A Study to Investigate the Safety and Efficacy of AT13387, Alone or in Combination With Imatinib, in Patients With GIST

The study consists of 3 parts: Part 1 is a dose escalation phase, Part 2 is a dose expansion phase and Part 3 is either a further dose expansion phase or a randomised phase in which half the patients receive AT13387 monotherapy and half continue to receive AT13387 in combination with imatinib. All patients will receive AT13387 given by intravenous infusion on Days 1, 8, and 15 of a 28-day cycle. Most patients will also receive imatinib 400 mg by mouth every day. Patients will have tumour imaging at baseline, and at 2, 4 and 6 months, and then at 2 month intervals until cycle 12, and then 3-monthly thereafter. Blood samples will be taken to measure plasma drug levels of AT13387 given in combination with imatinib