Optimizing chemotherapeutic drug delivery strategies relies, in part, on identification of the most clinically effective sequence, dose, and duration of drug exposure. The combination of dose intensive etoposide (VP-16) followed by cyclophosphamide has clinical efficacy in the treatment of advanced breast cancer. However, molecular mechanisms that underlie the effectiveness of this combination of chemotherapeutic agents have not been investigated. In this study we investigated regulation of BAX and BCL-2 expression by VP-16 and cyclophosphamide as a potential mechanism for the induction of breast cancer cell death induced by this regimen. There was a dose and time dependent increase in BAX expression in the breast cancer cell lines MCF-7, MDA-MB-435S, and MDA-MB-A231 following in vitro treatment with 50-100 microM VP-16. Elevation of BAX protein expression in the presence of VP-16 alone did not correlate with reduced viability or induction of apoptosis in MCF-7, MDA-MB-435S, or MDA-MB-A231. VP-16 did effectively block the breast cancer cell lines evaluated (MCF-7 and MDA-MB-435S) at G2/M phase of the cell cycle, confirming activity of the drug in vitro. MCF-7 and MDA-MB-435S cells that were pre-treated with VP-16 and subsequently exposed to 1.0-12.0 microg/ml 4-hydroperoxycyclophosphamide (4HC), an active metabolite of cyclophosphamide, had markedly reduced viability when compared to matched controls treated with either VP-16 or 4HC individually. Consistent with this loss of viability, exposure of all three cell lines to the combination of VP-16 and 4HC resulted in higher BAX protein levels than those observed following treatment with either single agent. This combination of chemotherapeutic agents also resulted in reduced BCL-2 expression. These observations suggest that combination chemotherapy may derive its efficacy, in part, through coordinated regulation of specific gene products associated with apoptosis. Characterization of molecular events that underlie susceptibility of specific tumor cells to combination chemotherapeutic regimens may lead to additional improvements in treatment strategies for this disease.