Inclusion Criteria:

- At least 18 years of age at the time of study randomization.

- Histologically confirmed metastatic and/or unresectable GIST.

- Measurable disease on CT or MRI as defined by RECIST.

- Documented radiographic progression or intolerance to imatinib and sunitinib.

- Clinical failure of the most recent prior therapy for GIST. Note: There is no limit
to the number of prior therapies a patient may have received.

- Eastern Cooperative Oncology Group (ECOG) performance status: 0 or 1.

- Hemoglobin ≥ 8.0 g/dL (80 g/L).

- Absolute Neutrophil Count ≥ 1500/µL (1.5 x 109/L).

- Platelets ≥ 100,000 /µL (100 x 109/L).

- ALT and AST ≤ 2.5 x upper limit of normal (ULN), or ≤ 5.0 x ULN if considered
secondary to liver metastases.

- Alkaline phosphatase ≤ 2.5 x ULN, or ≤ 5.0 x ULN if considered secondary to liver
metastases.

- Serum bilirubin ≤ 1.5 x ULN.

- PT and PTT ≤ 1.5 x ULN unless the patient is receiving warfarin. If the patient is
receiving warfarin, the INR must be within therapeutic range.

- Serum creatinine ≤ 1.5 x ULN.

Exclusion Criteria:

- Previous administration of other known heat shock protein 90 (Hsp90) inhibitors.

- Surgery, radiotherapy, or lesion ablative procedure to the only area of measurable
disease.

- Initiation or discontinuation of concurrent medication that is a potent CYP3A
inhibitor less than 2 weeks prior to administration of IPI-504 or placebo.

- History of any of the following within the last 6 months: cardiac disease such as
acute coronary syndrome or unstable angina, symptomatic congestive heart failure,
uncontrolled hypertension, cirrhotic liver disease, cerebrovascular accident, or any
other significant co-morbid condition or disease which, in the judgment of the
investigator, would place the patient at undue risk or interfere with the study.

- Grade 3 or 4 hemorrhagic event within the last 6 months.

- Known human immunodeficiency virus positivity.

- Sinus bradycardia (resting heart rate < 50 bpm) secondary to intrinsic conduction
system disease.

- QTcF ≥ 470 milliseconds, or previous history of clinically significant QTc
prolongation while taking other medications.

- History of prior malignancies within the past 3 years other than non-melanomatous
skin cancers that have been controlled, prostate cancer that has been treated and has
not recurred, non-muscle-invasive bladder cancer, and carcinoma in situ of the
cervix.

- Active or recent history (within 3 months) of keratitis or keratoconjunctivitis
confirmed by ophthalmology or optometry exam.

- Presence of Left Bundle Branch Block, Right Bundle Branch Block plus left anterior
hemiblock, bifascicular block, or 3rd degree heart block. This does not include
patients with a history of these events with adequate control by pacemaker.

- Known CNS metastases.

- Women who are pregnant or lactating.