We were interested in developing oncolytic adenoviral vectors that can be administered systemically for the treatment of breast cancer. To restrict viral replication in breast tumor cells, we constructed mhTERTAd.sTbetaRFc, a 01/07-based adenoviral vector expressing the soluble form of transforming growth factor-beta (TGFbeta) receptor II fused with the human Fc IgG1 (sTGFbetaRIIFc) gene, in which viral replication is under the control of a modified human telomerase reverse transcriptase (mhTERT) promoter. In addition, mhTERTAd.sTbetaRFc-mediated sTGFbetaRIIFc production targets the TGFbeta pathway known to contribute to the tumor progression of breast cancer metastasis. We chose to use the mhTERT promoter because it was found to be relatively more active (approximately 20 times) in breast cancer cells compared with normal human cells. We showed that infection of MDA-MB-231 and MCF-7 breast cancer cells for 48 h with mhTERTAd.sTbetaRFc produced high levels of sTGFbetaRIIFc (greater than 1 microg ml(-1)) in the medium. Breast cancer cells produced nearly a 6000-fold increase in viral titers during the 48 h infection period. However, mhTERTAd.sTbetaRFc replication was attenuated in normal cells. Infection of breast cancer cells with a replication-deficient virus Ad(E1(-)).sTbetaRFc also produced high levels of sTGFbetaRIIFc, but under these conditions, no detectable viral replication was observed. Adenoviral-mediated production of sTGFbetaRIIFc was shown to bind with TGFbeta-1, and to abolish the effects of TGFbeta-1 on downstream SMAD-3 phosphorylation. The administration of mhTERTAd.sTbetaRFc intravenously into MDA-MB-231 human xenograft-bearing mice resulted in a significant inhibition of tumor growth and production of sTGFbetaRIIFc in the blood. Conversely, intravenous injection of Ad(E1(-)).sTbetaRFc did not show a significant inhibition of tumor growth, but resulted in sTGFbetaRIIFc in the blood, suggesting that viral replication along with sTGFbetaRIIFc protein production is critical in inducing the inhibition of tumor growth. These results warrant future investigation of mhTERTAd.sTbetaRFc as an antitumor agent in vivo.