Prostate cancer is the leading neoplasm in American men. Two hundred thousand new cases will be diagnosed in 1994 resulting in 38,000 deaths. Thus far, no effective treatment has been developed for patients with advanced, and especially hormone refractory, disease. The development of novel therapeutic strategies is paramount. The use of suicide genes to selectively eradicate tumor cells has shown promise. Driven by tumor tissue-specific promoters, these genes can be transcribed selectively in the neoplastic cells and subsequently cause their destruction. The applicant has characterized a prostate tissue-specific promoter, derived from the 5' flanking sequence of human prostate-specific antigen (PSA) gene. This promoter element has shown strong activity in PSA- producing prostate cancer cells, but not in non-PSA-producing cells. Since the tumor cells from vast majority of patients with advanced prostate disease produce PSA, utilizing this PSA promoter to drive a therapeutic gene exclusively in prostate tissue should alleviate concerns of systemic toxicity while being extremely effective. He has increased the promoter activity four- to five- fold by inserting the cytomegalovirus (CMV) enhancer upstream. Simultaneously, he has analyzed different delivery vectors to maximize gene transfer into prostate tumor cells, and found that the adenoviral vector offers significant advantages due to its high infectivity and gene expression. He will now create an adenoviral construct with a suicide effector gene driven by the CMV-PSA promoter, in the hope that this will selectively eradicate prostate tumor cells. The thymidine kinase gene (TK) has been successfully used to treat several human neoplasms. When a tumor cell transfected with TK is exposed to ganciclovir, DNA replication is disrupted and the cell is eventually destroyed. Unfortunately, prostate cancer cells replicate slowly and may not be susceptible to the actions of a therapeutic gene that requires active cell division. In this case, a suicide gene with direct lytic activity may be better. An example of this is diphtheria toxin A. Hence, the applicant will try both types of agents and assess which is most effective for prostate cancer gene therapy: the TK gene and the DTA gene, alone and in combination with TK. This proposed strategy will be tested in both cell culture and animal models. From in vitro experiments will determine the efficacy and selectivity of the chosen suicide gene. With this basic knowledge, we will then infect tumor- bearing SCID mice with our tissue-specific promoter-driven adenoviral vector to define in vivo selectivity and safety.
