Synergistic Antitumor Effects of a Combination of Interferon and Tamoxifen on Estrogen Receptor-positive and Receptor-negative Human Tumor Cell Lines in Vivo and in Vitro

Solid tumors are relatively resistant to growth inhibition by interferons (IFNs). To enhance sensitivity, we assessed combinations of IFNs with tamoxifen in estrogen receptor-positive (ER-positive) and ER-negative human tumor xenografts. In nude mice, the growth of MCF-7 human breast tumors (ER-positive) and NIH-OVCAR-3 ovarian tumors (functionally ER-negative) was suppressed completely when tamoxifen and IFN-alpha or IFN-beta was started 2 days after tumor inoculation. Established, 6-week-old MCF-7 and NIH-OVCAR-3 tumors regressed when treated with the combination of IFN-beta and tamoxifen but not with single-agent therapy. Treatment with the combination also resulted in an augmented antitumor response in vivo in an ER-negative breast tumor (MDA-MB-231), a colon carcinoma (HT-29), and a melanoma (SK-MEL-1). Antiproliferative studies in vitro suggested that growth of both MCF-7 and NIH-OVCAR-3 cells was inhibited to a greater degree by combination treatment with human IFN-alpha and tamoxifen or IFN-beta and tamoxifen compared with single agents. Median effect analysis defined synergy. Four ER-negative carcinomas (MDA-MB-231, MDA-MB-468, BT-20, and HT-29) also exhibited synergistic growth inhibition in response to the drug combination. The response of these four cell lines was particularly striking. Tamoxifen as a single agent had little effect (up to 2.0 microM) but caused enhanced antiproliferative activity when added to IFN-beta. Sequential treatment of MCF-7 cells in vitro with tamoxifen followed by IFN-beta was more effective at inhibiting growth than treatment with IFN-beta followed by tamoxifen, suggesting that tamoxifen modulated the anticellular response to IFN-beta rather than the converse. Similar results were obtained with IFN-alpha. Cell cycle analysis indicated that 7 days of exposure to the combination resulted in MCF-7 cell fragmentation and death. Together with our recent studies demonstrating enhancement of IFN-stimulated gene expression (ISG) by tamoxifen pretreatment in IFN-resistant cells, these data suggest that combination treatment with tamoxifen and IFNs may increase ISG expression in IFN-resistant tumors, leading to augmented antitumor effects. These effects appear to be independent of ER expression.