AIMS: We sought to determine the feasibility of conducting percutaneous coronary intervention (PCI) in high-risk acute coronary syndrome (ACS) patients utilising the REG1 system consisting of pegnivacogin, an aptameric factor IXa inhibitor, and its controlling agent anivamersen. METHODS AND RESULTS: In RADAR, ACS patients were randomised to pegnivacogin 1 mg/kg with 25%, 50%, 75%, or 100% anivamersen reversal or unfractionated heparin. Of the 640 patients randomised, 388 (61%) underwent PCI. Major modified ACUITY 30-day bleeding rates were 18% (25% reversal), 12% (50% reversal), 9% (75% reversal), and 7% (100% reversal), compared with 11% with heparin. The corresponding total bleeding rates were 68%, 39%, 35%, 34%, and 38% (heparin). Ischaemic events were less frequent in those receiving pegnivacogin versus heparin (4.4% vs. 7.3%, p=0.3). Thirty-day urgent TVR (1.1% vs. 0.9%, p=1.0), myocardial infarction (4.0% vs. 6.4%, p=0.3), and angiographic complication (11.2% and 10.8%, p=0.9) rates were similar with pegnivacogin and heparin. There were no incidences of clot formation on guidewires or catheters. CONCLUSIONS: High-level factor IXa inhibition in ACS patients undergoing PCI, with at least 50% reversal, has a favourable bleeding profile and appears effective at suppressing ischaemic events and thrombotic complications. Larger phase trials in PCI are warranted. CLINICAL TRIALS REGISTRATION: ClinicalTrials.gov NCT00932100.