An estimated 131,000 new cases of colorectal cancer were diagnosed in the United States in 1997. The current standard of practice among most North American oncologists is to administer a potentially curative course of chemotherapy in most patients with stage III colorectal cancer and a palliative course of chemotherapy in many patients with stage IV disease. The two drugs that are currently approved by the FDA for the treatment of colon cancer are fluorouracil(Drug information on fluorouracil) (5-FU) and irinotecan(Drug information on irinotecan) (CPT-11 [Camptosar]).
Fluorouracil is known to be active in the setting of advanced colorectal cancer. Pooled results from 10 trials in which colorectal cancer patients with advanced disease were treated with intermittent-bolus 5-FU combined with leucovorin led to an overall response rate of 23% and a median survival of 12 months. Fluorouracil plus leucovorin is also an effective adjuvant therapy. In patients with resected stage III colon cancer, 6 months of treatment with 5-FU/leucovorin for 5 consecutive days each month decreased the odds of recurrence by approximately 33%.[3,4]
Irinotecan also is active in advanced colorectal cancer. A recently completed North Central Cancer Treatment Group (NCCTG) trial reported a 26% response rate (95% confidence interval [CI], 11.9% to 44.6%) to irinotecan in patients who had not previously received chemotherapy for advanced colorectal cancer. In the cohort of patients with advanced colorectal cancer that was refractory to 5-FU and leucovorin, this NCCTG trial reported a response rate of 13% with irinotecan (95% CI, 7.1% to 22.1%). Both of these outcomes are typical of published reports cited elsewhere in this monograph.
Based on the NCCTG data and results from other reported studies, in 1996 the FDA approved irinotecan for the treatment of patients with advanced colorectal cancer that is refractory to 5-FU. To date, the activity of irinotecan, either as a single agent or in combination with other chemotherapeutic agents, has not been tested as adjuvant therapy for stage II or III patients; therefore, its potential value in such patients remains unknown. Future trials integrating irinotecan into postsurgical treatment regimens for stage III patients are in the planning stages under the aegis of the National Cancer Institutes cooperative research group clinical trial program.
The NCCTG and other trials have clearly shown that some tumors that grow during treatment with 5-FU and leucovorin will shrink when exposed to irinotecan. This observation implies a lack of tumor cross-resistance to the two agents, at least in a proportion of metastatic human colon cancers, and led to a number of experiments in which irinotecan and 5-FU were combined in vitro and in human tumor xenograft models.
Promising findings of these experiments led to treatment trials employing a number of different dose and schedule strategies in patients with advanced colorectal cancer. These trials were designed to determine the maximum tolerated doses (MTDs) and response rates of the combination of irinotecan and 5-FU in patients with advanced colorectal cancer, as well as to provide survival data.
Because diarrhea is a major dose-limiting side effect of both irinotecan and 5-FU/leucovorin, the possibility of life- threatening, additive, or even synergistic toxicity exists. Investigators using the combination of the three drugs have taken this into account in their phase I trial designs.
This review addresses three main topics. First, data on interactions between 5-FU and irinotecan observed in a number of cell culture and human tumor xenograft experiments will be summarized. Second, recent or ongoing North American trials in which irinotecan and 5-FU plus leucovorin have been administered concurrently, sequentially, and in an alternating fashion will be reviewed. Finally, relevant toxicity data, as well as current knowledge about the mechanisms and treatments of irinotecan- and 5-FU-induced gastrointestinal toxicity, will be presented and discussed.
Fluorouracil inhibits the enzyme thymidylate synthase (TS), thus interfering with the formation of new strands of DNA during replication. Fluorouracil is also incorporated into RNA and DNA, where it inhibits DNA synthesis. Leucovorin is a reduced folate, which, when combined with 5-FU, augments 5-FU cytotoxicity by increasing the inhibition of TS by the 5-FU metabolite 5-fluoro-2¢-deoxyuridine monophosphate (FdUMP).
The role of 5-FU in the treatment of colorectal cancer has been explored for over 40 years. Modulation of 5-FU by leucovorin has been demonstrated to improve response rate, but not median survival, over bolus 5-FU alone. Nevertheless, 5-FU, with or without leucovorin, has become the standard therapy for patients with advanced colon cancer in North America.
Irinotecan binds and stabilizes topoisomerase I as it works to uncoil DNA during cell division. This stabilized complex of inhibitor-enzyme-DNA halts advancing DNA replication forks, resulting in double-strand DNA breaks and consequent apoptosis. Irinotecan is converted by carboxylesterases to its more active metabolite, 7-ethyl-10 hydroxy-camptothecin, or SN-38.[9,10]
Irinotecan Plus 5-FU and Leucovorin
The clinical activity of both 5-FU/leucovorin and irinotecan in patients with metastatic colorectal cancer makes the combination of these therapies clinically attractive. There is reason to believe, however, that this therapeutic combination may result in antagonism. Fluorouracil is converted to FdUMP, leading to inhibition of TS, depletion of deoxythymidine triphosphate (dTTP) pools, and inhibition of DNA synthesis, resulting in G1/S cell-cycle arrest. The active metabolite of irinotecan, SN-38, stabilizes the covalent complex between topoisomerase I and nuclear DNA, leading to DNA double-strand breaks and accumulation of cells in the G2 phase of the cell cycle.
Hence, 5-FU may inhibit the DNA synthesis required for the cytotoxicity of SN-38. Conversely, SN-38 may cause cells to accumulate in the G2 rather than in the S phase, when they would be most sensitive to 5-FU plus leucovorin.
Mullany et al addressed these issues in studies of SN-38 plus 5-FU and leucovorin in HCT-8 cells in vitro. They observed a sequence-dependent interaction of SN-38 and 5-FU plus leucovorin. The combination was less than additive when cells were treated with simultaneous administration of SN-38 and 5-FU plus leucovorin or when 5-FU plus leucovorin was followed by SN-38, but was synergistic when SN-38 was followed by 5-FU plus leucovorin. Other groups have found similar results.[12-18]
Mullany et al further demonstrated that SN-38 resulted in accumulation of cells in the S phase when therapeutically achievable nanomolar concentrations of SN-38 were used for time periods corresponding to three serum half-lives in vivo. Moreover, this S-phase-slowing was accompanied by inhibition of TS activity in the face of constant TS protein levels. Further studies revealed that SN-38 increased dTTP pools.
Taken together, these results are consistent with a model in which elevated dTTP pools account for the observed synergy of SN-38 followed by 5-FU plus leucovorin. The increased dTTP pools after SN-38 therapy inhibit TS by depletion of the normal substrate, deoxyuridine monophosphate (dUMP), and enhance 5-FU-associated inhibition of TS. In addition, the dTTP-associated inhibition of thymidine kinase decreases salvage thymidylate synthesis. Thus, the preclinical studies define a rational schedule for clinical evaluation of irinotecan plus 5-FU and leucovorin and explain, at least in part, the mechanism of this interaction.