Molecular Neurosurgery With Targeted Toxins

Malignant gliomas are among the most challenging of all cancers to treat successfully, being characterized not only by aggressive proliferation and expansion but also by inexorable tumor invasion into distant brain tissue. Although considerable progress has been made in the treatment of these tumors with combinations of surgery, radiotherapy, and chemotherapy, these efforts have not been curative. Neurosurgeons as oncologists have increasingly turned their attention to therapies on a molecular scale. Of particular interest to neurosurgeons is the ability to deliver therapy locally to the tumor site or to take advantage of existing immunological mediators, enhancing drug concentrations or therapeutic cell numbers while bypassing the blood-brain barrier to maximize efficacy and minimize systemic toxicity. Exciting local-therapy approaches have been proposed for these devastating tumors. In this review, we discuss the potential applications of bioreactors, neural stem cells, immunotherapies, biodegradable polymers, and convection-enhanced drug delivery in the treatment of malignant gliomas. These approaches are at different stages of readiness for application in clinical neurosurgery, and their eventual effects on the morbidity and mortality rates of gliomas among human patients are difficult to ascertain from successes in animal models. Nevertheless, we are entering an exciting era of "nanoneurosurgery," in which molecular therapies such as those discussed here may routinely complement existing surgical, radiological, and chemotherapeutic approaches to the treatment of neuro-oncological disease. The potential to deploy any of a number of eloquently devised molecular therapies may provide renewed hope for neurosurgeons treating malignant gliomas.

Molecular Neurosurgery: Understanding and Harnessing Human Chemical Structure and Function

Different vector systems that have been used and/or specifically developed for central nervous system (CNS) gene transfer studies are briefly discussed along with their advantages and disadvantages with respect to potential clinical application. These include retroviruses, recombinant herpes simplex virus, adenoviruses, adenoassociated viruses, encapsulation of plasmid deoxyribonucleic acid into cationic liposomes, and neural and oliogodendroglial stem cells. Particular attention has been paid to relate the modality of a specific CNS gene therapy to the strategy for adequate delivery of genetic material to the brain for either global or localized CNS neurodegenerative chronic disorder, as well as for CNS tumors and stroke. Techniques to circumvent the "impermeable" blood-brain barrier and how to breach the more versatile blood-brain-tumor barrier to deliver the genetic material to the target CNS cells are reviewed and include the following: 1) local stereotactic CNS injection/infusion of viral vectors, administration of vector producer cells, or cell replacement; 2) local administration of genetic material into the cerebrospinal fluid ventriculocisternal system; 3) osmotic opening of the blood-brain barrier; 4) local intra-arterial infusion; and 5) administration of blood-brain-tumor barrier permeabilizers, such as a bradykinin B2 agonist RMP-7. It is concluded that gene therapy for several brain disorders holds great potential, as suggested mainly by in vitro experiments and, to some extent, by a limited number of animal experiments. However, several drawbacks currently hamper the application of gene therapy under the clinical setting. The problems associated with gene therapy that still present major obstacles are as follows: 1) inefficient transfection of host cells by viral vectors; 2) restricted delivery of genetic material across vascular barriers of the CNS and brain tumors; 3) nonselective expression of the transgene; and 4) in situ CNS regulation of the transgene expression in a therapeutically controlled manner, as imposed by the course and phenotype of the CNS disease.

Molecular Neurosurgery in the Pituitary Gland

Recent advances in cellular and molecular biology and better understanding of genetic and biochemical bases of different central nervous system (CNS) disorders have made gene therapy of the CNS a realistic goal. Concept approaches for gene therapy of CNS disorders are reviewed and include the following: 1) gene replacement with a single normal allele to correct the inherited global neurodegenerative disorders, such as enzyme deficiencies; 2) brain repair to restore the function of a particular subset of cells that were lost because of a neurodegenerative process; 3) gene therapy of brain tumors; and 4) gene therapy of stroke. Techniques of viral vector-mediated CNS transfer of a therapeutic gene, transplantation of genetically modified cells, fetal embryonic implantation and/or implantation of genetically engineered neural progenitor cells, and production of a specific enzyme, neurotransmitter, and/or growth factor are discussed with respect to the therapeutic potential for global and localized CNS neurodegenerative disorders and stroke. Transfection of the CNS tumor cells with the drug susceptibility ("suicide") gene and/or "toxic" gene and antisense strategies and a concept of adoptive immunotherapy of brain tumors are also discussed. Other approaches, such as transfer of drug-resistant genes and monoclonal antibody gene transfer, are briefly discussed. In addition to summarizing current principles of gene therapy for several groups of CNS disorders, the issues that remain to be resolved in clinical reality, such as delivery of the genetic material and regulation of the cellular expression of the transgene, and the negatives associated with the concepts of gene therapy, such as transient gene expression, toxicity of viral proteins, drawbacks of antisense therapy, and the problem of immune response to the transfected protein, have been also identified.

Contemporary Applications of Functional and Stereotactic Techniques for Molecular Neurosurgery

The term molecular neurosurgery has been applied to several different experimental strategies, including a variety of genetic manipulations. For the purposes of this book, the term is used to refer to the use of targeted cytotoxins to produce highly selective neural lesions. Used in this sense, the term is relevant to both experimental and potential clinical applications. The body of work addressed in this volume grew out of initial experiments in the laboratory of Donald J. Reis in 1980–1981. The initial experimental challenge was how to selectively destroy baroreceptor afferents that make up a small portion of the vagus and glossopharyngeal nerves. The strategy chosen was to develop a technique using toxin retrogradely transported from an application site on the peripheral baroreceptor nerves in the neck. First attempts used low-molecular-weight cytotoxic drugs, such as doxorubicin, and were unsuccessful. Reasoning that the initial lack of success reflected inadequate delivery of toxin to the cell bodies, a plan was developed to attach these drugs to a well-transported agent, such as wheat germ agglutinin, which at the time was introduced as a highly effective anatomical tracer (1). However, a simpler option seemed attractive. If a lectin such as wheat germ agglutinin was well transported, then perhaps a toxic lectin such as ricin or abrin would work. In retrospect, Harper and colleagues (2) had previously shown evidence for retrograde axonal transport of ricin, but this publication was discovered only after the initial suicide transport experiments applied ricin to the vagus nerve (3).

Patients with glioma(which constitute 33% of all brain tumor cases) still have a poor prognosis, especially in the case of malignant(anaplastic astrocytoma and glioblastoma). This poor prognosis is related to the fact that malignant glioma cells aggressively infiltrate into normal brain tissues, making total removal of the tumor impossible. The median survival time of glioblastoma patients is less than 2 years, despite multimodality treatment with extensive surgical resection and adjuvant therapies using radiation and immunochemotherapy. In order to overcome this formidable neoplasm, the effectiveness of molecular neurosurgery using gene therapy has been investigated since 1992. In this paper, the current state of gene therapy for malignant brain tumors are described, and the future direction of this fascinating approach is discussed.