Current Cancer Drug Targets - Volume 12, Issue 3, 2012

Malignant gliomas are the most common type of primary brain tumors and prognosis of patients with these cancers remains poor despite standard treatment with radiotherapy and temozolomide. The structure and functioning of the Blood Brain Barrier (BBB) in brain tumours are controversial and have significant implications for designing new therapeutic strategies. Therefore, it is crucial to develop a strategy to allow drug delivery across the BBB. One way is to use direct delivery strategies with liposomal formulations. Molecular targeted therapy provides new effective treatment options with minimal toxicity; however, the involvement and the interaction of several signalling pathways determine difficulties to detect prevalent targets that can be successfully addressed by novel agents. Therefore, the development of targeted therapy for gliomas has been particularly challenging. Multitargeted kinase inhibitors, novel monoclonal antibodies, and new vaccines have been developed. Standard treatments and current development of new therapies for malignant gliomas are reviewed, focusing specifically on growth factors and their receptors, as well as the downstream intracellular effector molecules. Although histological and molecular genetic variability exists within malignant gliomas, there are alterations in specific cellular signal transduction pathways or cellular functions which are common in most gliomas. This finding has led to trials of novel molecularly targeted therapeutic agents alone, and in various combinations, for patients with malignant gliomas. Brain metastases, which occur in approximately 20-40% of individuals with systemic cancer, represent a significant cause of morbidity and mortality and overwhelm all other types of brain tumors in terms of incidence and public health impact. Recent advances in the treatment of many malignancies have frequently been due to the incorporation of molecular targeted agents into the treatment regimen. The role of target-based agents in the treatment of brain metastases from breast cancer is not still clear even if this tumor disease has largely taken advantage from the wide-spread use of target-based agents such as trastuzumab or lapatinib. We will also discuss the results derived from the trials using EGF-R-targeted agents and anti-VEGF bevacizumab in the treatment of brain metastases from non small cell lung cancer (NSCLC). Given our growing experience with the feasibility and safety of multimodal chemo-radiotherapy in the treatment of brain metastases, as well as the fact that intracranial responses often track extracranial responses in chemotherapy-naive patients, we will also discuss the disease-specific treatment for patients with brain metastases derived from different primary tumor sites. This special issue provides the current understanding of potential new therapeutic targets and keeps in consideration the action and the potential benefits of some promising target-based agents for the treatment of either primary or metastatic brain tumors.

Nanotechnology-based drug delivery was born as a chance for pharmaceutical weapons to be delivered in the body sites where drug action is required. Specifically, the incorporation of anti-cancer agents in nanodevices of 100-300 nm allows their delivery in tissues that have a fenestrated vasculature and a reduced lymphatic drainage. These two features are typical of neoplastic tissues and, therefore, allow the accumulation of nanostructured devices in tumours. An important issue of anti-cancer pharmacological strategies is the overcoming of anatomical barriers such as the bloodbrain- barrier (BBB) that protects brain from toxicological injuries but, at the same time, makes impossible for most of the pharmacological agents with anti-cancer activity to reach tumour cells placed in the brain and derived from either primary tumours or metastases. In fact, only highly lipophilic molecules can passively diffuse through BBB to reach central nervous system (CNS). Another possibility is to use nanotechnological approaches as powerful tools to across BBB, by both prolonging the plasma half-life of the drugs and crossing fenestrations of BBB damaged by brain metastases. Moreover, modifications of nanocarrier surface with specific endogenous or exogenous ligands can promote the crossing of intact BBB as in the case of primary brain tumours. This aim can be achieved through the binding of the nanodevices to carriers or receptors expressed by the endothelial cells of BBB and that can favour the internalization of the nanostructured devices delivering anti-cancer drugs. This review summarizes the most meaningful advances in the field of nanotechnologies for brain delivery of drugs.

Glioblastoma (glioblastoma multiforme; GBM; WHO Grade IV) accounts for the majority of primary malignant brain tumors in adults. Amplification and mutation of the epidermal growth factor receptor (EGFR) gene represent signature genetic abnormalities encountered in GBM. A range of potential therapies that target EGFR or its mutant constitutively active form, EGFR, including tyrosine kinase inhibitors (TKIs), monoclonal antibodies, vaccines, and RNA-based agents, are currently in development or in clinical trials for the treatment of GBM. Data from experimental studies evaluating these therapies have been very promising; however, their efficacy in the clinic has so far been limited by both upfront and acquired drug resistance. This review discusses the current status of anti-EGFR agents and the recurrent problem of resistance to these agents that strongly indicates that a multiple target approach will provide a more favorable future for these types of targeted therapies in GBM.

Central nervous system (CNS) metastases from breast cancer (BC) represent an important cause of diseaserelated morbidity and mortality. For BC patients who develop CNS metastases, local control measures (both surgery and radiation) are essentially palliative and usually poorly effective, with systemic therapies often failing to achieve optimal control mainly due to the presence of the blood-brain barrier which hampers adequate penetration of therapeutic agents into the brain. However, recent evidence suggests that the status of the human epidermal growth factor receptor-2 (HER2) strongly influences the incidence of CNS metastases and the survival of BC patients from the time of development of CNS metastases, with HER2-positive (HER2+) patients generally experiencing higher rates of CNS metastases and prolonged overall survival compared to patients with HER2-negative disease. This phenomenon likely reflects the difficult CNS drug-penetration and improved control of extra-CNS disease following the clinical use of the anti-HER2 monoclonal antibody trastuzumab. Importantly, this HER2-based survival difference has important implications when planning the optimal treatment of BC patients with CNS metastases. To date, although no systemic therapy has been specifically approved for the treatment of CNS metastases from BC, several targeted agents are being clinically developed for this purpose. In the present review we will discuss the targeted therapies that are under investigation for the treatment of CNS metastases from BC, highlighting the different implications based on whether a given agent is being developed to target CNS metastases from HER2+ or HER2-negative breast cancer.

The prognosis of malignant glioma and metastatic brain tumours is still extremely poor, despite recent advances in therapeutic strategies with molecular-targeted agents. Poly(ADP-ribose) polymerase (PARP) inhibitors are a promising, novel class of anticancer drugs to be used either as single agents or in combination with chemotherapy and radiotherapy. PARP-1 and PARP-2 are the only PARP proteins that bind to DNA single strand breaks (SSBs), facilitating the repair process by the base excision repair. For this reason, PARPs have been extensively investigated as targets of novel drugs that may be used to enhance the antitumour activity of SSBs inducing agents, such as the methylating compound temozolomide, which is the drug of choice for glioblastoma, or ionizing radiations. Moreover, PARP inhibitors exert cytotoxic effects in monotherapy in BRCA mutated tumours, which are defective in the homologous recombination (HR) repair. Finally, recent studies have shown that inhibition of PARP function might also induce anti-angiogenic effects which might contribute to impair tumour growth. Many clinical trials with PARP inhibitors are ongoing for the treatment of a variety of advanced solid tumours, including primary or secondary brain tumours. This review discusses the implications of targeting PARP on the design of new treatment regimens.

Brain metastases (BM) are a common occurrence in patients with non-small cell lung cancer (NSCLC). Standard therapy options include whole brain radiotherapy and, in selected patients, surgery or stereotactic radiosurgery. The role of systemic treatment is controversial. There is a strong clinical rationale for the use of targeted therapies, because patients often have a poor performance status, and are not candidates for cytotoxic chemotherapy or radiotherapy, yet treatment is required to improve the extra-cranial disease. The efficacy of epidermal growth factor receptor (EGFR) inhibitors in the treatment of patients with BM from NSCLC has been reported mainly in case reports or small retrospective case series, with only a few prospective trials. Current evidence suggests that the use of EGFR tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib should be considered in patients with asymptomatic CNS involvement, when clinical characteristics suggest a high likelihood of response; these characteristics are adenocarcinoma histology, never-smoker status, female gender and East Asian ethnicity. Upfront therapy with EGFR TKIs should be strongly considered in asymptomatic patients harboring activating EGFR mutations. In symptomatic BM, radiotherapy (RT) remains the standard treatment. Based on currently available data, treatment with concurrent RT and EGFR TKIs should be investigated in experimental trials only.

Brain metastases represent the most common intracranial tumors in the adults. Its incidence outnumbers that of primary brain tumors by a tenfold factor. Estimated cumulative incidence is between 10 to 20% of all cancer patients, which would represent over 170 000 new cases in the US. Typically, patients with multiple brain metastases are exposed to whole brain radiation therapy, as a palliative measure. Resulting median survival improvement is modest, ranging from 3 to 5 months. This survival has not been altered despite 3 decades of clinical research aiming at improving outcome of these patients. The role of standard chemotherapy in the treatment of brain metastases has always been marginal, as the penetration of chemotherapy beyond the BBB (blood-brain barrier) is considered limited. Whereas the BBB is universally recognized as a physiological entity, its role in the treatment of brain metastases remains controversial. Metastatic lesions often depict a homogeneous intense enhancement on either CT or MRI, thus implying that the brain tumor barrier (BTB) is breached. Although there is no doubt that the BBB and BTB suffer from variable degrees of breach in integrity in the presence of malignant brain tumors, impediment to drug delivery remains, and strategy to optimize delivery must be considered if one is to really impact patient’s outcome in the treatment of these diseases. The intended purpose of this paper is to review current data on the role of the BBB in the treatment of CNS metastatic disease.

Brain tumors, primary and metastatic, are a cause of significant mortality and morbidity. Radiotherapy (RT) forms an integral part of the treatment of brain tumors. Intrinsic relative tumor radio-resistance, normal tissue tolerance and impact on neurocognitive function, all limit the efficacy of RT. Radiosensitizers can potentially increase efficacy on tumors while maintaining normal tissue toxicity, with or without inherent cytotoxicity. This article reviews the evolution of evidence with use of non-cytotoxic radiosensitizers in brain radiotherapy and their status at the end of the first decade of this millennium. Considering, the era of development and mechanism of action, these agents are classified as first, second and third-generation non-cytotoxic radiosensitizers. The last millennium involved elaboration of first-generation compounds including halogenated pyrimidines, hypoxic cell sensitizers (e.g. imidazoles) and glycolytic inhibitors (e.g. lonidamine). The first decade of this millennium has highlighted redox modulators like motexafin gadolinium and newer hypoxic cell sensitizers like efaproxiral, which have shown promise. However, phase III trials and meta-analyses have not identified a clear winner though the second-generation has shown some rays of hope. Recent research has focused on expanding the horizon by studying modulation of newer molecular pathways like DNA repair, microtubule stabilization, cytokine function and nuclear factor-kappa beta (NF-KB) in order to increase RT efficacy. The review concludes by summarizing the class of evidence and the level of recommendation available for use of non-cytotoxic radiosensitizers in brain RT.

Despite advances in multidisciplinary approaches, the prognosis for most patients with malignant gliomas is poor. Malignant gliomas are highly vascularized tumors with elevated expression of vascular endothelial growth factor (VEGF), an important mediator of angiogenesis. Recent studies of bevacizumab, an anti-VEGF monoclonal antibody, alone or associated with chemotherapy, have demonstrated high response rates and prolongation of median and 6-month progression-free survival. Clinical evaluation of several multitarget small molecule tyrosine kinase inhibitors is ongoing. Other promising antiangiogenic compounds are cilengitide and continuous temozolomide. Toxicity is acceptable. Open issues are represented by patterns of tumor progression, resistance mechanisms and biomarkers.

Lung cancer is the leading cause of cancer-related mortality worldwide, and non-small cell lung cancer (NSCLC) accounts for about 85% of all new lung cancer diagnosis. The majority of people with NSCLC are unsuitable for surgery since most patients have metastatic disease at diagnosis. About 60% of brain metastases arise from lung cancer. Therapeutic approaches to brain metastases include surgery, whole brain radiotherapy (WBRT), stereotactic radiosurgery, chemotherapy and new biologic agents. Angiogenesis is essential for the development and progression of cancer, and vascular endothelial growth factor (VEGF) is a critical mediator of tumour angiogenesis. One of the targeted approaches most widely studied in the treatment of NSCLC is the inhibition of angiogenesis. Bevacizumab, an anti-VEGF recombinant humanized monoclonal antibody, is the first targeted agent which, when combined with chemotherapy, has shown superior efficacy versus chemotherapy alone as first-line treatment of advanced non-squamous NSCLC patients. Patients with central nervous system (CNS) metastases have initially been excluded from bevacizumab trials for the risk of cerebral haemorrhage as a result of the treatment. Nevertheless, the available data suggest an equal risk of intracranial bleeding in patients with CNS metastases treated with or without bevacizumab therapy. Several other anti-angiogenetic drugs are being investigated in the treatment of advanced NSCLC patients, but results of their activity specifically in CNS metastases are still lacking. This review will focus on the potential role of bevacizumab and other anti-angiogenetic agents in the treatment of brain metastases from NSCLC.