Terbium-161 for PSMA-targeted radionuclide therapy of prostate cancer

Exploration of synergic solvent extraction and extraction chromatographic materials for the separation of 161Tb from enriched 160Gd targets

One rare earth element (REE) radionuclide of considerable interest for nuclear medicine is 161Tb (t1/2 = 6.95 day, β-max = 593 keV), which can be produced using nuclear reactors through an indirect neutron capture reaction, 64160Gd(n,γ)64161Gd(t1/2=3.7min,β-)65161Tb, using enriched 160Gd targets. A significant challenge in implementing 161Tb and new REE radionuclides into radiopharmaceutical agents is the efficient separation of the product and target materials to achieve a high specific activity. This study explored the use of synergic extractant combinations to enhance the extraction and separation efficiencies of Gd(III) and Tb(III). Three neutral organophosphorus extractants, octyl (phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO), dibutyl N,N-diethylcarbamylmethylenephosphonate (DBDECMP), and dihexyl N,N-diethylcarbamylmethylenephosphonate (DHDECMP), were investigated as synergic agents in solvent extraction (SX) when combined with one of two β-diketones, 2-thenoyltrifluoroacetone (HTTA) and 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one (HP). The separation factors (SFTb/Gd) of Tb(III) and Gd(III) for each system are reported as a function of extractant concentration. The system with the highest SFTb/Gd was found to be 0.1 M HTTA and 0.1 M DBDECMP at pH 2.00, which displayed a SFTb/Gd of 3.7 ± 0.1. This extractant combination was sorbed onto a macroporous support at various extractant ratios using rotary evaporator methods, and their Gd(III) and Tb(III) uptake behavior was characterized. The prepared EXC resins exhibited excellent metal retention in column experiments, demonstrating promising potential for separating Gd(III) and Tb(III), while recovering approximately 96.6 % of 161Tb with <5 % of the initial Gd(III) mass. Thermodynamic studies revealed an endothermic adsorption progress with increased uptake and decreased Gibbs free energy (ΔG) with rising temperature. Overall, this work successfully demonstrated the conversion and characterization of synergic SX systems into novel synergic EXC resins for adjacent REE separations.

Therapeutic evaluation of [212Pb]Pb-AB001 and [177Lu]Lu-PSMA-617 in a mouse model of disseminated prostate cancer

BACKGROUND

Metastatic castration-resistant prostate cancer (mCRPC) frequently leads to bone and soft tissue metastases, leading to poor prognosis. The beta-emitting radioligand [177Lu]Lu-PSMA-617 targets the prostate-specific membrane antigen (PSMA) and may be less efficient against micrometastatic disease. The alpha-emitting radioligand [212Pb]Pb-AB001 could offer enhanced treatment by delivering high energy over a short range. This study compared the efficacy of [212Pb]Pb-AB001 and [177Lu]Lu-PSMA-617 in a mouse model of disseminated prostate cancer.

METHODS

Binding and internalisation of radioligands were evaluated in PC-3 PIP-luc cells. A mouse model was established by intracardiac injection of these cells. Treatments with 0.24‒1.0 MBq [212Pb]Pb-AB001 or 22‒66 MBq [177Lu]Lu-PSMA-617 were initiated 7 d post-cell inoculation. Metastatic burden was measured using bioluminescence imaging, and PSMA-targeted uptake was determined with [18F]F-PSMA-1007 µPET/µCT. Gamma-autoradiography evaluated [212Pb]Pb-AB001 distribution, and bone metastases were identified by radiography.

RESULTS

Both radioligands displayed comparable in vitro binding. In vivo studies revealed metastatic formation in clinically relevant organs. µPET/µCT demonstrated increased [18F]F-PSMA-1007 uptake in metastases, matching the bioluminescence imaging results. Focal [212Pb]Pb-AB001 distribution in the metastatic xenograft indicated heterogeneously distributed micrometastases in the organs. A median survival up to 47 d was achieved with [212Pb]Pb-AB001, compared to 25 d for controls and 27 d for [177Lu]Lu-PSMA-617. An activity-dependent reduction in bone metastases was observed for [177Lu]Lu-PSMA-617, while no bone lesions were detected in [212Pb]Pb-AB001-treated mice.

CONCLUSION

[212Pb]Pb-AB001 showed significant efficacy against micrometastases and advantages over [177Lu]Lu-PSMA-617 in preventing or treating early bone metastases for the investigated injected activities. This implies clinical potential for treating mCRPC, including patients at risk of early metastatic disease, but further studies including dosimetry and toxicity analyses are required with regards to activity levels.

Potent candidates for Targeted Alpha Therapy (TAT)

Targeted Alpha Therapy (TAT) holds significant promise as a localized treatment for cancer. Encouraging clinical results from using peptides and antibodies labeled with alpha emitters to treat patients with metastatic cancers, particularly those who have not responded to other therapies, provide compelling evidence of TAT's potential. To fully realize the benefits of TAT, it is essential to carefully select appropriate radionuclides and targeting delivery systems to maximize therapeutic efficacy while minimizing nonspecific toxicity to healthy tissues. This review explores key radiochemical, radiopharmaceutical, and radiation-biological considerations for current TAT candidates, and proposes additional potential candidates, establishing a foundation and criteria for the ongoing development of TAT radiopharmaceuticals.

Current Status and Future Perspectives of Nuclear Medicine in Prostate Cancer from Imaging to Therapy: A Comprehensive Review

Nuclear medicine has emerged as a critical modality in the diagnostic and therapeutic management of urological malignancies, particularly prostate cancer. Advances in single-photon emission computed tomography/computed tomography (CT) and positron emission tomography/CT (PET/CT) have enhanced tumor assessment across staging, treatment response, and recurrence settings. Molecular imaging, which offers insights beyond traditional anatomical imaging, is increasingly integral in specific clinical scenarios. Theranostic nuclear medicine, which combines diagnostic imaging with targeted therapy, has become a well-established treatment option, particularly for patients with metastatic castration-resistant prostate cancer (mCRPC). The development of the prostate-specific membrane antigen (PSMA) radioligands has revolutionized clinical management by enabling precise disease staging and delivering effective radioligand therapy (RLT). Ongoing research aims to refine the role of PSMA PET imaging in staging and treatment monitoring, while optimizing PSMA-targeted RLT for broader clinical use. Given that prostate cancer remains highly prevalent, the anticipated increase in the demand for RLT presents both challenges and opportunities for nuclear medicine services globally. Theranostic approaches exemplify personalized medicine by enabling the tailoring of treatments to individual tumor biology, thereby improving survival outcomes and maintaining patients' quality of life with minimal toxicity. Although the current focus is on advanced disease, future research holds promise for expanding these strategies to earlier stages, potentially enhancing curative prospects. This evolving field not only signifies a paradigm shift in the care of prostate cancer patients but also underscores the growing importance of nuclear medicine in delivering precision oncology.

State of the art and future perspectives of new radionuclides in Nuclear Medicine. Part II

The state of the art and future perspectives of new radionuclides in Nuclear Medicine continue to evolve, driven by the development of isotopes with innovative applications in theragnostics. In this second part of the continuing education series, the clinical and therapeutic applications of terbium, actinium, and bismuth are analyzed in depth. The use of the four terbium isotopes (terbium-149, terbium-152, terbium-155, and terbium-161) is described, offering a versatile system for both diagnosis and treatment due to their chemical similarity to lutetium-177, along with the challenges related to their production and availability. Additionally, actinium-225, a powerful alpha-emitting radionuclide, is reviewed for its growing role in Targeted Alpha Therapy (TAT), particularly in prostate cancer and neuroendocrine tumors. Finally, bismuth-213, derived from actinium-225, is analyzed for its short half-life, making it a viable option for localized and selective therapies. Despite technical and production challenges, these radionuclides are driving the evolution of precision medicine, expanding therapeutic and diagnostic possibilities in Nuclear Medicine.

Mitochondria-tropic radioconjugates to enhance the therapeutic potential of terbium-161

BACKGROUND

Strategies that focus on delivering Auger electron emitters to highly radiosensitive intracellular targets-such as the nucleus, cell membrane, or mitochondria-are gaining attention. Targeting these organelles could enhance therapeutic efficacy while minimizing off-target toxicity by allowing lower administered doses. In this context, this study explores the therapeutic potential of 161Tb-labeled radiocomplexes that integrate the mitochondria-targeting triphenylphosphonium (TPP) moiety with a prostate-specific membrane antigen (PSMA) targeting vector. The goal is to assess these dual-targeted radiocomplexes for their ability to deliver conversion electrons (CE) and Auger electrons (AEs) to prostate cancer (PCa) cells, specifically targeting the mitochondria to enhance therapeutic efficacy.

RESULTS

Two novel radiocomplexes, [161Tb]Tb-TPP-PSMA and [161Tb]Tb-TPP-G3-PSMA, were synthesized with high radiochemical yield and purity. The proposed structures were validated using HPLC and ESI-MS analysis, with their natTb counterparts serving as reference compounds. In vitro experiments included cellular uptake, internalization, mitochondrial uptake, and DNA damage assays in PSMA-positive PCa cell lines. Clonogenic assays were performed to evaluate cell survival post-treatment. In vivo studies were conducted using SCID/Beige mice bearing PCa xenografts and involved µSPECT/CT imaging and radiometabolite analysis to evaluate biodistribution, pharmacokinetics, tumor uptake and in vivo stability of the radiocomplexes. Both [161Tb]Tb-TPP-PSMA and [161Tb]Tb-TPP-G3-PSMA showed high radiochemical stability and were efficiently internalized by PSMA-positive cells, while showing minimal uptake in PSMA-negative cells. These dual-targeted radiocomplexes demonstrated significantly higher mitochondrial uptake compared to the non-TPP-containing [161Tb]Tb-PSMA-617, leading to increased DNA damage and enhanced radiocytotoxicity. In vivo, the dual-targeted complexes demonstrated PSMA-specific tumor uptake and pharmacokinetics comparable to [161Tb]Tb-PSMA-617, with effective clearance from non-target tissues.

CONCLUSIONS

The TPP-modified 161Tb-radiocomplexes effectively targeted the mitochondria of PSMA-positive PCa cells, leading to increased DNA damage and reduced cell viability compared to single-targeted radiocomplexes. These findings suggest that dual-targeting strategies, which combine PSMA and mitochondrial targeting, can enhance the therapeutic potential of radiopharmaceuticals for prostate cancer treatment.

Preclinical investigation of [149Tb]Tb-DOTATATE and [149Tb]Tb-DOTA-LM3 for tumor-targeted alpha therapy

PURPOSE

Terbium-149 is a short-lived α-particle emitter, potentially useful for tumor-targeted therapy. The aim of this study was to investigate terbium-149 in combination with the somatostatin receptor (SSTR) agonist DOTATATE and the SSTR antagonist DOTA-LM3. The radiopeptides were evaluated to compare their therapeutic efficacy in vitro and in vivo.

METHODS

Terbium-149 was produced at ISOLDE/CERN and chemically purified at the Paul Scherrer Institute. Radiolabeling of somatostatin analogues with [149Tb]TbCl3 was performed under standard labeling conditions at pH 4.5. Cell viability (MTT) and survival assays (colony forming) assays were performed after 16-18 h exposure of SSTR-positive AR42J rat pancreatic tumor cells to various activity concentrations of [149Tb]Tb-DOTATATE and [149Tb]Tb-DOTA-LM3. DNA double-strand breaks were determined using immunofluorescence imaging of γ-H2A.X and 53BP1. Therapy studies were performed with AR42J tumor-bearing mice injected with 1 × 5 MBq or 2 × 5 MBq of the respective radiopeptide. The tolerability of up to 40 MBq [149Tb]Tb-DOTATATE or 40 MBq [149Tb]Tb-DOTA-LM3 was assessed with regard to undesired effects to the bone marrow and kidneys in immunocompetent mice without tumors.

RESULTS

The radiolabeling of peptides was achieved at molar activities of up to 20 MBq/nmol at ≥ 98% radiochemical purity. AR42J cell viability was reduced in an activity-dependent manner, with [149Tb]Tb-DOTA-LM3 being slightly more potent than [149Tb]Tb-DOTATATE (EC50: 0.5 vs. 1.2 kBq/mL). Both radiopeptides induced a similar number of γ-H2A.X and 53BP1 foci per nuclei, which indicated DNA damage in AR42J tumor cells. Injection of tumor-bearing mice with 1 × 5 MBq radiopeptide resulted in median survival times of 16.5 days and 19 days for [149Tb]Tb-DOTATATE and [149Tb]Tb-DOTA-LM3, respectively, as compared to only 8 days for untreated control mice. Application of 2 × 5 MBq of the radiopeptides further extended the median survival times to 30 days and 29 days, respectively. The blood cell counts and values for blood plasma biomarkers of treated mice without tumors were similar to those of untreated controls. Renal accumulation of [99mTc]Tc-DMSA was similar in all mice, indicating normal kidney function.

CONCLUSION

149Tb-based radiopeptides effectively reduced the viability of tumor cells in vitro as well as the tumor growth in mice without causing relevant adverse events, irrespective of whether the SSTR agonist or antagonist was employed. These data encourage further preclinical application of terbium-149 to evaluate its potential in combination with other tumor-targeting agents.

Preclinical comparison of (radio)lanthanides using mass spectrometry and nuclear imaging techniques: biodistribution of lanthanide-based tumor-targeting agents and lanthanides in ionic form

PURPOSE

With the growing interest in exploring radiolanthanides for nuclear medicine applications, the question arises as to whether they are generally interchangeable without affecting a biomolecule's pharmacokinetic properties. The goal of this study was to investigate similarities and differences of four (radio)lanthanides simultaneously applied as complexes of biomolecules or in ionic form.

METHODS

Inductively coupled plasma mass spectrometry (ICP-MS) was employed for the simultaneous detection of four lanthanides (Ln = lutetium, terbium, gadolinium and europium) in biological samples. In vitro tumor cell uptake and in vivo biodistribution studies were performed with Ln-DOTATATE, Ln-DOTA-LM3, Ln-PSMA-617 and Ln-OxFol-1. AR42J cells, PC-3 PIP cells and KB cells expressing the somatostatin receptor, the prostate-specific membrane antigen and the folate receptor, respectively, were used in vitro as well as to obtain the respective tumor mouse models for in vivo studies. The distribution of lanthanides in ionic form was investigated in immunocompetent mice. Dual-isotope SPECT/CT imaging studies were performed with mice administered with the radiolabeled biomolecules or chloride salts of lutetium-177 and terbium-161.

RESULTS

Similar in vitro cell uptake was observed for all four lanthanide complexes of each biomolecule into the respective tumor cell lines. AR42J tumor uptake of Ln-DOTATATE and Ln-DOTA-LM3 in mice showed similar values for all lanthanide complexes (3.8‒5.1% ID/g and 4.5‒5.0% ID/g; 1 h p.i., respectively). Accumulation of Ln-PSMA-617 in PC-3 PIP tumors (24-25% ID/g; 1 h p.i.) and of Ln-OxFol-1 in KB tumors (28-31% ID/g; 24 h p.i.) were also equal for the four lanthanide complexes of each biomolecule. After injection of lanthanide chloride salts (LnCl3; Ln = natLu, natTb, natGd, natEu), the liver uptake was different for each metal (~ 12% ID/g, ~ 22% ID/g, ~ 31% ID/g and ~ 37% ID/g; 24 h p.i., respectively) which could be ascribed to the radii of the respective lanthanide ions. In the bones, accumulation was considerably higher for lutetium than for other lanthanides (25 ± 5% ID/g vs. 14‒15% ID/g; 24 h p.i.). These data were confirmed visually by 177Lu/161Tb-based dual-isotope SPECT/CT images.

CONCLUSIONS

The presented study confirmed similar properties of Ln-complexes, suggesting that lutetium-177 can be replaced by other radiolanthanides, most probably without affecting the tissue distribution profile of the resultant radiopharmaceuticals. On the other hand, the different radii of the lanthanide ions affected their uptake and resorption mechanisms in liver and bones when injected in uncomplexed form.

Preparation and Characterization of an Engineered FGF1 Conjugated to 161Tb for Targeting of FGFRs

The fibroblast growth factor receptor family members, FGFR1-4, are frequently overexpressed in various solid tumors, including breast cancer and sarcomas. This overexpression highlights the potential of the family of FGFRs as promising targets for cancer therapy. However, conventional FGFR kinase inhibitors often encounter challenges such as limited efficacy or drug resistance. In this study, we pursue an alternative strategy by designing a conjugate of the FGFR ligand FGF1 with the radioisotope 161Tb, for targeted therapy in FGFR-overexpressing cancer cells. FGF1 was engineered (eFGF1) to incorporate a single cysteine at the C terminus for site-specific labeling with a DOTA chelator. eFGF1-DOTA was mixed with the radioisotope 161Tb under mild conditions, resulting in a labeling efficiency above 90%. The nonradioactive ligands were characterized by mass spectrometry, while radioligands were characterized by thin-layer chromatography. The targeting function of the radioligands was assessed through confocal microscopy, flow cytometry, and Western blot analysis, focusing on binding to cancer cells and the activation of downstream signaling pathways related to FGFR. When compared to MCF-7 and RD cell lines with low FGFR expression, eFGF1-DOTA-Tb[161Tb] radioligands demonstrated significantly higher accumulation in FGFR-overexpressing cell lines (MCF-7 FGFR1 and RMS559), leading to enhanced cytotoxicity. Besides radionuclides, eFGF1 can also deliver doxorubicin (DOX) into cancer cells. Considering these characteristics, eFGF1-DOTA-Tb[161Tb] and eFGF1-DOX emerge as promising candidates for FGFR-targeted cancer therapy, and further evaluation in vivo is warranted.

Terbium-161 in nuclear medicine: Preclinical and clinical progress in comparison with lutetium-177

The use of new radiopharmaceuticals labeled with lutetium-177 represents a successful translation of experimental results into clinical practice. Recent experimental data suggests that terbium-161 might well follow the example of lutetium-177 regarding applicability in nuclear medicine. Similarly to lutetium-177, the terbium-161 emits beta particles and gamma-radiation, although terbium-161 emits short-ranged conversion and Auger electrons, creating an effect that may eliminate smaller tumor metastases more effectively than lutetium-177. Terbium-161 may exert a higher radiobiological effect in the target tissues in comparison with lutetium-177, a difference which makes possible a reduction in the doses of radioactivity administered. Further, due to the similar chemical properties of lutetium-177 and terbium-161, similar radiolabeling techniques can be used. The differences found in preclinical experiments on radiotoxicity of the counterparts seem to be minor. Despite intensive progress, the number of preclinical studies on 161Tb-labeled agents is still not comparable to studies on lutetium-177. Clinical trials with 161Tb-labeled radiopharmaceuticals focused on the treatment of prostate cancer and selected neuroendocrine tumors have already begun, although none of them have been completed yet.

Advances in Preclinical Research of Theranostic Radiopharmaceuticals in Nuclear Medicine

Theranostics of nuclear medicine refers to the combination of radionuclide imaging and internal irradiation therapy, which is currently a research hotspot and an important direction for the future development of nuclear medicine. Radiopharmaceutical is a vital component of nuclear medicine and serves as one of the fundamental pillars of molecular imaging and precision medicine. At present, a variety of radiopharmaceuticals have been developed for various targets such as fibroblast activation protein (FAP), prostate-specific membrane antigen (PSMA), somatostatin receptor 2 (SSTR2), C-X-C motif chemokine receptor 4 (CXCR4), human epidermal growth factor-2 (HER2), and integrin αvβ3, and some of them have been successfully applied in clinical practice. The radiopharmaceutical with theranostic function plays an important role in the diagnosis, treatment, efficacy evaluation, and prognosis prediction of cancers and is the key to realize the personalized treatment of tumors. This Review summarizes the preclinical research progress of theranostic radiopharmaceuticals toward the above targets in the field of nuclear medicine and discusses the prospects and development directions of radiopharmaceuticals in the future.

Cancer theragnostics: closing the loop for advanced personalized cancer treatment through the platform integration of therapeutics and diagnostics

Cancer continues to be one of the leading causes of death worldwide, and conventional cancer therapies such as chemotherapy, radiation therapy, and surgery have limitations. RNA therapy and cancer vaccines hold considerable promise as an alternative to conventional therapies for their ability to enable personalized therapy with improved efficacy and reduced side effects. The principal approach of cancer vaccines is to induce a specific immune response against cancer cells. However, a major challenge in cancer immunotherapy is to predict which patients will respond to treatment and to monitor the efficacy of the vaccine during treatment. Theragnostics, an integration of diagnostic and therapeutic capabilities into a single hybrid platform system, has the potential to address these challenges by enabling real-time monitoring of treatment response while allowing endogenously controlled personalized treatment adjustments. In this article, we review the current state-of-the-art in theragnostics for cancer vaccines and RNA therapy, including imaging agents, biomarkers, and other diagnostic tools relevant to cancer, and their application in cancer therapy development and personalization. We also discuss the opportunities and challenges for further development and clinical translation of theragnostics in cancer vaccines.

Pursuing theranostics: a multimodal architecture approach

Theranostics is a field of nuclear medicine which uses the same targeting vector and chelating system for both a diagnostic and therapeutic radionuclide, allowing for uniformity in imaging and treatment. This growing field requires the development of more flexible chelate systems that permit novel targeting strategies. Toward this end, a multimodal architecture has been realized, making use of a phosphazene-based core and click chemistry to achieve a flexible and customizable scaffold. The six arm phosphazene-based core can scaffold six DTPA chelating motifs or a mixed set of 3 : 3 DTPA : DFO chelates resulting in two multimodal compounds, pDbDt and pDbDtDf, respectively. Terbium complexes displayed strong luminescence, supporting that the structures act as an organic antenna for luminescence. Metal displacement titration studies confirmed the desired structures as well as the capability for heterometallic labeling of the structures. These structures were found to have high thermal and biological stability in vitro. Radiolabeling of each compound resulted in high molar activity labeling of each compound: 169 MBq nmol-1: [161Tb]Tb-pDbDt, 170 MBq nmol-1: [89Zr]Zr-pDbDtDf, and the mixed radiolabeling illustrated chelation of both radionuclides in a 1 : 1 ratio. This multimodal architecture is promising as a heterometallic structure for coupling of both a diagnostic and a therapeutic radionuclide with a highly customizable core structure.

161Terbium-Labeled Gold Nanoparticles as Nanoscale Brachytherapy Agents Against Breast Cancer

Due to their intriguing emission profile, Terbium-161 (161Tb) radiopharmaceuticals seem to bring significant advancement in theranostic applications to cancer treatment. The combination of 161Tb with nanoscale brachytherapy as an approach for cancer treatment is particularly advantageous and promising. Herein, we propose the application of a hybrid nanosystem comprising gold decorated (Au@TADOTAGA) iron oxide nanoflowers as a form of injectable nanobrachytherapy for the local treatment of breast cancer. More specifically, Au@TADOTAGA and NFAu@TADOTAGA NPs were efficiently radiolabeled with 161Tb, and their in vitro stability was assessed up to 21 d post-radiolabeling. Furthermore, their cytotoxic profile against 4T1 breast cancer cells was evaluated, and their ex vivo biodistribution characteristics were revealed after intratumoral injection in the same animal model. The enhanced retention at the tumor site urged us to evaluate the therapeutic effect of the [161Tb]Tb-NFAu@TADOTAGA nanosystem after intratumoral administration to 4T1-tumor-bearing mice, over a period of 24 days. Three different therapeutic protocols were performed in order to identify which therapeutic approach would offer the optimum results and identify the proposed nanosystem as a promising nanoscale brachytherapy agent.

Biological determinants of PSMA expression, regulation and heterogeneity in prostate cancer

Prostate-specific membrane antigen (PSMA) is an important cell-surface imaging biomarker and therapeutic target in prostate cancer. The PSMA-targeted theranostic 177Lu-PSMA-617 was approved in 2022 for men with PSMA-PET-positive metastatic castration-resistant prostate cancer. However, not all patients respond to PSMA-radioligand therapy, in part owing to the heterogeneity of PSMA expression in the tumour. The PSMA regulatory network is composed of a PSMA transcription complex, an upstream enhancer that loops to the FOLH1 (PSMA) gene promoter, intergenic enhancers and differentially methylated regions. Our understanding of the PSMA regulatory network and the mechanisms underlying PSMA suppression is evolving. Clinically, molecular imaging provides a unique window into PSMA dynamics that occur on therapy and with disease progression, although challenges arise owing to the limited resolution of PET. PSMA regulation and heterogeneity - including intertumoural and inter-patient heterogeneity, temporal changes, lineage dynamics and the tumour microenvironment - affect PSMA theranostics. PSMA response and resistance to radioligand therapy are mediated by a number of potential mechanisms, and complementary biomarkers beyond PSMA are under development. Understanding the biological determinants of cell surface target regulation and heterogeneity can inform precision medicine approaches to PSMA theranostics as well as other emerging therapies.

Au@109Pd Core-Shell Nanoparticles Conjugated to Panitumumab for the Combined β--Auger Electron Therapy of Triple-Negative Breast Cancer

Apart from HER2-positive, triple-negative breast cancer (TNBC) is the second most highly invasive type of breast cancer. Although TNBC does not overexpress HER2 receptors, it has been observed that EGFR protein expression is present in this specific type of tumor, making it an attractive target for immune and radiopharmaceutical treatments. In our current study, we used 109Pd (T1/2 = 13.7 h) in the form of a 109Pd/109mAg in vivo generator as a source of β- particles and Auger electrons in targeted radionuclide therapy for TNBC. 109Pd, obtained through neutron irradiation of the 108Pd target, was deposited onto 15 nm gold nanoparticles to form Au@109Pd core-shell nanoparticles, which were then conjugated to the panitumumab antibody. Au@109Pd-PEG-panitumumab nanoparticles were bound, internalized, and partially routed to the nucleus in MDA-MB-231 human breast cancer cells overexpressing EGFR receptors. The Au@109Pd-panitumumab radioconjugate significantly reduced the metabolic activity of MDA-MB-231 cells in a dose-dependent manner. In conclusion, we have found that Au@109Pd-PEG-panitumumab nanoparticles show potential as a therapeutic agent for combined β--Auger electron targeted radionuclide therapy of TNBC. The simultaneous emission of β-, conversion, and Auger electrons from the 109Pd/109mAg generator, similar to 161Tb conjugates, significantly enhances the therapeutic effect. The partial localization of these nanoparticles into the cell nucleus, provided by the panitumumab vector, ensures effective therapy with Auger electrons. This is particularly important for the treatment of drug-resistant TNBC cells.

The production and separation of 161Tb with high specific activity at the University of Utah

Targeted radiotherapy (TRT) is an increasingly prominent area of research in nuclear medicine, particularly in the context of treating cancerous tumors. One radionuclide of considerable interest for TRT is terbium-161 (t1/2 = 6.95 days), which undergoes beta emission and shares similar decay properties as 177Lu (FDA-approved as LUTATHERA® and PLUVICTO®). Besides beta emission, 161Tb also emits a significant number of conversion and Auger electrons further enhancing its therapeutic potential. Terbium-161 can be produced using nuclear reactors through an indirect neutron capture reaction, G64160dn,γG64161d→3.66min,β-T65161b, from 160Gd targets. However, a key challenge in utilizing 161Tb for TRT lies in effectively separating target and product materials to attain high specific activity for radiolabeling. Here, we detail the production of no-carrier added 161Tb using low flux research reactors (mean thermal (<0.625 eV) neutron flux: 1.356×1012n∙cm-2∙s-1) like the University of Utah TRIGA Reactor, using enriched 160Gd2O3 targets (1.5 ± 0.3 μCi of 161Tb per mg of 160Gd target per hour of irradiation). We also developed a separation technique based on cation exchange and extraction chromatography, suitable for mCi level irradiations with targets exceeding 200 mg. In a simulated full-scale irradiation, 161Tb was successfully isolated from large mass targets using cation exchange (AG 50W-X8, with 2-hydroxyisobutyric acid at 70 mM, pH 4.75) and extraction chromatography (LN Resin, 0.5-0.75 M HNO3) methods. This resulted in high apparent molar activities of [161Tb]Tb-DOTA (113 ± 3 MBq/nmol), demonstrating high purity 161Tb relevant for potential future preclinical applications.

In Vitro and Preclinical Systematic Dose-Effect Studies of Auger Electron- and β Particle-Emitting Radionuclides and External Beam Radiation for Cancer Treatment

PURPOSE

Despite a rise in clinical use of radiopharmaceutical therapies, the biological effects of radionuclides and their relationship with absorbed radiation dose are poorly understood. Here, we set out to define this relationship for Auger electron emitters [99mTc]TcO4- and [123I]I- and β--particle emitter [188Re]ReO4-. Studies were carried out using genetically modified cells that permitted direct radionuclide comparisons.

METHODS AND MATERIALS

Triple-negative MDA-MB-231 breast cancer cells expressing the human sodium iodide symporter (hNIS) and green fluorescent protein (GFP; MDA-MB-231.hNIS-GFP) were used. In vitro radiotoxicity of [99mTc]TcO4-, [123I]I-, and [188Re]ReO4- was determined using clonogenic assays. Radionuclide uptake, efflux, and subcellular location were used to calculate nuclear absorbed doses using the Medical Internal Radiation Dose (MIRD) formalism. In vivo studies were performed using female NSG mice bearing orthotopic MDA-MB-231.hNIS-GFP tumors and compared with X-ray-treated (12.6-15 Gy) and untreated cohorts. Absorbed dose per unit activity in tumors and sodium iodide symporter-expressing organs was extrapolated to reference human adult models using OLINDA/EXM.

RESULTS

[99mTc]TcO4- and [123I]I- reduced the survival fraction only in hNIS-expressing cells, whereas [188Re]ReO4- reduced survival fraction in hNIS-expressing and parental cells. [123I]I- required 2.4- and 1.5-fold lower decays/cell to achieve 37% survival compared with [99mTc]TcO4- and [188Re]ReO4-, respectively, after 72 hours of incubation. Additionally, [99mTc]TcO4-, [123I]I-, and [188Re]ReO4- had superior cell killing effectiveness in vitro compared with X-rays. In vivo, X-ray led to a greater median survival compared with [188Re]ReO4- and [123I]I- (54 days vs 45 and 43 days, respectively). Unlike the X-ray cohort, no metastases were visualized in the radionuclide-treated cohorts. Extrapolated human absorbed doses of [188Re]ReO4- to a 1 g tumor were 13.8- and 11.2-fold greater than for [123I]I- in female and male models, respectively.

CONCLUSIONS

This work reports reference dose-effect data using cell and tumor models for [99mTc]TcO4-, [123I]I-, and [188Re]ReO4- for the first time. We further demonstrate the tumor-controlling effects of [123I]I- and [188Re]ReO4- in comparison with external beam radiation therapy.

Comparison of the dosimetry and cell survival effect of 177Lu and 161Tb somatostatin analog radiopharmaceuticals in cancer cell clusters and micrometastases

BACKGROUND

177Lu-based radiopharmaceuticals (RPs) are the most used for targeted radionuclide therapy (TRT) due to their good response rates. However, the worldwide availability of 177Lu is limited. 161Tb represents a potential alternative for TRT, as it emits photons for SPECT imaging, β--particles for therapy, and also releases a significant yield of internal conversion (IE) and Auger electrons (AE). This research aimed to evaluate cell dosimetry with the MIRDcell code considering a realistic localization of three 161Tb- and 177Lu-somatostatin (SST) analogs in different subcellular regions as reported in the literature, various cell cluster sizes (25-1000 µm of radius) and percentage of labeled cells. Experimental values of the α- and β-survival coefficients determined by external beam photon irradiation were used to estimate the survival fraction (SF) of AR42J pancreatic cell clusters and micrometastases.

RESULTS

The different localization of RPs labeled with the same radionuclide within the cells, resulted in only slight variations in the dose absorbed by the nuclei (ADN) of the labeled cells with no differences observed in either the unlabeled cells or the SF. ADN of labeled cells (MDLC) produced by 161Tb-RPs were from 2.8-3.7 times higher than those delivered by 177Lu-RPs in cell clusters with a radius lower than 0.1 mm and 10% of labeled cells, due to the higher amount of energy emitted by 161Tb-disintegration in form of IE and AE. However, the 161Tb-RPs/177Lu-RPs MDLC ratio decreased below 1.6 in larger cell clusters (0.5-1 mm) with > 40% labeled cells, due to the significantly higher 177Lu-RPs cross-irradiation contribution. Using a fixed number of disintegrations, SFs of 161Tb-RPs in clusters with > 40% labeled cells were lower than those of 177Lu-RPs, but when the same amount of emitted energy was used no significant differences in SF were observed between 177Lu- and 161Tb-RPs, except for the smallest cluster sizes.

CONCLUSIONS

Despite the emissions of IE and AE from 161Tb-RPs, their localization within different subcellular regions exerted a negligible influence on the ADN. The same cell damage produced by 177Lu-RPs could be achieved using smaller quantities of 161Tb-RPs, thus making 161Tb a suitable alternative for TRT.

Evaluation of a novel 177Lu-labelled therapeutic Affibody molecule with a deimmunized ABD domain and improved biodistribution profile

PURPOSE

Fusion of Affibody molecules with an albumin-binding domain (ABD) provides targeting agents, which are suitable for radionuclide therapy. To facilitate clinical translation, the low immunogenic potential of such constructs with targeting properties conserved is required.

METHODS

The HER2-targeting Affibody molecule ZHER2:2891 was fused with a deimmunized ABD variant and DOTA was conjugated to a unique C-terminal cysteine. The novel construct, PEP49989, was labelled with 177Lu. Affinity, specificity, and in vivo targeting properties of [177Lu]Lu-PEP49989 were characterised. Experimental therapy in mice with human HER2-expressing xenografts was evaluated.

RESULTS

The maximum molar activity of 52 GBq/µmol [177Lu]Lu-PEP49989 was obtained. [177Lu]Lu-PEP49989 bound specifically to HER2-expressing cells in vitro and in vivo. The HER2 binding affinity of [177Lu]Lu-PEP49989 was similar to the affinity of [177Lu]Lu-ABY-027 containing the parental ABD035 variant. The renal uptake of [177Lu]Lu-PEP49989 was 1.4-fold higher, but hepatic and splenic uptake was 1.7-2-fold lower than the uptake of [177Lu]Lu-ABY-027. The median survival of xenograft-bearing mice treated with 21 MBq [177Lu]Lu-PEP49989 (> 90 days) was significantly longer than the survival of mice treated with vehicle (38 days) or trastuzumab (45 days). Treatment using a combination of [177Lu]Lu-PEP49989 and trastuzumab increased the number of complete tumour remissions. The renal and hepatic toxicity was minimal to mild.

CONCLUSION

In preclinical studies, [177Lu]Lu-PEP49989 demonstrated favourable biodistribution and a strong antitumour effect, which was further enhanced by co-treatment with trastuzumab.

Radiochemistry and Complex Formation of the Cyclen-Derived Chelator DOTI-Me with Mn2+, Cu2+, Zn2+, Ga3+, In3+, Tb3+, and Lu3

In this work, we describe the complex formation and radiochemistry of the cyclen-based chelator DOTI-Me bearing four methylimidazole arms. Radiolabeling properties were evaluated for 52gMn, 64Cu, 68Ga, 111In, 161Tb, and 177Lu, and DOTI-Me showed distinct differences to the structurally related H4DOTA. While radiochemical conversions (RCCs) for 52gMn and 111In were comparable to those of H4DOTA, DOTI-Me was not suited for 68Ga. Conversely, quantitative RCCs were achieved for 64Cu at ambient temperature, while elevated temperatures were required for complexation with H4DOTA. For 161Tb and 177Lu, good but not quantitative RCCs were obtained with DOTI-Me. With the exemption of 68Ga3+, radiolabeled complexes showed high stability in ligand challenge experiments and in human serum. X-ray analysis of the nonradioactive complexes revealed the formation of 8-coordinate Mn2+ and In3+ DOTI-Me complexes. Cu2+ adopted a unique distorted square-pyramidal 2 + 3 with the neutral DOTI-Me ligand and a Jahn-Teller distorted 4 + 2 coordination geometry for the diprotonated H2DOTI-Me2+ cation, respectively. For Zn2+, the complex with HDOTI-Me+ showed a distorted 4 + 3 pentagonal bipyramidal geometry. Summarizing, the ligand DOTI-Me may be an interesting alternative to H4DOTA for 52gMn, 64Cu, 111In, 161Tb, and 177Lu, covering diagnostic as well as therapeutic radionuclides. Further studies of targeted radiopharmaceuticals based on the DOTI-Me scaffold in combination with the set of radiometals presented herein are thus warranted.

Terbium sisters: current development status and upscaling opportunities

The interest in terbium radionuclides, which can be used in nuclear medicine, has increased tremendously over the last decade. Several research studies have shown the potential of four terbium radionuclides 149,152,155,161Tb both for cancer diagnosis as well as therapy. The comparison of 161Tb and 177Lu showed 161Tb as the preferred candidate not only for standard radiotherapy, but also for the treatment of minimal residual disease. Nevertheless, among the terbium sisters, currently, only 161Tb has an established production protocol where its no-carrier-added form is obtained via neutron irradiation of enriched 160Gd targets. The other terbium radioisotopes face challenges related to production capacity and production yield, which currently restricts their use in nuclear medicine. The purpose of this review is to report on recent research on the production and separation of terbium sisters and to assess the prospects for upscaling their production for nuclear medicine applications.

The Emission of Internal Conversion Electrons Rather Than Auger Electrons Increased the Nucleus-Absorbed Dose for 161Tb Compared with 177Lu with a Higher Dose Response for [161Tb]Tb-DOTA-LM3 Than for [161Tb]Tb-DOTATATE

Preclinical data have shown that 161Tb-labeled peptides targeting the somatostatin receptor are therapeutically more effective for peptide receptor radionuclide therapy than are their 177Lu-labeled counterparts. To further substantiate this enhanced therapeutic effect, we performed cellular dosimetry to quantify the absorbed dose to the cell nucleus and compared dose-response curves to evaluate differences in relative biological effectiveness in vitro. Methods: CA20948 cell survival was assessed after treatment with [161Tb]Tb- and [177Lu]Lu-DOTATATE (agonist) and with [161Tb]Tb- and [177Lu]Lu-DOTA-LM3 (antagonist) via a clonogenic assay. Cell binding, internalization, and dissociation assays were performed up to 7 d to acquire time-integrated activity coefficients. Separate S values for each type of particle emission (Auger/internal conversion [IC] electrons and β- particles) were computed via Monte Carlo simulations, while considering spheric cells. Once the absorbed dose to the cell nucleus was calculated, survival curves were fitted to the appropriate linear or linear-quadratic model and corresponding relative biological effectiveness was evaluated. Results: Although the radiopeptide uptake was independent of the radionuclide, [161Tb]Tb-DOTATATE and [161Tb]Tb-DOTA-LM3 delivered a 3.6 and 3.8 times higher dose to the nucleus, respectively, than their 177Lu-labeled counterparts on saturated receptor binding. This increased nucleus-absorbed dose was mainly due to the additional emission of IC and not Auger electrons by 161Tb. When activity concentrations were considered, both [161Tb]Tb-DOTATATE and [161Tb]Tb-DOTA-LM3 showed a lower survival fraction than did labeling with 177Lu. When the absorbed dose to the nucleus was considered, no significant difference could be observed between the dose-response curves for [161Tb]Tb- and [177Lu]Lu-DOTATATE. [161Tb]Tb-DOTA-LM3 showed a linear-quadratic dose response, whereas [161Tb]Tb-DOTATATE showed only a linear dose response within the observed dose range, suggesting additional cell membrane damage by Auger electrons. Conclusion: The IC, rather than Auger, electrons emitted by 161Tb resulted in a higher absorbed dose to the cell nucleus and lower clonogenic survival for [161Tb]Tb-DOTATATE and [161Tb]Tb-DOTA-LM3 than for the 177Lu-labeled analogs. In contrast, [161Tb]Tb-DOTATATE showed no higher dose response than [177Lu]Lu-DOTATATE, whereas for [161Tb]Tb-DOTA-LM3 an additional quadratic response was observed. Because of this quadratic response, potentially caused by cell membrane damage, [161Tb]Tb-DOTA-LM3 is a more effective radiopeptide than [161Tb]Tb-DOTATATE for labeling with 161Tb.

Quantitative SPECT imaging of 155Tb and 161Tb for preclinical theranostic radiopharmaceutical development

BACKGROUND

Element-equivalent matched theranostic pairs facilitate quantitative in vivo imaging to establish pharmacokinetics and dosimetry estimates in the development of preclinical radiopharmaceuticals. Terbium radionuclides have significant potential as matched theranostic pairs for multipurpose applications in nuclear medicine. In particular, 155Tb (t1/2 = 5.32 d) and 161Tb (t1/2 = 6.89 d) have been proposed as a theranostic pair for their respective applications in single photon emission computed tomography (SPECT) imaging and targeted beta therapy. Our study assessed the performance of preclinical quantitative SPECT imaging with 155Tb and 161Tb. A hot rod resolution phantom with rod diameters ranging between 0.85 and 1.70 mm was filled with either 155Tb (21.8 ± 1.7 MBq/mL) or 161Tb (23.6 ± 1.9 MBq/mL) and scanned with the VECTor preclinical SPECT/CT scanner. Image performance was evaluated with two collimators: a high energy ultra high resolution (HEUHR) collimator and an extra ultra high sensitivity (UHS) collimator. SPECT images were reconstructed from photopeaks at 43.0 keV, 86.6 keV, and 105.3 keV for 155Tb and 48.9 keV and 74.6 keV for 161Tb. Quantitative SPECT images of the resolution phantoms were analyzed to report inter-rod contrast, recovery coefficients, and contrast-to-noise metrics.

RESULTS

Quantitative SPECT images of the resolution phantom established that the HEUHR collimator resolved all rods for 155Tb and 161Tb, and the UHS collimator resolved rods ≥ 1.10 mm for 161Tb and ≥ 1.30 mm for 155Tb. The HEUHR collimator maintained better quantitative accuracy than the UHS collimator with recovery coefficients up to 92%. Contrast-to-noise metrics were also superior with the HEUHR collimator.

CONCLUSIONS

Both 155Tb and 161Tb demonstrated potential for applications in preclinical quantitative SPECT imaging. The high-resolution collimator achieves < 0.85 mm resolution and maintains quantitative accuracy in small volumes which is advantageous for assessing sub organ activity distributions in small animals. This imaging method can provide critical quantitative information for assessing and optimizing preclinical Tb-radiopharmaceuticals.

Developments in radionanotheranostic strategies for precision diagnosis and treatment of prostate cancer

BACKGROUND

Prostate Cancer (PCa) is the second most diagnosed urological cancer among men worldwide. Conventional methods used for diagnosis of PCa have several pitfalls which include lack of sensitivity and specificity. On the other hand, traditional treatment of PCa poses challenges such as long-term side effects and the development of multidrug resistance (MDR).

MAIN BODY

Hence, there is a need for novel PCa agents with the potential to lessen the burden of these adverse effects on patients. Nanotechnology has emerged as a promising approach to support both early diagnosis and effective treatment of tumours by ensuring precise delivery of the drug to the targeted site of the disease. Most cancer-related biological processes occur on the nanoscale hence application of nanotechnology has been greatly appreciated and implemented in the management and therapeutics of cancer. Nuclear medicine plays a significant role in the non-invasive diagnosis and treatment of PCa using appropriate radiopharmaceuticals. This review aims to explore the different radiolabelled nanomaterials to enhance the specific delivery of imaging and therapeutic agents to cancer cells. Thereafter, the review appraises the advantages and disadvantages of these modalities and then discusses and outlines the benefits of radiolabelled nanomaterials in targeting cancerous prostatic tumours. Moreover, the nanoradiotheranostic approaches currently developed for PCa are discussed and finally the prospects of combining radiopharmaceuticals with nanotechnology in improving PCa outcomes will be highlighted.

CONCLUSION

Nanomaterials have great potential, but safety and biocompatibility issues remain. Notwithstanding, the combination of nanomaterials with radiotherapeutics may improve patient outcomes and quality of life.

109Pd/109mAg in-vivo generator in the form of nanoparticles for combined β- - Auger electron therapy of hepatocellular carcinoma

BACKGROUND

Convenient therapeutic protocols for hepatocellular carcinoma (HCC) are often ineffective due to late diagnosis and high tumor heterogeneity, leading to poor long-term outcomes. However, recently performed studies suggest that using nanostructures in liver cancer treatment may improve therapeutic effects. Inorganic nanoparticles represent a unique material that tend to accumulate in the liver when introduced in-vivo. Typically, this is a major drawback that prevents the therapeutic use of nanoparticles in medicine. However, in HCC tumours, this may be advantageous because nanoparticles may accumulate in the target organ, where the leaky vasculature of HCC causes their accumulation in tumour cells via the EPR effect. On the other hand, recent studies have shown that combining low- and high-LET radiation emitted from the same radionuclide, such as 161Tb, can increase the effectiveness of radionuclide therapy. Therefore, to improve the efficacy of radionuclide therapy for hepatocellular carcinoma, we suggest utilizing radioactive palladium nanoparticles in the form of 109Pd/109mAg in-vivo generator that simultaneously emits β- particles and Auger electrons.

RESULTS

Palladium nanoparticles with a size of 5 nm were synthesized using 109Pd produced through neutron irradiation of natural palladium or enriched 108Pd. Unlike the 109Pd-cyclam complex, where the daughter radionuclide diffuses away from the molecules, 109mAg remains within the nanoparticles after the decay of 109Pd. In vitro cell studies using radioactive 109Pd nanoparticles revealed that the nanoparticles accumulated inside cells, reaching around 50% total uptake. The 109Pd-PEG nanoparticles exhibited high cytotoxicity, even at low levels of radioactivity (6.25 MBq/mL), resulting in almost complete cell death at 25 MBq/mL. This cytotoxic effect was significantly greater than that of PdNPs labeled with β- (131I) and Auger electron emitters (125I). The metabolic viability of HCC cells was found to be correlated with cell DNA DSBs. Also, successful radioconjugate anticancer activity was observed in three-dimensional tumor spheroids, resulting in a significant treatment response.

CONCLUSION

The results indicate that nanoparticles labeled with 109Pd can be effectively used for combined β- - Auger electron-targeted radionuclide therapy of HCC. Due to the decay of both components (β- and Auger electrons), the 109Pd/109mAg in-vivo generator presents a unique potential in this field.

Clinical Trial Protocol for VIOLET: A Single-Center, Phase I/II Trial Evaluation of Radioligand Treatment in Patients with Metastatic Castration-Resistant Prostate Cancer with [161Tb]Tb-PSMA-I&T

[177Lu]Lu-PSMA is an effective class of therapy for patients with metastatic castration-resistant prostate cancer (mCRPC); however, progression is inevitable. The limited durability of response may be partially explained by the presence of micrometastatic deposits, which are energy-sheltered and receive low absorbed radiation with 177Lu due to the approximately 0.7-mm mean pathlength. 161Tb has abundant emission of Auger and conversion electrons that deposit a higher concentration of radiation over a shorter path, particularly to single tumor cells and micrometastases. 161Tb has shown in vitro and in vivo efficacy superior to that of 177Lu. We aim to demonstrate that [161Tb]Tb-PSMA-I&T will deliver effective radiation to sites of metastatic prostate cancer with an acceptable safety profile. Methods: This single-center, single-arm, phase I/II trial will recruit 30 patients with mCRPC. Key eligibility criteria include a diagnosis of mCRPC with progression after at least one line of taxane chemotherapy (unless medically unsuitable) and androgen receptor pathway inhibitor; prostate-specific membrane antigen-positive disease on [68Ga]Ga-PSMA-11 or [18F]DCFPyL PET/CT (SUVmax ≥ 20); no sites of discordance on [18F]FDG PET/CT; adequate bone marrow, hepatic, and renal function; an Eastern Cooperative Oncology Group performance status of no more than 2, and no prior treatment with another radioisotope. The dose escalation is a 3 + 3 design to establish the safety of 3 prespecified activities of [161Tb]Tb-PSMA-I&T (4.4, 5.5, and 7.4 GBq). The maximum tolerated dose will be defined as the highest activity level at which a dose-limiting toxicity occurs in fewer than 2 of 6 participants. The dose expansion will include 24 participants at the maximum tolerated dose. Up to 6 cycles of [161Tb]Tb-PSMA-I&T will be administered intravenously every 6 wk, with each subsequent activity reduced by 0.4 GBq. The coprimary objectives are to establish the maximum tolerated dose and safety profile (Common Terminology Criteria for Adverse Events version 5.0) of [161Tb]Tb-PSMA-I&T. Secondary objectives include measuring absorbed radiation dose (Gy), evaluating antitumor activity (prostate-specific antigen 50% response rate, radiographic and prostate-specific antigen progression-free survival, overall survival, objective response rate), and evaluating pain (Brief Pain Inventory-Short Form) and health-related quality of life (Functional Assessment of Cancer Therapy-Prostate and Functional Assessment of Cancer Therapy-Radionuclide Therapy). Conclusion: Enrollment was completed in February 2024. Patients are still receiving [161Tb]Tb-PSMA-I&T.

Palladium-103 (103Pd/103mRh), a promising Auger-electron emitter for targeted radionuclide therapy of disseminated tumor cells - absorbed doses in single cells and clusters, with comparison to 177Lu and 161Tb

Early use of targeted radionuclide therapy (TRT) to eradicate disseminated tumor cells (DTCs) might offer cure. Selection of appropriate radionuclides is required. This work highlights the potential of 103Pd (T1/2 = 16.991 d) which decays to 103mRh (T1/2 = 56.12 min) then to stable 103Rh with emission of Auger and conversion electrons. Methods: The Monte Carlo track structure code CELLDOSE was used to assess absorbed doses in single cells (14-μm diameter; 10-μm nucleus) and clusters of 19 cells. The radionuclide was distributed on the cell surface, within the cytoplasm, or in the nucleus. Absorbed doses from 103Pd, 177Lu and 161Tb were compared after energy normalization. The impact of non-uniform cell targeting, and the potential benefit from dual-targeting was investigated. Additional results related to 103mRh, if used directly, are provided. Results: In the single cell, and depending on radionuclide distribution, 103Pd delivered 7- to 10-fold higher nuclear absorbed dose and 9- to 25-fold higher membrane dose than 177Lu. In the 19-cell clusters, 103Pd absorbed doses also largely exceeded 177Lu. In both situations, 161Tb stood in-between 103Pd and 177Lu. Non-uniform targeting, considering four unlabeled cells within the cluster, resulted in moderate-to-severe dose heterogeneity. For example, with intranuclear 103Pd, unlabeled cells received only 14% of the expected nuclear dose. Targeting with two 103Pd-labeled radiopharmaceuticals minimized dose heterogeneity. Conclusion: 103Pd, a next-generation Auger emitter, can deliver substantially higher absorbed doses than 177Lu to single tumor cells and cell clusters. This may open new horizons for the use of TRT in adjuvant or neoadjuvant settings, or for targeting minimal residual disease.

Advances in PSMA Alpha Theragnostics

Alpha theranostics offer an attractive alternative form of therapy, which has best been investigated and documented with 225Ac-PSMA in patients with prostate cancer. Advantages offered by targeted alpha therapy include overcoming radiation resistance, oxygen independence, effecting double-stranded DNA breakages within the tumors with anticipated improved clinical outcomes and an acceptable side effect profile. The previous Seminars article on this topic, published in 2020, had to rely mostly on published case reports and small observational studies. In the last few years, however, several meta-analyses have emerged that evaluate the safety and efficacy of 225Ac-PSMA in prostate cancer patients, followed most recently by a multi-center retrospective study initiated by WARMTH. The findings of these publications, together with the exploration of TAT offered in clinical conditions other than as a last resort, is the focus of this updated overview. Unresolved clinical issues that remain, include the appropriate selection of patients that would benefit most from treatment with 225Ac-PSMA, treatment timing within the disease landscape, optimal dosing schedule, dosimetry, when and how to best use combination therapies and minimization and treatment of side effects, particularly that of xerostomia.

Alpha and Beta Radiation for Theragnostics

Targeted radionuclide therapy (TRT) has significantly evolved from its beginnings with iodine-131 to employing carrier molecules with beta emitting isotopes like lutetium-177. With the success of Lu-177-DOTATATE for neuroendocrine tumors and Lu-177-PSMA-617 for prostate cancer, several other beta emitting radioisotopes, such as Cu-67 and Tb-161, are being explored for TRT. The field has also expanded into targeted alpha therapy (TAT) with agents like radium-223 for bone metastases in prostate cancer, and several other alpha emitter radioisotopes with carrier molecules, such as Ac-225, and Pb-212 under clinical trials. Despite these advancements, the scope of TRT in treating diverse solid tumors and integration with other therapies like immunotherapy remains under investigation. The success of antibody-drug conjugates further complements treatments with TRT, though challenges in treatment optimization continue.

Separation of terbium as a first step towards high purity terbium-161 for medical applications

Terbium-161 is a medical radiolanthanide that has a beta decay energy and half-life similar to that of lutetium-177, which makes it a promising alternative for therapeutic purposes. The production route using an enriched gadolinium-160 target necessitates the purification of terbium-161 from the untransmuted target material as well as from its stable decay product, dysprosium-161. The separation of neighbouring lanthanides is challenging due to their similar chemical properties and prominent trivalent oxidation states. In this work, the aim is to change the oxidation state of terbium, resulting in the altering of chemical properties that ease the intragroup separation. To this end, a novel separation method is investigated, involving the electrochemical oxidation of terbium (3+) to terbium (4+) followed by anion exchange chromatography. The electrolysis conditions are set to the highest achievable conversion rate, followed by a dilution step during which the pH and electrolyte concentration are slightly lowered to obtain conditions that are compatible with the separation method. XAS analysis is done to characterize the carbonato complex of both oxidation states and to further elucidate the separation mechanism. The results show that the separation approach of combining electrochemical oxidation with anion exchange chromatography is promising for the purification of 161Tb for medical use.

Prostate-Specific Membrane Antigen-Targeted Therapy in Prostate Cancer: History, Combination Therapies, Trials, and Future Perspective

In the last decades, the development of PET/CT radiopharmaceuticals, targeting the Prostate-Specific Membrane Antigen (PSMA), changed the management of prostate cancer (PCa) patients thanks to its higher diagnostic accuracy in comparison with conventional imaging both in staging and in recurrence. Alongside molecular imaging, PSMA was studied as a therapeutic agent targeted with various isotopes. In 2021, results from the VISION trial led to the Food and Drug Administration (FDA) approval of [177Lu]Lu-PSMA-617 as a novel therapy for metastatic castration-resistant prostate cancer (mCRPC) and set the basis for a radical change in the future perspectives of PCa treatment and the history of Nuclear Medicine. Despite these promising results, primary resistance in patients treated with single-agent [177Lu]Lu-PSMA-617 remains a real issue. Emerging trials are investigating the use of [177Lu]Lu-PSMA-617 in combination with other PCa therapies in order to cover the multiple oncologic resistance pathways and to overcome tumor heterogeneity. In this review, our aim is to retrace the history of PSMA-targeted therapy from the first preclinical studies to its future applications in PCa.

177Lu-Labeled Iron Oxide Nanoparticles Functionalized with Doxorubicin and Bevacizumab as Nanobrachytherapy Agents against Breast Cancer

The use of conventional methods for the treatment of cancer, such as chemotherapy or radiotherapy, and approaches such as brachytherapy in conjunction with the unique properties of nanoparticles could enable the development of novel theranostic agents. The aim of our current study was to evaluate the potential of iron oxide nanoparticles, coated with alginic acid and polyethylene glycol, functionalized with the chemotherapeutic agent doxorubicin and the monoclonal antibody bevacizumab, to serve as a nanoradiopharmaceutical agent against breast cancer. Direct radiolabeling with the therapeutic isotope Lutetium-177 (177Lu) resulted in an additional therapeutic effect. Functionalization was accomplished at high percentages and radiolabeling was robust. The high cytotoxic effect of our radiolabeled and non-radiolabeled nanostructures was proven in vitro against five different breast cancer cell lines. The ex vivo biodistribution in tumor-bearing mice was investigated with three different ways of administration. The intratumoral administration of our functionalized radionanoconjugates showed high tumor accumulation and retention at the tumor site. Finally, our therapeutic efficacy study performed over a 50-day period against an aggressive triple-negative breast cancer cell line (4T1) demonstrated enhanced tumor growth retention, thus identifying the developed nanoparticles as a promising nanobrachytherapy agent against breast cancer.

[161Tb]Tb-PSMA-617 radioligand therapy in patients with mCRPC: preliminary dosimetry results and intra-individual head-to-head comparison to [177Lu]Lu-PSMA-617

Rationale: Evaluation of alternative radionuclides for use in prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) is currently focusing on 161Tb, which may provide advantages by emitting additional Auger and conversion electrons. In this pilot study, we present preliminary dosimetry data for [161Tb]Tb-PSMA-617 RLT in a direct comparison with [177Lu]Lu-PSMA-617. Method: Six patients with metastatic castration-resistant prostate cancer (mCRPC) underwent treatment with [177Lu]Lu-PSMA-617 and subsequently - after inadequate response - with [161Tb]Tb-PSMA-617. Whole-body planar and SPECT imaging-based dosimetry of organs at risk (kidneys and salivary glands) and tumor lesions were calculated using IDAC for 177Lu and OLINDA/EXM for 161Tb. The therapeutic index (TI) of mean tumor-absorbed doses over relevant organs at risk was calculated. Results: Mean absorbed doses to organs at risk of PSMA-RLT were slightly higher for [161Tb]Tb-PSMA-617 compared to [177Lu]Lu-PSMA-617 (kidneys: 0.643 ± 0.247 vs. 0.545 ± 0.231 Gy/GBq, factor 1.18; parotid gland: 0.367 ± 0.198 vs. 0.329 ± 0.180 Gy/GBq, factor 1.10), but markedly higher regarding tumor lesions (6.10 ± 6.59 vs 2.59 ± 3.30 Gy/GBq, factor 2.40, p < 0.001). Consequently, the mean TI was higher for [161Tb]Tb-PSMA-617 compared to [177Lu]Lu-PSMA-617 for both, the kidneys (11.54 ± 9.74 vs. 5.28 ± 5.13, p = 0.002) and the parotid gland (16.77 ± 13.10 vs. 12.51 ± 18.09, p = 0.008). Conclusion: In this intra-individual head-to-head pilot study, [161Tb]Tb-PSMA-617 delivered higher tumor-absorbed doses and resulted in superior TI compared to [177Lu]Lu-PSMA-617. This preliminary data support 161Tb as a promising radionuclide for PSMA-RLT in mCRPC.

Quantitative calibration of Tb-161 SPECT/CT in view of personalised dosimetry assessment studies

BACKGROUND

Terbium-161 (161Tb)-based radionuclide therapy poses an alternative to current Lutetium-177 (177Lu) approaches with the additional benefit of secondary Auger and conversion electron emissions capable of delivering high doses of localised damage to micro-metastases including single cells. Quantitative single-photon emission computed tomography, paired with computed tomography (SPECT/CT), enables quantitative measurement from post-therapy imaging. In view of dosimetry extrapolations, a Tb-161 sensitivity SPECT/CT camera calibration was performed using a method previously validated for 177Lu.

METHODS

Serial imaging of a NEMA/IEC body phantom with Tb-161 was performed on SPECT/CT with low-energy high-resolution collimators employing a photopeak of 75 keV with a 20% width. Quantitative stability and recovery coefficients were investigated over a sequence of 19 scans with buffered 161Tb solution at total phantom activity ranging from 70 to 4990 MBq.

RESULTS

Sphere recovery coefficients were 0.60 ± 0.05, 0.52 ± 0.07, 0.45 ± 0.07, 0.39 ± 0.07, 0.28 ± 0.08, and 0.20 ± 0.08 for spheres 37, 28, 22, 17, 13, and 10mm, respectively, when considered across all activity and scan durations with dual-energy window scatter correction. Whole-field reconstructed sensitivity was calculated as 1.42E-5 counts per decay. Qualitatively, images exhibited no visual artefacts and were comparable to 177Lu SPECT/CT.

CONCLUSIONS

Quantitative SPECT/CT of 161Tb is feasible over a range of activities enabling dosimetry analogous to 177Lu whilst also producing suitable imaging for clinical review. This has been incorporated into a prospective trial of 161Tb-PSMA for men with metastatic prostate cancer.

Terbium radionuclides for theranostic applications in nuclear medicine: from atom to bedside

Terbium features four clinically interesting radionuclides for application in nuclear medicine: terbium-149, terbium-152, terbium-155, and terbium-161. Their identical chemical properties enable the synthesis of radiopharmaceuticals with the same pharmacokinetic character, while their distinctive decay characteristics make them valuable for both imaging and therapeutic applications. In particular, terbium-152 and terbium-155 are useful candidates for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, respectively; whereas terbium-149 and terbium-161 find application in α- and β--/Auger electron therapy, respectively. This unique characteristic makes the terbium family ideal for the "matched-pair" principle of theranostics. In this review, the advantages and challenges of terbium-based radiopharmaceuticals are discussed, covering the entire chain from radionuclide production to bedside administration. It elaborates on the fundamental properties of terbium, the production routes of the four interesting radionuclides and gives an overview of the available bifunctional chelators. Finally, we discuss the preclinical and clinical studies as well as the prospects of this promising development in nuclear medicine.

Cyanopyridoimidazole/1,2-Aminothiol Click Reaction: A Novel Bioorthogonal Reaction for Synthesis of Radiotracers

We herein described the design and synthesis of the cyanopyridoimidazoles (CPIs) as new bioorthogonal click reagents toward 1,2-aminothiol groups. Kinetic and density functional theory-based studies of the synthetic compounds revealed that incorporating an electron-withdrawing substituent into the CPI scaffold lowers its lowest unoccupied molecular orbital energy, consequently increasing reactivity. Optimized CPI 8a showed rapid reactivity and high stability in physiological conditions and has been demonstrated to be suitable for various radiotracer synthetic methods. Based on the new bioorthogonal reaction, a [67Ga]Ga-labeled prostate-specific membrane antigen-targeted probe was successfully prepared for in vivo imaging of prostate cancer in an animal model.

Comparative Study of a Decadentate Acyclic Chelate, HOPO-O10, and Its Octadentate Analogue, HOPO-O8, for Radiopharmaceutical Applications

Radiolanthanides and actinides are aptly suited for the diagnosis and treatment of cancer via nuclear medicine because they possess unique chemical and physical properties (e.g., radioactive decay emissions). These rare radiometals have recently shown the potential to selectively deliver a radiation payload to cancer cells. However, their clinical success is highly dependent on finding a suitable ligand for stable chelation and conjugation to a disease-targeting vector. Currently, the commercially available chelates exploited in the radiopharmaceutical design do not fulfill all of the requirements for nuclear medicine applications, and there is a need to further explore their chemistry to rationally design highly specific chelates. Herein, we describe the rational design and chemical development of a novel decadentate acyclic chelate containing five 1,2-hydroxypyridinones, 3,4,3,3-(LI-1,2-HOPO), referred to herein as HOPO-O10, based on the well-known octadentate ligand 3,4,3-(LI-1,2-HOPO), referred to herein as HOPO-O8, a highly efficient chelator for 89Zr[Zr4+]. Analysis by 1H NMR spectroscopy and mass spectrometry of the La3+ and Tb3+ complexes revealed that HOPO-O10 forms bimetallic complexes compared to HOPO-O8, which only forms monometallic species. The radiolabeling properties of both chelates were screened with [135La]La3+, [155/161Tb]Tb3+, [225Ac]Ac3+ and, [227Th]Th4+. Comparable high specific activity was observed for the [155/161Tb]Tb3+ complexes, outperforming the gold-standard 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, yet HOPO-O10 surpassed HOPO-O8 with higher [227Th]Th4+ affinity and improved complex stability in a human serum challenge assay. A comprehensive analysis of the decadentate and octadentate chelates was performed with density functional theory for the La3+, Ac3+, Eu3+, Tb3+, Lu3+, and Th4+ complexes. The computational simulations demonstrated the enhanced stability of Th4+-HOPO-O10 over Th4+-HOPO-O8. This investigation reveals the potential of HOPO-O10 for the stable chelation of large tetravalent radioactinides for nuclear medicine applications and provides insight for further chelate development.

The Radiolabeling of [161Tb]-PSMA-617 by a Novel Radiolabeling Method and Preclinical Evaluation by In Vitro/In Vivo Methods

Background

Prostate cancer (PC) is the most common type of cancer in elderly men, with a positive correlation with age. As resistance to treatment has developed, particularly in the progressive stage of the disease and in the presence of microfocal multiple bone metastases, new generation radionuclide therapies have emerged. Recently, [161Tb], a radiolanthanide introduced for treating micrometastatic foci, has shown great promise for treating prostate cancer.

Results

In this study, Terbium-161 [161Tb]Tb was radiolabeled with prostate-specific membrane antigen (PSMA)-617 ([161Tb]-PSMA-617) and the therapeutic efficacy of the radiolabeled compound investigated in vitro and in vivo. [161Tb]-PSMA-617 was found to have a radiochemical yield of 97.99 ± 2.01% and was hydrophilic. [161Tb]-PSMA-617 was also shown to have good stability, with a radiochemical yield of over 95% up to 72 hours. In vitro, [161Tb]-PSMA-617 showed a cytotoxic effect on LNCaP cells but not on PC-3 cells. In vivo, scintigraphy imaging visualized the accumulation of [161Tb]-PSMA-617 in the prostate, kidneys, and bladder.

Conclusions

The results suggest that [161Tb]-PSMA-617 can be an effective radiolabeled agent for the treatment of PSMA positive foci in prostate cancer.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biannual highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

MAIN BODY

This selection of highlights provides commentary on 21 different topics selected by each coauthoring Editorial Board member addressing a variety of aspects ranging from novel radiochemistry to first-in-human application of novel radiopharmaceuticals.

CONCLUSION

Trends in radiochemistry and radiopharmacy are highlighted. Hot topics cover the entire scope of EJNMMI Radiopharmacy and Chemistry, demonstrating the progress in the research field in many aspects.

Study of thulium-167 cyclotron production: a potential medically-relevant radionuclide

Introduction: Targeted Radionuclide Therapy is used for the treatment of tumors in nuclear medicine, while sparing healthy tissues. Its application to cancer treatment is expanding. In particular, Auger-electron emitters potentially exhibit high efficacy in treating either small metastases or single tumor cells due to their short range in tissue. The aim of this paper is to study the feasibility of a large-scale production of thulium-167, an Auger-electron emitter radionuclide, in view of eventual systematic preclinical studies. Methods: Proton-irradiated enriched erbium-167 and erbium-168 oxides were used to measure the production cross sections of thulium-165, thulium-166, thulium-167, and thulium-168 utilizing an 18-MeV medical cyclotron equipped with a Beam Transport Line (BTL) at the Bern medical cyclotron laboratory. The comparison between the experimental and the TENDL 2021 theoretical cross-section results were in good agreement. Additional experiments were performed to assess the production yields of thulium radioisotopes in the BTL. Thulium-167 production yield was also measured irradiating five different target materials (167 Er 2 O 3, 168 Er 2 O 3, nat Tm 2 O 3, nat Yb 2 O 3, 171 Yb 2 O 3) with proton beams up to 63 MeV at the Injector II cyclotron of Paul Scherrer Institute. Results and Discussion: Our experiments showed that an 8-h irradiation of enriched ytterbium-171 oxide produced about 420 MBq of thulium-167 with a radionuclidic purity of 99.95% after 5 days of cooling time with a proton beam of about 53 MeV. Larger activities of thulium-167 can be achieved using enriched erbium-168 oxide with a 23-MeV proton beam, obtaining about 1 GBq after 8-h irradiation with a radionuclidic purity of < 99.5% 5 days post end of bombardment.

Au@109Pd core-shell nanoparticle conjugated to trastuzumab for the therapy of HER2+ cancers: studies on the applicability of 109Pd/109mAg in vivo generator in combined β- auger electron therapy

BACKGROUND

In radionuclide therapy, to enhance therapeutic efficacy, an intriguing alternative is to ensure the simultaneous implementation of low- and high-LET radiation emitted from a one radionuclide. In the present study, we introduce the concept of utilizing 109Pd (T1/2 = 13.7 h) in the form of a 109Pd/109mAg in vivo generator. In this system, 109Pd emits beta particles of medium energy, while 109mAg releases a cascade of conversion and Auger electrons. 109Pd was utilized in the form of 15 nm gold nanoparticles, which were coated with a monolayer of 109Pd. In this system, the 109Pd atoms are on the surface of the nanoparticle, while the 109mAg atoms generated in the decay reaction possess the capability for unhindered emission of Auger electrons.

RESULTS

109Pd, obtained through neutron irradiation of natural palladium, was deposited onto 15-nm gold nanoparticles, exceeding a efficiency rate of 95%. In contrast to previously published data on in vivo generators based on chelators, where the daughter radionuclide diffuses away from the molecules, daughter radionuclide 109mAg remains on the surface of gold nanoparticles after the decay of 109Pd. To obtain a radiobioconjugate with an affinity for HER2 receptors, polyethylene glycol chains and the monoclonal antibody trastuzumab were attached to the Au@Pd nanoparticles. The synthesized bioconjugate contained an average of 9.5 trastuzumab molecules per one nanoparticle. In vitro cell studies indicated specific binding of the Au@109Pd-PEG-trastuzumab radiobioconjugate to the HER2 receptor on SKOV-3 cells, resulting in 90% internalization. Confocal images illustrated the accumulation of Au@109Pd-PEG-trastuzumab in the perinuclear area surrounding the cell nucleus. Despite the lack of nuclear localization, which is necessary to achieve an effective cytotoxic effect of Auger electrons, a substantial cytotoxic effect, significantly greater than that of pure β- and pure Auger electron emitters was observed. We hypothesize that in the studied system, the cytotoxic effect of the Auger electrons could have also occurred through the damage to the cell's nuclear membrane by Auger electrons emitted from nanoparticles accumulated in the perinuclear area.

CONCLUSION

The obtained results show that trastuzumab-functionalized 109Pd-labeled nanoparticles can be suitable for the application in combined β--Auger electron targeted radionuclide therapy. Due to both components decay (β- and conversion/Auger electrons), the 109Pd/109mAg in vivo generator presents unique potential in this field. Despite the lack of nuclear localization, which is highly required for efficient Auger electron therapy, an adequate cytotoxic effect was attained.

Albumin-Binding and Conventional PSMA Ligands in Combination with 161Tb: Biodistribution, Dosimetry, and Preclinical Therapy

The favorable decay characteristics of 161Tb attracted the interest of clinicians in using this novel radionuclide for radioligand therapy (RLT). 161Tb decays with a similar half-life to 177Lu, but beyond the emission of β--particles and γ-rays, 161Tb also emits conversion and Auger electrons, which may be particularly effective to eliminate micrometastases. The aim of this study was to compare the dosimetry and therapeutic efficacy of 161Tb and 177Lu in tumor-bearing mice using SibuDAB and PSMA-I&T, which differ in their blood residence time and tumor uptake. Methods: [161Tb]Tb-SibuDAB and [161Tb]Tb-PSMA-I&T were evaluated in vitro and investigated in biodistribution, imaging, and therapy studies using PC-3 PIP tumor-bearing mice. The 177Lu-labeled counterparts served for dose calculations and comparison of therapeutic efficacy. The tolerability of RLT in mice was monitored on the basis of body mass, blood plasma parameters, blood cell counts, and the histology of relevant organs and tissues. Results: The prostate-specific membrane antigen (PSMA)-targeting radioligands, irrespective of whether labeled with 161Tb or 177Lu, showed similar in vitro data and comparable tissue distribution profiles. As a result of the albumin-binding properties, [161Tb]Tb/[177Lu]Lu-SibuDAB had an enhanced blood residence time and higher tumor uptake (62%-69% injected activity per gram at 24 h after injection) than [161Tb]Tb/[177Lu]Lu-PSMA-I&T (30%-35% injected activity per gram at 24 h after injection). [161Tb]Tb-SibuDAB inhibited tumor growth more effectively than [161Tb]Tb-PSMA-I&T, as can be ascribed to its 4-fold increased absorbed tumor dose. At any of the applied activities, the 161Tb-based radioligands were therapeutically more effective than their 177Lu-labeled counterparts, as agreed with the approximately 40% increased tumor dose of 161Tb compared with that of 177Lu. Under the given experimental conditions, no obvious adverse events were observed. Conclusion: The data of this study indicate the promising potential of 161Tb in combination with SibuDAB for RLT of prostate cancer. Future clinical studies using 161Tb-based RLT will shed light on a potential clinical benefit of 161Tb over 177Lu.

Membrane and Nuclear Absorbed Doses from 177Lu and 161Tb in Tumor Clusters: Effect of Cellular Heterogeneity and Potential Benefit of Dual Targeting-A Monte Carlo Study

Early use of targeted radionuclide therapy to eradicate tumor cell clusters and micrometastases might offer cure. However, there is a need to select appropriate radionuclides and assess the potential impact of heterogeneous targeting. Methods: The Monte Carlo code CELLDOSE was used to assess membrane and nuclear absorbed doses from 177Lu and 161Tb (β--emitter with additional conversion and Auger electrons) in a cluster of 19 cells (14-μm diameter, 10-μm nucleus). The radionuclide distributions considered were cell surface, intracytoplasmic, or intranuclear, with 1,436 MeV released per labeled cell. To model heterogeneous targeting, 4 of the 19 cells were unlabeled, their position being stochastically determined. We simulated situations of single targeting, as well as dual targeting, with the 2 radiopharmaceuticals aiming at different targets. Results: 161Tb delivered 2- to 6-fold higher absorbed doses to cell membranes and 2- to 3-fold higher nuclear doses than 177Lu. When all 19 cells were targeted, membrane and nuclear absorbed doses were dependent mainly on radionuclide location. With cell surface location, membrane absorbed doses were substantially higher than nuclear absorbed doses, both with 177Lu (38-41 vs. 4.7-7.2 Gy) and with 161Tb (237-244 vs. 9.8-15.1 Gy). However, when 4 cells were not targeted by the cell surface radiopharmaceutical, the membranes of these cells received on average only 9.6% of the 177Lu absorbed dose and 2.9% of the 161Tb dose, compared with a cluster with uniform cell targeting, whereas the impact on nuclear absorbed doses was moderate. With an intranuclear radionuclide location, the nuclei of unlabeled cells received only 17% of the 177Lu absorbed dose and 10.8% of the 161Tb dose, compared with situations with uniform targeting. With an intracytoplasmic location, nuclear and membrane absorbed doses to unlabeled cells were one half to one quarter those obtained with uniform targeting, both for 177Lu and for 161Tb. Dual targeting was beneficial in minimizing absorbed dose heterogeneities. Conclusion: To eradicate tumor cell clusters, 161Tb may be a better candidate than 177Lu. Heterogeneous cell targeting can lead to substantial heterogeneities in absorbed doses. Dual targeting was helpful in reducing dose heterogeneity and should be explored in preclinical and clinical studies.

Can we reach suitable 161Tb purity for medical applications using the 160Gd(d,n) reaction?

Terbium is a chemical element that has several radioactive isotopes with suitable physical characteristics to be used in medical applications either for imaging or for therapy. This makes terbium a promising element to implement the theranostic approach. For therapeutic applications, 161Tb (T1/2 = 6.89 d) is suitable for targeted β-therapy. The main production route is through neutron capture reaction in nuclear reactors. In this work, we explored an alternative production route, the 160Gd(d,n)161Tb reaction. We have measured its production cross-section as well as those of possible co-produced contaminants, with a special focus on 160Tb (T1/2 = 72.3 d). To achieve this, cross-section measurements were made from natural gadolinium target. Production yields of 10.3 MBq/μA/h for the 161Tb and 1.5 MBq/μA/h for the 160Tb were obtained at 20 MeV. A161Tb radionuclidic purity of 86% was achieved over the 8 MeV-20 MeV energy range. The co-production of other terbium isotopes limits the interest of using higher energies. Based on the limited purity of 161Tb using the 160Gd(d,n)161Tb reaction, we conclude that it is not a production route suitable for medical applications. Although, this may be reconsidered when mass separation technique with high efficiency will be available.

Emerging Role of Nuclear Medicine in Prostate Cancer: Current State and Future Perspectives

Prostate cancer is the most frequent epithelial neoplasia after skin cancer in men starting from 50 years and prostate-specific antigen (PSA) dosage can be used as an early screening tool. Prostate cancer imaging includes several radiological modalities, ranging from ultrasonography, computed tomography (CT), and magnetic resonance to nuclear medicine hybrid techniques such as single-photon emission computed tomography (SPECT)/CT and positron emission tomography (PET)/CT. Innovation in radiopharmaceutical compounds has introduced specific tracers with diagnostic and therapeutic indications, opening the horizons to targeted and very effective clinical care for patients with prostate cancer. The aim of the present review is to illustrate the current knowledge and future perspectives of nuclear medicine, including stand-alone diagnostic techniques and theragnostic approaches, in the clinical management of patients with prostate cancer from initial staging to advanced disease.