Eszter Boros

In recent efforts to develop optimal 64Cu chelators with ideal thermodynamic stability and high kinetic inertness, focus has shifted to the common problem of reduction-induced demetalation of 64Cu(I). The most common approach to remedy this issue is to make the reduction of 64Cu(II) to 64Cu(I) inaccessible, with anionic ligands comprising hard Lewis bases like carboxylates. However, an alternative approach has been to focus on the stability of the 64Cu(I) species, most commonly achieved by incorporating soft sulfur donors, such as thiols or thioethers, into the chelator framework. Herein, we present a novel copper chelator, PyN2S, comprising both N and S donors, allowing for an N3S binding mode. Five new ligands are introduced, and their coordination chemistry with copper is explored. Thermodynamic stability constants with Cu2+ and Zn2+ are presented, as well as solution and solid-state characterization of the Cu(II) complexes. Finally, the radiochemistry of the PyN2S ligands with 64Cu was explored, including both in vitro characterization and in vivo biodistribution in mice. Overall, the PyN2S framework has demonstrated promising stability for both Cu(II) and Cu(I), and quantitative radiolabeling with 64Cu was achieved, while in vivo biodistribution revealed the highest uptake in renal clearance pathway organs.

We introduce a peptide prodrug activation strategy that employs the trivalent Lewis-acidic metal ions Ga3+, Fe3+, In3+, and Co3+, chelated by triazacyclononanediacetic acid monoamides. The metal chelates trigger rate-tunable, metal-mediated, autolytic amide bond cleavage (TMAC) adjacent to N-methylated and canonical amino acids. Our studies reveal that the four TMAC-compatible metal ions induce amide bond hydrolysis with divergent pH dependence and mechanistic pathways. We subsequently linked Ga3+, Fe3+, In3+, and Co3+ complexes to the N-terminus of a potent cytotoxic cargo, monomethyl auristatin E (MMAE), to produce the proof-of-concept prodrug PD1. In vitro toxicity studies establish IC50 values in HeLa and PC-3 cells that broadly correlate with complex cleavage rates; the metal ion-free PD1 conjugate, the Zn-PD1 complex, as well as the Ga3+, Fe3+, In3+, and Co3+ chelate hydrolysis products alone confer no toxicity. Attachment of a cancer-targeting peptide to the metal chelate, furnishing PD2, enables targeted delivery of the therapeutic payload in vivo, compatible with noninvasive nuclear imaging using positron emission tomography with 68Ga or single-photon computed tomography with 111In. The corresponding natGa-PD2 complex selectively suppressed tumor growth in vivo with minimal off-target toxicity. These studies establish metal identity as a general design parameter for programming amide bond cleavage and introduce TMAC as a versatile platform to engender peptide prodrugs with tunable release kinetics.

We introduce a solid‐phase synthesis platform for the rapid, modular, and scalable synthesis of the synthetic metallophore KO‐NH ₂ , which is readily recognized by bacterial siderophore transporters, but has optimized affinity for Ga ³⁺ over Fe ³⁺ . Four synthetic protochelin (PC) analogues, KK‐COOH, KO‐NH ₂ , KO‐COOH, and OK‐COOH were constructed. Growth recovery, radioactive uptake, and covalent photocrosslinker studies in Escherichia coli identified CirA and Fiu as the dominant outer membrane transport proteins capable of binding KO‐NH ₂ ‐linked, bifunctional constructs. Computational studies further supported that the structural framework of PC can be effectively mimicked by KO or OK peptides. Thermodynamic stability studies revealed an atypical inversion of thermodynamic stability trends, with logK _GaL of 39.9 and a logK _FeL of 36, demonstrating improved stability with Ga ³⁺ . Studies in naïve mice with ⁶⁸ Ga‐labeled derivatives identify the lead [ ⁶⁸ Ga][Ga(KO‐NH ₂ )] ³⁻ , which remained intact in vivo and demonstrated selective, enhanced uptake in an E. coli soft tissue model of infection (1.28 % ID/g, with a 3:1 ratio of infected to inflamed/normal muscle tissue). Linkage of KO‐NH ₂ to the antibiotic ampicillin produces an efficacious antibiotic, which supersedes the potency of the parent antibiotic ampicillin 4‐fold (MIC ₉₈ = 1.9 µM) effectively treats a soft tissue infection of ampicillin‐resistant E. coli .

We introduce a solid‐phase synthesis platform for the rapid, modular, and scalable synthesis of the synthetic metallophore KO‐NH ₂ , which is readily recognized by bacterial siderophore transporters, but has optimized affinity for Ga ³⁺ over Fe ³⁺ . Four synthetic protochelin (PC) analogues, KK‐COOH, KO‐NH ₂ , KO‐COOH, and OK‐COOH were constructed. Growth recovery, radioactive uptake, and covalent photocrosslinker studies in Escherichia coli identified CirA and Fiu as the dominant outer membrane transport proteins capable of binding KO‐NH ₂ ‐linked, bifunctional constructs. Computational studies further supported that the structural framework of PC can be effectively mimicked by KO or OK peptides. Thermodynamic stability studies revealed an atypical inversion of thermodynamic stability trends, with logK _GaL of 39.9 and a logK _FeL of 36, demonstrating improved stability with Ga ³⁺ . Studies in naïve mice with ⁶⁸ Ga‐labeled derivatives identify the lead [ ⁶⁸ Ga][Ga(KO‐NH ₂ )] ³⁻ , which remained intact in vivo and demonstrated selective, enhanced uptake in an E. coli soft tissue model of infection (1.28 % ID/g, with a 3:1 ratio of infected to inflamed/normal muscle tissue). Linkage of KO‐NH ₂ to the antibiotic ampicillin produces an efficacious antibiotic, which supersedes the potency of the parent antibiotic ampicillin 4‐fold (MIC ₉₈ = 1.9 µM) effectively treats a soft tissue infection of ampicillin‐resistant E. coli .

To inform development of separation and generator technology relevant to the accelerator (and generator) production of 43Sc, 44Sc (from 44Ti), 45Ti, and 47Sc with fundamental, experimental exploration of the speciation and coordination chemistry of Ti and Sc.

Lanthanide coordination complexes are harnessed for biological imaging due to their oxidative stability in aqueous media, favorable relaxometric behavior, and accessible luminescence emissions within biomedically relevant, optical-imaging wavelength ranges. In contrast with multimodal imaging strategies that rely on exchanging lanthanide ions to access distinct modalities, we exploit the unique redox chemistry of the Eu^2+/3+ pair to demonstrate the feasibility of multimodal imaging within a single chelate scaffold. We constructed a series of polypyridine-containing macrocyclic ligands that readily coordinate both Eu²⁺ and Eu³⁺ ions. Characterization of the corresponding chelates by X-ray crystallography, cyclic voltammetry, electron paramagnetic resonance spectroscopy, and photophysical measurements were conducted. The Eu²⁺-containing complexes of acetamide-functionalized, polypyridine-containing 18-membered macrocycles exhibit relaxometric properties comparable to clinical Gd³⁺ contrast agents for magnetic resonance imaging. Upon oxidation to Eu³⁺, the complexes display characteristic luminescence with quantum yields ranging from 1.3 to 13.9%. In situ sensitization with a Cherenkov-emitting radionuclide efficiently produces Eu³⁺ emission at concentrations comparable to those employed for Eu²⁺-enhanced magnetic resonance imaging. The complex that provided the greatest signal-to-noise ratio in magnetic resonance in vitro imaging studies, and when oxidized, produced a detectable, optical imaging signal with as little as 5 nmol of complex. A subsequent study in a murine xenograft tumor model demonstrated the feasibility of conducting sequential magnetic resonance and optical imaging experiments following single dose administration of the redox–switchable complex.

Siderophore-mediated, transmembrane uptake of iron in bacteria is a complex mechanism of essential nutrient acquisition. In addition to transporters that specifically recognize and shuttle endogenous siderophores, bacteria possess transmembrane transporters that efficiently internalize siderophore-metal complexes produced by other organisms. The discovery of transmembrane transporters using indirect methods has enabled the identification of high-affinity transporters but limits access to lower-affinity, promiscuous transport systems. Therefore, covalent tagging strategies that enable the direct identification of target proteins are desirable. To this end, we examined the co-crystal structure of Fe-D1, a ciprofloxacin-linked ferrioxamine complex bound to thePseudomonas aeruginosa outer-membrane transporter FoxA with affinity for ferrioxamine B and E. Using a rational design approach, we identified ideal structural characteristics for ferrioxamine-based, covalent photo-cross-linker probes with reactivity toward tyrosine- and aspartate-rich binding sites within transporter domains. Fe and Ga complexes of DFO-azir-05 and DFO-azir-06 efficiently tag FoxA inEscherichia coli Lemo21 (DE3) mutants overexpressing FoxA. Subsequently, DFO-azir-06 was successfully used to directly tag and identify the ferrioxamine-binding proteins FoxA and FpvB inP. aeruginosa PAO1, as well as the two main transporters of ferrioxamine B inE. coli K-12, FhuA and FhuE. Moreover, DFO-azir-06 revealed a putative new role of cobalamine transporter BtuB in the transport of DFO derivatives. In conclusion, we demonstrate that careful structural design of covalent, photo-cross-linking siderophore conjugates can provide unprecedented access to the elucidation of siderophore-mediated metal ion uptake in bacteria.

Manganese-based magnetic resonance imaging (MRI) contrast agents were developed using chelators derived from the 2,11-diaza[3.3](2,6)pyridinophane macrocycle to enhance Mn²⁺ relaxivity. The synthesized chelators were assessed to determine how coordination number and pendant arm selection affect the efficiency of the resulting Mn-based MRI contrast agents. Two compounds demonstrated significant potential: [Mn(TE-1)] exhibited relaxivity comparable to clinical gadolinium-based agents (3.6 mM^–1 s^–1 at 1.4 T), while [Mn(TE-4)] displayed rapid renal clearance and strong contrast enhancement in the kidney and bladder. In vivo PET imaging with ⁵²Mn confirmed that [Mn(TE-4)] undergoes renal excretion, supporting its further development as an MRI or PET imaging agent. The lipophilic [Mn(TE-1)] exhibited both biliary and renal excretion, indicating its suitability as a blood pool MRI agent and for potential immuno-PET applications.