Drug delivery to brain tumours: challenges and progress

Nearly 12.5 million new cancer cases are diagnosed worldwide each year. Although new treatments have been developed, most new anticancer drugs that are effective outside the brain have failed in clinical trials against brain tumours, in part due to poor penetration across the blood–brain barrier and the blood–brain tumour barrier. This review will discuss the challenges of drug delivery across the blood–brain barrier/blood–brain tumour barrier to cancer cells, as well as progress made thus far. This will include a biochemical modulation strategy that transiently opens the barrier to increase anticancer drug delivery selectively to brain tumours. It will also briefly discuss a quantitative non-invasive method to measure permeability changes and tumour response to treatment in the human brain.

Introduction

Every year in the US, ∼ 20,000 new primary and nearly 200,000 secondary (metastatic) brain tumour cases are reported. Worldwide numbers are more distressing. Even after surgical resection, brain cancer invariably recurs, severely shortening life expectancy [1]. Conventional treatment using radiation and intravenous chemotherapy often prove unsuccessful primarily because the anticancer drugs fail to cross the blood–brain barrier (BBB) in sufficient quantities [2]. Therefore, understanding the biochemical regulation of the BBB in its normal and abnormal states (in and around tumours) is of great importance as efforts continue to deliver therapeutic compounds to brain cancers. The focus is now on targeted cancer therapy by not only supplementing conventional chemotherapy and radiotherapy, but also by preventing toxicity in normal tissues and drug resistance. In particular, successful treatment of brain tumours involves efficient anticancer drug delivery to brain tumours across the blood–brain tumour barrier (BTB). Although the BTB is ‘leaky’ in the tumour centre, the established microvessels (capillaries) feeding the proliferating tumour edge and the brain tissue surrounding the tumour is nearly as impermeable as the BBB [3]. Therefore, the BTB still poses a major obstacle to anticancer drug delivery to tumours. In this article, the challenges involved in and the progress made, especially in the past decade, towards delivering therapeutic drugs selectively to brain tumours will be reviewed.

The cerebral microvessels/capillaries that form the BBB protect the brain from toxic agents in the blood but also pose a significant hindrance to the delivery of small and large therapeutic molecules. Pardridge reported that the BBB blocks delivery of > 98% of CNS drugs [2,4]. The National Institutes of Health (NIH) cited as high priority goals to understand the function of the BBB and BTB, develop novel drug delivery approaches for molecular-targeted therapy, and to further develop methods to non-invasively image the response of brain tumours to treatment. Different strategies have been developed to circumvent the physiological barrier that is posed by the BBB, often based on a conception of the barrier as being controlled by what is called the neurovascular unit. This consists of endothelial cells (ECs), tight junctional proteins connecting the ECs, glia, pericytes and astrocytic foot processes, which interact with neurons (Figure 1). For the most part, research seeks to understand the interaction among the constituents of the BBB and neurons in normal and pathological conditions [5]. By using in vitro and in vivo models, researchers seek to achieve a better understanding of the effects of neurological disorders on the BBB and, thereby, improve our knowledge of BBB biology. The goal is to better comprehend the initiation and progression of neurological disease and to develop approaches to effectively treat brain diseases such as brain tumours.

Novel cancer therapies include antiangiogenic agents, immunotherapy, bacterial agents, viral oncolysis, cyclin-dependent kinases and receptor tyrosine kinase inhibitors, antisense agents, gene therapy and combinations of various methods. Amazing clinical success in treating some types of cancer has been achieved using immunotherapy-based anticancer agents such as cytokines, monoclonal antibodies and cancer vaccines. For example, one promising treatment uses antisense oligonucleotides, such as small interfering RNAs, which have been used in various clinical trials for cancer: but only for cancers outside the brain [6]. Despite these promising approaches, the BBB still causes a significant complication to brain cancer treatment. As whole-brain gene microarrays have detected fewer BBB-specific transcripts [7], the focus of work carried out by Ningaraj and others is on cerebrovascular genomics and proteomic research. The ideal approach is to isolate brain capillaries in normal and diseased brain tissue and then to analyse genomic and proteomic differences. Generally human brain tissue from a temporal lobectomy of a trauma or epilepsy patient is considered to be normal tissue in these studies as it is unethical to obtain normal, healthy human brain tissue [8].

Drug delivery to brain tumours: challenges

Drug delivery research focuses on several innovative methods, including nanoparticles [9], microparticles as carriers of anticancer agents, PEG technology, encapsulating anticancer drugs in liposomes, and monoclonal antibodies for the delivery of anticancer payloads [2]. Focusing on brain cancer, one area of research has focused on ECs, which are a major component of the neurovascular unit [10,11]. However, many issues that are related to ECs are still not well understood, including gene and protein profiling in normal brain and brain tumour capillary ECs [12,13]. This research is hampered due to the complexities that are involved in isolating pure ECs devoid of pericytes, neurons and tumour cell populations, as well as from the differences between and within brain tumours. For instance, significant differences were found between normal human brain and brain tumour capillaries, including differential expression of calcium-activated potassium (KCa) [14,15] and ATP-sensitive potassium (KATP) channels [16]. Recent progress in the molecular targeting of tumour-specific antigens with specific agents, however, can be exploited by identifying additional novel targets for modulating BBB/BTB permeability. Future studies will seek to determine whether there are significant differences in the expression levels (induced or suppressed) of certain genes and proteins between normal and brain tumour capillary ECs.

Studies were executed using cerebrovascular genomics and proteomics in a laser-captured microscopy-dissected pure capillary EC population isolated from human normal brain and metastatic breast and lung tumour tissues [17,18]. This approach may elucidate differences in gene clusters and their expression products between the capillary ECs of normal brain and brain tumours [19]. Known [2] and novel BBB/BTB-specific genes and proteins can then be used to better understand BBB/BTB permeability regulation in human brain tumour tissues. Developing novel drug delivery modalities to brain tumours across the BBB/BTB is crucial, especially now as receptor tyrosine kinase inhibitors have been shown to have clinical benefit in patients with cancers outside the brain [20-22], whereas their clinical efficacy against cancer in the brain is modest at best [22,23], mainly due to their failure to cross the BBB/BTB

In a preliminary unpublished work, employing laser-captured microscopy technology, pure capillary ECs were isolated from human ‘normal brain’ (trauma brain tissue) and brain tumour tissues (NS Ningaraj et al.). An EC-rich sample was subjected to MALDI-TOF mass spectrometry proteomic analysis (Figure 2A and 2B) to compare capillaries from normal brain and cancer tissue and to highlight differences in protein expression between healthy and tumour tissue. Such pattern-specific tissue expression may provide the platform for further investigation into overall brain vascular biology as it pertains to conditions such as angiogenesis, cell adhesion, metastasis, cell–cell communication and local inflammation. Genomic and proteomic studies should facilitate the development of novel drug and gene delivery methods, including gene therapy via vectors or modified autologous cell transfer in the brain based on the unique properties of the BBB and BTB.