DOSE ESCALATION: Patients receive irinotecan hydrochloride intravenously (IV) over 90 minutes on days 1 and 8 and veliparib orally (PO) twice daily (BID) on days -1 to 14 (days 3-14 of course 1 only). Courses repeat every 21 days in the absence of disease progression or unacceptable toxicity.