617. Prostate Cancer Radiosensitization by Targeting Poly(ADP-ribose) Polymerase

Abstract

Identification of radiosensitizing biological compounds will enable the optimization of radiation therapy for the treatment of advanced prostate cancer by enhancing the therapeutic response to radiation while circumventing the problem of systemic toxicity associated with the doses of radiation currently used for radioresponsive lesions. Therefore, targeting molecular components that are critically involved in the maintenance of genome stability is a promising approach directed at overcoming an intrinsic tumor cell resistance to DNA-damaging treatments. Poly(ADP-ribose) polymerase (PARP) is a DNA-binding enzyme which plays important roles in the maintenance of genome stability, immediate cellular responses to DNA damage, and apoptosis. A DNA-binding domain of PARP (PARP-DBD) acts as a dominant-negative mutant by binding to DNA strand breaks irreversibly and sensitizing mammalian cells to DNA-damaging agents1,2. To direct the expression of human PARP-DBD to prostate we developed recombinant plasmids expressing the PARP-DBD under the control of the 5'-flanking sequences of the human prostate-specific antigen (PSA) gene. In vitro studies revealed that PSA promoter driven expression of the PARP-DBD showed prostate tissue specificity and androgen responsiveness and sensitized LNCaP cells to DNA-damaging agents, such as ionizing radiation and etoposide3. Subsequently, we evaluated therapeutic potential of PARP-DBD as a gene engineered radiosensitizer for prostate cancer using human prostate tumor xenografts grown in athymic mice. To assess the efficiency of this strategy in vivo, we developed a cationic liposome-mediated gene delivery of PARP-DBD plasmid in tumor xenografts of PSA producing and androgen sensitive prostate cancer cells (LNCaP and 22Rv1). Tumor bearing mice were treated with intratumoral liposome-complexed PARP-DBD (LE-PARP-DBD), ionizing radiation (IR) or a combination of LE-PARP-DBD and IR. Control groups received blank liposomes or were left untreated. Administration of LE-PARP-DBD resulted in expression of dominant-negative mutant of PARP in tumor cells and enhanced radiation-induced inhibition of tumor growth. These results provide a proof-of-principle for a novel therapeutic strategy to control prostate cancer using genetically engineered PARP mutant.