Background: Otamixaban is an intravenous direct factor Xa inhibitor. We aimed to assess its efficacy and safety in non-ST-elevation acute coronary syndromes and to identify the optimum dose range for further assessment in a phase 3 study. Methods: In this double-blind, phase 2 trial undertaken in 196 sites in 36 countries, 3241 patients with non-ST-elevation acute coronary syndromes were randomly assigned via a central, telephone-based interactive voice response system to one of five doses of otamixaban (0·08 mg/kg bolus followed by infusions of 0·035 [n=125], 0·070 , 0·105 , 0·140 , or 0·175  mg/kg/h) or to a control of unfractionated heparin (60 IU/kg intravenous bolus followed by an infusion of 12 IU/kg/h) plus eptifibatide (180 μg/kg intravenous bolus followed by an infusion of 1·0-2·0 μg/kg/min [n=449]). Both investigators and patients were unaware of treatment allocation. Enrolment into the lowest dose group was stopped early at the recommendation of the Data Monitoring Committee. The primary efficacy endpoint was a composite of death, myocardial infarction, urgent revascularisation, or bailout glycoprotein IIb/IIIa inhibitor use up to 7 days. The primary safety endpoint was TIMI major or minor bleeding not related to coronary-artery bypass grafting. Efficacy analyses were by intention to treat; safety analyses were in treated patients. This study is registered with ClinicalTrials.gov, number NCT00317395. Findings: Rates of the primary efficacy endpoint in the five otamixaban doses were 7·2% (nine of 125) with 0·035 mg/kg/h, 4·6% (31/676) with 0·070 mg/kg/h, 3·8% (25/662) with 0·105 mg/kg/h, 3·6% (24/658) with 0·140 mg/kg/h, and 4·3% (29/671) with 0·175 mg/kg/h (p=0·34 for trend). In the control group, the rate was 6·2% (28/449), yielding relative risks for the five otamixaban doses of 1·16 (95% CI 0·56-2·38), 0·74 (0·45-1·21), 0·61 (0·36-1·02), 0·58 (0·34-1·00), and 0·69 (0·42-1·15), respectively. Rates of the primary safety endpoint in the five otamixaban doses were 1·6% (two of 122), 1·6% (11/669), 3·1% (20/651), 3·4% (22/651), and 5·4% (36/664), respectively (p=0·0001 for trend); the rate in the control group was 2·7% (12/448). Interpretation: In patients with non-ST-elevation acute coronary syndromes, otamixaban infusions of 0·100-0·140 mg/kg/h might reduce ischaemic events and have a safety profile similar to unfractionated heparin plus eptifibatide. Further testing in a phase 3 trial is warranted. Funding: Sanofi-Aventis.
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