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Aggressive breast cancers often fail or acquire resistance to radiotherapy. To develop new strategies to improve the outcome of aggressive breast cancer patients, we studied how PARP inhibition radiosensitizes breast cancer models to proton therapy, which is a radiotherapy modality that generates more DNA damage in the tumor than standard radiotherapy using photons. Two human BRCA1-mutated breast cancer cell lines and their isogenic BRCA1-recovered pairs were treated with a PARP inhibitor and irradiated with photons or protons. Protons (9.9 and 3.85 keV/µm) induced higher cell kill independent of BRCA1 status. PARP inhibition amplified the cell kill effect to both photons and protons (9.9 and 3.85 keV/µm) independent of BRCA1 status. Numbers of γH2AX foci, micronuclei, and cGAS-positive micronuclei were significantly higher in BRCA1-mutated cells. Cell cycle distribution and stress-induced senescence were not affected by PARP inhibition in our cell lines. In vivo, the combination of protons (3.99 keV/µm) and PARP inhibition induced the greatest tumor growth delay and the highest survival. We found that PARP inhibition increases radiosensitization independent of BRCA1 status for both protons and photons. The combination of protons and PARP inhibition was the most effective in decreasing clonogenic cell survival, increasing DNA damage, and delaying tumor growth., Competing Interests: Declarations. Competing interests: GOS has or had research agreements with Alpha Tau Medical, Artios Pharma, Convergent Radiotherapy and Radiosurgery, TAE Life Sciences and grant funding from DoD and NIH. SFS had contracted research agreements with Alpha Tau Medical, Artios Pharma, TAE Life Sciences, ExactSciences and grant funding from the Emerson Collective Foundation and NIH. SJB had grant funding with the American Association of Physicists in Medicine. MBK, SJB, EM, DBF, DKJM, BXT, IQ, RK, MM, PCM declare no competing interests., (© 2024. The Author(s).)

Objective. To develop an empirical model to predict carbon ion (C-ion) relative biological effectiveness (RBE). Approach. We used published cell survival data comprising 360 cell line/energy combinations to characterize the linear energy transfer (LET) dependence of cell radiosensitivity parameters describing the dose required to achieve a given survival level, e.g. 5% (D 5% ), which are linearly correlated between photon and C-ion radiations. Based on the LET response of the metrics D 5% and D 37% , we constructed a model containing four free parameters that predicts cells' linear quadratic model (LQM) survival curve parameters for C-ions, α C and β C , from the reference LQM parameters for photons, α X and β X , for a given C-ion LET value. We fit our model's free parameters to the training dataset and assessed its accuracy via leave-one out cross-validation. We further compared our model to the local effect model (LEM) and the microdosimetric kinetic model (MKM) by comparing its predictions against published predictions made with those models for clinically relevant LET values in the range of 23-107 keV μ m -1 . Main Results. Our model predicted C-ion RBE within ±7%-15% depending on cell line and dose which was comparable to LEM and MKM for the same conditions. Significance. Our model offers comparable accuracy to the LEM or MKM but requires fewer input parameters and is less computationally expensive and whose implementation is so simple we provide it coded into a spreadsheet. Thus, our model can serve as a pragmatic alternative to these mechanistic models in cases where cell-specific input parameters cannot be obtained, the models cannot be implemented, or for which their computational efficiency is paramount., (Creative Commons Attribution license.)

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.

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., (©2022 by Radiation Research Society. All rights of reproduction in any form reserved.)

Background: Proton relative biological effectiveness (RBE) is known to depend on physical factors of the proton beam, such as its linear energy transfer (LET), as well as on cell-line specific biological factors, such as their ability to repair DNA damage. However, in a clinical setting, proton RBE is still considered to have a fixed value of 1.1 despite the existence of several empirical models that can predict proton RBE based on how a cell's survival curve (linear-quadratic model [LQM]) parameters α and β vary with the LET of the proton beam. Part of the hesitation to incorporate variable RBE models in the clinic is due to the great noise in the biological datasets on which these models are trained, often making it unclear which model, if any, provides sufficiently accurate RBE predictions to warrant a departure from RBE = 1.1., Purpose: Here, we introduce a novel model of proton RBE based on how a cell's intrinsic radiosensitivity varies with LET, rather than its LQM parameters., Methods and Materials: We performed clonogenic cell survival assays for eight cell lines exposed to 6 MV x-rays and 1.2, 2.6, or 9.9 keV/µm protons, and combined our measurements with published survival data (n = 397 total cell line/LET combinations). We characterized how radiosensitivity metrics of the form D SF% , (the dose required to achieve survival fraction [SF], e.g., D 10% ) varied with proton LET, and calculated the Bayesian information criteria associated with different LET-dependent functions to determine which functions best described the underlying trends. This allowed us to construct a six-parameter model that predicts cells' proton survival curves based on the LET dependence of their radiosensitivity, rather than the LET dependence of the LQM parameters themselves. We compared the accuracy of our model to previously established empirical proton RBE models, and implemented our model within a clinical treatment plan evaluation workflow to demonstrate its feasibility in a clinical setting., Results: Our analyses of the trends in the data show that D SF% is linearly correlated between x-rays and protons, regardless of the choice of the survival level (e.g., D 10% , D 37% , or D 50% are similarly correlated), and that the slope and intercept of these correlations vary with proton LET. The model we constructed based on these trends predicts proton RBE within 15%-30% at the 68.3% confidence level and offers a more accurate general description of the experimental data than previously published empirical models. In the context of a clinical treatment plan, our model generally predicted higher RBE-weighted doses than the other empirical models, with RBE-weighted doses in the distal portion of the field being up to 50.7% higher than the planned RBE-weighted doses (RBE = 1.1) to the tumor., Conclusions: We established a new empirical proton RBE model that is more accurate than previous empirical models, and that predicts much higher RBE values in the distal edge of clinical proton beams., (© 2022 American Association of Physicists in Medicine.)

Purpose: We assessed whether adding sodium borocaptate (BSH) or 4-borono-l-phenylalanine (BPA) to cells irradiated with proton beams influenced the biological effectiveness of those beams against prostate cancer cells to investigate if the alpha particles generated through proton-boron nuclear reactions would be sufficient to enhance the biological effectiveness of the proton beams., Methods: We measured clonogenic survival in DU145 cells treated with 80.4-ppm BSH or 86.9-ppm BPA, or their respective vehicles, after irradiation with 6-MV X-rays, 1.2-keV/μm (low linear energy transfer [LET]) protons, or 9.9-keV/μm (high-LET) protons. We also measured γH2AX and 53BP1 foci in treated cells at 1 and 24 h after irradiation with the same conditions., Results: We found that BSH radiosensitized DU145 cells across all radiation types. However, no difference was found in relative radiosensitization, characterized by the sensitization enhancement ratio or the relative biological effectiveness, for vehicle- versus BSH-treated cells. No differences were found in numbers of γH2AX or 53BP1 foci or γH2AX/53BP1 colocalized foci for vehicle- versus BSH-treated cells across radiation types. BPA did not radiosensitize DU145 cells nor induced any significant differences when comparing vehicle- versus BPA-treated cells for clonogenic cell survival or γH2AX and 53BP1 foci or γH2AX/53BP1 colocalized foci., Conclusions: Treatment with 11 B, at concentrations of 80.4 ppm from BSH or 86.9 ppm from BPA, had no effect on the biological effectiveness of proton beams in DU145 prostate cancer cells. Our results agree with published theoretical calculations indicating that the contribution of alpha particles from such reactions to the total absorbed dose and biological effectiveness is negligible. We also found that BSH radiosensitized DU145 cells to X-rays, low-LET protons, and high-LET protons but that the radiosensitization was not related to DNA damage., (© 2022 American Association of Physicists in Medicine.)