Patients with advanced colorectal cancer often are treated with systemic cytotoxic therapy using fluorouracil (5-FU), oxaliplatin, irinotecan, and FOLFOX or FOLFIRI combination protocols. Additionally, signaling pathways that are active in colorectal cancer can be therapeutically targeted. Herein, we examined whether chemotherapy impacts on WNT, MAPK and NOTCH signaling pathways in xenograft models of colon cancer. Furthermore, we tested whether combining chemotherapy with MAPK and NOTCH inhibition has superior therapeutic effects. We show that colon cancer cells with high WNT, MAPK and NOTCH activity are variably affected but generally persist in xenograft tumors under different chemotherapeutic regimens, indicating limited effects of cytotoxic therapy on oncogenic signaling pathways. Although these results provided a rationale to additionally target pathway activity, we found no significant increase in therapy response when combining MAPK and NOTCH inhibition with fluorouracil chemotherapy. We attribute this finding to a decrease in tumor cell proliferation upon MAPK and NOTCH inhibition, resulting in reduced effectiveness of cytotoxic treatment. Therapeutic benefits of combining chemotherapy with targeting of oncogenic signaling pathways must therefore be critically evaluated for patients with colorectal cancer.
