Replication -competent retroviral vectors for highly efficient and tumor -selective gene deliver

Many strategies of cancer gene therapy have involved the use of disabled retroviruses as vehicles for gene delivery. However, completely replication-defective viruses infect a limited number of tumor cells and deliver genes at levels too low to be therapeutically useful. We have developed a uniquely stable replication-competent retroviral (RCR) vector which achieves in vitro and in vivo transduction efficiencies much higher than those of conventional replication-defective retroviral vectors.;The significantly higher transduction levels achieved by RCR vectors have the potential to greatly enhance the efficacy of gene therapy for the treatment of tumors. Therefore we constructed RCR vectors carrying the yeast cytosine deaminase, the E. coli purine nucleoside phosphorylase, or the herpes simplex virus thymidine kinase, and tested their functionality by examining cell survival and metabolic activity of RCR vector-infected tumor cells in vitro and growth of RCR-transduced tumors in vivo after treatment with the respective prodrugs. In vitro studies showed highly efficient cell killing by each of the RCR vectors with initial inoculum as low as 0.1% of the target cell population. In in vivo studies employing mouse intracranial glioma models, significant tumor regression and markedly improved survival were observed in treated mice compared to control mice (P < 0.0001).;RCR vectors can be engineered to achieve tumor-restricted replication, however their safety can be further enhanced by redirecting the tropism of the virus envelope. We therefore tested the targeting capability of RCR vectors containing immunoglobulin-binding domain of Staphylococcal protein A inserted into the envelope, to enable the modular use of antibodies of various specificities for vector targeting. The results indicate that the targeted envelope can mediate efficient anchorage of vector to the cells through the antibody, but the ability of the targeted envelope to mediate vector entry into the cells appears to be greatly attenuated. Application of antibody-mediated targeting to the initial localization of RCR vectors to tumor sites will thus require optimized vector design.