Background: Veliparib (V) is a novel orally bioavailable PARP-1 and -2 inhibitor which potentially synergizes with temozolomide (T). A single arm phase II study combining C and T has shown promising activity in well-differentiated NETs with overall response rate of 43%. Hence, we propose to combine V with C and T optimizing the interaction of V with T by dosing the V during the T to produce improved clinical activity in pts with wdNETs. Methods: This is an investigator initiated, single institution, phase I, dose-escalation trial using a modified Toxicity Probability Interval (mTPI) design. The primary objective is to estimate the maximum tolerable dose (MTD), which is defined as the dose level at which the probability of dose limiting toxicities (DLTs) is 0.33. Secondary objectives include assessing safety profile, antitumor activity and progression free survival. Inclusion criteria: Pathologically confirmed metastatic unresectable well-differentiated (low grade and intermediate grade) NETs that demonstrate progressive disease in the prior 12 months, ECOG PS 0-2 and adequate organ function. Exclusion criteria: severe hypersensitivity reaction to C, 5-FU, T or dacarbazine or uncontrolled chronic illness. Prior treatment with both C and T. Administration and design: We initially will fix doses of C and T at C: 750 mg/m² po twice a day (BID) on days 1-14 and T at 100 mg/m2 po BID on days 10-14. V is given orally on days 10-14 along with T. We have 5 planned V dose levels (20, 40, 60, 80, 100 mg). If dose level 1 is too toxic we will go to C 600 mg/m², T 75 mg and V 20 mg. Patients will be enrolled in cohorts of 3. The dose escalation/de-escalation will be as per pre-specified guidelines from the mTPI design. The planned sample size is 21 based on the anticipated number of patients available for this study within 18 months. This trial will be terminated when the maximum sample size (21) is reached or if the lowest dose is deemed too toxic. The study will start enrolling patients October 2015.