The human epidermal growth factor receptor 2 (HER2)‑targeting agent, lapatinib, combined with oral fluoropyrimidine capecitabine, has been previously demonstrated to be an effective treatment option for patients with trastuzumab‑resistant HER2‑positive metastatic breast cancer. To investigate the molecular mechanisms associated with the interactions between lapatinib and capecitabine, the effect of treatment with lapatinib and phosphatidylinositol‑4,5‑bisphosphate 3‑kinase (PI3K) inhibitors on the expression of E2F transcription factor 1 (E2F1) and thymidylate synthase (TS), which is associated with an increased response to 5‑fluorouracil (5‑FU)‑based chemotherapy, was determined in HER2‑positive breast cancer cells. The results of reverse transcription‑quantitative polymerase chain reaction demonstrated that administration of lapatinib and PI3K inhibitors decreased the mRNA expression of TS and E2F1, a transcription factor that promotes TS gene expression, in SKBR3 and T47D cell lines. Furthermore, treatment with lapatinib and PI3K inhibitors also suppressed the mRNA expression of ribonucleotide reductase M1 subunit (RRM1), an important determinant of gemcitabine resistance, and DNA topoisomerase II‑α (TOP2A), a molecular target of anthracyclines, in SKBR3 and T47D cell lines. Western blot analysis further demonstrated that the phosphorylation of Akt was inhibited by lapatinib, and the results of the MTT assay revealed that the combination of lapatinib with either 5‑FU or gemcitabine demonstrated synergistic antitumor effects, whereas a combinatory treatment of lapatinib with epirubicin, a typical anthracycline drug, exhibited antagonistic antitumor effects in HER2‑positive breast cancer cells. These results indicate that the synergistic antitumor effects exhibited by combinatory treatment of lapatinib with capecitabine may be induced via the suppression of E2F1‑mediated TS expression.