Schedule-dependent synergistic effects of 5-fluorouracil and selumetinib in KRAS or BRAF mutant colon cancer models

Combination of MEK inhibitor and 5-FU had showed limited efficacy in clinical trials. We previously reported that acquired resistance to 5-FU was related with continued activation of salvage pathway. Here we investigated whether combination of 5-FU and a MEK inhibitor had treatment sequence-dependent synergistic effects in KRAS or BRAF mutant colon cancer models. Treatment with 5-FU followed by selumetinib (FS) induced highest cell death and synergy compared with reverse (SF) and concomitant (cFS) treatment in six cell lines. SF or cFS combination induced synergy in 1 or 2 cell lines, respectively, in which the synergy was less than that by FS combination. FS enhanced apoptosis and decreased anchorage-independent growth. Induction of thymidine kinase 1, a rate-limiting enzyme in salvage pathway, by 5-FU was abrogated by subsequent treatment with selumetinib, and ERK reactivation after selumetinib was prohibited by pretreatment with 5-FU. FS altered mRNA expression in groups of genes distinct from SF. Administration of 5-FU (10 or 30 mg/kg/day) for 7 days, followed by selumetinib (10 or 25 mg/kg/day) for another 7 days, in colo205 and HCT8 xenograft models significantly decreased tumor growth compared with a single agent. However, co-administration in the reverse sequence did not show the difference in tumor size compared with the treatment of single agent. Decreased expression of Ki67 was observed in tumors from mice treated with FS. Our results suggest that sequential administration of 5-FU plus selumetinib would be a promising strategy for patients having KRAS or BRAF mutant colon cancers.