Inclusion Criteria:

- Patients must have histologically or cytologically confirmed adenocarcinoma of the GE
junction or stomach, which is beyond the scope of surgical resection

- Patients 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 >=
20 mm with conventional techniques or as >= 10 mm with spiral CT scan

- Patients may have received adjuvant therapy for resected disease

- Patients who have received no prior systemic therapy for advanced disease are
eligible to receive PS341 + irinotecan

- Patients are eligible for PS-341 as a single agent if they have received any
number of prior regimens for gastric cancer

- Life expectancy of greater than 6 weeks

- ECOG performance status =< 2 (Karnofsky >= 60%)

- Leukocytes >= 3,000/uL

- Absolute neutrophil count >= 1,500/uL

- Platelets >=100,000/uL

- Total bilirubin within normal institutional limits

- AST(SGOT)/ALT(SGPT) =< 2.5 X institutional upper limit of normal

- Creatinine within normal institutional limits

- When possible, patients will undergo an esophagogastroduodenscopy (EGD) with biopsy
prior to therapy then at 24 hours after starting PS-341

- Women of child-bearing potential and men must agree to use adequate contraception
(hormonal or barrier method of birth control; abstinence) prior to study entry and
for the duration of study participation; should a woman become pregnant or suspect
she is pregnant while participating in this study, she should inform her treating
physician immediately

- Ability to understand and the willingness to sign a written informed consent document

Exclusion Criteria:

- Patients who have had chemotherapy or radiotherapy within 4 weeks (6 weeks for
nitrosoureas or mitomycin C) prior to entering the study or those who have not
recovered from adverse events due to agents administered more than 4 weeks earlier

- Patients may not be receiving any other investigational agents, concurrent radiation
therapy, or other chemotherapy; patients who have had photodynamic therapy either
within 4 weeks of study entry or concurrently are ineligible, unless it was used to
relieve esophageal obstruction that could not be treated with laser, stent or
dilatation

- Patients with known brain metastases should be excluded from this clinical trial
because of their poor prognosis and because they often develop progressive neurologic
dysfunction that would confound the evaluation of neurologic and other adverse events

- History of allergic reactions attributed to compounds of similar chemical or biologic
composition to PS 341 or other agents used in the study

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

- Pregnant women are excluded from this study; breastfeeding should be discontinued if
the mother is treated with PS341

- HIV-positive patients receiving combination anti-retroviral therapy are excluded from
the study

- Patients with a concurrent malignancy are excluded (except for early stage squamous
cell carcinoma of the skin or cervix which can be treated locally); patients with an
advanced malignancy within the past five years are not eligible; patients with a
prior malignancy who have been disease free for five years are eligible