Final analysis of the EORTC Soft Tissue and Bone Sarcoma Group, the Australasian Gastro-Intestinal Trials Group, UNICANCER, French Sarcoma Group, Italian Sarcoma Group, and Spanish Group for Research on Sarcomas confirms the efficacy of adjuvant treatment with imatinib in patients with localised gastrointestinal stromal tumour (GIST) in terms of prolonged relapse-free survival (RFS). In the high-risk subgroup, there was a trend toward a better long-term imatinib failure-free survival (IFFS). According to Paolo G. Casali of the Fondazione IRCCS Istituto Nazionale Tumori in Milan, Italy and colleagues who published the study findings in the Annals of Oncology, this is consistent with the German/Scandinavian study results and support current standard treatment with adjuvant imatinib for 3 years in GIST patients with a significant risk of relapse.

This collaborative randomised clinical study of adjuvant imatinib was launched in 2004. The study team included all GIST patients with an intermediate- or high-risk of relapse, following the consensus classification used at the time. When designed the study, the investigators believed that a benefit in RFS was more than likely, and decided that a meaningful benefit would have been in place only if it resulted in an increase in the cure rate, or, at least, in a substantial delay of relapses, provided no decrease in the time to progression was to occur when re-challenging relapsing patients with imatinib. Therefore, they chose overall survival (OS) as the study primary endpoint.

However, at a planned interim analysis in March 2009, the study Independent Data Monitoring Committee concluded that keeping OS as the primary endpoint would be incompatible with a reasonable duration of the study. The study team then decided another primary endpoint, IFFS, as an estimate of the time to resistance to imatinib, defined as a survival interval to the date of switching to an alternate tyrosine kinase inhibitor at any time during or following the adjuvant period.

The interim analysis was published in 2015, while continuing the study follow-up to the planned final analysis that is now reported. It is a randomised, open label, multicentre phase III study performed at 112 hospitals in 12 countries. Patients were randomised to 2 years of imatinib, 400 mg daily, or no further therapy after surgery.

In total, 908 patients were randomised between January 2005 and October 2008 (454 patients to imatinib and 454 patients to observation), but 835 patients were eligible. With a median follow-up of 9.1 years, 5 year IFFS was 87% and 10 year 75% in the imatinib arm versus 83% and 74% in the control arm (hazard ratio [HR] 0.87, 95.7% confidence interval [CI] 0.65, 1.15; p=0.31).

The RFS was 70% versus 63% at 5 years and 63% vs 61% at 10 years (HR 0.71, 95% CI 0.57, 0.89; p=0.002).

The OS was 93% versus 92% at 5 years and 80% versus 78% at 10 years (HR 0.88, 95% CI 0.65, 1.21; p=0.43).

Among 526 patients with high-risk GIST by local pathology, 10-year IFFS and RFS were 69% versus 61%, and 48% versus 43%, respectively.

For both OS and its potential surrogate defined in this study as IFFS, the data did not show an effect in favour of adjuvant treatment with imatinib overall, but a trend was observed in the high-risk GIST. The authors concluded that although the difference was not statistically significant and the surrogacy value of such an endpoint is not validated, this may be seen as supportive for the results reported by the Scandinavian/German study, showing a sustained small but significant long-term OS benefit in high-risk GIST patients treated with 3 years of adjuvant imatinib and may attenuate a mismatch between RFS and OS that undoubtedly marks the efficacy of adjuvant targeted therapy in GIST.

Mikael Eriksson of the Department of Oncology, Skåne University Hospital and Lund University in Lund, Sweden and Heikki Joensuu of the Department of Oncology, Helsinki University Hospital and University of Helsinki in Helsinki, Finland wrote in an accompanied editorial article that the results from the three randomised studies suggest that careful patient selection for adjuvant imatinib is of critical importance and that administration of adjuvant imatinib for a long enough duration is likely needed.

Currently 3 years of adjuvant imatinib remains the standard of care for patients who are at high-risk for GIST recurrence, despite macroscopically complete surgery, and who have a GIST mutational status that suggests sensitivity to imatinib. However, evaluation of the safety and efficacy of adjuvant imatinib of duration longer than 3 years remains a high priority, and two such studies are currently ongoing.

The collaborative intergroup study publication was supported by the EORTC Charitable Trust. Supporting funding was received from Novartis.