Novel Vector to Promote Radiosensitivity and Apoptosis in Prostate Carcinoma Cells

PUBLIC ABSTRACT

Prostate cancer is the most common malignancy and the second leading cause of death in males. Presently, no single therapeutic regimen has proven successful in eradicating the local tumor or preventing recurrence and metastasis. The lack of viable treatment options necessitates novel treatment approaches. Gene-based therapy for the management of prostate cancer provides a promising therapeutic option for both primary and recurrent disease. A new therapeutic approach employs viruses to deliver and replace tumor suppressor genes, i.e., genes that suppress the cancerous properties of tumor cells. To enhance the efficacy of this strategy, a gene promoter capable of selectively targeting gene expression to cancer cells, without harming normal cells, would be of immense clinical value and is the focus of this project.

Using the molecular biological approach of subtraction hybridization, a novel gene, progression elevated gene-3 (PEG-3), has been identified that displays increased expression as a tumor cell acquires aggressive properties. Moreover, forced expression of PEG-3 in cancer, but not in normal, cells results in more rapid tumor formation in animal models and increased blood vessel formation, a hallmark of aggressive cancers. To define the mechanism by which PEG-3 induces these profound effects specifically in cancer cells, our laboratory has isolated the region of the PEG-3 gene that controls its expression, the PEG-3 promoter (PEG-Prom). When the PEG-Prom is transferred into normal and cancer cells, this promoter is selectively active in tumor cells. This observation is noteworthy and suggests that the PEG-Prom could be used to directly target expression of defined genes selectively in cancer cells. Of added significance, PEG-3 expression is induced by ionizing radiation and the activity of the PEG-Prom is significantly augmented, predominantly in cancer cells, by radiation.

Subtraction hybridization has also identified a novel tumor suppressor gene associated with human melanoma progression; melanoma differentiation associated gene-7, mda-7/IL-24. When mda-7/IL-24 is expressed, by means of a replication incompetent adenovirus (Ad), in multiple cancer subtypes, including prostate cancer, growth is suppressed and programmed cell death (apoptosis) is induced. In contrast, mda-7/IL-24 exhibits no harmful effects in normal epithelial, including prostate, and fibroblast cells. Based on these observations, a Phase I clinical trial has been completed and mda-7/IL-24 has been found to be safe in patients, while inducing significant tumor cell death. Based on these relevant studies, Phase II clinical trials have begun to define antitumor activity of Ad.mda-7 in patients with diverse cancers.

Overexpression of another important molecule, Bax, induces apoptosis in diverse cell types. The expression of Bax is induced by radiation and the combination of Bax and radiation provides synergistic killing effects in multiple cancer cell lines including prostate cancer.

The present proposal is directed toward the development of a novel therapy for prostate cancer that employs a dual treatment regimen of ionizing radiation and adenovirus-based targeted gene expression. A replication-incompetent bipartite adenovirus will be constructed in which the expressions of mda-7/IL-24 and Bax will be controlled by the PEG-Prom (Ad.PEG-mda-7.Bax). The use of the PEG-Prom in the presence of radiation will provide enhanced expression of both mda-7/IL-24 and Bax in tumor cells. This will profoundly increase the level of killing provided by radiation, mda-7/IL-24, or Bax alone in cancer cells. Moreover, the cytokine (secreted) property of mda-7/IL-24 should allow cancer-specific killing and radiosensitization in both primary and potentially metastatic prostate cancers. The ability of (Ad.PEG-mda-7.Bax) to inhibit prostate cancer cell survival will be tested in the presence and absence of radiation both in vitro and in vivo in prostate cancer xenografts in athymic nude mice.

Ad.PEG-mda-7.Bax, in combination with radiation, offers potential for developing improved therapies for prostate cancer. Successful completion of the proposed studies will provide a solid framework for translation of this novel vector into the clinic as an effective therapy for patients with prostate cancer.