Cancer: Novel Viral-based Gene Therapy Targets Tumors, Leave Normal Cells Alone

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.