Introduction and Objective: Viral vectors that replicate exclusively in targeted tissues would minimize the relative side effects of cytotoxic gene therapy. Transcription of probasin (PB) gene is regulated by androgen and is highly specific to prostate epithelium. Here, we have investigated the feasibility of a PB-targeting replication-competent retroviral (RCR) vector to achieve both high transduction efficiency and strict cell type specificity. We have also investigated an optimized, PB-targeted RCR vector to inhibit prostate cancer growth as part of a suicide gene therapy strategy. Methods: Prostate tissue-targeted vectors were constructed by replacing murine leukemia virus enhancer/promoter of RCR vector with a recombinant double PB promoter and GFP marker gene. GFP expression was determined by immunohistochemical method in tumors established with various cancer cell lines including LNCaP (androgen receptor-positive, AR+) cells, PC-3 (AR-) cells, mammary carcinoma NMU (AR+) cells and bladder cancer 5637 (AR-) cells. Tumor bearing mice underwent an intratumoral injection of either wild type or tissue-targeted RCR vectors (3.2 x 105 transducing units/ml). For therapeutic experiments, pre-established LNCaP tumors were treated with PB-targeted RCR vector encoding E. coli purine nucleoside phosphorylase (PNP) gene followed by systemic administered fludarabine phosphate (FP). Results: Tumors injected with wild type RCR vectors demonstrated 80 to 100% GFP expression regardless of cell type. LNCaP tumors injected with PB-targeted RCR vectors showed up to 100% GFP transduction, whereas no tumors generated from other cell types demonstrated any evidence of viral transduction. Biodistribution studies revealed no detectable PCR products of virus in any tested distant organs of mice treated with PB-targeted RCR vectors. Targeted RCR suicide gene therapy studies demonstrated that the mean tumor volume in mice treated with PB-targeted RCR-PNP/FP (101 ± 30 mm3) was significantly smaller than that in mice treated with either PB-targeted RCR-GFP/FP (300 ± 70 mm3) or PB-targeted RCR-PNP/PBS (469 ± 86 mm3) 35 days after viral exposure (p=0.0227 and p<0.0001, respectively). Re-treatment of mice treated with PB-targeted RCR-PNP/FP with a second course of FP treatment, again demonstrated the ability to significantly suppress tumor growth. Conclusions: Our results indicate that PB-targeted RCR vectors can provide efficient and specific transduction of AR+ prostate cancers. Furthermore, PB-targeted RCR-PNP/FP treatment effectively inhibits LNCaP tumor growth, providing a novel therapeutic strategy for the management of prostate cancer.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]