Microsatellite instability predicts improved response to adjuvant therapy with irinotecan, 5-fluorouracil and leucovorin in stage III colon cancer

Abstract

10003 Background: Colon cancers exhibiting a high level of microsatellite instability (MSI-H) show distinct clinicopathological features, including both better prognosis and reduced response to 5-fluorouracil (5-FU)-based chemotherapy. We investigated the impact of adjuvant chemotherapy containing irinotecan in patients with MSI-H colon cancers. Methods: CALGB protocol 89803 randomized 1264 patients with resected stage III colon cancer to receive post-operative 5-FU and leucovorin (LV) with or without irinotecan. Paraffin blocks containing primary tumor and normal tissue were collected. Microsatellite instablility was assessed using a panel of mono- and di-nucleotide markers. Disease free survival (DFS) was measured from trial entry until documented disease progression or death from any cause. A statistical significance level of 0.2 was used in screening to generate hypotheses regarding MSI status and outcome. Median follow-up at analysis was 3.8 years. Overall C89803 showed no advantage for addition of irinotecan to 5-FU/LV. Results: Patients with and without tumor samples analyzed did not differ by treatment, age, gender, primary site, T-stage, differentiation, # positive nodes, or mucinous type. Of 482 tumors analyzed, 75 (16%) demonstrated MSI-H. MSI-H cancers were more likely to be located in the proximal colon (p<0.0001), of high histologic grade (p<0.0001) and mucinous histology (p<0.0001), and also had increased numbers of tumor-containing lymph nodes (mean # positive nodes/case = 3.5 for MSI Low/Stable vs. 4.7 for MSI-H; p = 0.04). At the time of analysis 143 of 482 patients (36%) analyzed experienced tumor recurrence and/or death due to any cause. For patients with MSI-H tumors, DFS was better in those treated with irinotecan in addition to 5-FU/LV (logrank p=0.18). Among patients with MSI Low/Stable tumors there was no difference in DFS between those treated with and without irinotecan (logrank p =0.39). Conclusions: Early results from CALGB protocol 89803 indicate that addition of postoperative irinotecan to 5-FU/LV may improve DFS in patients with stage III colon cancers that exhibit MSI-H. Longer follow-up is required to confirm this finding. [Table: see text]