Co-chairs: William M. Pardridge, M.D., and Edward H. Oldfield, M.D.
Peter M. Black
Ronald G. Blasberg
E. Antonio Chiocca
Pam Del Maestro
Ramon Gilberto Gonzalez
|Frederick F. Lang, Jr.
Edward A. Neuwelt
Sam D. Rabkin
STATEMENT OF THE PROBLEM
An important problem in the treatment of human brain tumors is posed by the need to deliver therapeutic agents to specific regions of the brain, distributing them within and targeting them to brain tumors. The molecules that might otherwise be effective in diagnosis and therapy either do not cross the blood-brain barrier (BBB) in the brain adjacent to the tumor or do not cross the blood-tumor barrier (BTB) in adequate amounts. Improving our knowledge of the basic molecular and cellular biology of the brain microvasculature, which constitutes the BBB and BTB in vivo, could lead to innovative new strategies for drug targeting to human brain tumors.
The magnitude of this challenge stems from the lack of emphasis on BBB research in both academic neuroscience and the pharmaceutical industry. Knowledge of the basic functions of the BBB and research on cerebrovascular biology lag behind those of neuronal or glial biology. An improved understanding of the brain vasculature and the BBB will play a crucial role in the development of new therapeutic and diagnostic approaches for the treatment of human brain tumors. In addition, there is a need for novel delivery strategies that are unique to the brain and that bypass the vasculature.
At one time, the BBB was not been considered to present a problem in the diagnosis and treatment of brain tumors because early scans of human brain tumors suggested that the BTB was "leaky." This leakiness is relative, however: as the size of the molecule increases, the rate of movement across the barrier decreases. Accordingly, antibodies that could be used as either diagnostic or therapeutic molecules do not cross the BTB in sufficient quantities to be effective. Anti-sense oligonucleotides, which could be used either to inhibit oncogenic signals or as anti-sense radiopharmaceuticals to image gene expression of the brain in vivo, also do not cross the BTB in sufficient amounts for activity. Gene therapies, whether of viral or nonviral formulations, are often too large to cross the BTB.
These problems are substantially greater for the BBB in the brain adjacent to the tumor, because even small molecules do not readily cross the BBB, which is the site of invasion of glioma cells into normal brain. Furthermore, the expression of drug-active efflux transporters, which are expressed at the BBB and the BTB, actively efflux chemotherapeutics from the brain back to the blood and may thereby prevent significant distribution of chemotherapeutic agents in the brain. It is partly for these reasons that most of the classical chemotherapeutic molecules that have been used to treat cancer outside the central nervous system (CNS) are ineffective in the treatment of brain tumors.
CHALLENGES AND QUESTIONS
Chief among the challenges to be addressed in brain tumor research is limited knowledge about the basic biology of brain endothelial cells, about the cells of origin and the developmental changes in gene and protein expression in brain and endothelial cells, and about the proliferative potential, turnover rate, and regional differences of brain endothelial cells. There is also limited knowledge of the cell interactions among brain endothelial cells, tumor cells, and cells of hematopoietic origin. The role of angiogenesis in tumor development and anti-angiogenesis research are important avenues for future studies of brain tumor therapy.
Mechanisms for drug targeting in the brain involve going either "through" or "behind" the BBB. Modalities for drug delivery through the BBB entail disruption of the BBB, either by osmotic means or biochemically by the use of vasoactive substances such as bradykinin. The potential for using BBB opening to target specific agents to brain tumors has just begun to be explored. Other strategies to go through the BBB may entail the use of endogenous transport systems, including carrier-mediated transporters such as glucose and amino acid carriers; receptor-mediated transcytosis for insulin or transferrin; and active efflux transporters such as p-glycoprotein. Strategies for drug delivery behind the BBB include intracerebral implantation and convection-enhanced distribution. There is a need to determine which strategies are most effective and how they can be improved for patients with brain tumors.
Cerebral edema is a serious complication in many patients with brain tumors. The molecular and gene-related mechanisms underlying the formation of cerebral edema need to be identified. Also needed are more quantitative methods to measure flux or transfer rate constants as indicators of vascular permeability in patients and experimental animals. We also need to improve our understanding of how to reverse edema and to better understand the mechanisms underlying the effects of current therapies.
Current in vitro models of the BBB are inadequate. Although brain endothelial cells co-cultured with astrocytes have been used to study the BBB, better in vitro models that retain the phenotype of brain endothelial cells will be valuable, as would an in vitro model of the BTB. Basic research on brain tumors would be facilitated by the development of appropriate models that simulate the human condition in situ.
RESEARCH AND SCIENTIFIC PRIORITIES
Priority 1: Develop strategies for delivering both small and large molecules to the CNS.
The transport of small molecules might be enhanced by designing drugs that have affinity for one of the carrier-mediated transporters within the BBB. Alternatively, drugs that inhibit the active efflux transporters may be useful as "co-drugs" to mediate the uptake of chemotherapeutic agents that are normally effluxed from brain to blood. Tumor-specific agents could be used with BBB disruption. Similar approaches might be used to develop new diagnostics for human brain tumor imaging. Peptide or antisense radiopharmaceuticals could be developed as molecular "Trojan horses" that bind to endogenous receptor-mediated transporters in the BBB and are transferred across the BBB by this mechanism.
An important mission for the future, not only for brain tumors but for the field of neuroscience in general, is the ability to "image any gene in any person." This might be done with antisense radiopharmaceuticals that are made transportable through the BBB. This is a "barrier" problem, because to be successful, an antisense molecule targeted at an mRNA molecule within the tumor cell must be transported across not only the BTB but also the tumor cell and organelle membrane "barriers."
Priority 2: Identify the genes and proteins expressed by the BBB and the BTB.
BBB genomics should be considered a high priority. Because only very abundant BBB-specific transcripts will be detected with whole-brain gene microarrays, BBB genomics research needs to start with the initial isolation of brain capillaries from animal or human brain, both normal and tumor derived. Comparison of capillaries from normal brain and brain tumor can help to elucidate the tissue-specific gene expression at the BTB and distinguish it from the tissue-specific gene expression at the BBB and normal brain. The elucidation of the pattern-specific tissue expression at the BBB or the BTB would provide the platform for further investigations on overall brain capillary biology and brain vasculature biology as they pertain to conditions such as angiogenesis, cell adhesion, antigen presentation, metastasis, and local inflammation.
Priority 3: Develop novel viral and non-viral strategies for brain tumor gene therapy.
Viral strategies include the use of adenovirus, herpes simplex virus, adeno-associated virus, and other virus vectors. To date, investigators conducting trials in humans have used virus gene formulations administered invasively, through intracerebral implantation. Work in experimental animals, however, has demonstrated that both intra-arterial and intravenous delivery can be efficacious and safe. Some of the new conditionally replicating virus vectors have the added advantage of virus amplification within the tumor after passage through the BBB, thus increasing the treatment volume.
The developments in Priorities 1 and 2 should be applied to improve the delivery of virus and non-virus vectors to tumors and to target the BTB. Current problems include the lack of information on the role of the immune system in limiting virus replication, enhancing tumor rejection, and potentially causing brain inflammation. Limitations also exist in the methodologies available for targeting specific tumor cells in order to minimize potential toxicity to normal brain and vasculature. Studies have demonstrated the synergistic effects of virus vectors with other modes of therapy, such as radiotherapy, chemotherapy, and immunotherapy, but to date these approaches have not been applied in humans.
The development of novel drug targeting systems in the brain that enhance brain tumor uptake of either small- or large-molecule diagnostics or therapeutic molecules requires the following:
• Novel forms of BTB disruption
• Drugs that access BBB carrier-mediated transport systems
• Drugs that inhibit BBB active efflux transporters such as p-glycoprotein
• New vectors (ligands) that are transported across the BBB by receptor-mediated transcytosis systems, which can act as "molecular Trojan horses" for transporting drugs across the BBB and BTBPriority 2
Isolated brain capillaries from either animal or human brain and human brain tumor should be used as the starting point for preparing BBB-specific gene arrays and cDNA libraries from BBB and BTB. BBB gene-specific proteomic programs can also be developed in parallel. The focus on these genomics or proteomic programs should be molecular-based strategies for investigation of the following:
• Differences in capillaries perfusing normal brain BBB and tumor capillaries (BTB)
• New endogenous BBB or BTB transporters for targeted drug delivery to brain tumors
• Novel mechanisms of tumor angiogenesis, invasion, cell adhesion, metastasis, and antigen presentationPriority 3
Development of vectors and strategies for gene/viral delivery to brain tumors, taking into account the unique character of brain vasculature, extracellular space, and cell diversity, will require the following:
• Virus or non-virus vectors that target brain tumors upon intraarterial or intravenous administration. Such strategies may utilize the ability of some virus vectors to cross the brain vasculature and specifically multiply within the tumor or may follow the development of novel BBB/BTB drug targeting systems.
• Vectors or molecules that specifically target the tumor vasculature without harm to the normal brain vasculature
• Strategies that utilize the above vectors in combination with radiotherapy, chemotherapy, or immunotherapyThe NCI and the NINDS are urged to adopt all three of these priorities, as they are interconnected. Gene and viral therapy, use of recombinant proteins, monoclonal antibodies, or antisense therapy may be successful in patients with the adaptation of novel BTB drug targeting systems applied to brain tumors. However, the discovery of novel BBB drug targeting systems will be accelerated by the classification of the tissue-specific gene expression at the BBB through a BBB genomics program. Further, the use of any biological agent for brain tumor therapy could have immunological problems that are different from those of similar therapies administered for non-CNS disease. Studies of this immunological response need to be supported in order to take these agents safely into clinical trial. Also needed are future training programs focusing on brain vascular biology in order to produce a generation of scientists who can integrate our knowledge of neuroscience and cerebrovascular biology.
Last updated February 9, 2005