Vascular Targeted Radioimmunotherapy for the Treatment of Glioblastoma

Targeted Alpha Therapy for Glioblastoma: Review on In Vitro, In Vivo and Clinical Trials

Glioblastoma (GB), a prevalent and highly malignant primary brain tumour with a very high mortality rate due to its resistance to conventional therapies and invasive nature, resulting in 5-year survival rates of only 4-17%. Despite recent advancements in cancer management, the survival rates for GB patients have not significantly improved over the last 10-20 years. Consequently, there exists a critical unmet need for innovative therapies. One promising approach for GB is Targeted Alpha Therapy (TAT), which aims to selectively deliver potentially therapeutic radiation doses to malignant cells and the tumour microenvironment while minimising radiation exposure to surrounding normal tissue with or without conventional external beam radiation. This approach has shown promise in both pre-clinical and clinical settings. A review was conducted following PRISMA 2020 guidelines across Medline, SCOPUS, and Embase, identifying 34 relevant studies out of 526 initially found. In pre-clinical studies, TAT demonstrated high binding specificity to targeted GB cells, with affinity rates between 60.0% and 84.2%, and minimal binding to non-targeted cells (4.0-5.6%). This specificity significantly enhanced cytotoxic effects and improved biodistribution when delivered intratumorally. Mice treated with TAT showed markedly higher median survival rates compared to control groups. In clinical trials, TAT applied to recurrent GB (rGB) displayed varying success rates in extending overall survival (OS) and progression-free survival. Particularly effective when integrated into treatment regimens for both newly diagnosed and recurrent cases, TAT increased the median OS by 16.1% in newly diagnosed GB and by 36.4% in rGB, compared to current standard therapies. Furthermore, it was generally well tolerated with minimal adverse effects. These findings underscore the potential of TAT as a viable therapeutic option in the management of GB.

Diffusing alpha-emitters radiation therapy in combination with temozolomide or bevacizumab in human glioblastoma multiforme xenografts

Glioblastoma multiforme (GBM) is at present an incurable disease with a 5-year survival rate of 5.5%, despite improvements in treatment modalities such as surgery, radiation therapy, chemotherapy [e.g., temozolomide (TMZ)], and targeted therapy [e.g., the antiangiogenic agent bevacizumab (BEV)]. Diffusing alpha-emitters radiation therapy (DaRT) is a new modality that employs radium-224-loaded seeds that disperse alpha-emitting atoms inside the tumor. This treatment was shown to be effective in mice bearing human-derived GBM tumors. Here, the effect of DaRT in combination with standard-of-care therapies such as TMZ or BEV was investigated. In a viability assay, the combination of alpha radiation with TMZ doubled the cytotoxic effect of each of the treatments alone in U87 cultured cells. A colony formation assay demonstrated that the surviving fraction of U87 cells treated by TMZ in combination with alpha irradiation was lower than was achieved by alpha- or x-ray irradiation as monotherapies, or by x-ray combined with TMZ. The treatment of U87-bearing mice with DaRT and TMZ delayed tumor development more than the monotherapies. Unlike other radiation types, alpha radiation did not increase VEGF secretion from U87 cells in culture. BEV treatment introduced several days after DaRT implantation improved tumor control, compared to BEV or DaRT as monotherapies. The combination was also shown to be superior when starting BEV administration prior to DaRT implantation in large tumors relative to the seed size. BEV induced a decrease in CD31 staining under DaRT treatment, increased the diffusive spread of 224Ra progeny atoms in the tumor tissue, and decreased their clearance from the tumor through the blood. Taken together, the combinations of DaRT with standard-of-care chemotherapy or antiangiogenic therapy are promising approaches, which may improve the treatment of GBM patients.

Radionuclide imaging and therapy directed towards the tumor microenvironment: a multi-cancer approach for personalized medicine

Targeted radionuclide theranostics is becoming more and more prominent in clinical oncology. Currently, most nuclear medicine compounds researched for cancer theranostics are directed towards targets expressed in only a small subset of cancer types, limiting clinical applicability. The identification of cancer-specific targets that are (more) universally expressed will allow more cancer patients to benefit from these personalized nuclear medicine–based interventions. A tumor is not merely a collection of cancer cells, it also comprises supporting stromal cells embedded in an altered extracellular matrix (ECM), together forming the tumor microenvironment (TME). Since the TME is less genetically unstable than cancer cells, and TME phenotypes can be shared between cancer types, it offers targets that are more universally expressed. The TME is characterized by the presence of altered processes such as hypoxia, acidity, and increased metabolism. Next to the ECM, the TME consists of cancer-associated fibroblasts (CAFs), macrophages, endothelial cells forming the neo-vasculature, immune cells, and cancer-associated adipocytes (CAAs). Radioligands directed at the altered processes, the ECM, and the cellular components of the TME have been developed and evaluated in preclinical and clinical studies for targeted radionuclide imaging and/or therapy. In this review, we provide an overview of the TME targets and their corresponding radioligands. In addition, we discuss what developments are needed to further explore the TME as a target for radionuclide theranostics, with the hopes of stimulating the development of novel TME radioligands with multi-cancer, or in some cases even pan-cancer, application.

Antibody-Based Formats to Target Glioblastoma: Overcoming Barriers to Protein Drug Delivery

Glioblastoma (GB) is recognized as the most aggressive form of primary brain cancer. Despite advances in treatment strategies that include surgery, radiation, and chemotherapy, the median survival time (∼15 months) of patients with GB has not significantly improved. The poor prognosis of GB is also associated with a very high chance of tumor recurrence (∼90%), and current treatment measures have failed to address the complications associated with this disease. However, targeted therapies enabled through antibody engineering have shown promise in countering GB when used in combination with conventional approaches. Here, we discuss the challenges in conventional as well as future GB therapeutics and highlight some of the known advantages of using targeted biologics to overcome these impediments. We also review a broad range of potential alternative routes that could be used clinically to administer anti-GB biologics to the brain through evasion of its natural barriers.

Radioimmunotherapy for solid tumors: spotlight on Glypican-1 as a radioimmunotherapy target

Radioimmunotherapy (i.e., the use of radiolabeled tumor targeting antibodies) is an emerging approach for the diagnosis, therapy, and monitoring of solid tumors. Often using paired agents, each targeting the same tumor molecule, but labelled with an imaging or therapeutic isotope, radioimmunotherapy has achieved promising clinical results in relatively radio-resistant solid tumors such as prostate. Several approaches to optimize therapeutic efficacy, such as dose fractionation and personalized dosimetry, have seen clinical success. The clinical use and optimization of a radioimmunotherapy approach is, in part, influenced by the targeted tumor antigen, several of which have been proposed for different solid tumors. Glypican-1 (GPC-1) is a heparan sulfate proteoglycan that is expressed in a variety of solid tumors, but whose expression is restricted in normal adult tissue. Here, we discuss the preclinical and clinical evidence for the potential of GPC-1 as a radioimmunotherapy target. We describe the current treatment paradigm for several solid tumors expressing GPC-1 and suggest the potential clinical utility of a GPC-1 directed radioimmunotherapy for these tumors.

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.

Radioimmunotherapy of glioblastoma multiforme - Current status and future prospects

Glioblastoma multiforme (GBM) or grade IV astrocytoma is the most diagnosed form of primary brain tumours in adults. Radioimmunotherapy (RIT), mostly in combination with conventional therapies, is presented in the current review as a therapeutic strategy of high potential in the management of GBM. A systematic literature search was performed following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) to identify clinical studies that employed a form of radioimmunotherapy using alpha- or beta-emitting radioisotopes. The available literature on RIT in GBM and high-grade gliomas is presented and discussed. The results suggest that this promising treatment approach merits further investigation in future clinical studies.

Current State of Targeted Radiometal-Based Constructs for the Detection and Treatment of Disease in the Brain

The continual development of radiopharmaceutical agents for the field of nuclear medicine is integral to promoting the necessity of personalized medicine. One way to greatly expand the selection of radiopharmaceuticals available is to broaden the range of radionuclides employed in such agents. Widening the scope of development to include radiometals with their variety of physical decay characteristics and chemical properties opens up a myriad of possibilities for new actively targeted molecules and bioconjugates. This is especially true to further advance the imaging and treatment of disease in the brain. Over the past few decades, imaging of disease in the brain has heavily relied on agents which exploit metabolic uptake. However, through utilizing the broad range of physical characteristics that radiometals offer, the ability to target other processes has become more available. The varied chemistries of radiometals also allows for them to incorporated into specifically designed diverse constructs. A major limitation to efficient treatment of disease in the brain is the ability for relevant agents to penetrate the blood-brain barrier. Thus, along with efficient disease targeting, there must be intentional thought put into overcoming this challenge. Here, we review the current field of radiometal-based agents aimed at either imaging or therapy of brain disease that have been evaluated through at least in vivo studies.

Targeted alpha immunotherapy of CD20-positive B-cell lymphoma model: dosimetry estimate of 225Ac-DOTA-rituximab using 64Cu-DOTA-rituximab

OBJECTIVE

The aim of this study was to evaluate the radiation dosimetry of alpha-emitter ²²⁵Ac-DOTA-rituximab using Monte Carlo simulation of ⁶⁴Cu-DOTA-rituximab.

METHODS

CD20 expression was evaluated in lymphoma cell lines (Jurkat and Raji). DOTA-rituximab was conjugated and then chelated by ⁶⁴Cu. Tumor xenograft models were established in BALB/c-nu mice. Animal PET/CT imaging was obtained after tail vein injection with and without a pre-dose of 2 mg of cold rituximab. Specific binding of tumors was evaluated by an organ distribution assay and autoradiography. CD20 expression in tumor tissues was evaluated by immunohistochemistry. The residence time was calculated using ⁶⁴Cu-DOTA-rituximab PET/CT acquisition data using OLINDA/EXM software. ²²⁵Ac-DOTA-rituximab tumor dosimetry was performed using Monte Carlo simulation with ⁶⁴Cu-DOTA-rituximab PET/CT images.

RESULTS

Specific binding of Raji cells (CD20 positive) was 90 times that of Jurkat cells (CD20 negative) (p < 0.0001). Immunoreactivity was more than 75%. PET/CT imaging with ⁶⁴Cu-DOTA-rituximab was specifically observed in tumors. The radioactivity of the tumor was much higher than that of other organs, and tumor uptake was related to CD20 expression. The predicted human dose for the administration of ⁶⁴Cu-DOTA-rituximab was measured as the effective dose (1.07E-02 mSv/MBq). In the tumor region, equivalent doses of ²²⁵Ac-DOTA-rituximab (14 Sv_RBE5/MBq) were much higher (74-fold) than those of ⁶⁴Cu-DOTA-rituximab (0.19 Sv_RBE5/MBq) (p < 0.01).

CONCLUSION

Tumor dosimetry of ²²⁵Ac-DOTA-rituximab can be estimated via the Monte Carlo simulation of ⁶⁴Cu-DOTA-rituximab. ²²⁵Ac-DOTA-rituximab can be employed for lymphoma as targeted alpha therapy.

Alpha emitting nuclides for targeted therapy

Targeted alpha therapy (TAT) is an area of research with rapidly increasing importance as the emitted alpha particle has a significant effect on inducing cytotoxic effects on tumor cells while mitigating dose to normal tissues. Two significant isotopes of interest within the area of TAT are thorium-227 and actinium-225 due to their nuclear characteristics. Both isotopes have physical half-lives suitable for coordination with larger biomolecules, and additionally actinium-225 has potential to serve as an in vivo generator. In this review, the authors will discuss the production, purification, labeling reactions, and biological studies of actinium-225 and thorium-227 complexes and clinical studies.

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

Evaluating 225Ac and 177Lu Radioimmunoconjugates against Antibody-Drug Conjugates for Small-Cell Lung Cancer

Interest in the use of ²²⁵Ac for targeted alpha therapies has increased dramatically over the past few years, resulting in a multitude of new isotope production and translational research efforts. However, ²²⁵Ac radioimmunoconjugate (RIC) research is still in its infancy, with most prior experience in hematologic malignancies and only one reported preclinical solid tumor study using ²²⁵Ac RICs. In an effort to compare ²²⁵Ac RICs to other current antibody conjugates, a variety of RICs are tested against intractable small-cell lung cancer (SCLC). We directly compare, in vitro and in vivo, two promising candidates of each α or β^- category, ²²⁵Ac and ¹⁷⁷Lu, versus pyrrolobenzodiazepine (PBD) nonradioactive benchmarks. The monoclonal antibody constructs are targeted to either delta like 3 protein (DLL3), a recently discovered SCLC target, or CD46 as a positive control. An immunocompromised maximum tolerated dose assay is performed on NOD SCID mice, along with tumor efficacy proof-of-concept studies in vivo. We overview the conjugation techniques required to create serum-stable RICs and characterize and compare in vitro cell killing with RICs conjugated to nonspecific antibodies (huIgG1) with either native or site-specific thiol loci against tumor antigen DLL3-expressing and nonexpressing cell lines. Using patient-derived xenografts of SCLC onto NOD SCID mice, solid tumor growth was controlled throughout 3 weeks before growth appeared, in comparison to PBD conjugate controls. NOD SCID mice showed lengthened survival using ²²⁵Ac compared to ¹⁷⁷Lu RICs, and PBD dimers showed full tumor suppression with nine out of ten mice. The exploration of RICs on a variety of antibody-antigen systems is necessary to direct efforts in cancer research toward promising candidates. However, the anti-DLL3-RIC system with ²²⁵Ac and ¹⁷⁷Lu appears to be not as effective as the anti-DLL3-PBD counterpart in SCLC therapy with matched antibodies and portrays the challenges in both SCLC therapy as well as the specialized utility of RICs in cancer treatment.

Preclinical PET Imaging of NTSR-1-Positive Tumors with 64Cu- and 68Ga-DOTA-Neurotensin Analogs and Therapy with an 225Ac-DOTA-Neurotensin Analog

Background: The aim of the study was to perform PET imaging and radiotherapy with a novel neurotensin derivative for neurotensin receptor 1 (NTSR-1)-positive tumors in an animal model. Materials and Methods: A di-DOTA analog of NT(6-13) with three unnatural amino acids was synthesized and radiolabeled with either ⁶⁴Cu or ⁶⁸Ga and tested for serum stability and tumor imaging in mice bearing NTSR-1-positive PC3, and HT29 xenografts. A dose-response therapy study was performed with 18.5, 37, and 74 kBq of ²²⁵Ac-di-DOTA-α,ɛ-Lys-NT(6-13). Results: ⁶⁸Ga-di-DOTA-α,ɛ-Lys-NT(6-13) was >99% stable in serum for 48 h, had an IC₅₀ of 5 nM using ¹²⁵I labeled NT(8-13) for binding to HT-29 cells, and high uptake in tumor models expressing NTSR-1. ⁶⁸Ga-di-DOTA-α,ɛ-Lys-NT(6-13) had an average %ID/g (n = 4) at 2 h of 4.0 for tumor, 0.5 for blood, 12.0 for kidney, and <1 for other tissues, resulting in a favorable T/B of 8. Mean survivals of tumor-bearing mice treated with 18.5 or 37 kBq of ²²⁵Ac-di-DOTA-α,ɛ-Lys-NT(6-13) were 81 and 93 d, respectively, versus 53 d for controls. Whole-body toxicity was seen for the 74 kBq dose. Conclusions: Based on the results of the animal model, di-DOTA-α,ɛ-Lys-NT(6-13) is a useful imaging agent for NTSR-1-positive tumors when radiolabeled with ⁶⁸Ga, and when radiolabeled with ²²⁵Ac, a potent therapeutic agent.

Construction of a thorium/actinium generator at the Canadian Nuclear Laboratories

Targeted Alpha Therapy (TAT) has demonstrated considerable promise in the treatment of a range of cancers in both preclinical and, more recently clinical research. In particular, work with the alpha-emitting radionuclide ²²⁵Ac has been effectively employed due to the relatively rapid decay cascade that leads to 4 alpha and 2 beta emissions. One limitation for TAT has been caused by access to the vital radionuclide. Traditionally, ²²⁵Ac has been sourced from thorium/actinium generators based on the alpha decay of stockpiles of ²²⁹Th. ²²⁹Th is itself the alpha-decay product from ²³³U. Due to proliferation issues associated with ²³³U, only three thorium/actinium generators have been reported in the literature, capable of supporting clinical research. This paper describes the construction and operation of a thorium/actinium radionuclide generator at the Canadian Nuclear Laboratories, capable of supporting preclinical and limited clinical research in the area of TAT. Thorium was recovered and purified by a combination of anion exchange and extraction chromatography from aged ²³³U stockpiles. A separation scheme for ²²⁵Ra and ²²⁵Ac has been developed, based upon the chemical composition of the thorium material to allow for regular, routine milkings capable of supplying up to 3.7 GBq (100 mCi) of radiochemically pure ²²⁵Ac annually. This routine separation is accomplished using a combination of anion exchange chromatography to separate Ac and Ra isotopes from Th and extraction chromatography employing TEVA and DGA-N resins to separate actinium from radium and breakthrough thorium.

TAG-72-Targeted α-Radionuclide Therapy of Ovarian Cancer Using 225Ac-Labeled DOTAylated-huCC49 Antibody

Radioimmunotherapy, an approach using radiolabeled antibodies, has had minimal success in the clinic with several β-emitting radionuclides for the treatment of ovarian cancer. Alternatively, radioimmunotherapy with α-emitters offers the advantage of depositing much higher energy over shorter distances but was thought to be inappropriate for the treatment of solid tumors, for which antibody penetration is limited to a few cell diameters around the vascular system. However, the deposition of high-energy α-emitters to tumor markers adjacent to a typical leaky tumor vascular system may have large antitumor effects at the tumor vascular level, and their reduced penetration in normal tissue would be expected to lower off-target toxicity. Methods: To evaluate this concept, DOTAylated-huCC49 was labeled with the α-emitter ²²⁵Ac to target tumor-associated glycoprotein 72-positive xenografts in a murine model of ovarian cancer. Results: ²²⁵Ac-labeled DOTAylated-huCC49 radioimmunotherapy significantly reduced tumor growth in a dose-dependent manner (1.85, 3.7, and 7.4 kBq), with the 7.4-kBq dose extending survival by more than 3-fold compared with the untreated control. Additionally, a multitreatment regime (1.85 kBq followed by 5 weekly doses of 0.70 kBq for a total of 5.4 kBq) extended survival almost 3-fold compared with the untreated control group, without significant off-target toxicity. Conclusion: These results establish the potential for antibody-targeted α-radionuclide therapy for ovarian cancer, which may be generalized to α-radioimmunotherapy in other solid tumors.

Beyond the Barrier: Targeted Radionuclide Therapy in Brain Tumors and Metastases

Brain tumors are notoriously difficult to treat. The blood-brain barrier provides a sanctuary site where residual and metastatic cancer cells can evade most therapeutic modalities. The delicate nature of the brain further complicates the decision of eliminating as much tumorous tissue as possible while protecting healthy tissue. Despite recent advances in immunotherapy, radiotherapy and systemic treatments, prognosis of newly diagnosed patients remains dismal, and recurrence is still a universal problem. Several strategies are now under preclinical and clinical investigation to optimize delivery and maximize the cytotoxic potential of pharmaceuticals with regards to brain tumors. This review provides an overview of targeted radionuclide therapy approaches for the treatment of primary brain tumors and brain metastases, with an emphasis on biological targeting moieties that specifically target key biomarkers involved in cancer development.

Potential for Nuclear Medicine Therapy for Glioblastoma Treatment

Glioblastoma is the most common malignant adult brain tumor and has a very poor patient prognosis. The mean survival for highly proliferative glioblastoma is only 10 to 14 months despite an aggressive current therapeutic approach known as Stupp's protocol, which consists of debulking surgery followed by radiotherapy and chemotherapy. Despite several clinical trials using anti-angiogenic targeted therapies, glioblastoma medical care remains without major progress in the last decade. Recent progress in nuclear medicine, has been mainly driven by advances in biotechnologies such as radioimmunotherapy, radiopeptide therapy, and radionanoparticles, and these bring a new promising arsenal for glioblastoma therapy. For therapeutic purposes, nuclear medicine practitioners classically use β^- particle emitters like ¹³¹I, ⁹⁰Y, ^186/188Re, or ¹⁷⁷Lu. In the glioblastoma field, these radioisotopes are coupled with nanoparticles, monoclonal antibodies, or peptides. These radiopharmaceutical compounds have resulted in a stabilization and/or improvement of the neurological status with only transient side effects. In nuclear medicine, the glioblastoma-localized and targeted internal radiotherapy proof-of-concept stage has been successfully demonstrated using β^- emitting isotopes. Similarly, α particle emitters like ²¹³Bi, ²¹¹At, or ²²⁵Ac appear to be an innovative and interesting alternative. Indeed, α particles deliver a high proportion of their energy inside or at close proximity to the targeted cells (within a few micrometers from the emission point versus several millimeters for β^- particles). This physical property is based on particle-matter interaction differences and results in α particles being highly efficient in killing tumor cells with minimal irradiation of healthy tissues and permits targeting of isolated tumor cells. The first clinical trials confirmed this idea and showed good therapeutic efficacy and less side effects, thus opening a new and promising era for glioblastoma medical care using α therapy. The objective of this literature review is focused on the developing field of nuclear medicine and aims to describe the various parameters such as targets, vectors, isotopes, or injection route (systemic and local) in relation to the clinical and preclinical results in glioblastoma pathology.

Intracavitary radioimmunotherapy of high-grade gliomas: present status and future developments

There is a distinct need for new and second-line therapies to delay or prevent local tumor regrowth after current standard of care therapy. Intracavitary radioimmunotherapy, in combination with radiotherapy, is discussed in the present review as a therapeutic strategy of high potential. We performed a systematic literature search following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA). The available body of literature on intracavitary radioimmunotherapy (iRIT) in glioblastoma and anaplastic astrocytomas is presented. Several past and current phase I and II clinical trials, using mostly an anti-tenascin monoclonal antibody labeled with I-131, have shown median overall survival of 19-25 months in glioblastoma, while adverse events remain low. Tenascin, followed by EGFR and variants, or smaller peptides have been used as targets, and most clinical studies were performed with I-131 or Y-90 as radionuclides while only recently Re-188, I-125, and Bi-213 were applied. The pharmacokinetics of iRIT, as well as the challenges encountered with this therapy, is comprehensively discussed. This promising approach deserves further exploration in future studies by incorporating several innovative modifications.

Leveraging Bioorthogonal Click Chemistry to Improve 225Ac-Radioimmunotherapy of Pancreatic Ductal Adenocarcinoma

PURPOSE

Interest in targeted alpha-therapy has surged due to α-particles' high cytotoxicity. However, the widespread clinical use of this approach could be limited by on-/off-target toxicities. Here, we investigated the inverse electron-demand Diels-Alder ligation between an 225Ac-labeled tetrazine radioligand and a trans-cyclooctene-bearing anti-CA19.9 antibody (5B1) for pretargeted α-radioimmunotherapy (PRIT) of pancreatic ductal adenocarcinoma (PDAC). This alternative strategy is expected to reduce nonspecific toxicities as compared with conventional radioimmunotherapy (RIT).Experimental Design: A side-by-side comparison of 225Ac-PRIT and conventional RIT using a directly 225Ac-radiolabeled immunoconjugate evaluates the therapeutic efficacy and toxicity of both methodologies in PDAC murine models.

RESULTS

A comparative biodistribution study of the PRIT versus RIT methodology underscored the improved pharmacokinetic properties (e.g., prolonged tumor uptake and increased tumor-to-tissue ratios) of the PRIT approach. Cerenkov imaging coupled to PRIT confirmed the in vivo biodistribution of 225Ac-radioimmunoconjugate but-importantly-further allowed for the ex vivo monitoring of 225Ac's radioactive daughters' redistribution. Human dosimetry was extrapolated from the mouse biodistribution and confirms the clinical translatability of 225Ac-PRIT. Furthermore, longitudinal therapy studies performed in subcutaneous and orthotopic PDAC models confirm the therapeutic efficacy of 225Ac-PRIT with the observation of prolonged median survival compared with control cohorts. Finally, a comparison with conventional RIT highlighted the potential of 225Ac-PRIT to reduce hematotoxicity while maintaining therapeutic effectiveness.

CONCLUSIONS

The ability of 225Ac-PRIT to deliver a radiotherapeutic payload while simultaneously reducing the off-target toxicity normally associated with RIT suggests that the clinical translation of this approach will have a profound impact on PDAC therapy.

Effect of densely ionizing radiation on cardiomyocyte differentiation from human-induced pluripotent stem cells

The process of human cardiac development can be faithfully recapitulated in a culture dish with human pluripotent stem cells, where the impact of environmental stressors can be evaluated. The consequences of ionizing radiation exposure on human cardiac differentiation are largely unknown. In this study, human-induced pluripotent stem cell cultures (hiPSCs) were subjected to an external beam of 3.7 MeV α-particles at low mean absorbed doses of 0.5, 3, and 10 cGy. Subsequently, the hiPSCs were differentiated into beating cardiac myocytes (hiPSC-CMs). Pluripotent and cardiac markers and morphology did not reveal differences between the irradiated and nonirradiated groups. While cell number was not affected during CM differentiation, cell number of differentiated CMs was severely reduced by ionizing radiation in a dose-responsive manner. β-adrenergic stimulation causes calcium (Ca²⁺) overload and oxidative stress. Although no significant increase in Ca²⁺ transient amplitude was observed in any group after treatment with 1 μmol/L isoproterenol, the incidence of spontaneous Ca²⁺ waves/releases was more frequent in hiPSC-CMs of the irradiated groups, indicating arrhythmogenic activities at the single cell level. Increased transcript expression of mitochondrial biomarkers (LONP1, TFAM) and mtDNA-encoded genes (MT-CYB, MT-RNR1) was detected upon differentiation of hiPSC-CMs suggesting increased organelle biogenesis. Exposure of hiPSC-CM cultures to 10 cGy significantly upregulated MT-CYB and MT-RNR1 expression, which may reflect an adaptive response to ionizing radiation. Our results indicate that important aspects of differentiation of hiPSCs into cardiac myocytes may be affected by low fluences of densely ionizing radiations such as α-particles.

α-Emitters for Radiotherapy: From Basic Radiochemistry to Clinical Studies-Part 1

With a short particle range and high linear energy transfer, α-emitting radionuclides demonstrate high cell-killing efficiencies. Even with the existence of numerous radionuclides that decay by α-particle emission, only a few of these can reasonably be exploited for therapeutic purposes. Factors including radioisotope availability and physical characteristics (e.g., half-life) can limit their widespread dissemination. The first part of this review will explore the diversity, basic radiochemistry, restrictions, and hurdles of α-emitters.

α-Emitters for Radiotherapy: From Basic Radiochemistry to Clinical Studies-Part 2

The use of radioactive sources to deliver cytotoxic ionizing radiation to disease sites dates back to the early 20th century, with the discovery of radium and its physiologic effects. α-emitters are of particular interest in the field of clinical oncology for radiotherapy applications. The first part of this review explored the basic radiochemistry, high cell-killing potency, and availability of α-emitting radionuclides, together with hurdles such as radiolabeling methods and daughter redistribution. The second part of this review will give an overview of the most promising and current uses of α-emitters in preclinical and clinical studies.

The interventional effect of new drugs combined with the Stupp protocol on glioblastoma: A network meta-analysis

OBJECTIVE

New therapeutic agents in combination with the standard Stupp protocol (a protocol about the temozolomide combined with radiotherapy treatment with glioblastoma was research by Stupp R in 2005) were assessed to evaluate whether they were superior to the Stupp protocol alone, to determine the optimum treatment regimen for patients with newly diagnosed glioblastoma.

PATIENTS AND METHODS

We implemented a search strategy to identify studies in the following databases: PubMed, Cochrane Library, EMBASE, CNKI, CBM, Wanfang, and VIP, and assessed the quality of extracted data from the trials included. Statistical software was used to perform network meta-analysis.

RESULTS

The use of novel therapeutic agents in combination with the Stupp protocol were all shown to be superior than the Stupp protocol alone for the treatment of newly diagnosed glioblastoma, ranked as follows: cilengitide 2000mg/5/week, bevacizumab in combination with irinotecan, nimotuzumab, bevacizumab, cilengitide 2000mg/2/week, cytokine-induced killer cell immunotherapy, and the Stupp protocol. In terms of serious adverse effects, the intervention group showed a 29% increase in the incidence of adverse events compared with the control group (patients treated only with Stupp protocol) with a statistically significant difference (RR=1.29; 95%CI 1.17-1.43; P<0.001). The most common adverse events were thrombocytopenia, lymphopenia, neutropenia, pneumonia, nausea, and vomiting, none of which were significantly different between the groups except for neutropenia, pneumonia, and embolism.

CONCLUSIONS

All intervention drugs evaluated in our study were superior to the Stupp protocol alone when used in combination with it. However, we could not conclusively confirm whether cilengitide 2000mg/5/week was the optimum regime, as only one trial using this protocol was included in our study.

Cure of Human Ovarian Carcinoma Solid Xenografts by Fractionated α-Radioimmunotherapy with 211At-MX35-F(ab')2: Influence of Absorbed Tumor Dose and Effect on Long-Term Survival

The goal of this study was to investigate whether targeted α-therapy can be used to successfully treat macrotumors, in addition to its established role for treating micrometastatic and minimal disease. We used an intravenous fractionated regimen of α-radioimmunotherapy in a subcutaneous tumor model in mice. We aimed to evaluate the absorbed dose levels required for tumor eradication and growth monitoring, as well as to evaluate long-term survival after treatment. Methods: Mice bearing subcutaneous tumors (50 mm³, NIH:OVCAR-3) were injected repeatedly (1-3 intravenous injections 7-10 d apart, allowing bone marrow recovery) with ²¹¹At-MX35-F(ab')₂ at different activities (close to acute myelotoxicity). Mean absorbed doses to tumors and organs were estimated from biodistribution data and summed for the fractions. Tumor growth was monitored for 100 d and survival for 1 y after treatment. Toxicity analysis included body weight, white blood cell count, and hematocrit. Results: Effects on tumor growth after fractionated α-radioimmunotherapy with ²¹¹At-MX35-F(ab')₂ was strong and dose-dependent. Complete remission (tumor-free fraction, 100%) was found for tumor doses of 12.4 and 16.4 Gy. The administered activities were high, and long-term toxicity effects (≤60 wk) were clear. Above 1 MBq, the median survival decreased linearly with injected activity, from 44 to 11 wk. Toxicity was also seen by reduced body weight. White blood cell count analysis after α-radioimmunotherapy indicated bone marrow recovery for the low-activity groups, whereas for high-activity groups the reduction was close to acute myelotoxicity. A decrease in hematocrit was seen at a late interval (34-59 wk after therapy). The main external indication of poor health was dehydration. Conclusion: Having observed complete eradication of solid tumor xenografts, we conclude that targeted α-therapy regimens may stretch beyond the realm of micrometastatic disease and be eradicative also for macrotumors. Our observations indicate that at least 10 Gy are required. This agrees well with the calculated tumor control probability. Considering a relative biological effectiveness of 5, this dose level seems reasonable. However, complete remission was achieved first at activity levels close to lethal and was accompanied by biologic effects that reduced long-term survival.

Remodeling the Vascular Microenvironment of Glioblastoma with α-Particles

Tumors escape antiangiogenic therapy by activation of proangiogenic signaling pathways. Bevacizumab is approved for the treatment of recurrent glioblastoma, but patients inevitably develop resistance to this angiogenic inhibitor. We previously investigated targeted α-particle therapy with ²²⁵Ac-E4G10 as an antivascular approach and showed increased survival and tumor control in a high-grade transgenic orthotopic glioblastoma model. Here, we investigated changes in tumor vascular morphology and functionality caused by ²²⁵Ac-E4G10.

METHODS

We investigated remodeling of the tumor microenvironment in transgenic Ntva glioblastoma mice using a therapeutic 7.4-kBq dose of ²²⁵Ac-E4G10. Immunofluorescence and immunohistochemical analyses imaged morphologic changes in the tumor blood-brain barrier microenvironment. Multicolor flow cytometry quantified the endothelial progenitor cell population in the bone marrow. Diffusion-weighted MR imaged functional changes in the tumor vascular network.

RESULTS

The mechanism of drug action is a combination of remodeling of the glioblastoma vascular microenvironment, relief of edema, and depletion of regulatory T and endothelial progenitor cells. The primary remodeling event is the reduction of both endothelial and perivascular cell populations. Tumor-associated edema and necrosis were lessened, resulting in increased perfusion and reduced diffusion. Pharmacologic uptake of dasatinib into tumor was enhanced after α-particle therapy.

CONCLUSION

Targeted antivascular α-particle radiation remodels the glioblastoma vascular microenvironment via a multimodal mechanism of action and provides insight into the vascular architecture of platelet-derived growth factor-driven glioblastoma.