Abstract: Background and purpose: Microsatellite instability (MSI) status is commonly applied to predict the prognosis and chemosensitivity in stage Ⅱ and stage Ⅲ colorectal cancer patients. However, researches of its function on metastasis colorectal cancer are limited. This study investigated its value on prognosis and chemosensitivity in metastatic colorectal cancer (CRC) patients. Methods: We retrospectively investigated tumor tissues from metastasis CRC patients who were treated with oxaliplatin and 5-FU-based therapy regimens (FOLFOX and XELOX). Immunostaining of proteins of the mismatch repair gene hMLH1, hMSH2, hMSH6 and hPMS2 was performed. Prognostic value and chemosensitivity in patients with MSI status were also determined. Results: Clinical features from 113 patients were analyzed. No correlation of overall survival (OS) and chemosensitivity with MSI status was found. We further investigated 79 patients with synchronous metastasis and palliatively tumor resection. Median progression free survival (PFS) from 22 MSI patients was significant longer than that in 57 MSS patients (19.9 months vs 7 months, P=0.005). No significant difference was seen in OS comparison (P=0.07). MSI status was also an independent prognostic factors of PFS by Cox multivariate analysis (MSS/ MSI，HR=2.079, P=0.043). Moreover, in this group, MSI patients had improved disease control rate (59.1% vs 31.6%, P=0.025) in chemosensitivity analysis than MSS patients. Conclusion: A better PFS in MSI patients with synchronous metastasis and palliative tumor resection was found after treated with oxaliplatin and 5-FU-based therapy and a better chemosensitivity in MSI patients was also found.

Key words: Advanced colorectal cancer, Microsatellite instability, Chemotherapy, Prognosis