Complementary Treatment of Breast Cancer Cells with Different Metastatic Potential with Iscador Qu in the Presence of Clinically Approved Anticancer Drugs

European mistletoe extract (Iscador Qu) has been studied for decades, but it has not ceased to arouse scientific interest. The purpose was to investigate the impact of Iscador Qu on the antiproliferative potential of 11 standard chemotherapeutic agents on two breast cancer cell lines: MCF-7 low-metastatic and MDA-MB-231 high-metastatic and control cell lines (MCF-10A). MTT-dye reduction assay, FACS analysis, and PI staining were utilized. The most promising combinations acting against the MDA-MB-231 cell line were observed upon the simultaneous application of Iscador Qu (80 µg/mL) and Docetaxel, with 4-fold reduction in IC50. An antagonistic effect was found under treatment with Cisplatin and Iscador Qu (1.5-fold increase in IC50). The response of the low-metastatic breast cancer cell line MCF-7 to the tested combinations was different compared to the high-metastatic one. The most pronounced cytotoxic effect was found for the combination of Oxaliplatin and Iscador Qu (20 µg/mL) (5.2-fold IC50 reduction). An antagonistic effect for MCF-7 line was also observed when combinations with Olaparib and Tamoxifen were applied. This in vitro study offers new combinations between Iscador Qu and standard chemotherapeutic agents that hold great promise in establishing breast cancer therapeutic protocols compared to traditional monotherapies.