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For the first time, synthesis of bifunctional [2.2.2]-cryptands (CRYPT) and demonstration of radiolabeling with lead(II) (Pb2+) isotopes are disclosed herein. The synthesis is convenient and high-yielding and gives access to three distinct bifunctional handles (azide (-N3), isothiocyanate (-NCS), and tetrazine (-Tz)) that can enable the construction of radioimmunoconjugates for targeted and pretargeted therapy. Proof-of-principle CRYPT radiolabeling was successful with lead-203 ([203Pb]Pb2+) and demonstrated complexation efficiency superior to that of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and efficiency comparable to that of the current industry standard TCMC (1,4,7,10-tetraaza-1,4,7,10-tetra-(2-carbamoylmethyl)-cyclododecane). In vitro human serum stability assays demonstrated excellent [203Pb]Pb-CRYPT stability over 72 h (91.7 ± 0.56%; n = 3). [203Pb]Pb-CRYPT-radioimmunoconjugates were synthesized from the corresponding CRYPT-immunoconjugate or by conjugating [203Pb]Pb-Tz-CRYPT to transcyclooctene modified trastuzumab (TCO-trastuzumab) via the inverse electron-demand Diels-Alder (IEEDA) reaction. This investigation reveals the potential for CRYPT ligands to become new industry standards for therapeutic and diagnostic radiometals in radiopharmaceutical elaboration.

Amide-based chelators DTPAm, EGTAm and ampam were synthesized to investigate which chelator most ideally coordinates [nat/203Pb]Pb2+ ions for potential radiopharmaceutical applications. 1H NMR spectroscopy was used to study each metal–ligand complex in the solution state. The 1H NMR spectrum of [Pb(DTPAm)]2+ revealed minimal isomerization and fluxional behaviour compared to [Pb(EGTAm)]2+ and [Pb(ampam)]2+, both of which showed fewer spectral changes indicative of less static behaviour. The solid-state coordination properties of each complex were also examined from single crystal structures that were studied by X-ray diffraction (XRD). In the solid-state, octadentate DTPAm coordinated Pb2+ to form an eight-coordinate hemidirected complex; octadentate EGTAm coordinated Pb2+ forming a ten-coordinate holodirected complex with a bidentate NO3− ion also coordinated to the metal centre; decadentate ampam completely encapsulated the Pb2+ ion to form a ten-coordinate holodirected complex with a C2 axis of symmetry. Potentiometric titrations were carried out to assess the thermodynamic stability of each metal–ligand complex. The pM values obtained for [Pb(DTPAm)]2+, [Pb(EGTAm)]2+ and [Pb(ampam)]2+ were 9.7, 7.2 and 10.2, respectively. The affinity of each chelator for Pb2+ ions was tested by [203Pb]Pb2+ radiolabeling studies to evaluate their prospects as chelators for [203/212Pb]Pb2+-based radiopharmaceuticals. DTPAm radiolabeled [203Pb]Pb2+ ions achieving molar activities as high as 3.5 MBq μmol−1 within 15 minutes, at 25 °C, whereas EGTAm and ampam produced lower molar activities of 0.25 MBq μmol−1 within 30 minutes, at 37 °C. EGTAm and ampam were therefore deemed unsuitable for [203/212Pb]Pb2+-based radiopharmaceutical applications, while DTPAm warrants further studies.

Background 64Cu is one of the few radioisotopes that can be used for both imaging and therapy, enabling theranostics with identical chemical composition. Development of stable chelators is essential to harness the potential of this isotope, challenged by the presence of endogenous copper chelators. Pyridyl type chelators show good coordination ability with copper, prompting the present study of a series of chelates DOTA-xPy (x = 1–4) that sequentially substitute carboxyl moieties with pyridyl moieties on a DOTA backbone. Results We found that the presence of pyridyl groups significantly increases 64Cu labeling conversion yield, with DOTA-2Py, −3Py and -4Py quantitatively complexing 64Cu at room temperature within 5 min (1 × 10− 4 M). [64Cu]Cu-DOTA-xPy (x = 2–4) exhibited good stability in human serum up to 24 h. When challenged with 1000 eq. of NOTA, no transmetallation was observed for all three 64Cu complexes. DOTA-xPy (x = 1–3) were conjugated to a cyclized α-melanocyte-stimulating hormone (αMSH) peptide by using one of the pendant carboxyl groups as a bifunctional handle. [64Cu]Cu-DOTA-xPy-αMSH retained good serum stability (> 96% in 24 h) and showed high binding affinity (Ki = 2.1–3.7 nM) towards the melanocortin 1 receptor. Conclusion DOTA-xPy (x = 1–3) are promising chelators for 64Cu. Further in vivo evaluation is necessary to assess the full potential of these chelators as a tool to enable further theranostic radiopharmaceutical development.

Recent clinical results have demonstrated remarkable treatment responses of late-stage cancer patients when treated with alpha-emitting radionuclides such as actinium-225 (225Ac). The resulting intense global effort to produce greater quantities of 225Ac has triggered a number of emerging technologies to produce this rare, yet important, radionuclide. Accelerator-based methods for increasing global 225Ac production capacity have focused on the high energy (>100 MeV) proton irradiation of thorium, despite the coproduction of the undesirable 227Ac byproduct at 0.1-0.3% of the 225Ac activity. We at TRIUMF have developed a process for the production of a 225Ra/225Ac generator from irradiated thorium that results in an 225Ac product with reduced 227Ac content. 225Ac was separated from irradiated thorium and coproduced radioactive spallation and fission products using a thorium peroxide precipitation method followed by cation exchange and extraction chromatography. Stable and radioactive tracer studies demonstrated the ability of this method to separate Ac from most other elements, providing a directly produced Ac product with measured 227Ac content of (0.15 ± 0.04)%. A second, indirectly produced Ac product with 227Ac content of

Background Lead-212 (212Pb, t1/2 = 10.6 h) and lead-203 (203Pb, t1/2 = 51.9 h) are an element-equivalent, or a matched theranostic radioisotope pair that show great potential for application in targeted radionuclide therapy (TRT) and single-photon emission computed tomography (SPECT), respectively. At TRIUMF we have produced both 203Pb and 212Pb using TRIUMF’s TR13 (13 MeV) and 500 MeV cyclotrons, and subsequently purified and evaluated both radioisotopes using a series of pyridine-modified DOTA analogues in comparison to the commercially available chelates DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and TCMC (1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane). Results Proton irradiation (12.8 MeV) of natural and enriched thallium-203 (203Tl) targets gave 203Pb saturation yields of 134 ± 25 and 483 ± 3 MBq/μA, respectively. Thorium-228 (228Th, t1/2 = 1.9 y), a by-product of 232Th proton spallation on TRIUMF’s main 500 MeV beamline (beamline 1A, BL1A), was recovered to build a 228Th/212Pb generator with the ability to deliver up to 9–10 MBq of 212Pb daily. Both lead isotopes were purified via solid phase extraction chromatography (Pb resin), and isolated in an acetate form ([203/212Pb]Pb(OAc)2) suitable for direct radiolabeling of chelators and bioconjugates. A series of cyclen-based chelators (herein referred to as DOTA-1Py, -2Py, and -3Py) along with established chelates DOTA and TCMC were evaluated for their ability to complex both 203Pb and 212Pb. All chelates incorporated 212Pb/203Pb efficiently, with higher radiolabeling yields observed for the 212Pb-complexes. Conclusion The production of 203Pb and 212Pb was established using TRIUMF 13 MeV and 500 MeV cyclotrons, respectively. Both production methods provided radiometals suitable for subsequent radiolabeling reactions using known and novel chelates. Furthermore, the novel chelate DOTA-3Py may be a good candidate for biomolecule conjugation and further theranostic 212Pb/203Pb studies.

Background 64Cu is one of the few radioisotopes that can be used for both imaging and therapy, enabling theranostics with identical chemical composition. Development of stable chelators is essential to harness the potential of this isotope, challenged by the presence of endogenous copper chelators. Pyridyl type chelators show good coordination ability with copper, prompting the present study of a series of chelates DOTA-xPy (x = 1–4) that sequentially substitute carboxyl moieties with pyridyl moieties on a DOTA backbone. Results We found that the presence of pyridyl groups significantly increases 64Cu labeling conversion yield, with DOTA-2Py, −3Py and -4Py quantitatively complexing 64Cu at room temperature within 5 min (1 × 10− 4 M). [64Cu]Cu-DOTA-xPy (x = 2–4) exhibited good stability in human serum up to 24 h. When challenged with 1000 eq. of NOTA, no transmetallation was observed for all three 64Cu complexes. DOTA-xPy (x = 1–3) were conjugated to a cyclized α-melanocyte-stimulating hormone (αMSH) peptide by using one of the pendant carboxyl groups as a bifunctional handle. [64Cu]Cu-DOTA-xPy-αMSH retained good serum stability (> 96% in 24 h) and showed high binding affinity (Ki = 2.1–3.7 nM) towards the melanocortin 1 receptor. Conclusion DOTA-xPy (x = 1–3) are promising chelators for 64Cu. Further in vivo evaluation is necessary to assess the full potential of these chelators as a tool to enable further theranostic radiopharmaceutical development. Supplementary Information The online version contains supplementary material available at 10.1186/s41181-020-00119-4.

Binding affinity and selectivity are critical properties of aptamers that must be optimized for any application. The sulforhodamine B binding RNA aptamer (SRB-2) is a somewhat promiscuous aptamer that can bind ligands that vary markedly in shape, size and charge. Here we categorize potential ligands based on their binding mode and structural characteristics required for high affinity and selectivity. Several known and potential ligands of SRB-2 were screened for binding affinity using LSPR, ITC and NMR spectroscopy. The study shows that rhodamine B has the ideal structural and electrostatic properties for selective and high-affinity binding of the SRB-2 aptamer.