Breaking Chemoresistance and Radioresistance with [213Bi]anti-CD45 Antibodies in Leukemia Cells

Current and emerging strategies for the management of advanced/metastatic lung neuroendocrine tumors

Pulmonary neuroendocrine tumors represent a spectrum of disease ranging from typical carcinoid tumors to small cell lung cancers. The incidence of low-grade pulmonary NETs has been increasing, leading to improved awareness and the need for more treatment options for this rare cancer. Somatostatin analogs continue to be the backbone of therapy and may be followed or accompanied by targeted therapy, chemotherapy, and immune therapy. The recent addition of peptide receptor radionuclide therapy (PRRT) to the treatment armamentarium of NETs has led to the development of targeted alpha therapy to overcome PRRT resistance and minimize off-target adverse effects. Herein, we aim to highlight current treatment options for patients with advanced low grade pulmonary NETs along with emerging therapies, sequencing of therapies, upcoming clinical trials, and the importance of a multidisciplinary team to improve patient outcomes.

DNA Damage by Radiopharmaceuticals and Mechanisms of Cellular Repair

DNA is an organic molecule that is highly vulnerable to chemical alterations and breaks caused by both internal and external factors. Cells possess complex and advanced mechanisms, including DNA repair, damage tolerance, cell cycle checkpoints, and cell death pathways, which together minimize the potentially harmful effects of DNA damage. However, in cancer cells, the normal DNA damage tolerance and response processes are disrupted or deregulated. This results in increased mutagenesis and genomic instability within the cancer cells, a known driver of cancer progression and therapeutic resistance. On the other hand, the inherent instability of the genome in rapidly dividing cancer cells can be exploited as a tool to kill by imposing DNA damage with radiopharmaceuticals. As the field of targeted radiopharmaceutical therapy (RPT) is rapidly growing in oncology, it is crucial to have a deep understanding of the impact of systemic radiation delivery by radiopharmaceuticals on the DNA of tumors and healthy tissues. The distribution and activation of DNA damage and repair pathways caused by RPT can be different based on the characteristics of the radioisotope and molecular target. Here we provide a comprehensive discussion of the biological effects of RPTs, with the main focus on the role of varying radioisotopes in inducing direct and indirect DNA damage and activating DNA repair pathways.

Radioimmunotherapy as a pathogen-agnostic treatment method for opportunistic mucormycosis infections

Invasive fungal infections (IFIs) such as mucormycosis are causing devastating morbidity and mortality in immunocompromised patients as anti-fungal agents do not work in the setting of a suppressed immune system. The coronavirus disease 2019 (COVID-19) pandemic has created a novel landscape for IFIs in post-pandemic patients, resulting from severe immune suppression caused by COVID-19 infection, comorbidities (diabetes, obesity) and immunosuppressive treatments such as steroids. The antigen-antibody interaction has been employed in radioimmunotherapy (RIT) to deliver lethal doses of ionizing radiation emitted by radionuclides to targeted cells and has demonstrated efficacy in several cancers. One of the advantages of RIT is its independence of the immune status of a host, which is crucial for immunosuppressed post-COVID-19 patients. In the present work we targeted the fungal pan-antigens 1,3-beta-glucan and melanin pigment, which are present in the majority of pathogenic fungi, with RIT, thus making such targeting pathogen-agnostic. We demonstrated in experimental murine mucormycosis in immunocompetent and immunocompromised mice that lutetium-177 (177Lu)-labelled antibodies to these two antigens effectively decreased the fungal burden in major organs, including the brain. These results are encouraging because they show the effectiveness of pathogen-agnostic RIT in significantly decreasing fungal burden in vivo, while they can also potentially be applied to treat the broad range of invasive fungal infections that express the pan-antigens 1,3-beta-glucan or melanin.

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

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

Development of [225Ac]Ac-DOTA-C595 as radioimmunotherapy of pancreatic cancer: in vitro evaluation, dosimetric assessment and detector calibration

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy which may benefit from radioimmunotherapy. Previously, [177Lu]Lu-DOTA-C595 has been developed as a beta-emitting radioimmunoconjugate to target cancer-specific mucin 1 epitopes (MUC1-CE) overexpressed on PDAC. However, the therapeutic effect may be enhanced by using an alpha-emitting radionuclide such as Actinium-225 (Ac-225). The short range and high linear energy transfer of alpha particles provides dense cellular damage and can overcome typical barriers related to PDAC treatment such as hypoxia. Despite the added cytotoxicity of alpha-emitters, their clinical implementation can be complicated by their complex decay chains, recoil energy and short-range impeding radiation detection. In this study, we developed and evaluated [225Ac]Ac-DOTA-C595 as an alpha-emitting radioimmunotherapy against PDAC using a series of in vitro experiments and conducted a preliminary dosimetric assessment and cross-calibration of detectors for the clinical implementation of Ac-225.

RESULTS

Cell binding and internalisation of [225Ac]Ac-DOTA-C595 was rapid and greatest in cells with strong MUC1-CE expression. Over 99% of PDAC cells had positive yH2AX expression within 1 h of [225Ac]Ac-DOTA-C595 exposure, suggesting a high level of DNA damage. Clonogenic assays further illustrated the cytotoxicity of [225Ac]Ac-DOTA-C595 in a concentration-dependent manner. At low concentrations of [225Ac]Ac-DOTA-C595, cells with strong MUC1-CE expression had lower cell survival than cells with weak MUC1-CE expression, yet survival was similar between cell lines at high concentrations irrespective of MUC1-CE expression. A dosimetric assessment was performed to estimate the dose-rate of 1 kBq of [225Ac]Ac-DOTA-C595 with consideration to alpha particles. Total absorption of 1 kBq of Ac-225 was estimated to provide a dose rate of 17.5 mGy/h, confirmed via both detector measurements and calculations.

CONCLUSION

[225Ac]Ac-DOTA-C595 was shown to target and induce a therapeutic effect in MUC1-CE expressing PDAC cells.

Cure of Micrometastatic B-Cell Lymphoma in a SCID Mouse Model Using 213Bi-Anti-CD20 Monoclonal Antibody

We studied the feasibility of using the α-emitting ²¹³Bi-anti-CD20 therapy with direct bioluminescent tracking of micrometastatic human B-cell lymphoma in a SCID mouse model. Methods: A highly lethal SCID mouse model of minimal-tumor-burden disseminated non-Hodgkin lymphoma (NHL) was established using human Raji lymphoma cells transfected to express the luciferase reporter. In vitro and in vivo radioimmunotherapy experiments were conducted. Single- and multiple-dose regimens were explored, and results with ²¹³Bi-rituximab were compared with various controls, including no treatment, free ²¹³Bi radiometal, unlabeled rituximab, and ²¹³Bi-labeled anti-HER2/neu (non-CD20-specific antibody). ²¹³Bi-rituximab was also compared in vivo with the low-energy β-emitter ¹³¹I-tositumomab and the high-energy β-emitter ⁹⁰Y-rituximab. Results: In vitro studies showed dose-dependent target-specific killing of lymphoma cells with ²¹³Bi-rituximab. Multiple in vivo studies showed significant and specific tumor growth delays with ²¹³Bi-rituximab versus free ²¹³Bi, ²¹³Bi-labeled control antibody, or unlabeled rituximab. Redosing of ²¹³Bi-rituximab was more effective than single dosing. With a single dose of therapy given 4 d after intravenous tumor inoculation, disease in all untreated controls, and in all mice in the 925-kBq ⁹⁰Y-rituximab group, progressed. With 3,700 kBq of ²¹³Bi-rituximab, 75% of the mice survived and all but 1 survivor was cured. With 2,035 kBq of ¹³¹I-tositumomab, 75% of the mice were tumor-free by bioluminescent imaging and 62.5% survived. Conclusion: Cure of micrometastatic NHL is achieved in most animals treated 4 d after intravenous tumor inoculation using either ²¹³Bi-rituximab or ¹³¹I-tositumomab, in contrast to the lack of cures with unlabeled rituximab or ⁹⁰Y-rituximab or if there was a high tumor burden before radioimmunotherapy. α-emitter-labeled anti-CD20 antibodies are promising therapeutics for NHL, although a longer-lived α-emitter may be of greater efficacy.

Thermosensitive Liposomes Encapsulating Nedaplatin and Picoplatin Demonstrate Enhanced Cytotoxicity against Breast Cancer Cells

Thermosensitive liposomes (TSL) have been used for localized temperature-responsive release of chemotherapeutics into solid cancers, with a minimum of one invention currently in clinical trials (phase III). In this study, TSL was designed using a lipid blend comprising 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), cholesterol, and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[maleimide(polyethylene glycol)-2000] (DSPE-PEG-2000) (molar ratio of 88:9:2.8:0.2). Either nedaplatin (ND) or p-sulfonatocalix[4]arene-nedaplatin was encapsulated in the aqueous inner layer of TSL to form (ND-TSL) or p-SC4-ND-TSL, respectively. The hydrophobic platinum-based drug picoplatin (P) was loaded into the external lipid bilayer of the TSL to develop P-TSL. The three nanosystems were studied in terms of size, PDI, surface charge, and on-shelf stability. Moreover, the entrapment efficiency (EE%) and release % at 37 and 40 °C were evaluated. In a 30 min in vitro release study, the maximum release of ND, p-SC4-ND, and picoplatin at 40 °C reached 74, 79, and 75%, respectively, compared to approximately 10% at 37 °C. This demonstrated temperature-triggered drug release from the TSL in all three developed systems. The designed TSL exhibited significant in vitro anticancer activity at 40 °C when tested on human mammary gland/breast adenocarcinoma cells (MDA-MB-231). The cytotoxicity of ND-TSL, p-SC4-ND-TSL, and P-TSL at 40 °C was approximately twice those observed at 37 °C. This study suggests that TSL is a promising nanoplatform for the temperature-triggered release of platinum-based drugs into cancer cells.

Targeted α-therapy in non-prostate malignancies

Progress in unraveling the complex biology of cancer, novel developments in radiochemistry, and availability of relevant α-emitters for targeted therapy have provided innovative approaches to precision cancer management. The approval of 223Ra dichloride for treatment of men with osseous metastatic castrate-resistant prostate cancer unleashed targeted α-therapy as a safe and effective cancer management strategy. While there is currently active research on new α-therapy regimens for prostate cancer based on the prostate-specific membrane antigen, there is emerging development of radiopharmaceutical therapy with a range of biological targets and α-emitting radioisotopes for malignancies other than the prostate cancer. This article provides a brief review of preclinical and first-in-human studies of targeted α-therapy in the cancers of brain, breast, lung, gastrointestinal, pancreas, ovary, and the urinary bladder. The data on leukemia, melanoma, myeloma, and neuroendocrine tumors will also be presented. It is anticipated that with further research the emerging role of targeted α-therapy in cancer management will be defined and validated.

Global experience with PSMA-based alpha therapy in prostate cancer

PURPOSE

This review discusses the current state of prostate-specific membrane antigen (PSMA)-based alpha therapy of metastatic castration-resistant prostate cancer (mCRPC). With this in-depth discussion on the growing field of PSMA-based alpha therapy (PAT), we aimed to increase the interactions between basic scientists and physician-scientists in order to advance the field.

METHODS

To achieve this, we discuss the potential, current status, and opportunities for alpha therapy and strategies, attempted to date, and important questions that need to be addressed. The paper reviews important concepts, including whom to treat, how to treat, what to expect regarding treatment outcome, and toxicity, and areas requiring further investigations.

RESULTS

There is much excitement about the potential of this field. Much of the potential exists because these therapies utilize unique mechanisms of action, difficult to achieve with other conventional therapies.

CONCLUSION

A better understanding of the strengths and limitations of PAT may help in creating an effective therapy for mCRPC and design a rational combinatorial approach to treatment by targeting different tumor pathways.

Breaking the Silence of Tumor Response: Future Prospects of Targeted Radionuclide Therapy

Therapy-induced tumor resistance has always been a paramount hurdle in the clinical triumph of cancer therapy. Resistance acquired by tumor through interventions of chemotherapeutic drugs, ionizing radiation, and immunotherapy in the patientsis a severe drawback and major cause of recurrence of tumor and failure of therapeutic responses. To counter acquired resistance in tumor cells, several strategies are practiced such as chemotherapy regimens, immunotherapy, and immunoconjugates, but the outcome is very disappointing for the patients as well as clinicians. Radionuclide therapy using alpha or beta-emitting radionuclide as payload became state-of-the-art for cancer therapy. With the improvement in dosimetric studies, development of high-affinity target molecules, and design of several novel chelating agents which provide thermodynamically stable complexes in vivo, the scope of radionuclide therapy has increased by leaps and bounds. Additionally, radionuclide therapy along with the combination of chemotherapy is gaining importance in pre-clinics, which is quite encouraging. Thus, it opens an avenue for newer cancer therapy modalities where chemotherapy, radiation therapy, and immunotherapy are unable to break the silence of tumor response. This article describes, in brief, the causes of tumor resistance and discusses the potential of radionuclide therapy to enhance tumor response.

Tuning the Kinetic Inertness of Bi3+ Complexes: The Impact of Donor Atoms on Diaza-18-Crown-6 Ligands as Chelators for 213Bi Targeted Alpha Therapy

The radionuclide ²¹³Bi can be applied for targeted α therapy (TAT): a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To use this radionuclide for this application, a bifunctional chelator (BFC) is needed to attach it to a biological targeting vector that can deliver it selectively to cancer cells. Here, we investigated six macrocyclic ligands as potential BFCs, fully characterizing the Bi³⁺ complexes by NMR spectroscopy, mass spectrometry, and elemental analysis. Solid-state structures of three complexes revealed distorted coordination geometries about the Bi³⁺ center arising from the stereochemically active 6s² lone pair. The kinetic properties of the Bi³⁺ complexes were assessed by challenging them with a 1000-fold excess of the chelating agent diethylenetriaminepentaacetic acid (DTPA). The most kinetically inert complexes contained the most basic pendent donors. Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations were employed to investigate this trend, suggesting that the kinetic inertness is not correlated with the extent of the 6s² lone pair stereochemical activity, but with the extent of covalency between pendent donors. Lastly, radiolabeling studies of ²¹³Bi (30-210 kBq) with three of the most promising ligands showed rapid formation of the radiolabeled complexes at room temperature within 8 min for ligand concentrations as low as 10^-7 M, corresponding to radiochemical yields of >80%, thereby demonstrating the promise of this ligand class for use in ²¹³Bi TAT.

Recent Advances in Radiometals for Combined Imaging and Therapy in Cancer

Nuclear medicine is defined as the use of radionuclides for diagnostic and therapeutic applications. The imaging modalities positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are based on γ-emissions of specific energies. The therapeutic technologies are based on β^- -particle-, α-particle-, and Auger electron emitters. In oncology, PET and SPECT are used to detect cancer lesions, to determine dosimetry, and to monitor therapy effectiveness. In contrast, radiotherapy is designed to irreparably damage tumor cells in order to eradicate or control the disease's progression. Radiometals are being explored for the development of diagnostic and therapeutic radiopharmaceuticals. Strategies that combine both modalities (diagnostic and therapeutic), referred to as theranostics, are promising candidates for clinical applications. This review provides an overview of the basic concepts behind therapeutic and diagnostic radiopharmaceuticals and their significance in contemporary oncology. Select radiometals that significantly impact current and upcoming cancer treatment strategies are grouped as clinically suitable theranostics pairs. The most important physical and chemical properties are discussed. Standard production methods and current radionuclide availability are provided to indicate whether a cost-efficient use in a clinical routine is feasible. Recent preclinical and clinical developments and outline perspectives for the radiometals are highlighted in each section.

Evaluation of STAT3 decoy oligodeoxynucleotides' synergistic effects on radiation and/or chemotherapy in metastatic breast cancer cell line

Resistance to radiotherapy and chemotherapy has been a major problem of conventional cancer therapies, which consequently leads to cancer relapse and cancer-related death. It is known that cancer stem cells (CSCs) play a key role in therapy resistance and CSC-based targeted therapies have been considered as a powerful tool for cancer treatment. In the current study, we investigated the synergistic effects of suppressing signal transducer and activator of transcription (STAT3) function by decoy ODNs on X-irradiation (XI) and methotrexate (MTX) exposure as a combinational therapy in triple-negative breast cancer (TNBC) MDA-MB-231 cells. Lipofectamine 2000® was used as a transfecting agent and the cells treated with Scramble ODNs (SCR) and decoy ODNs were subjected to irradiation with 2 Gy at single/fractionated (XI group) doses, different concentration of MTX group, and X-irradiation-methotrexate (XI/MTX group). Synergistic effects of STAT3 SCR and decoy ODNs on cells were investigated by cell viability (MTT), cell cycle profile, apoptosis rate, migration, and invasion assays. Statistical analysis of obtained data showed that STAT3 decoy ODNs significantly decreased the cell viability, arrested the growth at G0/G1 phase, increased apoptosis rate, and reduced migrated and invaded cells through transwell membrane, in XI, MTX, and XI/MTX exposed groups. Since STAT3 is a master transcription factor in breast cancer cells stemness, aggressiveness, TNBC's heterogeneity, and therapy resistance; therefore, inhibition of this transcription factor by decoy ODNs could increase antitumor efficiencies of XI and MTX exposure strategies. Accordingly, this method could have the potential to increase the efficiency of combination therapies.

Atomic Nanogenerators in Targeted Alpha Therapies: Curie's Legacy in Modern Cancer Management

Atomic in vivo nanogenerators such as actinium-225, thorium-227, and radium-223 are of increasing interest and importance in the treatment of patients with metastatic cancer diseases. This is due to their peculiar physical, chemical, and biological characteristics, leading to astonishing responses in otherwise resistant patients. Nevertheless, there are still a few obstacles and hurdles to be overcome that hamper the broader utilization in the clinical setting. Next to the limited supply and relatively high costs, the in vivo complex stability and the fate of the recoiling daughter radionuclides are substantial problems that need to be solved. In radiobiology, the mechanisms underlying treatment efficiency, possible resistance mechanisms, and late side effect occurrence are still far from being understood and need to be unraveled. In this review, the current knowledge on the scientific and clinical background of targeted alpha therapies is summarized. Furthermore, open issues and novel approaches with a focus on the future perspective are discussed. Once these are unraveled, targeted alpha therapies with atomic in vivo nanogenerators can be tailored to suit the needs of each patient when applying careful risk stratification and combination therapies. They have the potential to become one of the major treatment pillars in modern cancer management.

225Ac-H4py4pa for Targeted Alpha Therapy

Herein, we present the syntheses and characterization of a new undecadendate chelator, H₄py4pa, and its bifunctional analog H₄py4pa-phenyl-NCS, conjugated to the monoclonal antibody, Trastuzumab, which targets the HER2+ cancer. H₄py4pa possesses excellent affinity for ²²⁵Ac (α, t_1/2 = 9.92 d) for targeted alpha therapy (TAT), where quantitative radiolabeling yield was achieved at ambient temperature, pH = 7, in 30 min at 10^-6 M chelator concentration, leading to a complex highly stable in mouse serum for at least 9 d. To investigate the chelation of H₄py4pa with large metal ions, lanthanum (La³⁺), which is the largest nonradioactive metal of the lanthanide series, was adopted as a surrogate for ²²⁵Ac to enable a series of nonradioactive chemical studies. In line with the ¹H NMR spectrum, the DFT (density functional theory)-calculated structure of the [La(py4pa)]^- anion possessed a high degree of symmetry, and the La³⁺ ion was secured by two distinct pairs of picolinate arms. Furthermore, the [La(py4pa)]^- complex also demonstrated a superb thermodynamic stability (log K_[La(py4pa)]^- ∼ 20.33, pLa = 21.0) compared to those of DOTA (log K_[La(DOTA)]^- ∼ 24.25, pLa = 19.2) or H₂macropa (log K_{[La(macropa)]}^- = 14.99, pLa ∼ 8.5). Moreover, the functional versatility offered by the bifunctional py4pa precursor permits facile incorporation of various linkers for bioconjugation through direct nucleophilic substitution. In this work, a short phenyl-NCS linker was incorporated to tether H₄py4pa to Trastuzumab. Radiolabeling studies, in vitro serum stability, and animal studies were performed in parallel with the DOTA-benzyl-Trastuzumab. Both displayed excellent in vivo stability and tumor specificity.

Targeted α-Therapy in Cancer Management: Synopsis of Preclinical and Clinical Studies

The approval of ²²³Ra dichloride (²²³RaCl₂) in 2013 was a principal event in introducing targeted α-therapy as a form of safe and effective management strategy in cancer. There is an increasing interest in research and development of new targeted α-therapy agents spearheaded by advancements in cancer biology, radiochemistry, and availability of clinically relevant α particles. There are active clinical studies on sequencing or combining ²²³RaCl₂ with other drug regimens in the setting of metastatic prostate cancer and in other cancers such as osteosarcoma and bone-dominant breast cancer. Targeted α-therapy strategy is also being actively explored through many preclinical and few early clinical studies using ²²⁵Ac, ²¹³Bi, ²¹¹At, ²²⁷Th, and ²¹²Pb. Investigations incorporating ²²⁵Ac are more robust and active at this time with promising results. The author provide a brief synopsis of the preclinical and clinical studies in the rapidly evolving field of targeted α-therapy in cancer management.

Targets for Therapy of Bladder Cancer

In 2018 bladder cancer (urothelial carcinoma) was ranked twelfth concerning worldwide diagnosis of malignancies. At the time point of diagnosis of bladder cancer, approximately 75% of patients present with a nonmuscle-invasive disease (NMIBC), while the remaining 25% show invasion of tumor cells in the muscle layer of the bladder wall (MIBC). Among NMIBC tumors, flat, high-grade carcinoma in situ (CIS) is a therapeutic challenge. CIS shows a tendency to invade the muscle tissue of the bladder wall and thus become a MIBC. Standard therapy of NMIBC (including CIS) is done via intravesical instillation of BCG (bacillus Calmette Guerin) inducing a local immune reaction that finally promotes elimination of bladder cancer cells. However, BCG treatment of NMIBC proves to be ineffective in approximately 40% of patients. Therefore, new therapeutic approaches for the treatment of bladder cancer are urgently needed. Among promising new treatment options that are currently being investigated are the use of immune checkpoint inhibitors, and targeted approaches attacking (among others) long noncoding RNAs, micro RNAs, cancer stem cells, PARP1, and receptor signaling pathways. Moreover, the use of antibody-drug-conjugates (ADCs) is investigated also in bladder cancer therapy. Another approach that has been successfully established in preclinical studies uses the cytotoxic power of the alpha-emitter Bi-213 coupled to an antibody targeting EGFR. Overexpression of EGFR has been demonstrated in the majority of patients suffering from CIS. Feasibility, safety, toxicity and therapeutic efficacy of intravesical instillation of Bi-213-anti-EGFR have been evaluated in a pilot study. Since the results of the pilot study proved to be promising, a further optimization of alpha-emitter immunotherapy in bladder cancer seems mandatory.

Alpha-Emitters and Targeted Alpha Therapy in Oncology: from Basic Science to Clinical Investigations

Alpha-emitters are radionuclides that decay through the emission of high linear energy transfer α-particles and possess favorable pharmacologic profiles for cancer treatment. When coupled with monoclonal antibodies, peptides, small molecules, or nanoparticles, the excellent cytotoxic capability of α-particle emissions has generated a strong interest in exploring targeted α-therapy in the pre-clinical setting and more recently in clinical trials in oncology. Multiple obstacles have been overcome by researchers and clinicians to accelerate the development of targeted α-therapies, especially with the recent improvement in isotope production and purification, but also with the development of innovative strategies for optimized targeting. Numerous studies have demonstrated the in vitro and in vivo efficacy of the targeted α-therapy. Radium-223 (²²³Ra) dichloride (Xofigo®) is the first α-emitter to have received FDA approval for the treatment of prostate cancer with metastatic bone lesions. There is a significant increase in the number of clinical trials in oncology using several radionuclides such as Actinium-225 (²²⁵Ac), Bismuth-213 (²¹³Bi), Lead-212 (²¹²Pb), Astatine (²¹¹At) or Radium-223 (²²³Ra) assessing their safety and preliminary activity. This review will cover their therapeutic application as well as summarize the investigations that provide the foundation for further clinical development.

Recent Research Trends on Bismuth Compounds in Cancer Chemoand Radiotherapy

Background:

Application of coordination chemistry in nanotechnology is a rapidly developing research field in medicine. Bismuth complexes have been widely used in biomedicine with satisfactory therapeutic effects, mostly in Helicobacter pylori eradication, but also as potential antimicrobial and anti-leishmanial agents. Additionally, in recent years, application of bismuth-based compounds as potent anticancer drugs has been studied extensively.

Methods:

Search for data connected with recent trends on bismuth compounds in cancer chemo- and radiotherapy was carried out using web-based literature searching tools such as ScienceDirect, Springer, Royal Society of Chemistry, American Chemical Society and Wiley. Pertinent literature is covered up to 2016.

Results:

In this review, based on 213 papers, we highlighted a number of current problems connected with: (i) characterization of bismuth complexes with selected thiosemicarbazone, hydrazone, and dithiocarbamate classes of ligands as potential chemotherapeutics. Literature results derived from 50 papers show that almost all bismuth compounds inhibit growth and proliferation of breast, colon, ovarian, lung, and other tumours; (ii) pioneering research on application of bismuth-based nanoparticles and nanodots for radiosensitization. Results show great promise for improvement in therapeutic efficacy of ionizing radiation in advanced radiotherapy (described in 36 papers); and (iii) research challenges in using bismuth radionuclides in targeted radioimmunotherapy, connected with choice of adequate radionuclide, targeting vector, proper bifunctional ligand and problems with 213Bi recoil daughters toxicity (derived from 92 papers).

Conclusion:

This review presents recent research trends on bismuth compounds in cancer chemo- and radiotherapy, suggesting directions for future research.

An Overview of Targeted Alpha Therapy with 225Actinium and 213Bismuth

Background:

Recent reports of the remarkable therapeutic efficacy of 225Ac-labeled PSMA-617 for therapy of metastatic castration-resistant prostate cancer have under-lined the clinical potential of targeted alpha therapy.

Objective and Conclusion:

This review describes methods for the production of 225Ac and its daughter nuclide 213Bi and summarizes the current clinical experience with both alpha emitters with particular focus on recent studies of targeted alpha therapy of bladder cancer, brain tu-mors, neuroendocrine tumors and prostate cancer.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Preclinical optimization of antibody-based radiopharmaceuticals for cancer imaging and radionuclide therapy-Model, vector, and radionuclide selection

Intact antibodies and their truncated counterparts (eg, Fab, scFv fragments) are generally exquisitely specific and selective vectors, enabling recognition of individual cancer-associated molecular phenotypes against a complex and dynamic biomolecular background. Complementary alignment of these advantages with unique properties of radionuclides is a defining paradigm in both radioimmunoimaging and radioimmunotherapy, which remain some of the most adept and promising tools for cancer diagnosis and treatment. This review discusses how translational potency can be maximized through rational selection of antibody-nuclide couples for radioimmunoimaging/therapy in preclinical models.

Modeling Cell and Tumor-Metastasis Dosimetry with the Particle and Heavy Ion Transport Code System (PHITS) Software for Targeted Alpha-Particle Radionuclide Therapy

The use of targeted radionuclide therapy for cancer is on the rise. While beta-particle-emitting radionuclides have been extensively explored for targeted radionuclide therapy, alpha-particle-emitting radionuclides are emerging as effective alternatives. In this context, fundamental understanding of the interactions and dosimetry of these emitted particles with cells in the tumor microenvironment is critical to ascertaining the potential of alpha-particle-emitting radionuclides. One important parameter that can be used to assess these metrics is the S-value. In this study, we characterized several alpha-particle-emitting radionuclides (and their associated radionuclide progeny) regarding S-values in the cellular and tumor-metastasis environments. The Particle and Heavy Ion Transport code System (PHITS) was used to obtain S-values via Monte Carlo simulation for cell and tumor metastasis resulting from interactions with the alpha-particle-emitting radionuclides, lead-212 (²¹²Pb), actinium-225 (²²⁵Ac) and bismuth-213 (²¹³Bi); these values were compared to the beta-particle-emitting radionuclides yttrium-90 (⁹⁰Y) and lutetium-177 (¹⁷⁷Lu) and an Auger-electron-emitting radionuclide indium-111 (¹¹¹In). The effect of cellular internalization on S-value was explored at increasing degree of internalization for each radionuclide. This aspect of S-value determination was further explored in a cell line-specific fashion for six different cancer cell lines based on the cell dimensions obtained by confocal microscopy. S-values from PHITS were in good agreement with MIRDcell S-values (cellular S-values) and the values found by Hindié et al. (tumor S-values). In the cellular model, ²¹²Pb and ²¹³Bi decay series produced S-values that were 50- to 120-fold higher than ¹⁷⁷Lu, while ²²⁵Ac decay series analysis suggested S-values that were 240- to 520-fold higher than ¹⁷⁷Lu. S-values arising with 100% cellular internalization were two- to sixfold higher for the nucleus when compared to 0% internalization. The tumor dosimetry model defines the relative merit of radionuclides and suggests alpha particles may be effective for large tumors as well as small tumor metastases. These results from PHITS modeling substantiate emerging evidence that alpha-particle-emitting radionuclides may be an effective alternative to beta-particle-emitting radionuclides for targeted radionuclide therapy due to preferred dose-deposition profiles in the cellular and tumor metastasis context. These results further suggest that internalization of alpha-particle-emitting radionuclides via radiolabeled ligands may increase the relative biological effectiveness of radiotherapeutics.

In vitro evaluation of 225 Ac-DOTA-substance P for targeted alpha therapy of glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most malignant form of brain tumors with dismal prognosis despite treatment by surgery combined with radiotherapy and chemotherapy. The neuropeptide Substance P (SP) is the physiological ligand of the neurokinin-1 receptor, which is highly expressed in glioblastoma cells. Thus, SP represents a potential ligand for targeted alpha therapy. In this study, a protocol for the synthesis of SP labeled with the alpha emitter ²²⁵ Ac was developed and binding affinity properties were determined. The effects of ²²⁵ Ac-DOTA-SP were investigated on human glioblastoma cell lines (T98G, U87MG, U138MG) as well as GBM stem cells. A significant dose-dependent reduction in cell viability was detected up to 6 days after treatment. Also, colony-forming capacity was inhibited at the lower doses tested. In comparison, treatment with the conventional agent temozolomide showed higher cell viability and colony-forming capacity. ²²⁵ Ac-DOTA-SP treatment caused induction of late apoptosis pathways. Cells were arrested to G2/M-phase upon treatment. Increasing doses and treatment time caused additional S-phase arrest. Similar results were obtained using human glioblastoma stem cells, known to show radioresistance. Our data suggest that ²²⁵ Ac-DOTA-SP is a promising compound for treatment of GBM.

Functionalized TiO2 nanoparticles labelled with 225Ac for targeted alpha radionuclide therapy

The ²²⁵Ac radioisotope exhibits very attractive nuclear properties for application in radionuclide therapy. Unfortunately, the major challenge for radioconjugates labelled with ²²⁵Ac is that traditional chelating moieties are unable to sequester the radioactive daughters in the bioconjugate which is critical to minimize toxicity to healthy, non-targeted tissues. In the present work, we propose to apply TiO₂ nanoparticles (NPs) as carrier for ²²⁵Ac and its decay products. The surface of TiO₂ nanoparticles with 25 nm diameter was modified with Substance P (5-11), a peptide fragment which targets NK1 receptors on the glioma cells, through the silan-PEG-NHS linker. Nanoparticles functionalized with Substance P (5-11) were synthesized with high yield in a two-step procedure, and the products were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermogravimetric analysis (TGA). The obtained results show that one TiO₂-bioconjugate nanoparticle contains in average 80 peptide molecules on its surface. The synthesized TiO₂-PEG-SP(5-11) conjugates were labelled with ²²⁵Ac by ion-exchange reaction on hydroxyl (OH) functional groups on the TiO₂ surface. The labelled bioconjugates almost quantitatively retain ²²⁵Ac in phosphate-buffered saline (PBS), physiological salt and cerebrospinal fluid (CSF) for up to 10 days. The leaching of ²²¹Fr, a first decay daughter of ²²⁵Ac, in an amount of 30% was observed only in CSF after 10 days. The synthesized ²²⁵Ac-TiO₂-PEG-SP(5-11) has shown high cytotoxic effect in vitro in T98G glioma cells; therefore, it is a promising new radioconjugate for targeted radionuclide therapy of brain tumours.

α- Versus β-Emitting Radionuclides for Pretargeted Radioimmunotherapy of Carcinoembryonic Antigen-Expressing Human Colon Cancer Xenografts

Pretargeted radioimmunotherapy (PRIT) with the β-emitting radionuclide ¹⁷⁷Lu is an attractive approach to treat carcinoembryonic antigen (CEA)-expressing tumors. The therapeutic efficacy of PRIT might be improved using α-emitting radionuclides such as ²¹³Bi. Herein, we report and compare the tumor-targeting properties and therapeutic efficacy of ²¹³Bi and ¹⁷⁷Lu for PRIT of CEA-expressing xenografts, using the bispecific monoclonal antibody TF2 (anti-CEA × anti-histamine-succinyl-glycine [HSG]) and the di-HSG-DOTA peptide IMP288. Methods: The in vitro binding characteristics of ²¹³Bi-IMP288 were compared with those of ¹⁷⁷Lu-IMP288. Tumor targeting of ²¹³Bi-IMP288 and ¹⁷⁷Lu-IMP288 was studied in mice bearing subcutaneous LS174T tumors that were pretargeted with TF2. Finally, the effect of ²¹³Bi-IMP288 (6, 12, or 17 MBq) and ¹⁷⁷Lu-IMP288 (60 MBq) on tumor growth and survival was assessed. Toxicity was determined by monitoring body weight, analyzing blood samples for hematologic and renal toxicity (hemoglobin, leukocytes, platelets, creatinine), and immunohistochemical analysis of the kidneys. Results: The in vitro binding characteristics of ²¹³Bi-IMP288 (dissociation constant, 0.45 ± 0.20 nM) to TF2-pretargeted LS174T cells were similar to those of ¹⁷⁷Lu-IMP288 (dissociation constant, 0.53 ± 0.12 nM). In vivo accumulation of ²¹³Bi-IMP288 in LS174T tumors was observed as early as 15 min after injection (9.2 ± 2.0 percentage injected dose [%ID]/g). ²¹³Bi-IMP288 cleared rapidly from the circulation; at 30 min after injection, the blood levels were 0.44 ± 0.28 %ID/g. Uptake in normal tissues was low, except for the kidneys, where uptake was 1.8 ± 1.1 %ID/g at 30 min after injection. The biodistribution of ²¹³Bi-IMP288 was comparable to that of ¹⁷⁷Lu-IMP288. Mice treated with a single dose of ²¹³Bi-IMP288 or ¹⁷⁷Lu-IMP288 showed significant inhibition of tumor growth. Median survival for the groups treated with phosphate-buffered saline, 6 MBq ²¹³Bi-IMP288, 12 MBq ²¹³Bi-IMP288, and 60 MBq ¹⁷⁷Lu-IMP288 was 22, 31, 45, and 42 d, respectively. Mice receiving 17 MBq ²¹³Bi-IMP288 showed significant weight loss, resulting in a median survival of only 24 d. No changes in hemoglobin, platelets, or leukocytes were observed in the treatment groups. However, immunohistochemical analysis of the kidneys of mice treated with 17 or 12 MBq ²¹³Bi-IMP288 showed signs of tubular damage, indicating nephrotoxicity. Conclusion: To our knowledge, this study shows for the first time that PRIT with TF2 and ²¹³Bi-IMP288 is feasible and at least as effective as ¹⁷⁷Lu-IMP288. However, at higher doses, kidney toxicity was observed. Future studies are warranted to determine the optimal dosing schedule to improve therapeutic efficacy while reducing renal toxicity.

Radioimmunotherapy of cancer with high linear energy transfer (LET) radiation delivered by radionuclides emitting α-particles or Auger electrons

Radioimmunotherapy (RIT) aims to selectively deliver radionuclides emitting α-particles, β-particles or Auger electrons to tumors by conjugation to monoclonal antibodies (mAbs) that recognize tumor-associated antigens/receptors. The approach has been most successful for treatment of non-Hodgkin's B-cell lymphoma but challenges have been encountered in extending these promising results to the treatment of solid malignancies. These challenges include the low potency of β-particle emitters such as ¹³¹I, ¹⁷⁷Lu or ⁹⁰Y which have been commonly conjugated to the mAbs, due to their low linear energy transfer (LET=0.1-1.0keV/μm). Furthermore, since the β-particles have a 2-10mm range, there has been dose-limiting non-specific toxicity to hematopoietic stem cells in the bone marrow (BM) due to the cross-fire effect. Conjugation of mAbs to α-particle-emitters (e.g. ²²⁵Ac, ²¹³Bi, ²¹²Pb or ²¹¹At) or Auger electron-emitters (e.g. ¹¹¹In, ⁶⁷Ga, ¹²³I or ¹²⁵I) would increase the potency of RIT due to their high LET (50-230keV/μm and 4 to 26keV/μm, respectively). In addition, α-particles have a range in tissues of 28-100μm and Auger electrons are nanometer in range which greatly reduces or eliminates the cross-fire effect compared to β-particles, potentially reducing their non-specific toxicity to the BM. In this review, we describe the results of preclinical and clinical studies of RIT of cancer using radioimmunoconjugates emitting α-particles or Auger electrons, and discuss the potential of these high LET forms of radiation to improve the outcome of cancer patients.

Combination of Antiretroviral Drugs and Radioimmunotherapy Specifically Kills Infected Cells from HIV-Infected Individuals

Eliminating virally infected cells is an essential component of any HIV eradication strategy. Radioimmunotherapy (RIT), a clinically established method for killing cells using radiolabeled antibodies, was recently applied to target HIV-1 gp41 antigen expressed on the surface of infected cells. Since gp41 expression by infected cells is likely downregulated in patients on antiretroviral therapy (ART), we evaluated the ability of RIT to kill ART-treated infected cells using both in vitro models and lymphocytes isolated from HIV-infected subjects. Human peripheral blood mononuclear cells (PBMCs) were infected with HIV and cultured in the presence of two clinically relevant ART combinations. Scatchard analysis of the 2556 human monoclonal antibody to HIV gp41 binding to the infected and ART-treated cells demonstrated sufficient residual expression of gp41 on the cell surface to warrant subsequent RIT. This is the first time the quantification of gp41 post-ART is being reported. Cells were then treated with Bismuth-213-labeled 2556 antibody. Cell survival was quantified by Trypan blue and residual viremia by p24 ELISA. Cell surface gp41 expression was assessed by Scatchard analysis. The experiments were repeated using PBMCs isolated from blood specimens obtained from 15 HIV-infected individuals: 10 on ART and 5 ART-naïve. We found that ²¹³Bi-2556 killed ART-treated infected PBMCs and reduced viral production to undetectable levels. ART and RIT co-treatment was more effective at reducing viral load in vitro than either therapy alone, indicating that gp41 expression under ART was sufficient to allow ²¹³Bi-2556 to deliver cytocidal doses of radiation to infected cells. This study provides proof of concept that ²¹³Bi-2556 may represent an innovative and effective targeting method for killing HIV-infected cells treated with ART and supports continued development of ²¹³Bi-2556 for co-administration with ART toward an HIV eradication strategy.

Relevance of radiobiological concepts in radionuclide therapy of cancer

PURPOSE

Radionuclide therapy (RNT) is a rapidly growing area of clinical nuclear medicine, wherein radionuclides are employed to deliver cytotoxic dose of radiation to the diseased cells/tissues. During RNT, radionuclides are either directly administered or delivered through biomolecules targeting the diseased site. RNT has been clinically used for diverse range of diseases including cancer, which is the focus of the review.

CONCLUSIONS

The major emphasis in RNT has so far been given towards developing peptides/antibodies and other molecules to conjugate a variety of therapeutic radioisotopes for improved targeting/delivery of radiation dose to the tumor cells. Despite that, many of the RNT approaches have not achieved their desired therapeutic success probably due to poor knowledge about complex and dynamic (i) fate of radiolabeled molecules; (ii) radiation dose delivered; (iii) cellular heterogeneity in tumor mass; and (iv) cellular radiobiological response. Based on understanding gathered during recent years, it may be stated that besides the absorbed dose, the net radiobiological response of tumor/normal cells also determines the clinical response of radiotherapeutic modalities including RNT. The radiosensitivity of tumor/normal cells is governed by radiobiological phenomenon such as radiation-induced bystander effect, genomic instability, adaptive response and low dose hyper-radiosensitivity. These concepts have been well investigated in the context of external beam radiotherapy, but their clinical implications during RNT have received meagre attention. In this direction, a few studies performed using in vitro and in vivo models envisage the possibilities of exploiting the radiobiological knowledge for improved therapeutic outcome of RNT.

Alpha Particles Induce Autophagy in Multiple Myeloma Cells

OBJECTIVES

Radiation emitted by the radionuclides in radioimmunotherapy (RIT) approaches induce direct killing of the targeted cells as well as indirect killing through the bystander effect. Our research group is dedicated to the development of α-RIT, i.e., RIT using α-particles especially for the treatment of multiple myeloma (MM). γ-irradiation and β-irradiation have been shown to trigger apoptosis in tumor cells. Cell death mode induced by (213)Bi α-irradiation appears more controversial. We therefore decided to investigate the effects of (213)Bi on MM cell radiobiology, notably cell death mechanisms as well as tumor cell immunogenicity after irradiation.

METHODS

Murine 5T33 and human LP-1 MM cell lines were used to study the effects of such α-particles. We first examined the effects of (213)Bi on proliferation rate, double-strand DNA breaks, cell cycle, and cell death. Then, we investigated autophagy after (213)Bi irradiation. Finally, a coculture of dendritic cells (DCs) with irradiated tumor cells or their culture media was performed to test whether it would induce DC activation.

RESULTS

We showed that (213)Bi induces DNA double-strand breaks, cell cycle arrest, and autophagy in both cell lines, but we detected only slight levels of early apoptosis within the 120 h following irradiation in 5T33 and LP-1. Inhibition of autophagy prevented (213)Bi-induced inhibition of proliferation in LP-1 suggesting that this mechanism is involved in cell death after irradiation. We then assessed the immunogenicity of irradiated cells and found that irradiated LP-1 can activate DC through the secretion of soluble factor(s); however, no increase in membrane or extracellular expression of danger-associated molecular patterns was observed after irradiation.

CONCLUSION

This study demonstrates that (213)Bi induces mainly necrosis in MM cells, low levels of apoptosis, and autophagy that might be involved in tumor cell death.

Application of 212Pb for Targeted α-particle Therapy (TAT): Pre-clinical and Mechanistic Understanding through to Clinical Translation

Targeted α-particle therapy (TAT), in which an α-particle emitting radionuclide is specifically directed to a biological target, is gaining more attention to treat cancers as new targets are validated. Bio-vectors such as monoclonal antibodies are able to selectively transport α-particles to destroy targeted cancer cells. TAT has the potential for an improved therapeutic ratio over β-particle targeted conjugate therapy. The short path length and the intense ionization path generated render α-emitters suitable for treatment and management of minimal disease such as micrometastases or residual tumor after surgical debulking. ²¹²Pb is the longer-lived parent radionuclide of ²¹²Bi and serves as an in vivo generator of ²¹²Bi. ²¹²Pb has demonstrated significant utility in both in vitro and in vivo models. Recent evaluation of ²¹²Pb-TCMC-trastuzumab in a Phase I clinical trial has demonstrated the feasibility of ²¹²Pb in TAT for the treatment of ovarian cancer patients. This review highlights progress in radionuclide production, radiolabeling chemistry, molecular mechanisms, and application of ²¹²Pb to targeted pre-clinical and clinical radiation therapy for the management and treatment of cancer.

Cell death by autophagy: emerging molecular mechanisms and implications for cancer therapy

Autophagy is a tightly-regulated catabolic process of cellular self-digestion by which cellular components are targeted to lysosomes for their degradation. Key functions of autophagy are to provide energy and metabolic precursors under conditions of starvation and to alleviate stress by removal of damaged proteins and organelles, which are deleterious for cell survival. Therefore, autophagy appears to serve as a pro-survival stress response in most settings. However, the role of autophagy in modulating cell death is highly dependent on the cellular context and its extent. There is an increasing evidence for cell death by autophagy, in particular in developmental cell death in lower organisms and in autophagic cancer cell death induced by novel cancer drugs. The death-promoting and -executing mechanisms involved in the different paradigms of autophagic cell death (ACD) are very diverse and complex, but a draft scenario of the key molecular targets involved in ACD is beginning to emerge. This review provides an up-to-date and comprehensive report on the molecular mechanisms of drug-induced autophagy-dependent cell death and highlights recent key findings in this exciting field of research.

188Re-HYNIC-trastuzumab enhances the effect of apoptosis induced by trastuzumab in HER2-overexpressing breast cancer cells

PURPOSE

The development of radioimmunotherapy has provided an impressive alternative approach in improving trastuzumab therapy. However, the mechanisms of trastuzumab and radiation treatment combined to increase therapeutic efficacy are poorly understood. Here, we try to examine the efficacy of cytotoxicity and apoptosis induction for (188)Re-HYNIC-trastuzumab in cancer cell lines with various levels of Her2.

MATERIALS AND METHODS

Fluorescence flow cytometry was used to detect the alterations of apoptosis induction after (188)Re-HYNIC-trastuzumab treatment in two breast cancer cell lines with different levels of HER2 (BT-474 and MCF-7) and a colorectal carcinoma cell line (HT-29) for control.

RESULTS

Our results indicated that (188)Re-HYNIC-trastuzumab led to cell death of breast cancer cells specifically in HER2 level-dependent and radioactivity dose-dependent fashions. In BT-474 cells, 370 kBq/ml of (188)Re-HYNIC-trastuzumab enhanced the cytotoxicity to a level nearly 100-fold that of trastuzumab-alone treatment. The results also revealed that the mitochondria-dependent pathway attenuated irradiation-induced apoptosis in HER2-expressing breast cancer cells after (188)Re-HYNIC-trastuzumab treatment. In contrast, only after 48 h of (188)Re-HYNIC-trastuzumab treatment, BT-474 cells exhibited typical apoptotic changes, including exposure of phospholipid phosphatidylserine on the cell surface, or fragmented DNA formation, in a radioactivity dose-dependent manner.

CONCLUSION

Briefly, our study demonstrates that (188)Re-labeled HYNIC-trastuzumab not only enhances cell death in a radioactivity dose-dependent fashion, but may also prolong the effects of apoptosis involved with the mitochondria-dependent pathway in HER2-overexpressing breast cancer cells. It is possible that the (188)Re-HYNIC-trastuzumab treatment induced a second round of apoptosis to prolong the effects of cell kill in these cancer cells. These data revealed that (188)Re-HYNIC-trastuzumab has the potential for use as a therapeutic radiopharmaceutical agent in HER2-overexpressing breast cancer cell treatment.

Branched amphiphilic peptide capsules: cellular uptake and retention of encapsulated solutes

Branched amphiphilic peptide capsules (BAPCs) are peptide nano-spheres comprised of equimolar proportions of two branched peptide sequences bis(FLIVI)-K-KKKK and bis(FLIVIGSII)-K-KKKK that self-assemble to form bilayer delimited capsules. In two recent publications we described the lipid analogous characteristics of our BAPCs, examined their initial assembly, mode of fusion, solute encapsulation, and resizing and delineated their capability to be maintained at a specific size by storing them at 4°C. In this report we describe the stability, size limitations of encapsulation, cellular localization, retention and, bio-distribution of the BAPCs in vivo. The ability of our constructs to retain alpha particle emitting radionuclides without any apparent leakage and their persistence in the peri-nuclear region of the cell for extended periods of time, coupled with their ease of preparation and potential tune-ability, makes them attractive as biocompatible carriers for targeted cancer therapy using particle emitting radioisotopes. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

²¹³Bi-DOTATOC receptor-targeted alpha-radionuclide therapy induces remission in neuroendocrine tumours refractory to beta radiation: a first-in-human experience

Purpose

Radiopeptide therapy using a somatostatin analogue labelled with a beta emitter such as ⁹⁰Y/¹⁷⁷Lu-DOTATOC is a new therapeutic option in neuroendocrine cancer. Alternative treatments for patients with refractory disease are rare. Here we report the first-in-human experience with ²¹³Bi-DOTATOC targeted alpha therapy (TAT) in patients pretreated with beta emitters.

Methods

Seven patients with progressive advanced neuroendocrine liver metastases refractory to treatment with ⁹⁰Y/¹⁷⁷Lu-DOTATOC were treated with an intraarterial infusion of ²¹³Bi-DOTATOC, and one patient with bone marrow carcinosis was treated with a systemic infusion of ²¹³Bi-DOTATOC. Haematological, kidney and endocrine toxicities were assessed according to CTCAE criteria. Radiological response was assessed with contrast-enhanced MRI and ⁶⁸Ga-DOTATOC-PET/CT. More than 2 years of follow-up were available in seven patients.

Results

The biodistribution of ²¹³Bi-DOTATOC was evaluable with 440 keV gamma emission scans, and demonstrated specific tumour binding. Enduring responses were observed in all treated patients. Chronic kidney toxicity was moderate. Acute haematotoxicity was even less pronounced than with the preceding beta therapies.

Conclusion

TAT can induce remission of tumours refractory to beta radiation with favourable acute and mid-term toxicity at therapeutic effective doses.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-014-2857-9) contains supplementary material, which is available to authorized users.

Radioimmunotherapy with α-particle-emitting radionuclides

α-particle-emitting radionuclides are highly cytotoxic and are thus promising candidates for use in targeted radioimmunotherapy of cancer. Due to their high linear energy transfer (LET) combined with a short path length in tissue, α-particles cause severe DNA double-strand breaks that are repaired inaccurately and finally trigger cell death. For radioimmunotherapy, α-emitters such as 225Ac, 211At, 212Bi/212Pb, 213Bi and 227Th are coupled to antibodies via appropriate chelating agents. The α-emitter immunoconjugates preferably target proteins that are overexpressed or exclusively expressed on cancer cells. Application of α-emitter immunoconjugates seems particularly promising in treatment of disseminated cancer cells and small tumor cell clusters that are released during the resection of a primary tumor. α-emitter immunoconjugates have been successfully administered in numerous experimental studies for therapy of ovarian, colon, gastric, blood, breast and bladder cancer. Initial clinical trials evaluating α-emitter immunoconjugates in terms of toxicity and therapeutic efficacy have also shown positive results in patients with melanoma, ovarian cancer, acute myeloid lymphoma and glioma. The present problems in terms of availability of therapeutically effiective α-emitters will presumably be solved by use of alternative production routes and installation of additional production facilities in the near future. Therefore, clinical establishment of targeted α-emitter radioimmunotherapy as one part of a multimodal concept for therapy of cancer is a promising, middle-term concept.

Cell death sensitization of leukemia cells by opioid receptor activation

Cyclic AMP (cAMP) regulates a number of cellular processes and modulates cell death induction. cAMP levels are altered upon stimulation of specific G-protein-coupled receptors inhibiting or activating adenylyl cyclases. Opioid receptor stimulation can activate inhibitory G_i-proteins which in turn block adenylyl cyclase activity reducing cAMP. Opioids such as D,L-methadone induce cell death in leukemia cells. However, the mechanism how opioids trigger apoptosis and activate caspases in leukemia cells is not understood. In this study, we demonstrate that downregulation of cAMP induced by opioid receptor activation using the opioid D,L-methadone kills and sensitizes leukemia cells for doxorubicin treatment. Enhancing cAMP levels by blocking opioid-receptor signaling strongly reduced D,L-methadone-induced apoptosis, caspase activation and doxorubicin-sensitivity. Induction of cell death in leukemia cells by activation of opioid receptors using the opioid D,L-methadone depends on critical levels of opioid receptor expression on the cell surface. Doxorubicin increased opioid receptor expression in leukemia cells. In addition, the opioid D,L-methadone increased doxorubicin uptake and decreased doxorubicin efflux in leukemia cells, suggesting that the opioid D,L-methadone as well as doxorubicin mutually increase their cytotoxic potential. Furthermore, we found that opioid receptor activation using D,L-methadone alone or in addition to doxorubicin inhibits tumor growth significantly in vivo. These results demonstrate that opioid receptor activation via triggering the downregulation of cAMP induces apoptosis, activates caspases and sensitizes leukemia cells for doxorubicin treatment. Hence, opioid receptor activation seems to be a promising strategy to improve anticancer therapies.

DNA double strand breaks as predictor of efficacy of the alpha-particle emitter Ac-225 and the electron emitter Lu-177 for somatostatin receptor targeted radiotherapy

Rationale

Key biologic effects of the alpha-particle emitter Actinium-225 in comparison to the beta-particle emitter Lutetium-177 labeled somatostatin-analogue DOTATOC in vitro and in vivo were studied to evaluate the significance of γH2AX-foci formation.

Methods

To determine the relative biological effectiveness (RBE) between the two isotopes (as - biological consequence of different ionisation-densities along a particle-track), somatostatin expressing AR42J cells were incubated with Ac-225-DOTATOC and Lu-177-DOTATOC up to 48 h and viability was analyzed using the MTT assay. DNA double strand breaks (DSB) were quantified by immunofluorescence staining of γH2AX-foci. Cell cycle was analyzed by flow cytometry. In vivo uptake of both radiolabeled somatostatin-analogues into subcutaneously growing AR42J tumors and the number of cells displaying γH2AX-foci were measured. Therapeutic efficacy was assayed by monitoring tumor growth after treatment with activities estimated from in vitro cytotoxicity.

Results

Ac-225-DOTATOC resulted in ED₅₀ values of 14 kBq/ml after 48 h, whereas Lu-177-DOTATOC displayed ED₅₀ values of 10 MBq/ml. The number of DSB grew with increasing concentration of Ac-225-DOTATOC and similarly with Lu-177-DOTATOC when applying a factor of 700-fold higher activity compared to Ac-225. Already 24 h after incubation with 2.5–10 kBq/ml, Ac-225-DOTATOC cell-cycle studies showed up to a 60% increase in the percentage of tumor cells in G2/M phase. After 72 h an apoptotic subG1 peak was also detectable. Tumor uptake for both radio peptides at 48 h was identical (7.5%ID/g), though the overall number of cells with γH2AX-foci was higher in tumors treated with 48 kBq Ac-225-DOTATOC compared to tumors treated with 30 MBq Lu-177-DOTATOC (35% vs. 21%). Tumors with a volume of 0.34 ml reached delayed exponential tumor growth after 25 days (44 kBq Ac-225-DOTATOC) and after 21 days (34 MBq Lu-177-DOTATOC).

Conclusion

γH2AX-foci formation, triggered by beta- and alpha-irradiation, is an early key parameter in predicting response to internal radiotherapy.

Targeted alpha-therapy using [Bi-213]anti-CD20 as novel treatment option for radio- and chemoresistant non-Hodgkin lymphoma cells

Radioimmunotherapy (RIT) is an emerging treatment option for non-Hodgkin lymphoma (NHL) producing higher overall response and complete remission rates compared with unlabelled antibodies. However, the majority of patients treated with conventional or myeloablative doses of radiolabelled antibodies relapse. The development of RIT with alpha-emitters is attractive for a variety of cancers because of the high linear energy transfer (LET) and short path length of alpha-radiation in human tissue, allowing higher tumour cell kill and lower toxicity to healthy tissues. In this study, we investigated the molecular effects of the alpha-emitter Bi-213 labelled to anti-CD20 antibodies ([Bi-213]anti-CD20) on cell cycle and cell death in sensitive and radio-/chemoresistant NHL cells. [Bi-213]anti-CD20 induced apoptosis, activated caspase-3, caspase-2 and caspase-9 and cleaved PARP specifically in CD20-expressing sensitive as well as in chemoresistant, beta-radiation resistant and gamma-radiation resistant NHL cells. CD20 negative cells were not affected by [Bi-213]anti-CD20 and unspecific antibodies labelled with Bi-213 could not kill NHL cells. Breaking radio-/chemoresistance in NHL cells using [Bi-213]anti-CD20 depends on caspase activation as demonstrated by complete inhibition of [Bi-213]anti-CD20-induced apoptosis with zVAD.fmk, a specific inhibitor of caspases activation. This suggests that deficient activation of caspases was reversed in radioresistant NHL cells using [Bi-213]anti-CD20. Activation of mitochondria, resulting in caspase-9 activation was restored and downregulation of Bcl-x_L and XIAP, death-inhibiting proteins, was found after [Bi-213]anti-CD20 treatment in radio-/chemosensitive and radio-/chemoresistant NHL cells. [Bi-213]anti-CD20 seems to be a promising radioimmunoconjugate to improve therapeutic success by breaking radio- and chemoresistance selectively in CD20-expressing NHL cells via re-activating apoptotic pathways through reversing deficient activation of caspases and the mitochondrial pathway and downregulation of XIAP and Bcl-x_L.

²¹³Bi-anti-EGFR radioimmunoconjugates and X-ray irradiation trigger different cell death pathways in squamous cell carcinoma cells

INTRODUCTION

Treatment of patients with squamous cell carcinoma of head and neck is hampered by resistance of tumor cells to irradiation. Additional therapies enhancing the effect of X-ray irradiation may be beneficial. Antibodies targeting EGFR have been shown to improve the efficacy of radiation therapy. Therefore, we analyzed cytotoxicity of (213)Bi-anti-EGFR immunoconjugates in combination with X-ray irradiation.

METHODS

The monoclonal anti-EGFR antibody matuzumab was coupled to CHX-A"-DTPA forming stable complexes with (213)Bi. Cytotoxicity of X-ray radiation, of treatment with (213)Bi-anti-EGFR monoclonal antibodies (MAb) or of a combined treatment regimen was assayed using cell proliferation and colony formation assays in UD-SCC5 cells. Key proteins of cell-cycle arrest and cell death were examined by Western blot analysis. Cell cycle analysis was performed by flow cytometry. DNA double-strand breaks were detected via γH2AX and quantified using Definiens™ software.

RESULTS

Irradiation with X-rays or treatment with (213)Bi-anti-EGFR-MAb resulted in median lethal dose (LD50) values of 12 Gy or 130 kBq/mL, respectively. Treatment with 37 kBq/mL of (213)Bi-anti-EGFR-MAb or 2 Gy of X-rays had only little effect on colony formation of UD-SCC5 cells. In contrast, a combined treatment regimen (37 kBq/mL plus 2 Gy) significantly decreased colony formation and enhanced the formation of DNA double-strand breaks. As revealed by flow cytometry, radiation treatments caused accumulation of cells in the G0/G1 phase. Both treatment with (213)Bi-anti-EGFR immunoconjugates and application of the combined treatment regimen triggered activation of genes of signaling pathways involved in cell-cycle arrest and induction of apoptosis like p21/Waf, GADD45, Puma and Bax, which were only marginally modulated by X-ray irradiation of cells.

CONCLUSIONS

(213)Bi-anti-EGFR-MAb enhances cytotoxicity of X-ray irradiation in UD-SCC5 cells most probably due to effective induction of DNA double-strand breaks. Induction of genes involved in cell-cycle arrest and cell death is almost exclusively due to (213)Bi-anti-EGFR-MAb and seems to be independent of p53 function.

Systemic treatment with 4-211Atphenylalanine enhances survival of rats with intracranial glioblastoma

OBJECTIVE

Increased amino acid transport in brain tumours is used for diagnostic purposes. It has been shown that the α-emitting radionuclide astatine-211 labeled to L-phenylalanine is taken up by glioblastoma cells. We here tested, if systemic treatment with 4-[211At]astatine-phenylalanine (At-Phe) has a beneficial effect on survival of rats with intracranial glioblastoma.

ANIMALS, METHODS

The rat glioblastoma cell line BT4Ca was implanted into the prefrontal cortex of female BDIX rats by stereotaxic microinjection (10,000 cells/3 µl; n = 83). 3 days after implantation At-Phe or phosphate buffered saline were injected intravenously. A third group was treated twice, i.e., on day 3 and 10. Health condition was assessed each day by using a score system. Rats were sacrificed on days 6, 10, 13 and 17 after implantation, or when showing premortal health condition to measure tumour volume and necrosis. The proliferation index (PI) was assessed after immunohistochemical staining of Ki-67.

RESULTS

Survival time of rats treated twice with At-Phe was significantly prolonged. Additionally, both At-Phe-treated groups remained significantly longer in a better health condition. Rats with poor health status had larger tumours than rats with fair health condition. Overall, irrespective of treatment the PI was reduced in rats with poor health condition. Necrosis was larger in rats treated twice with At-Phe.

CONCLUSION

Intravenous treatment with At-Phe enhanced survival time of rats with intracranial glioblastomas and improved health condition. These results encourage studies using local treatment of intracranial glioblastoma with At-Phe, either by repeated local injection or by intracavital application after tumour resection.

Anti-CD33-antibodies labelled with the alpha-emitter Bismuth-213 kill CD33-positive acute myeloid leukaemia cells specifically by activation of caspases and break radio- and chemoresistance by inhibition of the anti-apoptotic proteins X-linked inhibitor of apoptosis protein and B-cell lymphoma-extra large

AIM

The emerging interest in radioimmunotherapies employing alpha-emitters for cancer treatment like high risk-leukaemia leads to the question of how these radionuclides exhibit their cytotoxicity. To clarify the molecular mechanisms of cell death induction, we investigated the molecular effects of the alpha-emitter Bismuth-213 (Bi-213) bound to a monoclonal anti-CD33-antibody ([Bi-213]anti-CD33) on the cell cycle and on apoptosis induction in sensitive as well as in beta- and gamma-radiation-resistant CD33-positive acute myeloid leukaemia (AML) cells.

METHODS

The cytotoxic potential of the radioimmunoconjugate [Bi-213]anti-CD33 was analysed in the CD33-expressing human AML cell line HL-60 and in radiation- and chemoresistant HL-60-derived cell lines. Cell cycle and apoptosis induction analyses were performed via flow cytometry. Activation of apoptosis pathways was determined by immunodetection.

RESULTS

[Bi-213]anti-CD33 induced apoptotic cell death in CD33-positive AML cells specifically. Molecular analyses revealed that the intrinsic mitochondrial pathway of apoptosis was activated resulting in caspase-9 activation. In the apoptotic executioner cascade caspase-3 was activated and its substrate poly (ADP-ribose) polymerase (PARP) was cleaved. Notably, [Bi-213]anti-CD33 overcame radio- and chemoresistance by reversing deficient activation of apoptosis pathways in resistant CD33-positive AML cells and by the downregulation of inhibitors of apoptosis B-cell lymphoma-extra large (Bcl-xL) and X-linked inhibitor of apoptosis protein (XIAP) involved in leukaemia resistance.

CONCLUSION

[Bi-213]anti-CD33 exhibits its cytotoxic effects specifically in CD33-expressing AML cells via induction of the intrinsic, mitochondrial pathway of apoptosis. The abrogation of chemo- and radioresistances and the reactivation of apoptotic pathways seem to be promising for the treatment of patients with so far untreatable resistant AML and underline the importance of this emerging therapeutic approach of targeted alpha-therapies.

212Pb-radioimmunotherapy potentiates paclitaxel-induced cell killing efficacy by perturbing the mitotic spindle checkpoint

BACKGROUND

Paclitaxel has recently been reported by this laboratory to potentiate the high-LET radiation therapeutic (212)Pb-TCMC-trastuzumab, which targets HER2. To elucidate mechanisms associated with this therapy, targeted α-particle radiation therapeutic (212)Pb-TCMC-trastuzumab together with paclitaxel was investigated for the treatment of disseminated peritoneal cancers.

METHODS

Mice bearing human colon cancer LS-174T intraperitoneal xenografts were pre-treated with paclitaxel, followed by treatment with (212)Pb-TCMC-trastuzumab and compared with groups treated with paclitaxel alone, (212)Pb-TCMC-HuIgG, (212)Pb-TCMC-trastuzumab and (212)Pb-TCMC-HuIgG after paclitaxel pre-treatment.

RESULTS

(212)Pb-TCMC-trastuzumab with paclitaxel given 24 h earlier induced increased mitotic catastrophe and apoptosis. The combined modality of paclitaxel and (212)Pb-TCMC-trastuzumab markedly reduced DNA content in the S-phase of the cell cycle with a concomitant increase observed in the G2/M-phase. This treatment regimen also diminished phosphorylation of histone H3, accompanied by an increase in multi-micronuclei, or mitotic catastrophe in nuclear profiles and positively stained γH2AX foci. The data suggests, possible effects on the mitotic spindle checkpoint by the paclitaxel and (212)Pb-TCMC-trastuzumab treatment. Consistent with this hypothesis, (212)Pb-TCMC-trastuzumab treatment in response to paclitaxel reduced expression and phosphorylation of BubR1, which is likely attributable to disruption of a functional Aurora B, leading to impairment of the mitotic spindle checkpoint. In addition, the reduction of BubR1 expression may be mediated by the association of a repressive transcription factor, E2F4, on the promoter region of BubR1 gene.

CONCLUSION

These findings suggest that the sensitisation to therapy of (212)Pb-TCMC-trastuzumab by paclitaxel may be associated with perturbation of the mitotic spindle checkpoint, leading to increased mitotic catastrophe and cell death.

Methodology for labeling proteins and peptides with lead-212 (212Pb)

INTRODUCTION

Alpha particles possess an exquisite degree of cytotoxicity when employed for targeted α-particle therapy (TAT) or radioimmunotherapy (RIT). (212)Pb, which acts as an in vivo generator of the α-emitting nuclide (212)Bi has shown great promise in pre-clinical studies when used to label the HER2 binding antibody, trastuzumab. Currently, the first RIT clinical trial employing (212)Pb radiolabeled trastuzumab is in progress. This report provides detailed current protocol operations and steps that were generated for use in the clinical trial as well as the relevant pre-clinical experimentation, and describes in detail the labeling of proteins or peptides with (212)Pb as provided via a (224)Ra based generator system.

METHODS

(212)Pb was eluted from the (224)Ra/(212)Pb generator using hydrochloric acid (2M). The generator eluate was evaporated and digested with nitric acid (8M) followed by extraction of the (212)Pb with dilute nitric acid (0.1M). The dilute nitric acid solution of (212)Pb was used to label the immunoconjugate Trastuzumab-TCMC (2-(4-isothiocyanatobenzyl-1,4,7,10-tetraaza-1,4,7,10,tetra-(2-carbamonylmethyl)-cyclododecane) at pH5.5.

RESULTS

Elution of (212)Pb from the generator was efficient yielding>90% of available (212)Pb. Trastuzumab-TCMC was efficiently labeled with a radiochemical yield of 94% ± 4% (n=7) by ITLC and an isolated yield of 73% ± 3% (n=7).

CONCLUSIONS

The results show the feasibility of generating radioimmunoconjugates and peptide conjugates for use as in vivo α generator systems in the clinic. The technology holds promise in applications involving the treatment of minimal disease such as micrometastases and residual tumor after surgical debulking, hematological cancers, infections, and compartmental cancers, such as ovarian cancer.

Preclinical evaluation of NETA-based bifunctional ligand for radioimmunotherapy applications using 212Bi and 213Bi: radiolabeling, serum stability, and biodistribution and tumor uptake studies

INTRODUCTION

Despite the great potential of targeted α-radioimmunotherapy (RIT) as demonstrated by pre-clinical and clinical trials, limited progress has been made on the improvement of chelation chemistry for (212)Bi and (213)Bi. A new bifunctional ligand 3p-C-NETA was evaluated for targeted α RIT using (212)Bi and (213)Bi.

METHODS

Radiolabeling of 3p-C-NETA with (205/6)Bi, a surrogate of (212)Bi and (213)Bi, was evaluated at pH5.5 and room temperature. In vitro stability of the (205/6)Bi-3p-C-NETA-trastuzumab conjugate was evaluated using human serum (pH7, 37 °C). Immunoreactivity and specific activity of the (205/6)Bi-3p-C-NETA-trastuzumab conjugate were measured. An in vivo biodistribution study was performed to evaluate the in vivo stability and tumor targeting properties of the (205/6)Bi-3p-C-NETA-trastuzumab conjugate in athymic mice bearing subcutaneous LS174T tumor xenografts.

RESULT

The 3p-C-NETA-trastuzumab conjugate was extremely rapid in complexing with (205/6)Bi, and the corresponding (205/6)Bi-3p-C-NETA-trastuzumab was stable in human serum. (205/6)Bi-3p-C-NETA-trastuzumab was prepared with a high specific activity and retained immunoreactivity. (205/6)Bi-3p-C-NETA-trastuzumab conjugate displayed excellent in vivo stability and targeting as evidenced by low normal organ and high tumor uptake.

CONCLUSION

The results of the in vitro and in vivo studies indicate that 3p-C-NETA is a promising chelator for RIT applications using (212)Bi and (213)Bi. Further detailed in vivo evaluations of 3p-C-NETA for targeted α RIT are warranted.

Sensitization of tumor to ²¹²Pb radioimmunotherapy by gemcitabine involves initial abrogation of G2 arrest and blocked DNA damage repair by interference with Rad51

PURPOSE

To elucidate the mechanism of the therapeutic efficacy of targeted α-particle radiation therapy using (212)Pb-TCMC-trastuzumab together with gemcitabine for treatment of disseminated peritoneal cancers.

METHODS AND MATERIALS

Mice bearing human colon cancer LS-174T intraperitoneal xenografts were pretreated with gemcitabine, followed by (212)Pb-TCMC-trastuzumab and compared with controls.

RESULTS

Treatment with (212)Pb-TCMC-trastuzumab increased the apoptotic rate in the S-phase-arrested tumors induced by gemcitabine at earlier time points (6 to 24 hours). (212)Pb-TCMC-trastuzumab after gemcitabine pretreatment abrogated G2/M arrest at the same time points, which may be associated with the inhibition of Chk1 phosphorylation and, in turn, cell cycle perturbation, resulting in apoptosis. (212)Pb-TCMC-trastuzumab treatment after gemcitabine pretreatment caused depression of DNA synthesis, DNA double-strand breaks, accumulation of unrepaired DNA, and down-regulation of Rad51 protein, indicating that DNA damage repair was blocked. In addition, modification in the chromatin structure of p21 may be associated with transcriptionally repressed chromatin states, indicating that the open structure was delayed at earlier time points.

CONCLUSION

These findings suggest that the cell-killing efficacy of (212)Pb-TCMC-trastuzumab after gemcitabine pretreatment may be associated with abrogation of the G2/M checkpoint, inhibition of DNA damage repair, and chromatin remodeling.

Molecular pathways: targeted α-particle radiation therapy

An α-particle, a (4)He nucleus, is exquisitely cytotoxic and indifferent to many limitations associated with conventional chemo- and radiotherapy. The exquisite cytotoxicity of α-radiation, the result of its high mean energy deposition [high linear energy transfer (LET)] and limited range in tissue, provides for a highly controlled therapeutic modality that can be targeted to selected malignant cells [targeted α-therapy (TAT)] with minimal normal tissue effects. A burgeoning interest in the development of TAT is buoyed by the increasing number of ongoing clinical trials worldwide. The short path length renders α-emitters suitable for treatment and management of minimal disease such as micrometastases or residual tumor after surgical debulking, hematologic cancers, infections, and compartmental cancers such as ovarian cancer or neoplastic meningitis. Yet, despite decades of study of high LET radiation, the mechanistic pathways of the effects of this modality remain not well defined. The modality is effectively presumed to follow a simple therapeutic mechanism centered on catastrophic double-strand DNA breaks without full examination of the actual molecular pathways and targets that are activated that directly affect cell survival or death. This Molecular Pathways article provides an overview of the mechanisms and pathways that are involved in the response to and repair of TAT-induced DNA damage as currently understood. Finally, this article highlights the current state of clinical translation of TAT as well as other high-LET radionuclide radiation therapy using α-emitters such as (225)Ac, (211)At, (213)Bi, (212)Pb, and (223)Ra.