Abstract
A major obstacle in cancer gene therapy is the limited efficiency of
in vivo
gene transfer by replication-defective retrovirus vectors in current use. One strategy for circumventing this difficulty would be to use vectors capable of replication within tumor tissues. We have developed a replication-competent retrovirus (RCR) vector derived from murine leukemia virus (MuLV). This vector utilizes a unique design strategy in which an internal ribosome entry site–transgene cassette is positioned between the
env
gene and the 3′ long terminal repeat (LTR). The ability of this vector to replicate and transmit a transgene was examined in culture and in a solid tumor model
in vivo
. The RCR vector exhibited replication kinetics similar to those of wild-type MuLV and mediated efficient delivery of the transgene throughout an entire population of cells in culture after an initial inoculation with 1 plaque-forming unit (PFU) of vector per 2000 cells. After injection of 6 × 10
3
PFU of vector into established subcutaneous tumors, highly efficient spread of the transgene was observed over a period of 7 weeks, in some cases resulting in spread of the transgene throughout the entire tumor. MuLV-based RCR vectors show significant advantages over standard replication-defective vectors in efficiency of gene delivery both in culture and
in vivo
. This represents the first example of the use of an RCR vector in an adult mammalian host, and their first application to transduction of solid tumors.
Low efficiency of gene delivery by currently used viral vectors remains a significant barrier to the success of gene therapy of cancer. To examine the possibility of achieving superior transduction efficiency utilizing retroviral vectors that are capable of replication, we inserted an IRES–transgene cassette between the
env
gene and 3′ LTR of an infectious full-length murine leukemia virus (MuLV) clone. We found that the resulting vector replicated with kinetics similar to those of wild-type MuLV, and was stable through multiple serial passages in cultured cells. Injection of this vector into established subcutaneous tumors in mice resulted in highly efficient transmission of the transgene and, in some cases, transduction of entire tumor masses. These results demonstrate the potential utility of replication-competent retroviral vectors for cancer gene therapy.
