Matching chelators to radiometals for radiopharmaceuticals

Tungsten-based nanoparticles as contrast agents for liver tumor detection using dual-energy computed tomography

Dual-energy computed tomography (DECT) is a commonly used imaging technique for detecting and diagnosing liver cancer. Currently, it is performed using clinically approved iodinated small molecule contrast agents (CAs). However, these iodinated CAs have several drawbacks, including sub-optimal contrast generation and contra-indication in patients with renal insufficiency. Herein, we synthesized tungsten-based CAs (i.e., WO3-x NPs) with excellent biocompatibility and investigated their effectiveness in DECT imaging. WO3-x NPs significantly enhanced the contrast between liver tumors and normal liver tissues as indicated by in vivo DECT imaging. Furthermore, WO3-x NPs exhibited excellent biocompatibility and minimal systemic toxicity. This study introduces a novel class of CAs for DECT and presents a promising method for accurate early diagnosis of liver tumors.

Anti-cancer bioprospecting on medicinal plants from Indonesia: A review

The Indonesian archipelago is home to the second largest biodiversity in the world and is inhabited by more than 300 ethnic groups with a total population of more than 270 million. The indigenous population still rely on traditional medicine practices, especially the use of plant-based remedies. Although modern science-based exploration on Indonesian medicinal plants started with the European settlement in the archipelago in the 16th century, it was not until the 1970's that the phytochemistry of Indonesian medicinal plants was recognized for its potency. The need for new cancer cures to increase the quality of human life has led to the bioprospecting of medicinal plants including those of Indonesian origin. Despite published reports on the anticancer potency of Indonesian medicinal plants, to date there has been no comprehensive review on this topic. In this manuscript, we review the phytochemical and pharmacological studies on medicinal plants from Indonesia related to cancer therapy. Established databases (GARUDA, SciFinder, and PubMed) were used to collate data from 1990 to 2022, resulting in the description of 134 medicinal plants and their phytochemical and pharmacological properties including examples containing potent agents against breast, leukaemia, cervix, lung, and colon cancer cell lines based on in vitro bioassays and in vivo evaluation. These findings provide valuable insights into the bioprospecting of Indonesian medicinal plant providing directions for future studies, including the development of new therapeutics, both as botanicals or by using conventional dosage.

Engineering Radioactive Microspheres for Intra‐Arterial Brachytherapy Using Radiation‐Induced Graft Polymerization

Intravascular brachytherapy requires advances in radio‐embolization technologies that combine brilliant radiostability efficacy with a facile and green synthesis route. A hybrid‐integrated radioactive microsphere strategy using phosphorylcholine‐modified lutetium‐177 coordinated polymeric microspheres (177Lu‐PCMs) is reported that are fabricated via radiation‐induced graft polymerization for imaging‐guided locoregional intravascular brachytherapy. The underlying formation mechanism of 177Lu‐PCMs is elucidated using first‐principles computations and density functional theory calculations, and 177Lu loading mechanisms are investigated with Near‐edge and extended X‐ray absorption fine structure spectroscopy. The engineered 177Lu‐PCMs exhibit excellent mechanical properties, good hydrophilicity, and controlled sphere diameter. These features provide advantages of ultra‐stable embolic radio‐theranostics, which is demonstrated in different preclinical rodent models and isolated human liver tumor tissues. During locoregional intra‐arterial brachytherapy, 177Lu‐PCMs can be visualized via SPECT to validate the in vivo biodistribution and retention in real time, achieving precise delivery, effective anti‐cancer treatment, and a distinguished safety profile without degradation, ectopic embolization, and adverse reactions. Therefore, this study offer a new avenue for the development of a highly innovative and translational approach for precision intra‐arterial brachytherapy.

Metallacages with 2,6-dipicolinoylbis(N,N-dialkylthioureas) as novel platforms in nuclear medicine for 68Ga, 177Lu and 198Au

BACKGROUND

Heterometallic gold metallacages are of great interest for the incorporation of several cations. Especially in nuclear medicine, those metallacages can serve as a platform for radionuclides relevant for imaging or therapy (e.g. 68Ga or 177Lu). Moreover, the radionuclide 198Au is an attractive beta emitter, for potential application in nuclear medicine. Here, we aim to synthesize a new set of gold metallacages and to study their ability to coordinate to 68Ga, 177Lu and 198Au.

RESULTS

New heterometallic gold metallacages of composition [M{Au(Lmorph-κS)}3] (M = La3+, Tb3+, Lu3+ or Y3+) and [Ga{Au(Lmorph-κS)}2]NO3 have been synthesized from 2,6-dipicolinoylbis(N,N-morpholinylthiourea) (H2Lmorph) with [AuCl(THT)] and the target M3+ metal ions in yields ranging from 33 (Lu) to 62% (Tb). The characterization of the compounds bases on ESI-MS, 1H NMR, IR, EA and single-crystal X-ray diffraction techniques (all except the Ga derivative). Selected gold cages derived from H2Lmorph were compared to previously reported gold cages that were derived from 2,6-dipicolinoylbis(N,N-diethylthiourea) (H2Ldiethyl). The tested metallacages show similar IC50 values close to that of auranofin in four different cancer cell lines (MCF-7, PC-3, U383, U343), e.g. 4.5 ± 0.7 µM for [Ga{Au(Ldiethyl)}2]NO3 on PC-3. The radiolabeling experiments thereof show high radiochemical purities with 68Ga and 198Au and low radiochemical purity with 177Lu.

CONCLUSIONS

The results indicate that these gold metallacages could serve as a novel platform for inclusion of different (radio)nuclides with potential theranostic applications in nuclear medicine.

The Curies' element: state of the art and perspectives on the use of radium in nuclear medicine

BACKGROUND

The alpha-emitter radium-223 (223Ra) is presently used in nuclear medicine for the palliative treatment of bone metastases from castration-resistant prostate cancer. This application arises from its advantageous decay properties and its intrinsic ability to accumulate in regions of high bone turnover when injected as a simple chloride salt. The commercial availability of [223Ra]RaCl2 as a registered drug (Xofigo®) is a further additional asset.

MAIN BODY

The prospect of extending the utility of 223Ra to targeted α-therapy of non-osseous cancers has garnered significant interest. Different methods, such as the use of bifunctional chelators and nanoparticles, have been explored to incorporate 223Ra in proper carriers designed to precisely target tumor sites. Nevertheless, the search for a suitable scaffold remains an ongoing challenge, impeding the diffusion of 223Ra-based radiopharmaceuticals.

CONCLUSION

This review offers a comprehensive overview of the current role of radium radioisotopes in nuclear medicine, with a specific focus on 223Ra. It also critically examines the endeavors conducted so far to develop constructs capable of incorporating 223Ra into cancer-targeting drugs. Particular emphasis is given to the chemical aspects aimed at providing molecular scaffolds for the bifunctional chelator approach.

Pentadentate and Hexadentate Pyridinophane Ligands Support Reversible Cu(II)/Cu(I) Redox Couples

Two new ligands were synthesized with the goal of copper stabilization, N,N'-(2-methylpyridine)-2,11-diaza[3,3](2,6)pyridinophane (PicN4) and N-(methyl),N'-(2-methylpyridine)-2,11-diaza[3,3](2,6)pyridinophane (PicMeN4), by selective functionalization of HN4 and TsHN4. These two ligands, when reacted with various copper salts, generated both Cu(II) and Cu(I) complexes. These ligands and Cu complexes were characterized by various methods, such as NMR, UV-Vis, MS, and EA. Each compound was also examined electrochemically, and each revealed reversible Cu(II)/Cu(I) redox couples. Additionally, stability constants were determined via spectrophotometric titrations, and radiolabeling and cytotoxicity experiments were performed to assess the chelators relevance to their potential use in vivo as 64Cu PET imaging agents.

A review of recent advancements in Actinium-225 labeled compounds and biomolecules for therapeutic purposes

In nuclear medicine, cancers that cannot be cured or can only be treated partially by traditional techniques like surgery or chemotherapy are killed by ionizing radiation as a form of therapeutic treatment. Actinium-225 is an alpha-emitting radionuclide that is highly encouraging as a therapeutic approach and more promising for targeted alpha therapy (TAT). Actinium-225 is the best candidate for tumor cells treatment and has physical characteristics such as high (LET) linear energy transfer (150 keV per μm), half-life (t1/2  = 9.92d), and short ranges (400-100 μm) which prevent the damage of normal healthy tissues. The introduction of various new radiopharmaceuticals and radioisotopes has significantly assisted the advancement of nuclear medicine. Ac-225 radiopharmaceuticals continuously demonstrate their potential as targeted alpha therapeutics. 225 Ac-labeled radiopharmaceuticals have confirmed their importance in medical and clinical areas by introducing [225 Ac]Ac-PSMA-617, [225 Ac]Ac-DOTATOC, [225 Ac]Ac-DOTA-substance-P, reported significantly improved response in patients with prostate cancer, neuroendocrine, and glioma, respectively. The development of these radiopharmaceuticals required a suitable buffer, incubation time, optimal pH, and reaction temperature. There is a growing need to standardize quality control (QC) testing techniques such as radiochemical purity (RCP). This review aims to summarize the development of the Ac-225 labeled compounds and biomolecules. The current state of their reported resulting clinical applications is also summarized as well.

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Implementing reversed-phase and hydrophilic interaction liquid chromatography into the characterization of DTPA-ramucirumab conjugate before radiolabeling

Radioimmunoconjugates represent a promising class of therapeutics and diagnostics. The characterization of intermediate chelator-antibody products, i.e., without the radionuclide, is frequently omitted, bringing significant uncertainty in the radioimmunoconjugate preparation. In the present study, we explored the utility of reversed-phase (RPLC) and hydrophilic interaction (HILIC) liquid chromatography with UV detection to characterize ramucirumab stochastically conjugated with p-SCN-Bn-CHX-A"-DTPA chelator (shortly DTPA). The conjugation was well reflected in RPLC chromatograms, while chromatograms from HILIC were significantly less informative. RPLC analyses at the intact level confirmed that the conjugation resulted in a heterogeneous mixture of modified ramucirumab. Moreover, the RPLC of DTPA-ramucirumab confirmed heterogeneous conjugation of all subunits. The peptide mapping did not reveal substantial changes after the conjugation, indicating that most parts of ramucirumab molecules remained unmodified and that the DTPA chelator was bound to various sites. Eventually, the RPLC method for analysis of intact ramucirumab was successfully applied to online monitoring of conjugation reaction in 1 h intervals for a total of 24 h synthesis, which readily reflected the structural changes of ramucirumab in the form of retention time shift by 0.21 min and increase in peak width by 0.22 min. The results were obtained in real-time, practically under 10 min per monitoring cycle. To the best of our knowledge, our study represents the first evaluation of RPLC and HILIC to assess the quality of intermediates during the on-site preparation of radioimmunoconjugates prior to radiolabeling.

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.

Development of Novel Trifunctional Chelating Agents That Enhance Tumor Retention of Radioimmunoconjugates

Chelator-containing radioimmunoconjugates (RICs) composed of monoclonal antibodies, chelators, and radiometals exhibit broad potential for cancer diagnosis or therapy. In this study, we developed novel trifunctional chelating agents that enhance the tumor retention of RICs, MDPEI2, and MDPEI4, which contain the metal chelator DOTA, a maleimide moiety, and diethylenetriamine (PEI2) or tetraethylenepentamine (PEI4), respectively, as a poly(ethylenimine) (PEI) scaffold for the addition of positive charges to the radiometabolites of RICs to reduce their release from tumor cells. Trastuzumab radiolabeled by [111In]In-MDPEI2 ([111In]In-TMDPEI2) or [111In]In-MDPEI4 ([111In]In-TMDPEI4) showed high immunoreactivity and lower rates of exportations of their radiometabolites from tumor cells than RICs without PEI scaffolds. The tumor uptake of [111In]In-TMDPEI2 and [111In]In-TMDPEI4 was enhanced compared with RICs without PEI scaffolds, and [111In]In-TMDPEI2 exhibited the highest tumor/blood ratio. These results indicate the utility of MDPEI2 to synthesize RICs with favorable tumor-targeting properties in vivo by controlling the radioactivity distribution in tumor cells.

An antibody-radionuclide conjugate targets fibroblast activation protein for cancer therapy

PURPOSE

Fibroblast activation protein is one of the most attractive targets for tumor diagnosis and therapy. There have been many successful clinical translations with small molecules and peptides, yet only a few anti-FAP antibody diagnostic or therapeutic agents have been reported. Antibodies often feature good tumor selectivity and long tumor retention, which may be a better-match with therapeutic radionuclides (e.g.,177Lu, 225Ac) for cancer therapy. Here we report a 177Lu-labeled anti-FAP antibody, PKU525, as a therapeutic radiopharmaceutical for FAP-targeted radiotherapy.

METHODS

The anti-FAP antibody is produced as a derivative of sibrotuzumab. The pharmacokinetics and blocking study are performed with 89Zr-labeled antibody by PET imaging. The conjugation strategies have been screened and tested with SPECT imaging through 177Lu-labeling. The biodistribution and radiotherapy studies are performed on 177Lu-labeled anti-FAP antibody in NU/NU mice-bearing HT-1080-FAP tumors.

RESULTS

A multiple time-point PET imaging study shows that the tumor accumulation of [89Zr]Zr-DFO-PKU525 is intense, selective, and relatively rapid. The time activity curve indicates that the tumor uptake continually increases until reaches the highest uptake (SUVmax = 18.4 ± 2.3, n = 4) at 192 h, then gradually declines. Radioactivity rapidly cleared from the blood, liver, and other major organs, resulting in high tumor-to-background ratios. An in vivo blocking experiment suggests that [89Zr]Zr-DFO-PKU525 is FAP-specific and the uptake in FAP-negative tumors is almost negligible. Ex vivo biodistribution study shows that the tumor uptake of [177Lu]Lu-DOTA-NCS-PKU525 is 23.04 ± 5.11% ID/g, 33.2 ± 6.36% ID/g, 19.87 ± 6.84% ID/g and 19.02 ± 5.90% ID/g at 24 h, 96 h, 168 h, and 240 h after injection (n = 5), which is corroborated with the PET imaging. In therapeutic assays, multiple doses of [177Lu]Lu-DOTA-NCS-PKU525 have been tested in tumor-bearing mice, and the data suggests that 3.7 MBq may be sufficient to completely suppress the tumor growth in mice without showing observable side effects.

CONCLUSION

A FAP-targeted antibody-radionuclide conjugate was developed and evaluated in vitro and in vivo. Its tumor accumulation is rapid and high with a clean background. It remarkably suppresses the tumors in mice while the side effect is almost negligible, showing that it is promising for further clinical translational studies.

Design and synthesis of a new bifunctional chelating agent: Application for Al 18F/177Lu complexation

Theranostic and personalized medicine are blooming strategies to improve oncologic patients' health care and facilitate early treatment. While 18F-radiochemistry for theranostic application is attractive due to its imaging properties, combining diagnosis by positron emission tomography (PET) via aluminum-fluoride-18 and β- therapy with lutetium-177 is relevant. Nevertheless, it requires the use of two different chelating agents, which are NOTA and DOTA for aluminum-fluoride-18 and lutetium-177 radiolabeling, respectively. To overcome this issue, we propose herein the synthesis of a new hybrid chelating agent named NO2A-AHM, which can be labeled with different types of emitters (β+, β- and γ) using the mismatched Al18F/177Lu pair. NO2A-AHM, is based on a hydrazine moiety functionalized by a NOTA cycle, a chelating arm, and a linker with a maleimide function. This design is chosen to increase the flexibility and allow the formation of 5 up to 7 coordination bonds with metal ions. Moreover, this agent can be coupled to targeting moieties containing a thiol function, such as peptides, to increase selectivity towards specific cancer cells. Experimental complexation and computational chemistry studies are performed to confirm the capacity of our chelating agent to label both aluminum-fluoride and lutetium using molecular modeling approaches at Density Functional Theory (DFT) level. The proof of concept of the ability of NO2A-AHM to complex both aluminum-fluoride-18, for PET imaging applications, and lutetium-177 for radiotherapy has shown encouraging results which is prominent for the development of a fully consistent theranostic approach.

Site-specific Conjugation of 6 DOTA Chelators to a CA19-9-targeting scFv-Fc Antibody for Imaging and Therapy

Antibodies conjugated with diagnostic/therapeutic radionuclides are attractive options for inoperable cancers lacking accurate imaging methods and effective therapeutics, such as pancreatic cancer. Hence, we have produced an antibody radionuclide conjugate termed TE-1132 comprising a α-CA19-9 scFv-Fc that is site-specifically conjugated at each C-terminus to 3 DOTA chelators via a cysteine-containing peptide linker. The smaller scFv-Fc size facilitates diffusivity within solid tumors, whereas the chelator-to-antibody ratio of six enabled 177Lu-radiolabeled TE-1132 to exhibit high radioactivity up to 520 MBq/nmol. In mice bearing BxPC3 tumors, immuno-SPECT/CT imaging of [111In]In-TE-1132 and the biodistribution of [177Lu]Lu-TE-1132 showed selective tumor accumulation. Single and multiple doses of [177Lu]Lu-TE-1132 effectively inhibited the BxPC3 tumor growth and prolonged the survival of mice with no irreversible body weight loss or hematopoietic damage. The adequate pharmacokinetic parameters, prominent tumor accumulation, and efficacy with good safety in mice encourage the further investigation of theranostic TE-1132 for treating pancreatic cancer.

Organotrifluoroborate enhances tumor targeting of fibroblast activation protein inhibitors for targeted radionuclide therapy

PURPOSE

Fibroblast activation protein (FAP) is a pan-cancer target and now the state-of-the-art to develop radiopharmaceuticals. FAP inhibitors have been of great success in developing imaging tracers. Yet, the overly rapid clearance cannot match with the long half-lives of regular therapeutic radionuclides. Though strategies that aim to elongate the circulation of FAPIs are being developed, here we describe an innovation that uses α-emitters of short half-lives (e.g., 213Bi) to pair the rapid pharmacokinetics of FAPIs.

METHODS

An organotrifluoroborate linker is engineered to FAPIs to give two advantages: (1) selectively increases tumor uptake and retention; (2) facile 18F-radiolabeling for positron emission tomography to guide radiotherapy with α-emitters, which can hardly be traced in general.

RESULTS

The organotrifluoroborate linker helps to improve the internalization in cancer cells, resulting in notably higher tumor uptake while the background is clean. In FAP-expressed tumor-bearing mice, this FAPI labeled with 213Bi, a short half-life α-emitter, exhibits almost complete suppression to tumor growth while the side effect is negligible. Additional data shows that this strategy is generally applicable to guide other α-emitters, such as 212Bi, 212Pb, and 149Tb.

CONCLUSION

The organotrifluoroborate linker may be of importance to optimize FAP-targeted radiopharmaceuticals, and the short half-lived α-emitters may be of choice for the rapid-cleared small molecule-based radiopharmaceuticals.

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.

Radiochemical and Biological Evaluation of 3p-C-NETA-ePSMA-16, a Promising PSMA-Targeting Agent for Radiotheranostics

Bifunctional chelators (BFCs) are a key element in the design of radiopharmaceuticals. By selecting a BFC that efficiently complexes diagnostic and therapeutic radionuclides, a theranostic pair possessing almost similar biodistribution and pharmacokinetic properties can be developed. We have previously reported 3p-C-NETA as a promising theranostic BFC, and the encouraging preclinical outcomes obtained with [¹⁸F]AlF-3p-C-NETA-TATE led us to conjugate this chelator to a PSMA-targeting vector for imaging and treatment of prostate cancer. In this study, we synthesized 3p-C-NETA-ePSMA-16 and radiolabeled it with different diagnostic (¹¹¹In, ¹⁸F) and therapeutic (¹⁷⁷Lu, ²¹³Bi) radionuclides. 3p-C-NETA-ePSMA-16 showed high affinity to PSMA (IC₅₀ = 4.61 ± 1.33 nM), and [¹¹¹In]In-3p-C-NETA-ePSMA-16 showed specific cell uptake (1.41 ± 0.20% ID/10⁶ cells) in PSMA expressing LS174T cells. Specific tumor uptake of [¹¹¹In]In-3p-C-NETA-ePSMA-16 was observed up to 4 h p.i. (1.62 ± 0.55% ID/g at 1 h p.i.; 0.89 ± 0.58% ID/g at 4 h p.i.) in LS174T tumor-bearing mice. Only a faint signal could be seen at 1 h p.i. in the SPECT/CT scans, whereas dynamic PET/CT scans performed after administration of [¹⁸F]AlF-3p-C-NETA-ePSMA-16 in PC3-Pip tumor xenografted mice resulted in a better tumor visualization and imaging contrast. Therapy studies with short-lived radionuclides such as ²¹³Bi could further elucidate the therapeutic potential of 3p-C-NETA-ePSMA-16 as a radiotheranostic.

Antibody and Nanobody Radiolabeling with Copper-64: Solid vs. Liquid Target Approach

Antibody and nanobody-based copper-64 radiopharmaceuticals are increasingly being proposed as theranostic tools in multiple human diseases. While the production of copper-64 using solid targets has been established for many years, its use is limited due to the complexity of solid target systems, which are available in only a few cyclotrons worldwide. In contrast, liquid targets, available in virtually in all cyclotrons, constitute a practical and reliable alternative. In this study, we discuss the production, purification, and radiolabeling of antibodies and nanobodies using copper-64 obtained from both solid and liquid targets. Copper-64 production from solid targets was performed on a TR-19 cyclotron with an energy of 11.7 MeV, while liquid target production was obtained by bombarding a nickel-64 solution using an IBA Cyclone Kiube cyclotron with 16.9 MeV on target. Copper-64 was purified from both solid and liquid targets and used to radiolabel NODAGA-Nb, NOTA-Nb, and DOTA-Trastuzumab conjugates. Stability studies were conducted on all radioimmunoconjugates in mouse serum, PBS, and DTPA. Irradiation of the solid target yielded 13.5 ± 0.5 GBq with a beam current of 25 ± 1.2 μA and an irradiation time of 6 h. On the other hand, irradiation of the liquid target resulted in 2.8 ± 1.3 GBq at the end of bombardment (EOB) with a beam current of 54.5 ± 7.8 μA and an irradiation time of 4.1 ± 1.3 h. Successful radiolabeling of NODAGA-Nb, NOTA-Nb, and DOTA-Trastuzumab with copper-64 from both solid and liquid targets was achieved. Specific activities (SA) obtained with the solid target were 0.11, 0.19, and 0.33 MBq/μg for NODAGA-Nb, NOTA-Nb, and DOTA-trastuzumab, respectively. For the liquid target, the corresponding SA values were 0.15, 0.12, and 0.30 MBq/μg. Furthermore, all three radiopharmaceuticals demonstrated stability under the testing conditions. While solid targets have the potential to produce significantly higher activity in a single run, the liquid process offers advantages such as speed, ease of automation, and the feasibility of back-to-back production using a medical cyclotron. In this study, successful radiolabeling of antibodies and nanobodies was achieved using both solid and liquid targets approaches. The radiolabeled compounds exhibited high radiochemical purity and specific activity, rendering them suitable for subsequent in vivo pre-clinical imaging studies.

Radiolabeling and in vivo evaluation of lanmodulin with biomedically relevant lanthanide isotopes

Short-lived, radioactive lanthanides comprise an emerging class of radioisotopes attractive for biomedical imaging and therapy applications. To deliver such isotopes to target tissues, they must be appended to entities that target antigens overexpressed on the target cell's surface. However, the thermally sensitive nature of biomolecule-derived targeting vectors requires the incorporation of these isotopes without the use of denaturing temperatures or extreme pH conditions; chelating systems that can capture large radioisotopes under mild conditions are therefore highly desirable. Herein, we demonstrate the successful radiolabeling of the lanthanide-binding protein, lanmodulin (LanM), with medicinally relevant radioisotopes: ¹⁷⁷Lu, ^132/135La and ⁸⁹Zr. Radiolabeling of the endogenous metal-binding sites of LanM, as well exogenous labeling of a protein-appended chelator, was successfully conducted at 25 °C and pH 7 with radiochemical yields ranging from 20-82%. The corresponding radiolabeled constructs possess good formulation stability in pH 7 MOPS buffer over 24 hours (>98%) in the presence of 2 equivalents of ^natLa carrier. In vivo experiments with [¹⁷⁷Lu]-LanM, [^132/135La]-LanM, and a prostate cancer targeting-vector linked conjugate, [^132/135La]-LanM-PSMA, reveal that endogenously labeled constructs produce bone uptake in vivo. Exogenous, chelator-tag mediated radiolabeling to produce [⁸⁹Zr]-DFO-LanM enables further study of the protein's in vivo behavior, demonstrating low bone and liver uptake, and renal clearance of the protein itself. While these results indicate that additional stabilization of LanM is required, this study establishes precedence for the radiochemical labeling of LanM with medically relevant lanthanide radioisotopes.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Nanoparticles labeled with gamma-emitting radioisotopes: an attractive approach for in vivo tracking using SPECT imaging

Providing accurate molecular imaging of the body and biological process is critical for diagnosing disease and personalizing treatment with the minimum side effects. Recently, diagnostic radiopharmaceuticals have gained more attention in precise molecular imaging due to their high sensitivity and appropriate tissue penetration depth. The fate of these radiopharmaceuticals throughout the body can be traced using nuclear imaging systems, including single-photon emission computed tomography (SPECT) and positron emission tomography (PET) modalities. In this regard, nanoparticles are attractive platforms for delivering radionuclides into targets because they can directly interfere with the cell membranes and subcellular organelles. Moreover, applying radiolabeled nanomaterials can decrease their toxicity concerns because radiopharmaceuticals are usually administrated at low doses. Therefore, incorporating gamma-emitting radionuclides into nanomaterials can provide imaging probes with valuable additional properties compared to the other carriers. Herein, we aim to review (1) the gamma-emitting radionuclides used for labeling different nanomaterials, (2) the approaches and conditions adopted for their radiolabeling, and (3) their application. This study can help researchers to compare different radiolabeling methods in terms of stability and efficiency and choose the best way for each nanosystem.

Evaluation of 89Zr-Labeled Anti-PD-L1 Monoclonal Antibodies Using DFO and Novel HOPO Analogues as Chelating Agents for Immuno-PET

Programmed death ligand 1 (PD-L1) is a type 1 transmembrane immunosuppressive protein that is expressed on a wide range of cell types, including cancer cells. Anti-PD-L1 antibodies have revolutionized cancer therapy and have led to improved outcomes for subsets of cancer patients, including triple-negative breast cancer (TNBC) patients. As a result, PET imaging of PD-L1 protein expression in cancer patients has been explored for noninvasive detection of PD-L1 expressing tumors as well as monitoring response to anti-PD-L1 immune checkpoint therapy. Previous studies have indicated that the in vivo stability and in vivo target detection of antibody-based radio-conjugates can be dramatically affected by the chelator used. These reports demonstrated that the chelator HOPO diminishes ⁸⁹Zr de-chelation compared to DFO. Herein, we report an improved HOPO synthesis and evaluated a series of novel analogues for thermal stability, serum stability, PD-L1-specific binding using the BT-549 TNBC cell line, PET imaging in vivo, as well as biodistribution of ⁸⁹Zr-labeled anti-PD-L1 antibodies in BT-549 xenograft murine models. A new chelator, C5HOPO, demonstrated high stability in vitro and afforded effective PD-L1 targeting in vivovia immuno-PET. These results demonstrated that an improved HOPO chelator is an effective chelating agent that can be utilized to image therapeutically relevant targets in vivo.

A Phosphine Oxide-Functionalized Cyclam as a Specific Copper(II) Chelator

Although cyclam-based ligands are among the strongest copper(II) chelators available, they also usually present good affinity for other divalent cations [Zn(II), Ni(II), and Co(II)], with no copper(II)-specific cyclam ligands having been described so far. As such a property is highly desirable in a wide range of applications, we present herein two novel phosphine oxide-appended cyclam ligands that could be efficiently synthesized through Kabachnik-Fields type reactions on protected cyclam precursors. Their copper(II) coordination properties were closely studied by different physicochemical techniques [electron paramagnetic resonance (EPR) and ultraviolet-visible (UV-vis) spectroscopies, X-ray diffraction, and potentiometry]. The mono(diphenylphosphine oxide)-functionalized ligand demonstrated a copper(II)-specific behavior, unprecedented within the cyclam family of ligands. This was evidenced by UV-vis complexation and competition studies with the parent divalent cations. Density functional theory calculations also confirmed that the particular ligand geometry in the complexes strongly favors copper(II) coordination over that of competing divalent cations, rationalizing the specificity observed experimentally.

3D Hollow Porous Radio‐Granular Hydrogels for SPECT Imaging‐Guided Cancer Intravascular Brachytherapy

Radioactive microspheres have shown excellent therapeutic effects in the treatment of advanced hepatocellular carcinoma (HCC) due to indiscriminate embolization and killing of tumor cells. However, limitations such as unstable loading, in vivo reflux, and untrackable radioactive microspheres restrict their clinical applicability. Herein, a novel injectable lutetium‐177‐labeled 3D hollow porous radio‐granular hydrogels with a double‐cross‐linked network (177Lu‐3D‐HPGH) are synthesized via microfluidics combined with ultraviolet photo‐cross‐linking technology is reported. The radiolabeling efficiency of 177Lu‐3D‐HPGH can reach 97.85%. The 3D hollow porous radio‐granular hydrogels exhibited uniform, controllable size, radio‐theranostics, and excellent underwater adhesion properties, avoiding unwanted radiation damage to non‐target organs. Particularly, the extended X‐ray absorption fine structure combined with the density functional theory calculation revealed the mechanism of 3D‐HPGH loading with 177Lu through Lu‐N/O coordination. Furthermore, rabbit orthotopic kidney and liver tumor models are used to verify the excellent embolization performance, radionuclide loading stability, anti‐reflux characteristics, anti‐tumor effect, and biosafety of 177Lu‐3D‐HPGH. Briefly, this facile, green, and safe synthesis strategy provides a superior choice for intravascular brachytherapy of HCC and has great application value and transformative potential in clinical diagnosis and treatment.

In vivo validation of 68Ga-labeled AMD3100 conjugates for PET imaging of CXCR4

INTRODUCTION

The chemokine receptor CXCR4 has been shown to be over-expressed in multiple types of cancer and is usually associated with aggressive phenotypes and poor prognosis. Successfully targeting and imaging the expression level of this receptor in tumours could inform treatment selection and facilitate patient stratification.

METHODS

Known conjugates of AMD3100 that are specific to CXCR4 have been radiolabelled with gallium-68 and evaluated in naïve and tumour-bearing mice. Tumour uptake of the radiotracers was compared to the known CXCR4-specific PET imaging agent, [68Ga]Pentixafor.

RESULTS

Ex vivo biodistribution in naïve animals showed CXCR4-mediated uptake in the liver with both radiotracers, confirmed by blocking experiments with the high affinity CXCR4 antagonist Cu2CB-Bicyclam (IC50 = 3 nM). PET/CT imaging studies revealed one tracer to have a higher accumulation in the tumour (SUVMean of 0.89 ± 0.14 vs 0.32 ± 0.11). CXCR4-specificity of the best performing tracer was confirmed by administration of a blocking dose of Cu2CB-Bicyclam, showing a 3- and 6-fold decrease in tumour and liver uptake, respectively.

CONCLUSION AND ADVANCES IN KNOWLEDGE

This initial study offers some interesting insights on the impact of some structural features on the pharmacokinetics and metabolic stability of the radiotracer. Additionally, as Pentixafor only binds to human CXCR4, the development of CXCR4-targeted imaging agents that bind to the receptor across different species could significantly help with preclinical evaluation of new CXCR4-specific therapeutics.

A tool for nuclear imaging of the SARS-CoV-2 entry receptor: molecular model and preclinical development of ACE2-selective radiopeptides

PURPOSE

The angiotensin converting enzyme-2 (ACE2)-entry receptor of SARS-CoV-2-and its homologue, the angiotensin-converting enzyme (ACE), play a pivotal role in maintaining cardiovascular homeostasis. Potential changes in ACE2 expression levels and dynamics after SARS-CoV-2 infection have been barely investigated. The aim of this study was to develop an ACE2-targeting imaging agent as a noninvasive imaging tool to determine ACE2 regulation.

METHODS

DOTA-DX600, NODAGA-DX600 and HBED-CC-DX600 were obtained through custom synthesis and labeled with gallium-67 (T_1/2 = 3.26 d) as a surrogate radioisotope for gallium-68 (T_1/2 = 68 min). ACE2- and ACE-transfected HEK cells were used for the in vitro evaluation of these radiopeptides. The in vivo tissue distribution profiles of the radiopeptides were assessed in HEK-ACE2 and HEK-ACE xenografted mice and imaging studies were performed using SPECT/CT.

RESULTS

The highest molar activity was obtained for [⁶⁷Ga]Ga-HBED-CC-DX600 (60 MBq/nmol), whereas the labeling efficiency of the other peptides was considerably lower (20 MBq/nmol). The radiopeptides were stable over 24 h in saline (> 99% intact peptide). All radiopeptides showed uptake in HEK-ACE2 cells (36-43%) with moderate ACE2-binding affinity (K_D value: 83-113 nM), but no uptake in HEK-ACE cells (< 0.1%) was observed. Accumulation of the radiopeptides was observed in HEK-ACE2 xenografts (11-16% IA/g) at 3 h after injection, but only background signals were seen in HEK-ACE xenografts (< 0.5% IA/g). Renal retention was still high 3 h after injection of [⁶⁷Ga]Ga-DOTA-DX600 and [⁶⁷Ga]Ga-NODAGA-DX600 (~ 24% IA/g), but much lower for [⁶⁷Ga]Ga-HBED-CC-DX600 (7.2 ± 2.2% IA/g). SPECT/CT imaging studies confirmed the most favorable target-to-nontarget ratio for [⁶⁷Ga]Ga-HBED-CC-DX600.

CONCLUSIONS

This study demonstrated ACE2 selectivity for all radiopeptides. [⁶⁷Ga]Ga-HBED-CC-DX600 was revealed as the most promising candidate due to its favorable tissue distribution profile. Importantly, the HBED-CC chelator enabled ⁶⁷Ga-labeling at high molar activity, which would be essential to obtain images with high signal-to-background contrast to detect (patho)physiological ACE2 expression levels in patients.

Molecular Design of 68Ga- and 89Zr-Labeled Anticalin Radioligands for PET-Imaging of PSMA-Positive Tumors

Anticalin proteins directed against the prostate-specific membrane antigen (PSMA), optionally having tailored plasma half-life using PASylation technology, show promise as radioligands for PET-imaging of xenograft tumors in mice. To investigate their suitability, the short-circulating unmodified Anticalin was labeled with ⁶⁸Ga (τ_1/2 = 68 min), using the NODAGA chelator, whereas the half-life extended PASylated Anticalin was labeled with ⁸⁹Zr (τ_1/2 = 78 h), using either the linear chelator deferoxamine (Dfo) or a cyclic derivative, fusarinine C (FsC). Different PSMA targeting Anticalin versions (optionally carrying the PASylation sequence) were produced carrying a single exposed N- or C-terminal Cys residue and site-specifically conjugated with the different radiochelators via maleimide chemistry. These protein conjugates were labeled with radioisotopes having distinct physical half-lives and, subsequently, applied for PET-imaging of subcutaneous LNCaP xenograft tumors in CB17 SCID mice. Uptake of the protein tracers into tumor versus healthy tissues was assessed by segmentation of PET data as well as biodistribution analyses. PET-imaging with both the ⁶⁸Ga-labeled plain Anticalin and the ⁸⁹Zr-labeled PASylated Anticalin allowed clear delineation of the xenograft tumor. The radioligand A3A5.1-PAS(200)-FsC·⁸⁹Zr, having an extended plasma half-life, led to a higher tumor uptake 24 h p.i. compared to the ⁶⁸Ga·NODAGA-Anticalin imaged 60 min p.i. (2.5% ID/g vs 1.2% ID/g). Pronounced demetallation was observed for the ⁸⁹Zr·Dfo-labeled PASylated Anticalin, which was ∼50% lower in the case of the cyclic radiochelator FsC (p < 0.0001). Adjusting the plasma half-life of Anticalin radioligands using PASylation technology is a viable approach to increase radioisotope accumulation within the tumor. Furthermore, ⁸⁹Zr-ImmunoPET-imaging using the FsC radiochelator is superior to that using Dfo. Our strategy for the half-life adjustment of a tumor-targeting Anticalin to match the physical half-life of the applied radioisotope illustrates the potential of small binding proteins as an alternative to antibodies for PET-imaging.

Direct Radiolabeling of Trastuzumab-Targeting Triblock Copolymer Vesicles with 89Zr for Positron Emission Tomography Imaging

Radiolabeled drug nanocarriers that can be easily imaged via positron emission tomography (PET) are highly significant as their in vivo outcome can be quantitatively PET-traced with high sensitivity. However, typical radiolabeling of most PET-guided theranostic vehicles utilizes modification with chelator ligands, which presents various challenges. In addition, unlike passive tumor targeting, specific targeting of drug delivery vehicles via binding affinity to overexpressed cancer cell receptors is crucial to improve the theranostic delivery to tumors. Herein, we developed 89Zr-labeled triblock copolymer polymersomes of 60 nm size through chelator-free radiolabeling. The polymersomes are assembled from poly(N-vinylpyrrolidone)5-b-poly(dimethylsiloxane)30-b-poly(N-vinylpyrrolidone)5 (PVPON5-PDMS30-PVPON5) triblock copolymers followed by adsorption of a degradable tannin, tannic acid (TA), on the polymersome surface through hydrogen bonding. TA serves as an anchoring layer for both 89Zr radionuclide and targeting recombinant humanized monoclonal antibody, trastuzumab (Tmab). Unlike bare PVPON5-PDMS30-PVPON5 polymersomes, TA- and Tmab-modified polymersomes demonstrated a high radiochemical yield of more than 95%. Excellent retention of 89Zr by the vesicle membrane for up to 7 days was confirmed by PET in vivo imaging. Animal biodistribution using healthy BALB/c mice confirmed the clearance of 89Zr-labeled polymersomes through the spleen and liver without their accumulation in bone, unlike the free nonbound 89Zr radiotracer. The 89Zr-radiolabeled polymersomes were found to specifically target BT474 HER2-positive breast cancer cells via the Tmab-TA complex on the vesicle surface. The noncovalent Tmab anchoring to the polymersome membrane can be highly advantageous for nanoparticle modification compared to currently developed covalent methods, as it allows easy and quick integration of a broad range of targeting proteins. Given the ability of these polymersomes to encapsulate and release anticancer therapeutics, they can be further expanded as precision-targeted therapeutic carriers for advancing human health through highly effective drug delivery strategies.

Preparation of near-infrared AIEgen-active fluorescent probes for mapping amyloid-β plaques in brain tissues and living mice

Fibrillar aggregates of the amyloid-β protein (Aβ) are the main component of the senile plaques found in brains of patients with Alzheimer's disease (AD). Development of probes allowing the noninvasive and high-fidelity mapping of Aβ plaques in vivo is critical for AD early detection, drug screening and biomedical research. QM-FN-SO3 (quinoline-malononitrile-thiophene-(dimethylamino)phenylsulfonate) is a near-infrared aggregation-induced-emission-active fluorescent probe capable of crossing the blood-brain barrier (BBB) and ultrasensitively lighting up Aβ plaques in living mice. Herein, we describe detailed procedures for the two-stage synthesis of QM-FN-SO3 and its applications for mapping Aβ plaques in brain tissues and living mice. Compared with commercial thioflavin (Th) derivatives ThT and ThS (the gold standard for detection of Aβ aggregates) and other reported Aβ plaque fluorescent probes, QM-FN-SO3 confers several advantages, such as long emission wavelength, large Stokes shift, ultrahigh sensitivity, good BBB penetrability and miscibility in aqueous biological media. The preparation of QM-FN-SO3 takes ~2 d, and the confocal imaging experiments for Aβ plaque visualization, including the preparation for mouse brain sections, take ~7 d. Notably, acquisition and analyses for in vivo visualization of Aβ plaques in mice can be completed within 1 h and require only a basic knowledge of spectroscopy and chemistry.

[68Ga]Ga-HBED-CC-FAPI Derivatives with Improved Radiolabeling and Specific Tumor Uptake

Fibroblast activation protein (FAP) is selectively expressed in tumors and highly important for maintaining the microenvironment in malignant tumors. Radioisotope-labeled FAP inhibitors (FAPIs) were proven to be useful for diagnosis and radionuclide therapy of cancer and are under active clinical investigations. Ga-HBED complex displays a higher in vivo stability constant (log K_GaL: 38.5), compared to that of Ga-DOTA (log K_GaL: 21.3). Such advantage in stability constant suggests that it may be useful for development of alternative FAPI imaging agents. In this study, previously reported [⁶⁸Ga]Ga-DOTA-FAPI-02 and -04 were converted to the corresponding [⁶⁸Ga]Ga-HBED-CC-FAPI-02 and -04 derivatives ([⁶⁸Ga]Ga-4, [⁶⁸Ga]Ga-5, [⁶⁸Ga]Ga-6, and [⁶⁸Ga]Ga-7). It was found that substituting the DOTA chelating group with HBED-CC led to several unique and desirable tumor-targeting properties: (1) robust, fast, and high yield labeling─readily adaptable to a kit formulation; (2) high stabilities in vitro; (3) excellent FAP binding affinities (IC₅₀ ranging between 4 and 7 nM) and improved cell uptake and retention (in HT1080 (FAP+) cells); and (4) excellent selective in vivo tumor uptake in nude mice bearing U87MG tumor. It appeared that Ga(III) chelation with HBED-CC improved the in vivo kinetics favoring higher tumor uptake and retention compared to the corresponding Ga-DOTA complex. Out of the four tested ligands the new [⁶⁸Ga]Ga-HBED-CC-FAPI dimer, [⁶⁸Ga]Ga-6, displayed the best tumor localization properties, and further studies are warranted to demonstrate that it is an alternative FAP imaging agent for cancer patients.

Advances in Radionuclides and Radiolabelled Peptides for Cancer Therapeutics

Radiopharmaceutical therapy, which can detect and treat tumours simultaneously, was introduced more than 80 years ago, and it has changed medical strategies with respect to cancer. Many radioactive radionuclides have been developed, and functional, molecularly modified radiolabelled peptides have been used to produce biomolecules and therapeutics that are vastly utilised in the field of radio medicine. Since the 1990s, they have smoothly transitioned into clinical application, and as of today, a wide variety of radiolabelled radionuclide derivatives have been examined and evaluated in various studies. Advanced technologies, such as conjugation of functional peptides or incorporation of radionuclides into chelating ligands, have been developed for advanced radiopharmaceutical cancer therapy. New radiolabelled conjugates for targeted radiotherapy have been designed to deliver radiation directly to cancer cells with improved specificity and minimal damage to the surrounding normal tissue. The development of new theragnostic radionuclides, which can be used for both imaging and therapy purposes, allows for more precise targeting and monitoring of the treatment response. The increased use of peptide receptor radionuclide therapy (PRRT) is also important in the targeting of specific receptors which are overexpressed in cancer cells. In this review, we provide insights into the development of radionuclides and functional radiolabelled peptides, give a brief background, and describe their transition into clinical application.

Increasing Reaction Rates of Water-Soluble Porphyrins for 64Cu Radiopharmaceutical Labeling

Searching for new compounds and synthetic routes for medical applications is a great challenge for modern chemistry. Porphyrins, natural macrocycles able to tightly bind metal ions, can serve as complexing and delivering agents in nuclear medicine diagnostic imaging utilizing radioactive nuclides of copper with particular emphasis on ⁶⁴Cu. This nuclide can, due to multiple decay modes, serve also as a therapeutic agent. As the complexation reaction of porphyrins suffers from relatively poor kinetics, the aim of this study was to optimize the reaction of copper ions with various water-soluble porphyrins in terms of time and chemical conditions, that would meet pharmaceutical requirements and to develop a method that can be applied for various water-soluble porphyrins. In the first method, reactions were conducted in a presence of a reducing agent (ascorbic acid). Optimal conditions, in which the reaction time was 1 min, comprised borate buffer at pH 9 with a 10-fold excess of ascorbic acid over Cu²⁺. The second approach involved a microwave-assisted synthesis at 140 °C for 1-2 min. The proposed method with ascorbic acid was applied for radiolabeling of porphyrin with ⁶⁴Cu. The complex was then subjected to a purification procedure and the final product was identified using high-performance liquid chromatography with radiometric detection.

Radiolabeled nanomaterial for cancer diagnostics and therapeutics: principles and concepts

In the last three decades, radiopharmaceuticals have proven their effectiveness for cancer diagnosis and therapy. In parallel, the advances in nanotechnology have fueled a plethora of applications in biology and medicine. A convergence of these disciplines has emerged more recently with the advent of nanotechnology-aided radiopharmaceuticals. Capitalizing on the unique physical and functional properties of nanoparticles, radiolabeled nanomaterials or nano-radiopharmaceuticals have the potential to enhance imaging and therapy of human diseases. This article provides an overview of various radionuclides used in diagnostic, therapeutic, and theranostic applications, radionuclide production through different techniques, conventional radionuclide delivery systems, and advancements in the delivery systems for nanomaterials. The review also provides insights into fundamental concepts necessary to improve currently available radionuclide agents and formulate new nano-radiopharmaceuticals.

PSMA-Targeted Nanotheranostics for Imaging and Radiotherapy of Prostate Cancer

Targeted nanotheranostic systems offer significant benefits due to the integration of diagnostic and therapeutic functionality, promoting personalized medicine. In recent years, prostate-specific membrane antigen (PSMA) has emerged as an ideal theranostic target, fueling multiple new drug approvals and changing the standard of care in prostate cancer (PCa). PSMA-targeted nanosystems such as self-assembled nanoparticles (NPs), liposomal structures, water-soluble polymers, dendrimers, and other macromolecules are under development for PCa theranostics due to their multifunctional sensing and therapeutic capabilities. Herein, we discuss the significance and up-to-date development of "PSMA-targeted nanocarrier systems for radioligand imaging and therapy of PCa". The review also highlights critical parameters for designing nanostructured radiopharmaceuticals for PCa, including radionuclides and their chelators, PSMA-targeting ligands, and the EPR effect. Finally, prospects and potential for clinical translation is discussed.

Radiochemical, Computational, and Spectroscopic Evaluation of High-Denticity Desferrioxamine Derivatives DFO2 and DFO2p toward an Ideal Zirconium-89 Chelate Platform

Desferrioxamine (DFO) has long been considered the gold standard chelator for incorporating [⁸⁹Zr]Zr⁴⁺ in radiopharmaceuticals for positron emission tomography (PET) imaging. To improve the stability of DFO with zirconium-89 and to expand its coordination sphere to enable binding of large therapeutic radiometals, we have synthesized the highest denticity DFO derivatives to date: dodecadentate DFO2 and DFO2p. In this study, we describe the synthesis and characterization of a novel DFO-based chelator, DFO2p, which is comprised of two DFO strands connected by an p-NO₂-phenyl linker and therefore contains double the chelating moieties of DFO (potential coordination number up to 12 vs 6). The chelator DFO2p offers an optimized synthesis comprised of only a single reaction step and improves water solubility relative to DFO2, but the shorter linker reduces molecular flexibility. Both DFO2 and DFO2p, each with 6 potential hydroxamate ligands, are able to reach a more energetically favorable 8-coordinate environment for Zr(IV) than DFO. The zirconium(IV) coordination environment of these complexes were evaluated by a combination of density functional theory (DFT) calculations and synchrotron spectroscopy (extended X-ray absorption fine structure), which suggest the inner-coordination sphere of zirconium(IV) to be comprised of the outermost four hydroxamate ligands. These results also confirm a single Zr(IV) in each chelator, and the hydroxide ligands which complete the coordination sphere of Zr(IV)-DFO are absent from Zr(IV)-DFO2 and Zr(IV)-DFO2p. Radiochemical stability studies with zirconium-89 revealed the order of real-world stability to be DFO2 > DFO2p ≫ DFO. The zirconium-89 complexes of these new high-denticity chelators were found to be far more stable than DFO, and the decreased molecular flexibility of DFO2p, relative to DFO2, could explain its decreased stability, relative to DFO2.

Exploration of 68Ga-DOTA-MAL as a Versatile Vehicle for Facile Labeling of a Variety of Thiol-Containing Bioactive Molecules

Efficient and site-specific radiolabeling reactions are essential in molecular probe synthesis. Thus, selecting an effective method for radiolabeling that does not affect bioactivity of the molecule is critical. Varieties of bifunctional chelating agents provide a solution in this matter. As a chemo-specific chelator, maleimido-mono-amide-DOTA (DOTA-Mal) holds significant potential for ⁶⁸Ga labeling of bioactive molecules; it can react specifically with free sulfhydryl groups under mild conditions. Compared with amino and carboxylic acid groups, free sulfhydryl groups are relatively less common in most biomolecules and can serve as site-specific radiolabeling targets. Labeling of ⁶⁸Ga usually employs a two-step labeling strategy; first, chelators are conjugated to the biomolecules, which is followed by radiolabeling. However, the bioactivity of biomolecules may be affected by harsh labeling conditions. In this study, three ⁶⁸Ga-labeled bioactive molecules, namely, ⁶⁸Ga-DOTA-RGD, ⁶⁸Ga-DOTA-FA, and ⁶⁸Ga-DOTA-BSA, were prepared using a novel strategy under mild conditions (pH of 8.0 at room temperature). Using this strategy, DOTA-Mal was labeled by ⁶⁸Ga before it reacted with the sulfhydryl group-containing biomolecules, which avoided damage to said biomolecules caused by the harsh reaction conditions required in ⁶⁸Ga-labeling procedures. The biological and chemical properties of these three radiotracers synthesized using this strategy are well manifested. Through a series of experiments, the effectiveness of this strategy is demonstrated, and we believe that this site-specific bioactivity-friendly reaction strategy will facilitate developments and translation applications of varieties of ⁶⁸Ga-labeled positron emission tomography probes.

Improved procedures for production and purification of 135La from enriched [135Ba]BaCO3 on a 16.5 MeV cyclotron

Lanthanum-135 (¹³⁵La) is a favorable Auger electron emitter with a high Auger electron yield and low gamma emission, making it promising for Auger electron radiotherapy. However, successful application requires reliable and scalable ¹³⁵La production. Up to now, metallic natural barium (^natBa) is a commonly used target material, but this material is sensitive to moisture and oxidation. BaCO₃ has also been tested, due to its higher chemical stability. However, BaCO₃ has poor thermal conductivity, limiting the applicable current and making high yield production challenging. In this study, we pressed a mixture of enriched [¹³⁵Ba]BaCO₃ and fine aluminum (Al) powder to provide a stable target with improved thermal conductivity compared to pure BaCO₃. After 4 h of irradiation with a 16.5 MeV proton beam at 20 μA current, 1.62 ± 0.18 GBq was produced from a 200 mg [¹³⁵Ba]BaCO₃:Al (1:2, w/w) target. This corresponded to a saturation yield of 11.91 ± 1.31 GBq (or 596 ± 66 MBq/μA). A purification procedure involving initial precipitation, followed by a single composite column containing a layer of TK200 resin and a second layer of branched DGA resin was developed, with 97.1 ± 3.6 % decay corrected ¹³⁵La recovery. [¹³⁵La]LaCl₃ was obtained in an effective molar activity of 79.6 ± 25.3 MBq/nmol (DOTA titration), 104.0 ± 40.4 MBq/nmol (DTPA titration) and 186.5 ± 83.8 MBq/nmol (CHX-A″-DTPA titration), and a radionuclidic purity (RNP) of >99.9 % at end of purification, hereby demonstrating a purity suitable for radiopharmaceutical use.

N,N-Alkylation Clarifies the Role of N- and O-Protonated Intermediates in Cyclen-Based 64Cu Radiopharmaceuticals

Radioisotopes of Cu, such as ⁶⁴Cu and ⁶⁷Cu, are alluring targets for imaging (e.g., positron emission tomography, PET) and radiotherapeutic applications. Cyclen-based macrocyclic polyaminocarboxylates are one of the most frequently examined bifunctional chelators in vitro and in vivo, including the FDA-approved ⁶⁴Cu radiopharmaceutical, Cu(DOTATATE) (Detectnet); however, connections between the structure of plausible reactive intermediates and their stability under physiologically relevant conditions remain to be established. In this study, we share the synthesis of a cyclen-based, N,N-alkylated spirocyclic chelate, H₂DO3A^C4H8, which serves as a model for N-protonation. Our combined experimental (in vitro and in vivo) and computational studies unravel complex pH-dependent speciation and enable side-by-side comparison of N- and O-protonated species of relevant ⁶⁴Cu radiopharmaceuticals. Our studies suggest that N-protonated species are not inherently unstable species under physiological conditions and demonstrate the potential of N,N-alkylation as a tool for the rational design of future radiopharmaceuticals.

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.

Radiometal chelators for infection diagnostics

Infection of native tissues or implanted devices is common, but clinical diagnosis is frequently difficult and currently available noninvasive tests perform poorly. Immunocompromised individuals (for example transplant recipients, or those with cancer) are at increased risk. No imaging test in clinical use can specifically identify infection, or accurately differentiate bacterial from fungal infections. Commonly used [18F]fluorodeoxyglucose (18FDG) positron emission computed tomography (PET/CT) is sensitive for infection, but limited by poor specificity because increased glucose uptake may also indicate inflammation or malignancy. Furthermore, this tracer provides no indication of the type of infective agent (bacterial, fungal, or parasitic). Imaging tools that directly and specifically target microbial pathogens are highly desirable to improve noninvasive infection diagnosis and localization. A growing field of research is exploring the utility of radiometals and their chelators (siderophores), which are small molecules that bind radiometals and form a stable complex allowing sequestration by microbes. This radiometal-chelator complex can be directed to a specific microbial target in vivo, facilitating anatomical localization by PET or single photon emission computed tomography. Additionally, bifunctional chelators can further conjugate therapeutic molecules (e.g., peptides, antibiotics, antibodies) while still bound to desired radiometals, combining specific imaging with highly targeted antimicrobial therapy. These novel therapeutics may prove a useful complement to the armamentarium in the global fight against antimicrobial resistance. This review will highlight current state of infection imaging diagnostics and their limitations, strategies to develop infection-specific diagnostics, recent advances in radiometal-based chelators for microbial infection imaging, challenges, and future directions to improve targeted diagnostics and/or therapeutics.

Leveraging a Dual Variable Domain Immunoglobulin to Create a Site-Specifically Modified Radioimmunoconjugate

Site-specifically modified radioimmunoconjugates exhibit superior in vitro and in vivo behavior compared to analogues synthesized via traditional stochastic methods. However, the development of approaches to site-specific bioconjugation that combine high levels of selectivity, simple reaction conditions, and clinical translatability remains a challenge. Herein, we describe a novel solution to this problem: the use of dual-variable domain immunoglobulins (DVD-IgG). More specifically, we report the synthesis, in vitro evaluation, and in vivo validation of a 177Lu-labeled radioimmunoconjugate based on HER2DVD, a DVD-IgG containing the HER2-targeting variable domains of trastuzumab and the catalytic variable domains of IgG h38C2. To this end, we first modified HER2DVD with a phenyloxadiazolyl methlysulfone-modified variant of the chelator CHX-A″-DTPA (PODS-CHX-A''-DTPA) and verified the site-specificity of the conjugation for the reactive lysines within the catalytic domains via chemical assay, MALDI-ToF mass spectrometry, and SDS-PAGE. The chelator-bearing immunoconjugate was subsequently labeled with [177Lu]Lu3+ to produce the completed radioimmunoconjugate, [177Lu]Lu-CHX-A″-DTPAPODS-HER2DVD, in >80% radiochemical conversion and a specific activity of 29.5 ± 7.1 GBq/μmol. [177Lu]Lu-CHX-A″-DTPAPODS-HER2DVD did not form aggregates upon prolonged incubation in human serum, displayed 87% stability to demetalation over a 7 days of incubation in serum, and exhibited an immunoreactive fraction of 0.95 with HER2-coated beads. Finally, we compared the pharmacokinetic profile of [177Lu]Lu-CHX-A″-DTPAPODS-HER2DVD to that of a 177Lu-labeled variant of trastuzumab in mice bearing subcutaneous HER2-expressing BT-474 human breast cancer xenografts. The in vivo performance of [177Lu]Lu-CHX-A″-DTPAPODS-HER2DVD matched that of 177Lu-labeled trastuzumab, with the former producing a tumoral activity concentration of 34.1 ± 12.1 %ID/g at 168 h and tumor-to-blood, tumor-to-liver, and tumor-to-kidney activity concentration ratios of 10.5, 9.6, and 21.8, respectively, at the same time point. Importantly, the DVD-IgG did not exhibit a substantially longer serum half-life than the traditional IgG despite its significantly larger size (202 kDa for the former vs 148 kDa for the latter). Taken together, these data suggest that DVD-IgGs represent a viable platform for the future development of highly effective site-specifically labeled radioimmunoconjugates for diagnostic imaging, theranostic imaging, and radioimmunotherapy.

Synthesis, radiolabeling, and preclinical in vivo evaluation of 68Ga-radiolabelled nanodiamonds

PURPOSE

Nanodiamonds (NDs) represent a new class of nanoparticles and have gained increasing interest in medical applications. Modifying the surface coating by attaching binding ligands or imaging probes can transform NDs into multi-modal targeting probes. This study evaluated the biokinetics and biodistribution of ⁶⁸Ga-radiolabelled NDs in a xenograft model.

PROCEDURES

NDs were coated with an albumin-derived copolymer modified with desferrioxamine to provide a chelator for radiolabeling. In vivo studies were conducted in AR42J tumor-bearing CD1 mice to evaluate biodistribution and tumor accumulation of the NDs.

RESULTS

Coated NDs were successfully radiolabeled using ⁶⁸Ga at room temperature with radiolabeling efficiencies up to 91.8 ± 3.2 % as assessed by radio-TLC. In vivo studies revealed the highest accumulation in the liver and spleen, whereas tumor radioactivity concentration was low.

CONCLUSIONS

Radiolabeling of coated NDs could be achieved. However, the obtained results indicate these coated NDs' limitations in their biodistribution within the conducted studies.

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

Potential of Nuclear Imaging Techniques to Study the Oral Delivery of Peptides

Peptides are small biomolecules known to stimulate or inhibit important functions in the human body. The clinical use of peptides by oral delivery, however, is very limited due to their sensitive structure and physiological barriers present in the gastrointestinal tract. These barriers can be overcome with chemical and mechanical approaches protease inhibitors, permeation enhancers, and polymeric encapsulation. Studying the success of these approaches pre-clinically with imaging techniques such as fluorescence imaging (IVIS) and optical microscopy is difficult due to the lack of in-depth penetration. In comparison, nuclear imaging provides a better platform to observe the gastrointestinal transit and quantitative distribution of radiolabeled peptides. This review provides a brief background on the oral delivery of peptides and states examples from the literature on how nuclear imaging can help to observe and analyze the gastrointestinal transit of oral peptides. The review connects the fields of peptide delivery and nuclear medicine in an interdisciplinary way to potentially overcome the challenges faced during the study of oral peptide formulations.

Development of the First Tritiated Tetrazine: Facilitating Tritiation of Proteins

Tetrazine (Tz)-trans-cyclooctene (TCO) ligation is an ultra-fast and highly selective reaction and it is particularly suited to label biomolecules under physiological conditions. As such, a ³ H-Tz based synthon would have wide applications for in vitro/ex vivo assays. In this study, we developed a ³ H-labeled Tz and characterized its potential for application to pretargeted autoradiography. Several strategies were explored to synthesize such a Tz. However, classical approaches such as reductive halogenation failed. For this reason, we designed a Tz containing an aldehyde and explored the possibility of reducing this group with NaBT₄ . This approach was successful and resulted in [³ H]-(4-(6-(pyridin-2-yl)-1,2,4,5-tetrazin-3-yl)phenyl)methan-t-ol with a radiochemical yield of 22 %, a radiochemical purity of 96 % and a molar activity of 0.437 GBq/μmol (11.8 Ci/mmol). The compound was successfully applied to pretargeted autoradiography. Thus, we report the synthesis of the first ³ H-labeled Tz and its successful application as a labeling building block.

Osteosarcoma-Derived Exosomes as Potential PET Imaging Nanocarriers for Lung Metastasis

Lung metastases represent the most adverse clinical factor and rank as the leading cause of osteosarcoma-related death. Nearly 80% of patients present lung micrometastasis at diagnosis not detected with current clinical tools. Herein, an exosome (EX)-based imaging tool is developed for lung micrometastasis by positron emission tomography (PET) using osteosarcoma-derived EXs as natural nanocarriers of the positron-emitter copper-64 (⁶⁴ Cu). Exosomes are isolated from metastatic osteosarcoma cells and functionalized with the macrocyclic chelator NODAGA for complexation with ⁶⁴ Cu. Surface functionalization has no effect on the physicochemical properties of EXs, or affinity for donor cells and endows them with favorable pharmacokinetics for in vivo studies. Whole-body PET/magnetic resonance imaging (MRI) images in xenografted models show a specific accumulation of ⁶⁴ Cu-NODAGA-EXs in metastatic lesions as small as 2-3 mm or in a primary tumor, demonstrating the exquisite tropism of EXs for homotypic donor cells. The targetability for lung metastasis is also observed by optical imaging using indocyanine green (ICG)-labeled EXs and D-luciferin-loaded EXs. These findings show that tumor-derived EXs hold great potential as targeted imaging agents for the noninvasive detection of small lung metastasis by PET. This represents a step forward in the biomedical application of EXs in imaging diagnosis with increased translational potential.