Advancing drug therapy for brain tumours: a current review of the pro-inflammatory peptide Substance P and its antagonists as anti-cancer agents.

Evidence for the involvement of the Substance P (SP)/NK1 receptor system in the development and progression of cancer strongly supports its potential as a therapeutic target in malignancies. Novel strategies for approaching cancer treatment are urgently required particularly with regard to tumours of the central nervous system (CNS), which are notoriously difficult to effectively treat and associated with extremely poor prognosis for many patients. This is due, in part, to the presence of the highly specialised blood-brain barrier, which is known to restrict common treatments such as chemotherapy and hinder early tumour diagnosis. Additionally, tumours of the CNS are difficult to surgically resect completely, often contributing to the resurgence of the disease many years later. Interestingly, despite the presence of the blood-brain barrier, circulating tumour cells are able to gain entry to the brain and form secondary brain tumours; however, the underlying mechanisms of this process remain unclear. Tachykinins, in particular Substance P, have been implicated in early blood-brain barrier disruption via neurogenic inflammation in a number of other CNS pathologies. Recent evidence also suggests that Substance P may play a central role in the development of CNS tumours. It has been well established that a number of tumour cells express Substance P, NK1 receptors and mRNA for the tachykinin NK1 receptor. This increase in the Substance P/NK1 receptor system is known to induce proliferation and migration of tumour cells as well as stimulate angiogenesis, thus contributing to tumour progression. Accordingly, the NK1 receptor antagonist presents a novel target for anti-cancer therapy for which a number of patents have been filed. This review will examine the role of Substance P in the development of CNS tumours and its potential application as an anti-cancer agent.

Treatment with the NK1 antagonist emend reduces blood brain barrier dysfunction and edema formation in an experimental model of brain tumors.

The neuropeptide substance P (SP) has been implicated in the disruption of the blood-brain barrier (BBB) and development of cerebral edema in acute brain injury. Cerebral edema accumulates rapidly around brain tumors and has been linked to several tumor-associated deficits. Currently, the standard treatment for peritumoral edema is the corticosteroid dexamethasone, prolonged use of which is associated with a number of deleterious side effects. As SP is reported to increase in many cancer types, this study examined whether SP plays a role in the genesis of brain peritumoral edema. A-375 human melanoma cells were injected into the right striatum of male Balb/c nude mice to induce brain tumor growth, with culture medium injected in animals serving as controls. At 2, 3 or 4 weeks following tumor cell inoculation, non-treated animals were perfusion fixed for immunohistochemical detection of Albumin, SP and NK1 receptor. A further subgroup of animals was treated with a daily injection of the NK1 antagonist Emend (3 mg/kg), dexamethasone (8 mg/kg) or saline vehicle at 3 weeks post-inoculation. Animals were sacrificed a week later to determine BBB permeability using Evan's Blue and brain water content. Non-treated animals demonstrated a significant increase in albumin, SP and NK1 receptor immunoreactivity in the peritumoral area as well as increased perivascular staining in the surrounding brain tissue. Brain water content and BBB permeability was significantly increased in tumor-inoculated animals when compared to controls (p<0.05). Treatment with Emend and dexamethasone reduced BBB permeability and brain water content when compared to vehicle-treated tumor-inoculated mice. The increase in peritumoral staining for both SP and the NK1 receptor, coupled with the reduction in brain water content and BBB permeability seen following treatment with the NK1 antagonist Emend, suggests that SP plays a role in the genesis of peritumoral edema, and thus warrants further investigation as a potential anti-edematous treatment.

The potential for substance P antagonists as anti-cancer agents in brain tumours.

Despite recent advances in cancer treatment and diagnosis, the prognosis for patients with CNS tumours remains extremely poor. This is, in part, due to the difficulty in completely removing tumours surgically, and also because of the presence of the blood brain barrier, which can prevent the entry of chemotherapeutic agents typically used in cancer treatment. Despite the presence of the blood brain barrier, tumour cells are capable of entering and colonising the brain to form secondary brain tumours. Additionally, tumour related disruption of the blood brain barrier is associated with the clinical presentation of many patients, with accompanying increases in intracranial pressure due, in part, to the development of vasogenic oedema. Vasogenic oedema results because the newly formed angiogenic vessels within brain tumours do not retain the highly selective properties of the blood brain barrier, and thus allow for the extravasation of plasma proteins and water into the brain parenchyma. Tachykinins, and in particular substance P, have been implicated in blood brain barrier disruption and the genesis of cerebral oedema in other CNS insults via a process known as neurogenic inflammation. Recent evidence suggests that substance P may play a similar role in CNS tumours. It has been well established that an upregulation of substance P and its receptors occurs in a number of different cancer types, including CNS neoplasms. In addition to disrupting blood brain barrier permeability, substance P and the NK1 receptors facilitate promotion of tumour growth and the development of cerebral oedema. Accordingly, recent patents describe the potential of NK1 receptor antagonists as anti-cancer agents suggesting that substance P may provide a novel cancer treatment target. This review will examine the role of substance P in the development of CNS tumours.

NK1 receptor antagonists and dexamethasone as anticancer agents in vitro and in a model of brain tumours secondary to breast cancer.

Emend, an NK1 antagonist, and dexamethasone are used to treat complications associated with metastatic brain tumours and their treatment. It has been suggested that these agents exert anticancer effects apart from their current use. The effects of the NK1 antagonists, Emend and N-acetyl-L-tryptophan, and dexamethasone on tumour growth were investigated in vitro and in vivo at clinically relevant doses. For animal experiments, a stereotaxic injection model of Walker 256 rat breast carcinoma cells into the striatum of Wistar rats was used. Emend treatment led to a decrease in tumour cell viability in vitro, although this effect was not replicated by N-acetyl-L-tryptophan. Dexamethasone did not decrease tumour cell viability in vitro but decreased tumour volume in vivo, likely to be through a reduction in tumour oedema, as indicated by the increase in tumour cell density. None of the agents investigated altered tumour cell replication or apoptosis in vivo. Inoculated animals showed increased glial fibrillary acidic protein and ionized calcium-binding adapter molecule 1 immunoreactivity indicative of astrocytes and microglia in the peritumoral area, whereas treatment with Emend and dexamethasone reduced the labelling for both glial cells. These results do not support the hypothesis that NK1 antagonists or dexamethasone exert a cytotoxic action on tumour cells, although these conclusions may be specific to this model and cell line.

Characterisation of Walker 256 breast carcinoma cells from two tumour cell banks as assessed using two models of secondary brain tumours.

BACKGROUND: Metastatic brain tumours are a common end stage of breast cancer progression, with significant associated morbidity and high mortality. Walker 256 is a rat breast carcinoma cell line syngeneic to Wistar rats and commonly used to induce secondary brain tumours. Previously there has been the assumption that the same cancer cell line from different cell banks behave in a similar manner, although recent studies have suggested that cell lines may change their characteristics over time in vitro. METHODS: In this study internal carotid artery injection and direct cerebral inoculation models of secondary brain tumours were used to determine the tumorigenicity of Walker 256 cells obtained from two cell banks, the American Type Culture Collection (ATCC), and the Cell Resource Centre for Medical Research at Tohoku University (CRCTU). RESULTS: Tumour incidence and volume, plus immunoreactivity to albumin, IBA1 and GFAP, were used as indicators of tumorigenicity and tumour interaction with the host brain microenvironment. CRCTU Walker 256 cells showed greater incidence, larger tumour volume, pronounced blood-brain barrier disruption and prominent glial response when compared to ATCC cell line. CONCLUSIONS: These findings indicate that immortalised cancer cell lines obtained from different cell banks may have diverse characteristics and behaviour in vivo.

Walker 256 tumour cells increase substance P immunoreactivity locally and modify the properties of the blood-brain barrier during extravasation and brain invasion.

It is not yet known how tumour cells traverse the blood-brain barrier (BBB) to form brain metastases. Substance P (SP) release is a key component of neurogenic inflammation which has been recently shown to increase the permeability of the BBB following CNS insults, making it a possible candidate as a mediator of tumour cell extravasation into the brain. This study investigated the properties of the BBB in the early stages of tumour cell invasion into the brain, and the possible involvement of SP. Male Wistar rats were injected with Walker 256 breast carcinoma cells via the internal carotid artery and euthanised at 1, 3, 6 and 9 days post tumour inoculation. Culture medium-injected animals served as controls at 1 and 9 days. Evidence of tumour cell extravasation across the BBB was first observed at 3 days post-inoculation, which corresponded with significantly increased albumin (p < 0.05) and SP immunoreactivity (p < 0.01) and significantly reduced endothelial barrier antigen labelling of microvessels when compared to culture medium control animals (p < 0.001). By day 9 after tumour cell inoculation, 100 % of animals developed large intracranial neoplasms that had significantly increased albumin in the peri-tumoral area (p < 0.001). The increased SP immunoreactivity and altered BBB properties at 3 days post-inoculation that coincided with early tumour invasion may be indicative of a mechanism for tumour cell extravasation into the brain. Thus, extravasation of tumour cells into the brain to form cerebral metastases may be a SP-mediated process.

Targeting classical but not neurogenic inflammation reduces peritumoral oedema in secondary brain tumours.

Dexamethasone, the standard treatment for peritumoral brain oedema, inhibits classical inflammation. Neurogenic inflammation, which acts via substance P (SP), has been implicated in vasogenic oedema in animal models of CNS injury. SP is elevated within and outside CNS tumours. This study investigated the efficacy of NK1 receptor antagonists, which block SP, compared with dexamethasone treatment, in a rat model of tumorigenesis. Dexamethasone reverted normal brain water content and reduced Evans blue and albumin extravasation, while NK1 antagonists did not ameliorate oedema formation. We conclude that classical inflammation rather than neurogenic inflammation drives peritumoral oedema in this brain tumour model.

Towards drug discovery for brain tumours: interaction of kinins and tumours at the blood brain barrier interface.

Cancers of the brain are intrinsically more complicated to treat than systemic malignancies due to the unique anatomical features of the brain. The blood-brain barrier prevents chemotherapeutic agents from reaching brain neoplasms, and angiogenesis occurs as the metabolic needs of the tumour increase, thus further complicating treatment. The newly formed blood vessels form the blood-tumour barrier and are distinct from the blood-brain barrier in that they are more permeable. Being more permeable, these abnormal blood vessels lead to the formation of peri-tumoural edema, which is the cause of much morbidity and mortality associated with central nervous system neoplasms. While the cause of the increased permeability is unclear, kinins have been implicated in regulating the permeability of normal vasculature. Kinins are also known to exert many inflammatory actions affecting both normal and angiogenic blood vessels, as well as tumour cells. The vasodilatory and vascular permeabilizing effects of kinins, and particularly bradykinin and substance P, have been investigated with regard to delivery of chemotherapeutic agents to neoplastic brain tissue through both vascular barriers. In contrast, kinin receptor antagonists have been found to exert effects on tumour cells that result in decreased angiogenesis, tumour cell motility and growth. Thus, many recent patents describe kinin activity on brain vasculature, which may play an integral role in the development of treatments for malignancies in the central nervous system through amelioration of angiogenesis. In conjunction, patents that discuss the ability of kinins to decrease tumour cell migration and proliferation demonstrate that kinins may offer novel approaches to brain tumour therapy in the future.