In Vivo Radionuclide Generators for Diagnostics and Therapy

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.

Prostate-specific Membrane Antigen: Alpha-labeled Radiopharmaceuticals

Novel prostate-specific membrane antigen (PSMA) ligands labeled with α-emitting radionuclides are sparking a growing interest in prostate cancer treatment. These targeted alpha therapies (TATs) have attractive physical properties that deem them effective in progressive metastatic castrate-resistant prostate cancer (mCRPC). Among the PSMA TAT radiopharmaceuticals, [225Ac]Ac-PSMA has been used extensively on a compassionate basis and is currently undergoing phase I trials. Notably, TAT has the potential to improve quality of life and has favorable antitumor activity and outcomes in multiple scenarios other than in mCRPC. In addition, resistance mechanisms to TAT may be amenable to combination therapies.

A new evaluation of the decay data for 166Ho

The β--emitter 166Ho is of interest as a potential radiolabel for therapeutic medical applications. A new decay data evaluation for 166Ho has been performed using the Decay Data Evaluation Project (DDEP) methodology. New recommended values for the half-life, γ-ray emission probabilities, β-- branching ratios, and other relevant nuclear and atomic data are provided. This paper provides a summary of the evaluation; the complete set of recommended data tables and detailed evaluator comments are available at the DDEP website.

Alpha-Emitting Radionuclides: Current Status and Future Perspectives

The use of radionuclides for targeted endoradiotherapy is a rapidly growing field in oncology. In particular, the focus on the biological effects of different radiation qualities is an important factor in understanding and implementing new therapies. Together with the combined approach of imaging and therapy, therapeutic nuclear medicine has recently made great progress. A particular area of research is the use of alpha-emitting radionuclides, which have unique physical properties associated with outstanding advantages, e.g., for single tumor cell targeting. Here, recent results and open questions regarding the production of alpha-emitting isotopes as well as their chemical combination with carrier molecules and clinical experience from compassionate use reports and clinical trials are discussed.

Production and purification of 43Sc and 47Sc from enriched [46Ti]TiO2 and [50Ti]TiO2 targets

The radioscandium isotopes, 43Sc and 47Sc, compose a promising elementally matched theranostic pair that can be used for the development of imaging and therapeutic radiopharmaceuticals with identical structures. This study aimed to investigate the production of high radionuclidic purity 43Sc from enriched [46Ti]TiO2 targets and 47Sc from enriched [50Ti]TiO2 targets and establish a target recycling technique. Enriched [46Ti]TiO2 targets were irradiated with 18 MeV protons, and enriched [50Ti]TiO2 targets were bombarded with 24 MeV protons. 43Sc and 47Sc were purified using ion chromatography attaining recovery yields of 91.7 ± 7.4% and 89.9 ± 3.9%, respectively. The average radionuclidic purity for 43Sc was 98.8 ± 0.3% and for 47Sc 91.5 ± 0.6%, while the average recovery of enriched titanium target material was 96 ± 4.0%. The highest apparent molar activity for [43Sc]Sc-DOTA was 23.2 GBq/µmol and 3.39 GBq/µmol for [47Sc]Sc-DOTA. This work demonstrates the feasibility of using enriched recycled [46Ti]TiO2 and [50Ti]TiO2 targets to produce high purity 43Sc and 47Sc as an elementally matched theranostic isotope pair.

Evaluation of 134Ce/134La as a PET Imaging Theranostic Pair for 225Ac α-Radiotherapeutics

225Ac-targeted α-radiotherapy is a promising approach to treating malignancies, including prostate cancer. However, α-emitting isotopes are difficult to image because of low administered activities and a low fraction of suitable γ-emissions. The in vivo generator 134Ce/134La has been proposed as a potential PET imaging surrogate for the therapeutic nuclides 225Ac and 227Th. In this report, we detail efficient radiolabeling methods using the 225Ac-chelators DOTA and MACROPA. These methods were applied to radiolabeling of prostate cancer imaging agents, including PSMA-617 and MACROPA-PEG4-YS5, for evaluation of their in vivo pharmacokinetic characteristics and comparison to the corresponding 225Ac analogs. Methods: Radiolabeling was performed by mixing DOTA/MACROPA chelates with 134Ce/134La in NH4OAc, pH 8.0, at room temperature, and radiochemical yields were monitored by radio-thin-layer chromatography. In vivo biodistributions of 134Ce-DOTA/MACROPA.NH2 complexes were assayed through dynamic small-animal PET/CT imaging and ex vivo biodistribution studies over 1 h in healthy C57BL/6 mice, compared with free 134CeCl3 In vivo, preclinical imaging of 134Ce-PSMA-617 and 134Ce-MACROPA-PEG4-YS5 was performed on 22Rv1 tumor-bearing male nu/nu-mice. Ex vivo biodistribution was performed for 134Ce/225Ac-MACROPA-PEG4-YS5 conjugates. Results: 134Ce-MACROPA.NH2 demonstrated near-quantitative labeling with 1:1 ligand-to-metal ratios at room temperature, whereas a 10:1 ligand-to-metal ratio and elevated temperatures were required for DOTA. Rapid urinary excretion and low liver and bone uptake were seen for 134Ce/225Ac-DOTA/MACROPA. NH2 conjugates in comparison to free 134CeCl3 confirmed high in vivo stability. An interesting observation during the radiolabeling of tumor-targeting vectors PSMA-617 and MACROPA-PEG4-YS5-that the daughter 134La was expelled from the chelate after the decay of parent 134Ce-was confirmed through radio-thin-layer chromatography and reverse-phase high-performance liquid chromatography. Both conjugates, 134Ce-PSMA-617 and 134Ce-MACROPA-PEG4-YS5, displayed tumor uptake in 22Rv1 tumor-bearing mice. The ex vivo biodistribution of 134Ce-MACROPA.NH2, 134Ce-DOTA and 134Ce-MACROPA-PEG4-YS5 corroborated well with the respective 225Ac-conjugates. Conclusion: These results demonstrate the PET imaging potential for 134Ce/134La-labeled small-molecule and antibody agents. The similar 225Ac and 134Ce/134La-chemical and pharmacokinetic characteristics suggest that the 134Ce/134La pair may act as a PET imaging surrogate for 225Ac-based radioligand therapies.

Core-shell structured gold nanoparticles as carrier for 166Dy/166Ho in vivo generator

Background

Radionuclide therapy (RNT) has become a very important treatment modality for cancer nowadays. Comparing with other cancer treatment options, sufficient efficacy could be achieved in RNT with lower toxicity. β⁻ emitters are frequently used in RNT due to the long tissue penetration depth of the β⁻ particles. The dysprosium-166/holmium-166 (¹⁶⁶Dy/¹⁶⁶Ho) in vivo generator shows great potential for treating large malignancies due to the long half-life time of the mother nuclide ¹⁶⁶Dy and the emission of high energy β⁻ from the daughter nuclide ¹⁶⁶Ho. However, the internal conversion occurring after β⁻ decay from ¹⁶⁶Dy to ¹⁶⁶Ho could cause the release of about 72% of ¹⁶⁶Ho when ¹⁶⁶Dy is bound to conventional chelators. The aim of this study is to develop a nanoparticle based carrier for ¹⁶⁶Dy/¹⁶⁶Ho in vivo generator such that the loss of the daughter nuclide ¹⁶⁶Ho induced by internal conversion is prevented. To achieve this goal, we radiolabelled platinum-gold bimetallic nanoparticles (PtAuNPs) and core–shell structured gold nanoparticles (AuNPs) with ¹⁶⁶Dy and studied the retention of both ¹⁶⁶Dy and ¹⁶⁶Ho under various conditions.

Results

The ¹⁶⁶Dy was co-reduced with gold and platinum precursor to form the ¹⁶⁶DyAu@AuNPs and ¹⁶⁶DyPtAuNPs. The ¹⁶⁶Dy radiolabelling efficiency was determined to be 60% and 70% for the two types of nanoparticles respectively. The retention of ¹⁶⁶Dy and ¹⁶⁶Ho were tested in MiliQ water or 2.5 mM DTPA for a period of 72 h. In both cases, more than 90% of both ¹⁶⁶Dy and ¹⁶⁶Ho was retained. The results show that the incorporation of ¹⁶⁶Dy in AuNPs can prevent the escape of ¹⁶⁶Ho released due to internal conversion.

Conclusion

We developed a chelator-free radiolabelling method for ¹⁶⁶Dy with good radiolabelling efficiency and very high stability and retention of the daughter nuclide ¹⁶⁶Ho. The results from this study indicate that to avoid the loss of the daughter radionuclides by internal conversion, carriers composed of electron-rich materials should be used.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00170-3.

PET in vivo generators 134Ce and 140Nd on an internalizing monoclonal antibody probe

The in vivo-generator radionuclides ¹⁴⁰Nd (t_1/2 = 3.4 d) and ¹³⁴Ce (t_1/2 = 3.2 d) were used to trace a urokinase-type plasminogen activator (uPA)-targeting mouse monoclonal antibody, ATN-291, in U87 MG xenograft tumor-bearing mice. ATN-291 is known to internalize on the uPA/uPA-receptor pair, making it an appropriate targeting vector for investigating the fate of in vivo generator daughters on internalizing probes. Ante-mortem and post-mortem PET imaging at 120 h post-injection gave no indication of redistribution of the positron emitting daughter nuclides ¹³⁴La and ¹⁴⁰Pr from tumor tissue (p > 0.5). The lack of redistribution indicates that the parent radionuclides ¹³⁴Ce and ¹⁴⁰Nd could be considered as long-lived PET-diagnostic matches to therapeutic radionuclides like ¹⁷⁷Lu, ¹⁶¹Tb and ²²⁵Ac when internalizing bioconjugates are employed.

212Pb: Production Approaches and Targeted Therapy Applications

Over the last decade, targeted alpha therapy has demonstrated its high effectiveness in treating various oncological diseases. Lead-212, with a convenient half-life of 10.64 h, and daughter alpha-emitter short-lived ²¹²Bi (T_1/2 = 1 h), provides the possibility for the synthesis and purification of complex radiopharmaceuticals with minimum loss of radioactivity during preparation. As a benefit for clinical implementation, it can be milked from a radionuclide generator in different ways. The main approaches applied for these purposes are considered and described in this review, including chromatographic, solution, and other techniques to isolate ²¹²Pb from its parent radionuclide. Furthermore, molecules used for lead’s binding and radiochemical features of preparation and stability of compounds labeled with ²¹²Pb are discussed. The results of preclinical studies with an estimation of therapeutic and tolerant doses as well as recently initiated clinical trials of targeted radiopharmaceuticals are presented.

A Whole-Body Physiologically Based Pharmacokinetic Model for Alpha Particle Emitting Bismuth in Rats

Introduction/Aim: α particle emitting bismuth (²¹²Bi) as decay product of ²¹²Pb-labeled pharmaceuticals has been effective in targeted α particle therapy (TAT). Estimating the contribution of ²¹²Bi released from its chelator to the absorbed doses in nontarget tissues is challenging in TAT. Physiologically based pharmacokinetic (PBPK) modeling can help overcome this limitation. Therefore, a whole-body ²¹²Bi-PBPK model was developed to describe the pharmacokinetics (PKs) of ²¹²Bi in rats. Materials and Methods: The rat ²¹²Bi-PBPK model was implemented using the modeling software SAAM II with data and parameter values from the literature. Besides other mechanisms, ²¹²Bi interactions with red blood cells, high molecular weight plasma protein, and intracellular biological thiols are described. Important PK parameters were fitted to time-activity data. Absorbed dose coefficients (ADCs) were calculated for injecting 0.774 fmol of ²¹²Bi. Results: ²¹²Bi uptake rates of liver, bone, small intestine, bone marrow, skin, and muscle were (0.86 ± 0.13), (3.85 ± 0.63), (0.27 ± 0.05), (1.44 ± 0.29), (0.04 ± 0.01), and (0.007 ± 0.007) per min with corresponding ADCs of 0.09, 0.03, 0.03, 0.07, 0.01, and 0.003 mGy/kBq, respectively. An ADC of 0.70 mGy/kBq was determined for kidneys. Conclusion: Kidneys are the dose-limiting organs in ²¹²Bi-based TAT. The ²¹²Bi-PBPK model is an effective tool to investigate the ²¹²Bi biodistribution in murine models. Integrating the ²¹²Bi-PBPK model into other murine and human PBPK models of α particle generators can help study the efficacy and safety of TAT.

An octadentate bis(semicarbazone) macrocycle: a potential chelator for lead and bismuth radiopharmaceuticals

A variant of 1,4,7,10-tetraazacyclododecane (cyclen) bearing two semicarbazone pendant groups has been prepared. The octadentate ligand forms complexes with Bi3+ and Pb2+. X-ray crystallography showed that the neutral ligand provides an eight-coordinate environment for both metal ions and intermolecular hydrogen bond interactions have influenced the coordination environments of both complexes in the solid state. NMR spectroscopy revealed a fluxional environment for both complexes. The ligand was radiolabeled with the α-emitting radioactive isotope 213Bi3+, which is used in systemic targeted radiotherapy. The resulting complex was stable in serum for at least 90 min (two decay half-lives). The Pb2+ complex has reasonably fast kinetics of formation (t1/2 = 20 min) at 25 °C and pH 7.4. The Bi3+ and Pb2+ complexes show kinetic stability in 1.2 M HCl (half-lives of 214 min and 47 min, respectively). This is the first description of a macrocycle bearing semicarbazone pendant groups and its utility in coordinating main group metals, specifically those with radiotherapeutic potential.

Targeted alpha therapy for chronic lymphocytic leukaemia and non-Hodgkin's lymphoma with the anti-CD37 radioimmunoconjugate 212Pb-NNV003

Relapse of chronic lymphocytic leukaemia and non-Hodgkin's lymphoma after standard of care treatment is common and new therapies are needed. The targeted alpha therapy with 212Pb-NNV003 presented in this study combines cytotoxic α-particles from 212Pb, with the anti-CD37 antibody NNV003, targeting B-cell malignancies. The goal of this study was to explore 212Pb-NNV003 for treatment of CD37 positive chronic lymphocytic leukaemia and non-Hodgkin's lymphoma in preclinical mouse models.An anti-proliferative effect of 212Pb-NNV003 was observed in both chronic lymphocytic leukaemia (MEC-2) and Burkitt's lymphoma (Daudi) cells in vitro. In biodistribution experiments, accumulation of 212Pb-NNV003 was 23%ID/g and 16%ID/g in Daudi and MEC-2 tumours 24 h post injection. In two intravenous animal models 90% of the mice treated with a single injection of 212Pb-NNV003 were alive 28 weeks post cell injection. Median survival times of control groups were 5-9 weeks. There was no significant difference between different specific activities of 212Pb-NNV003 with regards to therapeutic effect or toxicity. For therapeutically effective activities, a transient haematological toxicity was observed. This study shows that 212Pb-NNV003 is effective and safe in preclinical models of CD37 positive chronic lymphocytic leukaemia and non-Hodgkin's lymphoma, warranting future clinical testing.

Review of Therapeutic Applications of Radiolabeled Functional Nanomaterials

In the last two decades, various nanomaterials have attracted increasing attention in medical science owing to their unique physical and chemical characteristics. Incorporating radionuclides into conventionally used nanomaterials can confer useful additional properties compared to the original material. Therefore, various radionuclides have been used to synthesize functional nanomaterials for biomedical applications. In particular, several α- or β-emitter-labeled organic and inorganic nanoparticles have been extensively investigated for efficient and targeted cancer treatment. This article reviews recent progress in cancer therapy using radiolabeled nanomaterials including inorganic, polymeric, and carbon-based materials and liposomes. We first provide an overview of radiolabeling methods for preparing anticancer agents that have been investigated recently in preclinical studies. Next, we discuss the therapeutic applications and effectiveness of α- or β-emitter-incorporated nanomaterials in animal models and the emerging possibilities of these nanomaterials in cancer therapy.

Radionuclide generators: the prospect of availing PET radiotracers to meet current clinical needs and future research demands

Targeted molecular imaging with positron emission tomography (PET) constitutes a successful technique for detecting and diagnosing disease conditions promptly and accurately, and for effectively prognosticating outcomes and treating patients with a tailored and more individualized intervention. In order to expand the success of PET in nuclear medicine, it is important to assure access to radiotracers of desired quantities and qualities. In this context, the benefit of accessing PET radiotracers through a radionuclide generator (RNG) cannot be overstated, as generators offer the potential of enriching the PET radiotracer arsenal at the medical centers both with and without onsite cyclotrons. While RNG technology to avail PET tracers is in its infancy, their use is expected to revitalize current PET practices and seems poised to broaden the palette of PET in nuclear medicine in the foreseeable future. In this review, we discuss the principles of RNGs, assess major parent/daughter pairs of interest for PET, RNGs currently in use in clinical PET, and identify the potentially useful RNGs which have made substantial progress or are likely to be used in daily clinical practices in the near future. Availability of the parent radionuclides required for PET RNGs is an important criterion and hence their production will also be reviewed. This overview outlines a critical assessment of RNGs to avail PET tracers, the contemporary status of RNGs, and key challenges and apertures to the near future.

Ensuring effective and sustainable radionuclide delivery and its impact on the development of nuclear medicine in the developing world with special reference to Nigeria

Recent activities of Boko Haram, a local extremist group in Nigeria, raise concerns about a nuclear terrorist attack. Whereas nuclear medicine (NM) relies on the timely delivery of radioactive sources, a robust security structure that assures public safety is the backbone for its beneficial use. NM radionuclides have short half-lives and carry an insignificant risk for acts of terrorism. Yet, their importation and delivery in Nigeria receive undue scrutiny in a bid to implement a strict nuclear security regime. These actions prevent timely delivery of radionuclides with direct consequences on quality and economic viability of nuclear medicine. There have been no accounts of terrorist acts accomplished with NM radionuclides. Thus, it is important the NM community question the current approach that has contributed to the loss of NM services in Nigeria and proposes a more logical strategy for securing their supply. We also highlight the need for developing local pragmatic solutions when implementing global recommendations in developing countries.

Selectivity Conversion of Protease Inhibitory Antibodies

Solid tumors are inherently difficult to treat because of large regions of hypoxia and are often chemotherapy- or radiotherapy-resistant. It seems that cancer stem cells reside in hypoxic and adjacent necrotic tumor areas. Therefore, new treatments that are highly selective for tumors and can eradicate cells in both hypoxic and necrotic tumor regions are desirable. Antibody α-radioconjugates couple an α-emitting radionuclide with the specificity of a tumor-targeting monoclonal antibody. The large mass and energy of α-particles result in radiation dose delivery within a smaller area independent of oxygen concentration, thus matching key criteria for killing hypoxic tumor cells. With advances in radionuclide production and chelation chemistry, α-radioconjugate therapy is regaining interest as a cancer therapy. Here, we will review current literature examining radioconjugate therapy specifically targeting necrotic and hypoxic tumor cells and outline how α-radioconjugate therapy could be used to treat tumor regions harboring more resistant cancer cell types.

Statement of Significance

Tumor-targeting antibodies are excellent vehicles for the delivery of toxic payloads directly to the tumor site. Tumor hypoxia and necrosis promote treatment recurrence, resistance, and metastasis. Targeting these areas with antibody α-radioconjugates would aid in overcoming treatment resistance.

Selectivity Conversion of Protease Inhibitory Antibodies

Background: Proteases are one of the largest pharmaceutical targets for drug developments. Their dysregulations result in a wide variety of diseases. Because proteolytic networks usually consist of protease family members that share high structural and catalytic homology, distinguishing them using small molecule inhibitors is often challenging. To achieve specific inhibition, this study described a novel approach for the generation of protease inhibitory antibodies. As a proof of concept, we aimed to convert a matrix metalloproteinase (MMP)-14 specific inhibitor to MMP-9 specific inhibitory antibodies with high selectivity. Methods: An error-prone single-chain Fv (scFv) library of an MMP-14 inhibitor 3A2 was generated for yeast surface display. A dual-color competitive FACS was developed for selection on MMP-9 catalytic domain (cdMMP-9) and counter-selection on cdMMP-14 simultaneously, which were fused/conjugated with different fluorophores. Isolated MMP-9 inhibitory scFvs were biochemically characterized by inhibition assays on MMP-2/-9/-12/-14, proteolytic stability tests, inhibition mode determination, competitive ELISA with TIMP-2 (a native inhibitor of MMPs), and paratope mutagenesis assays. Results: We converted an MMP-14 specific inhibitor 3A2 into a panel of MMP-9 specific inhibitory antibodies with dramatic selectivity shifts of 690-4,500 folds. Isolated scFvs inhibited cdMMP-9 at nM potency with high selectivity over MMP-2/-12/-14 and exhibited decent proteolytic stability. Biochemical characterizations revealed that these scFvs were competitive inhibitors binding to cdMMP-9 near its reaction cleft via their CDR-H3s. Conclusions: This study developed a novel approach able to convert the selectivity of inhibitory antibodies among closely related protease family members. This methodology can be directly applied for mAbs inhibiting many proteases of biomedical importance.

Pretargeting in nuclear imaging and radionuclide therapy: Improving efficacy of theranostics and nanomedicines

Pretargeted nuclear imaging and radiotherapy have recently attracted increasing attention for diagnosis and treatment of cancer with nanomedicines. This is because it conceptually offers better imaging contrast and therapeutic efficiency while reducing the dose to radiosensitive tissues compared to conventional strategies. In conventional imaging and radiotherapy, a directly radiolabeled nano-sized vector is administered and allowed to accumulate in the tumor, typically on a timescale of several days. In contrast, pretargeting is based on a two-step approach. First, a tumor-accumulating vector carrying a tag is administered followed by injection of a fast clearing radiolabeled agent that rapidly recognizes the tag of the tumor-bound vector in vivo. Therefore, pretargeting circumvents the use of long-lived radionuclides that is a necessity for sufficient tumor accumulation and target-to-background ratios using conventional approaches. In this review, we give an overview of recent advances in pretargeted imaging strategies. We will critically reflect on the advantages and disadvantages of current state-of-the-art conventional imaging approaches and compare them to pretargeted strategies. We will discuss the pretargeted imaging concept and the involved chemistry. Finally, we will discuss the steps forward in respect to clinical translation, and how pretargeted strategies could be applied to improve state-of-the-art radiotherapeutic approaches.

Progress in Targeted Alpha-Particle Therapy. What We Learned about Recoils Release from In Vivo Generators

This review summarizes recent progress and developments as well as the most important pitfalls in targeted alpha-particle therapy, covering single alpha-particle emitters as well as in vivo alpha-particle generators. It discusses the production of radionuclides like ²¹¹At, ²²³Ra, ²²⁵Ac/²¹³Bi, labelling and delivery employing various targeting vectors (small molecules, chelators for alpha-emitting nuclides and their biomolecular targets as well as nanocarriers), general radiopharmaceutical issues, preclinical studies, and clinical trials including the possibilities of therapy prognosis and follow-up imaging. Special attention is given to the nuclear recoil effect and its impacts on the possible use of alpha emitters for cancer treatment, proper dose estimation, and labelling chemistry. The most recent and important achievements in the development of alpha emitters carrying vectors for preclinical and clinical use are highlighted along with an outlook for future developments.

Neodymium-140 DOTA-LM3: Evaluation of an In Vivo Generator for PET with a Non-Internalizing Vector

¹⁴⁰Nd (t_1/2 = 3.4 days), owing to its short-lived positron emitting daughter ¹⁴⁰Pr (t_1/2 = 3.4 min), has promise as an in vivo generator for positron emission tomography (PET). However, the electron capture decay of ¹⁴⁰Nd is chemically disruptive to macrocycle-based radiolabeling, meaning that an in vivo redistribution of the daughter ¹⁴⁰Pr is expected before positron emission. The purpose of this study was to determine how the delayed positron from the de-labeled ¹⁴⁰Pr affects preclinical imaging with ¹⁴⁰Nd. To explore the effect, ¹⁴⁰Nd was produced at CERN-ISOLDE, reacted with the somatostatin analogue, DOTA-LM3 (1,4,7,10- tetraazacyclododecane, 1,4,7- tri acetic acid, 10- acetamide N - p-Cl-Phecyclo(d-Cys-Tyr-d-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)d-Tyr-NH2) and injected into H727 xenograft bearing mice. Comparative pre- and post-mortem PET imaging at 16 h postinjection was used to quantify the in vivo redistribution of ¹⁴⁰Pr following ¹⁴⁰Nd decay. The somatostatin receptor-positive pancreas exhibited the highest tissue accumulation of ¹⁴⁰Nd-DOTA-LM3 (13% ID/g at 16 h) coupled with the largest observed redistribution rate, where 56 ± 7% (n = 4, mean ± SD) of the in situ produced ¹⁴⁰Pr washed out of the pancreas before decay. Contrastingly, the liver, spleen, and lungs acted as strong sink organs for free ¹⁴⁰Pr³⁺. Based upon these results, we conclude that ¹⁴⁰Nd imaging with a non-internalizing vector convolutes the biodistribution of the tracer with the accumulation pattern of free ¹⁴⁰Pr. This redistribution phenomenon may show promise as a probe of the cellular interaction with the vector, such as in determining tissue dependent internalization behavior.