Schedule-dependent activity of 5-fluorouracil and irinotecan combination in the treatment of human colorectal cancer: in vitro evidence and a phase I dose-escalating clinical trial

Abstract

Several schedules of 5-fluorouracil (FU) and irinotecan (IRI) have been shown to improve overall survival in advanced colorectal cancer (CRC). Preclinical evidence suggests that the sequential administration of IRI and FU produces synergistic activity, although their clinical use has not been fully optimised. We investigated the interaction between short-term exposure to SN-38, the active metabolite of IRI, and prolonged exposure to FU in human CRC HT-29 cells and observed that the synergism of action between the two agents can be increased by extending the time of cell exposure to FU and reducing the interval between administration of the two agents. Based on these findings, we performed a phase I trial in 25 advanced CRC patients using a modified IRI/FU regimen as first-line therapy and evaluated three dose levels of IRI (150-300 mg/m(2)) and two of continuous infusion of FU (800-1000 mg/m(2)) in a 3-weekly schedule. The most severe grade III-IV toxicities were neutropoenia in four cycles and diarrhoea in three. One patient achieved complete response (4%), 12 a partial response (48%), the overall response rate was 52% (+/-20, 95% CI); seven of 25 patients had stable disease (28%), the overall disease control was 80% (+/-16, 95% CI). This modified IRI/FU schedule is feasible and exhibits potentially interesting clinical activity.

Figure 1

The schedule-dependent synergism between SN-38 and FU. (A) Cytotoxicity evaluated by MTT assay after exposure of human colon carcinoma HT-29 cells to increasing concentrations of FU (10⁻⁴−100 μM) for 24 h, the sequential combination of EC₃₀ SN-38 (0.11 μM) for 6 h and increasing concentrations of FU for 24 h using both sequences and the concomitant treatment with EC₃₀ SN-38 for 6 h and increasing concentrations of FU for 24 h. (B) Cytotoxicity evaluated by MTT assay after exposure of HT-29 to increasing concentrations of SN-38 (10⁻⁶−10 μM) for 6 h, the sequential combination of EC₃₀ FU (5.2 μM) for 24 h and increasing concentrations of SN-38 for 6 h using both sequences and the concomitant treatment with EC₃₀ FU for 24 h and increasing concentrations of SN-38 for 6 h. (C) Plot of the CIs vs the cytotoxicity, calculated from data reported in (A,B), using the methods described by Chou et al (20–21) and under the assumption of a mutually exclusive drug interaction. CI>1, antagonism; CI=1, additive effect; CI<1, synergism. (D) Plot of the CIs vs the cytotoxicity calculated from MTT assay data of HT-29 cells exposed sequentially to SN-38 for 6 h followed by FU for 24, 48, 72 and 96 h. (E) Plot of the CIs vs the cytotoxicity calculated from MTT assay data of HT-29 cells exposed to SN-38 for 6 h followed by FU for 24 h after an interval between the two drugs of 0, 24, 48, 72 and 96 h during which cells were incubated in a drug-free medium. (F) Plot of the CIs vs the cytotoxicity obtained from MTT assay data of HT-29, HT-29 SN-38R and HT-29 FUR cells exposed to the sequence of SN-38 followed by FU for 96 h. Insert: Thymidylate synthase (TS) protein expression in wild-type HT-29 (line 1), HT-29 SN-38R (line 2) and HT-29 FUR (line 3) cells.