Drug Studies: Gene Therapy, Cancer-Killing Viruses and New Drugs Highlight Novel Approaches to Cancer Treatment

LOS ANGELES - Studies presented at the 2007 meeting of the American Association for Cancer Research show how researchers are using the new, as well as the natural, to help design and test new drugs to treat cancer.

For example, researchers are marrying the latest technologies and drug design together to figure out if a drug is having a biological impact, what the effect is, when it stops working and what can be done about it. They have “watched” as an experimental angiogenesis inhibitor shrank deadly brain tumors and when it began to fail. By reading blood proteins they discovered why that happened, and how a combination of therapies might work better.

Scientists are also turning to existing “natural” biological systems to help them design next era cancer therapies. Several research groups are making progress in turning viruses into smart search-and-destroy tumor busters that will leave normal cells alone, and others are finding that marijuana’s active ingredient can tweak receptors on the most common form of lung cancer and reduce cancer growth.


Eradication of resistant prostate cancer by a novel gene therapy approach: Abstract 4182

A research team at Columbia University has designed a novel viral-based gene therapy they say blasts through a body, targeting both primary and distant tumors, while leaving normal cells untouched. In the 15 mice they tested, injections of the therapy in tumors on one side of the mouse eliminated those cancers as well as tumors on the other side of the animal’s body, producing a cure in all of the mice.

This study tested this “dual cancer-specific targeting strategy” with aggressive therapy resistant prostate cancer. The researchers have also shown it works in animals with breast, and melanoma tumors.

An earlier version of the therapy showed powerful effects in a phase I clinical trial, said Paul B. Fisher, M.Ph., Ph.D., professor clinical pathology at Columbia. This improved treatment appears to be a much “smarter bomb with potential of treating metastatic and therapy-resistant cancers,” he said.

“The beauty of this approach is that two methods are being used to destroy a tumor,” said Devanand Sarkar, M.B.B.S, Ph.D., the study’s primary author, associate research scientist at Columbia. “The virus we designed replicates within a tumor, and at the same time produces a massive amount of a cancer killing compound. Either action alone is damaging and potentially deadly, but together they are lethal.”

Columbia researchers built the therapy around their earlier, pivotal discovery of a cytokine (a signaling protein) called melanoma differentiation associated gene-7/interleukin-24 (mda-7/IL-24). A technology developed in the Fisher laboratory, “subtraction hybridization,” applied to human melanoma, induced the cancer to revert to a more normal state, allowing comparison of genes expressed in both states. They discovered mda-7/IL-24 was progressively down-regulated as melanoma developed. In its normal state, the cytokine may affect growth and immune regulation, whereas expression at high levels kills cancer cells.

The investigators altered an adenovirus to carry the mda-7/IL-24 into tumors that normally did not express the gene, and based on successful animal studies, this cytokine was tested for safety in patients with advanced melanoma and other solid cancers. “Interestingly, this phase I clinical trial produced a significant clinical response,” Fisher said.

To make the treatment more potent, they then paired the mda-7/IL-24 gene with a “replication competent” adenovirus, a virus that can multiply within cells. After such a microbe enters a cell, it can reproduce and cause the cell to burst, releasing more viral particles. During replication, the mda/IL-24 gene is also reproduced and then expressed, delivering huge quantities of active mda/IL-24 locally and systemically.

Finally, the researchers worked out a strategy to ensure that the loaded virus would only replicate within cancer cells. They manipulated the viral genome again, and substituted its normal promoter (E1A) with a promoter (PEG-3) that they discovered could only be activated by transcription factors found in cancer cells. That means that if the virus may enter a normal cell, it won’t replicate and the cell will not die, the researchers say. It also suggests that the therapy will work in a variety of cancers “because virtually all cancers we have tested contain the necessary transcription factors that activate the PEG-3 promoter,” Fisher said.

When the viral gene therapy was injected into tumors growing in the mice, the virus replicated and produced mda-7/IL-24, which then killed the tumors, releasing millions of newly produced, loaded viral particles throughout the blood circulation to settle into distant tumors where the process was repeated. It also worked on prostate cancer resistant to other therapy because the two-pronged attack “overwhelmed their defense mechanisms,” Sarker said.

Although Sarkar and Fisher say the results are exciting, they stress that additional research is needed prior to testing the therapy in humans, including experiment in mice with an intact immune system. While a primary immune system response against the virus may eliminate some of the loaded particles, the researchers say that the mda-7/IL-24 will likely heighten a secondary therapeutic immune response, offering a much stronger cancer-killing potential.


AZD2171, a pan-VEGF receptor tyrosine kinase inhibitor, normalizes tumor vasculature and alleviates edema in glioblastoma patients: Abstract 2118

A phase II clinical trial of an angiogenesis inhibitor to treat glioblastoma has shown promise in a majority of patients tested, say researchers at Massachusetts General Hospital and Harvard Medical School. But they also say that the novel imaging and biomarker studies they performed as treatment was underway have revealed why the treatment, AZD2171, ultimately failed, and what might improve the response.

The imaging studies, which used specially adapted Magnetic Resonance Imaging (MRI) scans, exposed a “window” during which the blood system feeding the tumor reverted to a more normal state, before morphing again into the leaky, dilated vessels that make drug treatment difficult.

The blood biomarker studies showed that as tumors stopped relying on vascular endothelial growth factors (VEGF) to pump up blood flow to them - and VEGF is what AZD2171 targets - they started using two other growth factors, neither of which had previously been recognized as important for human tumor blood vessel growth.

The study is unique because it is the first to test AZD2171 in glioblastoma patients, and to find that it “offered promising benefits such as tumor shrinkage and reduction of brain swelling,” said Tracy Batchelor, M.D., chief of neuro-oncology at Massachusetts General Hospital.

Of 31 patients who participated, more than half experienced tumor shrinkage of 50 percent or more, and median time to tumor regrowth was 111 days. “This was not a randomized study, but compared to historical benchmarks, in which response to conventional therapies is approximately 10 percent and progression is usually 63 days, these results are encouraging,” Dr. Batchelor said.

The agent, which has been tested in other tumor types but is not yet approved, also reduced edema, or swelling, in the brain, he said. Because of that, some patients were able to stop using steroids, which can cause debilitating side effects.

The clinical trial also provided insights into how AZD2171 functions and how the therapy might be improved, the researchers say. MRI scans taken before, during, and after treatment provided a timeline picture of AZD2171’s effectiveness, and then loss of function as tumors began to resist the agent.

“This was beautiful,” said Rakesh Jain, Ph.D., professor of tumor biology at Harvard Medical School. “We were able to see changes within 24 hours of taking a single dose.”

Jain and his colleagues have spent years documenting how developing cancers promote blood growth factor signaling, which causes blood vessel architecture to go seriously awry: vessels loop back on each other, send blood in the wrong direction, and become enlarged as well as leaky due to holes that develop. They have found regions in solid tumors in which blood flows briskly, and others in which there is little or none. “If we try to deliver drugs to those latter areas, they do not arrive,” Jain said.

Still, cancer cells are alive in those hypoxic regions, and, in fact, they morph into much more aggressive cells, he said. It is also in these areas where cancer stem cells might hide. “Buried deep in this hostile environment are the cells responsible for invasion and metastasis,” Jain said.

The blood biomarker studies allowed them to track what was happening in the tumors. The researchers discovered that as expression of VEGF proteins decreased, levels of two other proteins increased as the tumor switched to other pathways. One of these proteins, fibroblast growth factor (FGF), was thought to be involved in angiogenesis, but the other, chemokine stroma-cell-derived factor 1 alpha (SDF1α), was a new discovery, Jain said. “We threw a net up with the biomarker studies and found the involvement of FGF, which had never been documented in patients, and SDF1α, which was not known to be one of several dozen pro-angiogenic molecules identified so far in such studies.”

“We all recognize that what we need to do now is combine this therapy with other types of treatments, either existing or to be developed, and to deliver these drug combinations during the window we have identified,” Dr. Batchelor said. “This might help us manage patients much more effectively.”


Δ-9 Tetrahydrocannabinol inhibits growth and metastasis of lung cancer: Abstract 4749

The active ingredient in marijuana cuts tumor growth in common lung cancer in half and significantly reduces the ability of the cancer to spread, say researchers at Harvard University who tested the chemical in both lab and mouse studies.

They say this is the first set of experiments to show that the compound, Δ-9 tetrahydrocannabinol (THC), inhibits EGF-induced growth and migration in epidermal growth factor receptor (EGFR) expressing non-small cell lung cancer cell lines. Lung cancers that over-express EGFR are usually highly aggressive and resistant to chemotherapy.

THC that targets cannabinoid receptors CB1 and CB2 is similar in function to endocannabinoids, which are cannabinoids that are naturally produced in the body and activate these receptors. The researchers suggest that THC or other designer agents that activate these receptors might be used in a targeted fashion to treat lung cancer.

“The beauty of this study is that we are showing that a substance of abuse, if used prudently, may offer a new road to therapy against lung cancer,” said Anju Preet, Ph.D., a researcher in the Division of Experimental Medicine.

Acting through cannabinoid receptors CB1 and CB2, endocannabinoids (as well as THC) are thought to play a role in variety of biological functions, including pain and anxiety control, and inflammation. Although a medical derivative of THC, known as Marinol, has been approved for use as an appetite stimulant for cancer patients, and a small number of U.S. states allow use of medical marijuana to treat the same side effect, few studies have shown that THC might have anti-tumor activity, Preet says. The only clinical trial testing THC as a treatment against cancer growth was a recently completed British pilot study in human glioblastoma.

In the present study, the researchers first demonstrated that two different lung cancer cell lines as well as patient lung tumor samples express CB1 and CB2, and that non-toxic doses of THC inhibited growth and spread in the cell lines. “When the cells are pretreated with THC, they have less EGFR stimulated invasion as measured by various in-vitro assays,” Preet said.

Then, for three weeks, researchers injected standard doses of THC into mice that had been implanted with human lung cancer cells, and found that tumors were reduced in size and weight by about 50 percent in treated animals compared to a control group. There was also about a 60 percent reduction in cancer lesions on the lungs in these mice as well as a significant reduction in protein markers associated with cancer progression, Preet says.

Although the researchers do not know why THC inhibits tumor growth, they say the substance could be activating molecules that arrest the cell cycle. They speculate that THC may also interfere with angiogenesis and vascularization, which promotes cancer growth.

Preet says much work is needed to clarify the pathway by which THC functions, and cautions that some animal studies have shown that THC can stimulate some cancers. “THC offers some promise, but we have a long way to go before we know what its potential is,” she said.


Targeted release of oncolytic measles virus by blood outgrowth endothelial cells in situ inhibits orthotopic gliomas: Abstract 4185

Researchers in Germany have hidden vaccine-grade measles virus inside artificially generated blood cells in order to devise a search-and-destroy therapy for human brain cancer that can’t be “seen” by the immune system.

They say their mouse experiments show a proof of principle that this non-pathogenic virus can attack glioma by getting inside tumor cells and replicating, destroying the common brain tumors from the inside out. This and other so-called “oncolytic” viruses are already being tested in clinical trials, but their effectiveness has been limited by an immediate human immune response, the researchers say.

“In an immune-competent patient, the immune system will fight the virus, and most adults are immune against measles since they have been vaccinated against the disease in childhood or have had measles,” said Christian Beltinger, M.D., an associate professor at the University Children’s Hospital in Ulm. “Although cancer patients are immune-compromised by their disease or because of therapy, they still may mount a sufficient attack against vaccine measles virus.”

To trick this immune surveillance, the researchers generated blood outgrowth endothelial cells (BOECs), which are produced outside of the body using human blood bathed in a cocktail of growth factors. “They do not naturally occur in the blood, but they are derived from endothelial progenitor cells, rare cells that are produced in the bone marrow and shed into the blood,” Dr. Beltinger said.

These cells are well suited for cancer therapy for two reasons, he said. If a vaccine measles virus is tucked within them, it can’t be reached by the immune system’s neutralizing antibodies. Also, they are endothelial progenitor cells, which are recruited in the body wherever new blood vessels are formed.

“Tumors need vessels to grow, hence they recruit these blood progenitor cells,” Dr. Beltinger said. “That makes them home to the tumors.”

BOECs have been used for other gene therapeutic approaches, such as for hemophilia, but this is the first time they have been adapted to carry vaccine measles virus, he said.

To test how well they functioned as a cancer therapy, the researchers injected U87 cells (the most commonly used human glioma cancer cell line) into the brains of immune-compromised mice. Once the tumors were established, BOECs recently infected with vaccine measles virus were injected around, but not into, the brain tumor. These loaded blood cells navigated through normal brain tissue to the tumor mass, and once inside, the BOECs released the virus into surrounding tumor cells. It then spread to other tumor cells.

Eventually the blood cells died. This delay of death, however, was sufficient to allow the infected cells to home to the tumor and release the virus, the researchers say.

They found that mice treated with BOECs survived significantly longer than mice receiving just empty blood cells or “naked” measles virus. But the researchers say that all mice eventually died, showing that the therapy could not completely eradicate the tumors.

“While these modified blood cells carrying vaccine measles virus look like a promising novel therapy for gliomas, it is still a preclinical experimental approach,” Dr. Beltinger said. "Potentially it could be used on most malignant gliomas, including glioblastomas, because the targeting of the virus can be genetically modified.