Preparation of 212Pb-labeled monoclonal antibody using a novel 224Ra-based generator solution

Antigen targeting and anti-tumor activity of a novel anti-CD146 212Pb internalizing alpha-radioimmunoconjugate against malignant peritoneal mesothelioma

Malignant mesothelioma, a highly aggressive cancer that primarily affects the serosal membranes, has limited therapeutic options, particularly for cavitary tumors, such as peritoneal and pleural malignant mesothelioma. Intracavitary administration of a radioimmunoconjugate to locally target mesothelioma cancer cells has been proposed as a treatment. CD146, upregulated in mesothelioma but not in healthy tissues, is a promising therapeutic target. This study characterized CD146 expression and binding/internalization kinetics of the CD146-targeting antibody OI-3 coupled with 212Pb (212Pb-TCMC-OI-3) in human mesothelioma cells. Flow cytometry showed that both chimeric (chOI-3) and murine (mOI-3) antibodies rapidly bound and internalized within 1-6 h in MSTO-211H cells. 212Pb-TCMC-chOI-3 exhibited 3.1- to 13.7-fold and 3.1- to 8.5-fold increased internalized 212Pb and 212Bi atoms per cell at 2 and 24 h, respectively, compared to isotype control, underscoring enhanced internalization efficiency. Intraperitoneal administration of 212Pb-TCMC-mOI-3 to mice with intraperitoneal MSTO-211H xenografts improved median survival by a ratio of 1.3 compared to non-binding 212Pb-TCMC-mIgG1. The ability of 212Pb-TCMC-mOI-3 to target and inhibit the growth of intraperitoneal mesothelioma xenografts supports targeted radionuclide therapy's efficacy for metastatic peritoneal mesothelioma. This study highlights the potential of localized CD146-targeted radioimmunotherapy for malignant mesothelioma, offering a new avenue for improving patient outcomes.

Transition and Post-Transition Radiometals for PET Imaging and Radiotherapy

Radiometals are an exciting class of radionuclides because of the large number of metallic elements available that have medically useful isotopes. To properly harness radiometals, they must be securely bound by chelators, which must be carefully matched to the radiometal ion to maximize radiolabeling performance and the stability of the resulting complex. This chapter focuses on practical aspects of radiometallation chemistry including chelator selection, radiolabeling procedures and conditions, radiolysis prevention, purification, quality control, requisite equipment and reagents, and useful tips.

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.

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

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

Radiometals in Imaging and Therapy: Highlighting Two Decades of Research

The present article highlights the important progress made in the last two decades in the fields of molecular imaging and radionuclide therapy. Advancements in radiometal-based positron emission tomography, single photon emission computerized tomography, and radionuclide therapy are illustrated in terms of their production routes and ease of radiolabeling. Applications in clinical diagnostic and radionuclide therapy are considered, including human studies under clinical trials; their current stages of clinical translations and findings are summarized. Because the metalloid astatine is used for imaging and radionuclide therapy, it is included in this review. In regard to radionuclide therapy, both beta-minus (β-) and alpha (α)-emitting radionuclides are discussed by highlighting their production routes, targeted radiopharmaceuticals, and current clinical translation stage.

Mathematical Modeling of In Vivo Alpha Particle Generators and Chelator Stability

Background: Targeted α particle therapy using long-lived in vivo α particle generators is cytotoxic to target tissues. However, the redistribution of released radioactive daughters through the circulation should be considered. A mathematical model was developed to describe the physicochemical kinetics of 212Pb-labeled pharmaceuticals and its radioactive daughters. Materials and Methods: A bolus of 212Pb-labeled pharmaceuticals injected in a developed compartmental model was simulated. The contributions of chelated and free radionuclides to the total released energy were investigated for different dissociation fractions of 212Bi for different chelators, for example, 36% for DOTA. The compartmental model was applied to describe a 212Bi retention study and to assess the stability of the 212Bi-1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (212Bi-DOTAM) complex after β- decay of 212Pb. Results: The simulation of the injection showed that α emissions contribute 75% to the total released energy, mostly from 212Po (72%). The simulation of the 212Bi retention study showed that (16 ± 5)% of 212Bi atoms dissociate from the 212Bi-DOTAM complexes. The fractions of energies released by free radionuclides were 21% and 38% for DOTAM and DOTA chelators, respectively. Conclusion: The developed α particle generator model allows for simulating the radioactive kinetics of labeled and unlabeled pharmaceuticals being released from the chelating system due to a preceding disintegration.

CD46 targeted 212Pb alpha particle radioimmunotherapy for prostate cancer treatment

We recently identified CD46 as a novel prostate cancer cell surface antigen that shows lineage independent expression in both adenocarcinoma and small cell neuroendocrine subtypes of metastatic castration resistant prostate cancer (mCRPC), discovered an internalizing human monoclonal antibody YS5 that binds to a tumor selective CD46 epitope, and developed a microtubule inhibitor-based antibody drug conjugate that is in a multi-center phase I trial for mCRPC (NCT03575819). Here we report the development of a novel CD46-targeted alpha therapy based on YS5. We conjugated 212Pb, an in vivo generator of alpha-emitting 212Bi and 212Po, to YS5 through the chelator TCMC to create the radioimmunoconjugate, 212Pb-TCMC-YS5. We characterized 212Pb-TCMC-YS5 in vitro and established a safe dose in vivo. We next studied therapeutic efficacy of a single dose of 212Pb-TCMC-YS5 using three prostate cancer small animal models: a subcutaneous mCRPC cell line-derived xenograft (CDX) model (subcu-CDX), an orthotopically grafted mCRPC CDX model (ortho-CDX), and a prostate cancer patient-derived xenograft model (PDX). In all three models, a single dose of 0.74 MBq (20 µCi) 212Pb-TCMC-YS5 was well tolerated and caused potent and sustained inhibition of established tumors, with significant increases of survival in treated animals. A lower dose (0.37 MBq or 10 µCi 212Pb-TCMC-YS5) was also studied on the PDX model, which also showed a significant effect on tumor growth inhibition and prolongation of animal survival. These results demonstrate that 212Pb-TCMC-YS5 has an excellent therapeutic window in preclinical models including PDXs, opening a direct path for clinical translation of this novel CD46-targeted alpha radioimmunotherapy for mCRPC treatment.

Bone Marrow Relative Biological Effectiveness for a 212Pb-labeled Anti-HER2/neu Antibody

PURPOSE

We have determined the in vivo relative biological effectiveness (RBE) of an alpha-particle-emitting radiopharmaceutical therapeutic agent (²¹²Pb-labeled anti-HER2/neu antibody) for the bone marrow, a potentially dose-limiting normal tissue.

METHODS AND MATERIALS

The RBE was measured in mice using femur marrow cellularity as the biological endpoint. External beam radiation therapy (EBRT), delivered by a small-animal radiation research platform was used as the reference radiation. Alpha-particle emissions were delivered by ²¹²Bi after the decay of its parent nuclide ²¹²Pb, which was conjugated onto an anti-HER2/neu antibody. The alpha-particle absorbed dose to the marrow after an intravenous administration (tail vein) of 122.1 to 921.3 kBq ²¹²Pb-TCMC-7.16.4 was calculated. The mice were sacrificed at 0 to 7 days after treatment and the radioactivity from the femur bone marrow was measured. Changes in marrow cellularity were assessed by histopathology.

RESULTS

The dose response for EBRT and ²¹²Pb-anti-HER2/neu antibody were linear-quadratic and linear, respectively. On transforming the EBRT dose-response relationship into a linear relationship using the equivalent dose in 2-Gy fractions of external beam radiation formalism, we obtained an RBE (denoted RBE2) of 6.4, which is independent of cellularity and absorbed dose.

CONCLUSIONS

Because hematologic toxicity is dose limiting in almost all antibody-based RPT, in vivo measurements of RBE are important in helping identify an initial administered activity in phase 1 escalation trials. Applying the RBE2 and assuming typical antibody clearance kinetics (biological half-life of 48 hours), using a modified blood-based dosimetry method, an average administered activity of approximately 185.5 MBq (5.0 mCi) per patient could be administered before hematologic toxicity is anticipated.

Dual targeting with 224Ra/212Pb-conjugates for targeted alpha therapy of disseminated cancers: A conceptual approach

Metastases are the primary cause of death among cancer patients and efficacious new treatments are sorely needed. Targeted alpha-emitting radiopharmaceuticals that are highly cytotoxic may fulfill this critical need. The focus of this paper is to describe and explore a novel technology that may improve the therapeutic effect of targeted alpha therapy by combining two radionuclides from the same decay chain in the same solution. We hypothesize that the dual targeting solution containing bone-seeking 224Ra and cell-directed complexes of progeny 212Pb is a promising approach to treat metastatic cancers with bone and soft tissue lesions as well as skeletal metastases of mixed lytic/osteoblastic nature. A novel liquid 224Ra/212Pb-generator for rapid preparation of a dual targeting solution is described. Cancer cell targeting monoclonal antibodies, their fragments, synthetic proteins or peptides can all be radiolabeled with 212Pb in the 224Ra-solution in transient equilibrium with daughter nuclides. Thus, 224Ra targets stromal elements in sclerotic bone metastases and 212Pb-chelated-conjugate targets tumor cells of metastatic prostate cancer or osteosarcoma. The dual targeting solution may also be explored to treat metastatic breast cancer or multiple myeloma after manipulation of bone metastases to a more osteoblastic phenotype by the use of bisphosphonates, denosumab, bortezomib or hormone therapy prior to treatment. This may improve targeting of bone-seeking 224Ra and render an augmented radiation dose deposited within metastases. Our preliminary preclinical studies provide conceptual evidence that the dual 224Ra-solution with bone or tumor-targeted delivery of 212Pb has potential to inhibit cancer metastases without significant toxicity. In some settings, the use of a booster dose of purified 212Pb-conjugate alone could be required to elevate the effect of this tumor cell directed component, if needed, e.g., in a fractionated treatment regimen, where the dual targeting solution will act as maintenance treatment.

An Experimental Generator for Production of High-Purity 212Pb for Use in Radiopharmaceuticals

The feasibility, performance, and radiation safety of an experimental generator were evaluated to efficiently produce ²¹²Pb intended for radiopharmaceuticals. Methods: The generator consisted of a flask with a removable cap containing a source of ²²⁴Ra or ²²⁸Th absorbed on quartz wool. Gaseous ²²⁰Rn emanated from the decaying source, which subsequently decayed to ²¹²Pb, which was adsorbed on the flask's interior surface. The ²¹²Pb was collected by washing the flask with 0.5-1 mL of 0.1 M HCl. Results: The generator collector flask trapped 62%-68% of the ²¹²Pb, of which more than 87% (tested up to 26 MBq) could be harvested. The obtained ²¹²Pb solution had a high purity (>99.98%) and could be used for the preparation of radioconjugates with more than 97% radiochemical purity. Future designs of the generator should aim to further reduce the risk of radon and γ-energy exposure to operators. Conclusion: The presented technology is a promising method for easy and convenient ²¹²Pb production.

Targeted alpha therapy with the 224Ra/212Pb-TCMC-TP-3 dual alpha solution in a multicellular tumor spheroid model of osteosarcoma

Osteosarcoma patients with overt metastases at primary diagnosis have a 5-year survival rate of less than 20%. TP-3 is a murine IgG2b monoclonal antibody with high affinity for an epitope residing on the p80 osteosarcoma cell surface membrane antigen. The tumor-associated antigen p80 is overexpressed in osteosarcomas, and has very low normal tissue expression. We propose a novel dual alpha targeting solution containing two radionuclides from the same decay chain, including the bone-seeking ²²⁴Ra, and cancer cell-surface seeking ²¹²Pb-TCMC-TP-3 for the treatment of osteoblastic bone cancers, circulating cancer cells and micrometastases. In this in vitro study, the cytotoxic effects of ²¹²Pb-TCMC-TP-3 (single alpha solution) and ²²⁴Ra/²¹²Pb-TCMC-TP-3 (dual alpha solution) were investigated in a multicellular spheroid model mimicking micrometastatic disease in osteosarcoma. OHS spheroids with diameters of 253 ± 98 μm treated with 4.5, 2.7, and 3.3 kBq/ml of ²¹²Pb-TCMC-TP-3 for 1, 4, and 24 h, respectively, were disintegrated within 3 weeks. The ²¹²Pb-TCMC-TP-3 induced a 7-fold delay in spheroid doubling time compared to a 28-times higher dose with the non-specific ²¹²Pb-TCMC-rituximab. The ²²⁴Ra/²¹²Pb-TCMC-TP-3 completely disintegrated spheroids with diameters of 218-476 μm within 3 and 2 weeks after 4 and 24 h incubation with 5 kBq/ml, respectively. Treatment with 1 kBq/ml of ²²⁴Ra/²¹²Pb-TCMC-TP-3 for 24 h caused an 11.4-fold reduction in spheroid viability compared with unconjugated ²²⁴Ra/²¹²Pb. The single and dual alpha solutions with TP-3 showed cytotoxicity in spheroids of clinically relevant size, which warrant further testing of the dual alpha solution using in vivo osteosarcoma models.

Factors Influencing the Therapeutic Efficacy of the PSMA Targeting Radioligand 212Pb-NG001

Simple Summary

Prostate-specific membrane antigen (PSMA) is a protein overexpressed in metastatic castration-resistant prostate cancer and a promising target for targeted radionuclide therapy. PSMA-targeted alpha therapy is of growing interest due to the high-emission energy and short range of alpha particles, resulting in a prominent cytotoxic potency. This study assesses the influence of various factors on the in vitro and in vivo therapeutic efficacy of the alpha particle generating PSMA-targeting radioligand ²¹²Pb-NG001.

Abstract

This study aimed to determine the influence of cellular PSMA expression, radioligand binding and internalization, and repeated administrations on the therapeutic effects of the PSMA-targeting radioligand ²¹²Pb-NG001. Cellular binding and internalization, cytotoxicity, biodistribution, and the therapeutic efficacy of ²¹²Pb-NG001 were investigated in two human prostate cancer cell lines with different PSMA levels: C4-2 (PSMA+) and PC-3 PIP (PSMA+++). Despite 10-fold higher PSMA expression on PC-3 PIP cells, cytotoxicity and therapeutic efficacy of the radioligand was only 1.8-fold better than for the C4-2 model, possibly explained by lower cellular internalization and less blood-rich stroma in PC-3 PIP xenografts. Mice bearing subcutaneous PC-3 PIP xenografts were treated with 0.2, 0.4, and 0.8 MBq of ²¹²Pb-NG001 that resulted in therapeutic indexes of 2.7, 3.0, and 3.5, respectively. A significant increase in treatment response was observed in mice that received repeated injections compared to the corresponding single dose (therapeutic indexes of 3.6 for 2 × 0.2 MBq and 4.4 for 2 × 0.4 MBq). The results indicate that ²¹²Pb-NG001 can induce therapeutic effects at clinically transferrable doses, both in the C4-2 model that resembles solid tumors and micrometastases with natural PSMA expression and in the PC-3 PIP model that mimics poorly vascularized metastases.

212Pb: Production Approaches and Targeted Therapy Applications

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

212Pb-conjugated anti-rat HER2/neu antibody against a neu-N derived murine mammary carcinoma cell line: cell kill and RBE in vitro

PURPOSE

In the current work, the RBE of a ²¹²Pb-conjugated anti-HER2/neu antibody construct has been evaluated, in vitro, by colony formation assay. The RBE was estimated by comparing two absorbed dose-survival curves: the first obtained from the conjugated ²¹²Pb experiments (test radiation), the second obtained by parallel experiments of single bolus irradiation of external beam (reference radiation).

MATERIALS AND METHODS

Mammary carcinoma NT2.5 cells were treated with (0-3.70) kBq/ml of radiolabeled antibody. Nonspecific binding was assessed with addition of excess amount of unlabeled antibody. The colony formation curves were converted from activity concentration to cell nucleus absorbed dose by simulating the decay and transport of all daughter and secondary particles of ²¹²Pb, using the Monte Carlo code GEANT 4.

RESULTS

The radiolabeled antibody yielded an RBE of 8.3 at 37% survival and a survival independent RBE (i.e. RBE2) of 9.9. Unbound/untargeted ²¹²Pb-labeled antibody, as obtained in blocking experiments yielded minimal alpha-particle radiation to cells. Conclusions: These results further highlight the importance of specific targeting toward achieving tumor cell kill and low toxicity to normal tissue.

A Novel Single-Step-Labeled 212Pb-CaCO3 Microparticle for Internal Alpha Therapy: Preparation, Stability, and Preclinical Data from Mice

Lead-212 is recognized as a promising radionuclide for targeted alpha therapy for tumors. Many studies of ²¹²Pb-labeling of various biomolecules through bifunctional chelators have been conducted. Another approach to exploiting the cytotoxic effect is coupling the radionuclide to a microparticle acting as a carrier vehicle, which could be used for treating disseminated cancers in body cavities. Calcium carbonate may represent a suitable material, as it is biocompatible, biodegradable, and easy to synthesize. In this work, we explored ²¹²Pb-labeling of various CaCO₃ microparticles and developed a protocol that can be straightforwardly implemented by clinicians. Vaterite microparticles stabilized by pamidronate were effective as ²¹²Pb carriers; labeling yields of ≥98% were achieved, and ²¹²Pb was strongly retained by the particles in an in vitro stability assessment. Moreover, the amounts of ²¹²Pb reaching the kidneys, liver, spleen, and skeleton of mice following intraperitoneal (i.p.) administration were very low compared to i.p. injection of unbound ²¹²Pb²⁺, indicating that CaCO₃-bound ²¹²Pb exhibited stability when administered intraperitoneally. Therapeutic efficacy was observed in a model of i.p. ovarian cancer for all the tested doses, ranging from 63 to 430 kBq per mouse. Lead-212-labeled CaCO₃ microparticles represent a promising candidate for treating intracavitary cancers.

Production and Supply of α-Particle-Emitting Radionuclides for Targeted α-Therapy

Encouraging results from targeted α-therapy have received significant attention from academia and industry. However, the limited availability of suitable radionuclides has hampered widespread translation and application. In the present review, we discuss the most promising candidates for clinical application and the state of the art of their production and supply. In this review, along with 2 forthcoming reviews on chelation and clinical application of α-emitting radionuclides, The Journal of Nuclear Medicine will provide a comprehensive assessment of the field.

Dosimetric impact of Ac-227 in accelerator-produced Ac-225 for alpha-emitter radiopharmaceutical therapy of patients with hematological malignancies: a pharmacokinetic modeling analysis

Background

Actinium-225 is an alpha-particle emitter under investigation for use in radiopharmaceutical therapy. To address limited supply, accelerator-produced ²²⁵Ac has been recently made available. Accelerator-produced ²²⁵Ac via ²³²Th irradiation (denoted ^225/7Ac) contains a low percentage (0.1–0.3%) of ²²⁷Ac (21.77-year half-life) activity at end of bombardment. Using pharmacokinetic modeling, we have examined the dosimetric impact of ²²⁷Ac on the use of accelerator-produced ²²⁵Ac for radiopharmaceutical therapy. We examine the contribution of ²²⁷Ac and its daughters to tissue absorbed doses. The dosimetric analysis was performed for antibody-conjugated ^225/7Ac administered intravenously to treat patients with hematological cancers. Published pharmacokinetic models are used to obtain the distribution of ^225/7Ac-labeled antibody and also the distribution of either free or antibody-conjugated ²²⁷Th.

Results

Based on our modeling, the tissue specific absorbed dose from ²²⁷Ac would be negligible in the context of therapy, less than 0.02 mGy/MBq for the top 6 highest absorbed tissues and less than 0.007 mGy/MBq for all other tissues. Compared to that from ²²⁵Ac, the absorbed dose from ²²⁷Ac makes up a very small component (less than 0.04%) of the total absorbed dose delivered to the 6 highest dose tissues: red marrow, spleen, endosteal cells, liver, lungs and kidneys when accelerator produced ^225/7Ac-conjugated anti-CD33 antibody is used to treat leukemia patients. For all tissues, the dominant contributor to the absorbed dose arising from the ²²⁷Ac is ²²⁷Th, the first daughter of ²²⁷Ac which has the potential to deliver absorbed dose both while it is antibody-bound and while it is free. CONCLUSIONS: These results suggest that the absorbed dose arising from ²²⁷Ac to normal organs would be negligible for an ^225/7Ac-labeled antibody that targets hematological cancer.

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

In situ Generated ²¹²Pb-PSMA Ligand in a ²²⁴Ra-Solution for Dual Targeting of Prostate Cancer Sclerotic Stroma and PSMA-positive Cells

Background: New treatments combating bone and extraskeletal metastases are needed for patients with metastatic castration-resistant prostate cancer. The majority of metastases overexpress prostate-specific membrane antigen (PSMA), making it an ideal candidate for targeted radionuclide therapy.

Objective: The aim of this study was to test a novel liquid 224Ra/212Pb-generator for the rapid preparation of a dual-alpha targeting solution. Here, PSMA-targeting ligands are labelled with 212Pb in the 224Ra-solution in transient equilibrium with daughter nuclides. Thus, natural bone-seeking 224Ra targeting sclerotic bone metastases and 212Pb-chelated PSMA ligands targeting PSMA-expressing tumour cells are obtained.

Methods: Two PSMA-targeting ligands, the p-SCN-Bn-TCMC-PSMA ligand (NG001), specifically developed for chelating 212Pb, and the most clinically used DOTA-based PSMA-617 were labelled with 212Pb. Radiolabelling and targeting potential were investigated in situ, in vitro (PSMA-positive C4-2 human prostate cancer cells) and in vivo (athymic mice bearing C4-2 xenografts).

Results: NG001 was rapidly labelled with 212Pb (radiochemical purity >94% at concentrations of ≥15 µg/ml) using the liquid 224Ra/212Pb-generator. The high radiochemical purity and stability of [212Pb]Pb-NG001 were demonstrated over 48 hours in the presence of ascorbic acid and albumin. Similar binding abilities of the 212Pb-labelled ligands were observed in C4-2 cells. The PSMA ligands displayed comparable tumour uptake after 2 hours, but NG001 showed a 3.5-fold lower kidney uptake than PSMA-617. Radium-224 was not chelated and, hence, showed high uptake in bones.

Conclusion: A fast method for the labelling of PSMA ligands with 212Pb in the 224Ra/212Pb-solution was developed. Thus, further in vivo studies with dual tumour targeting by alpha-particles are warranted.

Evaluation of the PSMA-Binding Ligand 212Pb-NG001 in Multicellular Tumour Spheroid and Mouse Models of Prostate Cancer

Radioligand therapy targeting the prostate-specific membrane antigen (PSMA) is rapidly evolving as a promising treatment for metastatic castration-resistant prostate cancer. The PSMA-targeting ligand p-SCN-Bn-TCMC-PSMA (NG001) labelled with ²¹²Pb efficiently targets PSMA-positive cells in vitro and in vivo. The aim of this preclinical study was to evaluate the therapeutic potential of ²¹²Pb-NG001 in multicellular tumour spheroid and mouse models of prostate cancer. The cytotoxic effect of ²¹²Pb-NG001 was tested in human prostate C4-2 spheroids. Biodistribution at various time points and therapeutic effects of different activities of the radioligand were investigated in male athymic nude mice bearing C4-2 tumours, while long-term toxicity was studied in immunocompetent BALB/c mice. The radioligand induced a selective cytotoxic effect in spheroids at activity concentrations of 3–10 kBq/mL. In mice, the radioligand accumulated rapidly in tumours and was retained over 24 h, while it rapidly cleared from nontargeted tissues. Treatment with 0.25, 0.30 or 0.40 MBq of ²¹²Pb-NG001 significantly inhibited tumour growth and improved median survival with therapeutic indexes of 1.5, 2.3 and 2.7, respectively. In BALB/c mice, no signs of long-term radiation toxicity were observed at activities of 0.05 and 0.33 MBq. The obtained results warrant clinical studies to evaluate the biodistribution, therapeutic efficacy and toxicity of ²¹²Pb-NG001.

Preclinical and Clinical Status of PSMA-Targeted Alpha Therapy for Metastatic Castration-Resistant Prostate Cancer

Bone, lymph node, and visceral metastases are frequent in castrate-resistant prostate cancer patients. Since such patients have only a few months' survival benefit from standard therapies, there is an urgent need for new personalized therapies. The prostate-specific membrane antigen (PSMA) is overexpressed in prostate cancer and is a molecular target for imaging diagnostics and targeted radionuclide therapy (theragnostics). PSMA-targeted α therapies (PSMA-TAT) may deliver potent and local radiation more selectively to cancer cells than PSMA-targeted β- therapies. In this review, we summarize both the recent preclinical and clinical advances made in the development of PSMA-TAT, as well as the availability of therapeutic α-emitting radionuclides, the development of small molecules and antibodies targeting PSMA. Lastly, we discuss the potentials, limitations, and future perspectives of PSMA-TAT.

Production, purification, and radiolabeling of the 203Pb/212Pb theranostic pair

Background

Lead-212 (²¹²Pb, t_1/2 = 10.6 h) and lead-203 (²⁰³Pb, t_1/2 = 51.9 h) are an element-equivalent, or a matched theranostic radioisotope pair that show great potential for application in targeted radionuclide therapy (TRT) and single-photon emission computed tomography (SPECT), respectively. At TRIUMF we have produced both ²⁰³Pb and ²¹²Pb using TRIUMF’s TR13 (13 MeV) and 500 MeV cyclotrons, and subsequently purified and evaluated both radioisotopes using a series of pyridine-modified DOTA analogues in comparison to the commercially available chelates DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and TCMC (1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane).

Results

Proton irradiation (12.8 MeV) of natural and enriched thallium-203 (²⁰³Tl) targets gave ²⁰³Pb saturation yields of 134 ± 25 and 483 ± 3 MBq/μA, respectively. Thorium-228 (²²⁸Th, t_1/2 = 1.9 y), a by-product of ²³²Th proton spallation on TRIUMF’s main 500 MeV beamline (beamline 1A, BL1A), was recovered to build a ²²⁸Th/²¹²Pb generator with the ability to deliver up to 9–10 MBq of ²¹²Pb daily. Both lead isotopes were purified via solid phase extraction chromatography (Pb resin), and isolated in an acetate form ([^203/212Pb]Pb(OAc)₂) suitable for direct radiolabeling of chelators and bioconjugates. A series of cyclen-based chelators (herein referred to as DOTA-1Py, -2Py, and -3Py) along with established chelates DOTA and TCMC were evaluated for their ability to complex both ²⁰³Pb and ²¹²Pb. All chelates incorporated ²¹²Pb/²⁰³Pb efficiently, with higher radiolabeling yields observed for the ²¹²Pb-complexes.

Conclusion

The production of ²⁰³Pb and ²¹²Pb was established using TRIUMF 13 MeV and 500 MeV cyclotrons, respectively. Both production methods provided radiometals suitable for subsequent radiolabeling reactions using known and novel chelates. Furthermore, the novel chelate DOTA-3Py may be a good candidate for biomolecule conjugation and further theranostic ²¹²Pb/²⁰³Pb studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-021-00121-4.

Calibration of sodium iodide detectors and reentrant ionization chambers for 212Pb activity in different geometries by HPGe activity determined samples

Lead-212 is a promising radionuclide for cancer therapy, but no primary ²¹²Pb activity standardization has been published. A need therefore exists for accurate estimation of injected doses of ²¹²Pb activity in equilibrium with progeny, when it comes to preclinical and clinical trials. In this study, ²¹²Pb activity was determined using a high purity germanium (HPGe) detector, which allowed the determination of geometry-specific calibration factors for commercially available reentrant ionization chambers (ICs) and sodium iodide (NaI) detectors.

Preparation of the alpha-emitting prostate-specific membrane antigen targeted radioligand [212 Pb]Pb-NG001 for prostate cancer

Prostate-specific membrane antigen (PSMA) is the most promising target for radioligand therapy of prostate cancer. The aim of this study was to prepare a small molecular ligand p-SCN-Bn-TCMC-PSMA (NG001) and compare it with the commonly used DOTA-based PSMA-617. The PSMA-targeting ability of the 212 Pb-labelled ligands was evaluated using PSMA-positive C4-2 human prostate cancer cells. Lead-212 is an in vivo generator of alpha particles by its daughter nuclides 212 Bi and 212 Po. NG001 was synthesized by conjugating the isothiocyanato group of p-SCN-Bn-TCMC to the amino group of a glutamate-urea-based PSMA-binding entity. Molecular size, chelator unit and chelator linking method are different in NG001 and PSMA-617. Both ligands were efficiently labelled with 212 Pb using a 224 Ra/212 Pb-solution generator in transient equilibrium with progeny. Lead-212-labelled NG001 was purified with a yield of 85.9±4.7% and with 0.7±0.2% of 224 Ra. Compared with [212 Pb]Pb-PSMA-617, [212 Pb]Pb-NG001 displayed a similar binding and internalization in C4-2 cells, with comparable tumour uptake in mice bearing C4-2 tumours, but almost a 2.5-fold lower kidney uptake. Due to the rapid normal tissue clearance and tumour cell internalization, any significant translocalization of 212 Bi was not detected in mice. In conclusion, the obtained results warrant further preclinical studies to evaluate the therapeutic efficacy of [212 Pb]Pb-NG001.

203/212Pb Theranostic Radiopharmaceuticals for Image-guided Radionuclide Therapy for Cancer

Receptor-targeted image-guided Radionuclide Therapy (TRT) is increasingly recognized as a promising approach to cancer treatment. In particular, the potential for clinical translation of receptor-targeted alpha-particle therapy is receiving considerable attention as an approach that can improve outcomes for cancer patients. Higher Linear-energy Transfer (LET) of alpha-particles (compared to beta particles) for this purpose results in an increased incidence of double-strand DNA breaks and improved-localized cancer-cell damage. Recent clinical studies provide compelling evidence that alpha-TRT has the potential to deliver a significantly more potent anti-cancer effect compared with beta-TRT. Generator-produced 212Pb (which decays to alpha emitters 212Bi and 212Po) is a particularly promising radionuclide for receptor-targeted alpha-particle therapy. A second attractive feature that distinguishes 212Pb alpha-TRT from other available radionuclides is the possibility to employ elementallymatched isotope 203Pb as an imaging surrogate in place of the therapeutic radionuclide. As direct non-invasive measurement of alpha-particle emissions cannot be conducted using current medical scanner technology, the imaging surrogate allows for a pharmacologically-inactive determination of the pharmacokinetics and biodistribution of TRT candidate ligands in advance of treatment. Thus, elementally-matched 203Pb labeled radiopharmaceuticals can be used to identify patients who may benefit from 212Pb alpha-TRT and apply appropriate dosimetry and treatment planning in advance of the therapy. In this review, we provide a brief history on the use of these isotopes for cancer therapy; describe the decay and chemical characteristics of 203/212Pb for their use in cancer theranostics and methodologies applied for production and purification of these isotopes for radiopharmaceutical production. In addition, a medical physics and dosimetry perspective is provided that highlights the potential of 212Pb for alpha-TRT and the expected safety for 203Pb surrogate imaging. Recent and current preclinical and clinical studies are presented. The sum of the findings herein and observations presented provide evidence that the 203Pb/212Pb theranostic pair has a promising future for use in radiopharmaceutical theranostic therapies for cancer.

Cationic radionuclides and ligands for targeted therapeutic radiopharmaceuticals

This review considers the already used and potential α- and β-emitting cationic radionuclides for targeted radionuclide therapy. Recent results of laboratory, preclinical and clinical applications of these radionuclides are discussed. As opposed to β-emitters, which are already used in nuclear medicine, α-emitters involved in targeted radiopharmaceuticals were subjected to clinical trials only recently and were found to be therapeutically effective. The review summarizes recent trends in the development of ligands as components of radiopharmaceuticals addressing specific features of short-lived cationic radionuclides applied in medicine. Despite a steadily growing number of chelating ligands, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and diethylenetriaminepentaacetic acid (DTPA) remain the most widely used agents in nuclear medicine. The drawbacks of these compounds restrict the application of radionuclides in medicine. Variations in the macrocycle size, the introduction and modification of substituents can significantly improve the chelating ability of ligands, enhance stability of radionuclide complexes with these ligands and eliminate the influence of ligands on the affinity of biological targeting vectors.

The bibliography includes 189 references.

Design and Challenges of Radiopharmaceuticals

This review describes general concepts with regard to radiopharmaceuticals for diagnostic or therapeutic applications that help to understand the specific challenges encountered during the design, (radio)synthesis, in vitro and in vivo evaluation and clinical translation of novel radiopharmaceuticals. The design of a radiopharmaceutical requires upfront decisions with regard to combining a suitable vector molecule with an appropriate radionuclide, considering the type and location of the molecular target, the desired application, and the time constraints imposed by the relatively short half-life of radionuclides. Well-designed in vitro and in vivo experiments allow nonclinical validation of radiotracers. Ultimately, in combination with a limited toxicology package, the radiotracer becomes a radiopharmaceutical for clinical evaluation, produced in compliance with regulatory requirements for medicines for intravenous (IV) injection.

Radiochemical aspects of alpha emitting radionuclides for medical application

The use of α-emitting radionuclides in targeted alpha therapy (TAT) holds great potential for treatment of human diseases, such as cancer, due to the short pathlength and high potency of the α particle, which can localize damage to targeted cells while minimizing effects to healthy surrounding tissues. In this review several potential α-emitting radionuclides having emission properties applicable to TAT are discussed from a radiochemical point of view. Overviews of production, radiochemical separation and chelation aspects relative to developing TAT radiopharmaceuticals are provided for the α-emitting radionuclides (and their generator systems) 211At, 224Ra/212Pb/212Bi, 225Ac/213Bi, 227Th/223Ra, 230U/226Th, 149Tb and 255Fm.

Development and dosimetry of 203Pb/212Pb-labelled PSMA ligands: bringing "the lead" into PSMA-targeted alpha therapy?

Purpose

The aims of this study were to develop a prostate-specific membrane antigen (PSMA) ligand for labelling with different radioisotopes of lead and to obtain an approximation of the dosimetry of a simulated ²¹²Pb-based alpha therapy using its ²⁰³Pb imaging analogue.

Methods

Four novel Glu-urea-based ligands containing the chelators p-SCN-Bn-TCMC or DO3AM were synthesized. Affinity and PSMA-specific internalization were studied in C4-2 cells, and biodistribution in C4-2 tumour-bearing mice. The most promising compound, ²⁰³Pb-CA012, was transferred to clinical use. Two patients underwent planar scintigraphy scans at 0.4, 4, 18, 28 and 42 h after injection, together with urine and blood sampling. The time–activity curves of source organs were extrapolated from ²⁰³Pb to ²¹²Pb and the calculated residence times of ²¹²Pb were forwarded to its unstable daughter nuclides. QDOSE and OLINDA were used for dosimetry calculations.

Results

In vitro, all ligands showed low nanomolar binding affinities for PSMA. CA09 and CA012 additionally showed specific ligand-induced internalization of 27.4 ± 2.4 and 15.6 ± 2.1 %ID/10⁶ cells, respectively. The ²⁰³Pb-labelled PSMA ligands were stable in serum for 72 h. In vivo, CA012 showed higher specific uptake in tumours than in other organs, and particularly showed rapid kidney clearance from 5.1 ± 2.5%ID/g at 1 h after injection to 0.9 ± 0.1%ID/g at 24 h. In patients, the estimated effective dose from 250–300 MBq of diagnostic ²⁰³Pb-CA012 was 6–7 mSv. Assuming instant decay of daughter nuclides, the equivalent doses projected from a therapeutic activity of 100 MBq of ²¹²Pb-CA012 were 0.6 Sv_RBE5 to the red marrow, 4.3 Sv_RBE5 to the salivary glands, 4.9 Sv_RBE5 to the kidneys, 0.7 Sv_RBE5 to the liver and 0.2 Sv_RBE5 to other organs; representative tumour lesions averaged 13.2 Sv_RBE5 (where RBE5 is relative biological effectiveness factor 5). Compared to clinical experience with ²¹³Bi-PSMA-617 and ²²⁵Ac-PSMA-617, the projected maximum tolerable dose was about 150 MBq per cycle.

Conclusion

²¹²Pb-CA012 is a promising candidate for PSMA-targeted alpha therapy of prostate cancer. The dosimetry estimate for radiopharmaceuticals decaying with the release of unstable daughter nuclides has some inherent limitations, thus clinical translation should be done cautiously.

Electronic supplementary material

The online version of this article (10.1007/s00259-018-4220-z) contains supplementary material, which is available to authorized users.

Ra-224 labeling of calcium carbonate microparticles for internal α-therapy: Preparation, stability, and biodistribution in mice

Internal therapy with α‐emitters should be well suited for micrometastatic disease. Radium‐224 emits multiple α‐particles through its decay and has a convenient 3.6 days of half‐life. Despite its attractive properties, the use of ²²⁴Ra has been limited to bone‐seeking applications because it cannot be stably bound to a targeting molecule. Alternative delivery systems for ²²⁴Ra are therefore of considerable interest. In this study, calcium carbonate microparticles are proposed as carriers for ²²⁴Ra, designed for local therapy of disseminated cancers in cavitary regions, such as peritoneal carcinomatosis. Calcium carbonate microparticles were radiolabeled by precipitation of ²²⁴Ra on the particle surface, resulting in high labeling efficiencies for both ²²⁴Ra and daughter ²¹²Pb and retention of more than 95% of these nuclides for up to 1 week in vitro. The biodistribution after intraperitoneal administration of the ²²⁴Ra‐labeled CaCO₃ microparticles in immunodeficient mice revealed that the radioactivity mainly remained in the peritoneal cavity. In addition, the systemic distribution of ²²⁴Ra was found to be strongly dependent on the amount of administered microparticles, with a reduced skeletal uptake of ²²⁴Ra with increasing dose. The results altogether suggest that the ²²⁴Ra‐labeled CaCO₃ microparticles have promising properties for use as a localized internal α‐therapy of cavitary cancers.