Colon cancer is one of the common malignant tumors in the digestive system, and the early diagnosis of colon cancer is very difficult. The main therapeutic methods of treating colon cancer are still radiotherapy and chemotherapy. As the first-line drug of clinical chemotherapy for colon cancer, 5-FU has not only poor efficacy, but also has significant side effects [3, 4]. A therapeutic alliance could possibly improve the effect of chemotherapy on these tumors, and meanwhile reduce the adverse reaction of using a single drug.
Attention has recently been paid to un-cytotoxic drugs in tumor therapy research, and there have been many developments in the research of the COX-2 inhibitor’s effects on inhibiting tumor growth [14–17]. Celecoxib is a kind of highly selective COX-2 inhibitor, and has stronger pharmacologic actions and less adverse reactions compared with traditional NSAIDS . Recent research has also found that selective COX-2 inhibitors can enhance the sensitivity of tumor cells to many kinds of chemotherapeutic drugs, thus reducing the IC50 of chemotherapeutic drugs by nearly 70% . By using cancer cell lines cultured in vitro as research targets, and observing the effect of the selective COX-2 inhibitor nimesulide on various anti-tumor drugs (including DDP and VP16), Zheng  found that there were synergistic effects when nimesulide was used in combination with cytotoxic chemotherapeutic drugs. Our research indicated that the apoptotic morphology change was most significant and had the highest tumor inhibition rate in the drug combination group.
In order to further investigate the mechanism of celecoxib’s chemotherapy sensitization, we examined the protein expression of cytochrome C, caspase-9 and caspase-3. Our results showed that cytochrome C, caspase-9 and caspase-3 were up-regulated after treatment with celecoxib and 5-FU, especially in combination. This reveals that the combined use of two drugs had synergistic, anti-tumor effects, whose mechanism of action may be associated with the activation of cytochrome C-dependent apoptosis signal pathway. Some papers also reported that NSAIDs, such as indomethacin and sulindac, could suppress the transmembrane protein ATP dependent efflux pump (multidrug resistance associated protein, MRP) p2 glucoprotein related to chemotherapeutic efflux, and inhibit the activity of glutathione transferase. Through these pathways, NSAIDs could inhibit chemotherapeutic efflux and decrease the multiple drug resistance of tumor cells against chemotherapeutics . In addition to animal experiments, there were some reports of the Phase I and Phase II clinical trials on celecoxib in combination with chemotherapy treatment for cancer [22–24]. Relevant test results indicated celecoxib has synergistic anti-tumor effects. A therapeutic alliance could reduce the dose and the adverse reaction of chemotherapy drugs. Some clinical research proved that combined application of celecoxib could effectively improve the three-year survival rate of patients with advanced colorectal cancer [25, 26].
In our research, we also found that celecoxib combined with 5-FU could inhibit COX-2 protein expression. In recent years, much evidence has documented that COX-2 could up-regulate apoptosis suppressor genes’ bcl-2 levels by activating MAP kinase and bcl-2 could dramatically inhibit activation of cytochrome C pathway [27, 28]. Our experiment showed that the synergistic anti-tumor effect with administration of 5-FU and celecoxib is not only related to improving the chemotherapy sensitization of 5-FU, but also may be associated with the effect by which 5-FU can enhance the ability of celecoxib to inhibit protein expression of COX-2. The chemotherapy agent 5-FU, which has been used against cancer for about 40 years, acted in several ways, but principally as a thymidylate synthase inhibitor. Interrupting the action of this enzyme blocked synthesis of the pyrimidine thymidine, which was a nucleoside required for DNA replication . Administration of 5-FU caused a scarcity in dTMP, so rapidly dividing cancerous cells undergo cell death via thymineless death. According to the results of this experiment, we speculate that 5-FU can promote the effect that celecoxib inhibits the protein expression of COX-2 by regulating the gene expression.
Current research has shown that NSAIDs, including selective COX-2 inhibitors, could augment the sensitivity of many kinds of tumors toward chemotherapeutics, but its mechanism was still unknown, and it may involve multiple molecular mechanisms. Our experiment utilized a combination to implement drug interference based on the model of xenograft tumors in nude mice, and proved that when celecoxib and 5-FU are used in combination they had synergic anti-tumor effects. Suppression of COX-2 and activation of the apoptosis signal might be related to these processes.