GBM radiosensitizers: dead in the water…or just the beginning?

Factors Influencing the Central Nervous System (CNS) Distribution of the Ataxia Telangiectasia Mutated and Rad3-Related Inhibitor Elimusertib (BAY1895344): Implications for the Treatment of CNS Tumors

Glioblastoma (GBM) is a disease of the whole brain, with infiltrative tumor cells protected by an intact blood-brain barrier (BBB). GBM has a poor prognosis despite aggressive treatment, in part due to the lack of adequate drug permeability at the BBB. Standard of care GBM therapies include radiation and cytotoxic chemotherapy that lead to DNA damage. Subsequent activation of DNA damage response (DDR) pathways can induce resistance. Various DDR inhibitors, targeting the key regulators of these pathways such as ataxia telangiectasia mutated and Rad3-related (ATR), are being explored as radio- and chemosensitizers. Elimusertib, a novel ATR kinase inhibitor, can prevent repair of damaged DNA, increasing efficacy of DNA-damaging cytotoxic therapies. Robust synergy was observed in vitro when elimusertib was combined with the DNA-damaging agent temozolomide; however, we did not observe improvement with this combination in in vivo efficacy studies in GBM orthotopic tumor-bearing mice. This in vitro-in vivo disconnect was explored to understand factors influencing central nervous system (CNS) distribution of elimusertib and reasons for lack of efficacy. We observed that elimusertib is rapidly cleared from systemic circulation in mice and would not maintain adequate exposure in the CNS for efficacious combination therapy with temozolomide. CNS distribution of elimusertib is partially limited by P-glycoprotein efflux at the BBB, and high binding to CNS tissues leads to low levels of pharmacologically active (unbound) drug in the brain. Acknowledging the potential for interspecies differences in pharmacokinetics, these data suggest that clinical translation of elimusertib in combination with temozolomide for treatment of GBM may be limited. SIGNIFICANCE STATEMENT: This study examined the disconnect between the in vitro synergy and in vivo efficacy of elimusertib/temozolomide combination therapy by exploring systemic and central nervous system (CNS) distributional pharmacokinetics. Results indicate that the lack of improvement in in vivo efficacy in glioblastoma (GBM) patient-derived xenograft (PDX) models could be attributed to inadequate exposure of pharmacologically active drug concentrations in the CNS. These observations can guide further exploration of elimusertib for the treatment of GBM or other CNS tumors.

Preclinical Systemic Pharmacokinetics, Dose Proportionality, and Central Nervous System Distribution of the ATM Inhibitor WSD0628, a Novel Radiosensitizer for the Treatment of Brain Tumors

Radiation therapy, a standard treatment option for many cancer patients, induces DNA double-strand breaks (DSBs), leading to cell death. Ataxia telangiectasia mutated (ATM) kinase is a key regulator of DSB repair, and ATM inhibitors are being explored as radiosensitizers for various tumors, including primary and metastatic brain tumors. Efficacy of radiosensitizers for brain tumors may be influenced by a lack of effective drug delivery across the blood-brain barrier. The objective of this study was to evaluate the systemic pharmacokinetics and mechanisms that influence the central nervous system (CNS) distribution of WSD0628, a novel and potent ATM inhibitor, in the mouse. Further, we have used these observations to form the basis of predicting effective exposures for clinical application. We observed a greater than dose proportional increase in exposure, likely due to saturation of clearance processes. Our results show that WSD0628 is orally bioavailable and CNS penetrant, with unbound partitioning in CNS (i.e., unbound tissue partition coefficient) between 0.15 and 0.3. CNS distribution is not limited by the efflux transporters P-glycoprotein and breast cancer resistant protein. WSD0628 is distributed uniformly among different brain regions. Thus, WSD0628 has favorable pharmacokinetic properties and potential for further exploration to determine the pharmacodynamics-pharmacokinetics efficacy relationship in CNS tumors. This approach will provide critical insights for the clinical translation of WSD0628 for the treatment of primary and secondary brain tumors. SIGNIFICANCE STATEMENT: This study evaluates the preclinical systemic pharmacokinetics, dose proportionality, and mechanisms influencing CNS distribution of WSD0628, a novel ATM inhibitor for the treatment of brain tumors. Results indicate that WSD0628 is orally bioavailable and CNS penetrant without efflux transporter liability. We also observed a greater than dose proportional increase in exposure in both the plasma and brain. These favorable pharmacokinetic properties indicate WSD0628 has potential for further exploration for use as a radiosensitizer in the treatment of brain tumors.

Knockdown of ATRX enhances radiosensitivity in glioblastoma

BACKGROUND

Glioblastoma are highly malignant type of primary brain tumors. Treatment for glioblastoma multiforme (GBM) generally involves surgery combined with chemotherapy and radiotherapy. However, the development of tumoral chemo- and radioresistance induces complexities in clinical practice. Multiple signaling pathways are known to be involved in radiation-induced cell survival. However, the role of alpha-thalassemia X-linked mutant retardation syndrome (ATRX), a chromatin remodeling protein, in GBM radioresistance remains unclear.

METHODS

In the present study, the ATRX mutation rate in patients with glioma was obtained from The Cancer Genome Atlas, while its expression analyzed using bioinformatics. Datasets were also obtained from the Gene Expression Omnibus, and ATRX expression levels following irradiation of GBM were determined. The effects of ATRX on radiosensitivity were investigated using a knockdown assays.

RESULTS

The present study demonstrated that the ATRX mutation rate in patients with GBM was significantly lower than that in patients with low-grade glioma, and that patients harboring an ATRX mutation exhibited a prolonged survival, compared with to those harboring the wild-type gene. Single-cell RNA sequencing demonstrated that ATRX counts increased 2 days after irradiation, with ATRX expression levels also increasing in U-251MG radioresistant cells. Moreover, the results of in vitro irradiation assays revealed that ATRX expression was increased in U-251MG cells, while ATRX knockdown was associated with increased levels of radiosensitivity.

CONCLUSIONS

High ATRX expression levels in primary GBM may contribute to high levels of radioresistance. Thus ATRX is a potential target for overcoming the radioresistance in GBM.

Glioblastoma Therapy: Past, Present and Future

Glioblastoma (GB) stands out as the most prevalent and lethal form of brain cancer. Although great efforts have been made by clinicians and researchers, no significant improvement in survival has been achieved since the Stupp protocol became the standard of care (SOC) in 2005. Despite multimodality treatments, recurrence is almost universal with survival rates under 2 years after diagnosis. Here, we discuss the recent progress in our understanding of GB pathophysiology, in particular, the importance of glioma stem cells (GSCs), the tumor microenvironment conditions, and epigenetic mechanisms involved in GB growth, aggressiveness and recurrence. The discussion on therapeutic strategies first covers the SOC treatment and targeted therapies that have been shown to interfere with different signaling pathways (pRB/CDK4/RB1/P16ink4, TP53/MDM2/P14arf, PI3k/Akt-PTEN, RAS/RAF/MEK, PARP) involved in GB tumorigenesis, pathophysiology, and treatment resistance acquisition. Below, we analyze several immunotherapeutic approaches (i.e., checkpoint inhibitors, vaccines, CAR-modified NK or T cells, oncolytic virotherapy) that have been used in an attempt to enhance the immune response against GB, and thereby avoid recidivism or increase survival of GB patients. Finally, we present treatment attempts made using nanotherapies (nanometric structures having active anti-GB agents such as antibodies, chemotherapeutic/anti-angiogenic drugs or sensitizers, radionuclides, and molecules that target GB cellular receptors or open the blood-brain barrier) and non-ionizing energies (laser interstitial thermal therapy, high/low intensity focused ultrasounds, photodynamic/sonodynamic therapies and electroporation). The aim of this review is to discuss the advances and limitations of the current therapies and to present novel approaches that are under development or following clinical trials.

How Much is Enough? Impact of Efflux Transporters on Drug delivery Leading to Efficacy in the Treatment of Brain Tumors

The lack of effective chemotherapeutic agents for the treatment of brain tumors is a serious unmet medical need. This can be attributed, in part, to inadequate delivery through the blood-brain barrier (BBB) and the tumor-cell barrier, both of which have active efflux transporters that can restrict the transport of many potentially effective agents for both primary and metastatic brain tumors. This review briefly summarizes the components and function of the normal BBB with respect to drug penetration into the brain and the alterations in the BBB due to brain tumor that could influence drug delivery. Depending on what is rate-limiting a compound's distribution, the limited permeability across the BBB and the subsequent delivery into the tumor cell can be greatly influenced by efflux transporters and these are discussed in some detail. Given these complexities, it is necessary to quantify the extent of brain distribution of the active (unbound) drug to compare across compounds and to inform potential for use against brain tumors. In this regard, the metric, Kp,uu, a brain-to-plasma unbound partition coefficient, is examined and its current use is discussed. However, the extent of active drug delivery is not the only determinant of effective therapy. In addition to Kp,uu, drug potency is an important parameter that should be considered alongside drug delivery in drug discovery and development processes. In other words, to answer the question - How much is enough? - one must consider how much can be delivered with how much needs to be delivered.

Antibody Delivery into the Brain by Radiosensitizer Nanoparticles for Targeted Glioblastoma Therapy

Background

Glioblastoma is the most lethal primary brain malignancy in adults. Standard of care treatment, consisting of temozolomide (TMZ) and adjuvant radiotherapy (RT), mostly does not prevent local recurrence. The inability of drugs to enter the brain, in particular antibody-based drugs and radiosensitizers, is a crucial limitation to effective glioblastoma therapy.

Methods

Here, we developed a combined strategy using radiosensitizer gold nanoparticles coated with insulin to cross the blood-brain barrier and shuttle tumor-targeting antibodies (cetuximab) into the brain.

Results

Following intravenous injection to an orthotopic glioblastoma mouse model, the nanoparticles specifically accumulated within the tumor. Combining targeted nanoparticle injection with TMZ and RT standard of care significantly inhibited tumor growth and extended survival, as compared to standard of care alone. Histological analysis of tumors showed that the combined treatment eradicated tumor cells, and decreased tumor vascularization, proliferation, and repair.

Conclusions

Our findings demonstrate radiosensitizer nanoparticles that effectively deliver antibodies into the brain, target the tumor, and effectively improve standard of care treatment outcome in glioblastoma.

Targeting the DNA damage response in pediatric malignancies

INTRODUCTION

High levels of DNA damage and mutations in DNA damage response genes creates a high reliance on DNA damage repair in various tumors. This creates a vulnerability for new cancer therapies. Although there is extensive data for the use of these agents in adult tumors, the evaluation of these compounds in the pediatric population remains in the early stages.

AREAS COVERED

In this review, we discuss the role of the DNA damage response as a therapeutic vulnerability in pediatric malignancies, provide a summary of clinical data for the use of DNA damage response inhibitors in cancer, and review how these compounds can be extended to the pediatric population.

EXPERT OPINION

A number of pediatric cancers rely on robust DNA damage repair to maintain cell viability. This provides a therapeutic vulnerability in cancer cells resistant to other traditional therapies. Unfortunately, although clinical evaluation of inhibitors of various components of the DNA damage response has been done in adults, pediatric data remains limited. Further studies are needed to evaluate the efficacy of these compounds in the pediatric population.

Multimodal targeting of glioma with functionalized nanoparticles

The most common and aggressive primitive intracranial tumor of the central nervous system is the glioma. The blood–brain barrier (BBB) has proven to be a significant obstacle to the effective treatment of glioma. To effectively treat glioma, different ways have been used to cross the BBB to deliver drugs to the brain. Drug delivery through nanocarriers proves to be an effective and non-invasive technique for the treatment of glioma and has great potential in the treatment of glioma. In this review, we will provide an overview of nanocarrier-mediated drug delivery and related glioma therapy. Nanocarrier-mediated drug delivery techniques to cross the BBB (liposomes, micelles, inorganic systems, polymeric nanoparticles, nanogel system, and biomimetic nanoparticles) are explored. Finally, the use of nanotherapeutic approaches in the treatment of glioblastoma including chemotherapy, radiotherapy, photothermal therapy, gene therapy, glioma genome editing, immunotherapy, chimeric antigen receptor (CAR) T-cells, immune checkpoint modulators, immune photothermal therapy, vaccine-based immunotherapy, and combination therapy is summarized. Furthermore, this article offers various views on the clinical applicability of nanomedicine.

Radiotherapy-drug combinations in the treatment of glioblastoma: a brief review

Glioblastoma (GBM) accounts for over 50% of gliomas and carries the worst prognosis of all solid tumors. Owing to the limited local control afforded by surgery alone, efficacious adjuvant treatments such as radiotherapy (RT) and chemotherapy are fundamental in achieving durable disease control. The best clinical outcomes are achieved with tri-modality treatment consisting of surgery, RT and systemic therapy. While RT-chemotherapy combination regimens are well established in oncology, this approach was largely unsuccessful in GBM until the introduction of temozolomide. The success of this combination has stimulated the search for other candidate drugs for concomitant use with RT in GBM. This review seeks to collate the current evidence for these agents and synthesize possible future directions for the field.

DDRugging glioblastoma: understanding and targeting the DNA damage response to improve future therapies

Glioblastoma is the most frequently diagnosed type of primary brain tumour in adults. These aggressive tumours are characterised by inherent treatment resistance and disease progression, contributing to ~ 190 000 brain tumour-related deaths globally each year. Current therapeutic interventions consist of surgical resection followed by radiotherapy and temozolomide chemotherapy, but average survival is typically around 1 year, with < 10% of patients surviving more than 5 years. Recently, a fourth treatment modality of intermediate-frequency low-intensity electric fields [called tumour-treating fields (TTFields)] was clinically approved for glioblastoma in some countries after it was found to increase median overall survival rates by ~ 5 months in a phase III randomised clinical trial. However, beyond these treatments, attempts to establish more effective therapies have yielded little improvement in survival for patients over the last 50 years. This is in contrast to many other types of cancer and highlights glioblastoma as a recognised tumour of unmet clinical need. Previous work has revealed that glioblastomas contain stem cell-like subpopulations that exhibit heightened expression of DNA damage response (DDR) factors, contributing to therapy resistance and disease relapse. Given that radiotherapy, chemotherapy and TTFields-based therapies all impact DDR mechanisms, this Review will focus on our current knowledge of the role of the DDR in glioblastoma biology and treatment. We also discuss the potential of effective multimodal targeting of the DDR combined with standard-of-care therapies, as well as emerging therapeutic targets, in providing much-needed improvements in survival rates for patients.

Brain Distribution of Berzosertib: An Ataxia Telangiectasia and Rad3-Related Protein Inhibitor for the Treatment of Glioblastoma

The effective treatment of brain tumors is a considerable challenge in part because of the presence of the blood-brain barrier (BBB) that limits drug delivery. Glioblastoma multiforme (GBM) is an aggressive and infiltrative primary brain tumor with an extremely poor prognosis after standard-of-care therapy with surgery, radiotherapy (RT), and chemotherapy. DNA damage response (DDR) pathways play a critical role in DNA repair in cancer cells, and inhibition of these pathways can potentially augment RT and chemotherapy tumor cell toxicity. The ataxia telangiectasia and Rad3-related protein (ATR) kinase is a key regulator of the DDR network and is potently and selectively inhibited by the ATR inhibitor berzosertib. Although in vitro studies demonstrate a synergistic effect of berzosertib in combination with temozolomide, in vivo efficacy studies have yet to recapitulate this observation using intracranial tumor models. In the current study, we demonstrate that delivery of berzosertib to the brain is restricted by efflux at the BBB. Berzosertib has a high binding affinity to brain tissue compared with plasma, thereby leading to low free drug concentrations in the brain. Berzosertib distribution is heterogenous within the tumor, wherein concentrations are substantially lower in normal brain and invasive tumor rim (wherein the BBB is intact) when compared with those in the tumor core (wherein the BBB is leaky). These results demonstrate that high tissue binding and limited and heterogenous brain distribution of berzosertib may be important factors that influence the efficacy of berzosertib therapy in GBM. SIGNIFICANCE STATEMENT: This study examined the brain delivery and efficacy of berzosertib in patient-derived xenograft models of glioblastoma multiforme (GBM). Berzosertib is actively effluxed at the blood-brain barrier and is highly bound to brain tissue, leading to low free drug concentrations in the brain. Berzosertib is heterogeneously distributed into different regions of the brain and tumor and, in this study, was not efficacious in vivo when combined with temozolomide. These factors inform the future clinical utility of berzosertib for GBM.

SOX transcription factors and glioma stem cells: Choosing between stemness and differentiation

Glioblastoma (GBM) is the most common, most aggressive and deadliest brain tumor. Recently, remarkable progress has been made towards understanding the cellular and molecular biology of gliomas. GBM tumor initiation, progression and relapse as well as resistance to treatments are associated with glioma stem cells (GSCs). GSCs exhibit a high proliferation rate and self-renewal capacity and the ability to differentiate into diverse cell types, generating a range of distinct cell types within the tumor, leading to cellular heterogeneity. GBM tumors may contain different subsets of GSCs, and some of them may adopt a quiescent state that protects them against chemotherapy and radiotherapy. GSCs enriched in recurrent gliomas acquire more aggressive and therapy-resistant properties, making them more malignant, able to rapidly spread. The impact of SOX transcription factors (TFs) on brain tumors has been extensively studied in the last decade. Almost all SOX genes are expressed in GBM, and their expression levels are associated with patient prognosis and survival. Numerous SOX TFs are involved in the maintenance of the stemness of GSCs or play a role in the initiation of GSC differentiation. The fine-tuning of SOX gene expression levels controls the balance between cell stemness and differentiation. Therefore, innovative therapies targeting SOX TFs are emerging as promising tools for combatting GBM. Combatting GBM has been a demanding and challenging goal for decades. The current therapeutic strategies have not yet provided a cure for GBM and have only resulted in a slight improvement in patient survival. Novel approaches will require the fine adjustment of multimodal therapeutic strategies that simultaneously target numerous hallmarks of cancer cells to win the battle against GBM.

Targeting the Regulatory Subunit R2Alpha of Protein Kinase A in Human Glioblastoma through shRNA-Expressing Lentiviral Vectors

Glioblastoma is the most malignant and most common form of brain tumor, still today associated with a poor 14-months median survival from diagnosis. Protein kinase A, particularly its regulatory subunit R2Alpha, presents a typical intracellular distribution in glioblastoma cells compared to the healthy brain parenchyma and this peculiarity might be exploited in a therapeutic setting. In the present study, a third-generation lentiviral system for delivery of shRNA targeting the regulatory subunit R2Alpha of protein kinase A was developed. Generated lentiviral vectors are able to induce an efficient and stable downregulation of R2Alpha in different cellular models, including non-stem and stem-like glioblastoma cells. In addition, our data suggest a potential correlation between silencing of the regulatory subunit of protein kinase A and reduced viability of tumor cells, apparently due to a reduction in replication rate. Thus, our findings support the role of protein kinase A as a promising target for novel anti-glioma therapies.

The Role of 2-Oxoglutarate Dependent Dioxygenases in Gliomas and Glioblastomas: A Review of Epigenetic Reprogramming and Hypoxic Response

Gliomas are a heterogeneous group of cancers that predominantly arise from glial cells in the brain, but may also arise from neural stem cells, encompassing low-grade glioma and high-grade glioblastoma. Whereas better diagnosis and new treatments have improved patient survival for many cancers, glioblastomas remain challenging with a highly unfavorable prognosis. This review discusses a super-family of enzymes, the 2-oxoglutarate dependent dioxygenase enzymes (2-OGDD) that control numerous processes including epigenetic modifications and oxygen sensing, and considers their many roles in the pathology of gliomas. We specifically describe in more detail the DNA and histone demethylases, and the hypoxia-inducible factor hydroxylases in the context of glioma, and discuss the substrate and cofactor requirements of the 2-OGDD enzymes. Better understanding of how these enzymes contribute to gliomas could lead to the development of new treatment strategies.

The Role of Hypoxia in Glioblastoma Radiotherapy Resistance

Glioblastoma (GB) (grade IV astrocytoma) is the most malignant type of primary brain tumor with a 16 months median survival time following diagnosis. Despite increasing attention regarding the development of targeted therapies for GB that resulted in around 450 clinical trials currently undergoing, radiotherapy still remains the most clinically effective treatment for these patients. Nevertheless, radiotherapy resistance (radioresistance) is commonly observed in GB patients leading to tumor recurrence and eventually patient death. It is therefore essential to unravel the molecular mechanisms underpinning GB cell radioresistance in order to develop novel strategies and combinational therapies focused on enhancing tumor cell sensitivity to radiotherapy. In this review, we present a comprehensive examination of the current literature regarding the role of hypoxia (O2 partial pressure less than 10 mmHg), a main GB microenvironmental factor, in radioresistance with the ultimate goal of identifying potential molecular markers and therapeutic targets to overcome this issue in the future.

Radiosensitizing high-Z metal nanoparticles for enhanced radiotherapy of glioblastoma multiforme

Radiotherapy is an essential step during the treatment of glioblastoma multiforme (GBM), one of the most lethal malignancies. The survival in patients with GBM was improved by the current standard of care for GBM established in 2005 but has stagnated since then. Since GBM is a radioresistant malignancy and the most of GBM recurrences occur in the radiotherapy field, increasing the effectiveness of radiotherapy using high-Z metal nanoparticles (NPs) has recently attracted attention. This review summarizes the progress in radiotherapy approaches for the current treatment of GBM, the physical and biological mechanisms of radiosensitization through high-Z metal NPs, and the results of studies on radiosensitization in the in vitro and in vivo GBM models using high-Z metal NPs to date.

N6-Isopentenyladenosine Enhances the Radiosensitivity of Glioblastoma Cells by Inhibiting the Homologous Recombination Repair Protein RAD51 Expression

Glioblastoma is among the most common malignant brain tumors and has a dismal prognosis due to the poor response to therapeutic regimens such as ionizing radiation and DNA-alkylating agents. In our study, we investigated the radiosensitizing activity of the N⁶-isopentenyladenosine (iPA), an naturally modified adenosine harboring an isopenenyl moiety, which shows antiproliferative effects on glioblastoma cell lines. We observed that co-treatment with ionizing radiation and iPA at micromolar concentration inhibited colony formation and viability of glioblastoma cell lines but not of non-malignant human cells. The combined treatment significantly attenuated the repair of radiation-induced DNA damage by inhibiting both the expression and irradiation-induced foci formation of RAD51, a key player in the homologous recombination repair process, leading to persistent DNA damage, as reflected by an increase of γ-H2AX foci. The radiosensitizing effect relied also on the inhibition of STAT5a/b activation, which is crucial for RAD51 expression, suggesting that iPA modulates the STAT5a/b-RAD51 axis following exposure to ionizing radiation. Overall, these data suggest that iPA, by acting through RAD51 inhibition at the mechanistic level, could function as a promising radiosensitizing agent and warrants further evaluation in prospective clinical trials.

Neural Stem Cells of the Subventricular Zone as the Origin of Human Glioblastoma Stem Cells. Therapeutic Implications

Human glioblastoma is the most aggressive type of primary malignant brain tumors. Standard treatment includes surgical resection followed by radiation and chemotherapy but it only provides short-term benefits and the prognosis of these brain tumors is still very poor. Glioblastomas contain a population of glioma stem cells (GSCs), with self-renewal ability, which are partly responsible for the tumor resistance to therapy and for the tumor recurrence after treatment. The human adult subventricular zone contains astrocyte-like neural stem cells (NSCs) that are probably reminiscent of the radial glia present in embryonic brain development. There are numerous molecules involved in the biology of subventricular zone NSCs that are also instrumental in glioblastoma development. These include cytoskeletal proteins, telomerase, tumor suppressor proteins, transcription factors, and growth factors. Interestingly, genes encoding these molecules are frequently mutated in glioblastoma cells. Indeed, it has been recently shown that NSCs in the subventricular zone are a potential cell of origin that contains the driver mutations of human glioblastoma. In this review we will describe common features between GSCs and subventricular zone NSCs, and we will discuss the relevance of this important finding in terms of possible future therapeutic strategies.