidence of an Underestimated Thermodynamic Stability

The complexes formed between Pb²⁺ and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) were reinvestigated in aqueous solutions using a combination of pH potentiometry, UV-vis spectroscopy, and NMR spectroscopy. The thermodynamic data were supported by kinetics assays. Differently protonated complexes, i.e., [PbH₃L]⁺, [PbH₂L], [PbHL]^-, and [PbL]^2-, were detected, and the corresponding stability constants (logβ) at T = 298 K and I = 0.1 M NaCl were 33.1 ± 0.2, 32.00 ± 0.06, 29.28 ± 0.06, and 25.3 ± 0.1, respectively. Results differed significantly from those previously reported by Chaves et al. (Talanta1992, 39, 249) and Pippin et al. (Inorg. Chim. Acta1995, 239, 43) in both the speciation and the overall complex stability; the latter in particular was found to be remarkably higher. The work disclosed herein provides revised data on the Pb²⁺-DOTA complexes, which should be used as a new stability benchmark during the development of lead chelators.

Dual-Labelling Strategies for Nuclear and Fluorescence Molecular Imaging: Current Status and Future Perspectives

Molecular imaging offers the possibility to investigate biological and biochemical processes non-invasively and to obtain information on both anatomy and dysfunctions. Based on the data obtained, a fundamental understanding of various disease processes can be derived and treatment strategies can be planned. In this context, methods that combine several modalities in one probe are increasingly being used. Due to the comparably high sensitivity and provided complementary information, the combination of nuclear and optical probes has taken on a special significance. In this review article, dual-labelled systems for bimodal nuclear and optical imaging based on both modular ligands and nanomaterials are discussed. Particular attention is paid to radiometal-labelled molecules for single-photon emission computed tomography (SPECT) and positron emission tomography (PET) and metal complexes combined with fluorescent dyes for optical imaging. The clinical potential of such probes, especially for fluorescence-guided surgery, is assessed.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

[213Bi]Bi3+/[111In]In3+-neunpa-cycMSH: Theranostic Radiopharmaceutical Targeting Melanoma─Structural, Radiochemical, and Biological Evaluation

With the emergence of [²²⁵Ac]Ac³⁺ as a therapeutic radionuclide for targeted α therapy (TAT), access to clinical quantities of the potent, short-lived α-emitter [²¹³Bi]Bi³⁺ (t_1/2 = 45.6 min) will increase over the next decade. With this in mind, the nonadentate chelator, H₄neunpa-NH₂, has been investigated as a ligand for chelation of [²¹³Bi]Bi³⁺ in combination with [¹¹¹In]In³⁺ as a suitable radionuclidic pair for TAT and single photon emission computed tomography (SPECT) diagnostics. Nuclear magnetic resonance (NMR) spectroscopy was utilized to assess the coordination characteristics of H₄neunpa-NH₂ on complexation of [^natBi]Bi³⁺, while the solid-state structure of [^natBi][Bi(neunpa-NH₃)] was characterized via X-ray diffraction (XRD) studies, and density functional theory (DFT) calculations were performed to elucidate the conformational geometries of the metal complex in solution. H₄neunpa-NH₂ exhibited fast complexation kinetics with [²¹³Bi]Bi³⁺ at RT achieving quantitative radiolabeling within 5 min at 10^-8 M ligand concentration, which was accompanied by the formation of a kinetically inert complex. Two bioconjugates incorporating the melanocortin 1 receptor (MC1R) targeting peptide Nle-CycMSH_hex were synthesized featuring two different covalent linkers for in vivo evaluation with [²¹³Bi]Bi³⁺ and [¹¹¹In]In³⁺. High molar activities of 7.47 and 21.0 GBq/μmol were achieved for each of the bioconjugates with [²¹³Bi]Bi³⁺. SPECT/CT scans of the [¹¹¹In]In³⁺-labeled tracer showed accumulation in the tumor over time, which was accompanied by high liver uptake and clearance via the hepatic pathway due to the high lipophilicity of the covalent linker. In vivo biodistribution studies in C57Bl/6J mice bearing B16-F10 tumor xenografts showed good tumor uptake (5.91% ID/g) at 1 h post-administration with [²¹³Bi][Bi(neunpa-Ph-Pip-Nle-CycMSH_hex)]. This study demonstrates H₄neunpa-NH₂ to be an effective chelating ligand for [²¹³Bi]Bi³⁺ and [¹¹¹In]In³⁺, with promising characteristics for further development toward theranostic applications.

When ring makes the difference: coordination properties of Cu2+/Cu+ complexes with sulfur-pendant polyazamacrocycles for radiopharmaceutical applications

The Cu2+/+ complexes formed by sulfur-containing polyazamacrocycles were studied in aqueous solution using potentiometry, UV-Vis, NMR, EPR, and cyclic voltammetry.

[nat/89Zr][Zr(pypa)]: Thermodynamically Stable and Kinetically Inert Binary Nonadentate Complex for Radiopharmaceutical Applications

H₄pypa is a nonadentate nonmacrocyclic chelator, which previously demonstrated high affinity for scandium-44, lutetium-177, and indium-111. Herein, we report the highly stable binary [Zr(pypa)] complex; the nonradioactive complex was synthesized and characterized in detail using high-resolution electrospray-ionization mass spectroscopy (HR-ESI-MS) and various nuclear magnetic resonance spectroscopies (NMR), which revealed C_2v symmetry of the complex. The geometry of [Zr(pypa)] was further detailed via X-ray crystallography and compared with the structure of [Fe(Hpypa)]. Despite a slow complexation rate with an association half-life of 31.4 h at pH 2 and room temperature, the [Zr(pypa)] complex is thermodynamically stable (log K_ML = 38.92, pZr = 39.4). Radiochemical studies demonstrated quantitative radiolabeling achieved at 10 μM chelator concentration within 2 h at 40 °C and pH = 7, antibody-compatible conditions. Of the utmost importance, [⁸⁹Zr][Zr(pypa)] is highly kinetically inert upon challenge with excess EDTA and DFO ligands, superior to [⁸⁹Zr][Zr(DFO)]⁺, and maintains inertness toward human serum.

Inorganic radiopharmaceutical chemistry of oxine

8-Hydroxyquinoline (8-HQ, oxine) is a small, monoprotic, bicyclic aromatic compound and its relative donor group orientation imparts impressive bidentate metal chelating abilities that have been exploited in a vast array of applications over decades. 8-HQ and its derivatives have been explored in medicinal applications including anti-neurodegeneration, anticancer properties, and antimicrobial activities. One long established use of 8-HQ in medicinal inorganic chemistry is the coordination of radioactive isotopes of metal ions in nuclear medicine. The metal-oxine complex with the single photon emission computed tomography (SPECT) imaging isotope [¹¹¹In]In³⁺ was developed in the 1970s and 1980s to radiolabel leukocytes for inflammation and infection imaging. The [¹¹¹In][In(oxine)₃] complex functions as an ionophore: a moderately stable lipophilic complex to enter cells; however, inside the cell environment [¹¹¹In]In³⁺ undergoes exchange and remains localized. As new developments have progressed towards radiopharmaceuticals capable of both imaging and therapy (theranostics), 8-HQ has been re-explored in recent years to investigate its potential to chelate larger radiometal ions with longer half-lives and different indications. Further, metal-oxine complexes have been used to study liposomes and other nanomaterials by tracking these nanomedicines in vivo. Expanding 8-HQ to multidentate ligands for highly thermodynamically stable and kinetically inert complexes has increased the possibilities of this small molecule in nuclear medicine. This article outlines the historic use of metal-oxine complexes in inorganic radiopharmaceutical chemistry, with a focus on recent advances highlighting the possibilities of developing higher denticity, targeted bifunctional chelators with 8-HQ.

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.

Improved separation scheme for 44Sc produced by irradiation of natCa targets with 12.8 MeV protons

INTRODUCTION

⁴⁴Sc is of great interest as a positron emission tomography (PET) radionuclide due to its suitable nuclear characteristics: E_β⁺max = 1.47 MeV, branching ratio 94.3% and convenient half-life of 3.97 h. Here, ⁴⁴Sc was produced via the widely used reaction ⁴⁴Ca (p,n)⁴⁴Sc using natural calcium as a target.

METHODS

The irradiation was performed at TRIUMF using the 13 MeV cyclotron. The separation consisted of a combination of DGA branched resin and Dowex 50Wx8 (200-400 mesh). The distribution coefficients of Sc³⁺ on Dowex 50Wx8 (NH₄⁺ form, 200-400 mesh) with ammonium α-hydroxyisobutyrate (pH = 4.8) medium were determined in this study.

RESULTS AND CONCLUSION

The tested scheme allows both a reliable separation of ⁴⁴Sc from the target material as well as from the other competitive metals and a final fraction with high specific activity. The achieved radiochemical yield was 95 ± 3%.

H2pyhox - Octadentate Bis(pyridyloxine)

A new versatile chelating ligand for intermediate size and softness radiometals [⁶⁴Cu]Cu²⁺ and [¹¹¹In]In³⁺, H₂pyhox, was synthesized by introducing pyridine as a new donor moiety to complement 8-hydroxyquinoline on an ethylenediamine backbone. The combination of pyridine and oxine as donor sets was explored through structural analysis, and crystals of the three metal complexes with Cu²⁺, La³⁺, and In³⁺ demonstrate how the ligand adapts to accommodate metal ions of different sizes and charge. Exhaustive in-batch UV solution studies characterized the protonation constants of the free ligand as well as the formation constants of the metal complexes with Cu²⁺, In³⁺, and La³⁺. Preliminary concentration-dependent radiolabeling studies with [¹¹¹In]In³⁺ and [⁶⁴Cu]Cu²⁺ show the robustness of H₂pyhox to successfully coordinate both radiometals under mild conditions (<15 min, room temperature, pH 6). H₂pyhox is the first oxinate ligand to successfully radiolabel [²²⁵Ac]Ac³⁺, albeit only at high concentrations (0.1-1 mM) with gentle heating to 37 °C. Whole serum, protein, and ligand challenge assays further demonstrate the kinetic inertness of the [¹¹¹In]In³⁺ and [⁶⁴Cu]Cu²⁺ radiometal-ligand complexes, confirming H₂pyhox to be a promising versatile radiopharmaceutical chelator.

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⁺.

Research Progress of Radiolabeled Asn-Gly-Arg (NGR) Peptides for Imaging and Therapy

Asn-Gly-Arg (NGR) motifs have vasculature-homing properties via interactions with the aminopeptidase N (CD13) expressed on tumor neovasculature. Numerous NGR peptides with different molecular scaffolds have been exploited for targeted delivery of different compounds for imaging and therapy. When conjugated with NGR, complexes recognize the CD13 receptor expressed on the tumor vasculature, which improves the specificity to tumor and avoids systematic toxic reactions. Both preclinical and clinical studies performed with these products suggest that NGR-mediated vascular targeting is an effective strategy for delivering bioactive amounts of cytokines to tumor endothelial cells. For molecular imaging, radiolabeled peptides have been the most successful approach and have been translated into clinic. This review describes current data on radiolabeled tumor vasculature-homing NGR peptides for imaging and therapy.

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.