To develop a novel gene therapeutic modality for the effective treatment of benign prostatic hyperplasia (BPH), we investigated the properties of toxic gene therapy utilizing prostate-specific antigen (PSA) promoter driving herpes simplex virus thymidine kinase (HSV-TK) suicide gene to induce highly selective molecular ablation of epithelial cells with minimal systemic toxicity in canine prostate. Replication-defective recombinant adenoviral vectors containing HSV-TK gene under transcriptional control of long PSA promoter (Ad-PSA-HSV-TK) were developed and delivered in an situ manner. Briefly, laparotomies were performed and Ad-PSA-HSV-TK (1 x 10(9) PFUs) was injected into the left lateral lobe of prostate only on days 1 and 7 with appropriate prodrug acyclovir in adult Beagle dogs. The therapeutic efficacy was evaluated on the 56th experimental day. The striking apoptosis of epithelial cells was identified in the treated left half of canine prostate on TUNEL assay. On immunohistochemical studies, there was markedly decreased number of PSA-secreting epithelial cells compared to control. Also significant atrophy of prostate glands, associated with dense infiltration of lymphocytes and plasma cells, was identified in the treated side. The PSA promoter-based suicide gene therapy induced highly selective and definite ablation of epithelial cells in benign canine prostate. Our novel approach could open opportunity of gene therapeutic modality for the treatment of clinical BPH.