Nearly 25% of all breast cancer is characterized by overexpression of HER2 (human epidermal growth factor receptor 2) which leads to overactivation of prosurvival signal transduction pathways, especially through Akt-mTOR-S6K kinases, and results in enhanced proliferation, migration, induction of angiogenesis, and apoptosis inhibition. Anti-HER2 targeted therapies, such as specific monoclonal antibodies or small-molecule tyrosine kinase inhibitors, even in combination, still seem to be insufficient due to incidence of primary or acquired resistance and prevalence of serious side-effects of these drugs. We assumed that combination of compounds that target different levels of the above-mentioned signal transduction pathway might be more effective in eradication of breast cancer cells. In our in vitro research we used a commercially available drug, lapatinib, acting at the level of the receptor in combination with 1 of the plant-derived isothiocyanates: sulforaphane, erucin, or sulforaphene, as it has been shown previously that sulforaphane inhibits Akt-mTOR-S6K1 pathway in breast cancer cells. We used 2 HER2 overexpressing breast cancer cell lines, SKBR-3 and BT-474. Combinations of the drug and isothiocyanates considerably decreased their viability. This action was synergistic and was accompanied by a decrease in phosphorylation of HER2, Akt, and S6. Combined treatment induced apoptosis more efficiently than either agent alone; however the most effective was a combination of lapatinib with erucin. These findings might support the optimization of therapy based on lapatinib treatment.