The purpose of this study is to determine a maximum tolerated dose and/or recommended phase 2 dose of a combination of imatinib and BYL719 in the treatment of 3rd line GIST patients.

Inclusion Criteria:

• Male or female patients ≥ 18 years of age -WHO performance status (PS) of 0-2 -Histologically confirmed diagnosis of GIST that is unresectable or metastatic -.Available tissue specimen: • Dose-escalation part: patients must have available archival tumor tissue which can be shipped during the course of the study. In the absence of archival tumor tissue, patients must agree to a fresh pre-treatment biopsy at screening. • Dose-expansion part: patients must have available archival tumor tissue which can be shipped during the course of the study and must agree to a fresh pre-treatment biopsy
• Failed prior therapy with imatinib followed by sunitinib for the treatment of unresectable or metastatic GIST. Note the following specific criteria for the two parts of the trial: • Dose-escalation part: patients who failed prior therapy with imatinib and then have failed therapy with sunitinib. Treatment failure may be due to either disease progression on therapy (both imatinib and sunitinib) or intolerance to therapy (sunitinib) • Dose-escalation part patients may have had additional lines of therapy than imatinib and sunitinib dose-expansion part: patients must have documented disease progression on both imatinib and sunitinib. In addition, patients may have had no more than two lines of prior therapy (i.e. treatment with imatinib followed by treatment with sunitinib). • Note: Adjuvant imatinib will not count as a prior course of imatinib for the purposes of this criterion 6. Radiological (CT/MRI) confirmation of disease progression (RECIST criteria) during prior therapy with imatinib and sunitinib will be required for patients entering the Dose-expansion part

Exclusion Criteria:

-Previous treatment with PI3K inhibitors -Patient has active uncontrolled or symptomatic central nervous system (CNS) metastases Note: A patient with controlled and asymptomatic CNS metastases may participate in this trial. As such, the patient must have completed any prior treatment for CNS metastases > 28 days (including radiotherapy and/or surgery) prior to start of treatment in this study and should not be receiving chronic corticosteroid therapy for the CNS metastases -Severe and/or uncontrolled concurrent medical condition that, in the opinion of the investigator, could cause unacceptable safety risks or compromise compliance with the protocol (e.g. acute or chronic liver, pancreatic disease, severe renal disease considered unrelated to study disease, chronic pulmonary disease including dyspnea at rest from any cause) -Patients with diabetes mellitus requiring insulin treatment and/or with clinical signs or with FPG >120mg/dL / 6.7mmol/L, or history of documented steroid-induced diabetes mellitus -Patient who has not recovered to grade 1 or better from any adverse events related to previous imatinib and/or sunitinib therapy before screening procedures are initiated