Proton Irradiation Increases the Necessity for Homologous Recombination Repair Along with the Indispensability of Non-Homologous End Joining

The fractionation dependence of tumor control in proton therapy for early-stage non-small cell lung cancer

Purpose

The relative biological effectiveness (RBE) of tumor control for proton beam therapy (PBT) compared to photon radiotherapy (RT) is typically assumed to be independent of fractionation. To test this, we modeled published PBT outcome results for early-stage non-small cell lung cancer (NSCLC) treatments across a range of fractionation schedules.

Materials and Methods

All published and analyzable cohorts were included (399 patients, 413 treated lesions). Two models were used to fit the data: a previously published tumor simulation model that fits photon RT results of NSCLC across all fractionation regimes and the Fowler LQ model with a kick-off time term. The treatment effect of each cohort was referenced to the photon equivalent dose through mechanistic model simulations in a 2 Gy/weekday scenario, with radiobiological parameters determined to simultaneously best-fit all fractionation results. The tumor control RBE of each published treatment schedule, compared to the modeled photon RT effect of the same schedule, was then estimated.

Results

For cohorts whose treatments lasted less than three weeks (i.e., 12 fractions or less), the RBE of PBT was in the range of 1.08 to 1.11. However, for fractionated treatments stretching over four weeks or more (20-25 fractions), the relative effectiveness was much lower, with RBEs in the range of 0.82-0.89. This conclusion was unchanged using the simpler Fowler LQ + time model.

Conclusions

The proton RBE for hypo-fractionated schedules was 20-30% higher than for conventional schedules. The derived radiobiological parameters of PBT differ significantly from those of photon RT, indicating that PBT is influenced differentially by radiobiological mechanisms which require further investigation.

Evaluating and reporting LET and RBE-weighted dose in proton therapy for glioma - The Dutch approach

BACKGROUND AND PURPOSE

With proton therapy, the relative biological effectiveness (RBE) accounts for increased DNA damage caused by higher linear energy transfer (LET) compared to photons. However, the LET and hence the RBE varies along the proton range, particularly at the Bragg peak, introducing challenges in proton treatment planning for brain tumors. The aim of this paper is to standardize evaluating and reporting LET and RBE in proton therapy for patients with grade 2 and 3 IDH mutant gliomas among the Dutch proton therapy centers.

MATERIALS AND METHODS

A working group, comprising experts from three Dutch proton therapy centers, conducted nine meetings between 2020 and 2023. A joint literature review supported the standardized evaluation and reporting of LET and RBE. Questionnaires sent out to the three Dutch proton centers in 2020 and 2023 provided input for discussions on clinical practices. Three clinical examples were chosen to illustrate the application of the recommended methodology in treatment planning.

RESULTS

Following the literature review, a guideline on evaluation and reporting using the dose averaged LET (LETd) of primary and secondary protons calculated in water normalized to unit density was established. The McNamara variable RBE model with an α/β value of 2 Gy was selected for reporting.

CONCLUSION

The study presents a harmonization of approaches to evaluating and reporting LET and variable RBE in a guideline for the three Dutch proton therapy centers, providing clarity for future clinical interpretation. Having chosen a single variable RBE model offers practicality, although its accuracy remains a topic of ongoing research.

Multi-Target Inhibitor CUDC-101 Impairs DNA Damage Repair and Enhances Radiation Response in Triple-Negative Breast Cell Line

BACKGROUND

Since the discovery that Histone deacetylase inhibitors (HDCAi) could enhance radiation response, a number of HDACi, mainly pan-HDAC inhibitors, have been studied either as monotherapy or in combination with X-ray irradiation or chemotherapeutic drugs in the management of breast cancer. However, studies on the combination of HDACi and proton radiation remain limited. CUDC-101 is a multitarget inhibitor of Histone deacetylases (HDACs), epidermal growth factor receptor (EGFR), and human epidermal growth factor receptor 2 (HER-2). In this paper, the effectiveness of CUDC-101 in enhancing radiation response to both proton and X-ray irradiation was studied.

METHODS

MCF-7, MDA-MB-231, and MCF-10A cell lines were pre-treated with CUDC-101 and exposed to 148 MeV protons, and X-rays were used as reference radiation. Colony survival, γ-H2AX foci, apoptosis, and cell cycle analysis assays were performed.

RESULTS

γ-H2AX foci assays showed increased sensitivity to CUDC-101 in the MDA-MB-231 cell line compared to the MCF-7 cell line. In both cell lines, induction of apoptosis was enhanced in CUDC-101 pre-treated cells compared to radiation (protons or X-rays) alone. Increased apoptosis was also noted in CUDC-101 pre-treated cells in the MCF-10A cell line. Cell cycle analysis showed increased G2/M arrest by CUDC-101 mono-treatment as well as combination of CUDC-101 and X-ray irradiation in the MDA-MB-231 cell line.

CONCLUSIONS

CUDC-101 effectively enhances response to both proton and X-ray irradiation, in the triple-negative MDA-MB-231 cell line. This enhancement was most notable when CUDC-101 was combined with proton irradiation. This study highlights that CUDC-101 holds potential in the management of triple-negative breast cancer as monotherapy or in combination with protons or X-ray irradiation.

ATR inhibition radiosensitizes cells through augmented DNA damage and G2 cell cycle arrest abrogation

Ataxia telangiectasia and Rad3-related protein (ATR) is a key DNA damage response protein that facilitates DNA damage repair and regulates cell cycle progression. As such, ATR is an important component of the cellular response to radiation, particularly in cancer cells, which show altered DNA damage response and aberrant cell cycle checkpoints. Therefore, ATR's pharmacological inhibition could be an effective radiosensitization strategy to improve radiotherapy. We assessed the ability of an ATR inhibitor, AZD6738, to sensitize cancer cell lines of various histologic types to photon and proton radiotherapy. We found that radiosensitization took place through persistent DNA damage and abrogated G2 cell cycle arrest. We also found that AZD6738 increased the number of micronuclei after exposure to radiotherapy. We found that combining radiation with AZD6738 led to tumor growth delay and prolonged survival relative to radiation alone in a breast cancer model. Combining AZD6738 with photons or protons also led to increased macrophage infiltration at the tumor microenvironment. These results provide a rationale for further investigation of ATR inhibition in combination with radiotherapy and with other agents such as immune checkpoint blockade.

Proton- compared to X-irradiation leads to more acinar atrophy and greater hyposalivation accompanied by a differential cytokine response

Proton therapy gives less dose to healthy tissue compared to conventional X-ray therapy, but systematic comparisons of normal tissue responses are lacking. The aim of this study was to investigate late tissue responses in the salivary glands following proton- or X-irradiation of the head and neck in mice. Moreover, we aimed at investigating molecular responses by monitoring the cytokine levels in serum and saliva. Female C57BL/6J mice underwent local fractionated irradiation with protons or X-rays to the maximally tolerated acute level. Saliva and serum were collected before and at different time points after irradiation to assess salivary gland function and cytokine expression. To study late responses in the major salivary glands, histological analyses were performed on tissues collected at day 105 after onset of irradiation. Saliva volume after proton and X-irradiation was significantly lower than for controls and remained reduced at all time points after irradiation. Protons caused reduced saliva production and fewer acinar cells in the submandibular glands compared to X-rays at day 105. X-rays induced a stronger inflammatory cytokine response in saliva compared to protons. This work supports previous preclinical findings and indicate that the relative biological effectiveness of protons in normal tissue might be higher than the commonly used value of 1.1.

Potential Benefits of Combining Proton or Carbon Ion Therapy with DNA Damage Repair Inhibitors

The use of charged particle radiotherapy is currently increasing, but combination therapy with DNA repair inhibitors remains to be exploited in the clinic. The high-linear energy transfer (LET) radiation delivered by charged particles causes clustered DNA damage, which is particularly effective in destroying cancer cells. Whether the DNA damage response to this type of damage is different from that elicited in response to low-LET radiation, and if and how it can be targeted to increase treatment efficacy, is not fully understood. Although several preclinical studies have reported radiosensitizing effects when proton or carbon ion irradiation is combined with inhibitors of, e.g., PARP, ATR, ATM, or DNA-PKcs, further exploration is required to determine the most effective treatments. Here, we examine what is known about repair pathway choice in response to high- versus low-LET irradiation, and we discuss the effects of inhibitors of these pathways when combined with protons and carbon ions. Additionally, we explore the potential effects of DNA repair inhibitors on antitumor immune signaling upon proton and carbon ion irradiation. Due to the reduced effect on healthy tissue and better immune preservation, particle therapy may be particularly well suited for combination with DNA repair inhibitors.

Radiobiology of Proton Therapy in Human Papillomavirus-Negative and Human Papillomavirus-Positive Head and Neck Cancer Cells

Photon-based radiotherapy (XRT) is one of the most frequently used treatment modalities for HPV-negative and HPV-positive locally advanced head and neck squamous cell carcinoma (HNSCC). However, locoregional recurrences and normal RT-associated toxicity remain major problems for these patients. Proton therapy (PT), with its dosimetric advantages, can present a solution to the normal toxicity problem. However, issues concerning physical delivery and the lack of insights into the underlying biology of PT hamper the full exploitation of PT. Here, we assessed the radiobiological processes involved in PT in HPV-negative and HPV-positive HNSCC cells. We show that PT and XRT activate the DNA damage-repair and stress response in both HPV-negative and HPV-positive cells to a similar extent. The activation of these major radiobiological mechanisms resulted in equal levels of clonogenic survival and mitotic cell death. Altogether, PT resulted in similar biological effectiveness when compared to XRT. These results emphasize the importance of dosimetric parameters when exploiting the potential of increased clinical effectiveness and reduced normal tissue toxicity in PT treatment.

Targeting OGG1 and PARG radiosensitises head and neck cancer cells to high-LET protons through complex DNA damage persistence

Complex DNA damage (CDD), containing two or more DNA lesions within one or two DNA helical turns, is a signature of ionising radiation (IR) and contributes significantly to the therapeutic effect through cell killing. The levels and complexity of CDD increases with linear energy transfer (LET), however, the specific cellular response to this type of DNA damage and the critical proteins essential for repair of CDD is currently unclear. We performed an siRNA screen of ~240 DNA damage response proteins to identify those specifically involved in controlling cell survival in response to high-LET protons at the Bragg peak, compared to low-LET entrance dose protons which differ in the amount of CDD produced. From this, we subsequently validated that depletion of 8-oxoguanine DNA glycosylase (OGG1) and poly(ADP-ribose) glycohydrolase (PARG) in HeLa and head and neck cancer cells leads to significantly increased cellular radiosensitivity specifically following high-LET protons, whilst no effect was observed after low-LET protons and X-rays. We subsequently confirmed that OGG1 and PARG are both required for efficient CDD repair post-irradiation with high-LET protons. Importantly, these results were also recapitulated using specific inhibitors for OGG1 (TH5487) and PARG (PDD00017273). Our results suggest OGG1 and PARG play a fundamental role in the cellular response to CDD and indicate that targeting these enzymes could represent a promising therapeutic strategy for the treatment of head and neck cancers following high-LET radiation.

Super-resolution GSDIM microscopy unveils distinct nanoscale characteristics of DNA repair foci under diverse genotoxic stress

DNA double-strand breaks initiate the DNA damage response (DDR), leading to the accumulation of repair proteins at break sites and the formation of the-so-called foci. Various microscopy methods, such as wide-field, confocal, electron, and super-resolution microscopy, have been used to study these structures. However, the impact of different DNA-damaging agents on their (nano)structure remains unclear. Utilising GSDIM super-resolution microscopy, here we investigated the distribution of fluorescently tagged DDR proteins (53BP1, RNF168, MDC1) and γH2AX in U2OS cells treated with γ-irradiation, etoposide, cisplatin, or hydroxyurea. Our results revealed that both foci structure and their nanoscale ultrastructure, including foci size, nanocluster characteristics, fluorophore density and localisation, can be significantly altered by different inducing agents, even ones with similar mechanisms. Furthermore, distinct behaviours of DDR proteins were observed under the same treatment. These findings have implications for cancer treatment strategies involving these agents and provide insights into the nanoscale organisation of the DDR.

Most DNA repair defects do not modify the relationship between relative biological effectiveness and linear energy transfer in CRISPR-edited cells

BACKGROUND

Cancer is a highly heterogeneous disease, driven by frequent genetic alterations which have significant effects on radiosensitivity. However, radiotherapy for a given cancer type is typically given with a standard dose determined from population-level trials. As a result, a proportion of patients are under- or over-dosed, reducing the clinical benefit of radiotherapy. Biological optimization would not only allow individual dose prescription but also a more efficient allocation of limited resources, such as proton and carbon ion therapy. Proton and ion radiotherapy offer an advantage over photons due to their elevated Relative Biological Effectiveness (RBE) resulting from their elevated Linear Energy Transfer (LET). Despite significant interest in optimizing LET by tailoring radiotherapy plans, RBE's genetic dependence remains unclear.

PURPOSE

The aim of this study is to better define the RBE/LET relationship in a panel of cell lines with different defects in DSB repair pathways, but otherwise identical biological features and genetic background to isolate these effects.

METHODS

Normal human cells (RPE1), genetically modified to introduce defects in DNA double-strand break (DSB) repair genes, ATM, BRCA1, DCLRE1C, LIG4, PRKDC and TP53, were used to map the RBE-LET relationship. Cell survival was measured with clonogenic assays after exposure to photons, protons (LET 1 and 12 keV/µm) and alpha particles (129 keV/µm). Gene knockout sensitizer enhancement ratio (SER) values were calculated as the ratio of the mean inactivation dose (MID) of wild-type cells to repair-deficient cells, and RBE values were calculated as the ratio of the MID of X-ray and particle irradiated cells. 53BP1 foci were used to quantify radiation-induced DSBs and their repair following irradiation.

RESULTS

Deletion of NHEJ genes had the greatest impact on photon sensitivity (ATM-/- SER = 2.0 and Lig4-/- SER = 1.8), with genes associated with HR having smaller effects (BRCA1-/- SER = 1.2). Wild-type cells showed RBEs of 1.1, 1.3, 5.0 for low- and high-LET protons and alpha particles respectively. SERs for different genes were independent of LET, apart from NHEJ knockouts which proved to be markedly hypersensitive across all tested LETs. Due to this hypersensitivity, the impact of high LET was reduced in cell models lacking the NHEJ repair pathway. HR-defective cells had moderately increased sensitivity across all tested LETs, but, notably, the contribution of HR pathway to survival appeared independent of LET. Analysis of 53BP1 foci shows that NHEJ-defective cells had the least DSB repair capacity after low LET exposure, and no visible repair after high LET exposure. HR-defective cells also had slower repair kinetics, but the impact of HR defects is not as severe as NHEJ defects.

CONCLUSIONS

DSB repair defects, particularly in NHEJ, conferred significant radiosensitivity across all LETs. This sensitization appeared independent of LET, suggesting that the contribution of different DNA repair pathways to survival does not depend on radiation quality.

Targeted nuclear irradiation with a proton microbeam induces oxidative DNA base damage and triggers the recruitment of DNA glycosylases OGG1 and NTH1

DNA is the major target of radiation therapy of malignant tumors. Ionizing radiation (IR) induces a variety of DNA lesions, including chemically modified bases and strand breaks. The use of proton beam therapy for cancer treatment is ramping up, as it is expected to reduce normal tissue damage. Thus, it is important to understand the molecular mechanisms of recognition, signaling, and repair of DNA damage induced by protons in the perspective of assessing not only the risk associated with human exposure to IR but also the possibility to improve the efficacy of therapy. Here, we used targeted irradiation of nuclear regions of living cells with controlled number of protons at a high spatio-temporal resolution to detect the induced base lesions and characterize the recruitment kinetics of the specific DNA glycosylases to DNA damage sites. We show that localized irradiation with 4 MeV protons induces, in addition to DNA double strand breaks (DSBs), the oxidized bases 7,8-dihydro-8-oxoguanine (8-oxoG) and thymine glycol (TG) at the site of irradiation. Consistently, the DNA glycosylases OGG1 and NTH1, capable of excising 8-oxoG and TG, respectively, and initiating the base excision repair (BER) pathway, are recruited to the site of damage. To our knowledge, this is the first direct evidence indicating that proton microbeams induce oxidative base damage, and thus implicating BER in the repair of DNA lesions induced by protons.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Proton Compared to X-Irradiation Induces Different Protein Profiles in Oral Cancer Cells and Their Derived Extracellular Vesicles

Extracellular vesicles (EVs) are membrane-bound particles released from cells, and their cargo can alter the function of recipient cells. EVs from X-irradiated cells have been shown to play a likely role in non-targeted effects. However, EVs derived from proton irradiated cells have not yet been studied. We aimed to investigate the proteome of EVs and their cell of origin after proton or X-irradiation. The EVs were derived from a human oral squamous cell carcinoma (OSCC) cell line exposed to 0, 4, or 8 Gy from either protons or X-rays. The EVs and irradiated OSCC cells underwent liquid chromatography-mass spectrometry for protein identification. Interestingly, we found different protein profiles both in the EVs and in the OSCC cells after proton irradiation compared to X-irradiation. In the EVs, we found that protons cause a downregulation of proteins involved in cell growth and DNA damage response compared to X-rays. In the OSCC cells, proton and X-irradiation induced dissimilar cell death pathways and distinct DNA damage repair systems. These results are of potential importance for understanding how non-targeted effects in normal tissue can be limited and for future implementation of proton therapy in the clinic.

DNA damage and repair dependencies of ionising radiation modalities

Radiotherapy is utilised in the treatment of ∼50% of all human cancers, which predominantly employs photon radiation. However, particle radiotherapy elicits significant benefits over conventional photons due to more precise dose deposition and increased linear energy transfer (LET) that generates an enhanced therapeutic response. Specifically, proton beam therapy (PBT) and carbon ion radiotherapy (CIRT) are characterised by a Bragg peak, which generates a low entrance radiation dose, with the majority of the energy deposition being defined within a small region which can be specifically targeted to the tumour, followed by a low exit dose. PBT is deemed relatively low-LET whereas CIRT is more densely ionising and therefore high LET. Despite the radiotherapy type, tumour cell killing relies heavily on the introduction of DNA damage that overwhelms the repair capacity of the tumour cells. It is known that DNA damage complexity increases with LET that leads to enhanced biological effectiveness, although the specific DNA repair pathways that are activated following the different radiation sources is unclear. This knowledge is required to determine whether specific proteins and enzymes within these pathways can be targeted to further increase the efficacy of the radiation. In this review, we provide an overview of the different radiation modalities and the DNA repair pathways that are responsive to these. We also provide up-to-date knowledge of studies examining the impact of LET and DNA damage complexity on DNA repair pathway choice, followed by evidence on how enzymes within these pathways could potentially be therapeutically exploited to further increase tumour radiosensitivity, and therefore radiotherapy efficacy.

Impact of proton therapy on the DNA damage induction and repair in hematopoietic stem and progenitor cells

Proton therapy is of great interest to pediatric cancer patients because of its optimal depth dose distribution. In view of healthy tissue damage and the increased risk of secondary cancers, we investigated DNA damage induction and repair of radiosensitive hematopoietic stem and progenitor cells (HSPCs) exposed to therapeutic proton and photon irradiation due to their role in radiation-induced leukemia. Human CD34+ HSPCs were exposed to 6 MV X-rays, mid- and distal spread-out Bragg peak (SOBP) protons at doses ranging from 0.5 to 2 Gy. Persistent chromosomal damage was assessed with the micronucleus assay, while DNA damage induction and repair were analyzed with the γ-H2AX foci assay. No differences were found in induction and disappearance of γ-H2AX foci between 6 MV X-rays, mid- and distal SOBP protons at 1 Gy. A significantly higher number of micronuclei was found for distal SOBP protons compared to 6 MV X-rays and mid- SOBP protons at 0.5 and 1 Gy, while no significant differences in micronuclei were found at 2 Gy. In HSPCs, mid-SOBP protons are as damaging as conventional X-rays. Distal SOBP protons showed a higher number of micronuclei in HSPCs depending on the radiation dose, indicating possible changes of the in vivo biological response.

Antioxidative stress protein SRXN1 can be used as a radiotherapy prognostic marker for prostate cancer

PURPOSE

To explore the mechanisms of radiotherapy resistance and search for prognostic biomarkers for prostate cancer.

METHODS

The GSE192817 and TCGA PRAD datasets were selected and downloaded from the GEO and UCSC Xena databases. Differential expression and functional annotation analyses were applied to 52 tumour cell samples from GSE192817. Then, the ssGSEA or GSVA algorithms were applied to quantitatively score the biological functional activity of samples in the GSE192817 and TCGA PRAD datasets, combined with specific gene sets collected from the Molecular Signatures Database (MSigDB). Subsequently, the Wilcoxon rank-sum test was used to compare the differences in ssGSEA or GSVA scores among cell types or PRAD patients. Moreover, radiotherapy resistance-associated gene screening was performed on DU145 and PC3 cells (prostate cancer cells), and survival analysis was used to evaluate the efficacy of these genes for predicting the prognosis of PRAD patients.

RESULTS

A total of 114 genes that were differentially expressed in more than two different cancer cell types and associated with either sham surgery or radiotherapy treatment (X-ray or photon irradiation) were detected in cancer cells from GSE192817. Comparison of DNA damage-related ssGSEA scores between sham surgery and radiotherapy treatment in prostate cancer cells (DU145 and PC3) showed that photon irradiation was potentially more effective than X-ray treatment. In the TCGA PRAD dataset, patients treated with radiotherapy had much higher "GOBP_CELLULAR_RESPONSE_TO_DNA_DAMAGE_STIMULUS", "GOBP_G2_DNA_DAMAGE_CHECKPOINT" and "GOBP_INTRA_S_DNA_DAMAGE_CHECKPOINT" GSVA scores, and the Wilcoxon rank-sum test p values were 0.0005, 0.0062 and 0.0800, respectively. Furthermore, SRXN1 was upregulated in DU145 cells (resistant to X-ray irradiation compared to PC3 cells) after radiotherapy treatment, and low SRXN1 expression in patients was beneficial to radiotherapy outcomes. The log-rank test p value for PFS was 0.0072.

CONCLUSIONS

Radiotherapy can damage DNA and induce oxidative stress to kill tumour cells. In this study, we found that SRXN1, as an antioxidative stress gene, plays an important role in radiotherapy for prostate cancer treatment, and this gene is also a potential biomarker for predicting the prognosis of patients treated with radiotherapy.

Does particle radiation have superior radiobiological advantages for prostate cancer cells? A systematic review of in vitro studies

BACKGROUND

Charged particle beams from protons to carbon ions provide many significant physical benefits in radiation therapy. However, preclinical studies of charged particle therapy for prostate cancer are extremely limited. The aim of this study was to comprehensively investigate the biological effects of charged particles on prostate cancer from the perspective of in vitro studies.

METHODS

We conducted a systematic review by searching EMBASE (OVID), Medline (OVID), and Web of Science databases to identify the publications assessing the radiobiological effects of charged particle irradiation on prostate cancer cells. The data of relative biological effectiveness (RBE), surviving fraction (SF), standard enhancement ratio (SER) and oxygen enhancement ratio (OER) were extracted.

RESULTS

We found 12 studies met the eligible criteria. The relative biological effectiveness values of proton and carbon ion irradiation ranged from 0.94 to 1.52, and 1.67 to 3.7, respectively. Surviving fraction of 2 Gy were 0.17 ± 0.12, 0.55 ± 0.20 and 0.53 ± 0.16 in carbon ion, proton, and photon irradiation, respectively. PNKP inhibitor and gold nanoparticles were favorable sensitizing agents, while it was presented poorer performance in GANT61. The oxygen enhancement ratio values of photon and carbon ion irradiation were 2.32 ± 0.04, and 1.77 ± 0.13, respectively. Charged particle irradiation induced more G0-/G1- or G2-/M-phase arrest, more expression of γ-H2AX, more apoptosis, and lower motility and/or migration ability than photon irradiation.

CONCLUSIONS

Both carbon ion and proton irradiation have advantages over photon irradiation in radiobiological effects on prostate cancer cell lines. Carbon ion irradiation seems to have further advantages over proton irradiation.

Targeted Inhibition of DNA-PKcs, ATM, ATR, PARP, and Rad51 Modulate Response to X Rays and Protons

Small molecule inhibitors are currently in preclinical and clinical development for the treatment of selected cancers, particularly those with existing genetic alterations in DNA repair and DNA damage response (DDR) pathways. Keen interest has also been expressed in combining such agents with other targeted antitumor strategies such as radiotherapy. Radiotherapy exerts its cytotoxic effects primarily through DNA damage-induced cell death; therefore, inhibiting DNA repair and the DDR should lead to additive and/or synergistic radiosensitizing effects. In this study we screened the response to X-ray or proton radiation in cell lines treated with DDR inhibitors (DDRis) targeting ATM, ATR, DNA-PKcs, Rad51, and PARP, with survival metrics established using clonogenic assays. We observed that DDRis generate significant radiosensitization in cancer and primary cells derived from normal tissue. Existing genetic defects in cancer cells appear to be an important consideration when determining the optimal inhibitor to use for synergistic combination with radiation. We also show that while greater radiosensitization can be achieved with protons (9.9 keV/µm) combined with DDRis, the relative biological effectiveness is unchanged or in some cases reduced. Our results indicate that while targeting the DDR can significantly radiosensitize cancer cells to such combinations, normal cells may also be equally or more severely affected, depending on the DDRi used. These data highlight the importance of identifying genetic defects as predictive biomarkers of response for combination treatment.

CD9- and CD81-positive extracellular vesicles provide a marker to monitor glioblastoma cell response to photon-based and proton-based radiotherapy

Glioblastoma multiforme (GBM) is the most aggressive tumor of the central nervous system with a poor prognosis. In the treatment of GBM tumors, radiotherapy plays a major role. Typically, GBM tumors cannot be cured by irradiation because of intrinsic resistance machanisms. An escalation of the irradiation dose in the GBM tumor is difficult due to the high risk of severe side effects in the brain. In the last decade, the development of new irradiation techniques, including proton-based irradiation, promised new chances in the treatment of brain tumors. In contrast to conventional radiotherapy, irradiation with protons allows a dosimetrically more confined dose deposition in the tumor while better sparing the normal tissue surrounding the tumor. A systematic comparison of both irradiation techniques on glioblastoma cells has not been performed so far. Despite the improvements in radiotherapy, it remains challenging to predict the therapeutical response of GBM tumors. Recent publications suggest extracellular vesicles (EVs) as promising markers predicting tumor response. Being part of an ancient intercellular communication system, virtually all cells release specifically composed EVs. The assembly of EVs varies between cell types and depends on environmental parameters. Here, we compared the impact of photon-based with proton-based radiotherapy on cell viability and phenotype of four different glioblastoma cell lines. Furthermore, we characterized EVs released by different glioblastoma cells and correlated released EVs with the cellular response to radiotherapy. Our results demonstrated that glioblastoma cells reacted more sensitive to irradiation with protons than photons, while radiation-induced cell death 72 h after single dose irradiation was independent of the irradiation modality. Moreover, we detected CD9 and CD81-positive EVs in the supernatant of all glioblastoma cells, although at different concentrations. The amount of released CD9 and CD81-positive EVs increased after irradiation when cells became apoptotic. Although secreted EVs of non-irradiated cells were not predictive for radiosensitivity, their increased EV release after irradiation correlated with the cytotoxic response to radiotherapy 72 h after irradiation. Thus, our data suggest a novel application of EVs in the surveillance of anti-cancer therapies.

Biological Mechanisms to Reduce Radioresistance and Increase the Efficacy of Radiotherapy: State of the Art

Cancer treatment with ionizing radiation (IR) is a well-established and effective clinical method to fight different types of tumors and is a palliative treatment to cure metastatic stages. Approximately half of all cancer patients undergo radiotherapy (RT) according to clinical protocols that employ two types of ionizing radiation: sparsely IR (i.e., X-rays) and densely IR (i.e., protons). Most cancer cells irradiated with therapeutic doses exhibit radio-induced cytotoxicity in terms of cell proliferation arrest and cell death by apoptosis. Nevertheless, despite the more tailored advances in RT protocols in the last few years, several tumors show a relatively high percentage of RT failure and tumor relapse due to their radioresistance. To counteract this extremely complex phenomenon and improve clinical protocols, several factors associated with radioresistance, of both a molecular and cellular nature, must be considered. Tumor genetics/epigenetics, tumor microenvironment, tumor metabolism, and the presence of non-malignant cells (i.e., fibroblast-associated cancer cells, macrophage-associated cancer cells, tumor-infiltrating lymphocytes, endothelial cells, cancer stem cells) are the main factors important in determining the tumor response to IR. Here, we attempt to provide an overview of how such factors can be taken advantage of in clinical strategies targeting radioresistant tumors.

A computational approach to quantifying miscounting of radiation-induced double-strand break immunofluorescent foci

Immunofluorescent tagging of DNA double-strand break (DSB) markers, such as γ-H2AX and other DSB repair proteins, are powerful tools in understanding biological consequences following irradiation. However, whilst the technique is widespread, there are many uncertainties related to its ability to resolve and reliably deduce the number of foci when counting using microscopy. We present a new tool for simulating radiation-induced foci in order to evaluate microscope performance within in silico immunofluorescent images. Simulations of the DSB distributions were generated using Monte Carlo track-structure simulation. For each DSB distribution, a corresponding DNA repair process was modelled and the un-repaired DSBs were recorded at several time points. Corresponding microscopy images for both a DSB and (γ-H2AX) fluorescent marker were generated and compared for different microscopes, radiation types and doses. Statistically significant differences in miscounting were found across most of the tested scenarios. These inconsistencies were propagated through to repair kinetics where there was a perceived change between radiation-types. These changes did not reflect the underlying repair rate and were caused by inconsistencies in foci counting. We conclude that these underlying uncertainties must be considered when analysing images of DNA damage markers to ensure differences observed are real and are not caused by non-systematic miscounting.

PyFoci is a tool that simulates distributions of fluorescently labeled DNA double-strand break marker protein foci and allows the estimation of miscounting under different radiation types, doses and microscopy settings.

An Integrated Approach Reveals DNA Damage and Proteotoxic Stress as Main Effects of Proton Radiation in S. cerevisiae

Proton radiotherapy (PRT) has the potential to reduce the normal tissue toxicity associated with conventional photon-based radiotherapy (X-ray therapy, XRT) because the active dose can be more directly targeted to a tumor. Although this dosimetric advantage of PRT is well known, the molecular mechanisms affected by PRT remain largely elusive. Here, we combined the molecular toolbox of the eukaryotic model Saccharomyces cerevisiae with a systems biology approach to investigate the physiological effects of PRT compared to XRT. Our data show that the DNA damage response and protein stress response are the major molecular mechanisms activated after both PRT and XRT. However, RNA-Seq revealed that PRT treatment evoked a stronger activation of genes involved in the response to proteotoxic stress, highlighting the molecular differences between PRT and XRT. Moreover, inhibition of the proteasome resulted in decreased survival in combination with PRT compared to XRT, not only further confirming that protons induced a stronger proteotoxic stress response, but also hinting at the potential of using proteasome inhibitors in combination with proton radiotherapy in clinical settings.

Ganetespib selectively sensitizes cancer cells for proximal and distal spread-out Bragg peak proton irradiation

Objective

Hypersensitivity towards proton versus photon irradiation was demonstrated in homologous recombination repair (HRR)-deficient cell lines. Hence, combined treatment concepts targeting HRR provide a rational for potential pharmaceutical exploitation. The HSP90 inhibitor ganetespib (STA-9090) downregulates a multitude of HRR-associated proteins and sensitizes for certain chemotherapeutics. Thus, the radiosensitizing effect of HSP90-inhibiting ganetespib was investigated for reference photon irradiation and proton irradiation at a proximal and distal position in a spread-out Bragg peak (SOBP).

Methods

A549 and FaDu cells were treated with low-dose (2 nM resp. 1 nM) ganetespib and irradiated with 200 kV photons. Proton irradiation was performed at a proximal and a distal position within a SOBP, with corresponding dose-averaged linear-energy transfer (LET_D) values of 2.1 and 4.5 keV/µm, respectively. Cellular survival data was fitted to the linear-quadratic model to calculate relative biological effectiveness (RBE) and the dose-modifying factor (DMF). Additionally, A549 cells were treated with increasing doses of ganetespib and investigated by flow cytometry, immunoblotting, and immunofluorescence microscopy to investigate cell cycle distribution, Rad51 protein levels, and γH2AX foci, respectively.

Results

Low-dosed ganetespib significantly sensitized both cancer cell lines exclusively for proton irradiation at both investigated LET_D, resulting in increased RBE values of 10–40%. In comparison to photon irradiation, the fraction of cells in S/G2/M phase was elevated in response to proton irradiation with 10 nM ganetespib consistently reducing this population. No changes in cell cycle distribution were detected in unirradiated cells by ganetespib alone. Protein levels of Rad51 are downregulated in irradiated A549 cells by 10 nM and also 2 nM ganetespib within 24 h. Immunofluorescence staining demonstrated similar induction and removal of γH2AX foci, irrespective of irradiation type or ganetespib administration.

Conclusion

Our findings illustrate a proton-specific sensitizing effect of low-dosed ganetespib in both employed cell lines and at both investigated SOBP positions. We provide additional experimental data on cellular response and a rational for future combinatorial approaches with proton radiotherapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13014-022-02036-z.

Charged Particle Irradiation for Pancreatic Cancer: A Systematic Review of In Vitro Studies

Purpose

Given the higher precision accompanied by optimized sparing of normal tissue, charged particle therapy was thought of as a promising treatment for pancreatic cancer. However, systematic preclinical studies were scarce. We aimed to investigate the radiobiological effects of charged particle irradiation on pancreatic cancer cell lines.

Methods

A systematic literature search was performed in EMBASE (OVID), Medline (OVID), and Web of Science databases. Included studies were in vitro English publications that reported the radiobiological effects of charged particle irradiation on pancreatic cancer cells.

Results

Thirteen carbon ion irradiation and seven proton irradiation in vitro studies were included finally. Relative biological effectiveness (RBE) values of carbon ion irradiation and proton irradiation in different human pancreatic cancer cell lines ranged from 1.29 to 4.5, and 0.6 to 2.1, respectively. The mean of the surviving fraction of 2 Gy (SF2) of carbon ion, proton, and photon irradiation was 0.18 ± 0.11, 0.48 ± 0.11, and 0.57 ± 0.13, respectively. Carbon ion irradiation induced more G2/M arrest and a longer-lasting expression of γH2AX than photon irradiation. Combination therapies enhanced the therapeutic effects of pancreatic cell lines with a mean standard enhancement ratio (SER) of 1.66 ± 0.63 for carbon ion irradiation, 1.55 ± 0.27 for proton irradiation, and 1.52 ± 0.30 for photon irradiation. Carbon ion irradiation was more effective in suppressing the migration and invasion than photon irradiation, except for the PANC-1 cells.

Conclusions

Current in vitro evidence demonstrates that, compared with photon irradiation, carbon ion irradiation offers superior radiobiological effects in the treatment of pancreatic cancer. Mechanistically, high-LET irradiation may induce complex DNA damage and ultimately promote genomic instability and cell death. Both carbon ion irradiation and proton irradiation confer similar sensitization effects in comparison with photon irradiation when combined with chemotherapy or targeted therapy.

Mechanisms and Review of Clinical Evidence of Variations in Relative Biological Effectiveness in Proton Therapy

Proton therapy is increasingly being used as a radiation therapy modality. There is uncertainty about the biological effectiveness of protons relative to photon therapies as it depends on several physical and biological parameters. Radiation oncology currently applies a constant and generic value for the relative biological effectiveness (RBE) of 1.1, which was chosen conservatively to ensure tumor coverage. The use of a constant value has been challenged particularly when considering normal tissue constraints. Potential variations in RBE have been assessed in several published reviews but have mostly focused on data from clonogenic cell survival experiments with unclear relevance for clinical proton therapy. The goal of this review is to put in vitro findings in relation to clinical observations. Relevant in vivo pathways determining RBE for tumors and normal tissues are outlined, including not only damage to tumor cells and parenchyma but also vascular damage and immune response. Furthermore, the current clinical evidence of varying RBE is reviewed. The assessment can serve as guidance for treatment planning, personalized dose prescriptions, and outcome analysis.

ATM and RAD51 Repair Pathways in Human Lymphocytes Irradiated with 70 MeV Therapeutic Proton Beam

The repair of radiation-induced DNA damage is a key factor differentiating patients in terms of the therapeutic efficacy and toxicity to surrounding normal tissue. Proton energy substantially determines the types of cancers that can be treated. The present work investigated the DNA double-strand break repair systems, represented by phosphorylated ATM and Rad51. The status of proton therapy energy used to treat major types of cancer is summarized. Here, human lymphocytes from eight healthy donors (male and female) were irradiated with a spread-out Bragg peak using a therapeutic 70 MeV proton beam or with reference X rays. For both types of radiation, the kinetics of pATM and Rad51 repair protein activation (0-24 h) were estimated as determinants of homologous and non-homologous double-strand break repair. Additionally, γ-H2AX was used as the gold standard marker of double-strand breaks. Our results showed that at 30 min postirradiation there was significantly greater accumulation of γ-H2AX (0.6-fold), pATM (2.0-fold), and Rad51 (0.6-fold) in the proton-irradiated cells compared with the X-ray-treated cells. At 24 h post irradiation, for both types of radiation and all investigated proteins, the foci number was still significantly higher when compared with control. Furthermore, the mean value of pATM and Rad51 repair effectiveness was higher in cells exposed to protons than in cells exposed to X rays; however, the difference was significant only for pATM. The largest inter-individual differences in the repair capabilities were noted for Rad51. The association between the frequency of repair protein foci and the frequency of lymphocyte viability at 1 h post irradiation showed a positive correlation for protons but a negative correlation for X rays. These findings indicate that the accumulation of radiation-induced repair protein foci after proton versus X-ray irradiation differs between patients, consequently affecting the cellular responses to particle therapy and conventional radiation therapy.

Avoidance or adaptation of radiotherapy in patients with cancer with Li-Fraumeni and heritable TP53-related cancer syndromes

The management of patients with cancer and Li-Fraumeni or heritable TP53-related cancer syndromes is complex because of their increased risk of developing second malignant neoplasms after genotoxic stresses such as systemic treatments or radiotherapy (radiosusceptibility). Clinical decision making also integrates the risks of normal tissue toxicity and sequelae (radiosensitivity) and tumour response to radiotherapy (radioresistance and radiocurability). Radiotherapy should be avoided in patients with cancer and Li-Fraumeni or heritable TP53 cancer-related syndromes, but overall prognosis might be poor without radiotherapy: radioresistance in these patients seems similar to or worse than that of the general population. Radiosensitivity in germline TP53 variant carriers seems similar to that in the general population. The risk of second malignant neoplasms according to germline TP53 variant and the patient's overall oncological prognosis should be assessed during specialised multidisciplinary staff meetings. Radiotherapy should be avoided whenever other similarly curative treatment options are available. In other cases, it should be adapted to minimise the risk of second malignant neoplasms in patients who still require radiotherapy despite its genotoxicity, in view of its potential benefit. Adaptations might be achieved through the reduction of irradiated volumes using proton therapy, non-ionising diagnostic procedures, image guidance, and minimal stray radiation. Non-ionising imaging should become more systematic. Radiotherapy approaches that might result in a lower probability of misrepaired DNA damage (eg, particle therapy biology and tumour targeting) are an area of investigation.

DNA Double Strand Break Repair Pathways in Response to Different Types of Ionizing Radiation

The superior dose distribution of particle radiation compared to photon radiation makes it a promising therapy for the treatment of tumors. However, the cellular responses to particle therapy and especially the DNA damage response (DDR) is not well characterized. Compared to photons, particles are thought to induce more closely spaced DNA lesions instead of isolated lesions. How this different spatial configuration of the DNA damage directs DNA repair pathway usage, is subject of current investigations. In this review, we describe recent insights into induction of DNA damage by particle radiation and how this shapes DNA end processing and subsequent DNA repair mechanisms. Additionally, we give an overview of promising DDR targets to improve particle therapy.

Does the uncertainty in relative biological effectiveness affect patient treatment in proton therapy?

Clinical treatment with protons uses the concept of relative biological effectiveness (RBE) to convert the absorbed dose into an RBE-weighted dose that equals the dose for radiotherapy with photons causing the same biological effect. Currently, in proton therapy a constant RBE of 1.1 is generically used. However, empirical data indicate that the RBE is not constant, but increases at the distal edge of the proton beam. This increase in RBE is of concern, as the clinical impact is still unresolved, and clinical studies demonstrating a clinical effect of an increased RBE are emerging. Within the European Particle Therapy Network (EPTN) work package 6 on radiobiology and RBE, a workshop was held in February 2020 in Manchester with one day of discussion dedicated to the impact of proton RBE in a clinical context. Current data on RBE effects, patient outcome and modelling from experimental as well as clinical studies were presented and discussed. Furthermore, representatives from European clinical proton therapy centres, who were involved in patient treatment, laid out their current clinical practice on how to consider the risk of a variable RBE in their centres. In line with the workshop, this work considers the actual impact of RBE issues on patient care in proton therapy by reviewing preclinical data on the relation between linear energy transfer (LET) and RBE, current clinical data sets on RBE effects in patients, and applied clinical strategies to manage RBE uncertainties. A better understanding of the variability in RBE would allow development of proton treatments which are safer and more effective.

A simple microscopy setup for visualizing cellular responses to DNA damage at particle accelerator facilities

Cellular responses to DNA double-strand breaks (DSBs) not only promote genomic integrity in healthy tissues, but also largely determine the efficacy of many DNA-damaging cancer treatments, including X-ray and particle therapies. A growing body of evidence suggests that activation of the mechanisms that detect, signal and repair DSBs may depend on the complexity of the initiating DNA lesions. Studies focusing on this, as well as on many other radiobiological questions, require reliable methods to induce DSBs of varying complexity, and to visualize the ensuing cellular responses. Accelerated particles of different energies and masses are exceptionally well suited for this task, due to the nature of their physical interactions with the intracellular environment, but visualizing cellular responses to particle-induced damage - especially in their early stages - at particle accelerator facilities, remains challenging. Here we describe a straightforward approach for real-time imaging of early response to particle-induced DNA damage. We rely on a transportable setup with an inverted fluorescence confocal microscope, tilted at a small angle relative to the particle beam, such that cells can be irradiated and imaged without any microscope or beamline modifications. Using this setup, we image and analyze the accumulation of fluorescently-tagged MDC1, RNF168 and 53BP1-key factors involved in DSB signalling-at DNA lesions induced by 254 MeV α-particles. Our results provide a demonstration of technical feasibility and reveal asynchronous initiation of accumulation of these proteins at different individual DSBs.

DNA double-strand break repair gene mutation and the risk of papillary thyroid microcarcinoma: a case-control study

Objective

To study the relationship between DNA double-strand break (DSB) repair gene mutations and the risk of papillary thyroid microcarcinoma (PTMC).

Methods

One hundred patients with PTMC or benign thyroid nodules (BTNs) at Henan Cancer Hospital were retrospectively analyzed. The DSB repair capacity of peripheral blood T lymphocytes in the two groups was assessed by flow cytometry. Data were compared using Student’s t-test to evaluate the relationship between DSB repair capacity and the risk of PTMC. Factors influencing DSB repair capacity were analyzed by multivariate logistic regression analysis. The relationship between PTMC and DSB repair capacity was analyzed by univariate analysis. Targeted next-generation DNA sequencing was applied to screen and analyze DSB repair genes related to PTMC.

Results

The DSB repair capacity was 31.30% in the PTMC group and 44.40% in the BTN group, with that of the former being significantly lower (P < 0.05). Multivariate logistic regression analysis of age, sex, obesity status, radiation and other factors showed that radiation exposure was positively correlated with reduced DSB repair capacity(OR = 3.642; 95% CI 1.484–8.935, P = 0.020). Moreover, univariate analysis showed that a reduction in DSB repair capacity was a risk factor for PTMC(OR = 2.333; 95% CI 1.027–5.300, P = 0.043).Targeted next-generation DNA sequencing was performed on the DSB repair genes discovered, and those that were mutated in association with PTMC were Rad50 and FANCA; Rad51 mutations were related to BTN.

Conclusion

Radiation exposure is positively associated with induced DSB repair gene mutations, which may cause a reduced capacity for DSB repair and eventually lead to PTMC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02032-5.

Comparative Therapeutic Exploitability of Acute Adaptation Mechanisms to Photon and Proton Irradiation in 3D Head and Neck Squamous Cell Carcinoma Cell Cultures

For better tumor control, high-precision proton beam radiation therapy is currently being intensively discussed relative to conventional photon therapy. Here, we assumed that radiation type-specific molecular response profiles in more physiological 3D, matrix-based head and neck squamous cell carcinoma (HNSCC) cell cultures can be identified and therapeutically exploited. While proton irradiation revealed superimposable clonogenic survival and residual DNA double strand breaks (DSB) relative to photon irradiation, kinome profiles showed quantitative differences between both irradiation types. Pharmacological inhibition of a subset of radiation-induced kinases, predominantly belonging to the mitogen-activated protein kinase (MAPK) family, failed to sensitize HNSCC cells to either proton or photon irradiation. Likewise, inhibitors for ATM, DNA-PK and PARP did not discriminate between proton and photon irradiation but generally elicited a radiosensitization. Conclusively, our results suggest marginal cell line-specific differences in the radiosensitivity and DSB repair without a superiority of one radiation type over the other in 3D grown HNSCC cell cultures. Importantly, radiation-induced activity changes of cytoplasmic kinases induced during the first, acute phase of the cellular radiation response could neither be exploited for sensitization of HNSCC cells to photon nor proton irradiation.

Downregulation of Mcl-1 by Panobinostat Potentiates Proton Beam Therapy in Hepatocellular Carcinoma Cells

Epigenetic modulation by histone deacetylase (HDAC) inhibitors is an attractive anti-cancer strategy for diverse hematological and solid cancers. Herein, we explored the relative effectiveness of the pan-HDAC inhibitor panobinostat in combination with proton over X-ray irradiation in HCC cells. Clonogenic survival assays revealed that radiosensitization of Huh7 and Hep3B cells by panobinostat was more evident when combined with protons than X-rays. Panobinostat increased G2/M arrest and production of intracellular reactive oxygen species, which was further enhanced by proton irradiation. Immunofluorescence staining of γH2AX showed that panobinostat enhanced proton-induced DNA damage. Panobinostat dose-dependently decreased expression of an anti-apoptotic protein, Mcl-1, concomitant with increasing acetylation of histone H4. The combination of panobinostat with proton irradiation enhanced apoptotic cell death to a greater extent than that with X-ray irradiation. Depletion of Mcl-1 by RNA interference enhanced proton-induced apoptosis and proton radiosensitization, suggesting a potential role of Mcl-1 in determining proton sensitivity. Together, our findings suggest that panobinostat may be a promising combination agent for proton beam therapy in HCC treatment.

Targeting the DNA replication stress phenotype of KRAS mutant cancer cells

Mutant KRAS is a common tumor driver and frequently confers resistance to anti-cancer treatments such as radiation. DNA replication stress in these tumors may constitute a therapeutic liability but is poorly understood. Here, using single-molecule DNA fiber analysis, we first characterized baseline replication stress in a panel of unperturbed isogenic and non-isogenic cancer cell lines. Correlating with the observed enhanced replication stress we found increased levels of cytosolic double-stranded DNA in KRAS mutant compared to wild-type cells. Yet, despite this phenotype replication stress-inducing agents failed to selectively impact KRAS mutant cells, which were protected by CHK1. Similarly, most exogenous stressors studied did not differentially augment cytosolic DNA accumulation in KRAS mutant compared to wild-type cells. However, we found that proton radiation was able to slow fork progression and preferentially induce fork stalling in KRAS mutant cells. Proton treatment also partly reversed the radioresistance associated with mutant KRAS. The cellular effects of protons in the presence of KRAS mutation clearly contrasted that of other drugs affecting replication, highlighting the unique nature of the underlying DNA damage caused by protons. Taken together, our findings provide insight into the replication stress response associated with mutated KRAS, which may ultimately yield novel therapeutic opportunities.

Clinical Progress in Proton Radiotherapy: Biological Unknowns

Simple Summary

Proton radiation therapy is a more recent type of radiotherapy that uses proton beams instead of classical photon or X-rays beams. The clinical benefit of proton therapy is that it allows to treat tumors more precisely. As a result, proton radiotherapy induces less toxicity to healthy tissue near the tumor site. Despite the experience in the clinical use of protons, the response of cells to proton radiation, the radiobiology, is less understood. In this review, we describe the current knowledge about proton radiobiology.

Abstract

Clinical use of proton radiation has massively increased over the past years. The main reason for this is the beneficial depth-dose distribution of protons that allows to reduce toxicity to normal tissues surrounding the tumor. Despite the experience in the clinical use of protons, the radiobiology after proton irradiation compared to photon irradiation remains to be completely elucidated. Proton radiation may lead to differential damages and activation of biological processes. Here, we will review the current knowledge of proton radiobiology in terms of induction of reactive oxygen species, hypoxia, DNA damage response, as well as cell death after proton irradiation and radioresistance.

Comparative Proton and Photon Irradiation Combined with Pharmacological Inhibitors in 3D Pancreatic Cancer Cultures

Simple Summary

Due to higher precision and consequent sparing of normal tissue, pancreatic cancer patients might profit from proton beam radiotherapy, a treatment modality increasingly used. Since molecular data upon proton irradiation in comparison to standard photon radiotherapy are limited in pancreatic cancer, the aims of our study were to unravel differences in the effectiveness of photon versus proton irradiation and to exploit radiation type-specific molecular changes for radiosensitizing 3D PDAC cell cultures. Although protons showed a slightly higher effectiveness and a stronger induction of molecular alterations than photons, our results revealed a radiation-type independent sensitization of molecular-targeted agents selected according to the discovered molecular, radiation-induced alterations.

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is a highly therapy-resistant tumor entity of unmet needs. Over the last decades, radiotherapy has been considered as an additional treatment modality to surgery and chemotherapy. Owing to radiosensitive abdominal organs, high-precision proton beam radiotherapy has been regarded as superior to photon radiotherapy. To further elucidate the potential of combination therapies, we employed a more physiological 3D, matrix-based cell culture model to assess tumoroid formation capacity after photon and proton irradiation. Additionally, we investigated proton- and photon-irradiation-induced phosphoproteomic changes for identifying clinically exploitable targets. Here, we show that proton irradiation elicits a higher efficacy to reduce 3D PDAC tumoroid formation and a greater extent of phosphoproteome alterations compared with photon irradiation. The targeting of proteins identified in the phosphoproteome that were uniquely altered by protons or photons failed to cause radiation-type-specific radiosensitization. Targeting DNA repair proteins associated with non-homologous endjoining, however, revealed a strong radiosensitizing potential independent of the radiation type. In conclusion, our findings suggest proton irradiation to be potentially more effective in PDAC than photons without additional efficacy when combined with DNA repair inhibitors.

Oncometabolites and the response to radiotherapy

Radiotherapy (RT) is applied in 45-60% of all cancer patients either alone or in multimodal therapy concepts comprising surgery, RT and chemotherapy. However, despite technical innovations approximately only 50% are cured, highlight a high medical need for innovation in RT practice. RT is a multidisciplinary treatment involving medicine and physics, but has always been successful in integrating emerging novel concepts from cancer and radiation biology for improving therapy outcome. Currently, substantial improvements are expected from integration of precision medicine approaches into RT concepts.Altered metabolism is an important feature of cancer cells and a driving force for malignant progression. Proper metabolic processes are essential to maintain and drive all energy-demanding cellular processes, e.g. repair of DNA double-strand breaks (DSBs). Consequently, metabolic bottlenecks might allow therapeutic intervention in cancer patients.Increasing evidence now indicates that oncogenic activation of metabolic enzymes, oncogenic activities of mutated metabolic enzymes, or adverse conditions in the tumor microenvironment can result in abnormal production of metabolites promoting cancer progression, e.g. 2-hyroxyglutarate (2-HG), succinate and fumarate, respectively. Interestingly, these so-called "oncometabolites" not only modulate cell signaling but also impact the response of cancer cells to chemotherapy and RT, presumably by epigenetic modulation of DNA repair.Here we aimed to introduce the biological basis of oncometabolite production and of their actions on epigenetic regulation of DNA repair. Furthermore, the review will highlight innovative therapeutic opportunities arising from the interaction of oncometabolites with DNA repair regulation for specifically enhancing the therapeutic effects of genotoxic treatments including RT in cancer patients.

Could Protons and Carbon Ions Be the Silver Bullets Against Pancreatic Cancer?

Pancreatic cancer is a very aggressive cancer type associated with one of the poorest prognostics. Despite several clinical trials to combine different types of therapies, none of them resulted in significant improvements for patient survival. Pancreatic cancers demonstrate a very broad panel of resistance mechanisms due to their biological properties but also their ability to remodel the tumour microenvironment. Radiotherapy is one of the most widely used treatments against cancer but, up to now, its impact remains limited in the context of pancreatic cancer. The modern era of radiotherapy proposes new approaches with increasing conformation but also more efficient effects on tumours in the case of charged particles. In this review, we highlight the interest in using charged particles in the context of pancreatic cancer therapy and the impact of this alternative to counteract resistance mechanisms.