The impact of microsatellite instability status and sidedness of the primary tumor on the effect of bevacizumab-containing chemotherapy in patients with metastatic colorectal cancer

Purpose: We aimed to evaluate the effect of bevacizumab in metastatic CRC (colorectal cancer) regarding to microsatellite instability (MSI) and the sidedness of the primary tumor. Materials and Methods: A total of 140 CRC patients were retrospectively analyzed, who received bevacizumab-containing chemotherapy between April 2008 and January 2013. MSI status and Kirsten RSAS (KRAS) mutational status were available in all 140 patients, but BRAF (the gene for the B-type Raf kinase) mutational status was only available in 74 patients (52.9%). Results: MSI-high (MSI-H) was detected in 4.3% of analyzed patients. Characteristics of patients, with the exception of BRAF mutational status, were generally similar between those with right- (RC) and left-sided colon cancer (LC). Right-sided tumors were significantly associated with a BRAF mutation (p=0.025). In addition, patient characteristics with a microsatellite stable (MSS) tumor were not different from those with an MSI-H tumor. For all 140 patients, the most commonly used regimen with bevacizumab was capecitabine plus oxaliplain (XELOX), irrespective of treatment line, followed by 5-fluorouracil, leucovorin, and irinotecan (FOLFIRI), 5-fluorouracil, leucovorin, and oxaliplatin (FOLFOX), intravenous 5-fluorouracil (5-FU) and capecitabine plus irinotecan (XELIRI). There was no significant difference between the MSI-H and MSS groups in treatment efficacy, including response rate (RR) and disease control rate (DCR). There was also no difference in RR and DCR according to the sidedness of the primary tumor. No significant difference in progression-free survival (PFS) was observed between MSI-H and MSS groups (5.93 months vs. 7.37 months; p=0.801) or between LC and RC groups (7.37 months vs. 5.83 months; p=0.801). Conclusions: The effect of bevacizumab was not different between LC and RC and between MSS and the MSI-H tumors.