Objectives: We sought to evaluate the safety and efficacy of recombinant nematode anticoagulant protein c2 (rNAPc2) in patients with non-ST-segment elevation acute coronary syndrome (nSTE-ACS). Background: Recombinant NAPc2 is a potent inhibitor of the tissue factor/factor VIIa complex that has the potential to reduce ischemic complications mediated by thrombin generation. Methods: A total of 203 patients were randomized 4:1 to double-blinded intravenous rNAPc2 or placebo every 48 h for a total of 1 to 3 doses in 8 ascending panels (1.5 to 10 μg/kg). All patients received aspirin, unfractionated heparin (UFH), or enoxaparin and early catheterization; clopidogrel and glycoprotein IIb/IIIa blockers were encouraged. Two subsequent open-label panels evaluated 10 μg/kg rNAPc2 with half-dose UFH (n = 26) and no UFH (n = 26). The primary end point was the rate of major plus minor bleeding. Pharmacokinetics, pharmacodynamics, continuous electrocardiography, and clinical events were assessed. Results: Recombinant NAPc2 did not significantly increase major plus minor bleeding (3.7% vs. 2.5%; p = NS) despite increasing the international normalized ratio in a dose-related fashion (trend p ≤ 0.0001). Higher-dose rNAPc2 (≥7.5 μg/kg) suppressed prothrombin fragment F1.2 generation compared with placebo and reduced ischemia by >50% compared to placebo and lower-dose rNAPc2. Thrombotic bailout requiring open-label anticoagulant occurred in 5 of 26 patients treated without UFH, but none in the half-dose UFH group (19% vs. 0%; p = 0.051). Conclusions: In patients with nSTE-ACS managed with standard antithrombotics and an early invasive approach, additional proximal inhibition of the coagulation cascade with rNAPc2 was well tolerated. rNAPc2 doses ≥7.5 μg/kg suppressed F1.2 and reduced ischemia, though some heparin may be necessary to avoid procedure-related thrombus formation. (Anticoagulation With rNAPc2 to Eliminate MACE/TIMI 32; http://www.clinicaltrial.gov/ct/show/NCT00116012?order=1; NCT00116012). © 2007 American College of Cardiology Foundation.