Optimized production, purification, and radiolabeling of the 203Pb/212Pb theranostic pair for nuclear medicine.

TRIUMF is one of the only laboratories in the world able to produce both lead-203 (203Pb, t1/2 = 51.9 h) and 212Pb (t1/2 = 10.6 h) onsite via its 13 and 500 MeV cyclotrons, respectively. Together, 203Pb and 212Pb form an element-equivalent theranostic pair that potentiate image-guided, personalized cancer treatment, using 203Pb as a single-photon emission computed tomography (SPECT) source, and 212Pb for targeted alpha therapy. In this study, improvements to 203Pb production were accomplished by manufacturing electroplated, silver-backed thallium (Tl) targets to improve target thermal stability, which allow for higher currents during irradiation. We implemented a novel, two-column purification method that employs selective Tl precipitation (203Pb only) alongside extraction and anion exchange chromatography to elute high specific activity and chemical purity 203/212Pb in a minimal volume of dilute acid, without the need for evaporation. Optimization of the purification method translated to improvements in radiolabeling yields and apparent molar activity of lead chelators TCMC (S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane) and Crypt-OH, a derivative of a [2.2.2]-cryptand.

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.

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.

Development of a multi faceted platform containing a tetrazine, fluorophore and chelator: synthesis, characterization, radiolabeling, and immuno-SPECT imaging.

BACKGROUND: Combining optical (fluorescence) imaging with nuclear imaging has the potential to offer a powerful tool in personal health care, where nuclear imaging offers in vivo functional whole-body visualization, and the fluorescence modality may be used for image-guided tumor resection. Varying chemical strategies have been exploited to fuse both modalities into one molecular entity. When radiometals are employed in nuclear imaging, a chelator is typically inserted into the molecule to facilitate radiolabeling; the availability of the chelator further expands the potential use of these platforms for targeted radionuclide therapy if a therapeutic radiometal is employed. Herein, a novel mixed modality scaffold which contains a tetrazine (Tz)--for biomolecule conjugation, fluorophore-for optical imaging, and chelator-for radiometal incorporation, in one construct is presented. The novel platform was characterized for its fluorescence properties, radiolabeled with single-photon emission computed tomography (SPECT) isotope indium-111 (111In3+) and therapeutic alpha emitter actinium-225 (225Ac3+). Both radiolabels were conjugated in vitro to trans-cyclooctene (TCO)-modified trastuzumab; biodistribution and immuno-SPECT imaging of the former conjugate was assessed. RESULTS: Key to the success of the platform synthesis was incorporation of a 4,4'-dicyano-BODIPY fluorophore. The route gives access to an advanced intermediate where final chelator-incorporated compounds can be easily accessed in one step prior to radiolabeling or biomolecule conjugation. The DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) conjugate was prepared, displayed good fluorescence properties, and was successfully radiolabeled with 111In & 225Ac in high radiochemical yield. Both complexes were then separately conjugated in vitro to TCO modified trastuzumab through an inverse electron demand Diels-Alder (IEDDA) reaction with the Tz. Pilot small animal in vivo immuno-SPECT imaging with [111In]In-DO3A-BODIPY-Tz-TCO-trastuzumab was also conducted and exhibited high tumor uptake (21.2 ± 5.6%ID/g 6 days post-injection) with low uptake in non-target tissues. CONCLUSIONS: The novel platform shows promise as a multi-modal probe for theranostic applications. In particular, access to an advanced synthetic intermediate where tailored chelators can be incorporated in the last step of synthesis expands the potential use of the scaffold to other radiometals. Future studies including validation of ex vivo fluorescence imaging and exploiting the pre-targeting approach available through the IEDDA reaction are warranted.

Evaluation of the Effect of Macrocyclic Ring Size on [203Pb]Pb(II) Complex Stability in Pyridyl-Containing Chelators.

As an element-equivalent theranostic pair, lead-203 (203Pb, 100% EC, half-life = 51.92 h) and lead-212 (212Pb, 100% ß-, half-life = 10.64 h), through the emission of γ rays and an α particle in its decay chain, respectively, can aid in the development of personalized targeted radionuclide treatment for advanced and currently untreatable cancers. With these isotopes currently being used in clinical trials, an understanding of the relationship between the chelator structure, ability to incorporate the radiometal, and metal-complex stability is needed to help design appropriate chelators for clinical use. Herein, we report an investigation into the effect of ring size in macrocyclic chelators where pyridine, an intermediate Lewis base, acts as an electron donor toward lead. Crown-4Py (4,7,13,16-tetrakis(pyridin-2-ylmethyl)-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane), cyclen-4Py (1,4,7,10-tetrakis(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane), and NOON-2Py (7,16-bis(pyridin-2-ylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) were synthesized and analyzed for their ability to coordinate Pb2+. Metal complex stability was investigated via [203Pb]Pb2+ radiolabeling studies, 1H NMR spectroscopy, X-ray crystallography, and potentiometry. With the smallest macrocyclic backbone, cyclen-4Py had the highest radiochemical yield, while, in descending order, crown-4Py and NOON-2Py had the lowest. Thermodynamic stability constants (log KML) of 19.95(3), 13.29(5), and 11.67 for [Pb(Cyclen-4Py)]2+, [Pb(Crown-4Py)]2+, and [Pb(NOON-2Py)]2+, respectively, correlated with their radiochemical yields. The X-ray crystal structure of the least stable complexes [Pb(NOON-2Py)]2+ revealed a hemidirected Pb2+ center, as reflected by a void within the coordination sphere, and [Pb(Crown-4Py)]2+ showed an average Pb-N pyridine interatomic distance of >3 Å. By contrast, the crystal structure of [Pb(Cyclen-4Py)]2+ showed shorter Pb-N pyridine interactions, and in solution, only one highly symmetric isomer existed for this complex, whereas conformational flexibility was observed for both [Pb(Crown-4Py)]2+ and [Pb(NOON-2Py)]2+ at the NMR timescale. This study illustrates the importance of the macrocyclic backbone size when incorporating bulky electron-donor groups into the design of a macrocyclic chelator as it affects the accessibility of lead to the donor arms. Our results show that cyclen-4Py is a promising chelator for future studies with this theranostic pair.