Revisiting the Radiobiology of Targeted Alpha Therapy

Abstract

Targeted alpha therapy (TAT) using alpha particle-emitting radionuclides is in the spotlight after the approval of ²²³RaCl₂ for patients with metastatic castration-resistant prostate cancer and the development of several alpha emitter-based radiopharmaceuticals. It is acknowledged that alpha particles are highly cytotoxic because they produce complex DNA lesions. Hence, the nucleus is considered their critical target, and many studies did not report any effect in other subcellular compartments. Moreover, their physical features, including their range in tissues (<100 μm) and their linear energy transfer (50-230 keV/μm), are well-characterized. Theoretically, TAT is indicated for very small-volume, disseminated tumors (e.g., micrometastases, circulating tumor cells). Moreover, due to their high cytotoxicity, alpha particles should be preferred to beta particles and X-rays to overcome radiation resistance. However, clinical studies showed that TAT might be efficient also in quite large tumors, and biological effects have been observed also away from irradiated cells. These distant effects are called bystander effects when occurring at short distance (<1 mm), and systemic effects when occurring at much longer distance. Systemic effects implicate the immune system. These findings showed that cells can die without receiving any radiation dose, and that a more complex and integrated view of radiobiology is required. This includes the notion that the direct, bystander and systemic responses cannot be dissociated because DNA damage is intimately linked to bystander effects and immune response. Here, we provide a brief overview of the paradigms that need to be revisited.

Keywords: bystander; cGAS-STING; lipid rafts; non-targeted effects; radiobiology; targeted alpha particle therapy; targeted alpha radiotherapy.

Figure 1

Overview of targeted and non-targeted effects of radiation. Alpha-particle irradiation of a cell population can induce targeted effects (in cells hit directly by particles) and non-targeted effects (in non-irradiated cells). Non-targeted effects can be detected at short distance (bystander effects) or at long distance (systemic effects). Genomic instability, another class of non-targeted effects, is not described here. In irradiated cells (targeted effects), alpha particles induce DSBs and non-DSB clustered DNA lesions (MDS) that are detected by ATM and activate the DNA damage response (11, 12). Alpha-particle irradiation of the cell membrane generates lipid peroxidation products (4-HNE, 4-hydroxy-2-nonenal; MDA, malondialdehyde) from polyunsaturated fatty acids (PUFA) (43). Alpha-particle irradiation can also activate acid sphingomyelinase (ASMase) and this leads to the rapid formation of ceramide through hydrolysis of sphingomyelin, a cell membrane phospholipid (38, 42). Ceramide-enriched large domains (lipid rafts) are formed by aggregation of ceramide, leading to activation of the mitogen-activated protein kinase (MAPK) pathway and its downstream effector, nuclear factor kappa B (NF-κB) (41). NF-κB induces the transcription of target genes, such as those encoding cytokines, COX-2, and inducible nitric oxide synthase (iNOS), followed by production of ROS and nitric oxide (NO) that contribute to oxidative stress (44). Irradiation can also increase the intracellular Ca²⁺ level (45) through release from the endoplasmic reticulum via calcium release mechanisms (46). Ca²⁺ can in turn activate protein kinase C, the MAPK pathway and transcription factors (NF-κB, AP1) that promote various downstream pathways (iNOS, COX-2). Mitochondria also are affected by alpha-particle irradiation (–52). Ca²⁺ can be taken up by mitochondria, leading to ROS and RNS increase, mitochondrial DNA damage, altered ATP synthesis, mitochondrial depolarization, and release of cytochrome C and caspase 3. Mitochondrial fission also has been observed. Targeted cells can communicate with bystander cells through gap junctions or through the release of soluble factors. Extracellular vesicles, including exosomes, containing nucleic acids, lipids and proteins, might be released, and contribute to bystander immunity. Systemic effects may involve the immune system through the release of DAMPs that are recognized by antigen-presenting cells (e.g., dendritic cells, DC, that present antigenic peptides to CD4⁺ and CD8⁺ T lymphocytes for immune response activation). Altogether these effects contribute to tumor cell death.

Figure 2

Example of bystander effects at tissue scale and abscopal effects in patients exposed to TAT. Examples of bystander effects: (1) Strong heterogeneous distribution of radioactivity (²¹²Pb-labeled monoclonal antibodies) in tumors of mice treated with TAT was unexpectedly accompanied by homogenous distribution of DNA damage, measured by immunofluorescence analysis of 53BP1 expression (modified from Ladjohounlou et al.). (2) Model of ²²³RaCl² (Xofigo)-induced bystander effects in a bone metastasis. (3) Systemic effect induced in a patient with cutaneous squamous cell carcinoma (cSCC) who received alpha brachytherapy to one tumor lesion. Surprisingly, another tumor lesion far from the radiation area also was cured (66) (modified from Bellia et al.).