Coexpression of a multidrug-resistance gene (MDR1) and herpes simplex virus thymidine kinase gene as part of a bicistronic messenger RNA in a retrovirus vector allows selective killing of MDR1-transduced cells.

A new retroviral vector, pSXLC/pHa, was constructed to coexpress drug-selectable markers with a second gene of interest as a part of a bicistronic mRNA in a retroviral vector using an internal ribosome entry site (IRES) from encephalomyocarditis virus. This system was used to develop a new retroviral vector pHa-MDR-IRES-TK which expresses a single mRNA from which translation of the MDR1 gene is cap dependent and translation of the herpes simplex virus thymidine kinase gene is IRES dependent. The pHa-MDR-IRES-TK transfectants showed high levels of P-glycoprotein expression and multidrug resistance. More than 95% of the vincristine-resistant cells transfected or transduced with pHa-MDR-IRES-TK showed hypersensitivity to ganciclovir, which selects against cells expressing herpes simplex virus thymidine kinase. An amphotropic retrovirus titer of 7.8 x 10(4)/ml was obtained with this vector. This safety-modified vector should be useful for introducing the MDR1 gene into bone marrow cells to protect normal cells from the toxic effects of cancer chemotherapy because this vector allows the elimination of cancer cells that have been unintentionally transduced with the MDR1 vector.