Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically or cytologically confirmed colorectal cancer

- Locally advanced and/or metastatic disease

- Disease considered incurable

- Suitable for treatment with single agent irinotecan hydrochloride as a palliative
intervention, as determined by the investigator

- Must have clinically and/or radiologically documented disease

- Patients whose only evidence of disease progression is tumor marker elevation
are not eligible

- Must have received no more than one prior oxaliplatin- and/or irinotecan
hydrochloride-based chemotherapy regimen given either with adjuvant, neoadjuvant, or
palliative intent

- One additional adjuvant fluoropyrimidine (fluorouracil or capecitabine) regimen
may have been given for relapsed or metastatic disease

- No untreated brain or meningeal metastases (CT scan required if there is a clinical
suspicion of CNS disease)

PATIENT CHARACTERISTICS:

- ECOG performance status 0-2

- Absolute granulocyte count ≥ 1,500/mm³

- Platelet count ≥ 100,000/mm³

- Bilirubin ≤ 1.5 times upper limit of normal (ULN)

- ALT and AST ≤ 2 times ULN (≤ 5 times ULN if liver metastasis is present)

- Serum creatinine ≤ 1.5 x ULN OR creatinine clearance ≥ 50 mL/min

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception during and for 90 days after
completion of study therapy

- Must reside within a 1½ hour drive from participating center

- No other invasive malignancies, unless curatively treated with no evidence of disease

- No GI tract disease resulting in an inability to absorb oral medication, including
the following:

- Uncontrolled inflammatory GI disease (e.g., Crohn's disease or ulcerative
colitis)

- Post-surgical malabsorption characterized by uncontrolled diarrhea that results
in weight loss and vitamin deficiency or requires IV hyperalimentation

- Pancreatic enzyme supplementation is allowed

- No untreated and/or uncontrolled hypertension, cardiovascular conditions, and/or
symptomatic cardiac dysfunction

- No active or uncontrolled infections

- No serious illnesses or medical conditions that would preclude study participation

- No known hypersensitivity to the study drugs or their components, atropine, or
loperamide

- Not known to be homozygous for the UGT1A1*28 allele

- No known deficiency in glucuronidation of bilirubin, such as Gilbert's syndrome

- No neuropathy ≥ grade 2

- Patients with persistent, stable, grade 3 sensory neuropathy, who meet other
eligibility criteria may be allowed at the discretion of the investigator

PRIOR CONCURRENT THERAPY:

- Recovered from all prior therapy

- No prior PARP inhibitor

- No prior radical pelvic irradiation

- No prior radiotherapy to ≥ 25% of bone marrow stores

- Prior irinotecan hydrochloride allowed provided the drug was not discontinued due to
toxic effects and the patient did not have severe irinotecan hydrochloride-related
toxicity (grade 4 or requiring hospitalization)

- At least 21 days since prior radiotherapy (exceptions may be made for low-dose,
nonmyelosuppressive radiotherapy)

- At least 30 days since prior chemotherapy

- At least 21 days since prior hormonal, immunologic, biologic, or signal transduction
inhibitor therapies

- More than 3 weeks since prior and no other concurrent investigational drugs or
anticancer therapy

- At least 14 days since prior major surgery

- Wound healing must have occurred

- At least 14 days since prior and no concurrent CYP3A4 enzyme-inducing or -inhibiting
drugs, including enzyme-inducing anticonvulsants, rifampin, rifabutin, St. John's
wort, atazanavir, or ketoconazole

- Dexamethasone is allowed for antiemetic prophylaxis