Development of replication -competent retroviral vectors for efficient, targeted gene therapy of cancer

Retroviruses were the first viruses to be adapted for use in gene transfer, and defective retroviral vectors remain the most commonly used vector in clinical trials of gene therapy. Nevertheless, the transduction efficiency of these vectors remains insufficient for therapeutically effective in vivo administration. As a means of enhancing gene transfer efficiency, retrovirus vectors could be rendered replication-competent. This would allow the in situ amplification of vector and would therefore presumably permit significantly improved levels of transduction. To investigate this possibility, we constructed a series of replication-competent retroviral vectors derived from murine leukemia virus containing a transgene between the env gene and the 3' LTR. In vitro studies demonstrated that these vectors may be used to stably transmit a functional transgene cassette of 1.3 kb through multiple serial passages with very high efficiency. Furthermore, injection of the same vector into solid tumors in mice resulted in levels of tumor cell transduction not possible using defective vectors. The use of such a vector as a therapeutic agent, however, would not be feasible unless there were means to confine replication to targeted tissues. In order to achieve tissue-targeted vector replication, we replaced sequences from the native transcriptional control region of the vector with sequences from the prostate-specific probasin promoter. Our results demonstrate that efficient vector replication can be combined with strict cell type-specificity to achieve high level targeted transduction in vitro.