"Off-Label Use" of the Siderophore Enterobactin Enables Targeted Imaging of Cancer with Radioactive Ti(IV)

The development of inert, biocompatible chelation methods is required to harness the emerging positron emitting radionuclide 45Ti for radiopharmaceutical applications. Herein, we evaluate the Ti(IV)-coordination chemistry of four catechol-based, hexacoordinate chelators using synthetic, structural, computational, and radiochemical approaches. The siderophore enterobactin (Ent) and its synthetic mimic TREN-CAM readily form mononuclear Ti(IV) species in aqueous solution at neutral pH. Radiolabeling studies reveal that Ent and TREN-CAM form mononuclear complexes with the short-lived, positron-emitting radionuclide 45Ti(IV), and do not transchelate to plasma proteins in vitro and exhibit rapid renal clearance in naïve mice. These features guide efforts to target the 45Ti isotope to prostate cancer tissue through the design, synthesis, and evaluation of Ent-DUPA, a small molecule conjugate composed of a prostate specific membrane antigen (PSMA) targeting peptide and a monofunctionalized Ent scaffold. The [45Ti][Ti(Ent-DUPA)]2- complex forms readily at room temperature. In a tumor xenograft model in mice, selective tumor tissue accumulation (8±5 %, n=5), and low off-target uptake in other organs is observed. Overall, this work demonstrates targeted imaging with 45Ti(IV), provides a foundation for advancing the application of 45Ti in nuclear medicine, and reveals that Ent can be repurposed as a 45Ti-complexing cargo for targeted nuclear imaging applications.

De Novo Approaches to the Solid-Phase Separation of Titanium(IV) and Scandium(III): Translating Speciation Data to Selective On-Bead Chelation toward Applications in Nuclear Medicine

The solution chemistry of the hydrolytic, early-transition-metal ions Ti4+ and Sc3+ represents a coordination chemistry challenge with important real-world implications, specifically in the context of 44Ti/44Sc and 45Ti/NatSc radiochemical separations. Unclear speciation of the solid and solution phases and tertiary mixtures of mineral acid, organic chelators, and solid supports are common confounds, necessitating tedious screening of multiple variables. Herein we describe how thermodynamic speciation data in solution informs the design of new solid-phase chelation approaches enabling separations of Ti4+ and Sc3+. The ligands catechol (benzene-1,2-diol) and deferiprone [3-hydroxy-1,2-dimethyl-4(1H)-pyridone] bind Ti4+ at significantly more acidic conditions (2-4 pH units) than Sc3+. Four chelating resins were synthesized using either catechol or deferiprone with two different solid supports. Of these, deferiprone appended to carboxylic acid polymer-functionalized silica (CA-Def) resin exhibited excellent binding affinity for Ti4+ across a wide range of HCl concentrations (1.0-0.001 M), whereas Sc3+ was only retained in dilute acidic conditions (0.01-0.001 M HCl). CA-Def resin produced separation factors of >100 (Ti/Sc) in 0.1-0.4 M HCl, and the corresponding Kd values (>1000) show strong retention of Ti4+. A model 44Ti/44Sc generator was produced, showing 65 ± 3% yield of 44Sc in 200 μL of 0.2 M HCl with a significant 44Ti breakthrough of 0.1%, precluding use in its current form. Attempts, however, removed natSc in loading fractions and a dilute (0.4 M HCl) wash and recovered 80% of the loaded 45Ti activity in 400 μL of 6 M HCl. The previously validated 45Ti chelator TREN-CAM was used for comparative proof-of-concept reactions with the CA-Def eluent (in HCl) and literature-reported hydroxamate-based resin eluents (in citric acid). CA-Def shows improved radiolabeling efficiency with an apparent molar activity (AMA) of 0.177 mCi nmol-1, exceeding the established methods (0.026 mCi nmol-1) and improving the separation and recovery of 45Ti for positron emission tomography imaging applications.

Radiometallation and photo-triggered release of ready-to-inject radiopharmaceuticals from the solid phase

The efficient, large-scale synthesis of radiometallated radiopharmaceuticals represents an emerging clinical need which, to date, is inherently limited by time consuming, sequential procedures to conduct isotope separation, radiochemical labeling and purification prior to formulation for injection into the patient. In this work, we demonstrate that a solid-phase based, concerted separation and radiosynthesis strategy followed by photochemical release of radiotracer in biocompatible solvents can be employed to prepare ready-to-inject, clinical grade radiopharmaceuticals. Optimization of resin base, resin loading, and radiochemical labeling capacity are demonstrated with ⁶⁷Ga and ⁶⁴Cu radioisotopes using a short model peptide sequence and further validated using two peptide-based radiopharmaceuticals with clinical relevance, targeting the gastrin-releasing peptide and the prostate specific membrane antigen. We also demonstrate that the solid-phase approach enables separation of non-radioactive carrier ions Zn²⁺ and Ni²⁺ present at 10⁵-fold excess over ⁶⁷Ga and ⁶⁴Cu by taking advantage of the superior Ga³⁺ and Cu²⁺ binding affinity of the solid-phase appended, chelator-functionalized peptide. Finally, a proof of concept radiolabeling and subsequent preclinical PET-CT study with the clinically employed positron emitter ⁶⁸Ga successfully exemplifies that Solid Phase Radiometallation Photorelease (SPRP) allows the streamlined preparation of radiometallated radiopharmaceuticals by concerted, selective radiometal ion capture, radiolabeling and photorelease.

A General Design Strategy Enabling the Synthesis of Hydrolysis-Resistant, Water-Stable Titanium(IV) Complexes

Despite its prevalence in the environment, the chemistry of the Ti⁴⁺ ion has long been relegated to organic solutions or hydrolyzed TiO₂ polymorphs. A knowledge gap in stabilizing molecular Ti⁴⁺ species in aqueous environments has prevented the use of this ion for various applications such as radioimaging, design of water-compatible metal-organic frameworks (MOFs), and aqueous-phase catalysis applications. Herein, we show a thorough thermodynamic screening of bidentate chelators with Ti⁴⁺ in aqueous solution, as well as computational and structural analyses of key compounds. In addition, the hexadentate analogues of catechol (benzene-1,2-diol) and deferiprone (3-hydroxy-1,2-dimethyl-4(1H)-pyridone), TREN-CAM and THP^Me respectively, were assessed for chelation of the ⁴⁵ Ti isotope (t_1/2 =3.08 h, β⁺ =85 %, E_β+ =439 keV) towards positron emission tomography (PET) imaging applications. Both were found to have excellent capacity for kit-formulation, and [⁴⁵ Ti]Ti-TREN-CAM was found to have remarkable stability in vivo.

An Unusual Pair: Facile Formation and In Vivo Validation of Robust Sc-18 F Ternary Complexes for Molecular Imaging

Fluorine-18 remains the most widely clinically utilized radionuclide globally for positron emission tomography (PET). The emergence of therapeutic isotopes for the management of disease has produced a pronounced interest in matched, theranostic isotope pairs that can be employed in tandem for the diagnosis and stratification of patients for subsequent radiotherapy. ¹⁸ F, however, does not have a suitable therapeutic isotopologue. Here, we demonstrate that the formation of [¹⁸ F][Sc-F] ternary complexes is feasible under mild, aqueous conditions, producing chemically robust radiopharmaceuticals in high radiochemical yield and specific activity. A corresponding in vivo study with a cancer-targeting [¹⁸ F][Sc-F] tracer indicates excellent in vivo stability and produces exquisite PET image quality, rendering the ¹⁸ F/⁴⁷ Sc isotope pair an unusual, yet chemically matched theranostic pair with excellent potential for clinical translation.

Aqueous chemistry of the smallest rare earth: Comprehensive characterization of radioactive and non-radioactive scandium complexes for biological applications

The aqueous chemistry of scandium(III) is of emerging interest for biological applications, specifically in nuclear medicine, as radioactive isotopes of scandium are becoming more readily accessible. In contrast to other rare earths, Sc³⁺ has no d or f electrons, limiting characterization of corresponding coordination complexes to spectroscopic techniques that do not rely on the characteristic electronic transitions of f-elements or transition metal ions. Herein, we provide a comprehensive overview on characterization techniques suitable to elucidate the solution behavior of small and macromolecular complexes of the smallest rare earth.

Homologous Structural, Chemical, and Biological Behavior of Sc and Lu Complexes of the Picaga Bifunctional Chelator: Toward Development of Matched Theranostic Pairs for Radiopharmaceutical Applications

The radioactive isotopes scandium-44/47 and lutetium-177 are gaining relevance for radioimaging and radiotherapy, resulting in a surge of studies on their coordination chemistry and subsequent applications. Although the trivalent ions of these elements are considered close homologues, dissimilar chemical behavior is observed when they are complexed by large ligand architectures due to discrepancies between Lu(III) and Sc(III) ions with respect to size, chemical hardness, and Lewis acidity. Here, we demonstrate that Lu and Sc complexes of 1,4-bis(methoxycarbonyl)-7-[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H₃mpatcn) and its corresponding bioconjugate picaga-DUPA can be employed to promote analogous structural features and, subsequently, biological properties for coordination complexes of these ions. The close homology was evidenced using potentiometric methods, computational modeling, variable temperature mass spectrometry, and pair distribution function analysis of X-ray scattering data. Radiochemical labeling, in vitro stability, and biodistribution studies with Sc-47 and Lu-177 indicate that the 7-coordinate ligand environment of the bifunctional picaga ligand is compatible with biological applications and the future investigation of β-emitting, picaga-chelated Sc and Lu isotopes for radiotherapy.