spRRV-TK vectors propagate efficiently and highly expresses transgene. (A) HSV-TK expression profiles in sRRVe-TK- or spRRVe-TK-transduced U-87 MG cells. Cell extracts were analyzed in 10% SDS-PAGE and the expression of TK was analyzed by immunoblotting with an anti-HSV-TK antibody. (B) In vitro cytotoxicity in U-87 MG cells transduced with 1 × 107 gc of spRRV-FL, sRRV-TK, or spRRV-TK. The cells were cultured for 10 days and then seeded onto 96-well plates in triplicate (2,000 cells/well). After 24 h, the transduced cells were exposed to 2 μg/ml of GCV for 5 days. The number of viable cells was measured daily by MTT assay. (C) In vivo spread of spRRV-TK in U-87 MG xenograft.