Inclusion Criteria:

- Participants must have histologically or cytologically confirmed advanced esophageal,
gastroesophageal, or gastric cancer. When possible, archived biopsy or resection
specimens must be available for correlative SNaPshot and FISH studies.

- Participants must have measurable disease, defined as at least one lesion that can be
accurately measured in at least one dimension (longest diameter to be recorded) as
20mm or greater with conventional techniques or as 10mm or greater with spiral CT
scan (per RECIST criteria).

- Participants must have progressed through prior 1st-line therapy. For the purposes
of this trial, neoadjuvant chemoradiation or peri-operative chemotherapy may be
considered as prior 1st-line treatment in the event of metastatic recurrence.

- 18 years of age or older

- Life expectancy of greater than 12 weeks

- ECOG Performance status of 1 or greater

- Participants must have normal organ and marrow function as defined in the protocol.

- Participants must have adequate peripheral IV access. Administration of STA-9090 via
indwelling catheters is prohibited at this time.

- No concurrent active primary or metastatic cancer other than superficial squamous
cell or basal cell skin cancer.

- At least 3 weeks or 5 half-lives must have elapsed between the most recent dose of
any prior anticancer therapy and the start date of treatment with STA-9090.
Participants must have resolution to baseline of all toxicities associated with prior
anticancer therapies.

- Women of child-bearing potential and men must agree to use adequate contraception
prior to study entry and for the duration of study participation.

Exclusion Criteria:

- Participants may not be receiving any other standard or investigational anticancer
agents, with the exception of hormonal therapy.

- Participants with known CNS metastases must have received whole-brain radiation or
other appropriate therapy not less than 4 weeks prior to starting the study drug and
exhibit clinical stability of brain disease.

- History of allergic reactions attributed to compounds of similar chemical or biologic
composition to STA-9090 or to the excipients PEG 300 and Polysorbate 80.

- Ventricular ejection fraction of 55% or less at baseline.

- Uncontrolled intercurrent illness including, but not limited to, ongoing or active
infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
arrhythmia other than chronic atrial fibrillation, or psychiatric illness/social
situations that would limit compliance with study requirements.

- Pregnant women are excluded from this study. Breastfeeding should be discontinued

- HIV-positive individuals on combination antiretroviral therapy are ineligible.