Objectives To examine the effect of microsatellite instability (MSI) on the outcome of fluoropyrimidine and oxaliplatin containing first-line chemotherapy in metastatic colorectal cancer (mCRC).

Methods Patients with mCRC and treated with fluoropyrimidine/oxaliplatin first-line chemotherapy were included in our study. Demographic data, tumour characteristics, chemotherapy regimens, treatment responses and progression-free survival (PFS) were collected from medical records. The MSI analysis was performed using fluorescence-based PCR, and divided into MSI-high (MSI-H) and MSI-low (MSI-L)/microsatellite stable (MSS). Statistical analysis used Kaplan-Meier method, log-rank test and multivariate Cox model.

Results From 1 January 2015 to 1 May 2016, a total of 192 patients with mCRC were included in our study. Among these, 14 (7.29%) exhibited MSI-H and 178 (92.71%) were MSI-L/MSS. The objective response rate (p=0.79), disease control rate (p=0.22) and PFS (p=0.22) of fluoropyrimidine/oxaliplatin first-line chemotherapy were not significantly different between MSI-H and MSI-L/MSS tumours. But MSI-H tumours had a trend to better disease control rate (71.43% vs 54.49%) and PFS (6.50 m vs 5.40 m) than MSI-L/MSS tumours. Multivariate analysis indicated that MSI was not a predictive factor for PFS (p=0.18).

Conclusion The effect of fluoropyrimidine/oxaliplatin first-line chemotherapy was not significantly different between MSI-H and MSI-L/MSS tumours. However, MSI-H tumours tended to have better disease control rate and PFS.