Randomized phase II study of 6 versus 12 months of adjuvant imatinib for patients with intermediate- or high-risk GIST.

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9 Background: The Z9001 study revealed adjuvant imatinib for 1 year significantly improved RFS in GIST patients (pts). The SSGXVIII study compared 3 years with 1 year of adjuvant imatinib for high risk GIST pts, but there was no study to evaluate shorter period of imatinib administration than 1 year. We conducted a randomized phase II study to compare 6 months (6-mo) with 12 months (12-mo) adjuvant imatinib for intermediate or high risk GIST pts. Methods: Inclusion criteria included ECOG-PS of 0 or 1, age between 20 and 79 years, and primary KIT-positive GIST with intermediate or high risk according to the Fletcher criteria. Pts were randomized assigned to the 6-mo or 12-mo treatment of imatinib 400 mg/day after complete resection. The primary endpoint was recurrence-free survival (RFS). The study was designed as a randomized screening trial to evaluate non-inferiority with margin of hazard ratio 1.67, 1-sided alpha 0.2 and power 0.8. Results: Ninety-two pts were randomly allocated the 6-mo group (n=45) or the 12-mo group (n=47) between Dec 2007 and Aug 2011, which was well balanced for baseline characteristics. One patient was ineligible due to non-GIST (desmoid) tumor at a central review. The proportions of pts completed their assigned adjuvant treatment were 80% in the 6-mo and 70% in the 12-mo group. The first interim analysis was conducted at Sep 2012 with the median follow-up time of 33 months. The 1- and 2-year RFS were 82% and 65% in the 6-mo group and 96% and 86% in the 12-mo group, respectively. Hazard ratio of recurrence was 1.81 (95%CI: 0.84-3.91), and the 2-sided log-rank p value was 0.12. Adjuvant imatinib was well tolerated, with one patient of Gr. 4 rash and no treatment-related death. Because of the lower efficacy of the 6-mo group than expected, the Data and Safety Monitoring Committee recommended the early release of first interim analysis results. Conclusions: Adjuvant Imatinib for 6-mo was inferior in efficacy to that for 12-mo in terms of RFS. Shortening of the adjuvant imatinib duration is not recommended for intermediate or high risk GIST pts. Clinical trial information: UMIN000000950.