Sc-HOPO: A Potential Construct for Use in Radioscandium-Based Radiopharmaceuticals

Development of a SnO2-based 44Ti/44Sc generator for medical applications

Towards application of 44Sc for diagnostic nuclear medicine, a 44Ti/44Sc generator based on an inorganic resin has been evaluated. Unlike other radionuclide generators used for medical applications, the long-term retention of the parent 44Ti is vital due to its long half life. Herein, tin dioxide (SnO2), a robust inorganic-based resin, has been synthesized and used as the stationary phase for a 44Ti/44Sc generator. The sorption behavior of 44Ti/44Sc was tested on SnO2 with varying acids, concentrations, and times. Preliminary batch study results showed >88 % 44Ti retention to the resin at lower acid concentrations (0.05 M HNO3 and 0.05 M HCl). A pilot generator was evaluated for a year, demonstrating 85.3 ± 2.8 % 44Sc elution yields and 0.71 ± 0.14 % 44Ti breakthrough in 5 M HNO3. Based on capacity studies, a 7.4 MBq (200 µCi) upscaled generator system was constructed for further evaluation of the SnO2 resin stability and the efficacy of the eluted 44Sc for radiolabeling. 44Sc could be regularly eluted from this generator in 5 M HNO3 with an overall average radiochemical yield 84.7 ± 9.5 %. Post-elution processing of the 44Sc with DGA-normal resin removed all 44Ti present and allowed for high 44Sc-DOTA labeling yields of 94.2 ± 0.5 %. Overall, SnO2 has been shown to be a viable material for a 44Ti/44Sc generator.

Cross section measurements for the production of 49,51Cr and 47Sc from proton irradiation of natural vanadium up to 24 MeV

Scandium-47 is a promising radionuclide for targeted radiotherapy and is also an elementally matched therapeutic partner to 43Sc and 44Sc, which are suitable for Positron Emission Tomography. The predominantly reported routes for the production of 47Sc employ expensive enriched titanium or calcium targets to achieve high radionuclidic purity. This study reports measurements of the excitation function of the natV(p,x)47Sc reaction at proton energies of 18-24 MeV to optimize bombardment parameters for the production of 47Sc using this promising approach. The cross-sections reported here demonstrate that irradiation of vanadium targets can produce >99% radionuclidically pure 47Sc with a proton energy of 24 MeV, albeit at modest yields.

PYTA: a universal chelator for advancing the theranostic palette of nuclear medicine

To clinically advance the growing arsenal of radiometals available to image and treat cancer, chelators with versatile binding properties are needed. Herein, we evaluated the ability of the py2[18]dieneN6 macrocycle PYTA to interchangeably bind and stabilize 225Ac3+, [177Lu]Lu3+, [111In]In3+ and [44Sc]Sc3+, a chemically diverse set of radionuclides that can be used complementarily for targeted alpha therapy, beta therapy, single-photon emission computed tomography (SPECT) imaging, and positron emission tomography (PET) imaging, respectively. Through NMR spectroscopy and X-ray diffraction, we show that PYTA possesses an unusual degree of flexibility for a macrocyclic chelator, undergoing dramatic conformational changes that enable it to optimally satisfy the disparate coordination properties of each metal ion. Subsequent radiolabeling studies revealed that PYTA quantitatively binds all 4 radiometals at room temperature in just minutes at pH 6. Furthermore, these complexes were found to be stable in human serum over 2 half-lives. These results surpass those obtained for 2 state-of-the-art chelators for nuclear medicine, DOTA and macropa. The stability of 225Ac-PYTA and [44Sc]Sc-PYTA, the complexes having the most disparity with respect to metal-ion size, was further probed in mice. The resulting PET images (44Sc) and ex vivo biodistribution profiles (44Sc and 225Ac) of the PYTA complexes differed dramatically from those of unchelated [44Sc]Sc3+ and 225Ac3+. These differences provide evidence that PYTA retains this size-divergent pair of radionuclides in vivo. Collectively, these studies establish PYTA as a new workhorse chelator for nuclear medicine and warrant its further investigation in targeted constructs.

Improving the In Vivo Stability of [52Mn]Mn(II) Complexes with 18-Membered Macrocyclic Chelators for PET Imaging

We report the [natMn/52Mn]Mn(II) complexes of the macrocyclic chelators PYAN [3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane] and CHXPYAN [(41R,42R,101R,102R)-3,5,9,11-tetraaza-1,7(2,6)-dipyridina-4,10(1,2)-dicyclohexanacyclododecaphane]. The X-ray crystal structures of Mn-PYAN and Mn-CHXPYAN evidence distorted octahedral geometries through coordination of the nitrogen atoms of the macrocycles. Cyclic voltammetry studies evidence reversible processes due to the Mn(II)/Mn(III) pair, indicating that the complexes are resistant to oxidation. CHXPYAN forms a more thermodynamically stable and kinetically inert Mn(II) complex than PYAN. Radiochemical studies with the radioactive isotope manganese-52 (52Mn, t1/2 = 5.6 days) evidenced better radiochemical yields for CHXPYAN than for PYAN. Both [52Mn]Mn(II) complexes remained stable in mouse and human serum, so in vivo stability studies were carried out. Positron emission tomography/computed tomography scans and biodistribution assays indicated that [52Mn]Mn-PYAN has a distribution pattern similar to that of [52Mn]MnCl2, showing persistent radioactivity accumulation in the kidneys. Conversely, [52Mn]Mn-CHXPYAN remained stable in vivo, clearing quickly from the liver and kidneys.

Evaluation of a novel hexadentate 1,2-hydroxypyridinone-based acyclic chelate, HOPO-O6-C4, for 43Sc/47Sc, 68Ga, and 45Ti radiopharmaceuticals

INTRODUCTION

Chelators play a crucial role in the development of metal-based radiopharmaceuticals, and with the continued interest in 68Ga and increasing availability of new radiometals such as 43Sc/47Sc and 45Ti, there is a growing demand for tailored chelators that can form stable complexes with these metals. This work reports the synthesis and characterization of a hexadentate tris-1,2-hydroxypyridonone chelator HOPO-O6-C4 and its in vitro and in vivo evaluation with the above mentioned radiometals.

METHODS

To investigate the affinity of HOPO-O6-C4, macroscopic studies were performed with Sc3+, and Ga3+ followed by DFT structural optimization of the Sc3+, Ga3+ and Ti4+ complexes. Further tracer studies with 43Sc (and 47Sc), 45Ti, and 68Ga were performed to determine the potential for positron emission tomography (PET) imaging with these complexes. In vitro stability studies followed by in vivo imaging and biodistribution studies were performed to understand the kinetic stability of the resultant radiometal-complexes of HOPO-O6-C4.

RESULTS

Promising radiolabeling results with HOPO-O6-C4 were obtained with 43Sc, 47Sc, 45Ti, and 68Ga radionuclides; rapid radiolabeling was observed at 37 °C and pH 7 in under 30-min. Apparent molar activity measurements were performed for radiolabeling of HOPO-O6-C4 with 43Sc (4.9 ± 0.26 GBq/μmol), 47Sc (1.58 ± 0.01 GBq/μmol), 45Ti (11.5 ± 1.6 GBq/μmol) and 68Ga (5.74 ± 0.7 GBq/μmol), respectively. Preclinical in vivo imaging studies resulted in promising results with [68Ga]Ga-HOPO-O6-C4 indicating a rapid clearance through hepatic excretion route and no decomplexation whereas [43Sc]Sc-HOPO-O6-C4, [47Sc]Sc-HOPO-O6-C4 and [45Ti]Ti-HOPO-O6-C4 showed modest and significant evidence of decomplexation, respectively.

CONCLUSIONS

The tris-1,2-HOPO chelator HOPO-O6-C4 is a promising scaffold for elaboration into a 68Ga- based radiopharmaceutical.

Metal-Ligand Interactions in Scandium Complexes with Radiopharmaceutical Applications

The radioisotopes of scandium (43Sc, 44Sc, and 47Sc) are potential candidates for use in imaging and therapy both separately and as elementally matched pairs for radiotheranostics. In the present study the bonding interactions of Sc3+ with 18 hepta- to decadentate ligands are compared using density functional theory (DFT) calculations. The bonding analysis is based on the natural bond orbital (NBO) model. The main contributions to the bonding were assessed using natural energy decomposition analysis (NEDA). Most of the ligands have anionic character (charges from 2- to 8-); thus the electrical term determines the major differences in the interaction energies. However, interesting features were found in the covalent contributions manifested by the ligand → Sc3+ charge transfer (CT) interactions. Significant differences could be observed in the energetic contributions of the N and O donors to the total CT.

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.