Triapine is a potent inhibitor of ribonucleotide reductase (RNR), which is required for the conversion of ribonucleotides to deoxyribonucleotides. Two phase I trials in patients with hematologic malignancies demonstrated that Triapine could reduce circulating blasts in the majority of patients without significant non-hematologic toxicity (Giles et al. Leuk Res 2003; 27:1077–1083; Karp et al. Blood 2002; 100:560a). Preclinical studies have shown that the combination of Triapine and ara-C produces additive or synergistic cytotoxicity against several tumor cell lines, potentially by increasing intracellular ara-CTP levels when Triapine inhibits RNR. Therefore, a phase I study of Triapine in combination with ara-C was conducted in 32 patients with relapsed or refractory acute leukemia and high-risk MDS (1 MDS, 28 AML, 2 Ph+ ALL, and 1 Ph- ALL) in order to determine the tolerability, safety, and maximum tolerated dose (MTD). Triapine® was administered at a dose of 105 mg/m2/d as a 6-hour infusion for 5 consecutive days (D1–5) followed immediately by ara-C 100 (n=4), 200 (n=6), 400 (n=7), or 800 (n=8) mg/m2/d as an 18-hour infusion for 5 consecutive days (D1–5). Median age was 59 years (range, 15–83 years). Median ECOG status 1 (range, 0–2). Median number of prior therapies was 2 (range 1–9), including prior autologous (n= 2) and allogeneic stem cell transplant (SCT) (n=4). Dose-limiting toxicities (DLTs) were observed at the 800 mg/m2 ara-C dose level (1 patient each with grade 4 mucositis; grade 4 neutropenic colitis, sepsis and death; grade 4 neuropathy; and grade 4 hyperbilirubinemia). Therefore, the study was amended to include an ara-C dose level of 600 mg/m2/d. Seven patients were treated at this dose level without development DLTs. Thirty-one patients received at least 1 course, 2 received 2 courses, and 4 received 3 courses; 1 patient withdrew prior to completion of 1 course of therapy. Of the 31 evaluable patients, 4 (13%) patients (3 AML, 1 Ph+ ALL) achieved a CR (1 at an ara-C dose of 800 mg/m2; 2 at 600 mg/m2; 1 at 200 mg/m2). Two of the responders also achieved a complete cytogenetic remission (including 1 Ph+ ALL). All responses occurred after 1 induction course. One patient went on to receive an allogeneic (SCT) and continues in CR. Duration of responses was 9, 20, 52+, and 73+ weeks. Mean Cmax and AUC achieved for Triapine were 1.13 μg/mL and 251.5 min•μg/mL. The most frequent toxicities included nausea &/or vomiting (66%), elevated liver enzymes (50%; gr. 3 in 22%), fever (47%), diarrhea (41%; gr. 3 in 6%), rash (31%; gr. 3 in 6%), mucositis (28%), hand-foot syndrome (16%; gr. 3 in 3%), and peripheral edema (16%; gr. 3 in 9%). Triapine and cytarabine has activity in patients with relapsed or refractory acute leukemias. The recommended phase II dose is Triapine 105 mg/m2/day for 5 consecutive days (D1–5) followed by ara-C 600 mg/m2/d for 5 consecutive days (D1–5).