BACKGROUND: Hypoxia is a common tumor condition correlated with therapeutic resistance. Ribonucleotide reductase (RR) is a rate-limiting enzyme for viral replication. We hypothesize that hypoxia-driven transcription of UL39, the gene encoding the large subunit of RR, would enhance herpes oncolytic viral therapy in breast cancer. METHODS: Hypoxia-inducible factor 1alpha (HIF-1alpha) ELISA was performed on MCF7 human breast cancer cells in hypoxia (1% O2) or normoxia (21% O2). A multimerized hypoxia-responsive enhancer was constructed (10xHRE) and functionally tested in a luciferase assay. 10xHRE was cloned upstream of the UL39 gene (10xHRE-UL39). MCF7 cells were transfected with 10xHRE-UL39, incubated in hypoxia or normoxia, and infected with G207. Cytotoxicity assays and viral titers were performed. RESULTS: HIF-1alpha levels increased 7-fold in hypoxic MCF7 cells (P < .001). 10xHRE increased luciferase gene expression 61-fold in hypoxia versus controls (P < .01). G207 cytotoxicity of 10xHRE-UL39-transfected, hypoxic MCF7 cells increased 74% versus mock-transfected, hypoxic MCF7 cells (P < .001). In normoxia, 10xHRE-UL39 transfection did not significantly improve G207 cytotoxicity. 10xHRE-UL39 transfection improved peak viral titers 69-fold in hypoxia (P < .005), with no significant difference in normoxia. CONCLUSION: Hypoxia-driven RR production significantly enhances G207 cytotoxicity in hypoxic breast cancer cells, which would otherwise be resistant to herpes viral therapy alone. Copyright 2004 Elsevier Inc.