Current clinical application of lutetium‑177 in solid tumors (Review)

Radionuclide-based therapy represents a novel treatment regimen for tumors. Among these therapies, lutetium-177 (177Lu) has gained significant attention due to its stability and safety, as well as its ability to emit both γ and β rays, allowing for both imaging with single photon emission computed tomography and tumor treatment. As a result, 177Lu can be used for both diagnosis and treatment for diseases such as prostatic and gastric cancer. Therefore, based on the available data, the present review provides a brief overview of the clinical applications of 177Lu-targeted radionuclide therapy in metastatic prostate cancer, neuroendocrine tumors and other types of solid tumors, and highlights the current therapeutic effect, reduction in damage to normal tissues and future research directions, including the development of new nuclides and the application of more nuclides in different tumors. In the future, such treatments could be used in more tumors.

Human ABC and SLC Transporters: The Culprit Responsible for Unspecific PSMA-617 Uptake?

[177Lu]Lu-PSMA-617 has recently been successfully approved by the FDA, the MHRA, Health Canada and the EMA as Pluvicto®. However, salivary gland (SG) and kidney toxicities account for its main dose-limiting side-effects, while its corresponding uptake and retention mechanisms still remain elusive. Recently, the presence of different ATP-binding cassette (ABC) transporters, such as human breast cancer resistance proteins (BCRP), multidrug resistance proteins (MDR1), multidrug-resistance-related proteins (MRP1, MRP4) and solute cassette (SLC) transporters, such as multidrug and toxin extrusion proteins (MATE1, MATE2-K), organic anion transporters (OAT1, OAT2v1, OAT3, OAT4) and peptide transporters (PEPT2), has been verified at different abundances in human SGs and kidneys. Therefore, our aim was to assess whether [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are substrates of these ABC and SLC transporters. For in vitro studies, the novel isotopologue ([α,β-3H]Nal)Lu-PSMA-617 was used in cell lines or vesicles expressing the aforementioned human ABC and SLC transporters for inhibition and uptake studies, respectively. The corresponding probe substrates and reference inhibitors were used as controls. Our results indicate that [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are neither inhibitors nor substrates of the examined transporters. Therefore, our results show that human ABC and SLC transporters play no central role in the uptake and retention of [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 in the SGs and kidneys nor in the observed toxicities.

Managing Catecholamine Release Syndrome During and Following Lu-177-DOTATATE in High-Risk Pheochromocytoma Patients

Pheochromocytomas and paragangliomas (PPGLs) are rare catecholamine-producing tumors that express somatostatin receptors (SSTR) that can be treated with lutetium-177 DOTATATE (Lu-177-TRT); however, treatment can be associated with life-threatening cardiovascular events. A patient case with management strategies for high-risk PPGL patients receiving Lu-177-TRT is described. The 78-year-old patient with metastatic paraganglioma was enrolled and treated under NCT03206060. Deemed to be at high risk, the patient was preemptively admitted to the intensive care unit (ICU) with central line access placed. Due to comorbidities, a reduced dose of 100 mCi x 4 cycles was used for this patient. Vital signs, blood work, and serum catecholamine levels were obtained at various time points. Despite reduced dosing, the patient still developed a severe hypertensive reaction with systolic blood pressure of 240 mmHg within minutes of Lu-177-TRT infusion, which was controlled with an intravenous nicardipine drip. The patient remained in the ICU for 24 hours post Lu-177-TRT before moving to an inpatient ward for an additional 24 hours. All subsequent infusions were performed using reduced doses with elective ICU admissions and were well-tolerated. Despite the increased risk, metastatic PPGL patients can be safely treated with proper staff training, monitoring, and preparation for intravenous medications, especially nicardipine.

Low-dose targeted radionuclide therapy synergizes with CAR T cells and enhances tumor response

Ionizing radiation has garnered considerable attention as a combination partner for immunotherapy due to its potential immunostimulatory effects. In contrast to the more commonly used external beam radiation, we explored the feasibility of combining chimeric antigen receptor (CAR) T cell therapy with targeted radionuclide therapy (TRT), which is achieved by delivering β-emitting 177Lu-DOTATATE to tumor via tumor-infiltrating CAR T cells that express somatostatin receptor 2 (SSTR2). We hypothesized that the delivery of radiation to tumors could synergize with CAR T therapy, resulting in enhanced antitumor immunity and tumor response. To determine the optimal dosage and timing of 177Lu-DOTATATE treatment, we measured CAR T cell infiltration and expansion in tumors longitudinally through positron emission tomography (PET) using a SSTR2-specific positron-emitting radiotracer,18F-NOTA-Octreotide. In animals receiving CAR T cells and a low-dose (2.5 Gy) of TRT following the administration of 177Lu-DOTATATE, we observed a rapid regression of large subcutaneous tumors, which coincided with a dramatic increase in serum proinflammatory cytokines. Tumor burden was also reduced when a higher radiation dose (6 Gy) was delivered to the tumor. However, this higher dose led to cell death in both the tumor and CAR T cells. Our study suggests that there may exist an optimum range of TRT dosage that can enhance T cell activity and sensitize tumor cells to T cell killing, which may result in more durable tumor control compared to a higher radiation dose.

Towards Effective Targeted Alpha Therapy for Neuroendocrine Tumours: A Review

This review article explores the evolving landscape of Molecular Radiotherapy (MRT), emphasizing Peptide Receptor Radionuclide Therapy (PRRT) for neuroendocrine tumours (NETs). The primary focus is on the transition from β-emitting radiopharmaceuticals to α-emitting agents in PRRT, offering a critical analysis of the radiobiological basis, clinical applications, and ongoing developments in Targeted Alpha Therapy (TAT). Through an extensive literature review, the article delves into the mechanisms and effectiveness of PRRT in targeting somatostatin subtype 2 receptors, highlighting both its successes and limitations. The discussion extends to the emerging paradigm of TAT, underlining its higher potency and specificity with α-particle emissions, which promise enhanced therapeutic efficacy and reduced toxicity. The review critically evaluates preclinical and clinical data, emphasizing the need for standardised dosimetry and a deeper understanding of the dose-response relationship in TAT. The review concludes by underscoring the significant potential of TAT in treating SSTR2-overexpressing cancers, especially in patients refractory to β-PRRT, while also acknowledging the current challenges and the necessity for further research to optimize treatment protocols.

A Proof-of-Concept Study on the Theranostic Potential of 177 Lu-labeled Biocompatible Covalent Polymer Nanoparticles for Cancer Targeted Radionuclide Therapy

Theranostic nanomedicine combined bioimaging and therapy probably rises more helpful and interesting opportunities for personalized medicine. In this work, 177 Lu radiolabeling and surface PEGylation of biocompatible covalent polymer nanoparticles (CPNs) have generated a new theranostic nanoformulation (177 Lu-DOTA-PEG-CPNs) for targeted diagnosis and treatment of breast cancer. The in vitro anticancer investigations demonstrate that 177 Lu-DOTA-PEG-CPNs possess excellent bonding capacity with breast cancer cells (4T1), inhibiting the cell viability, leading to cell apoptosis, arresting the cell cycle, and upregulating the reactive oxygen species (ROS), which can be attributed to the good targeting ability of the nanocarrier and the strong relative biological effect of the radionuclide labelled compound. Single photon emission computed tomography/ computed tomography (SPECT/CT) imaging and in vivo biodistribution based on 177 Lu-DOTA-PEG-CPNs reveal that notable radioactivity accumulation at tumor site in murine 4T1 models with both intravenous and intratumoral administration of the prepared radiotracer. Significant tumor inhibition has been observed in mice treated with 177 Lu-DOTA-PEG-CPNs, of which the median survival was highly extended. More strikingly, 50 % of mice intratumorally injected with 177 Lu-DOTA-PEG-CPNs was cured and showed no tumor recurrence within 90 days. The outcome of this work can provide new hints for traditional nanomedicines and promote clinical translation of 177 Lu radiolabeled compounds efficiently.

Comparative Study of the Effect of Radiation Delivered by Lutetium-177 or Actinium-225 on Anti-GD2 Chimeric Antigen Receptor T Cell Viability and Functions

BACKGROUND AND PURPOSE

Chimeric antigen receptor (CAR) T cells have been relatively ineffective against solid tumors. Low-dose radiation which can be delivered to multiple sites of metastases by targeted radionuclide therapy (TRT) can elicit immunostimulatory effects. However, TRT has never been combined with CAR T cells against solid tumors in a clinical setting. This study investigated the effects of radiation delivered by Lutetium-177 (177Lu) and Actinium-225 (225Ac) on the viability and effector function of CAR T cells in vitro to evaluate the feasibility of such therapeutic combinations. After the irradiation of anti-GD2 CAR T cells with various doses of radiation delivered by 177Lu or 225Ac, their viability and cytotoxic activity against GD2-expressing human CHLA-20 neuroblastoma and melanoma M21 cells were determined by flow cytometry. The expression of the exhaustion marker PD-1, activation marker CD69 and the activating receptor NKG2D was measured on the irradiated anti-GD2 CAR T cells. Both 177Lu and 225Ac displayed a dose-dependent toxicity on anti-GD2 CAR T cells. However, radiation enhanced the cytotoxic activity of these CAR T cells against CHLA-20 and M21 irrespective of the dose tested and the type of radionuclide. No significant changes in the expression of PD-1, CD69 and NKG2D was noted on the CAR T cells following irradiation. Given a lower CAR T cell viability at equal doses and an enhancement of cytotoxic activity irrespective of the radionuclide type, 177Lu-based TRT may be preferred over 225Ac-based TRT when evaluating a potential synergism between these therapies in vivo against solid tumors.

Preclinical Evaluation of a Radiotheranostic Single-Domain Antibody Against Fibroblast Activation Protein α

Fibroblast activation protein α (FAP) is highly expressed on cancer-associated fibroblasts of epithelial-derived cancers. Breast, colon, and pancreatic tumors often show strong desmoplastic reactions, which result in a dominant presence of stromal cells. FAP has gained interest as a target for molecular imaging and targeted therapies. Single-domain antibodies (sdAbs) are the smallest antibody-derived fragments with beneficial pharmacokinetic properties for molecular imaging and targeted therapy. Methods: We describe the generation, selection, and characterization of a sdAb against FAP. In mice, we assessed its imaging and therapeutic potential after radiolabeling with tracer-dose 131I and 68Ga for SPECT and PET imaging, respectively, and with 131I and 225Ac for targeted radionuclide therapy. Results: The lead sdAb, 4AH29, exhibiting picomolar affinity for a distinct FAP epitope, recognized both purified and membrane-bound FAP protein. Radiolabeled versions, including [68Ga]Ga-DOTA-4AH29, [225Ac]Ac-DOTA-4AH29, and [131I]I-guanidinomethyl iodobenzoate (GMIB)-4AH29, displayed radiochemical purities exceeding 95% and effectively bound to recombinant human FAP protein and FAP-positive GM05389 human fibroblasts. These radiolabeled compounds exhibited rapid and specific accumulation in human FAP-positive U87-MG glioblastoma tumors, with low but specific uptake in lymph nodes, uterus, bone, and skin (∼2-3 percentage injected activity per gram of tissue [%IA/g]). Kidney clearance of unbound [131I]I-GMIB-4AH29 was fast (<1 %IA/g after 24 h), whereas [225Ac]Ac-DOTA-4AH29 exhibited slower clearance (8.07 ± 1.39 %IA/g after 24 h and 2.47 ± 0.18 %IA/g after 96 h). Mice treated with [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 demonstrated prolonged survival compared with those receiving vehicle solution. Conclusion: [68Ga]Ga-DOTA-4AH29 and [131I]I-GMIB-4AH29 enable precise FAP-positive tumor detection in mice. Therapeutic [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 exhibit strong and sustained tumor targeting, resulting in dose-dependent therapeutic effects in FAP-positive tumor-bearing mice, albeit with kidney toxicity observed later for [225Ac]Ac-DOTA-4AH29. This study confirms the potential of radiolabeled sdAb 4AH29 as a radiotheranostic agent for FAP-positive cancers, warranting clinical evaluation.

3D Cell Culture Models

In vitro therapeutic efficacy studies are commonly conducted in cell monolayers. However, three-dimensional (3D) tumor spheroids are known to better represent in vivo tumors. This study used [177Lu]Lu-PSMA-I&T, an already clinically applied radiopharmaceutical for targeted radionuclide therapy against metastatic castrate-resistant prostate cancer, to demonstrate the differences in the radiobiological response between 2D and 3D cell culture models of the prostate cancer cell lines PC-3 (PSMA negative) and LNCaP (PSMA positive). After assessing the target expression in both models via Western Blot, cell viability, reproductive ability, and growth inhibition were assessed. To investigate the geometric effects on dosimetry for the 2D vs. 3D models, Monte Carlo simulations were performed. Our results showed that PSMA expression in LNCaP spheroids was highly preserved, and target specificity was shown in both models. In monolayers of LNCaP, no short-term (48 h after treatment), but only long-term (14 days after treatment) radiobiological effects were evident, showing decreased viability and reproductive ability with the increasing activity. Further, LNCaP spheroid growth was inhibited with the increasing activity. Overall, treatment efficacy was higher in LNCaP spheroids compared to monolayers, which can be explained by the difference in the resulting dose, among others.

Theranostic Imaging Surrogates for Targeted Alpha Therapy: Progress in Production, Purification, and Applications

This article highlights recent developments of SPECT and PET diagnostic imaging surrogates for targeted alpha particle therapy (TAT) radiopharmaceuticals. It outlines the rationale for using imaging surrogates to improve diagnostic-scan accuracy and facilitate research, and the properties an imaging-surrogate candidate should possess. It evaluates the strengths and limitations of each potential imaging surrogate. Thirteen surrogates for TAT are explored: 133La, 132La, 134Ce/134La, and 226Ac for 225Ac TAT; 203Pb for 212Pb TAT; 131Ba for 223Ra and 224Ra TAT; 123I, 124I, 131I and 209At for 211At TAT; 134Ce/134La for 227Th TAT; and 155Tb and 152Tb for 149Tb TAT.

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.

Cellular lethal damage of 64Cu incorporated in mammalian genome evaluated with Monte Carlo methods

Purpose

Targeted Radionuclide Therapy (TRT) with Auger Emitters (AE) is a technique that allows targeting specific sites on tumor cells using radionuclides. The toxicity of AE is critically dependent on its proximity to the DNA. The aim of this study is to quantify the DNA damage and radiotherapeutic potential of the promising AE radionuclide copper-64 (64Cu) incorporated into the DNA of mammalian cells using Monte Carlo track-structure simulations.

Methods

A mammalian cell nucleus model with a diameter of 9.3 μm available in TOPAS-nBio was used. The cellular nucleus consisted of double-helix DNA geometrical model of 2.3 nm diameter surrounded by a hydration shell with a thickness of 0.16 nm, organized in 46 chromosomes giving a total of 6.08 giga base-pairs (DNA density of 14.4 Mbp/μm3). The cellular nucleus was irradiated with monoenergetic electrons and radiation emissions from several radionuclides including 111In, 125I, 123I, and 99mTc in addition to 64Cu. For monoenergetic electrons, isotropic point sources randomly distributed within the nucleus were modeled. The radionuclides were incorporated in randomly chosen DNA base pairs at two positions near to the central axis of the double-helix DNA model at (1) 0.25 nm off the central axis and (2) at the periphery of the DNA (1.15 nm off the central axis). For all the radionuclides except for 99mTc, the complete physical decay process was explicitly simulated. For 99mTc only total electron spectrum from published data was used. The DNA Double Strand Breaks (DSB) yield per decay from direct and indirect actions were quantified. Results obtained for monoenergetic electrons and radionuclides 111In, 125I, 123I, and 99mTc were compared with measured and calculated data from the literature for verification purposes. The DSB yields per decay incorporated in DNA for 64Cu are first reported in this work. The therapeutic effect of 64Cu (activity that led 37% cell survival after two cell divisions) was determined in terms of the number of atoms incorporated into the nucleus that would lead to the same DSBs that 100 decays of 125I. Simulations were run until a 2% statistical uncertainty (1 standard deviation) was achieved.

Results

The behavior of DSBs as a function of the energy for monoenergetic electrons was consistent with published data, the DSBs increased with the energy until it reached a maximum value near 500 eV followed by a continuous decrement. For 64Cu, when incorporated in the genome at evaluated positions (1) and (2), the DSB were 0.171 ± 0.003 and 0.190 ± 0.003 DSB/decay, respectively. The number of initial atoms incorporated into the genome (per cell) for 64Cu that would cause a therapeutic effect was estimated as 3,107 ± 28, that corresponds to an initial activity of 47.1 ± 0.4 × 10-3 Bq.

Conclusion

Our results showed that TRT with 64Cu has comparable therapeutic effects in cells as that of TRT with radionuclides currently used in clinical practice.

Current Status of Radiolabeled Monoclonal Antibodies Targeting PSMA for Imaging and Therapy

Prostate cancer (PCa) is one of the most prevalent cancer diagnoses among men in the United States and in several other developed countries. The prostate specific membrane antigen (PSMA) has been recognized as a promising molecular target in PCa, which has led to the development of specific radionuclide-based tracers for imaging and radiopharmaceuticals for PSMA targeted therapy. These compounds range from small molecule ligands to monoclonal antibodies (mAbs). Monoclonal antibodies play a crucial role in targeting cancer cell-specific antigens with a high degree of specificity while minimizing side effects to normal cells. The same mAb can often be labeled in different ways, such as with radionuclides suitable for imaging with Positron Emission Tomography (β+ positrons), Gamma Camera Scintigraphy (γ photons), or radiotherapy (β- electrons, α-emitters, or Auger electrons). Accordingly, the use of radionuclide-based PSMA-targeting compounds in molecular imaging and therapeutic applications has significantly grown in recent years. In this article, we will highlight the latest developments and prospects of radiolabeled mAbs that target PSMA for the detection and treatment of prostate cancer.

DNA Repair Inhibitors: Potential Targets and Partners for Targeted Radionuclide Therapy

The present review aims to explore the potential targets/partners for future targeted radionuclide therapy (TRT) strategies, wherein cancer cells often are not killed effectively, despite receiving a high average tumor radiation dose. Here, we shall discuss the key factors in the cancer genome, especially those related to DNA damage response/repair and maintenance systems for escaping cell death in cancer cells. To overcome the current limitations of TRT effectiveness due to radiation/drug-tolerant cells and tumor heterogeneity, and to make TRT more effective, we propose that a promising strategy would be to target the DNA maintenance factors that are crucial for cancer survival. Considering their cancer-specific DNA damage response/repair ability and dysregulated transcription/epigenetic system, key factors such as PARP, ATM/ATR, amplified/overexpressed transcription factors, and DNA methyltransferases have the potential to be molecular targets for Auger electron therapy; moreover, their inhibition by non-radioactive molecules could be a partnering component for enhancing the therapeutic response of TRT.

Ga68-FAPI Imaging and Lu177-FAPI Therapy in a Case of Metastatic Solitary Fibrous Tumor

Solitary fibrous tumor (SFT) of the central nervous system (previously called hemangiopericytoma) is a rare mesenchymal tumor. Malignant SFT has a tendency to recur after surgery and can metastasize to distant organs. Treatment options for metastatic disease are limited. This case demonstrated high expression of FAP (fibroblast activating protein) in all metastatic sites with Ga-FAPI positron emission tomography-computed tomography imaging. Subsequently, the patient was treated with Lu177-FAPI-targeted radionuclide therapy. There was significant clinical response. There was mild partial morphological response seen on follow-up imaging.

Preclinical Evaluation of a 64Cu-Based Theranostic Approach in a Murine Model of Multiple Myeloma

Although the concept of theranostics is neither new nor exclusive to nuclear medicine, it is a particularly promising approach for the future of nuclear oncology. This approach is based on the use of molecules targeting specific biomarkers in the tumour or its microenvironment, associated with optimal radionuclides which, depending on their emission properties, allow the combination of diagnosis by molecular imaging and targeted radionuclide therapy (TRT). Copper-64 has suitable decay properties (both β⁺ and β- decays) for PET imaging and potentially for TRT, making it both an imaging and therapy agent. We developed and evaluated a theranostic approach using a copper-64 radiolabelled anti-CD138 antibody, [⁶⁴Cu]Cu-TE1PA-9E7.4 in a MOPC315.BM mouse model of multiple myeloma. PET imaging using [⁶⁴Cu]Cu-TE1PA-9E7.4 allows for high-resolution PET images. Dosimetric estimation from ex vivo biodistribution data revealed acceptable delivered doses to healthy organs and tissues, and a very encouraging tumour absorbed dose for TRT applications. Therapeutic efficacy resulting in delayed tumour growth and increased survival without inducing major or irreversible toxicity has been observed with 2 doses of 35 MBq administered at a 2-week interval. Repeated injections of [⁶⁴Cu]Cu-TE1PA-9E7.4 are safe and can be effective for TRT application in this syngeneic preclinical model of MM.

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.

Click chemistry: a transformative technology in nuclear medicine

The 2022 Nobel Prize in Chemistry was awarded to Professors K. Barry Sharpless, Morten Meldal and Carolyn Bertozzi for their pioneering roles in the advent of click chemistry. Sharpless and Meldal worked to develop the canonical click reaction-the copper-catalyzed azide-alkyne cycloaddition-while Bertozzi opened new frontiers with the creation of the bioorthogonal strain-promoted azide-alkyne cycloaddition. These two reactions have revolutionized chemical and biological science by facilitating selective, high yielding, rapid and clean ligations and by providing unprecedented ways to manipulate living systems. Click chemistry has affected every aspect of chemistry and chemical biology, but few disciplines have been impacted as much as radiopharmaceutical chemistry. The importance of speed and selectivity in radiochemistry make it an almost tailor-made application of click chemistry. In this Perspective, we discuss the ways in which the copper-catalyzed azide-alkyne cycloaddition, the strain-promoted azide-alkyne cycloaddition and a handful of 'next-generation' click reactions have transformed radiopharmaceutical chemistry, both as tools for more efficient radiosyntheses and as linchpins of technologies that have the potential to improve nuclear medicine.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

Nuclear medicine in pheochromocytoma and paraganglioma: at a crossroads with precision medicine

Precision medicine (PM) aims to maximize the risk-benefit balance of medical decisions by integrating individual patient and disease characteristics. This approach is gaining increasing recognition from clinicians, healthcare systems, pharmaceutical companies, patients, and governments. Nuclear medicine plays a critical role in PM by its virtue of providing critical information at every step of disease management, digital markers, and companion diagnostics/therapeutics. It is anticipated that technological breakthroughs and new tracers will continue to position nuclear medicine among the significant players in PM.

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.

Synthesis and radiolabelling of PSMA-targeted derivatives containing GYK/MVK cleavable linkers

Targeted radionuclide therapy (TRT) is a promising strategy to treat different types of cancer. TRT relies on a targeting vector used to deliver a therapeutic radionuclide specifically to the tumour site. Several low molecular weight ligands targeting the prostate-specific membrane antigen (PSMA) have been synthesized, but their pharmacokinetic properties still need to be optimized. Hereby, we describe the synthesis of new conjugates, featuring the cleavable linkers Gly-Tyr-Lys (GYK) and Met-Val-Lys (MVK), to reduce the dose delivered to the kidneys. Compounds were synthesized by solid-phase peptide synthesis (SPPS) and obtained in greater than 95% chemical purity. Radiolabelling was performed with both In-111 and Lu-177 to validate potential use of the compounds as both imaging and therapeutic agents. Radiochemical purity greater than 80% was obtained for both nuclides, but significant radiolysis was observed for the methionine-containing analogue. The results obtained thus far with the GYK-PSMA conjugate could warrant further biological investigations.

Targeted Radiation and Immune Therapies-Advances and Opportunities for the Treatment of Prostate Cancer

Prostate cancer is the most diagnosed malignancy in men in the United States and the second leading cause of cancer-related death. For localized disease, radiation therapy is a standard treatment that is often curative. For metastatic disease, radiation therapy has been primarily used for palliation, however, several newer systemic radiation therapies have been demonstrated to significantly improve patient outcomes and improve survival. In particular, several targeted radionuclide therapies have been approved for the treatment of advanced-stage cancer, including strontium-89, samarium-153, and radium-223 for bone-metastatic disease, and lutetium-177-labeled PSMA-617 for patients with prostate-specific membrane antigen (PSMA)-expressing metastatic castration-resistant prostate cancer (mCRPC). Contrarily, immune-based treatments have generally demonstrated little activity in advanced prostate cancer, with the exception of the autologous cellular vaccine, sipuleucel-T. This has been attributed to the presence of an immune-suppressive prostate cancer microenvironment. The ability of radiation therapy to not only eradicate tumor cells but also potentially other immune-regulatory cells within the tumor immune microenvironment suggests that targeted radionuclide therapies may be well poised to combine with immune-targeted therapies to eliminate prostate cancer metastases more effectively. This review provides an overview of the recent advances of targeted radiation agents currently approved for prostate cancer, and those being investigated in combination with immunotherapy, and discusses the challenges as well as the opportunities in this field.

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

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

131I-Labeled Anti-HER2 Nanobody for Targeted Radionuclide Therapy of HER2-Positive Breast Cancer

Purpose

The unique structure of nanobodies is advantageous for the development of radiopharmaceuticals for nuclear medicine. Nanobodies targeted to human epidermal growth factor receptor 2 (HER2) can be used as tools for the imaging and therapy of HER2-overexpressing tumors. In this study, we aimed to describe the generation of a ¹³¹I-labeled anti-HER2 nanobody as a targeted radionuclide therapy (TRNT) agent for HER2-positive breast cancer.

Methods

The anti-HER2 nanobody NM-02 was labeled with ¹³¹I using the iodogen method, and its radiochemical purity and stability in vitro were assessed. The pharmacokinetic profile of ¹³¹I-NM-02 was investigated in normal mice. Tumor accumulation, biodistribution, and therapeutic potential of ¹³¹I-NM-02 were evaluated in HER2-positive SKBR3 xenografts; HER2-negative MB-MDA-231 xenografts were used as the control group.

Results

¹³¹I-NM-02 could be readily prepared with satisfactory radiochemical purity and stability in vitro. Apparent tumor uptake was observed in HER2-positive tumor-bearing mice with rapid blood clearance and favorable biodistribution. ¹³¹I-NM-02 could significantly inhibit tumor growth and extend the life of these mice with good organ compatibility. Negligible tumor accumulation and inhibitory effects of ¹³¹I-NM-02 were observed in the negative control group.

Conclusion

¹³¹I-NM-02 has the potential to be explored as a novel tool for TRNT of HER2-positive breast cancer.

Targeting mTORC1 Activity to Improve Efficacy of Radioligand Therapy in Cancer

Radioligand therapy (RLT) represents an effective strategy to treat malignancy by cancer-selective delivery of radioactivity following systemic application. Despite recent therapeutic successes, cancer radioresistance and insufficient delivery of the radioactive ligands, as well as cytotoxicity to healthy organs, significantly impairs clinical efficacy. To improve disease management while minimizing toxicity, in recent years, the combination of RLT with molecular targeted therapies against cancer signaling networks showed encouraging outcomes. Characterization of the key deregulated oncogenic signaling pathways revealed their convergence to activate the mammalian target of rapamycin (mTOR), in which signaling plays an essential role in the regulation of cancer growth and survival. Therapeutic interference with hyperactivated mTOR pathways was extensively studied and led to the development of mTOR inhibitors for clinical applications. In this review, we outline the regulation and oncogenic role of mTOR signaling, as well as recapitulate and discuss mTOR complex 1 (mTORC1) inhibition to improve the efficacy of RLT in cancer.

Mathematical Model for Evaluation of Tumor Response in Targeted Radionuclide Therapy with 211At Using Implanted Mouse Tumor

Recent introduction of alpha-emitting radionuclides in targeted radionuclide therapy has stimulated the development of new radiopharmaceuticals. Preclinical evaluation using an animal experiment with an implanted tumor model is frequently used to examine the efficiency of the treatment method and to predict the treatment response before clinical trials. Here, we propose a mathematical model for evaluation of the tumor response in an implanted tumor model and apply it to the data obtained from the previous experiment of ²¹¹At treatment in a thyroid cancer mouse model. The proposed model is based on the set of differential equations, describing the kinetics of radiopharmaceuticals, the tumor growth, and the treatment response. First, the tumor growth rate was estimated from the control data without injection of ²¹¹At. The kinetic behavior of the injected radionuclide was used to estimate the radiation dose profile to the target tumor, which can suppress the tumor growth in a dose-dependent manner. An additional two factors, including the time delay for the reduction of tumor volume and the impaired recovery of tumor regrowth after the treatment, were needed to simulate the temporal changes of tumor size after treatment. Finally, the parameters obtained from the simulated tumor growth curve were able to predict the tumor response in other experimental settings. The model can provide valuable information for planning the administration dose of radiopharmaceuticals in clinical trials, especially to determine the starting dose at which efficacy can be expected with a sufficient safety margin.

Exploring the Potential of High-Molar-Activity Samarium-153 for Targeted Radionuclide Therapy with [(153)Sm]Sm-DOTA-TATE

Samarium-153 is a promising theranostic radionuclide, but low molar activities (Am) resulting from its current production route render it unsuitable for targeted radionuclide therapy (TRNT). Recent efforts combining neutron activation of 152Sm in the SCK CEN BR2 reactor with mass separation at CERN/MEDICIS yielded high-Am 153Sm. In this proof-of-concept study, we further evaluated the potential of high-Am 153Sm for TRNT by radiolabeling to DOTA-TATE, a well-established carrier molecule binding the somatostatin receptor 2 (SSTR2) that is highly expressed in gastroenteropancreatic neuroendocrine tumors. DOTA-TATE was labeled with 153Sm and remained stable up to 7 days in relevant media. The binding specificity and high internalization rate were validated on SSTR2-expressing CA20948 cells. In vitro biological evaluation showed that [153Sm]Sm-DOTA-TATE was able to reduce CA20948 cell viability and clonogenic potential in an activity-dependent manner. Biodistribution studies in healthy and CA20948 xenografted mice revealed that [153Sm]Sm-DOTA-TATE was rapidly cleared and profound tumor uptake and retention was observed whilst these were limited in normal tissues. This proof-of-concept study showed the potential of mass-separated 153Sm for TRNT and could open doors towards wider applications of mass separation in medical isotope production.

3p-C-NETA: A versatile and effective chelator for development of Al18F-labeled and therapeutic radiopharmaceuticals

Background: Radiolabeled somatostatin analogues (e.g. [⁶⁸Ga]Ga-DOTATATE and [¹⁷⁷Lu]Lu-DOTATATE) have been used to diagnose, monitor, and treat neuroendocrine tumour (NET) patients with great success. [¹⁸F]AlF-NOTA-octreotide, a promising ¹⁸F-labeled somatostatin analogue and potential alternative for ⁶⁸Ga-DOTA-peptides, is under clinical evaluation. However, ideally, the same precursor (combination of chelator-linker-vector) can be used for production of both diagnostic and therapeutic radiopharmaceuticals with very similar (e.g. Al¹⁸F-method in combination with therapeutic radiometals ²¹³Bi/¹⁷⁷Lu) or identical (e.g. complementary Tb-radionuclides) pharmacokinetic properties, allowing for accurate personalised dosimetry estimation and radionuclide therapy of NET patients. In this study we evaluated 3p-C-NETA, as potential theranostic Al¹⁸F-chelator and present first results of radiosynthesis and preclinical evaluation of [¹⁸F]AlF-3p-C-NETA-TATE. Methods: 3p-C-NETA was synthesized and radiolabeled with diagnostic (⁶⁸Ga, Al¹⁸F) or therapeutic (¹⁷⁷Lu, ¹⁶¹Tb, ²¹³Bi, ²²⁵Ac and ⁶⁷Cu) radionuclides at different temperatures (25-95 °C). The in vitro stability of the corresponding radiocomplexes was determined in phosphate-buffered saline (PBS) and human serum. 3p-C-NETA-TATE was synthesized using standard solid/liquid-phase peptide synthesis. [¹⁸F]AlF-3p-C-NETA-TATE was synthesized in an automated AllinOne® synthesis module and the in vitro stability of [¹⁸F]AlF-3p-C-NETA-TATE was evaluated in formulation buffer, PBS and human serum. [¹⁸F]AlF-3p-C-NETA-TATE pharmacokinetics were evaluated using µPET/MRI in healthy rats, with [¹⁸F]AlF-NOTA-Octreotide as benchmark. Results: 3p-C-NETA quantitatively sequestered ¹⁷⁷Lu, ²¹³Bi and ⁶⁷Cu at 25 °C while heating was required to bind Al¹⁸F, ⁶⁸Ga, ¹⁶¹Tb and ²²⁵Ac efficiently. The [¹⁸F]AlF-, [¹⁷⁷Lu]Lu- and [¹⁶¹Tb]Tb-3p-C-NETA-complex showed excellent in vitro stability in both PBS and human serum over the study period. In contrast, [⁶⁷Cu]Cu- and [²²⁵Ac]Ac-, [⁶⁸Ga]Ga-3p-C-NETA were stable in PBS, but not in human serum. [¹⁸F]AlF-3p-C-NETA-TATE was obtained in good radiochemical yield and radiochemical purity. [¹⁸F]AlF-3p-C-NETA-TATE displayed good in vitro stability for 4 h in all tested conditions. Finally, [¹⁸F]AlF-3p-C-NETA-TATE showed excellent pharmacokinetic properties comparable with the results obtained for [¹⁸F]AlF-NOTA-Octreotide. Conclusions: 3p-C-NETA is a versatile chelator that can be used for both diagnostic applications (Al¹⁸F) and targeted radionuclide therapy (²¹³Bi, ¹⁷⁷Lu, ¹⁶¹Tb). It has the potential to be the new theranostic chelator of choice for clinical applications in nuclear medicine.

Novel Radiolabeled TMTP1 for Long-Acting Hepatocellular Carcinoma Therapeutics

Currently, the 5-year survival rate for patients with advanced hepatocellular carcinoma (HCC) is very low. Therefore, there is an urgent need to find new strategies for the treatment of HCC. TMTP1 (NVVRQ) is a tumor-homing peptide that has been shown to target a range of highly metastatic tumor cells. In this study, a novel radiotherapeutic probe, [¹⁷⁷Lu]Lu-DOTA-EB-TMTP1, was synthesized and used to explore the antitumor efficacy in an HCC tumor model. The albumin-binding TMTP1 radioligand was achieved with >98% radiochemical purity. Long tumor retention property of [¹⁷⁷Lu]Lu-DOTA-EB-TMTP1 was exhibited in single photon emission computed tomography (SPECT) imaging and biodistribution study. The [¹⁷⁷Lu]Lu-DOTA-EB-TMTP1 showed significant accumulation in the SMMC-7721 HCC tumor with an uptake value of 9.67 ± 1.27 %ID/g at 8 h and a T/M ratio of 6.4. In radiotherapy studies, 30 days after injection of [¹⁷⁷Lu]Lu-DOTA-EB-TMTP1, the tumor inhibition rate reached 93.2 ± 0.10 and 94.9 ± 0.04% in the 18.5 and 29.6 MBq high-dose groups, respectively. These preclinical data suggest that [¹⁷⁷Lu]Lu-DOTA-EB-TMTP1 may be an effective treatment option for HCC and should be further evaluated in human trials.

Radionuclide Delivery Strategies in Tumor Treatment: A Systematic Review

The aim of this review was to assess recent progress in targeted radionuclide tumor therapy, focusing on the best delivery strategies. A literature search was conducted in PubMed, Web of Science, and Scopus using the terms "radionuclides", "liposomes", "avidin-biotin interaction", "theranostic", and "molecular docking". The 10 year filter was applied, except for the avidin-biotin interaction. Data were retrieved from both preclinical and clinical settings. Three targeting strategies were considered: pretargeting, liposomes, and ligands. Pretargeting can be achieved by exploiting the avidin-biotin interaction. This strategy seems very promising, although it has been investigated mainly in resectable tumors. Radiolabeled liposomes have attracted new interest as probes to identify the most suitable patients for treatment with liposomal formulations of common chemotherapeutics. The use of ligands for the delivery of radiotherapeutics to a specific target is still the most appealing strategy for treating tumors. The most appropriate ligand can be identified by virtually simulating its interaction with the receptor. All strategies showed great potential for use in targeted radionuclide therapy, but they also have numerous drawbacks. The most promising option is probably the one based on the use of new ligands.

Immune Checkpoint Inhibitor-Mediated Cancer Theranostics with Radiolabeled Anti-Granzyme B Peptide

Although immune checkpoint inhibitors (ICI) have revolutionized cancer management, patient response can be heterogeneous, and the development of ICI resistance is increasingly reported. Novel treatment strategies are necessary not only to expand the use of ICI to previously unresponsive tumor types but also to overcome resistance. Targeted radionuclide therapy may synergize well with ICIs since it can promote a pro-inflammatory tumor microenvironment. We investigated the use of a granzyme B targeted peptide (GZP) as a cancer theranostic agent, radiolabeled with 68Ga (68Ga-GZP) as a PET imaging agent and radiolabeled with 90Y (90Y-GZP) as a targeted radionuclide therapy agent for combinational therapy with ICI in murine models of colon cancer. Our results demonstrate that GZP increasingly accumulates in tumor tissue after ICI and that the combination of ICI with 90Y-GZP promotes a dose-dependent response, achieving curative response in some settings and increased overall survival.

Fast beta-emitter Monte Carlo simulations and full patient dose calculations of targeted radionuclide therapy: introducing egs_mird

BACKGROUND

Targeted Radionuclide Therapy (TRT) is a fast-growing field garnering much interest, with several clinical trials currently underway, that has a steady increase in development of treatment techniques. Unfortunately, within the field and many clinical trials, the dosimetry calculation techniques used remain relatively simple, often using a mix of S-value calculations and kernel convolutions.

PURPOSE

The common TRT calculation techniques, although very quick, can often ignore important aspects of patient anatomy and radionuclide distribution, as well as the interplay there-in. This paper introduces egs_mird, a new Monte Carlo (MC) application built in EGSnrc which allows users to model full patient tissue and density (using clinical CT images) and radionuclide distribution (using clinical PET images) for fast and detailed dose TRT calculation.

METHODS

The novel application egs_mird is introduced along with a general outline of the structure of egs_mird simulations. The general structure of the code, and the track-length estimator scoring implementation for variance reduction, is described. A new egs++ source class egs_internal_source, created to allow detailed patient-wide source distribution, and a modified version of egs_radionuclide_source, changed to be able to work with egs_internal_source, are also described. The new code is compared to other MC calculations of S-values kernels of ¹³¹ I, ⁹⁰ Y, and ¹⁷⁷ Lu in the literature, along with further self-validation using a histogram variant of the electron Fano test. Several full patient prostate ¹⁷⁷ Lu TRT prostate cancer treatment simulations are performed using a single set of patient DICOM CT and [¹⁸ F]-DCFPyL PET data.

RESULTS

Good agreement is found between S-value kernels calculated using egs_mird with egs_internal_source and those found in the literature. Calculating 1000 doses (individual voxel uncertainties of ∼0.05%) in a voxel grid Fano test for monoenergetic 500 keV electrons and ¹⁷⁷ Lu electrons results in 94 and 99% of the doses being within 0.1% of the expected dose, respectively. For a hypothetical ¹⁷⁷ Lu treatment, patient prostate, rectum, bone marrow, and bladder dose volume histograms (DVHs) results did not vary significantly when using the track-length estimator and not modelling electron transport, modelling bone marrow explicitly (rather than using generic tissue compositions), and reducing activity to voxels containing partial or full calcifications to half or none, respectively. Dose profiles through different regions demonstrate there are some differences with model choices not seen in the DVH. Simulations using the track-length estimator can be completed in under 15 minutes (∼30 minutes when using standard interaction scoring).

CONCLUSION

This work shows egs_mird to be a reliable MC code for computing TRT doses as realistically as the patient CT and PET data allow. Furthermore, the code can compute doses to sub-1% uncertainty within 15 minutes, with little to no optimization. Thus, this work supports the use of egs_mird for dose calculations in TRT. This article is protected by copyright. All rights reserved.

Perspective on the Use of DNA Repair Inhibitors as a Tool for Imaging and Radionuclide Therapy of Glioblastoma

Simple Summary

The current routine treatment for glioblastoma (GB), the most lethal high-grade brain tumor in adults, aims to induce DNA damage in the tumor. However, the tumor cells might be able to repair that damage, which leads to therapy resistance. Fortunately, DNA repair defects are common in GB cells, and their survival is often based on a sole backup repair pathway. Hence, targeted drugs inhibiting essential proteins of the DNA damage response have gained momentum and are being introduced in the clinic. This review gives a perspective on the use of radiopharmaceuticals targeting DDR kinases for imaging in order to determine the DNA repair phenotype of GB, as well as for effective radionuclide therapy. Finally, four new promising radiopharmaceuticals are suggested with the potential to lead to a more personalized GB therapy.

Abstract

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

Novel NK1R-Targeted 68Ga-/177Lu-Radioconjugates with Potential Application against Glioblastoma Multiforme: Preliminary Exploration of Structure-Activity Relationships

Locoregionally administered, NK1 receptor (NK1R) targeted radionuclide therapy is a promising strategy for the treatment of glioblastoma multiforme. So far, the radiopharmaceuticals used in this approach have been based on the endogenous agonist of NK1R, Substance P or on its close analogues. Herein, we used a well-known, small molecular NK1R antagonist, L732,138, as the basis for the radiopharmaceutical vector. First, 14 analogues of this compound were evaluated to check whether extending the parent structure with linkers of different lengths would not deteriorate the NK1R binding. The tested analogues had affinity similar to or better than the parent compound, and none of the linkers had a negative impact on the binding. Next, five DOTA conjugates were synthesized and used for labelling with ⁶⁸Ga and ¹⁷⁷Lu. The obtained radioconjugates turned out to be fairly lipophilic but showed rather limited stability in human plasma. Evaluation of the receptor affinity of the (radio)conjugates showed that neither the chelator nor the metal negatively impacts the NK1R binding. The ¹⁷⁷Lu-radioconjugates exhibited the binding characteristics towards NK1R similar or better than that of the ¹⁷⁷Lu-labelled derivative of Substance P, which is in current clinical use. The experimental results presented herein, along with their structural rationalization provided by modelling, give insight for the further molecular design of small molecular NK1R-targeting vectors.

Fibroblast activation protein-based theranostics in cancer research: A state-of-the-art review

In recent years, quinoline-based fibroblast activation protein (FAP) inhibitors (FAPI) have shown promising results in the diagnosis of cancer and several other diseases, making them the hotspot of much productive research. This review summarizes the literature for the state-of-the-art FAPI-PET imaging for cancer diagnosis compared with fluorodeoxyglucose (FDG)-PET. We also summarize the use of FAPI-PET for therapeutic regimen improvement and fibroblast activation protein (FAP)-targeted molecule modification strategies, as well as preliminary clinical studies regarding FAP-targeted radionuclide therapy. Our qualitative summary of the literature to date can inform future research directions, medical guidelines, and optimal clinical decision-making.

A High Separation Factor for 165Er from Ho for Targeted Radionuclide Therapy

Background: Radionuclides emitting Auger electrons (AEs) with low (0.02–50 keV) energy, short (0.0007–40 µm) range, and high (1–10 keV/µm) linear energy transfer may have an important role in the targeted radionuclide therapy of metastatic and disseminated disease. Erbium-165 is a pure AE-emitting radionuclide that is chemically matched to clinical therapeutic radionuclide ¹⁷⁷Lu, making it a useful tool for fundamental studies on the biological effects of AEs. This work develops new biomedical cyclotron irradiation and radiochemical isolation methods to produce ¹⁶⁵Er suitable for targeted radionuclide therapeutic studies and characterizes a new such agent targeting prostate-specific membrane antigen. Methods: Biomedical cyclotrons proton-irradiated spot-welded Ho_(m) targets to produce ¹⁶⁵Er, which was isolated via cation exchange chromatography (AG 50W-X8, 200–400 mesh, 20 mL) using alpha-hydroxyisobutyrate (70 mM, pH 4.7) followed by LN2 (20–50 µm, 1.3 mL) and bDGA (50–100 µm, 0.2 mL) extraction chromatography. The purified ¹⁶⁵Er was radiolabeled with standard radiometal chelators and used to produce and characterize a new AE-emitting radiopharmaceutical, [¹⁶⁵Er]PSMA-617. Results: Irradiation of 80–180 mg ^natHo targets with 40 µA of 11–12.5 MeV protons produced ¹⁶⁵Er at 20–30 MBq·µA⁻¹·h⁻¹. The 4.9 ± 0.7 h radiochemical isolation yielded ¹⁶⁵Er in 0.01 M HCl (400 µL) with decay-corrected (DC) yield of 64 ± 2% and a Ho/¹⁶⁵Er separation factor of (2.8 ± 1.1) · 10⁵. Radiolabeling experiments synthesized [¹⁶⁵Er]PSMA-617 at DC molar activities of 37–130 GBq·µmol⁻¹. Conclusions: A 2 h biomedical cyclotron irradiation and 5 h radiochemical separation produced GBq-scale ¹⁶⁵Er suitable for producing radiopharmaceuticals at molar activities satisfactory for investigations of targeted radionuclide therapeutics. This will enable fundamental radiation biology experiments of pure AE-emitting therapeutic radiopharmaceuticals such as [¹⁶⁵Er]PSMA-617, which will be used to understand the impact of AEs in PSMA-targeted radionuclide therapy of prostate cancer.

CS1-specific single-domain antibodies labeled with Actinium-225 prolong survival and increase CD8+ T cells and PD-L1 expression in Multiple Myeloma

Multiple myeloma (MM) is a hematological malignancy characterized by the presence of clonal plasma cells in the bone marrow niche. Despite significant therapeutic advances, MM remains incurable for the majority of patients. Targeted radionuclide therapy (TRNT) has emerged as a promising treatment option to eradicate residual cancer cells. In this study, we developed and characterized single-domain antibodies (sdAbs) against the MM-antigen CS1 and evaluated its therapeutic potential in MM using TRNT. We first validated CS1 as potential target for TRNT. CS1 is expressed in normal and malignant plasma cells in different disease stages including progression and relapse. It is expressed in dormant as well as proliferating MM cells, while low expression could be observed in environmental cells. Biodistribution studies demonstrated the specific uptake of anti-CS1 sdAbs in tissues of 5TMM cell infiltration including bone, spleen and liver. TRNT using anti-CS1 sdAbs labeled with actinium-225 significantly prolonged survival of syngeneic, immunocompetent 5T33MM mice. In addition, we observed an increase in CD8⁺ T-cells and more overall PD-L1 expression on immune and non-immune cells, implying an interferon gamma signature using actinium-225 labeled CS1-directed sdAbs. In this proof-of-principle study, we highlight, for the first time, the therapeutic potential and immunomodulating effects of anti-CS1 radionuclide therapy to target residual MM cells.

Multifunctional Cyanine-Based Theranostic Probe for Cancer Imaging and Therapy

Cancer is one of the leading causes of death in the world. A cancer-targeted multifunctional probe labeled with the radionuclide has been developed to provide multi-modalities for NIR fluorescence and nuclear imaging (PET, SPECT), for photothermal therapy (PTT), and targeted radionuclide therapy of cancer. In this study, synthesis, characterization, in vitro, and in vivo biological evaluation of the cyanine-based probe (DOTA-NIR790) were demonstrated. The use of cyanine dyes for the selective accumulation of cancer cells were used to achieve the characteristics of tumor markers. Therefore, all kinds of organ tumors can be targeted for diagnosis and treatment. The DOTA-NIR790 labeled with lutetium-111 could detect original or metastatic tumors by using SPECT imaging and quantify tumor accumulation. The β-emission of ¹⁷⁷Lu-DOTA-NIR790 can be used for targeted radionuclide therapy of tumors. The DOTA-NIR790 enabled imaging by NIR fluorescence and by nuclear imaging (SPECT) to monitor in real-time the tumor accumulation and the situation of cancer therapy, and to guide the surgery or the photothermal therapy of the tumor. The radionuclide-labeled heptamethine cyanine based probe (DOTA-NIR790) offers multifunctional modalities for imaging and therapies of cancer.

A Mathematical Modeling Approach for Targeted Radionuclide and Chimeric Antigen Receptor T Cell Combination Therapy

Simple Summary

Targeted radionuclide therapy (TRT) and immunotherapy, an example being chimeric antigen receptor T cells (CAR-Ts), represent two potent means of eradicating systemic cancers. Although each one as a monotherapy might have a limited effect, the potency can be increased with a combination of the two therapies. The complications involved in the dosing and scheduling of these therapies make the mathematical modeling of these therapies a suitable solution for designing combination treatment approaches. Here, we investigate a mathematical model for TRT and CAR-T cell combination therapies. Through an analysis of the mathematical model, we find that the tumor proliferation rate is the most important factor affecting the scheduling of TRT and CAR-T cell treatments with faster proliferating tumors requiring a shorter interval between the two therapies.

Abstract

Targeted radionuclide therapy (TRT) has recently seen a surge in popularity with the use of radionuclides conjugated to small molecules and antibodies. Similarly, immunotherapy also has shown promising results, an example being chimeric antigen receptor T cell (CAR-T) therapy in hematologic malignancies. Moreover, TRT and CAR-T therapies possess unique features that require special consideration when determining how to dose as well as the timing and sequence of combination treatments including the distribution of the TRT dose in the body, the decay rate of the radionuclide, and the proliferation and persistence of the CAR-T cells. These characteristics complicate the additive or synergistic effects of combination therapies and warrant a mathematical treatment that includes these dynamics in relation to the proliferation and clearance rates of the target tumor cells. Here, we combine two previously published mathematical models to explore the effects of dose, timing, and sequencing of TRT and CAR-T cell-based therapies in a multiple myeloma setting. We find that, for a fixed TRT and CAR-T cell dose, the tumor proliferation rate is the most important parameter in determining the best timing of TRT and CAR-T therapies.

Nanobody-Based Theranostic Agents for HER2-Positive Breast Cancer: Radiolabeling Strategies

The overexpression of human epidermal growth factor 2 (HER2) in breast cancer (BC) has been associated with a more aggressive tumor subtype, poorer prognosis and shorter overall survival. In this context, the development of HER2-targeted radiotracers is crucial to provide a non-invasive assessment of HER2 expression to select patients for HER2-targeted therapies, monitor response and identify those who become resistant. Antibodies represent ideal candidates for this purpose, as they provide high contrast images for diagnosis and low toxicity in the therapeutic setting. Of those, nanobodies (Nb) are of particular interest considering their favorable kinetics, crossing of relevant biological membranes and intratumoral distribution. The purpose of this review is to highlight the unique characteristics and advantages of Nb-based radiotracers in BC imaging and therapy. Additionally, radiolabeling methods for Nb including direct labeling, indirect labeling via prosthetic group and indirect labeling via complexation will be discussed, reporting advantages and drawbacks. Furthermore, the preclinical to clinical translation of radiolabeled Nbs as promising theranostic agents will be reported.

Oligonucleotide-Functionalized Gold Nanoparticles for Synchronous Telomerase Inhibition, Radiosensitization, and Delivery of Theranostic Radionuclides

Telomerase represents an attractive target in oncology as it is expressed in cancer but not in normal tissues. The oligonucleotide inhibitors of telomerase represent a promising anticancer strategy, although poor cellular uptake can restrict their efficacy. In this study, gold nanoparticles (AuNPs) were used to enhance oligonucleotide uptake. "match" oligonucleotides complementary to the telomerase RNA template subunit (hTR) and "scramble" (control) oligonucleotides were conjugated to diethylenetriamine pentaacetate (DTPA) for 111In-labeling. AuNPs (15.5 nm) were decorated with a monofunctional layer of oligonucleotides (ON-AuNP) or a multifunctional layer of oligonucleotides, PEG(polethylene glycol)800-SH (to reduce AuNP aggregation) and the cell-penetrating peptide Tat (ON-AuNP-Tat). Match-AuNP enhanced the cellular uptake of radiolabeled oligonucleotides while retaining the ability to inhibit telomerase activity. The addition of Tat to AuNPs increased nuclear localization. 111In-Match-AuNP-Tat induced DNA double-strand breaks and caused a dose-dependent reduction in clonogenic survival of telomerase-positive cells but not telomerase-negative cells. hTR inhibition has been reported to sensitize cancer cells to ionizing radiation, and 111In-Match-AuNP-Tat therefore holds promise as a vector for delivery of radionuclides into cancer cells while simultaneously sensitizing them to the effects of the emitted radiation.

Modeling early radiation DNA damage occurring during [177Lu]Lu-DOTA-[Tyr3]octreotate radionuclide therapy

The aim of this study was to build a simulation framework to evaluate the number of DNA double-strand breaks (DSBs) induced by in vitro targeted radionuclide therapy (TRT). This work represents the first step toward exploring underlying biologic mechanisms and the influence of physical and chemical parameters to enable a better response prediction in patients. We used this tool to characterize early DSB induction by ¹⁷⁷Lu-DOTATATE, a commonly used TRT for neuroendocrine tumors. Methods: A multiscale approach was implemented to simulate the number of DSBs produced over 4 h by the cumulated decays of ¹⁷⁷Lu distributed according to the somatostatin receptor binding. The approach involves 2 sequential simulations performed with Geant4/Geant4-DNA. The radioactive source is sampled according to uptake experiments on the distribution of activities within the medium and the planar cellular cluster, assuming instant and permanent internalization. A phase space is scored around the nucleus of the central cell. Then, the phase space is used to generate particles entering the nucleus containing a multiscale description of the DNA in order to score the number of DSBs per particle source. The final DSB computations are compared with experimental data, measured by immunofluorescent detection of p53-binding protein 1 foci. Results: The probability of electrons reaching the nucleus was significantly influenced by the shape of the cell compartment, causing a large variance in the induction pattern of DSBs. A significant difference was found in the DSBs induced by activity distributions in cell and medium, as is explained by the specific energy (z¯) distributions. The average number of simulated DSBs was 14 DSBs per cell (range, 7–24 DSBs per cell), compared with 13 DSBs per cell (range, 2–30 DSBs per cell) experimentally determined. We found a linear correlation between the mean absorbed dose to the nucleus and the number of DSBs per cell: 0.014 DSBs per cell mGy⁻¹ for internalization in the Golgi apparatus and 0.017 DSBs per cell mGy⁻¹ for internalization in the cytoplasm. Conclusion: This simulation tool can lead to a more reliable absorbed-dose–to–DNA correlation and help in prediction of biologic response.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives

Simple Summary

One of the most frequently diagnosed cancer in men is adenocarcinoma of the prostate. Once the disease is metastatic, only very limited treatment options are available, resulting in a very short median survival time of 13 months; however, this reality is gradually changing due to the discovery of prostate-specific membrane antigen (PSMA), a protein that is present in cancerous prostate tissue. Researchers have developed pharmaceuticals specific for PSMA, ranging from antibodies (mAb) to low-molecular weight molecules coupled to beta minus and alpha-emitting radionuclides for their use in targeted radionuclide therapy (TRT). TRT offers the possibility of selectively removing cancer tissue via the emission of radiation or radioactive particles within the tumour. In this article, the major milestones in PSMA ligand research and the therapeutic developments are summarised, together with a future perspective on the enhancement of current therapeutic approaches.

Abstract

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

Production of Sm-153 With Very High Specific Activity for Targeted Radionuclide Therapy

Samarium-153 (153Sm) is a highly interesting radionuclide within the field of targeted radionuclide therapy because of its favorable decay characteristics. 153Sm has a half-life of 1.93 d and decays into a stable daughter nuclide (153Eu) whereupon β- particles [E = 705 keV (30%), 635 keV (50%)] are emitted which are suitable for therapy. 153Sm also emits γ photons [103 keV (28%)] allowing for SPECT imaging, which is of value in theranostics. However, the full potential of 153Sm in nuclear medicine is currently not being exploited because of the radionuclide's limited specific activity due to its carrier added production route. In this work a new production method was developed to produce 153Sm with higher specific activity, allowing for its potential use in targeted radionuclide therapy. 153Sm was efficiently produced via neutron irradiation of a highly enriched 152Sm target (98.7% enriched, σth = 206 b) in the BR2 reactor at SCK CEN. Irradiated target materials were shipped to CERN-MEDICIS, where 153Sm was isolated from the 152Sm target via mass separation (MS) in combination with laser resonance enhanced ionization to drastically increase the specific activity. The specific activity obtained was 1.87 TBq/mg (≈ 265 times higher after the end of irradiation in BR2 + cooling). An overall mass separation efficiency of 4.5% was reached on average for all mass separations. Further radiochemical purification steps were developed at SCK CEN to recover the 153Sm from the MS target to yield a solution ready for radiolabeling. Each step of the radiochemical process was fully analyzed and characterized for further optimization resulting in a high efficiency (overall recovery: 84%). The obtained high specific activity (HSA) 153Sm was then used in radiolabeling experiments with different concentrations of 4-isothiocyanatobenzyl-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA). Even at low concentrations of p-SCN-Bn-DOTA, radiolabeling of 0.5 MBq of HSA 153Sm was found to be efficient. In this proof-of-concept study, we demonstrated the potential to combine neutron irradiation with mass separation to supply high specific activity 153Sm. Using this process, 153SmCl3 suitable for radiolabeling, was produced with a very high specific activity allowing application of 153Sm in targeted radionuclide therapy. Further studies to incorporate 153Sm in radiopharmaceuticals for targeted radionuclide therapy are ongoing.

Production, Purification, and Applications of a Potential Theranostic Pair: Cobalt-55 and Cobalt-58m

The emerging success of [⁶⁸Ga/¹⁷⁷Lu]Ga/Lu-DOTATATE as a theranostic pair has spurred interest in other isotopes as potential theranostic combinations. Here, we review cobalt-55 and cobalt-58m as a potential theranostic pair. Radionuclidically pure cobalt-55 and cobalt-58m have been produced on small cyclotrons with high molar activity. In vitro, DOTATOC labeled with cobalt has shown greater affinity for SSTR2 than DOTATOC labeled with gallium and yttrium. Similarly, [^58mCo]Co-DOTATATE has shown improved cell-killing capabilities as compared to DOTATATE labeled with either indium-111 or lutetium-177. Finally, PET imaging with an isotope such as cobalt-55 allows for image acquisition at much later timepoints than gallium, allowing for an increased degree of biological clearance of non-bound radiotracer. We discuss the accelerator targetry and radiochemistry used to produce cobalt-55,58m, emphasizing the implications of these techniques to downstream radiotracers being developed for imaging and therapy.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

The application of theranostics in different stages of prostate cancer

Despite the remarkable achievements in treating metastatic prostate cancer over the last two decades, castrate-resistant status is still considered the lethal stage of the disease. Theranostics combines a targeting compound (ligand) with a therapeutic radioisotope (radioactive particle) injected into the blood to target the cancer cells. The most studied radioligand is ¹⁷⁷Lu-PSMA-617, which targets PSMA, a protein found in prostate cancer cells. This new approach has shown promising results in treating metastatic castration-resistant prostate cancer. Currently, many trials are using PSMA-targeting radioligands in combination with conventional therapies in advanced prostate cancer or even in the earlier stages of the disease. Other preclinical trials are exploring the possibility of using newer ligands or radioisotopes to treat prostate cancer to increase the specificity and efficacy of this treatment.

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.