Development of [225Ac]Ac-PSMA-I&T for Targeted Alpha Therapy According to GMP Guidelines for Treatment of mCRPC

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

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

Ac-225 radiochemistry through the lens of [225Ac]Ac-DOTA-TATE

BACKGROUND

Targeted alpha therapy with Ac-225 showed to be effective in treating metastatic cancers. However, the complex decay chain requires optimized radiolabeling and quality control. This study aims to determine critical parameters and establish optimal labeling and accurate measuring techniques for radiochemical yield and purity with DOTA-TATE as a model molecule. Ac-225 sources were analyzed for metals (ΣFe, Zn, Cu) and quantified by UPLC. Optimization of radiolabeling kinetics for clinical conditions was performed in regards to temperature (20-90 °C), heating time (5-60 min), pH (2.5-10, with/without excess of metal ions), buffers, quenchers, volume (0.1-10 mL) and molar activity (90-540 kBq/nmol). The quality control was investigated using radio-TLC/HPLC by changing gradient to evaluate peak separation, radiolysed peptide and impurity separation.

RESULTS

Metal ingrowth was observed in Ac-225 stocks (n = 3), (time of arrival: 17.9, 36.8 and 101.4 nmol per 10 MBq). Optimal radiochemical yields were achieved with > 80 °C (20 min) at pH 8.5 (15 mM TRIS) up to 270 kBq. Labeling at a high pH showed a higher RCY, even in presence of an excess of metals. High stability (RCP > 90%) was achieved after addition of quenchers (cysteine, methionine, ascorbate, histidine, or gentisic acid (35 mM)) up to 24 h. For optimal determination of the radiochemical purity (indirect HPLC) fifty fractions are required.

CONCLUSION

The quality of Ac-225 labeled DOTA-radiopharmaceuticals is highly dependent on the pH and stabilization (buffer/quencher). Within this research it is demonstrated that optimized quality control methods and accurate measurement of the radiolabeling kinetics are crucial to ensure safe implementation for patient treatment.

Gut microbiota, metabolites, and cytokines in relation to the risk of prostate cancer in the Asian population

Purpose

Studies have shown that gut microbiota is involved in the tumorigenesis and development of prostate cancer. We aimed to perform a comprehensive analysis of causal associations of gut microbiota, metabolites, and cytokines with prostate cancer in the Asian population.

Patients and methods

Genome-wide association study (GWAS) summary datasets were collected from the public databases. There were 418 bacterial traits, 452 metabolites, 91 cytokines, 5408 cases of prostate cancer from East Asia, and 109,347 controls included. Mendelian randomization (MR) analyses were performed to investigate their causal relationships. Sensitivity analyses were conducted to test the reliability of MR results. Furthermore, the FinnGen database was used to assess the generalizability of our findings based on Asians.

Results

There were a total of 17 bacterial traits, 28 metabolites (including 2 microbiota-associated metabolites), and 9 cytokines to be significantly associated with prostate cancer in Asians (P < 0.05). Further MR analyses of these positive results indicated that G_Ruminococcaceae UCG014/TNFSF10 axis, G_Anaerofilum/TNFRSF14 axis, G_Erysipelotrichaceae UCG003/TNFSF10 axis, and P_Proteobacteria/cholesterol axis were key signaling pathways involved in the progression of prostate cancer. Notably, G_Ruminococcaceae UCG014/TNFSF10 axis and G_Anaerofilum/TNFRSF14 axis were found to act as protective factors, while the other two signaling axes played a crucial role in promoting the progression of prostate cancer. Sensitivity analyses further confirmed the reliability of our findings. Using the European population as outcome, we further assessed the generalizability of our conclusions and found limited applicability to Europeans.

Conclusions

We found that there were causal associations of gut microbiota, metabolites, and cytokines with prostate cancer in Asians. The causal effects of gut microbiota on prostate cancer were partially mediated by metabolites and cytokines. These findings might contribute to the development of new therapeutic strategies for prostate cancer.

Improvement of End-of-Synthesis Radiochemical Purity of 177Lu-DOTA-PSMA-Ligands with Alternative Synthesis Approaches: Conversion Upswing and Side-Products Minimization

BACKGROUND

Radiochemical purity is a key criterion for the quality of radiopharmaceuticals used in clinical practice. The joint improvement of analytical methods capable of identifying related radiochemical impurities and determining the actual radiochemical purity, as well as the improvement of synthesis methods to minimize the formation of possible radiochemical impurities, is integral to the implementation of high-tech nuclear medicine procedures. PSMA-targeted radionuclide therapy with lutetium-177 has emerged as an effective treatment option for prostate cancer, and [177Lu]Lu-PSMA-617 and [177Lu]Lu-PSMAI&T have achieved global recognition as viable radiopharmaceuticals. Recently, it was shown that specific radiochemical impurities can form during the synthesis of [177Lu]Lu-PSMA-617 because of a spontaneous, thermally mediated condensation of the Glu-C(O)-Lys fragment, resulting in the formation of three different cyclic forms (with no affinity for PSMA). During this study, we identified another impurity, a product of detachment of the Glu-CO fragment from PSMA-617, caused by heating. The total content of all four thermally mediated degradation products may reach 9-11% during classical incubation for 30 min at 95 °C, reducing the radiochemical purity to an unacceptable level (albeit with high levels of radiochemical conversion). It is reasonable to assume that the formation of similar impurities is characteristic of all PSMA-specific vectors that contain Glu-C(O)-Lys pharmacophores. Because the formation of these impurities directly depends on the temperature and incubation time, to reduce their content in the reaction mixture at the end of the synthesis, it is necessary to select conditions to achieve a high level of radiochemical conversion for the minimum possible time and/or at the minimum sufficient temperature.

METHODS

In this study, using [177Lu]Lu-PSMA-617 as an example, we evaluated the efficiency of alternative methods of synthesis with microwave heating and co-solvent (ethanol) addition to ensure radiochemical yield and radiochemical purity in the shortest possible time and at the minimum necessary and sufficient synthesis temperature.

RESULTS

Both approaches achieved a significant reduction in the impurities content, while achieving satisfactory synthesis yields in a short time. In addition to improving the synthesis parameters and radiochemical purity, the use of microwave heating and the addition of ethanol reduces the negative influence of other auxiliaries on labeling kinetics. Notably, the addition of ethanol under certain conditions allowed [177Lu]Lu-PSMA-617 to be synthesized at room temperature for only 10 min. This makes it possible to achieve exceptionally high real radiochemical purity of the preparations, determined only by the quality of the original precursor. The approaches considered in this study can be successfully applied to improve the synthesis process and quality parameters of the finished product, both for known radiopharmaceuticals and for those under development.

Optimization Processes of Clinical Chelation-Based Radiopharmaceuticals for Pathway-Directed Targeted Radionuclide Therapy in Oncology

Targeted radionuclide therapy (TRT) for internal pathway-directed treatment is a game changer for precision medicine. TRT improves tumor control while minimizing damage to healthy tissue and extends the survival for patients with cancer. The application of theranostic-paired TRT along with cellular phenotype and genotype correlative analysis has the potential for malignant disease management. Chelation chemistry is essential for the development of theranostic-paired radiopharmaceuticals for TRT. Among image-guided TRT, 68Ga and 99mTc are the current standards for diagnostic radionuclides, while 177Lu and 225Ac have shown great promise for β- and α-TRT, respectively. Their long half-lives, potent radiobiology, favorable decay schemes, and ability to form stable chelation conjugates make them ideal for both manufacturing and clinical use. The current challenges include optimizing radionuclide production processes, coordinating chelation chemistry stability of theranostic-paired isotopes to reduce free daughters [this pertains to 225Ac daughters 221Fr and 213Bi]-induced tissue toxicity, and improving the modeling of micro dosimetry to refine dose-response evaluation. The empirical approach to TRT delivery is based on standard radionuclide administered activity levels, although clinical trials have revealed inconsistent outcomes and normal-tissue toxicities despite equivalent administered activities. This review presents the latest optimization methods for chelation-based theranostic radiopharmaceuticals, advancements in micro-dosimetry, and SPECT/CT technologies for quantifying whole-body uptake and monitoring therapeutic response as well as cytogenetic correlative analyses.

Design of 225Ac-PSMA for targeted alpha therapy in prostate cancer

The first alpha emitting radiopharmaceutical, 223RaCl2, radium dichloride, was approved 10 years ago into the clinical armament of treating bone metastases in metastatic castration-resistant prostate cancer (mCRPC). In addition to this, the first beta-emitting radionuclide Lu-177 chelated with a prostate-specific membrane antigen (PSMA) compound, got last year its marketing approval for the third line treatment of mCRPC. Therefore, there is great excitement about combining alpha-emitters and prostate cancer targeting PSMA compounds. This review describes the clinical history of alpha-emitting PSMA in treating mCRPC. Here, we present the potential, current status, and opportunities for 225Ac-PSMA therapy. The work reviews the basic concepts, current treatment outcome, and toxicity, and areas requiring further investigations such as dosimetric aspects in clinical studies covering more than 400 patients. In general, approximately two-thirds of the patients benefit from this third-line therapy. There is also successful evidence of using 225Ac-PSMA in the second-line of prostate cancer management. The future potential of 225Ac-PSMA therapy and targeted alpha therapy (TAT) of cancer in general is enormous. According to our overview the clinical experience with 225Ac-PSMA therapy to date has shown great benefit and physicians dedicated to theragnostics are anxiously waiting for new applications. Hopefully, this review helps in deeper understanding of the strengths and limitations of TAT and may help in creating effective therapy protocols.

Imaging and therapy in prostate cancer using prostate specific membrane antigen radioligands

Prostate specific membrane antigen (PSMA) directed PET imaging has rapidly transformed prostate cancer workup over the past decade and paved the way for a theranostic approach using 177Lu-labelled PSMA radioligand therapy (RLT). This review gives an overview of the underlying principles behind PSMA as a target; the current use of PSMA PET in prostate cancer imaging and benefits compared to conventional imaging; and therapeutic applications including optimisation of patient selection. It also explores the evidence base of PSMA PET for other indications not in routine clinical use and the future of PSMA-directed RLT.

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.

Time-efficient HPLC Validation Methodology for the Qualitative Analysis of 68Ga PSMA-11 in Routine Clinical Usage under Isocratic Method

Background

Prostate-specific membrane antigen (PSMA) has shown to be a promising agent for prostate cancer imaging under PET-CT. With the automation in radiolabeling with 68Ga, using iTG 68Ge/68Ga generator, it has helped introduce various new diagnostic agents and achieve good manufacturing practices (GMP) simultaneously. However, before any radiopharmaceutical is put into clinical usage, it should always be checked for its radiochemical purity and other quality parameters before injecting in the patient. Chromatography techniques such as Gas Chromatography (GC), High-Performance Liquid Chromatography (HPLC), and Thin-Layer Chromatography (TLC) are the most frequently utilized separation technique for purity analysis. A rapid quality control HPLC based methodology was required for radiopharmaceuticals.

Aim & Objective

In our current setting, we conducted quality control analysis and standardized and validated HPLC method for the routine quality check of 68Ga-PSMA-11.

Materials and Methods

The QC of 68Ga PSMA-11 was performed under ITLC and HPLC.

Results

Linearity, accuracy, precision and specificity were assessed and quantified in accordance with International conference on harmonisation of technical requirements for registration of pharmaceuticals for human use (Q2 (R1) ICH) guidelines, which can be implemented in resource-limited settings to check the quality.

Conclusion

The current HPLC based methodology is rapid, with a retention time of 2.24 min, rendering it a favorable analytical standard operating procedure for QC analysis of 68Ga-PSMA-11.

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.

Performance evaluation of Gallium-68 radiopharmaceuticals production using liquid target PETtrace 800 cyclotron

Due to increased demand, cyclotron has an expanding role in producing Gallium-68 (68Ga) radiopharmaceuticals using solid and liquid targets. Though the liquid target produces lower end-of-bombardment activity compared to the solid target, our study presents the performance of 68Ga radiopharmaceuticals production using the liquid target by evaluating the end-of-bombardment activity and the end-of-purification activity of [68Ga]GaCl3. We also present the effect of increasing irradiation time, which significantly improves the end-of-synthesis yield. From the result obtained, the end-of-bombardment activity produced was 4.48 GBq, and the [68Ga]GaCl3 end-of-purification activity produced was 2.51 GBq with below-limit metallic impurities. Increasing the irradiation time showed a significant increase in the end-of-synthesis activity from 1.33 GBq to 1.95 GBq for [68Ga]Ga-PSMA-11 and from 1.13 GBq to 1.74 GBq for [68Ga]Ga-DOTA-TATE. Based on the improvements made, the liquid target production of 68Ga radiopharmaceuticals is feasible and reproducible to accommodate up to 5 patients per production. In addition, this work also discusses the issues encountered, together with the possible corrective and preventative measures.

Automated radiolabeling and handling of 177 Lu- and 225 Ac-PSMA-617 using a robotic pipettor

While automated modules for F-18 and C-11 radiosyntheses are standardized with features such as multiple reactors, vacuum connection and semi-preparative HPLC, labeling and processing of compounds with radiometals such as Zr-89, Lu-177 and Ac-225 often do not require complex manipulations and are frequently performed manually by a radiochemist. These procedures typically involve transferring solutions to and from vials using pipettes followed by heating of the reaction mixture, and do not require all the features found in most commercial automated synthesis units marketed as F-18 or C-11 modules. Here we present an efficient automated method for performing radiosyntheses involving radiometals by adapting a commercially available robotic pipettor originally developed for high-throughput processing of biological samples. While a robotic pipettor is less costly than a radiosynthesis module, it holds many similar advantages over manual radiosynthesis such as minimization of operator error, lower operator exposure rates, and abbreviated synthesis times, among others. To demonstrate the feasibility of using the OpenTrons OT-2 robotic pipettor to perform automated radiosyntheses, we radiolabeled and formulated 177 Lu-PSMA-617 and 225 Ac-PSMA-617 on the system. The OT-2 was then used to help streamline the quality control process for both products, further minimizing manual handling by and exposure to the radiochemist.

Implementing Ac-225 labelled radiopharmaceuticals: practical considerations and (pre-)clinical perspectives

BACKGROUND

In the past years, there has been a notable increase in interest regarding targeted alpha therapy using Ac-225, driven by the observed promising clinical anti-tumor effects. As the production and technology has advanced, the availability of Ac-225 is expected to increase in the near future, making the treatment available to patients worldwide.

MAIN BODY

Ac-225 can be labelled to different biological vectors, whereby the success of developing a radiopharmaceutical depends heavily on the labelling conditions, purity of the radionuclide source, chelator, and type of quenchers used to avoid radiolysis. Multiple (methodological) challenges need to be overcome when working with Ac-225; as alpha-emission detection is time consuming and highly geometry dependent, a gamma co-emission is used, but has to be in equilibrium with the mother-nuclide. Because of the high impact of alpha emitters in vivo it is highly recommended to cross-calibrate the Ac-225 measurements for used quality control (QC) techniques (radio-TLC, HPLC, HP-Ge detector, and gamma counter). More strict health physics regulations apply, as Ac-225 has a high toxicity, thereby limiting practical handling and quantities used for QC analysis.

CONCLUSION

This overview focuses specifically on the practical and methodological challenges when working with Ac-225 labelled radiopharmaceuticals, and underlines the required infrastructure and (detection) methods for the (pre-)clinical application.

Targeted Alpha Therapy: All We Need to Know about 225Ac's Physical Characteristics and Production as a Potential Theranostic Radionuclide

The high energy of α emitters, and the strong linear energy transfer that goes along with it, lead to very efficient cell killing through DNA damage. Moreover, the degree of oxygenation and the cell cycle state have no impact on these effects. Therefore, α radioisotopes can offer a treatment choice to individuals who are not responding to β- or gamma-radiation therapy or chemotherapy drugs. Only a few α-particle emitters are suitable for targeted alpha therapy (TAT) and clinical applications. The majority of available clinical research involves 225Ac and its daughter nuclide 213Bi. Additionally, the 225Ac disintegration cascade generates γ decays that can be used in single-photon emission computed tomography (SPECT) imaging, expanding the potential theranostic applications in nuclear medicine. Despite the growing interest in applying 225Ac, the restricted global accessibility of this radioisotope makes it difficult to conduct extensive clinical trials for many radiopharmaceutical candidates. To boost the availability of 225Ac, along with its clinical and potential theranostic applications, this review attempts to highlight the fundamental physical properties of this α-particle-emitting isotope, as well as its existing and possible production methods.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Actinium-225 Targeted Agents: Where Are We Now?

α-particle targeted radionuclide therapy has shown promise for optimal cancer management, an exciting new era for brachytherapy. Alpha-emitting nuclides can have significant advantages over gamma- and beta-emitters due to their high linear energy transfer (LET). While their limited path length results in more specific tumor 0kill with less damage to surrounding normal tissues, their high LET can produce substantially more lethal double strand DNA breaks per radiation track than beta particles. Over the last decade, the physical and chemical attributes of Actinium-225 (225Ac) including its half-life, decay schemes, path length, and straightforward chelation ability has peaked interest for brachytherapy agent development. However, this has been met with challenges including source availability, accurate modeling for standardized dosimetry for brachytherapy treatment planning, and laboratory space allocation in the hospital setting for on-demand radiopharmaceuticals production. Current evidence suggests that a simple empirical approach based on 225Ac administered radioactivity may lead to inconsistent outcomes and toxicity. In this review article, we highlight the recent advances in 225Ac source production, dosimetry modeling, and current clinical studies.

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

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

Clinical Experience with [225Ac]Ac-PSMA Treatment in Patients with [177Lu]Lu-PSMA-Refractory Metastatic Castration-Resistant Prostate Cancer

For patients with advanced-stage metastatic castration-resistant prostate cancer (mCRPC) who do not respond to [177Lu]Lu-PSMA therapy, there are limited treatment options. Clinical results obtained with [225Ac]Ac-PSMA are promising. We retrospectively analyzed the outcomes of patients treated with [225Ac]Ac-PSMA between December 2018 and October 2022. Methods: We evaluated the treatment results of 23 patients (mean age, 70.3 ± 8.8 y) with mCRPC who were refractory to treatment with [177Lu]Lu-PSMA (2-9 cycles). The safety profile was assessed according to Common Technology Criteria for Adverse Events version 5.0. Treatment efficacy was assessed using prostate-specific membrane antigen PET progression criteria and prostate-specific antigen (PSA) response according to Prostate Cancer Working Group 2 criteria after the first cycle of [225Ac]Ac-PSMA treatment. Results: All patients received androgen-deprivation therapy, whereas 22 (96%) and 19 (83%) patients received chemotherapy and second-generation antiandrogen therapy, respectively. One patient received 4 cycles, 2 received 3 cycles, 8 received 2 cycles, and 12 received 1 cycle of [225Ac]Ac-PSMA. The median interval between cycles was 13 wk (range, 8-28 wk). [225Ac]Ac-PSMA was administered with a mean activity of 7.6 MBq (range, 6.2-10.0 MBq) in each cycle. Patients were at an advanced stage of disease, and tumor burden was very high. Although the best PSA response was observed in 5 patients (26%) after [225Ac]Ac-PSMA treatment, there was at least some level of decline in PSA observed in 11 patients (58%; n = 19). Treatment response was assessed in patients who underwent [68Ga]Ga-PSMA PET/CT imaging. After the first cycle of treatment (n = 18), 50% of patients (n = 9) showed disease progression according to prostate-specific membrane antigen PET progression criteria, and the disease control rate was calculated to be 50%. Median progression-free survival was 3.1 mo, and median overall survival was 7.7 mo. Grade 3 hematologic toxicity occurred in 1 patient, and grade 3 nephrotoxicity was observed in another patient. Parotid SUVmax decreased by 33%, although all patients complained of dry mouth before treatment. Conclusion: We observed that [225Ac]Ac-PSMA therapy was safe and showed potential even in cases with advanced-stage mCRPC in which all other treatment options were completed.

161Tb-PSMA Unleashed: a Promising New Player in the Theranostics of Prostate Cancer

Radiotheranostics with 177Lu-PSMA have changed the treatment paradigm in patients with prostate cancer, becoming the new standard in certain settings. Terbium-161 (161Tb) has been recently investigated as a potential radionuclide for radiotheranostics in various types of cancer, including metastatic castration-resistant prostate cancer (mCRPC). The nuclear medicine team at King Hussein Cancer Center (KHCC) in Amman, Jordan, recently published the first-in-human SPECT/CT imaging results following a well-tolerated dose of 161Tb-PSMA radioligand therapy with no treatment-related adverse events, adding to the potential of radiotheranostics in prostate cancer. Two clinical trials for 161Tb-PSMA radioligand therapy in prostate cancer are currently underway and will provide valuable insights. This review will shed light on the expanding field of radiotheranostics in prostate cancer, which is not without challenges, and will discuss how the introduction of a new therapeutic option like 161Tb-PSMA may help to combat these challenges and build on the proven success of 177Lu-PSMA-based radiotheranostics for the benefit of prostate cancer patients worldwide.

First preclinical evaluation of [225Ac]Ac-DOTA-JR11 and comparison with [177Lu]Lu-DOTA-JR11, alpha versus beta radionuclide therapy of NETs

BACKGROUND

The [¹⁷⁷Lu]Lu-DOTA-TATE mediated peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors (NETs) is sometimes leading to treatment resistance and disease recurrence. An interesting alternative could be the somatostatin antagonist, [¹⁷⁷Lu]Lu-DOTA-JR11, that demonstrated better biodistribution profile and higher tumor uptake than [¹⁷⁷Lu]Lu-DOTA-TATE. Furthermore, treatment with alpha emitters showed improvement of the therapeutic index of PRRT due to the high LET offered by the alpha particles compared to beta emitters. Therefore, [²²⁵Ac]Ac-DOTA-JR11 can be a potential candidate to improve the treatment of NETs (Graphical abstract). DOTA-JR11 was radiolabeled with [²²⁵Ac]Ac(NO₃)₃ and [¹⁷⁷Lu]LuCl₃. Stability studies were performed in phosphate buffered saline (PBS) and mouse serum. In vitro competitive binding assay has been carried out in U2OS-SSTR2 + cells for ^natLa-DOTA-JR11, ^natLu-DOTA-JR11 and DOTA-JR11. Ex vivo biodistribution studies were performed in mice inoculated with H69 cells at 4, 24, 48 and 72 h after injection of [²²⁵Ac]Ac-DOTA-JR11. A blocking group was included to verify uptake specificity. Dosimetry of selected organs was determined for [²²⁵Ac]Ac-DOTA-JR11 and [¹⁷⁷Lu]Lu-DOTA-JR11.

RESULTS

[²²⁵Ac]Ac-DOTA-JR11 has been successfully prepared and obtained in high radiochemical yield (RCY; 95%) and radiochemical purity (RCP; 94%). [²²⁵Ac]Ac-DOTA-JR11 showed reasonably good stability in PBS (77% intact radiopeptide at 24 h after incubation) and in mouse serum (~ 81% intact radiopeptide 24 h after incubation). [¹⁷⁷Lu]Lu-DOTA-JR11 demonstrated excellent stability in both media (> 93%) up to 24 h post incubation. Competitive binding assay revealed that complexation of DOTA-JR11 with ^natLa and ^natLu did not affect its binding affinity to SSTR2. Similar biodistribution profiles were observed for both radiopeptides, however, higher uptake was noticed in the kidneys, liver and bone for [²²⁵Ac]Ac-DOTA-JR11 than [¹⁷⁷Lu]Lu-DOTA-JR11.

CONCLUSION

[²²⁵Ac]Ac-DOTA-JR11 showed a higher absorbed dose in the kidneys compared to [¹⁷⁷Lu]Lu-DOTA-JR11, which may limit further studies with this radiopeptide. However, several strategies can be explored to reduce nephrotoxicity and offer opportunities for future clinical investigations with [²²⁵Ac]Ac-DOTA-JR11.

Recent Innovations and Nano-Delivery of Actinium-225: A Narrative Review

The actinium-225 (²²⁵Ac) radioisotope exhibits highly attractive nuclear properties for application in radionuclide therapy. However, the ²²⁵Ac radionuclide presents multiple daughter nuclides in its decay chain, which can escape the targeted site, circulate in plasma, and cause toxicity in areas such as kidneys and renal tissues. Several ameliorative strategies have been devised to circumvent this issue, including nano-delivery. Alpha-emitting radionuclides and nanotechnology applications in nuclear medicine have culminated in major advancements that offer promising therapeutic possibilities for treating several cancers. Accordingly, the importance of nanomaterials in retaining the ²²⁵Ac daughters from recoiling into unintended organs has been established. This review expounds on the advancements of targeted radionuclide therapy (TRT) as an alternative anticancer treatment. It discusses the recent developments in the preclinical and clinical investigations on ²²⁵Ac as a prospective anticancer agent. Moreover, the rationale for using nanomaterials in improving the therapeutic efficacy of α-particles in targeted alpha therapy (TAT) with an emphasis on ²²⁵Ac is discussed. Quality control measures in the preparation of ²²⁵Ac-conjugates are also highlighted.

Innovation in Radionuclide Therapy for the Treatment of Prostate Cancers: Radiochemical Perspective and Recent Therapeutic Practices

Prostate cancer represents the second cause of death by cancer in males in western countries. While early-stage diseases are accessible to surgery and/or external radiotherapy, advanced metastatic prostate cancers are primarily treated with androgen deprivation therapy, to which new generation androgen receptor antagonists or taxane-based chemotherapies are added in the case of tumor relapse. Nevertheless, patients become invariably resistant to castration with a median survival that rarely exceeds 3 years. This fostered the search for alternative strategies, independent of the androgen receptor signaling pathway. In this line, radionuclide therapies may represent an interesting option as they could target either the microenvironment of sclerotic bone metastases with the use of radiopharmaceuticals containing samarium-153, strontium-89 or radium-223 or tumor cells expressing the prostate-specific membrane antigen (PSMA), a protein found at the surface of prostate cancer cells. This review gives highlights the chemical properties of radioligands targeting prostate cancer cells and recapitulates the clinical trials evaluating the efficacy of radionuclide therapies, alone or in combination with other approved treatments, in patients with castration-resistant prostate tumors. It discusses some of the encouraging results obtained, especially the benefit on overall survival that was reported with [¹⁷⁷Lu]-PSMA-617. It also addresses the specific requirements for the use of this particular class of drugs, both in terms of medical staff coordination and adapted infrastructures for efficient radioprotection.

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

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

siAKR1C3@PPA complex nucleic acid nanoparticles inhibit castration-resistant prostate cancer in vitro

Introduction

AKR1C3, as a crucial androgenic enzyme, implicates the androgen biosynthesis and promoting prostate cancer cell growth in vitro. This study provides a new gene therapy strategy for targeting AKR1C3 to treat castration-resistant prostate cancer.

Methods

siAKR1C3@PPA is assembled from PEG3500, PAMAM, Aptamer-PSMA, and siRNA for AKR1C3. We analyzed the relationship between AKR1C3 expression and the survival rate of prostate cancer patients based on the GEPIA online database to perform disease-free survival, and found that AKR1C3 may be an important factor leading to poor prognosis in prostate cancer. Considering AKR1C3 as a therapeutic target for castration-resistant prostate cancer, we constructed a complex nucleic acid nanoparticle, siAKR1C3@PPA to investigate the inhibitory effect on castration-resistant prostate cancer.

Results

Aptamer-PSMA acts as a target to guide siAKR1C3@PPA into PSMA-positive prostate cancer cells and specifically down regulate AKR1C3. Cyclin D1 was decreased as a result of siAKR1C3@PPA treatment. Changes in Cyclin D1 were consistent with decreased expression of AKR1C3 in LNCaP-AKR1C3 cells and 22RV1 cells. Furthermore, in the LNCaP-AKR1C3 group, 1070 proteins were upregulated and 1015 proteins were downregulated compared to the LNCaP group according to quantitative 4D label-free proteomics. We found 42 proteins involved in cell cycle regulation. In a validated experiment, we demonstrated that PCNP and CINP were up-regulated, and TERF2 and TP53 were down-regulated by western blotting.

Conclusion

We concluded that siAKR1C3@PPA may arrest the cell cycle and affect cell proliferation.

Alpha-peptide receptor radionuclide therapy using actinium-225 labeled somatostatin receptor agonists and antagonists

Peptide receptor radionuclide therapy (PRRT) has over the last two decades emerged as a very promising approach to treat neuroendocrine tumors (NETs) with rapidly expanding clinical applications. By chelating a radiometal to a somatostatin receptor (SSTR) ligand, radiation can be delivered to cancer cells with high precision. Unlike conventional external beam radiotherapy, PRRT utilizes primarily β or α radiation derived from nuclear decay, which causes damage to cancer cells in the immediate proximity by irreversible direct or indirect ionization of the cells' DNA, which induces apoptosis. In addition, to avoid damage to surrounding normal cells, PRRT privileges the use of radionuclides that have little penetrating and more energetic (and thus more ionizing) radiations. To date, the most frequently radioisotopes are β- emitters, particularly Yttrium-90 (90Y) and Lutetium-177 (177Lu), labeled SSTR agonists. Current development of SSTR-targeting is triggering the shift from using SSTR agonists to antagonists for PRRT. Furthermore, targeted α-particle therapy (TAT), has attracted special attention for the treatment of tumors and offers an improved therapeutic option for patients resistant to conventional treatments or even beta-irradiation treatment. Due to its short range and high linear energy transfer (LET), α-particles significantly damage the targeted cancer cells while causing minimal cytotoxicity toward surrounding normal tissue. Actinium-225 (225Ac) has been developed into potent targeting drug constructs including somatostatin-receptor-based radiopharmaceuticals and is in early clinical use against multiple neuroendocrine tumor types. In this article, we give a review of preclinical and clinical applications of 225Ac-PRRT in NETs, discuss the strengths and challenges of 225Ac complexes being used in PRRT; and envision the prospect of 225Ac-PRRT as a future alternative in the treatment of NETs.

Gamma counting protocols for the accurate quantification of 225Ac and 213Bi without the need for a secular equilibrium between parent and gamma-emitting daughter

BACKGROUND

Quantification of actinium-225 through gamma counter measurements, when there is no secular equilibrium between actinium-225 and its gamma emitting daughters bismuth-213 and/or francium-221, can provide valuable information regarding the possible relocation of recoiled daughters such that related radiotoxicity effects can be evaluated. This study proposes a multiple time-point protocol using the bismuth-213 photopeak with measurements before secular equilibrium between actinium-225 and bismuth-213, and a single time-point protocol using both the francium-221 and bismuth-213 photopeak while assuming secular equilibrium between actinium-225 and francium-221 but not between bismuth-213 and actinium-225.

RESULTS

Good agreement (i.e. 3% accuracy) was obtained when relying on a multiple time-points measurement of bismuth-213 to quantify both actinium-225 and excess of bismuth-213. Following scatter correction, actinium-225 can be accurately quantified using the francium-221 in a single time-point measurement within 3% of accuracy. The analysis performed on the stability data of [225Ac]Ac-DEPA and [225Ac]Ac-DOTA complexes, before secular equilibrium between bismuth-213 and actinium-225 was formed, revealed considerable amounts of unbound bismuth-213 (i.e. more than 90%) after 24 h of the radiolabeling most likely due to the recoiled daughter effect.

CONCLUSION

Both protocols were able to accurately estimate 225Ac-activities provided the francium-221 energy window was corrected for the down scatter of the higher-energy gamma-emissions by bismuth-213. This could prove beneficial to study the recoiled daughter effect and redistribution of free bismuth-213 by monitoring the accumulation or clearance of bismuth-213 in different tissues during biodistribution studies or in patient samples during clinical studies. On the other hand, the single gamma counter measurement protocol, although required a 30 min waiting time, is more time and cost efficient and therefore more appropriate for standardized quality control procedures of 225Ac-labeled radiopharmaceuticals.

Advances in 177Lu-PSMA and 225Ac-PSMA Radionuclide Therapy for Metastatic Castration-Resistant Prostate Cancer

For patients with metastatic castration-resistant prostate cancer (mCRPC), the survival benefit of classic treatment options with chemotherapy and drugs targeting androgen signaling is limited. Therefore, beta and alpha radionuclide therapy (RNT) have emerged as novel treatment options for patients with mCRPC. Radioligands target the prostate-specific membrane antigen (PSMA) epitopes, which are upregulated up to a thousand times more in prostate cancer cells compared to the cells in normal tissues. For this reason, PSMA is an excellent target for both imaging and therapy. Over the past years, many studies have investigated the treatment effects of lutetium-177 labeled PSMA (177Lu-PSMA) and actinium-225 labeled PSMA (225Ac-PSMA) RNT in patients with mCRPC. While promising results have been achieved, this field is still in development. In this review, we have summarized and discussed the clinical data of 177Lu-PSMA and 225Ac-PSMA RNT in patients with mCRPC.

Quality control of [177Lu]Lu-PSMA preparations using HPLC: effect of sample composition and ligand on recovery

Background

[¹⁷⁷Lu]Lu-PSMA is used for the treatment of metastatic castration-resistant prostate cancer. For in-house productions, quality control methods are essential for ensuring product quality, and thus patient safety. During HPLC method development for quality control of [¹⁷⁷Lu]Lu-PSMA-I&T, we noticed an unpredictable variability in peak area and height with replicate measurements. After a run, irremovable radioactivity was measured over the whole the length of the HPLC column, with slightly higher activity at the beginning and end of the column. The uniform distribution suggests that [¹⁷⁷Lu]Lu-PSMA-I&T interacts with the column. As a result of the interaction, incomplete and variable recovery of injected activity was observed leading to the variability in peak area and height. Therefore the aim of this study was to (1) investigate the effect of sample composition on the interaction of [¹⁷⁷Lu]Lu-PSMA-I&T to the HPLC column (measured as recovery, peak area, and peak height), and (2) to compare this with same concentrations of the well-known [¹⁷⁷Lu]Lu-PSMA-617.

Results

Sample composition significantly affects recovery of [¹⁷⁷Lu]Lu-PSMA-I&T, leading to a change in peak area and height. Recovery was 24% when diluted with 0.1 mM octreotide, 38% with water, and increased to 95% when diluted with 0.7 mM unlabeled PSMA-I&T. Peak area and height decreased to 26% and 17% when diluted in octreotide and to 41% and 29% when diluted in water, compared to a dilution in PSMA-I&T. Further experiments showed that recovery (and consequently peak area and peak height) reached a plateau of > 99% at concentrations of 0.27 mM and higher. [¹⁷⁷Lu]Lu-PSMA-617 also interacts with the HPLC column, leading to lower, but less variable, recovery (9%). The low recovery of [¹⁷⁷Lu]Lu-PSMA-617 could not be prevented with addition of unlabeled PSMA-617.

Conclusion

[¹⁷⁷Lu]Lu-PSMA-I&T can undergo an irreversible binding with an HPLC column resulting in a decreased recovery. The recovery is can be highly dependent on sample composition. The addition of a surplus of unlabeled PSMA-I&T leads to an accurate analysis of [¹⁷⁷Lu]Lu-PSMA-I&T.

H2BZmacropa-NCS: A Bifunctional Chelator for Actinium-225 Targeted Alpha Therapy

Actinium-225 (225Ac) is one of the most promising radionuclides for targeted alpha therapy (TAT). With a half-life of 9.92 days and a decay chain that emits four high-energy α particles, 225Ac is well-suited for TAT when conjugated to macromolecular targeting vectors that exhibit extended in vivo circulation times. The implementation of 225Ac in these targeted constructs, however, requires a suitable chelator that can bind and retain this radionuclide in vivo. Previous work has demonstrated the suitability of a diaza-18-crown-6 macrocyclic chelator H2macropa for this application. Building upon these prior efforts, in this study, two rigid variants of H2macropa, which contain either one (H2BZmacropa) or two (H2BZ2macropa) benzene rings within the macrocyclic core, were synthesized and investigated for their potential use for 225Ac TAT. The coordination chemistry of these ligands with La3+, used as a nonradioactive model for Ac3+, was carried out. Both NMR spectroscopic and X-ray crystallographic studies of the La3+ complexes of these ligands revealed similar structural features to those found for the related complex of H2macropa. Thermodynamic stability constants of the La3+ complexes, however, were found to be 1 and 2 orders of magnitude lower than those of H2macropa for H2BZmacropa and H2BZ2macropa, respectively. The decrease in thermodynamic stability was rationalized via the use of density functional theory calculations. 225Ac radiolabeling and serum stability studies with H2BZmacropa showed that this chelator compares favorably with H2macropa. Based on these promising results, a bifunctional version of this chelator, H2BZmacropa-NCS, was synthesized and conjugated to the antibody codrituzumab (GC33), which targets the liver cancer biomarker glypican-3 (GPC3). The resulting GC33-BZmacropa conjugate and an analogous GC33-macropa conjugate were evaluated for their 225Ac radiolabeling efficiencies, antigen-binding affinities, and in vivo biodistribution in HepG2 liver cancer tumor-bearing mice. Although both conjugates were comparably effective in their radiolabeling efficiencies, [225Ac]Ac-GC33-BZmacropa showed slightly poorer serum stability and biodistribution than [225Ac]Ac-GC33-macropa. Together, these results establish H2BZmacropa-NCS as a new bifunctional chelator for the preparation of 225Ac radiopharmaceuticals.

In vitro dose effect relationships of actinium-225- and lutetium-177-labeled PSMA-I&T

PURPOSE

Targeting the prostate-specific membrane antigen (PSMA) using lutetium-177-labeled PSMA-specific tracers has become a very promising novel therapy option for prostate cancer (PCa). The efficacy of this therapy might be further improved by replacing the β-emitting lutetium-177 with the α-emitting actinium-225. Actinium-225 is thought to have a higher therapeutic efficacy due to the high linear energy transfer (LET) of the emitted α-particles, which can increase the amount and complexity of the therapy induced DNA double strand breaks (DSBs). Here we evaluated the relative biological effectiveness of [²²⁵Ac]Ac-PSMA-I&T and [¹⁷⁷Lu]Lu-PSMA-I&T by assessing in vitro binding characteristics, dosimetry, and therapeutic efficacy.

METHODS AND RESULTS

The PSMA-expressing PCa cell line PC3-PIP was used for all in vitro assays. First, binding and displacement assays were performed, which revealed similar binding characteristics between [²²⁵Ac]Ac-PSMA-I&T and [¹⁷⁷Lu]Lu-PSMA-I&T. Next, the assessment of the number of 53BP1 foci, a marker for the number of DNA double strand breaks (DSBs), showed that cells treated with [²²⁵Ac]Ac-PSMA-I&T had slower DSB repair kinetics compared to cells treated with [¹⁷⁷Lu]Lu-PSMA-I&T. Additionally, clonogenic survival assays showed that specific targeting with [²²⁵Ac]Ac-PSMA-I&T and [¹⁷⁷Lu]Lu-PSMA-I&T caused a dose-dependent decrease in survival. Lastly, after dosimetric assessment, the relative biological effectiveness (RBE) of [²²⁵Ac]Ac-PSMA-I&T was found to be 4.2 times higher compared to [¹⁷⁷Lu]Lu-PSMA-I&T.

CONCLUSION

We found that labeling of PSMA-I&T with lutetium-177 or actinium-225 resulted in similar in vitro binding characteristics, indicating that the distinct biological effects observed in this study are not caused by a difference in uptake of the two tracers. The slower repair kinetics of [²²⁵Ac]Ac-PSMA-I&T compared to [¹⁷⁷Lu]Lu-PSMA-I&T correlates to the assumption that irradiation with actinium-225 causes more complex, more difficult to repair DSBs compared to lutetium-177 irradiation. Furthermore, the higher RBE of [²²⁵Ac]Ac-PSMA-I&T compared to [¹⁷⁷Lu]Lu-PSMA-I&T underlines the therapeutic potential for the treatment of PCa.

Total Tumor Volume on 18F-PSMA-1007 PET as Additional Imaging Biomarker in mCRPC Patients Undergoing PSMA-Targeted Alpha Therapy with 225Ac-PSMA-I&T

Background: PSMA-based alpha therapy using 225Ac-PSMA-I&T provides treatment for metastatic castration-resistant prostate cancer (mCRPC), even after the failure of 177Lu-PSMA radioligand therapy (RLT). In clinical routine, the total tumor volume (TTV) on PSMA PET impacts therapy outcomes and plays an increasing role in mCRPC patients. Hence, we aimed to assess TTV and its changes during 225Ac-PSMA-I&T RLT. Methods: mCRPC patients undergoing RLT with 225Ac-PSMA-I&T with available 18F-PSMA-1007 PET/CT prior to therapy initiation were included. TTV was assessed in all patients using established cut-off values. Image derived, clinical and biochemistry parameters (PSA, LDH, AP, pain score) were analyzed prior to and after two cycles of 225Ac-PSMA. Changes in TTV and further parameters were directly compared and then correlated with established response criteria, such as RECIST 1.1 or mPERCIST. Results: 13 mCRPC patients were included. The median overall survival (OS) was 10 months. Prior to 225Ac-PSMA RLT, there was no significant correlation between TTV with other clinical parameters (p > 0.05 each). Between short-term survivors (STS, <10 months OS) and long-term survivors (LTS, ≥10 months OS), TTV and PSA were comparable (p = 0.592 & p = 0.286, respectively), whereas AP was significantly lower in the LTS (p = 0.029). A total of 7/13 patients completed two cycles and underwent a follow-up 18F-PSMA-1007 PET/CT. Among these patients, there was a significant decrease in TTV (median 835 vs. 201 mL, p = 0.028) and PSA (median 687 ng/dL vs. 178 ng/dL, p = 0.018) after two cycles of 225Ac-PSMA RLT. Here, percentage changes of TTV after two cycles showed no direct correlation to all other clinical parameters (p > 0.05 each). In two patients, new PET-avid lesions were detected on 18F-PSMA-1007 PET/CT. However, TTV and PSA were decreasing or stable. Conclusion: PET-derived assessment of TTV is an easily applicable imaging biomarker independent of other established parameters prior to 225Ac-PSMA RLT in these preliminary follow-up data. Even after the failure of 177Lu-PSMA, patients with extensive TTV seem to profit from RLT. All but one patient who was eligible for ≥2 cycles of 225Ac-PSMA-RLT demonstrated drastic TTV decreases without direct correlation to other biomarkers, such as serum PSA changes. Changes in TTV might hence improve the response assessment compared to standard classifiers by reflecting the current tumor load independent of the occurrence of new lesions.

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.