Prostate cancer is the second leading cause of cancer death among American men and is responsible for around 40,000 deaths per year. This disease is more prevalent in older men, and with increasing life expectancy, prostate cancer is an escalating health problem. Localized prostate cancer can be treated by surgical removal of the prostate, or radiotherapy. However, for many men, the disease has spread beyond the prostate by the time of diagnosis. For these individuals, treatment with compounds that block the action of male hormones (androgens) is generally effective but rarely curative. Unfortunately, almost all prostate cancers become resistant to hormonal manipulation eventually, usually within a few years. Many chemotherapy agents have been tested in prostate cancer clinical trials, but none have produced any significant increase in survival rate. Therefore, new therapeutic agents are much needed for the treatment of hormone-independent prostate cancer.

Prostate cancer, like all cancers, develops when cells grow without the normal control mechanisms. This is usually a result of inappropriate or increased expression of proteins involved in cell proliferation, or because of inactivation of proteins that would normally ensure regulation of cell growth. Ideally, a therapeutic agent would target only these dysregulated proteins (or the genes that encode them) in order to inhibit the growth of cancer cells while leaving healthy cells undamaged.

Oligonucleotides are short fragments of DNA that can be synthesized chemically. They have been tested as agents that target the dysregulated proteins by binding to the specific DNA or RNA molecules that encode them. One of these ¿antisense¿ oligonucleotides is now approved by the Food and Drug Administration for clinical use, and many others are being tested in human clinical trials. These trials have shown that oligonucleotides are safe and have fewer side effects than most cancer chemotherapy drugs. However, these antisense oligonucleotides have not achieved optimal activity, partly because they bind to molecules other than their target DNA or RNA.

We discovered that certain types of oligonucleotides could completely inhibit the growth of prostate cancer cells growing in cell culture. Because of the DNA sequence of these oligonucleotides, we postulated that they form unusual structures and work by a different mechanism from the ones described above. Therefore, in our Phase I research, we proposed to study the activity, mechanism, and structure of these novel oligonucleotides.

We have now confirmed that the novel oligonucleotides form unusual folded structures that are very stable under biological conditions. We have also shown that these oligonucleotides bind to specific cellular proteins that probably recognize their unique three-dimensional shape (in contrast to antisense oligonucleotides, which bind to RNA by recognizing the sequence). Most importantly, we have identified the oligonucleotide-binding protein as nucleolin, a protein involved in cell growth. This protein is present at much higher levels in cancer cells than in most normal cells and is thus a good target for therapeutic intervention. The novel oligonucleotides can also inhibit the growth of other types of cancer cells (for example, breast cancer cells) but are less active at inhibiting the growth of non-cancer cells. Because of their high activity, potential selectivity for cancer cells over normal cells, and novel mechanism of action, these oligonucleotides represent promising new agents in the fight against prostate cancer.

In the Phase II studies, we propose to pursue the clinical development of the novel oligonucleotides. Before human clinical trials can begin, it will be necessary to demonstrate activity in animal models. Therefore we propose to test efficacy of the oligonucleotides in rat and mouse models of prostate cancer. Preliminary data suggest a high likelihood of success. Our preliminary studies also show that the novel oligonucleotides can work in combination with traditional chemotherapy drugs to increase their activity. Therefore, we will test these combinations in cultured cells and in animals. In a broader perspective, Phase I studies have identified nucleolin as a novel target protein for the therapy of prostate cancer. The studies proposed in Phase II will lead to further understanding of the role of this protein in prostate cancer biology and may lead to new diagnostic, prognostic, or therapeutic approaches for prostate cancer. Because small molecule drugs may offer a more practical alternative to oligonucleotides, we will also use a computer-assisted drug design approach to identify potential small molecule inhibitors of nucleolin.

Since discovering the novel growth inhibitory oligonucleotides less than 3 years ago, the investigators on this proposal have made remarkable progress in understanding their mechanism and structure. The combination of their expertise in cancer biology and treatment, structural biology, and drug design methods has yielded productive research in the Phase I studies and supports the success of Phase II studies.