the TP10 Cardiac Surgery Study GroupBackground-This study was undertaken to determine whether soluble human complement receptor type 1 (TP10), a potent inhibitor of complement activation, would reduce morbidity and mortality in high-risk patients undergoing cardiac surgery on cardiopulmonary bypass (CPB). Methods-This was a randomized multicenter, prospective, placebo-controlled, double-blind study in which 564 high-risk patients undergoing cardiac surgery on CPB received an intravenous bolus of TP10 (1, 3, 5, 10 mg/kg) or placebo immediately before CPB. The primary endpoint was the composite events of death, myocardial infarction (MI), prolonged (Ն24 hours) intra-aortic balloon pump support (IABP), and prolonged intubation. Results-TP10 significantly inhibited complement activity after 10 to 15 minutes of CPB and this inhibition persisted for 3 days postoperatively. However, there was no difference in the primary endpoint between the 2 groups (33.7% placebo versus 31.4% TP10; Pϭ0.31). The primary composite endpoint was, however, reduced in all male TP10 patients by 30% (Pϭ0.025). TP10 reduced the incidence of death or MI in males by 36% (Pϭ0.026), the incidence of death or MI in CABG males by 43% (Pϭ0.043) and the need for prolonged IABP support in male CABG and valve patients by 100% (Pϭ0.019). There was, however, no improvement seen in female TP10 patients. There were no significant differences in adverse events between the groups. Conclusion-TP10 effectively inhibits complement activation during CPB; however, this was not associated with an improvement in the primary endpoint of the study. Nevertheless, TP10 did significantly decrease the incidence of mortality and MI in male patients.