EGFRvIII-mediated radioresistance through a strong cytoprotective response

Aptamer-functionalized quantum dots as theranostic nanotools against cancer and bacterial infections: A comprehensive overview of recent trends

Aptamers (Apts) are synthetic nucleic acid ligands that can be engineered to target various molecules, including amino acids, proteins, and pharmaceuticals. Through a series of adsorption, recovery, and amplification steps, Apts are extracted from combinatorial libraries of synthesized nucleic acids. Using aptasensors in bioanalysis and biomedicine can be improved by combining them with nanomaterials. Moreover, Apt-associated nanomaterials, including liposomes, polymeric, dendrimers, carbon nanomaterials, silica, nanorods, magnetic NPs, and quantum dots (QDs), have been widely used as promising nanotools in biomedicine. Following surface modifications and conjugation with appropriate functional groups, these nanomaterials can be successfully used in aptasensing. Advanced biological assays can use Apts immobilized on QD surfaces through physical interaction and chemical bonding. Accordingly, modern QD aptasensing platforms rely on interactions between QDs, Apts, and targets to detect them. QD-Apt conjugates can be used to directly detect prostate, ovarian, colorectal, and lung cancers or simultaneously detect biomarkers associated with these malignancies. Tenascin-C, mucin 1, prostate-specific antigen, prostate-specific membrane antigen, nucleolin, growth factors, and exosomes are among the cancer biomarkers that can be sensitively detected using such bioconjugates. Furthermore, Apt-conjugated QDs have shown great potential for controlling bacterial infections such as Bacillus thuringiensis, Pseudomonas aeruginosa, Escherichia coli, Acinetobacter baumannii, Campylobacter jejuni, Staphylococcus aureus, and Salmonella typhimurium. This comprehensive review discusses recent advancements in the design of QD-Apt bioconjugates and their applications in cancer and bacterial theranostics.

The DNA Double-Strand Break Repair in Glioma: Molecular Players and Therapeutic Strategies

Gliomas are the most frequent type of tumor in the central nervous system, which exhibit properties that make their treatment difficult, such as cellular infiltration, heterogeneity, and the presence of stem-like cells responsible for tumor recurrence. The response of this type of tumor to chemoradiotherapy is poor, possibly due to a higher repair activity of the genetic material, among other causes. The DNA double-strand breaks are an important type of lesion to the genetic material, which have the potential to trigger processes of cell death or cause gene aberrations that could promote tumorigenesis. This review describes how the different cellular elements regulate the formation of DNA double-strand breaks and their repair in gliomas, discussing the therapeutic potential of the induction of this type of lesion and the suppression of its repair as a control mechanism of brain tumorigenesis.

Radiotherapy as a tool to elicit clinically actionable signalling pathways in cancer

A variety of targeted anticancer agents have been successfully introduced into clinical practice, largely reflecting their ability to inhibit specific molecular alterations that are required for disease progression. However, not all malignant cells rely on such alterations to survive, proliferate, disseminate and/or evade anticancer immunity, implying that many tumours are intrinsically resistant to targeted therapies. Radiotherapy is well known for its ability to activate cytotoxic signalling pathways that ultimately promote the death of cancer cells, as well as numerous cytoprotective mechanisms that are elicited by cellular damage. Importantly, many cytoprotective mechanisms elicited by radiotherapy can be abrogated by targeted anticancer agents, suggesting that radiotherapy could be harnessed to enhance the clinical efficacy of these drugs. In this Review, we discuss preclinical and clinical data that introduce radiotherapy as a tool to elicit or amplify clinically actionable signalling pathways in patients with cancer.

SOCS proteins and their roles in the development of glioblastoma

Glioblastoma multiforme (GBM) is the most common type of primary brain tumor in adults. GBM is characterized by a high degree of malignancy and aggressiveness, as well as high morbidity and mortality rates. GBM is currently treatable via surgical resection, chemotherapy and radiotherapy, but the prognosis of patients with GBM is poor. The suppressor of cytokine signaling (SOCS) protein family comprises eight members, including SOCS1-SOCS7 and cytokine-inducible SH2-containing protein. SOCS proteins regulate the biogenesis of GBM via the JAK/STAT and NF-κB signaling pathways. Driven by NF-κB, the expression of SOCS proteins can serve as a negative regulator of the JAK/STAT signaling pathway and exerts a potential inhibitory effect on GBM. In GBM, E3 ubiquitin ligase is involved in the regulation of cellular functions, such as the receptor tyrosine kinase (RTK) survival signal, in which SOCS proteins negatively regulate RTK signaling, and kinase overexpression or mutation can lead to the development of malignancies. Moreover, SOCS proteins affect the proliferation and differentiation of GBM cells by regulating the tumor microenvironment. SOCS proteins also serve specific roles in GBM of different grades and different isocitrate dehydrogenase mutation status. The aim of the present review was to describe the biogenesis and function of the SOCS protein family, the roles of SOCS proteins in the microenvironment of GBM, as well as the role of this protein family and E3 ubiquitin ligases in GBM. Furthermore, the role of SOCS proteins as diagnostic and prognostic markers in GBM and their potential role as GBM therapeutics were explored.

Combining radiotherapy with targeted therapies in non-small cell lung cancer: focus on anti-EGFR, anti-ALK and anti-angiogenic agents

The combination of radiotherapy (RT) with targeted agents in non-small cell lung cancer (NSCLC) has been expected to improve the therapeutic ratio and tumor control. The EGFR blockade enhances the antitumor effect of RT. The ALK inhibition elicits anti-proliferative, pro-apoptotic and antiangiogenic effects in ALK-positive NSCLC cell lines, enhanced by the exposure to RT. The antiangiogenic agents normalize pathological tumor vessels, thus decrease tumor cell hypoxia and improve radiosensitivity. To date, however, none of the targeted agents combined with RT has shown proven clinical benefit over standard chemoradiation (CRT) in locally advanced NSCLC. The risk of potential excessive toxicity related to the therapeutic combination of RT and targeted agents cannot be ignored. Well-designed clinical trials may allow development of more effective combination strategies. Another potential application of combined RT and targeted therapies in oncogene-driven NSCLC is metastatic oligoprogressive or oligopersistent disease. The use of RT in oligoprogressive oncogene-driven NSCLC, while continuing first line targeted therapy, can potentially eradicate resistant cell clones and provide survival benefit. Likewise, the consolidation of oligopersistent foci (molecularly resistant to first line targeted therapy) may potentially interfere with the natural course of the disease by avoiding or delaying progression. We discuss here the molecular and radiobiological mechanisms of combining RT and targeted agents, and summarize current clinical experience.

Inhibition of the Spectraplakin Protein Microtubule Actin Crosslinking Factor 1 Sensitizes Glioblastomas to Radiation

Background

Microtubule actin crosslinking factor 1 (MACF1) is a spectraplakin cytoskeletal crosslinking protein whose function and role in cancer biology has lacked investigation. Recent studies have identified MACF1 as a novel target in glioblastomas expressed in tissue from tumor patient explants but not normal brain tissue and when silenced has an antitumorigenic impact on these tumors. Radiation as a single agent therapy to treat glioblastomas has been used for decades and has done little to improve survival of individuals diagnosed with this disease. However, contemporary clinical radiotherapy protocols have provided evidence that combinatorial radiotherapy approaches confer a therapeutic benefit in glioblastoma patients. In this study MACF1 was investigated as a radiosensitization target in glioblastomas.

Methods

To provide context of MACF1 in glioblastomas, The Cancer Genome Atlas expression analyses were performed in conjunction with genes associated with glioblastoma evolution, while a genetic inhibitory approach, cell migratory assays, and immunofluorescence procedures were used to evaluate responses to MACF1 suppression with radiation. Additionally, expression analyses were conducted to assess co-expression of mTOR signaling pathway regulators and MACF1 in glioblastoma patient samples.

Results

Our amalgamation approach demonstrated that negative regulation of MACF1, which was positively correlated with epidermal growth factor receptor and p70s6k expression, enhanced the sensitivity of glioblastoma cells to radiation as a consequence of reducing glioblastoma cell viability and migration. Mechanistically, the antitumorigenic effects on glioblastoma cell behaviors after radiation and impairing MACF1 function were associated with decreased expression of ribosomal protein S6, a downstream effector of p70s6k.

Conclusion

MACF1 represents a diagnostic marker with target specificity in glioblastomas that can enhance the efficacy of radiation while minimizing normal tissue toxicity. This approach could potentially expand combinatorial radiation strategies for glioblastoma treatments via impairment of translational regulatory processes that contribute to poor patient survival.

Spinal cord astrocytomas: progresses in experimental and clinical investigations for developing recovery neurobiology-based novel therapies

Spinal cord astrocytomas (SCAs) have discernibly unique signatures in regards to epidemiology, clinical oncological features, genetic markers, pathophysiology, and research and therapeutic challenges. Overall, there are presently very limited clinical management options for high grade SCAs despite progresses made in validating key molecular markers and standardizing tumor classification. The endeavors were aimed to improve diagnosis, therapy design and prognosis assessment, as well as to define more effective oncolytic targets. Efficacious treatment for high grade SCAs still remains an unmet medical demand. This review is therefore focused on research state updates that have been made upon analyzing clinical characteristics, diagnostic classification, genetic and molecular features, tumor initiation cell biology, and current management options for SCAs. Particular emphasis was given to basic and translational research endeavors targeting SCAs, including establishment of experimental models, exploration of unique profiles of SCA stem cell-like tumor survival cells, characterization of special requirements for effective therapeutic delivery into the spinal cord, and development of donor stem cell-based gene-directed enzyme prodrug therapy. We concluded that precise understanding of molecular oncology, tumor survival mechanisms (e.g., drug resistance, metastasis, and cancer stem cells/tumor survival cells), and principles of Recovery Neurobiology can help to create clinically meaningful experimental models of SCAs. Establishment of such systems will expedite the discovery of efficacious therapies that not only kill tumor cells but simultaneously preserve and improve residual neural function.

Effects of Aptamer to U87-EGFRvIII Cells on the Proliferation, Radiosensitivity, and Radiotherapy of Glioblastoma Cells

Glioblastoma multiforme (GBM) is the most prevalent and lethal malignant intracranial tumor in the brain, with very poor prognosis and survival. The epidermal growth factor receptor variant III (EGFRvIII) contributes to increased oncogenicity that does not occur through binding EGFR ligands and instead occurs through constitutive activation, which enhances glioma tumorigenicity and resistance to targeted therapy. Aptamers are nucleic acids with high affinity and specificity to targets selected by systematic evolution of ligands by exponential enrichment (SELEX), and are usually developed as antagonists of disease-associated factors. Herein, we generated a DNA aptamer U2, targeting U87-EGFRvIII cells, and demonstrated that U2 alters the U87-EGFRvIII cell growth, radiosensitivity, and radiotherapy of glioblastoma cells. We detected U2 and U87-EGFRvIII cells by flow cytometry and confocal microscopy to explore the binding ability of U2 to U87-EGFRvIII cells. Then, we found that aptamer U2 inhibits the proliferation, migration, invasion, and downstream signaling of U87-EGFRvIII cells. Moreover, the U2 aptamer can increase the radiosensitivity of U87-EGFRvIII in vitro and has a better antitumor effect on 188Re-U2 in vivo. Therefore, the results revealed the promising potential of the U2 aptamer to be a new type of drug candidate and aptamer-targeted drug delivery system for glioblastoma therapy.

Akt/mTOR mediated induction of bystander effect signaling in a nucleus independent manner in irradiated human lung adenocarcinoma epithelial cells

Cytoplasm is an important target for the radiation-induced bystander effect (RIBE). In the present work, the critical role of protein kinase B (Akt)/mammalian target of rapamycin (mTOR) pathway in the generation of RIBE signaling after X-ray irradiation and the rapid phosphorylation of Akt and mTOR was observed in the cytoplasm of irradiated human lung adenocarcinoma epithelial (A549) cells. Targeting A549 cytoplasts with individual protons from a microbeam showed that RIBE signal(s) mediated by the Akt/mTOR pathway were generated even in the absence of a cell nucleus. These results provide a new insight into the mechanisms driving the cytoplasmic response to irradiation and their impact on the production of RIBE signal(s).

EGFR/EGFRvIII Dual-Targeting Peptide-Mediated Drug Delivery for Enhanced Glioma Therapy

Tumor-homing peptides have been widely used to mediate active targeted drug delivery. l-AE is a reported targeting peptide demonstrating high binding affinity to epidermal growth factor receptor (EGFR) and mutation variant III (EGFRvIII) overexpressed on neovasculature, vasculogenic mimicry, tumor cells, and tumor stem cells. To improve its proteolytic stability, a d-peptide ligand (termed d-AE, the enantiomer of l-AE) was developed. d-AE was confirmed to bind receptors EGFR and EGFRvIII with targeting capability comparable to l-AE. In vivo biodistribution demonstrated the superiority of d-AE in prolonged circulation and enhanced intratumoral accumulation. Furthermore, stabilized peptide modification endowed micelles higher transcytosis efficiency and penetrating capability on blood-brain tumor barrier/U87 tumor spheroids coculture model. When paclitaxel (PTX) was loaded, d-AE-micelle/PTX demonstrated excellent antitumor effect in comparison to Taxol, micelle/PTX, and l-AE-micelle/PTX. These findings indicated that the multitargeted drug delivery system enabled by d-AE ligand provides a promising way for glioma therapy.

Aptamer-conjugated PEGylated quantum dots targeting epidermal growth factor receptor variant III for fluorescence imaging of glioma

The extent of resection is a significant prognostic factor in glioma patients. However, the maximum safe resection level is difficult to determine due to the inherent infiltrative character of tumors. Recently, fluorescence-guided surgery has emerged as a new technique that allows safe resection of glioma. In this study, we constructed a new kind of quantum dot (QD)-labeled aptamer (QD-Apt) nanoprobe by conjugating aptamer 32 (A32) to the QDs surface, which can specially bind to the tumors. A32 is a single-stranded DNA capable of binding to the epidermal growth factor receptor variant III (EGFRvIII) specially distributed on the surface of glioma cells. To detect the expression of EGFRvIII in human brain tissues, 120 specimens, including 110 glioma tissues and 10 normal brain tissues, were examined by immunohistochemistry, and the results showed that the rate of positive expression of EGFRvIII in the glioma tissues was 41.82%, and 0.00% in normal brain tissues. Besides, the physiochemical properties of QD-Apt nanoparticles (NPs) were thoroughly characterized. Biocompatibility of the NPs was evaluated, and the results suggested that the QD-Apt was nontoxic in vivo and vitro. Furthermore, the use of the QD-Apt in labeling glioma cell lines and human brain glioma tissues, and target gliomas in situ was also investigated. We found that not only could QD-Apt specially bind to the U87-EGFRvIII glioma cells but also bind to human glioma tissues in vitro. Fluorescence imaging in vivo with orthotopic glioma model mice bearing U87-EGFRvIII showed that QD-Apt could penetrate the blood-brain barrier and then selectively accumulate in the tumors through binding to EGFRvIII, and consequently, generate a strong fluorescence, which contributed to the margins of gliomas that were visualized clearly, and thus, help the surgeons realize the maximum safe resection of glioma. In addition, QD-Apt can also be applied in preoperative diagnosis and postoperative examination of glioma. Therefore, these achievements facilitate the use of tumor-targeted fluorescence imaging in the diagnosis, surgical resection, and postoperative examination of glioma.

A brief perspective of drug resistance toward EGFR inhibitors: the crystal structures of EGFRs and their variants

The EGFR is one of the most popular targets for anticancer therapies and many drugs, such as erlotinib and gefitinib, have got enormous success in clinical treatments of cancer in past decade. However, the efficacy of these agents is often limited because of the quick emergence of drug resistance. Fundamental structure researches of EGFR in recent years have generally elucidated the mechanism of drug resistance. In this review, based on systematic resolution of full structures of EGFR and their variants via single crystal x-ray crystallography, the working and drug resistance mechanism of EGFR-targeted drugs are fully illustrated. Moreover, new strategies for avoiding EGFR drug resistance in cancer treatments are also discussed.

Radiolabeled novel mAb 4G1 for immunoSPECT imaging of EGFRvIII expression in preclinical glioblastoma xenografts

Epidermal growth factor receptor mutant III (EGFRvIII) is exclusively expressed in tumors, such as glioblastoma, breast cancer and hepatocellular carcinoma, but never in normal organs. Increasing evidence suggests that EGFRvIII has clinical significance in glioblastoma prognosis due to its enhanced tumorigenicity and chemo/radio resistance, thus the development of an imaging approach to early detect EGFRvIII expression with high specificity is urgently needed. To illustrate this point, we developed a novel anti-EGFRvIII monoclonal antibody 4G1 through mouse immunization, cell fusion and hybridoma screening and then confirmed its specificity and affinity by a serial of assays. Following biodistribution and small animal single-photon emission computed tomography (SPECT/CT) imaging of 125I-4G1 in EGFRvIII positive/negative tumor-bearing mice were performed and evaluated to verify the tumor accumulation of this radiotracer. The biodistribution indicated that 125I-4G1 showed prominent tumor accumulation at 24 h post-injection, which reached maximums of 11.20 ± 0.75% ID/g and 13.98 ± 0.57% ID/g in F98npEGFRvIII and U87vIII xenografts, respectively. In contrast, 125I-4G1 had lower tumor accumulation in F98npEGFR and U87MG xenografts. Small animal SPECT/CT imaging revealed that 125I-4G1 had a higher tumor uptake in EGFRvIII-positive tumors than that in EGFRvIII-negative tumors. This study demonstrates that radiolabeled 4G1 can serve as a valid probe for the imaging of EGFRvIII expression, and would be valuable into the clinical translation for the diagnosis, prognosis, guiding therapy, and therapeutic efficacy evaluation of tumors.

EGFRvIII does not affect radiosensitivity with or without gefitinib treatment in glioblastoma cells

BACKGROUND

Glioblastomas (GBM) are often characterized by an elevated expression of the epidermal growth factor receptor variant III (EGFRvIII). We used GBM cell lines with native EGFRvIII expression to determine whether this EGFR variant affects radiosensitivity with or without EGFR targeting.

METHODS

Experiments were performed with GBM cell lines lacking (LN229, U87MG, U251, CAS-1) or endogenously expressing EGFRvIII (BS153, DKMG). The two latter cell lines were also used to establish sublines with a low (-) or a high proportion (+) of cells expressing EGFRvIII. EGFR signaling and the cell cycle were analyzed using Western blot and flow cytometry; cell survival was assessed by colony forming assay and double-strand break repair capacity by immunofluorescence.

RESULTS

DKMG and BS153 parental cells with heterogeneous EGFRvIII expression were clearly more radiosensitive compared to other GBM cell lines without EGFRvIII expression. However, no significant difference was observed in cell proliferation, clonogenicity or radiosensitivity between the EGFRvIII- and + sublines derived from DKMG and BS153 parental cells. Expression of EGFRvIII was associated with decreased DSB repair capacity for BS153 but not for DKMG cells. The effects of EGFR targeting by gefitinib alone or in combination with irradiation were also found not to depend on EGFRvIII expression. Gefitinib was only observed to influence the proliferation of EGFRvIII- BS153 cells.

CONCLUSION

The data indicate that EGFRvIII does not alter radiosensitivity with or without anti-EGFR treatment.

Retargeted human avidin-CAR T cells for adoptive immunotherapy of EGFRvIII expressing gliomas and their evaluation via optical imaging

There has been significant progress in the design of chimeric antigen receptors (CAR) for adoptive immunotherapy targeting tumor-associated antigens. However, the challenge of monitoring the therapy in real time has been continually ignored. To address this issue, we developed optical molecular imaging approaches to evaluate a recently reported novel CAR strategy for adoptive immunotherapy against glioma xenografts expressing EGFRvIII. We initially biotinylated a novel anti-EGFRvIII monoclonal antibody (biotin-4G1) to pre-target EGFRvIII⁺ gliomas and then redirect activated avidin-CAR expressing T cells against the pre-targeted biotin-4G1. By optical imaging study and bio-distribution analysis, we confirmed the specificity of pre-target and target and determined the optimal time for T cells adoptive transfer in vivo. The results showed this therapeutic strategy offered efficient therapy effect to EGFRvIII⁺ glioma-bearing mice and implied that optical imaging is a highly useful tool in aiding in the instruction of clinical CAR-T cells adoptive transfer in future.

Targeted therapy combined with radiotherapy in non-small-cell lung cancer: a review of the Oncologic Group for the Study of Lung Cancer (Spanish Radiation Oncology Society)

In recent years, major advances in our understanding of the molecular biology of lung cancer, together with significant improvements in radiotherapy technologies, have revolutionized the treatment of non-small cell lung cancer (NSCLC). This has led to the development of new therapies that target molecular mutations specific to each tumor type, acting on the cell surface antigens or intracellular signaling pathways, or directly affecting cell survival. At the same time, ablative dose radiotherapy can be delivered safely in the context of metastatic disease. In this article, the GOECP/SEOR (Oncological Group for Study of Lung Cancer/Spanish Society of Radiation Oncology) reviews the role of new targeted therapies used in combination with radiotherapy in patients with locally advanced (stage III) NSCLC and in patients with advanced, metastatic (stage IV) NSCLC.

Cross talk of tyrosine kinases with the DNA damage signaling pathways

Tyrosine kinases respond to extracellular and intracellular cues by activating specific cellular signaling cascades to regulate cell cycle, growth, proliferation, differentiation and survival. Likewise, DNA damage response proteins (DDR) activated by DNA lesions or chromatin alterations recruit the DNA repair and cell cycle checkpoint machinery to restore genome integrity and cellular homeostasis. Several new examples have been uncovered in recent studies which reveal novel epigenetic and non-epigenetic mechanisms by which tyrosine kinases interact with DDR proteins to dictate cell fate, i.e. survival or apoptosis, following DNA damage. These studies reveal the ability of tyrosine kinases to directly regulate the activity of DNA repair and cell cycle check point proteins by tyrosine phosphorylation. In addition, tyrosine kinases epigenetically regulate DNA damage signaling pathways by modifying the core histones as well as chromatin modifiers at critical tyrosine residues. Thus, deregulated tyrosine kinase driven epigenomic alterations have profound implications in cancer, aging and genetic disorders. Consequently, targeting oncogenic tyrosine kinase induced epigenetic alterations has gained significant traction in overcoming cancer cell resistance to various therapies. This review discusses mechanisms by which tyrosine kinases interact with DDR pathways to regulate processes critical for maintaining genome integrity as well as clinical strategies for targeted cancer therapies.

Radiosensitivity enhancement of radioresistant glioblastoma by epidermal growth factor receptor antibody-conjugated iron-oxide nanoparticles

The epidermal growth factor receptor deletion variant EGFRvIII is known to be expressed in a subset of patients with glioblastoma (GBM) tumors that enhances tumorigenicity and also accounts for radiation and chemotherapy resistance. Targeting the EGFRvIII deletion mutant may lead to improved GBM therapy and better patient prognosis. Multifunctional magnetic nanoparticles serve as a potential clinical tool that can provide cancer cell targeted drug delivery, imaging, and therapy. Our previous studies have shown that an EGFRvIII-specific antibody and cetuximab (an EGFR- and EGFRvIII-specific antibody), when bioconjugated to IONPs (EGFRvIII-IONPs or cetuximab-IONPs respectively), can simultaneously provide sensitive cancer cell detection by magnetic resonance imaging (MRI) and targeted therapy of experimental GBM. In this study, we investigated whether cetuximab-IONPs can additionally allow for the radiosensitivity enhancement of GBM. Cetuximab-IONPs were used in combination with single (10 Gy × 1) or multiple fractions (10 Gy × 2) of ionizing radiation (IR) for radiosensitization of EGFRvIII-overexpressing human GBM cells in vitro and in vivo after convection-enhanced delivery (CED). A significant GBM antitumor effect was observed in vitro after treatment with cetuximab-IONPs and subsequent single or fractionated IR. A significant increase in overall survival of nude mice implanted with human GBM xenografts was found after treatment by cetuximab-IONP CED and subsequent fractionated IR. Increased DNA double strands breaks (DSBs), as well as increased reactive oxygen species (ROS) formation, were felt to represent the mediators of the observed radiosensitization effect with the combination therapy of IR and cetuximab-IONPs treatment.

Targeting EGFR-PI3K-AKT-mTOR signaling enhances radiosensitivity in head and neck squamous cell carcinoma

INTRODUCTION

Head and neck squamous cell carcinoma (HNSCC) is frequently characterized by high resistance to radiotherapy, which critically depends on both altered signaling pathways within tumor cells and their dynamic interaction with the tumor microenvironment.

AREAS COVERED

This review covers EGFR-phosphoinositide 3-kinase (PI3K)-protein kinase B (AKT)-mechanistic target of rapamycin (mTOR) signaling in HNSCC. The role of each pathway node in radioresistance is discussed. Preclinical and clinical innovative aspects of targeting EGFR-PI3K-AKT and mTOR are demonstrated. Ongoing clinical trials and future perspectives are presented.

EXPERT OPINION

Different cellular signaling pathways seem to mediate radioresistance in advanced HNSCC and various molecular targeted therapies are currently being investigated to sensitize tumor cells to radiotherapy. Recently, new insights in the mutational landscape of HNSCC unraveled critical alterations in putative oncogenes and tumor suppressor genes and have emphasized the importance of PI3K and the corresponding upstream and downstream signaling pathways in pathogenesis and treatment response. The frequent activation of the EGFR-PI3K-AKT-mTOR pathway in HNSCC and its implication in the context of radiosensitivity make this pathway one of the most promising targets in the therapy of HNSCC patients. Clinical studies targeting EGFR and mTOR in combination with radiotherapy are under investigation.

Phosphorylation of AKT(Ser473) serves as an independent prognostic marker for radiosensitivity in advanced head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is frequently characterized by high resistance to radiotherapy, which critically depends on both altered signaling pathways within tumor cells and their dynamic interaction with the tumor microenvironment. This study evaluated the prognostic value of the phosphorylation status of AKT on Ser473 and Thr308 for the clinical outcome of patients with advanced HNSCC on radiotherapy. Furthermore, we investigated the impact of AKT(Ser473) phosphorylation [p-AKT(Ser473)] in the context of radioresistance using ex vivo tissue cultures that resemble the complex tissue architecture and paracrine interaction with the tumor microenvironment. In a cohort of 120 patients with advanced HNSCC, who were treated with primary or adjuvant radiotherapy, a significant association was found between relative p-AKT(Ser473) levels and overall survival (p = 0.006) as well as progression-free survival (p = 0.021), while no significant correlation was revealed for relative p-AKT(Thr308) levels. In ex vivo tissue cultures p-AKT(Ser473) levels were increased upon irradiation and treatment with the PI3K inhibitor LY294002 inhibited both basal and irradiation induced AKT(Ser473) phosphorylation. Strikingly, pretreatment with LY294002 sensitized tissue cultures derived from primary and recurrent tumors to radiotherapy as determined by impaired tumor cell proliferation and enhanced DNA damage. In conclusion, phosphorylation status of AKT(Ser473) in tumor specimens serves as a novel biomarker to identify patients with advanced HNSCC at high risk for treatment failure following radiotherapy, and our data from ex vivo tissue cultures support the assumption that pharmacological inhibition of AKT(Ser473) phosphorylation might circumvent radioresistance to improve efficiency and reduce toxicity of current treatment modalities.

EGFR-dependent mechanisms in glioblastoma: towards a better therapeutic strategy

Glioblastoma is a particularly resilient cancer, and while therapies may be able to reach the brain by crossing the blood-brain barrier, they then have to deal with a highly invasive tumor that is very resistant to DNA damage. It seems clear that in order to kill aggressive glioma cells more efficiently and with fewer side effects on normal tissue, there must be a shift from classical cytotoxic chemotherapy to more targeted therapies. Since the epidermal growth factor receptor (EGFR) is altered in almost 50% of glioblastomas, it currently represents one of the most promising therapeutic targets. In fact, it has been associated with several distinct steps in tumorigenesis, from tumor initiation to tumor growth and survival, and also with the regulation of cell migration and angiogenesis. However, inhibitors of the EGFR kinase have produced poor results with this type of cancer in clinical trials, with no clear explanation for the tumor resistance observed. Here we will review what we know about the expression and function of EGFR in cancer and in particular in gliomas. We will also evaluate which are the possible molecular and cellular escape mechanisms. As a result, we hope that this review will help improve the design of future EGFR-targeted therapies for glioblastomas.

Concomitant EGFR inhibitors combined with radiation for treatment of non-small cell lung carcinoma

Epidermal growth factor receptor (EGFR) is considered to be one of the key driver genes in non-small cell lung cancer (NSCLC). Several clinical trials have shown great promise of EGFR tyrosine kinase inhibitors (TKIs) in the first-line treatment of NSCLC. Many advances have been made in the understanding of EGFR signal transduction network and the interaction between EGFR and tumor microenvironment in mediating cancer survival and development. The concomitant targeted therapy and radiation is a new strategy in the treatment of NSCLC. A number of preclinical studies have demonstrated synergistic anti-tumor activity in the combination of EGFR inhibitors and radiotherapy in vitro and in vivo. In the present review, we discuss the rationale of the combination of EGFR inhibitors and radiotherapy in the treatment of NSCLC.

Current concepts in clinical radiation oncology

Based on its potent capacity to induce tumor cell death and to abrogate clonogenic survival, radiotherapy is a key part of multimodal cancer treatment approaches. Numerous clinical trials have documented the clear correlation between improved local control and increased overall survival. However, despite all progress, the efficacy of radiation-based treatment approaches is still limited by different technological, biological, and clinical constraints. In principle, the following major issues can be distinguished: (1) The intrinsic radiation resistance of several tumors is higher than that of the surrounding normal tissue, (2) the true patho-anatomical borders of tumors or areas at risk are not perfectly identifiable, (3) the treatment volume cannot be adjusted properly during a given treatment series, and (4) the individual heterogeneity in terms of tumor and normal tissue responses toward irradiation is immense. At present, research efforts in radiation oncology follow three major tracks, in order to address these limitations: (1) implementation of molecularly targeted agents and 'omics'-based screening and stratification procedures, (2) improvement of treatment planning, imaging, and accuracy of dose application, and (3) clinical implementation of other types of radiation, including protons and heavy ions. Several of these strategies have already revealed promising improvements with regard to clinical outcome. Nevertheless, many open questions remain with individualization of treatment approaches being a key problem. In the present review, the current status of radiation-based cancer treatment with particular focus on novel aspects and developments that will influence the field of radiation oncology in the near future is summarized and discussed.

Combination epidermal growth factor receptor inhibition and radical radiotherapy for NSCLC

Non-small cell lung cancer (NSCLC) remains the most common cause of cancer-related death in the developed world. Despite advances in therapy with conventional modalities, over 85% of patients will die from their disease within 5 years of diagnosis. For patients with inoperable lung cancer, the addition of chemotherapy to radical radiotherapy yields a small but significant 10% survival benefit at 3 years. However, the systemic toxicity of chemotherapy is common and may be severe. Over the past 20 years, dramatic improvements in our understanding of the molecular etiology of cancer have enabled the development of novel targeted therapies. Overexpression of the epidermal growth factor receptor (EGFR) in lung cancer correlates with an aggressive disease course and poor tumor response to radiotherapy. Strategies to inhibit this molecular switch have become a focus for drug development. Preclinical efficacy has been repeatedly demonstrated with anti-EGFR monoclonal antibodies and small molecule tyrosine kinase inhibitors, and responses have been documented in the clinic with acceptable toxicity. Phase III trials combining EGFR tyrosine kinase inhibitors with radical chemoradiation are recruiting at present. This review addresses the current challenges of discovering how best to use these new anticancer therapies, with particular emphasis on the enhancement of existing therapeutic strategies such as radical radiotherapy, factors relating to patient selection and prediction of clinical response.

A minority subpopulation of CD133(+) /EGFRvIII(+) /EGFR(-) cells acquires stemness and contributes to gefitinib resistance

AIMS

To study the contribution of epidermal growth factor receptor variant III (EGFRvIII) to glioblastoma multiforme (GBM) stemness and gefitinib resistance.

METHODS

CD133(+) and CD133(-) cells were separated from EGFRvIII(+) clinical specimens of three patients with newly diagnosed GBM. Then, RT-PCR was performed to evaluate EGFRvIII and EGFR expression in CD133(+) and CD133(-) cells. The tumorigenicity and stemness of CD133(+) cells was verified by intracranial implantation of 5 × 10(3) cells into immunodeficient NOD/SCID mice. Finally, cells were evaluated for their sensitivity to EGFR tyrosine kinase inhibition by gefitinib.

RESULTS

RT-PCR results showed that the sorted CD133(+) cells expressed EGFRvIII exclusively, while the CD133(-) cells expressed both EGFRvIII and EGFR. At 6-8 weeks postimplantation, CD133(+) /EGFRvIII(+) /EGFR(-) cells formed intracranial tumors. Cell counting kit-8 results showed that the IC50 values of the three isolated EGFRvIII(+) cell lines treated with gefitinib were 14.44, 16.00, and 14.66 μM, respectively, whereas the IC50 value of an isolated EGFRvIII(-) cell line was 8.57 μM.

CONCLUSIONS

EGFRvIII contributes to the stemness of cancer stem cells through coexpression with CD133 in GBMs. Furthermore, CD133(+) /EGFRvIII(+) /EGFR(-) cells have the ability to initiate tumor formation and may contribute to gefitinib resistance.

Dacomitinib, an irreversible Pan-ErbB inhibitor significantly abrogates growth in head and neck cancer models that exhibit low response to cetuximab

Aberrant epidermal growth factor (EGF) signaling is associated with tumor growth in squamous cell carcinoma of the head and neck in humans (HNSCC), and is a major focus of targeted therapy. Cetuximab, a monoclonal antibody against EGFR, has been successful at prolonging survival but has only a 10% tumor shrinkage response rate in a clinical setting. The goal of this study was to compare dacomitinib (PF-00299804), a next generation small molecule tyrosine kinase inhibitor that irreversibly blocks multiple HER family receptors (HER-1 (EGFR), HER-2 and HER-4 tyrosine kinases), to cetuximab, the current FDA approved anti-EGFR medication for HNSCC and erlotinib, an EGFR specific small molecule tyrosine kinase inhibitor. Dacomitinib, erlotinib and cetuximab were tested in a panel of 27 HNSCC cell lines. Treatment with 100 ug/ml of cetuximab or 1 uM of erlotinib inhibited growth by at least 50% in 7/27 cell lines, while treatment with 1 uM of dacomitinib had similar growth inhibition in 17/27 lines. Cell lines representing three levels of sensitivity to dacomitinib were further examined using Western blots, cell cycle and apoptosis analysis. Treatment with 100 nM of dacomitinib reduced EGFR activity and downstream AKT and ERK pathways more effectively than treatment with 100 ug/ml of cetuximab in all ten tested lines. Although both compounds induced apoptosis at similar levels, dacomitinib caused greater G0/G1 arrest. Sensitivity to EGFR blockade was associated with levels of EGFR and ERK and was not associated with common oncogenic mutations and copy number variations. Phosphorylated and total EGFR and ERK levels correlate with sensitivity to both cetuximab and dacomitinib. Three of the four lines in the exquisitely sensitive group had the highest levels of phosphorylated and total EGFR and ERK among the ten lines selected, while the three resistant lines collectively had the lowest levels. Neither pAKT nor tAKT was associated with sensitivity.

Rindopepimut: an evidence-based review of its therapeutic potential in the treatment of EGFRvIII-positive glioblastoma

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults and is universally fatal. Despite surgical resection, radiotherapy, and systemic chemotherapy, the median overall survival is less than 15 months. As current therapies are not tumor-specific, treatment commonly results in toxicity. The epidermal growth factor receptor variant III (EGFRvIII) is a naturally occurring mutant of EGFR and is expressed on approximately 20% to 30% of GBMs. As it is not expressed on normal cells, it is an ideal therapeutic target. Rindopepimut is a peptide vaccine which elicits EGFRvIII-specific humoral and cellular immune responses. Phase I and II clinical trials have demonstrated significantly higher progression-free and overall survival times in vaccinated patients with EGFRvIII-expressing GBM tumors. Side effects are minimal and mainly consist of hypersensitivity reactions. Due to the efficacy and safety of rindopepimut, it is a promising therapy for patients with GBM. Currently, rindopepimut is undergoing clinical testing in an international Phase III trial for newly diagnosed GBM and a Phase II trial for relapsed GBM.

Personalized Combined Modality Therapy for Locally Advanced Non-small Cell Lung Cancer

Locally advanced non-small cell lung cancer (NSCLC) is a heterogeneous disease, and we have embarked on an era where patients will benefit from individualized therapeutic strategies based on identifiable molecular characteristics of the tumor. The landmark studies demonstrating the importance of molecular characterization of tumors for NSCLC patients, the promising molecular pathways, and the potential molecular targets/agents for treatment of this disease will be reviewed. Understanding these issues will aid in the development of rationally designed clinical trials, so as to determine best means of appropriately incorporating these molecular strategies, to the current standard of radiation and chemotherapy regimens, for the treatment of locally advanced NSCLC.

Targeting of a conformationally exposed, tumor-specific epitope of EGFR as a strategy for cancer therapy

Epidermal growth factor receptor (EGFR) and its most common extracellular mutant, EGFRvIII, are important therapeutic targets in multiple cancer types. A number of monoclonal antibodies and small-molecule inhibitors against these receptors are now used for anticancer treatments. New insights into the structure and function of these receptors illustrate how they can be targeted in novel ways, with expected improvements in the therapeutic efficacy. Monoclonal antibody 806 (mAb806) is an antibody that targets a conformationally exposed epitope of wild-type EGFR when it is overexpressed on tumor cells or in the presence of oncogenic mutations such as EGFRvIII. The mechanism of action of mAb806, which allows for EGFR inhibition without normal tissue toxicity, creates opportunities for combination therapy and strongly suggests mAb806 will be a superior targeted delivery system for antitumor agents. Targeting of the epitope for mAb806 also appears to be an improved strategy to inhibit tumors that express EGFRvIII. This concept of conformational epitope targeting by antibodies reflects an underlying interplay between the structure and biology of different conformational forms of the EGFR family.

DNA double-strand break - induced pro-survival signaling

Radiation and other types of DNA damaging agents induce a plethora of signaling events simultaneously originating from the nucleus, cytoplasm, and plasma membrane. As a result, this presents a dilemma when seeking to determine causal relationships and better insight into the intricacies of stress signaling. ATM plays critical roles in both nuclear and cytoplasmic signaling, of which, the DNA damage response (DDR) is the best characterized. We have recently created experimental conditions where the DNA damage signal alone can be studied while minimizing the influence from the extranuclear compartment. We have been able to document pro-survival and growth promoting signaling (via ATM-AKT-ERK) resulting from low levels of DSBs (equivalent to ≤2 Gy). More extensive DSBs (>2 Gy eq.) result in phosphatase-mediated ERK dephosphorylation, and thus shutdown of ERK signaling. In contrast, radiation does not result in such dephosphorylation even at very high doses. We propose that phosphatases are inactivated perhaps as a result of reactive oxygen species, which does not occur in response to 'pure' DNA damage. Our findings suggest that clinically relevant radiation doses, which are intended to halt tumor growth and induce cell death, are unable to inhibit tumor pro-survival signaling via ERK dephosphorylation.

Cediranib enhances control of wild type EGFR and EGFRvIII-expressing gliomas through potentiating temozolomide, but not through radiosensitization: implications for the clinic

Glioblastomas (GBM) frequently overexpress the epidermal growth factor receptor (wtEGFR) or its mutant, EGFRvIII, contributing to chemo- and radioresistance. The current standard of care is surgery followed by radiation therapy with concurrent temozolomide (TMZ) followed by adjuvant TMZ. New treatment strategies for GBM include blockade of EGFR signaling and angiogenesis. Cediranib is a highly potent receptor tyrosine kinase inhibitor that inhibits all three VEGF receptors. This study investigated the radiosensitizing potential of cediranib in combination with TMZ in U87 GBM xenografts expressing wtEGFR or EGFRvIII. U87 GBM cells stably transfected with either wtEGFR or EGFRvIII were injected into the hind limbs of nude mice. Cediranib was dosed at 3 mg/kg daily five times a week orally for 2 weeks. TMZ was dosed at 10 mg/kg once only on day 0. Radiotherapy (RT) consisted of 3 fractions of 5 Gy (days 0-2). Cediranib did not radiosensitize either tumor type; however, cediranib did enhance the effectiveness of TMZ in both transfectants. Our results suggest that combining cediranib with temozolomide in the clinic will lead to improved tumor control.

Epidermal growth factor receptor in glioma: signal transduction, neuropathology, imaging, and radioresistance

Aberrant epidermal growth factor receptor (EGFR) signaling is common in cancer. Increased expression of wild type and mutant EGFR is a widespread feature of diverse types of cancer. EGFR signaling in cancer has been the focus of intense investigation for decades primarily for two reasons. First, aberrant EGFR signaling is likely to play an important role in the pathogenesis of cancer, and therefore, the mechanisms of EGFR-mediated oncogenic signaling are of interest. Second, the EGFR signaling system is an attractive target for therapeutic intervention. EGFR gene amplification and overexpression are a particularly striking feature of glioblastoma (GBM), observed in approximately 40% of tumors. GBM is the most common primary malignant tumor of the central nervous system in adults. In approximately 50% of tumors with EGFR amplification, a specific EGFR mutant (EGFRvIII, also known as EGFR type III, de2-7, Delta EGFR) can be detected. This mutant is highly oncogenic and is generated from a deletion of exons 2 to 7 of the EGFR gene, which results in an in-frame deletion of 267 amino acids from the extracellular domain of the receptor. EGFRvIII is unable to bind ligand, and it signals constitutively. Although EGFRvIII has the same signaling domain as the wild type receptor, it seems to generate a distinct set of downstream signals that may contribute to an increased tumorigenicity. In this review, we discuss recent progress in key aspects of EGFR signaling in GBM, focusing on neuropathology, signal transduction, imaging of the EGFR, and the role of the EGFR in mediating resistance to radiation therapy in GBM.

MAb 806 enhances the efficacy of ionizing radiation in glioma xenografts expressing the de2-7 epidermal growth factor receptor

PURPOSE

Mutations of the epidermal growth factor receptor (EGFR) are common in glioma. The most frequent mutation, de2-7 EGFR/EGFRvIII, occurs in approximately 40% of high-grade gliomas and confers resistance to ionizing radiation (IR). We have previously shown that mAb 806, a novel EGFR-specific antibody, is able to inhibit the growth of U87MG.Δ2-7 glioma xenografts expressing the de2-7 EGFR and may have potential as a therapeutic.

METHODS AND MATERIALS

Nude mice bearing U87MG.Δ2-7 xenografts were treated with mAb 806 and/or IR. Comparison of tumor volumes, the effect of treatment on angiogenesis as determined by mean vessel density, and expression changes in prosurvival protein pAkt between treatment groups were undertaken.

RESULTS

Treatment of mice bearing U87MG.Δ2-7 xenografts with mAb 806 and IR resulted in schedule-dependent radiosensitization. Maximal benefit was obtained when antibody treatment was given before irradiation, with the greatest inhibition of both tumor angiogenesis and tumor growth. Combination treatment mediated radiosensitization by selectively blocking the phosphorylation of the prosurvival protein Akt at serine 473, a process that is independent of DNA-dependent protein kinase catalytic subunit.

CONCLUSIONS

Our results provide a rationale for the use of mAb 806 in combination with IR for the treatment of glioma and potentially other solid tumors bearing the de2-7 EGFR.

An epidermal growth factor receptor variant III-targeted vaccine is safe and immunogenic in patients with glioblastoma multiforme

Conventional therapies for glioblastoma multiforme (GBM) fail to target tumor cells exclusively, such that their efficacy is ultimately limited by nonspecific toxicity. Immunologic targeting of tumor-specific gene mutations, however, may allow more precise eradication of neoplastic cells. The epidermal growth factor receptor variant III (EGFRvIII) is a consistent and tumor-specific mutation widely expressed in GBMs and other neoplasms. The safety and immunogenicity of a dendritic cell (DC)-based vaccine targeting the EGFRvIII antigen was evaluated in this study. Adults with newly diagnosed GBM, who had undergone gross-total resection and standard conformal external beam radiotherapy, received three consecutive intradermal vaccinations with autologous mature DCs pulsed with an EGFRvIII-specific peptide conjugated to keyhole limpet hemocyanin. The dose of DCs was escalated in cohorts of three patients. Patients were monitored for toxicity, immune response, radiographic and clinical progression, and death. No allergic reactions or serious adverse events were seen. Adverse events were limited to grade 2 toxicities. The maximum feasible dose of antigen-pulsed mature DCs was reached at 5.7 x 10(7) +/- 2.9 x 10(7) SD without dose-limiting toxicity. EGFRvIII-specific immune responses were evident in most patients. The mean time from histologic diagnosis to vaccination was 3.6 +/- 0.6 SD months. Median time to progression from vaccination was 6.8 months [95% confidence interval (C.I.(95)), 2.5-8.8], and median survival time from vaccination was 18.7 months (C.I.(95), 14.5-25.6). Overall median survival from time of histologic diagnosis was 22.8 months (C.I.(95), 17.5-29). This study establishes the EGFRvIII mutation as a safe and immunogenic tumor-specific target for immunotherapy.

Challenges in clinical design of immunotherapy trials for malignant glioma

Glioblastoma multiforme (GBM) is the most common and lethal primary malignant brain tumor. The traditional treatments for GBM, including surgery, radiation, and chemotherapy, only modestly improve patient survival. Therefore, immunotherapy has emerged as a novel therapeutic modality. Immunotherapeutic strategies exploit the immune system's ability to recognize and mount a specific response against tumor cells, but not normal cells. Current immunotherapeutic approaches for glioma can be divided into 3 categories: immune priming (active immunotherapy), immunomodulation (passive immunotherapy), and adoptive immunotherapy. Immune priming sensitizes the patient's immune cells to tumor antigens using various vaccination protocols. In the case of immunomodulation, strategies are aimed at reducing suppressive cytokines in the tumor microenvironment or using immune molecules to specifically target tumor cells. Adoptive immunotherapy involves harvesting the patient's immune cells, followed by ex vivo activation and expansion before reinfusion. This article provides an overview of the interactions between the central nervous system and the immune system, and discusses the challenges facing current immunotherapeutic strategies.

Targeting EGFR resistance networks in head and neck cancer

A core set of oncoproteins is overexpressed or functionally activated in many types of cancer, and members of this group have attracted significant interest as subjects for development of targeted therapeutics. For some oncoproteins such as EGFR/ErbB1, both small molecule and antibody agents have been developed and applied in the clinic for over a decade. Analysis of clinical outcomes has revealed an initially unexpected complexity in the response of patients to these agents. Diverse factors, including developmental lineage of the tumor progenitor cell, co-mutation or epigenetic modulation of genes encoding proteins in an extended EGFR signaling network or regulating core survival responses in individual tumors, and environmental factors including inflammatory agents and viral infection, all have been identified as modulating response to treatment with EGFR-targeted drugs. Second and third generation therapeutic strategies increasingly incorporate knowledge of cancer type-specific signaling environments, in a more personalized treatment approach. This review takes squamous cell carcinoma of the head and neck (SCCHN) as a specific example of an EGFR-involved cancer with idiosyncratic biological features that influence design of treatment modalities, with particular emphasis on commonalities and differences with other cancer types.

Binding of cetuximab to the EGFRvIII deletion mutant and its biological consequences in malignant glioma cells

BACKGROUND AND PURPOSE

Despite the clinical use of cetuximab, a chimeric antibody against EGFR, little is known regarding its interaction with EGFRvIII, a frequently expressed deletion mutant of EGFR. Therefore, we investigated the interaction and the functional consequences of cetuximab treatment on glioma cells stably expressing EGFRvIII.

MATERIALS AND METHODS

The human glioma cell line U373 genetically modified to express EGFRvIII was used to measure the binding of cetuximab and its internalization using flow cytometry and confocal microscopy. Proliferation and cell survival were analyzed by cell growth and clonogenic survival assays.

RESULTS

Cetuximab is able to bind to EGFRvIII and causes an internalization of the receptor and decreases its expression levels. Furthermore, in contrast to EGF, cetuximab was able to activate EGFRvIII which was evidenced by multiple phosphorylation sites and its downstream signaling targets. Despite this activation, the growth rate and the radiosensitivity of the EGFRvIII-expressing glioma cells were not modulated.

CONCLUSIONS

Cetuximab binds to EGFRvIII and leads to the initial activation, internalization and subsequent downregulation of EGFRvIII, but it does not seem to modulate the proliferation or radiosensitivity of EGFRvIII-expressing glioma cells. Thus, approaches to treat EGFRvIII-expressing glioma cells should be evaluated more carefully.

Toward effective immunotherapy for the treatment of malignant brain tumors

The immunologic treatment of cancer has long been heralded as a targeted molecular therapeutic with the promise of eradicating tumor cells with minimal damage to surrounding normal tissues. However, a demonstrative example of the efficacy of immunotherapy in modulating cancer progression is still lacking for most human cancers. Recent breakthroughs in our understanding of the mechanisms leading to full T-cell activation, and recognition of the importance of overcoming tumor-induced immunosuppressive mechanisms, have shed new light on how to generate effective anti-tumor immune responses in humans, and sparked a renewed and enthusiastic effort to realize the full potential of cancer immunotherapy. The immunologic treatment of invasive malignant brain tumors has not escaped this re-invigorated endeavor, and promising therapies are currently under active investigation in dozens of clinical trials at several institutions worldwide. This review will focus on some of the most important breakthroughs in our understanding of how to generate potent anti-tumor immune responses, and some of the clear challenges that lie ahead in achieving effective immunotherapy for the majority of patients with malignant brain tumors. A review of immunotherapeutic strategies currently under clinical evaluation, as well as an outline of promising novel approaches on the horizon, is included to provide perspective on the active and stalwart progress toward effective immunotherapy for the treatment of malignant brain tumors.

EGFRvIII and DNA double-strand break repair: a molecular mechanism for radioresistance in glioblastoma

Glioblastoma multiforme (GBM) is the most lethal of brain tumors and is highly resistant to ionizing radiation (IR) and chemotherapy. Here, we report on a molecular mechanism by which a key glioma-specific mutation, epidermal growth factor receptor variant III (EGFRvIII), confers radiation resistance. Using Ink4a/Arf-deficient primary mouse astrocytes, primary astrocytes immortalized by p53/Rb suppression, as well as human U87 glioma cells, we show that EGFRvIII expression enhances clonogenic survival following IR. This enhanced radioresistance is due to accelerated repair of DNA double-strand breaks (DSB), the most lethal lesion inflicted by IR. The EGFR inhibitor gefitinib (Iressa) and the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 attenuate the rate of DSB repair. Importantly, expression of constitutively active, myristylated Akt-1 accelerates repair, implicating the PI3K/Akt-1 pathway in radioresistance. Most notably, EGFRvIII-expressing U87 glioma cells show elevated activation of a key DSB repair enzyme, DNA-dependent protein kinase catalytic subunit (DNA-PKcs). Enhanced radioresistance is abrogated by the DNA-PKcs-specific inhibitor NU7026, and EGFRvIII fails to confer radioresistance in DNA-PKcs-deficient cells. In vivo, orthotopic U87-EGFRvIII-derived tumors display faster rates of DSB repair following whole-brain radiotherapy compared with U87-derived tumors. Consequently, EGFRvIII-expressing tumors are radioresistant and continue to grow following whole-brain radiotherapy with little effect on overall survival. These in vitro and in vivo data support our hypothesis that EGFRvIII expression promotes DNA-PKcs activation and DSB repair, perhaps as a consequence of hyperactivated PI3K/Akt-1 signaling. Taken together, our results raise the possibility that EGFR and/or DNA-PKcs inhibition concurrent with radiation may be an effective therapeutic strategy for radiosensitizing high-grade gliomas.

Pro-survival AKT and ERK signaling from EGFR and mutant EGFRvIII enhances DNA double-strand break repair in human glioma cells

The epidermal growth factor receptor (EGFR) is frequently dysregulated in malignant glioma that leads to increased resistance to cancer therapy. Upregulation of wild type or expression of mutant EGFR is associated with tumor radioresistance and poor clinical outcome. EGFR variant III (EGFRvIII) is the most common EGFR mutation in malignant glioma. Radioresistance is thought to be, at least in part, the result of a strong cytoprotective response fueled by signaling via AKT and ERK that is heightened by radiation in the clinical dose range. Several groups including ours have shown that this response may modulate DNA repair. Herein, we show that expression of EGFRvIII promoted gamma-H2AX foci resolution, a surrogate for double-strand break (DSB) repair, and thus enhanced DNA repair. Conversely, small molecule inhibitors targeting EGFR, MEK, and the expression of dominant-negative EGFR (EGFR-CD533) significantly reduced the resolution of gamma-H2AX foci. When homologous recombination repair (HRR) and non-homologous end joining (NHEJ) were specifically examined, we found that EGFRvIII stimulated and CD533 compromised HRR and NHEJ, respectively. Furthermore, NHEJ was blocked by inhibitors of AKT and ERK signaling pathways. Moreover, expression of EGFRvIII and CD533 increased and reduced, respectively, the formation of phospho-DNA-PKcs and -ATM repair foci, and RAD51 foci and expression levels, indicating that DSB repair is regulated at multiple levels. Altogether, signaling from EGFR and EGFRvIII promotes both HRR and NHEJ that is likely a contributing factor towards the radioresistance of malignant gliomas.

Tumor-specific immunotherapy targeting the EGFRvIII mutation in patients with malignant glioma

Conventional therapies for malignant gliomas (MGs) fail to target tumor cells exclusively, such that their efficacy is ultimately limited by non-specific toxicity. Immunologic targeting of tumor-specific gene mutations, however, may allow more precise eradication of neoplastic cells. The epidermal growth factor receptor variant III (EGFRvIII) is a consistent tumor-specific mutation that is widely expressed in MGs and other neoplasms. This mutation encodes a constitutively active tyrosine kinase that enhances tumorgenicity and migration and confers radiation and chemotherapeutic resistance. This in-frame deletion mutation splits a codon resulting in the creation of a novel glycine at the fusion junction between normally distant parts of the molecule and producing a sequence re-arrangement which creates a tumor-specific epitope for cellular or humoral immunotherapy in patients with MGs. We have previously shown that vaccination with a peptide that spans the EGFRvIII fusion junction is an efficacious immunotherapy in syngeneic murine models, but patients with MGs have a profound immunosuppression that may inhibit the ability of antigen presenting cells (APCs), even those generated ex vivo, to induce EGFRvIII-specific immune responses. In this report, we summarize our results in humans targeting this mutation in two consecutive and one multi-institutional Phase II immunotherapy trials. These trials demonstrated that vaccines targeting EGFRvIII are capable of inducing potent T- and B-cell immunity in these patients, and lead to an unexpectedly long survival time. Most importantly, vaccines targeting EGFRvIII were universally successful at eliminating tumor cells expressing the targeted antigen without any evidence of symptomatic collateral toxicity. These studies establish the tumor-specific EGFRvIII mutation as a novel target for humoral- and cell-mediated immunotherapy in a variety of cancers. The recurrence of EGFRvIII-negative tumors in our patients, however, highlights the need for targeting a broader repertoire of tumor-specific antigens.

Epidermal growth factor receptor vIII expression in U87 glioblastoma cells alters their proteasome composition, function, and response to irradiation

Little is known about the factors that influence the proteasome structures in cells and their activity, although this could be highly relevant to cancer therapy. We have previously shown that, within minutes, irradiation inhibits substrate degradation by the 26S proteasome in most cell types. Here, we report an exception in U87 glioblastoma cells transduced to express the epidermal growth factor receptor vIII (EGFRvIII) mutant (U87EGFRvIII), which does not respond to irradiation with 26S proteasome inhibition. This was assessed using either a fluorogenic substrate or a reporter gene, the ornithine decarboxylase degron fused to ZsGreen (cODCZsGreen), which targets the protein to the 26S proteasome. To elucidate whether this was due to alterations in proteasome composition, we used quantitative reverse transcription-PCR to quantify the constitutive (X, Y, Z) and inducible 20S subunits (Lmp7, Lmp2, Mecl1), and 11S (PA28alpha and beta) and 19S components (PSMC1 and PSMD4). U87 and U87EGFRvIII significantly differed in expression of proteasome subunits, and in particular immunosubunits. Interestingly, 2 Gy irradiation of U87 increased subunit expression levels by 16% to 324% at 6 hours, with a coincident 30% decrease in levels of the proteasome substrate c-myc, whereas they changed little in U87EGFRvIII. Responses similar to 2 Gy were seen in U87 treated with a proteasome inhibitor, NPI0052, suggesting that proteasome inhibition induced replacement of subunits independent of the means of inhibition. Our data clearly indicate that the composition and function of the 26S proteasome can be changed by expression of the EGFRvIII. How this relates to the increased radioresistance associated with this cell line remains to be established.

Targeted therapy for malignant gliomas

The identification of markers that are associated with tumour but not normal tissue has allowed the development of highly-specific targeted therapies. Monoclonal antibodies, either alone or linked to radioisotopes or toxins, have provided a powerful tool for research, as well as the basis for promising therapeutic agents with less side effects than standard radiotherapy or chemotherapy. A new class of drugs, the tyrosine kinase inhibitors, which interfere with the function of key molecules in cancer-promoting pathways, have had a dramatic effect in haematological malignancy and are being trialled in solid tumours, including glioma. Although the problem of achieving specific, high-level delivery of these various agents to tumours in the brain remains a major issue, encouraging early results with some targeted agents support the attractive theoretical principles of this new paradigm. Further work to identify new molecular targets and to develop agents exploiting them, is needed, as well as confirmation of their safety and efficacy by clinical trials.

Stimulated PI3K-AKT signaling mediated through ligand or radiation-induced EGFR depends indirectly, but not directly, on constitutive K-Ras activity

Previous results showed an inducible radiation sensitivity selectively observable for K-RAS-mutated cell lines as a function of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor blockade of phosphatidylinositol 3-kinase (PI3K)-AKT signaling. Therefore, the role of K-Ras activity for a direct (i.e., through activation of PI3K by K-Ras) or an indirect stimulation of PI3K-AKT signaling (through K-Ras activity-dependent EGFR ligand production) was investigated by means of small interfering RNA and inhibitor approaches as well as ELISA measurements of EGFR ligand production. K-RASmt tumor cells presented a constitutively activated extracellular signal-regulated kinase-1/2 signaling, resulting in enhanced production and secretion of the EGFR ligand amphiregulin (AREG). Medium supernatants conditioned by K-RASmt tumor cells equally efficiently stimulated EGFR signaling into the PI3K-AKT and mitogen-activated protein kinase pathways. Knocking down K-Ras expression by specific small interfering RNA markedly affected autocrine production of AREG, but not PI3K-AKT signaling, after treatment of K-RAS-mutated or wild-type cells with EGFR ligands or exposure to ionizing radiation. These results indicate that PI3K-mediated activation of AKT in K-RASmt human tumor cells as a function of EGFR ligand or radiation stimulus is independent of a direct function of K-Ras enzyme activity but depends on a K-Ras-mediated enhanced production of EGFR ligands (i.e., most likely AREG) through up-regulated extracellular signal-regulated kinase-1/2 signaling. The data provide new differential insight into the importance of K-RAS mutation in the context of PI3K-AKT-mediated radioresistance of EGFR-overexpressing or EGFR-mutated tumors.