Induction of lethal bystander effects in human breast cancer cell cultures by DNA-incorporated Iodine-125 depends on phenotype

Abscopal Effects, Clastogenic Effects and Bystander Effects: 70 Years of Non-Targeted Effects of Radiation

In vitro and in vivo observations accumulated over several decades have firmly shown that the biological effects of ionizing radiation can spread from irradiated cells/tissues to non-targeted cells/tissues. Redox-modulated intercellular communication mechanisms that include a role for secreted factors and gap junctions, can mediate these non-targeted effects. Clearly, the expression of such effects and their transmission to progeny cells has implications for issues related to radiation protection. Their elucidation is also relevant towards enhancing the efficacy of cancer radiotherapy and reducing its impact on the development of normal tissue toxicities. In addition, the study of non-targeted effects is pertinent to our basic understanding of intercellular communications under conditions of oxidative stress. This review will trace the history of non-targeted effects of radiation starting with early reports of abscopal effects which described radiation induced effects in tissues distant from the site of radiation exposure. A related effect involved the production of clastogenic factors in plasma following irradiation which can induce chromosome damage in unirradiated cells. Despite these early reports suggesting non-targeted effects of radiation, the classical paradigm that a direct deposition of energy in the nucleus was required still dominated. This paradigm was challenged by papers describing radiation induced bystander effects. This review will cover mechanisms of radiation-induced bystander effects and the potential impacts on radiation protection and radiation therapy.

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.

An approach to assessing the contribution of the high LET effect in strategies for Auger endoradiotherapy

Purpose: The interest in exploiting Auger emitters in cancer therapy stems from their high linear energy transfer (LET)-type radiation damage to DNA. However, the design of Auger-emitter labeled vehicles that target the Auger cascade specifically to the DNA of tumour cells is challenging. Here we suggest a possible approach to evaluate tumour-targeting Auger-labeled conjugates by assessing the impact of a radioprotector known to be effective in protecting from low LET radiation, but not high LET radiation. Given some similarity between the energy spectrum of Auger electrons and that of secondary electrons from soft X-rays, we report the results of radioprotection experiments with 25 kVp X-rays. Materials and methods: Clonogenic survival curves for cultured human keratinocytes were established for three different irradiation conditions: ¹³⁷Cs γ-rays, 25 kVp X-rays and 320 kVp X-rays, and the effect of including a new radioprotector, denoted "2PH", was investigated.Results: The extent of radioprotection by 2PH was comparable for all radiation conditions, although RBE was higher (about 1.7) for soft X-rays. Conclusions: Radioprotectors like 2PH will help to evaluate Auger endoradiotherapy strategies, by determining the relative contributions of the high-LET effects (not protected), compared to other components, such as Auger electrons not effectively targeted to DNA.

Predicting response of micrometastases with MIRDcell V3: proof of principle with 225Ac-DOTA encapsulating liposomes that produce different activity distributions in tumor spheroids

PURPOSE

The spatial distribution of radiopharmaceuticals within multicellular clusters is known to have a significant effect on their biological response. Most therapeutic radiopharmaceuticals distribute nonuniformly in tissues which makes predicting responses of micrometastases challenging. The work presented here analyzes published temporally dependent nonuniform activity distributions within tumor spheroids treated with actinium-225-DOTA encapsulating liposomes (225Ac-liposomes) and uses these data in MIRDcell V3.11 to calculate absorbed dose distributions and predict biological response. The predicted responses are compared with experimental responses.

METHODS

Four types of liposomes were prepared having membranes with different combinations of release (R) and adhesion (A) properties. The combinations were R-A-, R-A+, R+A-, and R+A+. These afford different penetrating properties into tissue. The liposomes were loaded with either carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) or 225Ac. MDA-MB-231 spheroids were treated with the CFDA-SE-liposomes, harvested at different times, and the time-integrated CFDA-SE concentration at each radial position within the spheroid was determined. This was translated into mean 225Ac decays/cell versus radial position, uploaded to MIRDcell, and the surviving fraction of cells in spherical multicellular clusters was simulated. The MIRDcell-predicted surviving fractions were compared with experimental fractional-outgrowths of the spheroids following treatment with 225Ac-liposomes.

RESULTS

The biological responses of the multicellular clusters treated with 225Ac-liposomes with physicochemical properties R+A+, R-A+, and R-A- were predicted by MIRDcell with statistically significant accuracy. The prediction for R+A- was not predicted accurately.

CONCLUSION

In most instances, MIRDcell predicts responses of spheroids treated with 225Ac-liposomes that result in different tissue-penetrating profiles of the delivered radionuclides.

Modeling bystander effects that cause growth delay of breast cancer xenografts in bone marrow of mice treated with radium-223

RATIONALE

The role of radiation-induced bystander effects in cancer therapy with alpha-particle emitting radiopharmaceuticals remains unclear. With renewed interest in using alpha-particle emitters to sterilize disseminated tumor cells, micrometastases, and tumors, a better understanding of the direct effects of alpha particles and the contribution of the bystander responses they induce is needed to refine dosimetric models that help predict clinical benefit. Accordingly, this work models and quantifies the relative importance of direct effects (DE) and bystander effects (BE) in the growth delay of human breast cancer xenografts observed previously in the tibiae of mice treated with 223RaCl2.

METHODS

A computational model of MDA-MB-231 and MCF-7 human breast cancer xenografts in the tibial bone marrow of mice administered 223RaCl2 was created. A Monte Carlo radiation transport simulation was performed to assess individual cell absorbed doses. The responses of the breast cancer cells to direct alpha particle irradiation and gamma irradiation were needed as input data for the model and were determined experimentally using a colony-forming assay and compared to the responses of preosteoblast MC3T3-E1 and osteocyte-like MLO-Y4 bone cells. Using these data, a scheme was devised to simulate the dynamic proliferation of the tumors in vivo, including DE and BE propagated from the irradiated cells. The parameters of the scheme were estimated semi-empirically to fit experimental tumor growth.

RESULTS

A robust BE component, in addition to a much smaller DE component, was required to simulate the in vivo tumor proliferation. We also found that the relative biological effectiveness (RBE) for cell killing by alpha particle radiation was greater for the bone cells than the tumor cells.

CONCLUSION

This modeling study demonstrates that DE of radiation alone cannot explain experimental observations of 223RaCl2-induced growth delay of human breast cancer xenografts. Furthermore, while the mechanisms underlying BE remain unclear, the addition of a BE component to the model is necessary to provide an accurate prediction of the growth delay. More complex models are needed to further comprehend the extent and complexity of 223RaCl2-induced BE.

Advancements in the use of Auger electrons in science and medicine during the period 2015-2019

Auger electrons can be highly radiotoxic when they are used to irradiate specific molecular sites. This has spurred basic science investigations of their radiobiological effects and clinical investigations of their potential for therapy. Focused symposia on the biophysical aspects of Auger processes have been held quadrennially. This 9th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes at Oxford University brought together scientists from many different fields to review past findings, discuss the latest studies, and plot the future work to be done. This review article examines the research in this field that was published during the years 2015-2019 which corresponds to the period since the last meeting in Japan. In addition, this article points to future work yet to be done. There have been a plethora of advancements in our understanding of Auger processes. These advancements range from basic atomic and molecular physics to new ways to implement Auger electron emitters in radiopharmaceutical therapy. The highly localized doses of radiation that are deposited within a 10 nm of the decay site make them precision tools for discovery across the physical, chemical, biological, and medical sciences.

Dose-Dependent Growth Delay of Breast Cancer Xenografts in the Bone Marrow of Mice Treated with 223Ra: The Role of Bystander Effects and Their Potential for Therapy

The role of radiation-induced bystander effects in radiation therapy remains unclear. With renewed interest in therapy with α-particle emitters, and their potential for sterilizing disseminated tumor cells (DTCs), it is critical to determine the contribution of bystander effects to the overall response so they can be leveraged for maximum clinical benefit. Methods: Female Foxn1_nu athymic nude mice were administered 0, 50, or 600 kBq/kg ²²³RaCl₂ to create bystander conditions. At 24 hours after administration, MDA-MB-231 or MCF-7 human breast cancer cells expressing luciferase were injected into the tibial marrow compartment. Tumor burden was tracked weekly via bioluminescence. Results: The MDA-MB-231 xenografts were observed to have a 10-day growth delay in the 600 kBq/kg treatment group only. In contrast, MCF-7 cells had 7- and 65-day growth delays in the 50 and 600 kBq/kg groups, respectively. Histologic imaging of the tibial marrow compartment, α-camera imaging, and Monte Carlo dosimetry modeling revealed DTCs both within and beyond the range of the α-particles emitted from ²²³Ra in bone for both MCF-7 and MDA-MB-231 cells. Conclusion: Taken together, these results support the participation of ²²³Ra-induced antiproliferative/cytotoxic bystander effects in delayed growth of DTC xenografts. They indicate that the delay depends on the injected activity and therefore is dose-dependent. They suggest using ²²³RaCl₂ as an adjuvant treatment for select patients at early stages of breast cancer.

Targeted and Off-Target (Bystander and Abscopal) Effects of Radiation Therapy: Redox Mechanisms and Risk/Benefit Analysis

SIGNIFICANCE

Radiation therapy (from external beams to unsealed and sealed radionuclide sources) takes advantage of the detrimental effects of the clustered production of radicals and reactive oxygen species (ROS). Research has mainly focused on the interaction of radiation with water, which is the major constituent of living beings, and with nuclear DNA, which contains the genetic information. This led to the so-called target theory according to which cells have to be hit by ionizing particles to elicit an important biological response, including cell death. In cancer therapy, the Poisson law and linear quadratic mathematical models have been used to describe the probability of hits per cell as a function of the radiation dose. Recent Advances: However, in the last 20 years, many studies have shown that radiation generates "danger" signals that propagate from irradiated to nonirradiated cells, leading to bystander and other off-target effects.

CRITICAL ISSUES

Like for targeted effects, redox mechanisms play a key role also in off-target effects through transmission of ROS and reactive nitrogen species (RNS), and also of cytokines, ATP, and extracellular DNA. Particularly, nuclear factor kappa B is essential for triggering self-sustained production of ROS and RNS, thus making the bystander response similar to inflammation. In some therapeutic cases, this phenomenon is associated with recruitment of immune cells that are involved in distant irradiation effects (called "away-from-target" i.e., abscopal effects).

FUTURE DIRECTIONS

Determining the contribution of targeted and off-target effects in the clinic is still challenging. This has important consequences not only in radiotherapy but also possibly in diagnostic procedures and in radiation protection.

Use of synchrotron medical microbeam irradiation to investigate radiation-induced bystander and abscopal effects in vivo

The question of whether bystander and abscopal effects are the same is unclear. Our experimental system enables us to address this question by allowing irradiated organisms to partner with unexposed individuals. Organs from both animals and appropriate sham and scatter dose controls are tested for expression of several endpoints such as calcium flux, role of 5HT, reporter assay cell death and proteomic profile. The results show that membrane related functions of calcium and 5HT are critical for true bystander effect expression. Our original inter-animal experiments used fish species whole body irradiated with low doses of X-rays, which prevented us from addressing the abscopal effect question. Data which are much more relevant in radiotherapy are now available for rats which received high dose local irradiation to the implanted right brain glioma. The data were generated using quasi-parallel microbeams at the biomedical beamline at the European Synchrotron Radiation Facility in Grenoble France. This means we can directly compare abscopal and "true" bystander effects in a rodent tumour model. Analysis of right brain hemisphere, left brain and urinary bladder in the directly irradiated animals and their unirradiated partners strongly suggests that bystander effects (in partner animals) are not the same as abscopal effects (in the irradiated animal). Furthermore, the presence of a tumour in the right brain alters the magnitude of both abscopal and bystander effects in the tissues from the directly irradiated animal and in the unirradiated partners which did not contain tumours, meaning the type of signal was different.

The complex interactions between radiation induced non-targeted effects and cancer

Radiation induced non-targeted effects have been widely investigated in the last two decades for their potential impact on low dose radiation risk. In this paper we will give an overview of the most relevant aspects related to these effects, starting from the definition of the low dose scenarios. We will underline the role of radiation quality, both in terms of mechanisms of interaction with the biological matter and for the importance of charged particles as powerful tools for low dose effects investigation. We will focus on cell communication, representing a common feature of non-targeted effects, giving also an overview of cancer models that have explicitly considered such effects.

What is the Role of the Bystander Response in Radionuclide Therapies?

Radionuclide therapy for cancer is undergoing a renaissance, with a wide range of radionuclide and clinical delivery systems currently under investigation. Dosimetry at the cellular and sub-cellular level is complex with inhomogeneity and incomplete targeting of all cells such that some tumor cells will receive little or no direct radiation energy. There is now sufficient preclinical evidence of a Bystander response which can modulate the biology of these un-irradiated cells with current research demonstrating both protective and inhibitory responses. Dependence upon fraction of irradiated cells has also been found and the presence of functional gap junctions appears to be import for several Bystander responses. The selection of either high or low LET radionuclides may be critical. While low LET radionuclides appear to have a Bystander response proportional to dose, the dose-response from high LET radionuclides are more complex. In media transfer experiments a “U” shaped response curve has been demonstrated for high LET treatments. However this “U” shaped response has not been seen with co-culture experiments and its relevance remains uncertain. For high LET treatments there is a suggestion that dose rate effects may also be important with inhibitory effects noted with 125I labelling study and a stimulatory seen with 123I labelling in one study.

Human cell responses to ionizing radiation are differentially affected by the expressed connexins

In multicellular organisms, intercellular communication is essential for homeostatic functions and has a major role in tissue responses to stress. Here, we describe the effects of expression of different connexins, which form gap junction channels with different permeabilities, on the responses of human cells to ionizing radiation. Exposure of confluent HeLa cell cultures to (137)Cs γ rays, 3.7 MeV α particles, 1000 MeV protons or 1000 MeV/u iron ions resulted in distinct effects when the cells expressed gap junction channels composed of either connexin26 (Cx26) or connexin32 (Cx32). Irradiated HeLa cells expressing Cx26 generally showed decreased clonogenic survival and reduced metabolic activity relative to parental cells lacking gap junction communication. In contrast, irradiated HeLa cells expressing Cx32 generally showed enhanced survival and greater metabolic activity relative to the control cells. The effects on clonogenic survival correlated more strongly with effects on metabolic activity than with DNA damage as assessed by micronucleus formation. The data also showed that the ability of a connexin to affect clonogenic survival following ionizing radiation can depend on the specific type of radiation. Together, these findings show that specific types of connexin channels are targets that may be exploited to enhance radiotherapeutic efficacy and to formulate countermeasures to the harmful effects of specific types of ionizing radiation.