Funded Grants

Development of Gene Delivery Systems for the Treatment of Brain Cancer

According to Webster's Dictionary, the definition of the word "hope" is "A wish or desire accompanied by confident expectation of its fulfillment". The wish or expectation of a brain cancer patient is the wish or desire to be cured. They would then expect the physician following the diagnosis to be able to provide a therapy that would fulfill this desire. Unfortunately, for the majority of brain cancers, this is not the case. At best what can be provided is a small extension of the person's life with the quality of life during this extended time period being poor. The other aspect is the frustration of physicians lacking the tools or therapies to provide hope for curing these patients. This is where I hope through my research to increase the arsenal of effective therapies for clinicians to treat this disease.

Before delving into how we will develop brain cancer therapies, let's better understand the deficiencies of the existing therapies. Diagnosis of brain tumors is the result of altered behavior, dizziness, disorientation, impairment of vision or persisting headaches. It can be any one of these symptoms or a group of symptoms. This is the result of the tumor impinging on a nerve or neural process in the brain. Usually, the tumor is large enough to image by either Magnetic Resonance Imaging (MRI) or Computer Assisted Tomagraph (CAT) scanning. These imaging instruments serve several purposes, one being to determine if the patient has a tumor and the second is to identify the location of the tumors so that they can be surgically removed by a neurosurgeon. These instruments do not provide the surgeon with information to distinguish tumor tissue from non?tumor tissue, resulting in incomplete surgical removal of the cancerous cells. Hence, the need for additional therapy to kill those residual cells either by radiation or chemotherapy.

Radiation kills tumor cells by damaging the DNA of the tumor. For effective radiation therapy, the DNA must be exposed in a state of replication. This occurs for a very short period of time in the life cycle of the tumor cell and cell division within the tumor is random. Such that at any one time there is a small fraction of cells undergoing division and consequently a small number of cells susceptible to radiation damage. This can be compensated for by applying multiple dose of radiation a varying time points. However, the radiation will affect non?cancer tissue resulting in limiting the doses that can be applied.

Chemotherapy requires that the drug cross the blood brain barrier, diffuse to the cancer cells and be taken up to illicit an effect. There is one new drug currently approved, TEMODAR (temozolimide) that is able to cross the blood brain barrier. One additional problem is that tumors develop drug resistance, so even if the drug were to get to the correct site, it would not be taken up by the cancer cells.

The ideal drug would be one that remained at the tumor site for a sufficient period of time to be effective, the drug would not be susceptible to export by the multidrug resistance machinery of the cells and would not be toxic to noncancer cells. Technology has been developed that addresses some of these features in that wafers composed of a biocompatible polymer are impregnated with a cancer drug, carmustine (BCNU), and placed in the tumor cavity after surgery. With time the drug leaches from the wafers and is taken up by cancer cells. The technology yields a slowing of the tumor growth but does not completely eliminate the tumor and as the drug becomes depleted, the tumor returns. The problem is that the wafers do not contain enough drug and readministration requires additional surgery to a site that is already traumatized from the initial surgery.

The application of gene therapy to treat this form of cancer deals directly with this supply of drug and the type of drug. Gene therapy is the delivery of genes encoding a therapeutic protein to cells. For cancer, the therapeutic protein can participate directly as a monkey wrench in disconnecting the events leading to cell division. Alternatively, it can be secreted and bind to neighboring cancer cells that sends a signal for the cell to either stop growing or self destruct.

Most cells in the human body do not divide, with the exception of the bone marrow and cells of the immune system. The reason that tumors become a problem is because the cells do divide and do so at a rapid aggressive pace. In cancer, the signals inside the cells that prevent cell division are either silenced or deleted. By restoring these signals, cell division can be halted or stalled. Halting or stalling the cells for an abnormal period of time is a signal to the cell that something is wrong and that triggers a self?destruct mechanism that leads to cell death.

Through gene therapy, these signals for halting cell division can be restored for a sufficient period of time and in the required quantity to trigger selfdestruction of the tumor cells. The key is in delivery. For the therapy to be effective, 100% of the cells need to either produce the therapeutic protein or have it bind to their surface. Direct administration of the therapeutic protein suffers from the same problem that the BCNU impregnated polymer wafers have, insufficient supply.

The next question is how to deliver the gene. Genes can be delivered directly to the tumors however, the injection volume is limited and only those cells in direct contact with the needle tract formed from the injection will receive the genes. A better approach is through the food supply of the tumor, i.e., the blood stream. Because the genes are large, they do not readily enter the cells. The gene can be coated with a material that tricks the cells into thinking it is food, very analogous to the "Trojan Horse". Once inside the cell, the gene becomes uncoated and in so doing becomes activated for producing the therapeutic protein. In the tumor, not only do the tumor cells divide but also the cells that make up the tumor blood vessels. Delivering the gene to these cells inhibits them from growing and in so doing, inhibits the formation of new blood vessels that are essential for supplying nutrients to newly dividing tumor cells. In addition, the therapeutic protein can be engineered such that it is secreted from the blood vessel cells into the space between the tumor cells thus providing direct access to the tumor cells. Even though the blood vessels are not dividing, they still have the capacity to produce the therapeutic protein. Hence, the gene therapy can cut off the food supply to the tumor indirectly inhibiting tumor growth and it can also interact with the tumor cells and directly inhibit tumor growth.

The other advantage of this technology is that it only impacts dividing cells. So even if it were to be taken up by non?cancerous cells, only those cells that were dividing would be affected. Gene delivery to other tissue can be minimized by injecting the gene delivery system into the blood stream through a tube that is upstream of the tumor. When tumor cells grow, their blood vessels are disorganized due to the rapid rate of growth. The location for insertion of the tube into the blood vessel is such that the first organ the coated genes see is the tumor vasculature. Because it is so disorganized, the genes become trapped in the tumor blood vessels and result in selectively being localized to the tumor. These coated genes can be readministered once a week, every other week or once a month.

This type of therapy can be used by itself or in combination with already approved therapy, such as surgery followed by radiation or simply in combination with radiation. The success of this therapy will not only create a new form of brain cancer therapy but it will also be applicable to the treatment of other types of cancer, such as metastases.