Oxyaapa: A Picolinate-Based Ligand with Five Oxygen Donors that Strongly Chelates Lanthanides

Rearmed Bifunctional Chelating Ligand for 225Ac/155Tb Precision-Guided Theranostic Radiopharmaceuticals─H4noneunpaX

Superior bifunctional chelating ligands, which can sequester both α-emitting radionuclides (225Ac, 213Bi) and their diagnostic companions (155Tb, 111In), remain a formidable challenge to translating targeted alpha therapy, with complementary diagnostic imaging, to the clinic. H4noneupaX, a chelating ligand with an unusual diametrically opposed arrangement of pendant donor groups, has been developed to this end. H4noneunpaX preferentially complexes Ln3+ and An3+ ions, forming thermodynamically stable (pLa = 17.8, pLu = 21.3) and kinetically inert complexes─single isomeric species by nuclear magnetic resonance and density functional theory. Metal binding versatility demonstrated in radiolabeling [111In]In3+, [155Tb]Tb3+, [177Lu]Lu3+, and [225Ac]Ac3+ achieved high molar activities under mild conditions. Efficient, scalable synthesis enabled in vivo evaluation of bifunctional H4noneunpaX conjugated to two octreotate peptides targeting neuroendocrine tumors. Single photon emission computed tomography/CT and biodistribution studies of 155Tb-radiotracers in AR42J tumor-bearing mice showed excellent image contrast, good tumor uptake, and high in vivo stability. H4noneunpaX shows significant potential for theranostic applications involving 225Ac/155Tb or 177Lu/155Tb.

Hyperbranched Polyester Polyfumaratomaleate Doped with Gd(III) and Dy(III) Ions: Synthesis, Structure and Properties

For the first time, metal-polymer complexes have been synthesized using hyperbranched polyester polyfumaratomaleate as a matrix, the structure of which has been established by 1H NMR, IR, electron spectroscopy, and elemental analysis methods. The formation of complexes with Gd(III) and Dy(III) ions involving fumarate and maleate groups of the polyester was proved by IR and electron spectroscopy methods. It was established that the structure of the coordination units has the form of a square antiprism. The compositions and conditional logarithms of the stability constants of the complexes were determined. It was established that complexation with lanthanide ions promotes emission enhancement in the ligand.

Chelating Rare-Earth Metals (Ln3+) and 225Ac3+ with the Dual-Size-Selective Macrocyclic Ligand Py2-Macrodipa

Radioisotopes of metallic elements, or radiometals, are widely employed in both therapeutic and diagnostic nuclear medicine. For this application, chelators that efficiently bind the radiometal of interest and form a stable metal-ligand complex with it are required. Toward the development of new chelators for nuclear medicine, we recently reported a novel class of 18-membered macrocyclic chelators that is characterized by their ability to form stable complexes with both large and small rare-earth metals (Ln3+), a property referred to as dual size selectivity. A specific chelator in this class called py-macrodipa, which contains one pyridyl group within its macrocyclic core, was established as a promising candidate for 135La3+, 213Bi3+, and 44Sc3+ chelation. Building upon this prior work, here we report the synthesis and characterization of a new chelator called py2-macrodipa with two pyridyl units fused into the macrocyclic backbone. Its coordination chemistry with the Ln3+ series was investigated by NMR spectroscopy, X-ray crystallography, density functional theory (DFT) calculations, analytical titrations, and transchelation assays. These studies reveal that py2-macrodipa retains the expected dual size selectivity and possesses an enhanced thermodynamic affinity for all Ln3+ compared to py-macrodipa. By contrast, the kinetic stability of Ln3+ complexes with py2-macrodipa is only improved for the light, large Ln3+ ions. Based upon these observations, we further assessed the suitability of py2-macrodipa for use with 225Ac3+, a large radiometal with valuable properties for targeted α therapy. Radiolabeling and stability studies revealed py2-macrodipa to efficiently incorporate 225Ac3+ and to form a complex that is inert in human serum over 3 weeks. Although py2-macrodipa does not surpass the state-of-the-art chelator macropa for 225Ac3+ chelation, it does provide another effective 225Ac3+ chelator. These studies shed light on the fundamental coordination chemistry of the Ln3+ series and may inspire future chelator design efforts.

Toward Bifunctional Chelators for Thallium-201 for Use in Nuclear Medicine

Auger electron therapy exploits the cytotoxicity of low-energy electrons emitted during radioactive decay that travel very short distances (typically <1 μm). 201Tl, with a half-life of 73 h, emits ∼37 Auger and other secondary electrons per decay and can be tracked in vivo as its gamma emissions enable SPECT imaging. Despite the useful nuclear properties of 201Tl, satisfactory bifunctional chelators to incorporate it into bioconjugates for molecular targeting have not been developed. H4pypa, H5decapa, H4neunpa-NH2, and H4noneunpa are multidentate N- and O-donor chelators that have previously been shown to have high affinity for 111In, 177Lu, and 89Zr. Herein, we report the synthesis and serum stability of [nat/201Tl]Tl3+ complexes with H4pypa, H5decapa, H4neunpa-NH2, and H4noneunpa. All ligands quickly and efficiently formed complexes with [201Tl]Tl3+ that gave simple single-peak radiochromatograms and showed greatly improved serum stability compared to DOTA and DTPA. [natTl]Tl-pypa was further characterized using nuclear magnetic resonance spectroscopy (NMR), mass spectroscopy (MS), and X-ray crystallography, showing evidence of the proton-dependent presence of a nine-coordinate complex and an eight-coordinate complex with a pendant carboxylic acid group. A prostate-specific membrane antigen (PSMA)-targeting bioconjugate of H4pypa was synthesized and radiolabeled. The uptake of [201Tl]Tl-pypa-PSMA in DU145 PSMA-positive and PSMA-negative prostate cancer cells was evaluated in vitro and showed evidence of bioreductive release of 201Tl and cellular uptake characteristic of unchelated [201Tl]TlCl. SPECT/CT imaging was used to probe the in vivo biodistribution and stability of [201Tl]Tl-pypa-PSMA. In healthy animals, [201Tl]Tl-pypa-PSMA did not show the myocardial uptake that is characteristic of unchelated 201Tl. In mice bearing DU145 PSMA-positive and PSMA-negative prostate cancer xenografts, the uptake of [201Tl]Tl-pypa-PSMA in DU145 PSMA-positive tumors was higher than that in DU145 PSMA-negative tumors but insufficient for useful tumor targeting. We conclude that H4pypa and related ligands represent an advance compared to conventional radiometal chelators such as DOTA and DTPA for Tl3+ chelation but do not resist dissociation for long periods in the biological environment due to vulnerability to reduction of Tl3+ and subsequent release of Tl+. However, this is the first report describing the incorporation of [201Tl]Tl3+ into a chelator-peptide bioconjugate and represents a significant advance in the field of 201Tl-based radiopharmaceuticals. The design of the next generation of chelators must include features to mitigate this susceptibility to bioreduction, which does not arise for other trivalent heavy radiometals.

Advancing Chelation Strategies for Large Metal Ions for Nuclear Medicine Applications

Nuclear medicine leverages radioisotopes of a wide range of elements, a significant portion of which are metals, for the diagnosis and treatment of disease. To optimally use radioisotopes of the metal ions, or radiometals, for these applications, a chelator that efficiently forms thermodynamically and kinetically stable complexes with them is required. The chelator also needs to attach to a biological targeting vector that locates pathological tissues. Numerous chelators suitable for small radiometals have been established to date, but chelators that work well for large radiometals are significantly less common. In this Account, we describe recent progress by us and others in the advancement of ligands for large radiometal chelation with emerging applications in nuclear medicine.First, we discuss and analyze the coordination chemistry of the chelator macropa, a macrocyclic ligand that contains the 18-crown-6 backbone and two picolinate pendent arms, with large metal ions in the context of nuclear medicine. This ligand is known for its unusual reverse size selectivity, the preference for binding large over small metal ions. The radiolabeling properties of macropa with large radiometals ²²⁵Ac³⁺, ^132/135La³⁺, ¹³¹Ba²⁺, ²²³Ra²⁺, ²¹³Bi³⁺, and related in vivo investigations are described. The development of macropa derivatives containing different pendent donors or rigidifying groups in the macrocyclic core is also briefly reviewed.Next, efforts to transform macropa into a radiopharmaceutical agent via covalent conjugation to biological targeting vectors are summarized. In this discussion, two types of bifunctional analogues of macropa reported in the literature, macropa-NCS and mcp-click, are presented. Their implementation in different radiopharmaceutical agents is discussed. Bioconjugates containing macropa attached to small-molecule targeting vectors or macromolecular antibodies are presented. The in vitro and in vivo evaluations of these constructs are also discussed.Lastly, chelators with dual size selectivity are described. This class of ligands exhibits good affinities for both large and small metal ions. This property is valuable for nuclear medicine applications that require the simultaneous chelation of both large and small radiometals with complementary therapeutic and diagnostic properties. Recently, we reported an 18-membered macrocyclic ligand called macrodipa that attains this selectivity pattern. This chelator, its second-generation analogue py-macrodipa, and their applications for chelating the medicinally relevant large ¹³⁵La³⁺, ²²⁵Ac³⁺, ²¹³Bi³⁺, and small ⁴⁴Sc³⁺ ions are also presented. Studies with these radiometals show that py-macrodipa can effectively radiolabel and stably retain both large and small radiometals. Overall, this Account makes the case for innovative ligand design approaches for novel emerging radiometal ions with unusual coordination chemistry properties.

Stable Chelation of the Uranyl Ion by Acyclic Hexadentate Ligands: Potential Applications for 230U Targeted α-Therapy

Uranium-230 is an α-emitting radionuclide with favorable properties for use in targeted α-therapy (TAT), a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To successfully implement this radionuclide for TAT, a bifunctional chelator that can stably bind uranium in vivo is required. To address this need, we investigated the acyclic ligands H2dedpa, H2CHXdedpa, H2hox, and H2CHXhox as uranium chelators. The stability constants of these ligands with UO22+ were measured via spectrophotometric titrations, revealing log βML values that are greater than 18 and 26 for the "pa" and "hox" chelators, respectively, signifying that the resulting complexes are exceedingly stable. In addition, the UO22+ complexes were structurally characterized by NMR spectroscopy and X-ray crystallography. Crystallographic studies reveal that all six donor atoms of the four ligands span the equatorial plane of the UO22+ ion, giving rise to coordinatively saturated complexes that exclude solvent molecules. To further understand the enhanced thermodynamic stabilities of the "hox" chelators over the "pa" chelators, density functional theory (DFT) calculations were employed. The use of the quantum theory of atoms in molecules revealed that the extent of covalency between all four ligands and UO22+ was similar. Analysis of the DFT-computed ligand strain energy suggested that this factor was the major driving force for the higher thermodynamic stability of the "hox" ligands. To assess the suitability of these ligands for use with 230U TAT in vivo, their kinetic stabilities were probed by challenging the UO22+ complexes with the bone model hydroxyapatite (HAP) and human plasma. All four complexes were >95% stable in human plasma for 14 days, whereas in the presence of HAP, only the complexes of H2CHXdedpa and H2hox remained >80% intact over the same period. As a final validation of the suitability of these ligands for radiotherapy applications, the in vivo biodistribution of their UO22+ complexes was determined in mice in comparison to unchelated [UO2(NO3)2]. In contrast to [UO2(NO3)2], which displays significant bone uptake, all four ligand complexes do not accumulate in the skeletal system, indicating that they remain stable in vivo. Collectively, these studies suggest that the equatorial-spanning ligands H2dedpa, H2CHXdedpa, H2hox, and H2CHXhox are highly promising candidates for use in 230U TAT.

Chelating the Alpha Therapy Radionuclides 225Ac3+ and 213Bi3+ with 18-Membered Macrocyclic Ligands Macrodipa and Py-Macrodipa

The radionuclides 225Ac3+ and 213Bi3+ possess favorable physical properties for targeted alpha therapy (TAT), a therapeutic approach that leverages α radiation to treat cancers. A chelator that effectively binds and retains these radionuclides is required for this application. The development of ligands for this purpose, however, is challenging because the large ionic radii and charge-diffuse nature of these metal ions give rise to weaker metal-ligand interactions. In this study, we evaluated two 18-membered macrocyclic chelators, macrodipa and py-macrodipa, for their ability to complex 225Ac3+ and 213Bi3+. Their coordination chemistry with Ac3+ was probed computationally and with Bi3+ experimentally via NMR spectroscopy and X-ray crystallography. Furthermore, radiolabeling studies were conducted, revealing the efficient incorporation of both 225Ac3+ and 213Bi3+ by py-macrodipa that matches or surpasses the well-known chelators macropa and DOTA. Incubation in human serum at 37 °C showed that ∼90% of the 225Ac3+-py-macrodipa complex dissociates after 1 d. The Bi3+-py-macrodipa complex possesses remarkable kinetic inertness reflected by an EDTA transchelation challenge study, surpassing that of Bi3+-macropa. This work establishes py-macrodipa as a valuable candidate for 213Bi3+ TAT, providing further motivation for its implementation within new radiopharmaceutical agents.

Py-Macrodipa: A Janus Chelator Capable of Binding Medicinally Relevant Rare-Earth Radiometals of Disparate Sizes

Nuclear medicine leverages different types of radiometals for disease diagnosis and treatment, but these applications usually require them to be stably chelated. Given the often-disparate chemical properties of these radionuclides, it is challenging to find a single chelator that binds all of them effectively. Toward addressing this problem, we recently reported a macrocyclic chelator macrodipa with an unprecedented "dual-size-selectivity" pattern for lanthanide (Ln³⁺) ions, characterized by its high affinity for both the large and the small Ln³⁺ ( J. Am. Chem. Soc, 2020, 142, 13500). Here, we describe a second-generation "macrodipa-type" ligand, py-macrodipa. Its coordination chemistry with Ln³⁺ was thoroughly investigated experimentally and computationally. These studies reveal that the Ln³⁺-py-macrodipa complexes exhibit enhanced thermodynamic and kinetic stabilities compared to Ln³⁺-macrodipa, while retaining the unusual dual-size selectivity. Nuclear medicine applications of py-macrodipa for chelating radiometals with disparate chemical properties were assessed using the therapeutic ¹³⁵La³⁺ and diagnostic ⁴⁴Sc³⁺ radiometals representing the two size extremes within the rare-earth series. Radiolabeling and stability studies demonstrate that the rapidly formed complexes of these radionuclides with py-macrodipa are highly stable in human serum. Thus, in contrast to gold standard chelators like DOTA and macropa, py-macrodipa can be harnessed for the simultaneous, efficient binding of radiometals with disparate ionic radii like La³⁺ and Sc³⁺, signifying a substantial achievement in nuclear medicine. This concept could enable the facile incorporation of a breadth of medicinally relevant radiometals into chemically identical radiopharmaceutical agents. The fundamental coordination chemistry learned from py-macrodipa provides valuable insight for future chelator development.

Na-Ln Heterometallic Coordination Polymers: Structure Modulation by Na+ Concentration and Efficient Detection to Tetracycline Antibiotics and 4-(Phenylazo)aniline

On the basis of the lanthanide metalloligand [Ln(ODA)₃]^3- (H₂ODA = oxydiacetic acid), three new Na-Ln heterometallic coordination polymers, [Ln(ODA)₃Na₂]_n [Ln = Eu (1) and Gd (2)] and [Tb(ODA)₃Na₃(H₂O)₂]_n (3), had been assembled by adjusting the concentration of Na⁺ ions in the reaction system. The investigations of fluorescence sensing showed that 1 could be a ratiometric probe to detect tetracycline (TC) and oxytetracycline (OTC) with high sensitivity and low detection limits, 71.92 ppb for the former and 45.54 ppb for the latter, and 3 could selectively sense 4-(phenylazo)aniline through the turn-off pathway with 14.59 ppb of detection limits. Moreover, the competing and circulating experiments indicated that both 1 and 3 had satisfactory antiinterference and recyclability for the corresponding analytes. All of these results implied that 1 and 3 should be potential fluorescent sensors for the detection of TC/OTC and 4-(phenylazo)aniline, and the possible sensing mechanism had also been discussed in depth.

Chemical Promiscuity of Non-Macrocyclic Multidentate Chelating Ligands for Radiometal Ions: H4neunpa-NH2 vs H4noneunpa

A comparative investigation of two structurally related potentially nonadentate chelating ligands, H₄neunpa-NH₂ and H₄noneunpa, has been undertaken to examine the influence of bifunctionalization on their coordination chemistry and metal ion selectivity. Significantly improved synthetic routes for each compound have been developed, employing straightforward high-yielding strategies. Radiolabeling studies with [⁴⁴Sc]Sc³⁺, [¹¹¹In]In³⁺, [¹⁷⁷Lu]Lu³⁺, and [²²⁵Ac]Ac³⁺ revealed a sharp contrast between the affinity of each chelator for large radiometal ions. H₄noneunpa demonstrated highly effective coordination of [¹⁷⁷Lu]Lu³⁺ and [²²⁵Ac]Ac³⁺ achieving quantitative radiochemical yields (>98%) at ligand concentrations of 10^-6 M (room temperature (RT), 10 min), with excellent stability when challenged in human serum, while H₄neunpa-NH₂ was unable to complex either metal ion effectively. Nuclear magnetic resonance (NMR) spectroscopy was employed to explore the coordination chemistry of each chelating ligand with nonradioactive metal ions, spanning a range of ionic radii and coordination numbers. A comprehensive conformational analysis of each metal complex was undertaken using density functional theory (DFT) calculations to explore the coordination geometries and explain the discrepancy in binding characteristics. Theoretical simulations revealed notable differences in the coordination geometry and apparent denticity of each ligand, which together account for the observed selectivity in metal binding and have important implications for the future design of complexes based upon this framework to target large radiometal ion coordination.

Macrocyclic Ligands with an Unprecedented Size-Selectivity Pattern for the Lanthanide Ions

Lanthanides (Ln3+) are critical materials used for many important applications, often in the form of coordination compounds. Tuning the thermodynamic stability of these compounds is a general concern, which is not readily achieved due to the similar coordination chemistry of lanthanides. Herein, we report two 18-membered macrocyclic ligands called macrodipa and macrotripa that show for the first time a dual selectivity toward both the light, large Ln3+ ions and the heavy, small Ln3+ ions, as determined by potentiometric titrations. The lanthanide complexes of these ligands were investigated by NMR spectroscopy and X-ray crystallography, which revealed the occurrence of a significant conformational toggle between a 10-coordinate Conformation A and an 8-coordinate Conformation B that accommodates Ln3+ ions of different sizes. The origin of this selectivity pattern was further supported by density functional theory (DFT) calculations, which show the complementary effects of ligand strain energy and metal-ligand binding energy that contribute to this conformational switch. This work demonstrates how novel ligand design strategies can be applied to tune the selectivity pattern for the Ln3+ ions.