Different biological responses to chemotherapy of colon cancer may result in a similar clinical response

Introduction: Irinotecan in combination with 5-FU is used as a „first line„ therapy for advanced colorectal cancer. We showed recently that the response to irinotecan in vitro is dependent on the p53 mutation status of the tumor: the p53wt colorectal cancer cell lines respond to treatment with a long-term arrest, the p53mut cells with a short term arrest followed by mitotic catastrophe and apoptosis. We investigated if the observed biological responses occur also in vivo and how they are reflected by the clinical response.

Methods: Selected cell lines with a known p53 status (HCT116 and LS174T versus HCT116p53 knock-out and HT–29) were used for treatment in vitro with SN–38, the active metabolite of irinotecan. Cell growth was followed by cell counting and the expression of cell cycle and apoptosis molecules by western blot and immunohistochemistry. The cell lines were injected into nude mice and the tumors were treated with one cycle of irinotecan (5 days, 50mg/kg). The tissue sections were investigated for the same parameters by immunohistochemistry within 1–2–3 days after treatment. The long-term response to therapy was followed be measuring the tumor size during two months following the start of treatment.

Results: The expression of the indicators of the cell cycle (cyclin B1, CDK1, p27) shows that cell lines with p53wt respond to treatment in vivo with a tetraploid G1 arrest, while in the cells with p53mut G2/M arrest is triggered but not maintained, after which cells prematurely enter mitosis. The long term tetraploid G1 arrest as well as the mitotic catastrophe lead to a similar delay (9–10 days) of the total cell growth. The same behavior of the cells is observed after treatment in vivo, as judged by the expression of the cell cycle and apoptosis markers in tumor tissue sections. Furthermore, the delay of tumor growth after treatment was similar (12–14 days) in the p53mut and the p53wt group.

Conclusions: Different biological response to irinotecan may result in a similar clinical response. The knowledge of the type of the biological response underlying the observed clinical response may allow a rational enhancement of the therapy, adapted to the genetic profile of the tumor.