Replication-competent retrovirus (RCR) vectors can propagate specifically within actively dividing cancer cells and thereby achieve highly efficient and tumor-selective gene delivery in vivo. To further enhance their tissue-specificity through transcriptional targeting, we have developed a prostate-specific RCR vector regulated by a synthetic variant of the probasin promoter (ARR2PB). We have previously shown that RCR vectors regulated by ARR2PB exhibit stringent specificity for androgen receptor (AR)-positive prostate cancer cells compared to untargeted RCR vectors. These RCR vectors were both derived from murine leukemia virus pseudotyped with the wild type amphotropic envelope glycoprotein (MLV(4070A)), which binds to PiT-2, a ubiquitous and highly conserved phosphate transport protein present on the surface of human and murine cells. We now report the development of RCR vectors pseudotyped with the heterologous gibbon ape leukemia virus envelope (MLV(GALV)), whose receptor is PiT-1, another member of the same phosphate transporter family. We have further constructed a targeted MLV(GALV) RCR vector regulated by the ARR2PB promoter and tested its selectivity for prostate cancer cells in vitro and in vivo. Probasin-targeted and untargeted MLV(4070A) and MLV(GALV) RCR vectors expressing GFP were inoculated onto LNCaP (AR-positive prostate cancer), PC3 (AR-negative prostate cancer) and WiDr (colon cancer) cells in vitro, and the replication kinetics and transduction efficiency of each vector was monitored by flow cytometry. Both ARR2PB-targeted vectors exhibited stringent specificity for LNCaP cells, and MLV(GALV) RCR vectors replicated more rapidly than MLV(4070A) RCR vectors. After injection of equal infectious units into pre-established subcutaneous LNCaP tumors in nude mice, the in vivo tumor transduction efficiency of ARR2PB-targeted MLV(GALV) RCR vectors reached significantly higher levels more rapidly than targeted MLV(4070A) vectors. To assess the impact of transcriptional targeting on the potential for extratumoral spread and resultant genotoxicity of RCR vectors in normal tissues, untargeted and ARR2PB-targeted RCR vectors were inoculated into nude mice by direct intravenous injection and vector biodistribution was examined 3 months later by quantitative real-time PCR analysis of genomic DNA. Untargeted RCR vector integration was detected in the spleen and bone marrow, but the ARR2PB-targeted vector showed no detectable extratumoral integration. Our results thus indicate that GALV-pseudotyped RCR vectors have the potential to further enhance the efficiency of replicative transduction and therapeutic gene transfer in prostate cancer, and that transcriptional targeting may improve the safety profile of RCR vectors in immunocompromised hosts.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]