This is a 2 arms study concerning patients with primary GIST who followed an Imatinib adjuvant treatment for 3 years after surgery and who have a high risk of recurrence.

In the first arm, patients will continue Imatinib treatment for 3 more years, allowing to determine if the continuation of this treatment is efficient for disease control, in terms of Disease Free Survival improvement.

In the second arm, patients will discontinue the Imatinib treatment, as standard practice. This arm will allow to determine if the re-introduction of Imatinib at relapse is still an efficient treatment for the control of disease.

Gastrointestinal stromal tumours (GISTs) are rare mesenchymal neoplasms, mostly diagnosed between 55 and 60 years of age, which account for 5% of all sarcomas. Worldwide annual incidence is approximately 12 cases per million people, corresponding to approximately 800 new cases per year in France.

A large majority of GISTs harbour activating mutations in the proto-oncogenes KIT and/or PDGFRA, both coding cell-surface cytokine receptors with tyrosine-protein kinase activity.

Imatinib mesilate (Glivec®, Novartis Pharma SAS) is a selective tyrosine kinase inhibitor, leading to inhibition of KIT and PDGFRA signalling pathways. The introduction of imatinib has revolutionised the therapeutic management of GIST patients and has provided an unprecedented demonstration of the clinical benefit of a targeted therapy for patients with advanced/metastatic solid tumors. First results from prospective trials conducted with imatinib in GIST patients have demonstrated a 300% increase in median overall survival, and a likely 100% increase in 5 and 10-year survival as compared to cytotoxic chemotherapy.

The successful use of imatinib in the treatment of advanced GISTs and the significant risk of recurrence of advanced GISTs have prompted the investigation of the clinical benefit of imatinib as a post-operative adjuvant therapy. Two prospective randomized Phase III trials have demonstrated that adjuvant imatinib treatment significantly prolong overall survival (OS) and recurrence-free survival (RFS) when given for 3 years. To date, imatinib is also indicated in the adjuvant setting after complete resection of primary, localized, KIT-positive GIST at high risk of recurrence. However, the optimal treatment duration remains unclear and it should be determined whether

1. prolonged use of adjuvant imatinib beyond 3 years may enable to reduce the risk of GIST recurrence and to improve overall survival, and
2. imatinib rechallenge is efficient for treating recurrence after completion of 3-year adjuvant imatinib therapy.

This trial is an open-label, randomized, multicenter phase III study aiming to determine the clinical impact of maintaining imatinib treatment beyond 3 years in the adjuvant setting for patients with resected GISTs at high risk of recurrence according to the National Comprehensive Cancer Network Task Force on GIST (NCCN) risk classification.

Inclusion Criteria:

• Age ≥ 18 years at the day of consenting to the study

• Patients must have histologically confirmed diagnosis of localized GIST with documented KIT (CD117) positivity (by polyclonal DAKO antibody staining)

• Documented macroscopically complete surgical R0 or R1 resection of primary GIST lesion with no evidence of residual lesions or metastases on the baseline CT-scan or MRI performed no more than 4 weeks before randomization.

• Risk of tumor recurrence ≥ 35% according to National Comprehensive Cancer Network Task Force on GIST (NCCN) risk classification (Demetri et al., 2010) (See Appendix 1)

• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2

• Patients must be under imatinib treatment (at 300 or 400 mg/day) initiated immediately after resection and maintained for 3 years (i.e. 36 months ± 3 months at the time of randomization) with no more than 3 consecutive months or 6 months in total of interruption during these past 3 years.

• Patients must have normal organ and bone marrow function at baseline as defined below:

  • absolute neutrophil count (ANC) ≥ 1.5 G/L, platelet count ≥ 100 G/L, and haemoglobin of ≥ 9 g/dL).

  • Serum total bilirubin ≤ 1.5 (upper limit of normal (ULN), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3 x ULN (or 5 x ULN in case of hepatic metastases at time of reintroduction)

  • Adequate renal function assessed by at least one of the following:

    • 1) Serum creatinine ≤ 1.5 x ULN or
    • 2) creatinine clearance estimate ≥ 50 mL/min (as calculated according to Cockcroft-Gault formula or MDRD formula for patients > 65 years).

• Recovered from prior anti-neoplasia treatment-related toxicity (persistent treatment-related toxicity < Grade 2 as per Common Toxicity Criteria for Adverse Effects (CTCAE) v4 are accepted)

• Women of childbearing potential are required to have a negative serum pregnancy test within 72 hours prior to randomization. A positive urine test must be confirmed by a serum pregnancy test

• Patient must use effective contraception at least 4 weeks prior to study entry, during the study participation and for at least 30 days post-treatment (not applicable for women of non-childbearing potential)

• Ability to understand and willingness for follow-up visits.

• Covered by a medical insurance.

• Signed and dated informed consent document indicating that the patient has been informed of all aspects of the trial prior to enrolment.

Exclusion Criteria:

• Pregnant or breastfeeding women
• Patient concurrently using other approved or investigational antineoplastic agents
• Any contra-indication to imatinib treatment as per Glivec® SPC
• Patient with GIST harboring the mutation D842V in PDGFRA
• Major concurrent disease affecting cardiovascular system, liver, kidneys, haematopoietic system or else considered as clinically important by the investigator and that could be incompatible with patient's participation in this trial or would likely interfere with study procedures or results.
• Prior history of other malignancies other than study disease (except for basal cell or squamous cell carcinoma of the skin or carcinoma in situ of the cervix) unless the patient has been free of the disease for at least 3 years.
• Patient receiving concurrent treatment with warfarin (acceptable alternative: low-molecular weight heparin) or any prohibited concomitant and/or concurrent medications
• Patient with Grade III/IV cardiac problems as defined by the New York Heart Association Criteria. (i.e., congestive heart failure, myocardial infarction within 6 months of study)
• Patient has a known diagnosis of human immunodeficiency virus (HIV) infection.
• Major surgery within 2 weeks prior to study entry