CD103+ regulatory T cells underlie resistance to radio-immunotherapy and impair CD8+ T cell activation in glioblastoma.

Glioblastomas are aggressive primary brain tumors with an inherent resistance to T cell-centric immunotherapy due to their low mutational burden and immunosuppressive tumor microenvironment. Here we report that fractionated radiotherapy of preclinical glioblastoma models induce a tenfold increase in T cell content. Orthogonally, spatial imaging mass cytometry shows T cell enrichment in human recurrent tumors compared with matched primary glioblastoma. In glioblastoma-bearing mice, α-PD-1 treatment applied at the peak of T cell infiltration post-radiotherapy results in a modest survival benefit compared with concurrent α-PD-1 administration. Following α-PD-1 therapy, CD103+ regulatory T cells (Tregs) with upregulated lipid metabolism accumulate in the tumor microenvironment, and restrain immune checkpoint blockade response by repressing CD8+ T cell activation. Treg targeting elicits tertiary lymphoid structure formation, enhances CD4+ and CD8+ T cell frequency and function and unleashes radio-immunotherapeutic efficacy. These results support the rational design of therapeutic regimens limiting the induction of immunosuppressive feedback pathways in the context of T cell immunotherapy in glioblastoma.

Study protocol: PreOperative Brain Irradiation in Glioblastoma (POBIG) - A phase I trial.

Background: Glioblastoma is a high-grade aggressive neoplasm whose outcomes have not changed in decades. In the current treatment pathway, tumour growth continues and remains untreated for several weeks post-diagnosis. Intensified upfront therapy could target otherwise untreated tumour cells and improve the treatment outcome. POBIG will evaluate the safety and feasibility of single-fraction preoperative radiotherapy for newly diagnosed glioblastoma, assessed by the maximum tolerated dose (MTD) and maximum tolerated irradiation volume (MTIV). Methods: POBIG is an open-label, dual-centre phase I dose and volume escalation trial that has received ethical approval. Patients with a new radiological diagnosis of glioblastoma will be screened for eligibility. This is deemed sufficient due to the high accuracy of imaging and to avoid treatment delay. Eligible patients will receive a single fraction of preoperative radiotherapy ranging from 6 to 14 Gy followed by their standard of care treatment comprising maximal safe resection and postoperative chemoradiotherapy (60 Gy/30 fr) with concurrent and adjuvant temozolomide). Preoperative radiotherapy will be directed to the part of the tumour that is highest risk for remaining as postoperative residual disease (hot spot). Part of the tumour will remain unirradiated (cold spot) and sampled separately for diagnostic purposes. Dose/volume escalation will be guided by a Continual Reassessment Method (CRM) model. Translational opportunities will be afforded through comparison of irradiated and unirradiated primary glioblastoma tissue. Discussion: POBIG will help establish the role of radiotherapy in preoperative modalities for glioblastoma. Trial registration: NCT03582514 (clinicaltrials.gov).

Visualising spatial heterogeneity in glioblastoma using imaging habitats.

Glioblastoma is a high-grade aggressive neoplasm characterised by significant intra-tumoral spatial heterogeneity. Personalising therapy for this tumour requires non-invasive tools to visualise its heterogeneity to monitor treatment response on a regional level. To date, efforts to characterise glioblastoma's imaging features and heterogeneity have focussed on individual imaging biomarkers, or high-throughput radiomic approaches that consider a vast number of imaging variables across the tumour as a whole. Habitat imaging is a novel approach to cancer imaging that identifies tumour regions or 'habitats' based on shared imaging characteristics, usually defined using multiple imaging biomarkers. Habitat imaging reflects the evolution of imaging biomarkers and offers spatially preserved assessment of tumour physiological processes such perfusion and cellularity. This allows for regional assessment of treatment response to facilitate personalised therapy. In this review, we explore different methodologies to derive imaging habitats in glioblastoma, strategies to overcome its technical challenges, contrast experiences to other cancers, and describe potential clinical applications.

Re-purposing the pro-senescence properties of doxorubicin to introduce immunotherapy in breast cancer brain metastasis.

An increasing number of breast cancer patients develop brain metastases (BM). Standard-of-care treatments are largely inefficient, and breast cancer brain metastasis (BCBM) patients are considered untreatable. Immunotherapies are not successfully employed in BCBM, in part because breast cancer is a "cold" tumor and also because the brain tissue has a unique immune landscape. Here, we generate and characterize immunocompetent models of BCBM derived from PyMT and Neu mammary tumors to test how harnessing the pro-senescence properties of doxorubicin can be used to prime the specific immune BCBM microenvironment. We reveal that BCBM senescent cells, induced by doxorubicin, trigger the recruitment of PD1-expressing T cells to the brain. Importantly, we demonstrate that induction of senescence with doxorubicin improves the efficacy of immunotherapy with anti-PD1 in BCBM in a CD8 T cell-dependent manner, thereby providing an optimized strategy to introduce immune-based treatments in this lethal disease. In addition, our BCBM models can be used for pre-clinical testing of other therapeutic strategies in the future.

Rapid early progression (REP) of glioblastoma is an independent negative prognostic factor: Results from a systematic review and meta-analysis.

Background: In patients with newly diagnosed glioblastoma, rapid early progression (REP) refers to tumor regrowth between surgery and postoperative chemoradiotherapy. This systematic review and meta-analysis appraised previously published data on REP to better characterize and understand it. Methods: Systematic searches of MEDLINE, EMBASE and the Cochrane database from inception to October 21, 2021. Studies describing the incidence of REP-tumor growth between the postoperative MRI scan and pre-radiotherapy MRI scan in newly diagnosed glioblastoma were included. The primary outcome was REP incidence. Results: From 1590 search results, 9 studies were included with 716 patients. The median age was 56.9 years (IQR 54.0-58.8 y). There was a male predominance with a median male-to-female ratio of 1.4 (IQR 1.1-1.5). The median number of days between MRI scans was 34 days (IQR 18-45 days). The mean incidence rate of REP was 45.9% (range 19.3%-72.0%) and significantly lower in studies employing functional imaging to define REP (P < .001). REP/non-REP groups were comparable with respect to age (P = .99), gender (P = .33) and time between scans (P = .81). REP was associated with shortened overall survival (HR 1.78, 95% CI 1.30-2.43, P < .001), shortened progression-free survival (HR 1.78, 95% CI 1.30-2.43, P < .001), subtotal resection (OR 6.96, 95% CI 4.51-10.73, P < .001) and IDH wild-type versus mutant tumors (OR 0.20, 95% CI 0.02-0.38, P = .03). MGMT promoter methylation was not associated with REP (OR 1.29, 95% CI 0.72-2.28, P = .39). Conclusions: REP occurs in almost half of patients with newly diagnosed glioblastoma and has a strongly negative prognostic effect. Future studies should investigate its biology and effective treatment strategies.

Protocol for live-cell imaging during Tumor Treating Fields treatment with Inovitro Live.

Tumor Treating Fields (TTFields) are an FDA-approved anticancer treatment using alternating electric fields. Here, we present a protocol to perform live-cell imaging (LCI) of cells during TTFields treatment with the Inovitro LiveTM system. The setup we describe dissipates TTFields-related heat production and can be used in conjunction with any LCI-compatible microscope setup. This approach will enable further elucidation of TTFields' mechanism of action at the molecular level and facilitate the development of promising combination strategies.

Early Therapeutic Interventions for Newly Diagnosed Glioblastoma: Rationale and Review of the Literature.

PURPOSE OF REVIEW: Glioblastoma is the commonest primary brain cancer in adults whose outcomes are amongst the worst of any cancer. The current treatment pathway comprises surgery and postoperative chemoradiotherapy though unresectable diffusely infiltrative tumour cells remain untreated for several weeks post-diagnosis. Intratumoural heterogeneity combined with increased hypoxia in the postoperative tumour microenvironment potentially decreases the efficacy of adjuvant interventions and fails to prevent early postoperative regrowth, called rapid early progression (REP). In this review, we discuss the clinical implications and biological foundations of post-surgery REP. Subsequently, clinical interventions potentially targeting this phenomenon are reviewed systematically. RECENT FINDINGS: Early interventions include early systemic chemotherapy, neoadjuvant immunotherapy, local therapies delivered during surgery (including Gliadel wafers, nanoparticles and stem cell therapy) and several radiotherapy techniques. We critically appraise and compare these strategies in terms of their efficacy, toxicity, challenges and potential to prolong survival. Finally, we discuss the most promising strategies that could benefit future glioblastoma patients. There is biological rationale to suggest that early interventions could improve the outcome of glioblastoma patients and they should be investigated in future trials.

Preoperative Stereotactic Radiosurgery for Glioblastoma.

Glioblastoma is a devastating primary brain tumor with a median overall survival of approximately 15 months despite the use of optimal modern therapy. While GBM has been studied for decades, modern therapies have allowed for a reduction in treatment-related toxicities, while the prognosis has largely been unchanged. Adjuvant stereotactic radiosurgery (SRS) was previously studied in GBM; however, the results were disappointing. SRS is a highly conformal radiation technique that permits the delivery of high doses of ionizing radiation in 1-5 sessions while largely sparing surrounding healthy tissues. Furthermore, studies have shown that the delivery of ablative doses of ionizing radiation within the central nervous system is associated with enhanced anti-tumor immunity. While SRS is commonly used in the definitive and adjuvant settings for other CNS malignancies, its role in the preoperative setting has become a topic of great interest due to the potential for reduced treatment volumes due to the treatment of an intact tumor, and a lower risk of nodular leptomeningeal disease and radiation necrosis. While early reports of SRS in the adjuvant setting for glioblastoma were disappointing, its role in the preoperative setting and its impact on the anti-tumor adaptive immune response is largely unknown. In this review, we provide an overview of GBM, discuss the potential role of preoperative SRS, and discuss the possible immunogenic effects of this therapy.

The impact of anatomical changes during photon or proton based radiation treatment on tumor dose in glioblastoma dose escalation trials.

PURPOSE/OBJECTIVE: Most dose-escalation trials in glioblastoma patients integrate the escalated dose throughout the standard course by targeting a specific subvolume. We hypothesize that anatomical changes during irradiation may affect the dose coverage of this subvolume for both proton- and photon-based radiotherapy. MATERIAL AND METHODS: For 24 glioblastoma patients a photon- and proton-based dose escalation treatment plan (of 75 Gy/30 fr) was simulated on the dedicated radiotherapy planning MRI obtained before treatment. The escalated dose was planned to cover the resection cavity and/or contrast enhancing lesion on the T1w post-gadolinium MRI sequence. To analyze the effect of anatomical changes during treatment, we evaluated on an additional MRI that was obtained during treatment the changes of the dose distribution on this specific high dose region. RESULTS: The median time between the planning MRI and additional MRI was 26 days (range 16-37 days). The median time between the planning MRI and start of radiotherapy was relatively short (7 days, range 3-11 days). In 3 patients (12.5%) changes were observed which resulted in a substantial deterioration of both the photon and proton treatment plans. All these patients underwent a subtotal resection, and a decrease in dose coverage of more than 5% and 10% was observed for the photon- and proton-based treatment plans, respectively. CONCLUSION: Our study showed that only for a limited number of patients anatomical changes during photon or proton based radiotherapy resulted in a potentially clinically relevant underdosage in the subvolume. Therefore, volume changes during treatment are unlikely to be responsible for the negative outcome of dose-escalation studies.

PSMA-11-PET/CT versus choline-PET/CT to guide stereotactic ablative radiotherapy for androgen deprivation therapy deferral in patients with oligometastatic prostate cancer.

BACKGROUND: In patients with oligometastatic recurrent prostate cancer, standard treatment is androgen deprivation therapy (ADT). However, ADT has many potential side effects that may result in impaired quality of life. Early identification to select patients suitable for stereotactic ablative radiotherapy (SABR) is of utmost importance to prevent or delay start of ADT and its side effects. Because Prostate-Specific Membrane Antigen-11-Positron Emission Tomography (PSMA-11-PET) has a higher sensitivity than choline-PET, we hypothesise that PSMA-11-PET based SABR results in longer response duration and subsequent longer delay in starting ADT than choline-PET. METHODS: Patients with oligometastatic (≤4 metastases) recurrent prostate cancer (with no local recurrence) based on PSMA-11-PET or choline-PET treated with SABR from January 2012 until December 2017 were included. Primary endpoint was ADT-free survival. Secondary endpoints were Prostate Specific Antigen (PSA) response after SABR and time to PSA rise after SABR. RESULTS: Fifty patients (n = 40 PSMA-11-PET and n = 10 choline-PET) with in total 72 lesions were included. Median follow-up was 24.3 months. PSMA-11-PET enabled eligibility of patients with lower PSA levels than choline-PET (median 1.8 versus 4.2 ng/mL, p = 0.03). The PSMA-11-PET group had a significant longer PSA response duration (median 34.0 months (95% confidence interval (CI), 16.0-52.0) versus 14.7 months (95% CI 4.7-24.7), p = 0.004) with a subsequent longer ADT-free survival (median 32.7 months (95% CI, 20.8-44.5) versus 14.9 months (95% CI, 5.7-24.1), p = 0.01). CONCLUSIONS: With PSMA-11-PET we are able to select patients with oligometastatic recurrent prostate cancer suitable for SABR in an earlier disease stage at lower PSA levels. PSMA-11-PET guided SABR resulted in a significant longer response duration and ADT-free survival compared with choline-PET and can therefore prevent or delay ADT related side effects.

Correction: Neoadjuvant olaparib targets hypoxia to improve radioresponse in a homologous recombination-proficient breast cancer model.

[This corrects the article DOI: 10.18632/oncotarget.20936.].

Radiosensitivity Is an Acquired Vulnerability of PARPi-Resistant BRCA1-Deficient Tumors.

The defect in homologous recombination (HR) found in BRCA1-associated cancers can be therapeutically exploited by treatment with DNA-damaging agents and PARP inhibitors. We and others previously reported that BRCA1-deficient tumors are initially hypersensitive to the inhibition of topoisomerase I/II and PARP, but acquire drug resistance through restoration of HR activity by the loss of end-resection antagonists of the 53BP1/RIF1/REV7/Shieldin/CST pathway. Here, we identify radiotherapy as an acquired vulnerability of 53BP1;BRCA1-deficient cells in vitro and in vivo. In contrast to the radioresistance caused by HR restoration through BRCA1 reconstitution, HR restoration by 53BP1 pathway inactivation further increases radiosensitivity. This highlights the relevance of this pathway for the repair of radiotherapy-induced damage. Moreover, our data show that BRCA1-mutated tumors that acquire drug resistance due to BRCA1-independent HR restoration can be targeted by radiotherapy. SIGNIFICANCE: These findings uncover radiosensitivity as a novel, therapeutically viable vulnerability of BRCA1-deficient mouse mammary cells that have acquired drug resistance due to the loss of the 53BP1 pathway.

Learning radiation oncology in Europe: Results of the ESTRO multidisciplinary survey.

INTRODUCTION: Radiotherapy education can be very different across Europe, despite the publication of the ESTRO core curricula in 2011. The purpose of the current study is to map the different RO European education systems, to report their perceived quality and to understand what could be improved to better teach RO. METHODS: An online survey consisting of 30 questions was sent to RO professionals under 40 years of age via email and social media. Clinicians, radiobiologists, physicists and radiation therapists (RTTs) were invited to answer questions regarding (1) demographics data, (2) duration, (3) organization, (4) content, (5) quality and potential improvements of national education programs. RESULTS: Four hundred and sixty three questionnaires were received from 34 European countries. All disciplines were represented: 45% clinicians (n = 210), 29% physicists (n = 135), 24% RTTs (n = 108) and 2% radiobiologists (n = 10). Male and female participants were well-balanced in each speciality, except for radiobiologists (80% males). Median age was 31.5 years old (range 21-40). A large range of the duration of the National RO education programs was observed: median = 9 years (range: 3-15). In half of the surveyed countries the European Credit Transfer System (ECTS), that facilitates mobility for trainees, has been implemented. Participants declared only a minority of countries have implemented the ESTRO Core Curriculum (n = 5). A quarter of participants indicated that their national education program is insufficient. CONCLUSION: This is the first study to examine the different RO education systems in Europe. Large differences in organization and duration of national education programs have been found, along with perceived quality across Europe within each speciality. These results show the necessity of a discussion on how to move forward in this diversity of education programs and the potential contribution that the ESTRO may fulfil.

Neoadjuvant olaparib targets hypoxia to improve radioresponse in a homologous recombination-proficient breast cancer model.

Clinical trials are studying the benefits of combining the PARP-1 inhibitor olaparib with chemotherapy and radiotherapy treatment in a variety of cancer increasing the therapeutic ratio for olaparib may come from its ability to modify the tumour microenvironment by targeting homologous recombination-deficient, hypoxic tumour clonogens, and/or increasing tumour-associated vasodilation to improve oxygenation. Herein, we investigated the effect of prolonged neoadjuvant exposure to olaparib on the tumor microenvironment using a genetically-engineered mouse p53-/- syngeneic breast cancer model, which is proficient in homology-directed DNA repair. We observed increased in vivo growth delay and decreased ex vivo clonogenic survival following pre-treatment with olaparib 50 mg/kg bid Olaparib for 7 days ending 48 hours prior to a radiation dose of 12Gy. This increased in vivo radioresponse was associated with a decreased hypoxic fraction. This study suggests that the radiation response in patients can be improved with limited toxicity if olaparib is given in a purely neoadjuvant setting to modify the tumor microenviroment prior to the start of the radiotherapy treatment. Consequently a significant gain can be achieved in therapeutic window and clinical studies are needed to confirm this preclinical data.

Radiosensitization by gold nanoparticles: Will they ever make it to the clinic?

The utilization of gold nanoparticles (AuNPs) as radiosensitizers has shown great promise in pre-clinical research. In the current review, the physical, chemical, and biological pathways via which AuNPs enhance the effects of radiation are presented and discussed. In particular, the impact of AuNPs on the 5 Rs in radiobiology, namely repair, reoxygenation, redistribution, repopulation, and intrinsic radiosensitivity, which determine the extent of radiation enhancement effects are elucidated. Key findings from previous studies are outlined. In addition, crucial parameters including the physicochemical properties of AuNPs, route of administration, dosing schedule of AuNPs and irradiation, as well as type of radiation therapy, are highlighted; the optimal selection and combination of these parameters enable the achievement of a greater therapeutic window for AuNP sensitized radiotherapy. Future directions are put forward as a means to provide guidelines for successful translation of AuNPs to clinical applications as radiosensitizers.

Significant Radiation Enhancement Effects by Gold Nanoparticles in Combination with Cisplatin in Triple Negative Breast Cancer Cells and Tumor Xenografts.

Gold nanoparticles (AuNPs) and cisplatin have been explored in concomitant chemoradiotherapy, wherein they elicit their effects by distinct and overlapping mechanisms. Cisplatin is one of the most frequently utilized radiosensitizers in the clinical setting; however, the therapeutic window of cisplatin-aided chemoradiotherapy is limited by its toxicity. The goal of this study was to determine whether AuNPs contribute to improving the treatment response when combined with fractionated cisplatin-based chemoradiation in both in vitro and in vivo models of triple-negative breast cancer (MDA-MB-231Luc+). Cellular-targeting AuNPs with receptor-mediated endocytosis (AuNP-RME) in vitro at a noncytotoxic concentration (0.5 mg/ml) or cisplatin at IC25 (12 µM) demonstrated dose enhancement factors (DEFs) of 1.25 and 1.14, respectively; the combination of AuNP-RME and cisplatin resulted in a significant DEF of 1.39 in vitro. Transmission electron microscopy (TEM) images showed effective cellular uptake of AuNPs at tumor sites 24 h after intratumoral infusion. Computed tomography (CT) images demonstrated that the intratumoral levels of gold remained stable up to 120 h after infusion. AuNPs (0.5 mg gold per tumor) demonstrated a radiation enhancement effect that was equivalent to three doses of cisplatin at IC25 (4 mg/kg), but did not induce intrinsic toxicity or increased radiotoxicity. Results from this study suggest that AuNPs are the true radiosensitizer in these settings. Importantly, AuNPs enhance the treatment response when combined with cisplatin-based fractionated chemoradiation. This combination of AuNPs and cisplatin provides a promising approach to improving the therapeutic ratio of fractionated radiotherapy.

Significant tumor shift in patients treated with stereotactic radiosurgery for brain metastasis.

INTRODUCTION: Linac-based stereotactic radiosurgery (SRS) for brain metastases may be influenced by the time interval between treatment preparation and delivery, related to risk of anatomical changes. We studied tumor position shifts and its relations to peritumoral volume edema changes over time, as seen on MRI. METHODS: Twenty-six patients who underwent SRS for brain metastases in our institution were included. We evaluated the occurrence of a tumor shift between the diagnostic MRI and radiotherapy planning MRI. For 42 brain metastases the tumor and peritumoral edema were delineated on the contrast enhanced T1weighted and FLAIR images of both the diagnostic MRI and planning MRI examinations. Centre of Mass (CoM) shifts and tumor borders were evaluated. We evaluated the influence of steroids on peritumoral edema and tumor volume and the correlation with CoM and tumor border changes. RESULTS: The median values of the CoM shifts and of the maximum distances between the tumor borders obtained from the diagnostic MRI and radiotherapy planning MRI were 1.3 mm (maximum shift of 5.0 mm) and 1.9 mm (maximum distance of 7.4 mm), respectively. We found significant correlations between the absolute change in edema volume and the tumor shift of the CoM (p < 0.001) and tumor border (p = 0.040). Patients who received steroids did not only had a decrease in peritumoral edema, but also had a median decrease in tumor volume of 0.02 cc while patients who did not receive steroids had a median increase of 0.06 cc in tumor volume (p = 0.035). CONCLUSION: Our results show that large tumor shifts of brain metastases can occur over time. Because shifts may have a significant impact on the local dose coverage, we recommend minimizing the time between treatment preparation and delivery for Linac based SRS.

Irradiation of localized squamous cell carcinoma of the nasal vestibule.

BACKGROUND: The purpose of this study was to evaluate the long-term results of primary radiotherapy treatment for squamous cell carcinoma (SCC) of the nasal vestibule. METHODS: Eighty-one patients were treated with external beam radiotherapy (EBRT) and/or interstitial radiotherapy (IRT) for a primary, localized, Wang classified SCC of the nasal vestibule. RESULTS: Median follow-up was 38 months. T1 tumors were treated with IRT: we observed 1 local failure (at 13 months) among 48 patients (5-year local control rate of 97%). Most T2 tumors (20 of 26) were treated with EBRT. There were 8 local recurrences among 26 patients (5-year local control rate of 68%). For the T3 tumors (n = 7; all treated with EBRT), we observed local recurrence in 2 patients (5-year local control rate of 53%). The late-term side effects were relatively mild. CONCLUSION: Local primary radiotherapy (either IRT for T1 or EBRT for T2/3) is an adequate treatment for SCC of the nasal vestibule with little late sequelae. © 2015 Wiley Periodicals, Inc. Head Neck 38: E1870-E1875, 2016.

Primary tumour standardised uptake value is prognostic in nonsmall cell lung cancer: a multivariate pooled analysis of individual data.

(18)F-fluoro-2-deoxy-d-glucose positron emission tomography (PET) complements conventional imaging for diagnosing and staging lung cancer. Two literature-based meta-analyses suggest that maximum standardised uptake value (SUVmax) on PET has univariate prognostic value in nonsmall cell lung cancer (NSCLC). We analysed individual data pooled from 12 studies to assess the independent prognostic value of binary SUVmax for overall survival.After searching the published literature and identifying unpublished data, study coordinators were contacted and requested to provide data on individual patients. Cox regression models stratified for study were used.Data were collected for 1526 patients (median age 64 years, 60% male, 34% squamous cell carcinoma, 47% adenocarcinoma, 58% stage I-II). The combined univariate hazard ratio for SUVmax was 1.43 (95% CI 1.22-1.66) and nearly identical if the SUV threshold was calculated stratifying for histology. Multivariate analysis of patients with stage I-III disease identified age, stage, tumour size and receipt of surgery as independent prognostic factors; adding SUV (HR 1.58, 95% CI 1.27-1.96) improved the model significantly. The only detected interaction was between SUV and stage IV disease.SUV seems to have independent prognostic value in stage I-III NSCLC, for squamous cell carcinoma and for adenocarcinoma.

Targeted radiosensitization by the Chk1 inhibitor SAR-020106.

PURPOSE: To explore the activity of a potent Chk1 inhibitor (SAR-020106) in combination with radiation. METHODS AND MATERIALS: Colony and mechanistic in vitro assays and a xenograft in vivo model. RESULTS: SAR-020106 suppressed-radiation-induced G2/M arrest and reduced clonogenic survival only in p53-deficient tumor cells. SAR-020106 promoted mitotic entry following irradiation in all cell lines, but p53-deficient cells were likely to undergo apoptosis or become aneuploid, while p53 wild-type cells underwent a postmitotic G1 arrest followed by subsequent normal cell cycle re-entry. Following combined treatment with SAR-020106 and radiation, homologous-recombination-mediated DNA damage repair was inhibited in all cell lines. A significant increase in the number of pan-γH2AX-staining apoptotic cells was observed only in p53-deficient cell lines. Efficacy was confirmed in vivo in a clinically relevant human head-and-neck cell carcinoma xenograft model. CONCLUSION: The Chk1 inhibitor SAR-020106 is a potent radiosensitizer in tumor cell lines defective in p53 signaling.