H3TPAN-Triazole-Bn-NH2: Tripicolinate Clicked-Bifunctional Chelate for [225Ac]/[111In] Theranostics

An 211At-labeled alpha-melanocyte stimulating hormone peptide analog for targeted alpha therapy of metastatic melanoma

PURPOSE

Patients who develop metastatic melanoma have a very poor prognosis, and new treatments are needed to improve the response rates. Melanocortin-1 receptor (MC1R) is a promising target for radionuclide therapy of metastatic melanoma, and alpha-melanocyte stimulating hormone (α-MSH) peptide analogs show high affinities to MC1Rs. Because targeted alpha therapy (TAT) can be a desirable treatment for metastatic melanoma, this study aimed to develop an 211At-labeled α-MSH peptide analog for TAT of metastatic melanoma.

METHODS

We designed an α-MSH analog labeled with 211At using a neopentyl glycol scaffold via a hydrophilic linker. Preliminary studies using 125I-labeled α-MSH analogs were performed to identify suitable hydrophilic linkers. Then, [211At]NpG-GGN4c was prepared using a procedure similar to that of the 125I-labeled counterpart, [125I]NpG-GGN4b. The biodistribution profile of [211At]NpG-GGN4c in B16F10 tumor-bearing mice was compared with that of [125I]NpG-GGN4b. B16F10 tumor-bearing mice were treated with a single dose of vehicle or [211At]NpG-GGN4c (1 or 0.4 MBq).

RESULTS

The D-Glu-D-Arg linker was identified as the optimal hydrophilic linker because of its high affinity for MC1R and good biodistribution profile, especially with low accumulation in the liver and intestine. [211At]NpG-GGN4c showed tumor accumulation comparable to that of [125I]NpG-GGN4b and maintained the tumor radioactivity retention from 1 to 3 h postinjection. [211At]NpG-GGN4c exhibited a dose-dependent inhibitory effect on B16F10 xenograft growth without apparent body weight loss.

CONCLUSION

[211At]NpG-GGN4c showed dose-dependent efficacy against B16F10 xenografts, suggesting that [211At]NpG-GGN4c is a promising TAT agent for treating metastatic melanoma.

Alpha Atlas: Mapping global production of α-emitting radionuclides for targeted alpha therapy

Targeted Alpha Therapy has shown great promise in cancer treatment, sparking significant interest over recent decades. However, its broad adoption has been impeded by the scarcity of alpha-emitters and the complexities related to their use. The availability of these radionuclides is often constrained by the intricate production processes and purification, as well as regulatory and logistical challenges. Moreover, the high cost and technical difficulties associated with handling and applying alpha-emitting radionuclides pose additional barriers to their clinical implementation. This Alpha Atlas provides an in-depth overview of the leading alpha-particle emitting radionuclide candidates for clinical use, focusing on their production processes and supply chains. By mapping the current facilities that produce and supply these radionuclides, this atlas aims to assist researchers, clinicians, and industries in initiating or scaling up the applications of alpha-emitters. The Alpha Atlas aspires to act as a strategic guide, facilitating collaboration and driving forward the integration of these potent therapeutic agents into cancer treatment practices.

Towards DFO*12-Preliminary Results of a New Chelator for the Complexation of Actinium-225

Background: Actinium-225 (225Ac) has gained interest in nuclear medicine for use in targeted alpha therapy (TAT) for the treatment of cancer. However, the number of suitable chelators for the stable complexation of 225Ac3+ is limited. The promising physical properties of 225Ac result in an increased demand for the radioisotope that is not matched by its current supply. To expand the possibilities for the development of 225Ac-based TAT therapeutics, a new hydroxamate-based chelator, DFO*12, is described. We report the DFT-guided design of dodecadentate DFO*12 and an efficient and convenient automated solid-phase synthesis for its preparation. To address the limited availability of 225Ac, a small-scale 229Th/225Ac generator was constructed in-house to provide [225Ac]AcCl3 for research. Methods: DFT calculations were performed in ORCA 5.0.1 using the BP86 functional with empirical dispersion correction D3 and Becke-Johnson damping (D3BJ). The monomer synthesis over three steps enabled the solid-phase synthesis of DFO*12. The small-scale 229Th/225Ac generator was realized by extracting 229Th from aged 233U material. Radiolabeling of DFO*12 with 225Ac was performed in 1 M TRIS pH 8.5 or 1.5 M NaOAc pH 4.5 for 30 min at 37 °C. Results: DFT calculations directed the design of a dodecadentate chelator. The automated synthesis of the chelator DFO*12 and the development of a small-scale 229Th/225Ac generator allowed for the radiolabeling of DFO*12 with 225Ac quantitatively at 37 °C within 30 min. The complex [225Ac]Ac-DFO*12 indicated good stability in different media for 20 h. Conclusions: The novel hydroxamate-based dodecadentate chelator DFO*12, together with the developed 229Th/225Ac generator, provide new opportunities for 225Ac research for future radiopharmaceutical development and applications in TAT.

Chelation of [111In]In3+ with the dual-size-selective macrocycles py-macrodipa and py2-macrodipa

Indium-111 (111In) is a diagnostic radiometal that is important in nuclear medicine for single-photon emission computed tomography (SPECT). In order to apply this radiometal, it needs to be stably chelated and conjugated to a targeting vector that delivers it to diseased tissue. Identifying effective chelators that are capable of binding and retaining [111In]In3+in vivo is an important research area. In this study, two 18-membered macrocyclic chelators, py-macrodipa and py2-macrodipa, were investigated for their ability to form stable coordination complexes with In3+ and to be effectively radiolabeled with [111In]In3+. The In3+ complexes of these two chelators were characterized by NMR spectroscopy, X-ray crystallography, and density functional theory calculations. These studies show that both py-macrodipa and py2-macrodipa form 8-coordinate In3+ complexes and attain an asymmetric conformation, consistent with prior studies on this ligand class with small rare earth metal ions. Spectrophotometric titrations were carried out to determine the thermodynamic stability constants (log KML) of [In(py-macrodipa)]+ and [In(py2-macrodipa)]+, which were found to be 18.96(6) and 19.53(5), respectively, where the values in parentheses are the errors of the last significant figures obtained from the standard deviation from three independent replicates. Radiolabeling studies showed that py-macrodipa and py2-macrodipa can quantitatively be radiolabeled with [111In]In3+ at 25 °C within 5 min, even at ligand concentrations as low as 1 μM. The in vitro stability of the radiolabeled complexes was investigated in human serum at 37 °C, revealing that ∼90% of [111In][In(py-macrodipa)]+ and [111In][In(py2-macrodipa)]+ remained intact after 7 days. The biodistribution of these radiolabeled complexes in mice was investigated, showing lower uptake in the kidneys, liver, and blood at the 24 h mark compared to [111In]InCl3. These results demonstrate the potential of py-macrodipa and py2-macrodipa as chelators for [111In]In3+, suggesting their value for SPECT radiopharmaceuticals.

Construction of the Bioconjugate Py-Macrodipa-PSMA and Its In Vivo Investigations with Large 132/135La3+ and Small 47Sc3+ Radiometal Ions

To harness radiometals in clinical settings, a chelator forming a stable complex with the metal of interest and targets the desired pathological site is needed. Toward this goal, we previously reported a unique set of chelators that can stably bind to both large and small metal ions, via a conformational switch. Within this chelator class, py-macrodipa is particularly promising based on its ability to stably bind several medicinally valuable radiometals including large 132/135La3+, 213Bi3+, and small 44Sc3+. Here, we report a 10-step organic synthesis of its bifunctional analogue py-macrodipa-NCS, which contains an amine-reactive -NCS group that is amenable for bioconjugation reactions to targeting vectors. The hydrolytic stability of py-macordipa-NCS was assessed, revealing a half-life of 6.0 d in pH 9.0 aqueous buffer. This bifunctional chelator was then conjugated to a prostate-specific membrane antigen (PSMA)-binding moiety, yielding the bioconjugate py-macrodipa-PSMA, which was subsequently radiolabeled with large 132/135La3+ and small 47Sc3+, revealing efficient and quantitative complex formation. The resulting radiocomplexes were injected into mice bearing both PSMA-expressing and PSMA-non-expressing tumor xenografts to determine their biodistribution patterns, revealing delivery of both 132/135La3+ and 47Sc3+ to PSMA+ tumor sites. However, partial radiometal dissociation was observed, suggesting that py-macrodipa-PSMA needs further structural optimization.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Rearmed Bifunctional Chelating Ligand for 225Ac/155Tb Precision-Guided Theranostic Radiopharmaceuticals─H4noneunpaX

Superior bifunctional chelating ligands, which can sequester both α-emitting radionuclides (225Ac, 213Bi) and their diagnostic companions (155Tb, 111In), remain a formidable challenge to translating targeted alpha therapy, with complementary diagnostic imaging, to the clinic. H4noneupaX, a chelating ligand with an unusual diametrically opposed arrangement of pendant donor groups, has been developed to this end. H4noneunpaX preferentially complexes Ln3+ and An3+ ions, forming thermodynamically stable (pLa = 17.8, pLu = 21.3) and kinetically inert complexes─single isomeric species by nuclear magnetic resonance and density functional theory. Metal binding versatility demonstrated in radiolabeling [111In]In3+, [155Tb]Tb3+, [177Lu]Lu3+, and [225Ac]Ac3+ achieved high molar activities under mild conditions. Efficient, scalable synthesis enabled in vivo evaluation of bifunctional H4noneunpaX conjugated to two octreotate peptides targeting neuroendocrine tumors. Single photon emission computed tomography/CT and biodistribution studies of 155Tb-radiotracers in AR42J tumor-bearing mice showed excellent image contrast, good tumor uptake, and high in vivo stability. H4noneunpaX shows significant potential for theranostic applications involving 225Ac/155Tb or 177Lu/155Tb.

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of 177Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of 225Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of 225Ac, its physical and radiochemical properties, as well as the place of 225Ac-DOTATOC and 225Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.