Evaluation of Helium Ion Radiotherapy in Combination with Gemcitabine in Pancreatic Cancer In Vitro

BACKGROUND

Pancreatic cancer is one of the most aggressive and lethal cancers. New treatment strategies are highly warranted. Particle radiotherapy could offer a way to overcome the radioresistant nature of pancreatic cancer because of its biological and physical characteristics. Within particles, helium ions represent an attractive therapy option to achieve the highest possible conformity while at the same time protecting the surrounding normal tissue. The aim of this study was to evaluate the cytotoxic efficacy of helium ion irradiation in pancreatic cancer in vitro.

METHODS

Human pancreatic cancer cell lines AsPC-1, BxPC-3 and Panc-1 were irradiated with photons and helium ions at various doses and treated with gemcitabine. Photon irradiation was performed with a biological cabin X-ray irradiator, and helium ion irradiation was performed with a spread-out Bragg peak using the raster scanning technique at the Heidelberg Ion Beam Therapy Center (HIT). The cytotoxic effect on pancreatic cancer cells was measured with clonogenic survival. The survival curves were compared to the predicted curves that were calculated via the modified microdosimetric kinetic model (mMKM).

RESULTS

The experimental relative biological effectiveness (RBE) of helium ion irradiation ranged from 1.0 to 1.7. The predicted survival curves obtained via mMKM calculations matched the experimental survival curves. Mainly additive cytotoxic effects were observed for the cell lines AsPC-1, BxPC-3 and Panc-1.

CONCLUSION

Our results demonstrate the cytotoxic efficacy of helium ion radiotherapy in pancreatic cancer in vitro as well as the capability of mMKM calculation and its value for biological plan optimization in helium ion therapy for pancreatic cancer. A combined treatment of helium irradiation and chemotherapy with gemcitabine leads to mainly additive cytotoxic effects in pancreatic cancer cell lines. The data generated in this study may serve as the radiobiological basis for future experimental and clinical works using helium ion radiotherapy in pancreatic cancer treatment.

Robustness of carbon-ion radiotherapy against DNA damage repair associated radiosensitivity variation based on a biophysical model

BACKGROUND

Interpatient variation of tumor radiosensitivity is rarely considered during the treatment planning process despite its known significance for the therapeutic outcome.

PURPOSE

To apply our mechanistic biophysical model to investigate the biological robustness of carbon ion radiotherapy (CIRT) against DNA damage repair interference (DDRi) associated patient-to-patient variability in radiosensitivity and its potential clinical advantages against conventional radiotherapy approaches.

METHODS AND MATERIALS

The "UNIfied and VERSatile bio response Engine" (UNIVERSE) was extended by carbon ions and its predictions were compared to a panel of in vitro and in vivo data including various endpoints and DDRi settings within clinically relevant dose and linear energy transfer (LET) ranges. The implications of UNIVERSE predictions were then assessed in a clinical patient scenario considering DDRi variance.

RESULTS

UNIVERSE tests well against the applied benchmarks. While in vitro survival curves were predicted with an R2 > 0.92, deviations from in vivo RBE data were less than 5.6% The conducted paradigmatic patient plan study implies a markedly reduced significance of DDRi based radiosensitivity variability in CIRT (13% change ofD 50 ${{D}_{50}}$ in target) compared to conventional radiotherapy (62%) and that boosting the LET within the target further amplifies this robustness of CIRT (8%). In the case of heightened tumor radiosensitivity, a dose de-escalation strategy for photons allows a reduction of the maximum effective dose within the normal tissue (NT) from aD 2 ${{D}_2}$ of 2.65 to 1.64 Gy, which lies below the level found for CIRT (D 2 ${{D}_2}$  = 2.41 Gy) for the analyzed plan and parameters. However, even after de-escalation, the integral effective dose in the NT is found to be substantially higher for conventional radiotherapy in comparison to CIRT (D m e a n ${{D}_{mean}}$ of 0.75, 0.46, and 0.24 Gy for the conventional plan, its de-escalation and CIRT, respectively).

CONCLUSIONS

The framework offers adequate predictions of in vitro and in vivo radiation effects of CIRT while allowing the consideration of DRRi based solely on parameters derived from photon data. The results of the patient planning study underline the potential of CIRT to minimize important sources of interpatient divergence in therapy outcome, especially when combined with techniques that allow to maximize the LET within the tumor. Despite the potential of de-escalation strategies for conventional radiotherapy to reduce the maximum effective dose in the NT, CIRT appears to remain a more favorable option due to its ability to reduce the integral effective dose within the NT.

Ionization detail parameters and cluster dose: a mathematical model for selection of nanodosimetric quantities for use in treatment planning in charged particle radiotherapy

Objective. To propose a mathematical model for applying ionization detail (ID), the detailed spatial distribution of ionization along a particle track, to proton and ion beam radiotherapy treatment planning (RTP).Approach. Our model provides for selection of preferred ID parameters (Ip) for RTP, that associate closest to biological effects. Cluster dose is proposed to bridge the large gap between nanoscopicIpand macroscopic RTP. Selection ofIpis demonstrated using published cell survival measurements for protons through argon, comparing results for nineteenIp:Nk,k= 2, 3, …, 10, the number of ionizations in clusters ofkor more per particle, andFk,k= 1, 2, …, 10, the number of clusters ofkor more per particle. We then describe application of the model to ID-based RTP and propose a path to clinical translation.Main results. The preferredIpwereN4andF5for aerobic cells,N5andF7for hypoxic cells. Significant differences were found in cell survival for beams having the same LET or the preferredNk. Conversely, there was no significant difference forF5for aerobic cells andF7for hypoxic cells, regardless of ion beam atomic number or energy. Further, cells irradiated with the same cluster dose for theseIphad the same cell survival. Based on these preliminary results and other compelling results in nanodosimetry, it is reasonable to assert thatIpexist that are more closely associated with biological effects than current LET-based approaches and microdosimetric RBE-based models used in particle RTP. However, more biological variables such as cell line and cycle phase, as well as ion beam pulse structure and rate still need investigation.Significance. Our model provides a practical means to select preferredIpfrom radiobiological data, and to convertIpto the macroscopic cluster dose for particle RTP.

Do We Preserve Tumor Control Probability (TCP) in FLASH Radiotherapy? A Model-Based Analysis

Reports of concurrent sparing of normal tissue and iso-effective treatment of tumors at ultra-high dose-rates (uHDR) have fueled the growing field of FLASH radiotherapy. However, iso-effectiveness in tumors is often deduced from the absence of a significant difference in their growth kinetics. In a model-based analysis, we investigate the meaningfulness of these indications for the clinical treatment outcome. The predictions of a previously benchmarked model of uHDR sparing in the "UNIfied and VERSatile bio response Engine" (UNIVERSE) are combined with existing models of tumor volume kinetics as well as tumor control probability (TCP) and compared to experimental data. The potential TCP of FLASH radiotherapy is investigated by varying the assumed dose-rate, fractionation schemes and oxygen concentration in the target. The developed framework describes the reported tumor growth kinetics appropriately, indicating that sparing effects could be present in the tumor but might be too small to be detected with the number of animals used. The TCP predictions show the possibility of substantial loss of treatment efficacy for FLASH radiotherapy depending on several variables, including the fractionation scheme, oxygen level, and DNA repair kinetics. The possible loss of TCP should be seriously considered when assessing the clinical viability of FLASH treatments.

Neuroprotective Effects of Ultra-High Dose Rate FLASH Bragg Peak Proton Irradiation

PURPOSE

To investigate brain tissue response to ultra-high dose rate (uHDR, FLASH) and standard dose rate (SDR) proton irradiations in the Bragg peak region.

METHODS AND MATERIALS

Active scanning uHDR delivery was established for proton beams for investigation of dose rate effects between clinical SDR and uHDR at ∼10 Gy in the Bragg peak region (dose-averaged linear energy transfer [LETD] ranging from 4.5 to 10.2 keV µm-1 ). Radiation- induced injury of neuronal tissue was assessed by studying the DNA double strand break repair kinetics surrogated by nuclear γH2AX staining (radiation induced foci [RIF]), microvascular density and structural integrity (MVD, CD31+ endothelium), and inflammatory microenvironmental response (CD68+ microglia/macrophages and high mobility group box protein 1[HMGB]) in healthy C57BL/6 mouse brains.

RESULTS

Averaged dose rates achieved were 0.17 Gy/s (SDR) and 120 Gy/s (uHDR). The fraction of RIF-positive cells increased after SDR ∼10-fold, whereas a significantly lower fraction of RIF-positive cells was found after uHDR versus SDR (∼2 fold, P < .0001). Moreover, uHDR substantially preserved the microvascular architecture and reduced microglia/macrophage regulated associated inflammation as compared with SDR.

CONCLUSIONS

The feasibility of uHDR raster scanning proton irradiation is demonstrated to elicit FLASH sparing neuroprotective effects compared to SDR in a preclinical in vivo model.

The Impact of Sub-Millisecond Damage Fixation Kinetics on the In Vitro Sparing Effect at Ultra-High Dose Rate in UNIVERSE

The impact of the exact temporal pulse structure on the potential cell and tissue sparing of ultra-high dose-rate irradiation applied in FLASH studies has gained increasing attention. A previous version of our biophysical mechanistic model (UNIVERSE: UNIfied and VERSatile bio response Engine), based on the oxygen depletion hypothesis, has been extended in this work by considering oxygen-dependent damage fixation dynamics on the sub-milliseconds scale and introducing an explicit implementation of the temporal pulse structure. The model successfully reproduces in vitro experimental data on the fast kinetics of the oxygen effect in irradiated mammalian cells. The implemented changes result in a reduction in the assumed amount of oxygen depletion. Furthermore, its increase towards conventional dose-rates is parameterized based on experimental data from the literature. A recalculation of previous benchmarks shows that the model retains its predictive power, while the assumed amount of depleted oxygen approaches measured values. The updated UNIVERSE could be used to investigate the impact of different combinations of pulse structure parameters (e.g., dose per pulse, pulse frequency, number of pulses, etc.), thereby aiding the optimization of potential clinical application and the development of suitable accelerators.

Development of Ultra-High Dose-Rate (FLASH) Particle Therapy

Research efforts in FLASH radiotherapy have increased at an accelerated pace recently. FLASH radiotherapy involves ultra-high dose rates and has shown to reduce toxicity to normal tissue while maintaining tumor response in pre-clinical studies when compared to conventional dose rate radiotherapy. The goal of this review is to summarize the studies performed to-date with proton, electron, and heavy ion FLASH radiotherapy, with particular emphasis on the physical aspects of each study and the advantages and disadvantages of each modality. Beam delivery parameters, experimental set-up, and the dosimetry tools used are described for each FLASH modality. In addition, modeling efforts and treatment planning for FLASH radiotherapy is discussed along with potential drawbacks when translated into the clinical setting. The final section concludes with further questions that have yet to be answered before safe clinical implementation of FLASH radiotherapy.

Combined DNA Damage Repair Interference and Ion Beam Therapy: Development, Benchmark, and Clinical Implications of a Mechanistic Biological Model

PURPOSE

Our purpose was to develop a mechanistic model that describes and predicts radiation response after combined DNA damage repair interference (DDRi) and particle radiation therapy.

METHODS AND MATERIALS

The heterogeneous dose distributions of protons and ⁴He ions were implemented into the "UNIfied and VERSatile bio-response Engine" (UNIVERSE). Predictions for monoenergetic and mixed fields over clinically relevant dose and linear energy transfer range were compared with experimental in vitro survival data measured in this work as well as data available in the literature, including different cell lines and DDR interferences. Ultimately, UNIVERSE predictions were investigated in a patient plan.

RESULTS

UNIVERSE accurately predicts survival of cell lines with and without DDRi in clinical settings of ion beam therapy based only on 3 parameters derived from photon data. With increasing dose or linear energy transfer, the radiosensitizing effect of DDRi decreases, resulting in diminished relative biological effect of ion beam radiation for cells subjected to DDRi in comparison to cells that are not. Similar trends were observed in patient plan recalculations; however, this analysis also suggests that DDRi + particle radiation therapy may better preserve the therapeutic window in comparison to DDRi + photon radiation therapy.

CONCLUSIONS

The presented framework represents the first mechanistic model of combined DDRi and particle radiation therapy comprehensively benchmarked in clinically relevant scenarios and a step toward more personalized treatment. It reveals potential differences between DDRi + photon radiation therapy versus DDRi + particle radiation therapy, which have not been described so far. UNIVERSE could aid in appraising the clinical viability of combined administration of radiosensitizing drugs and charged particle therapy, as well as the identification of patients with known DDR deficiencies in the tumor who might benefit from therapy with light ions, freeing limited space at heavy ion therapy centers.

Radioresistance and Transcriptional Reprograming of Invasive Glioblastoma Cells

PURPOSE

Infiltrative growth pattern is a hallmark of glioblastoma (GBM). Radiation therapy aims to eradicate microscopic residual GBM cells after surgical removal of the visible tumor bulk. However, in-field recurrences remain the major pattern of therapy failure. We hypothesized that the radiosensitivity of peripheral invasive tumor cells (peri) may differ from the predominantly investigated tumor bulk.

METHODS AND MATERIALS

Invasive GBM populations were generated via debulking of the visible tumor core and serial orthotopic transplantation of peri cells, and sustained proinvasive phenotype of peri cells was confirmed in vitro by scratch assay and time lapse imaging. In parallel, invasive GBM cells were selected by transwell assay and from peri cells of patient-derived 3-dimensional spheroid cultures. Transcriptome analysis deciphered a GBM invasion-associated gene signature, and functional involvement of key pathways was validated by pharmacologic inhibition.

RESULTS

Compared with the bulk cells, invasive GBM populations acquired a radioresistant phenotype characterized by increased cell survival, reduced cell apoptosis, and enhanced DNA double-strand break repair proficiency. Transcriptome analysis revealed a reprograming of invasive cells toward augmented activation of epidermal growth factor receptor- and nuclear factor-κB-related pathways, whereas metabolic processes were downregulated. An invasive GBM score derived from this transcriptional fingerprint correlated well with patient outcome. Inhibition of epidermal growth factor receptor and nuclear factor-κB signaling resensitized invasive cells to irradiation. Invasive cells were eradicated with similar efficacy by particle therapy with carbon ions.

CONCLUSIONS

Our data indicate that invasive tumor cells constitute a phenotypically distinct and highly radioresistant GBM subpopulation with prognostic impact that may be vulnerable to targeted therapy and carbon ions.

FLASH dose-rate helium ion beams: first in vitro investigations

PURPOSE

To establish and investigate the impact of dose, linear energy transfer (LET) and O₂ concentration on biological response to ultra-high dose-rate (uHDR, FLASH) helium ion beams compared to standard dose-rate (SDR) irradiation.

METHODS AND MATERIALS

Beam delivery settings for raster-scanned helium ions at both uHDR and SDR were tuned to achieve >100 Gy/s and ∼0.1 Gy/s, respectively. For both SDR and uHDR, plan optimization and calibration for 10 × 10mm² fields was performed to assess in vitro response at LET range of 4.5-16 keV/µm. Clonogenic survival assay was conducted at doses ranging from 2 Gy to 12 Gy in two human lung epithelial cell lines (A549 and H1437). Radiation induced nuclear γH2AX foci (RIF) were assessed in both epithelial cell lines and primary human pulmonary fibroblasts.

RESULTS

Average dose-rates achieved were 185 Gy/s and 0.12 Gy/s for uHDR and SDR, respectively. No differences in cellular response to SDR vs. uHDR were observed for all tested doses at 21% O2, as well as at 2 and 4 Gy at 1% O2. In contrast, at 1% O2 and dose threshold of ≳8Gy cell survival was higher and correlated with reduced nuclear γH2AX RIF signal indicating FLASH sparing effect in the investigated cell lines irradiated with uHDR as compared to SDR .

CONCLUSION

The first uHDR delivery of raster-scanned particle beams was achieved using helium ions, reaching FLASH-level dose-rates of >100 Gy/s.  Baseline oxygen levels and delivered dose (≳ 8 Gy) play a pivotal role, irrespective of the studied cell lines, for observation of a sparing effect for helium ions.

Study of the intracellular nanoparticle-based radiosensitization mechanisms in F98 glioma cells treated with charged particle therapy through synchrotron-based infrared microspectroscopy

The use of nanoparticles (NP) as dose enhancers in radiotherapy (RT) is a growing research field. Recently, the use of NP has been extended to charged particle therapy in order to improve the performance in radioresistant tumors. However, the biological mechanisms underlying the synergistic effects involved in NP-RT approaches are not clearly understood. Here, we used the capabilities of synchrotron-based Fourier Transform Infrared Microspectroscopy (SR-FTIRM) as a bio-analytical tool to elucidate the NP-induced cellular damage at the molecular level and at a single-cell scale. F98 glioma cells doped with AuNP and GdNP were irradiated using several types of medical ion beams (proton, helium, carbon and oxygen). Differences in cell composition were analyzed in the nucleic acids, protein and lipid spectral regions using multivariate methods (Principal Component Analysis, PCA). Several NP-induced cellular modifications were detected, such as conformational changes in secondary protein structures, intensity variations in the lipid CHx stretching bands, as well as complex DNA rearrangements following charged particle therapy irradiations. These spectral features seem to be correlated with the already shown enhancement both in the DNA damage response and in the reactive oxygen species (ROS) production by the NP, which causes cell damage in the form of protein, lipid, and/or DNA oxidations. Vibrational features were NP-dependent due to the NP heterogeneous radiosensitization capability. Our results provided new insights into the molecular changes in response to NP-based RT treatments using ion beams, and highlighted the relevance of SR-FTIRM as a useful and precise technique for assessing cell response to innovative radiotherapy approaches.

Assessment of RBE-Weighted Dose Models for Carbon Ion Therapy Toward Modernization of Clinical Practice at HIT: In Vitro, in Vivo, and in Patients

PURPOSE

Present-day treatment planning in carbon ion therapy is conducted with assumptions for a limited number of tissue types and models for effective dose. Here, we comprehensively assess relative biological effectiveness (RBE) in carbon ion therapy and associated models toward the modernization of current clinical practice in effective dose calculation.

METHODS

Using 2 human (A549, H460) and 2 mouse (B16, Renca) tumor cell lines, clonogenic cell survival assay was performed for examination of changes in RBE along the full range of clinical-like spread-out Bragg peak (SOBP) fields. Prediction power of the local effect model (LEM1 and LEM4) and the modified microdosimetric kinetic model (mMKM) was assessed. Experimentation and analysis were carried out in the frame of a multidimensional end point study for clinically relevant ranges of physical dose (D), dose-averaged linear energy transfer (LET_d), and base-line photon radio-sensitivity (α/β)_x. Additionally, predictions were compared against previously reported RBE measurements in vivo and surveyed in patient cases.

RESULTS

RBE model prediction performance varied among the investigated perspectives, with mMKM prediction exhibiting superior agreement with measurements both in vitro and in vivo across the 3 investigated end points. LEM1 and LEM4 performed their best in the highest LET conditions but yielded overestimations and underestimations in low/midrange LET conditions, respectively, as demonstrated by comparison with measurements. Additionally, the analysis of patient treatment plans revealed substantial variability across the investigated models (±20%-30% uncertainty), largely dependent on the selected model and absolute values for input tissue parameters α_x and β_x.

CONCLUSION

RBE dependencies in vitro, in vivo, and in silico were investigated with respect to various clinically relevant end points in the context of tumor-specific tissue radio-sensitivity assignment and accurate RBE modeling. Discovered model trends and performances advocate upgrading current treatment planning schemes in carbon ion therapy and call for verification via clinical outcome analysis with large patient cohorts.

Modeling the Effect of Hypoxia and DNA Repair Inhibition on Cell Survival After Photon Irradiation

Mechanistic approaches to modeling the effects of ionizing radiation on cells are on the rise, promising a better understanding of predictions and higher flexibility concerning conditions to be accounted for. In this work we modified and extended a previously published mechanistic model of cell survival after photon irradiation under hypoxia to account for radiosensitization caused by deficiency or inhibition of DNA damage repair enzymes. The model is shown to be capable of describing the survival data of cells with DNA damage repair deficiency, both under norm- and hypoxia. We find that our parameterization of radiosensitization is invariant under change of oxygen status, indicating that the relevant parameters for both mechanisms can be obtained independently and introduced freely to the model to predict their combined effect.

Biophysical modeling and experimental validation of relative biological effectiveness (RBE) for 4He ion beam therapy

BACKGROUND

Helium (⁴He) ion beam therapy provides favorable biophysical characteristics compared to currently administered particle therapies, i.e., reduced lateral scattering and enhanced biological damage to deep-seated tumors like heavier ions, while simultaneously lessened particle fragmentation in distal healthy tissues as observed with lighter protons. Despite these biophysical advantages, raster-scanning ⁴He ion therapy remains poorly explored e.g., clinical translational is hampered by the lack of reliable and robust estimation of physical and radiobiological uncertainties. Therefore, prior to the upcoming ⁴He ion therapy program at the Heidelberg Ion-beam Therapy Center (HIT), we aimed to characterize the biophysical phenomena of ⁴He ion beams and various aspects of the associated models for clinical integration.

METHODS

Characterization of biological effect for ⁴He ion beams was performed in both homogenous and patient-like treatment scenarios using innovative models for estimation of relative biological effectiveness (RBE) in silico and their experimental validation using clonogenic cell survival as the gold-standard surrogate. Towards translation of RBE models in patients, the first GPU-based treatment planning system (non-commercial) for raster-scanning ⁴He ion beams was devised in-house (FRoG).

RESULTS

Our data indicate clinically relevant uncertainty of ±5-10% across different model simulations, highlighting their distinct biological and computational methodologies. The in vitro surrogate for highly radio-resistant tissues presented large RBE variability and uncertainty within the clinical dose range.

CONCLUSIONS

Existing phenomenological and mechanistic/biophysical models were successfully integrated and validated in both Monte Carlo and GPU-accelerated analytical platforms against in vitro experiments, and tested using pristine peaks and clinical fields in highly radio-resistant tissues where models exhibit the greatest RBE uncertainty. Together, these efforts mark an important step towards clinical translation of raster-scanning ⁴He ion beam therapy to the clinic.

Carbon irradiation overcomes glioma radioresistance by eradicating stem cells and forming an antiangiogenic and immunopermissive niche

Tumor radioresistance leading to local therapy failure remains a major obstacle for successful treatment of high-grade glioma. We hypothesized that distinct radiobiological features of particle therapy with carbon ions may circumvent glioma radioresistance. We demonstrate that carbon irradiation (CIR) efficiently eradicates radioresistant patient-derived glioma stem cells (GSCs), leading to growth inhibition and prolonged survival. The impact of CIR at the tumor-stroma interface was further investigated in 2 syngeneic mouse and 2 orthotopic GSC xenograft models. Intriguingly, tumor regressions and long-term local controls were observed at doses greater than or equal to 15-Gy CIR. Fractionated CIR further prolonged survival. The enhanced relative biological effectiveness of CIR in vivo was attributed to its potent antiangiogenic effects and eradication of radioresistant hypoxic tumor cells. Blockade of the HIF1-α/stromal cell-derived factor 1/CXCR4 axis by CIR reduced the recruitment of microglia and myeloid-derived suppressor cells (CD11b+Gr1+). Consequently, CIR abrogated M2-like immune polarization and enhanced the influx of CD8+ cells, generating an immunopermissive niche. We report that radiotherapy with carbon ions could surmount several central glioma resistance mechanisms by eradicating hypoxic and stem cell-like tumor cells, as well as modulating the glioma niche toward an antiangiogenic and less immunosuppressive state. Conclusively, potentially novel rationales for CIR in conquering glioma radioresistance are provided.

Next generation multi-scale biophysical characterization of high precision cancer particle radiotherapy using clinical proton, helium-, carbon- and oxygen ion beams

The growing number of particle therapy facilities worldwide landmarks a novel era of precision oncology. Implementation of robust biophysical readouts is urgently needed to assess the efficacy of different radiation qualities. This is the first report on biophysical evaluation of Monte Carlo simulated predictive models of prescribed dose for four particle qualities i.e., proton, helium-, carbon- or oxygen ions using raster-scanning technology and clinical therapy settings at HIT. A high level of agreement was found between the in silico simulations, the physical dosimetry and the clonogenic tumor cell survival. The cell fluorescence ion track hybrid detector (Cell-Fit-HD) technology was employed to detect particle traverse per cell nucleus. Across a panel of radiobiological surrogates studied such as late ROS accumulation and apoptosis (caspase 3/7 activation), the relative biological effectiveness (RBE) chiefly correlated with the radiation species-specific spatio-temporal pattern of DNA double strand break (DSB) formation and repair kinetic. The size and the number of residual nuclear γ-H2AX foci increased as a function of linear energy transfer (LET) and RBE, reminiscent of enhanced DNA-damage complexity and accumulation of non-repairable DSB. These data confirm the high relevance of complex DSB formation as a central determinant of cell fate and reliable biological surrogates for cell survival/ RBE. The multi-scale simulation, physical and radiobiological characterization of novel clinical quality beams presented here constitutes a first step towards development of high precision biologically individualized radiotherapy.

Overcoming hypoxia-induced tumor radioresistance in non-small cell lung cancer by targeting DNA-dependent protein kinase in combination with carbon ion irradiation

Background

Hypoxia-induced radioresistance constitutes a major obstacle for a curative treatment of cancer. The aim of this study was to investigate effects of photon and carbon ion irradiation in combination with inhibitors of DNA-Damage Response (DDR) on tumor cell radiosensitivity under hypoxic conditions.

Methods

Human non-small cell lung cancer (NSCLC) models, A549 and H1437, were irradiated with dose series of photon and carbon ions under hypoxia (1% O₂) vs. normoxic conditions (21% O₂). Clonogenic survival was studied after dual combinations of radiotherapy with inhibitors of DNA-dependent Protein Kinase (DNAPKi, M3814) and ATM serine/threonine kinase (ATMi).

Results

The OER at 30% survival for photon irradiation of A549 cells was 1.4. The maximal oxygen effect measured as survival ratio was 2.34 at 8 Gy photon irradiation of A549 cells. In contrast, no significant oxygen effect was found after carbon ion irradiation. Accordingly, the relative effect of 6 Gy carbon ions was determined as 3.8 under normoxia and. 4.11 under hypoxia. ATM and DNA-PK inhibitors dose dependently sensitized tumor cells for both radiation qualities. For 100 nM DNAPKi the survival ratio at 4 Gy more than doubled from 1.59 under normoxia to 3.3 under hypoxia revealing a strong radiosensitizing effect under hypoxic conditions. In contrast, this ratio only moderately increased after photon irradiation and ATMi under hypoxia. The most effective treatment was combined carbon ion irradiation and DNA damage repair inhibition.

Conclusions

Carbon ions efficiently eradicate hypoxic tumor cells. Both, ATMi and DNAPKi elicit radiosensitizing effects. DNAPKi preferentially sensitizes hypoxic cells to radiotherapy.

Electronic supplementary material

The online version of this article (10.1186/s13014-017-0939-0) contains supplementary material, which is available to authorized users.

Optimization of Monte Carlo particle transport parameters and validation of a novel high throughput experimental setup to measure the biological effects of particle beams

PURPOSE

Accurate modeling of the relative biological effectiveness (RBE) of particle beams requires increased systematic in vitro studies with human cell lines with care towards minimizing uncertainties in biologic assays as well as physical parameters. In this study, we describe a novel high-throughput experimental setup and an optimized parameterization of the Monte Carlo (MC) simulation technique that is universally applicable for accurate determination of RBE of clinical ion beams. Clonogenic cell-survival measurements on a human lung cancer cell line (H460) are presented using proton irradiation.

METHODS

Experiments were performed at the Heidelberg Ion Therapy Center (HIT) with support from the Deutsches Krebsforschungszentrum (DKFZ) in Heidelberg, Germany using a mono-energetic horizontal proton beam. A custom-made variable range selector was designed for the horizontal beam line using the Geant4 MC toolkit. This unique setup enabled a high-throughput clonogenic assay investigation of multiple, well defined dose and linear energy transfer (LETs) per irradiation for human lung cancer cells (H460) cultured in a 96-well plate. Sensitivity studies based on application of different physics lists in conjunction with different electromagnetic constructors and production threshold values to the MC simulations were undertaken for accurate assessment of the calculated dose and the dose-averaged LET (LET_d ). These studies were extended to helium and carbon ion beams.

RESULTS

Sensitivity analysis of the MC parameterization revealed substantial dependence of the dose and LET_d values on both the choice of physics list and the production threshold values. While the dose and LET_d calculations using FTFP_BERT_LIV, FTFP_BERT_EMZ, FTFP_BERT_PEN and QGSP_BIC_EMY physics lists agree well with each other for all three ions, they show large differences when compared to the FTFP_BERT physics list with the default electromagnetic constructor. For carbon ions, the dose corresponding to the largest LET_d value is observed to differ by as much as 78% between FTFP_BERT and FTFP_BERT_LIV. Furthermore, between the production threshold of 700 μm and 5 μm, proton dose varies by as much as 19% corresponding to the largest LET_d value sampled in the current investigation. Based on the sensitivity studies, the FTFP_BERT physics list with the low energy Livermore electromagnetic constructor and a production threshold of 5 μm was employed for determining accurate dose and LET_d . The optimized MC parameterization results in a different LET_d dependence of the RBE curve for 10% SF of the H460 cell line irradiated with proton beam when compared with the results from a previous study using the same cell line. When the MC parameters are kept consistent between the studies, the proton RBE results agree well with each other within the experimental uncertainties.

CONCLUSIONS

A custom high-throughput, high-accuracy experimental design for accurate in vitro cell survival measurements was employed at a horizontal beam line. High sensitivity of the physics-based optimization establishes the importance of accurate MC parameterization and hence the conditioning of the MC system on a case-by-case basis. The proton RBE results from current investigations are observed to agree with a previous measurement made under different experimental conditions. This establishes the consistency of our experimental findings across different experiments and institutions.

Comparative analysis of the effects of a sphingosine kinase inhibitor to temozolomide and radiation treatment on glioblastoma cell lines

Glioblastoma multiforme (GBM) exhibits high resistance to the standard treatment of temozolomide (TMZ) combined with radiotherapy, due to its remarkable cell heterogeneity. Accordingly, there is a need to target alternative molecules enhancing specific GBM autocrine or paracrine mechanisms and amplifying the effect of standard treatment. Sphingosine 1-phosphate (S1P) is such a lipid target molecule with an important role in cell invasion and proliferation. Sphingosine kinase inhibitors (SKI) prevent S1P formation and induce increased production of reactive oxygen species (ROS), which may potentiate radiation cytotoxicity. We analyzed the effect of SKI singular versus combined treatments with TMZ and radiation on 2 human GBM cell lines characterized by a lack of MGMT expression and low or high expression of the anti-oxidant enzyme, glutathione peroxidase 1 (GPx1). Effects were drug concentration-, cell line-dependent and partly ROS-mediated. Clonogenic survival assay demonstrates that SKI was more effective than TMZ in increasing the sensitivity of U87 cells, which express low GPx1 amount, to a 2 Gy X-ray dose. Addition of both SKI and TMZ drastically decreased U87 cells survival compared with the combination temozolomide/radiation. SKI less effectively than TMZ sensitized LN229 cells to the 2 Gy X-ray dose. Its combination to TMZ in absence of irradiation was as efficient as TMZ combination with X-ray. We provide first evidence for SKI as an alternative or complementary treatment to TMZ, and for efficient combinations of low doses of drugs and X-ray. These may help as novel bi-modal and tri-modal therapies to contend with GBM heterogeneity.

Deciphering the Acute Cellular Phosphoproteome Response to Irradiation with X-rays, Protons and Carbon Ions

Radiotherapy is a cornerstone of cancer therapy. The recently established particle therapy with raster-scanning protons and carbon ions landmarks a new era in the field of high-precision cancer medicine. However, molecular mechanisms governing radiation induced intracellular signaling remain elusive. Here, we present the first comprehensive proteomic and phosphoproteomic study applying stable isotope labeling by amino acids in cell culture (SILAC) in combination with high-resolution mass spectrometry to decipher cellular response to irradiation with X-rays, protons and carbon ions. At protein expression level limited alterations were observed 2 h post irradiation of human lung adenocarcinoma cells. In contrast, 181 phosphorylation sites were found to be differentially regulated out of which 151 sites were not hitherto attributed to radiation response as revealed by crosscheck with the PhosphoSitePlus database.Radiation-induced phosphorylation of the p(S/T)Q motif was the prevailing regulation pattern affecting proteins involved in DNA damage response signaling. Because radiation doses were selected to produce same level of cell kill and DNA double-strand breakage for each radiation quality, DNA damage responsive phosphorylation sites were regulated to same extent. However, differential phosphorylation between radiation qualities was observed for 55 phosphorylation sites indicating the existence of distinct signaling circuitries induced by X-ray versus particle (proton/carbon) irradiation beyond the canonical DNA damage response. This unexpected finding was confirmed in targeted spike-in experiments using synthetic isotope labeled phosphopeptides. Herewith, we successfully validated uniform DNA damage response signaling coexisting with altered signaling involved in apoptosis and metabolic processes induced by X-ray and particle based treatments.In summary, the comprehensive insight into the radiation-induced phosphoproteome landscape is instructive for the design of functional studies aiming to decipher cellular signaling processes in response to radiotherapy, space radiation or ionizing radiation per se Further, our data will have a significant impact on the ongoing debate about patient treatment modalities.

Data-driven RBE parameterization for helium ion beams

Helium ion beams are expected to be available again in the near future for clinical use. A suitable formalism to obtain relative biological effectiveness (RBE) values for treatment planning (TP) studies is needed. In this work we developed a data-driven RBE parameterization based on published in vitro experimental values. The RBE parameterization has been developed within the framework of the linear-quadratic (LQ) model as a function of the helium linear energy transfer (LET), dose and the tissue specific parameter (α/β)ph of the LQ model for the reference radiation. Analytic expressions are provided, derived from the collected database, describing the RBEα = αHe/αph and Rβ = βHe/βph ratios as a function of LET. Calculated RBE values at 2 Gy photon dose and at 10% survival (RBE10) are compared with the experimental ones. Pearson's correlation coefficients were, respectively, 0.85 and 0.84 confirming the soundness of the introduced approach. Moreover, due to the lack of experimental data at low LET, clonogenic experiments have been performed irradiating A549 cell line with (α/β)ph = 5.4 Gy at the entrance of a 56.4 MeV u(-1)He beam at the Heidelberg Ion Beam Therapy Center. The proposed parameterization reproduces the measured cell survival within the experimental uncertainties. A RBE formula, which depends only on dose, LET and (α/β)ph as input parameters is proposed, allowing a straightforward implementation in a TP system.

Correlation of Particle Traversals with Clonogenic Survival Using Cell-Fluorescent Ion Track Hybrid Detector

Development of novel approaches linking the physical characteristics of particles with biological responses are of high relevance for the field of particle therapy. In radiobiology, the clonogenic survival of cells is considered the gold standard assay for the assessment of cellular sensitivity to ionizing radiation. Toward further development of next generation biodosimeters in particle therapy, cell-fluorescent ion track hybrid detector (Cell-FIT-HD) was recently engineered by our group and successfully employed to study physical particle track information in correlation with irradiation-induced DNA damage in cell nuclei. In this work, we investigated the feasibility of Cell-FIT-HD as a tool to study the effects of clinical beams on cellular clonogenic survival. Tumor cells were grown on the fluorescent nuclear track detector as cell culture, mimicking the standard procedures for clonogenic assay. Cell-FIT-HD was used to detect the spatial distribution of particle tracks within colony-initiating cells. The physical data were associated with radiation-induced foci as surrogates for DNA double-strand breaks, the hallmark of radiation-induced cell lethality. Long-term cell fate was monitored to determine the ability of cells to form colonies. We report the first successful detection of particle traversal within colony-initiating cells at subcellular resolution using Cell-FIT-HD.

High resistance to X-rays and therapeutic carbon ions in glioblastoma cells bearing dysfunctional ATM associates with intrinsic chromosomal instability

PURPOSE

To investigate chromosomal instability and radiation response mechanisms in glioblastoma cells.

METHODS AND MATERIALS

We undertook a comparative analysis of two patient-derived glioblastoma cell lines. Their resistance to low and high linear energy transfer (LET) radiation was assessed using clonogenic survival assay and their intrinsic chromosome instability status using fluorescence in situ hybridization. DNA damage was analyzed by pulsed-field gel electrophoresis and by γ-H2AX foci quantification. Expression of DNA damage response proteins was assessed by immunoblot.

RESULTS

Increased radioresistance to X-rays as well as carbon ions was observed in glioblastoma cells exhibiting high levels of naturally occurring chromosomal instability and impaired Ataxia-telangiectasia mutated (ATM) signaling, as reflected by lack of phosphorylation of ATM, CHK2 and p53 after double-strand breaks induction.

CONCLUSION

Our results indicate the existence of highly radioresistant glioblastoma cells, characterized by dysfunctional ATM signaling and high levels of intrinsic chromosomal instability.

Modelling of cell killing due to sparsely ionizing radiation in normoxic and hypoxic conditions and an extension to high LET radiation

PURPOSE

An approach for describing cell killing with sparsely ionizing radiation in normoxic and hypoxic conditions based on the initial number of randomly distributed DNA double-strand breaks (DSB) is proposed. An extension of the model to high linear energy transfer (LET) radiation is also presented.

MATERIALS AND METHODS

The model is based on the probabilities that a given DNA giant loop has one DSB or at least two DSB. A linear combination of these two classes of damage gives the mean number of lethal lesions. When coupled with a proper modelling of the spatial distribution of DSB from ion tracks, the formalism can be used to predict cell response to high LET radiation in aerobic conditions.

RESULTS

Survival data for sparsely ionizing radiation of cell lines in normoxic/hypoxic conditions were satisfactorily fitted with the proposed parametrization. It is shown that for dose ranges up to about 10 Gy, the model describes tested experimental survival data as good as the linear-quadratic model does. The high LET extension yields a reasonable agreement with data in aerobic conditions.

CONCLUSIONS

A new survival model has been introduced that is able to describe the most relevant features of cellular dose-response postulating two damage classes.

Glutathione peroxidase 1 activity dictates the sensitivity of glioblastoma cells to oxidative stress

The high intratumoral and intertumoral heterogeneity of glioblastoma (GBM) leads to resistance to different therapies, and hence, selecting an effective therapy is very challenging. We hypothesized that the antioxidant enzyme status is a significant feature of GBM heterogeneity. The most important reactive oxygen/nitrogen species (ROS/RNS) detoxification mechanisms include superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPx). Expression and activity of these enzymes and the cellular response to induced oxidative stress were systematically analyzed and compared between GBM cells and nontransformed glial cells of both human and murine origin. Regardless of cell type or species, all tested cells expressed similar amount of catalase and MnSOD. All except one, GBM cell lines exhibited a deficiency in GPx1 expression and activity. Analysis of GBM tissue sections indicated a heterogeneous profile of weak to moderate expression of GPx1 in tumor cells. GPx1 deficiency led to an accumulation of ROS/RNS and subsequent death of GBM cells after induction of oxidative stress. Astrocytes, microglia/macrophages, and glioma stem cell lines expressed active GPx1 and resisted ROS/RNS-mediated cell death. Pharmacological inhibition or siRNA silencing of GPx1 partially reverted this resistance in astrocytes, indicating the contribution of various antioxidant molecules besides GPx1. The GPx1-expressing GBM cell line on the contrary, became extremely sensitive to oxidative stress after GPx1 inhibition. Altogether, these results highlight GPx1 as a crucial element over other antioxidant enzymes for oxidative stress regulation in GBM cells. Mapping the antioxidant enzyme status of GBM may prove to be a useful tool for personalized ROS/RNS inducing therapies.

TNF-alpha- and TRAIL-resistant glioma cells undergo autophagy-dependent cell death induced by activated microglia

The role of microglia, the brain resident macrophages, in glioma biology is still ill-defined. Despite their cytotoxic potential, these cells that significantly infiltrate the tumor mass seem to support tumor growth rather than tumor eradication. A proper activation of microglia anti-tumor activities within the tumor may provide a valuable additional arm of defense to immunotherapies against brain tumors. We herewith report a detailed characterization of (lipopolysaccharide and interferon-gamma)-induced anti-tumor activities of mouse primary microglia towards two TNF-alpha and TRAIL resistant glioma cell lines, in cell monolayer or spheroid cultures and in collagen-embedded tumor explants. Irrespective of the mouse strain, stimulated microglia secreted proteic factors that decreased proliferation and migration of these glioma cells and efficiently killed them. Death occurred specifically in glioma cells as demonstrated by the lack of toxicity of microglia supernatant towards primary cultures of astrocytes or neurons. Cell death was characterized by the early accumulation of acidic vesicles, phosphatidylserine exposure, appearance of double-membrane cytoplasmic vesicles, extensive zeiosis and a very late loss of DNA in cells that had lost membrane integrity. Inhibition of autophagosome formation efficiently protected glioma cells from death whereas caspase inhibition could only prevent DNA loss but not cytotoxicity. Death however, resulted from a blockade by microglia supernatant of the basal autophagic flux present in the glioma cells. These observations demonstrate that glioma cells resistant to apoptotic death ligands could be successfully and specifically killed through autophagy-dependent death induced by appropriately activated microglia.