Limited generation of activated protein C during infusion of the protein C activator thrombin analog W215A/E217A in primates

Anticoagulation with activated protein C (APC) reduces the mortality of severe sepsis. We investigated whether the circulating protein C (PC) pool could be utilized for sustained anticoagulation by endogenous APC. To generate APC without procoagulant effects, we administered the anticoagulant thrombin mutant W215A;E217A (WE) to baboons. In preliminary studies, administration of high dose WE (110 μg kg^-1 i.v. bolus every 120 min; n = 2) depleted PC levels by 50% and elicited transient APC bursts and anticoagulation. The response to WE became smaller with each successive injection. Low dose WE infusion (5 μg kg^-1 loading + 5 μg kg^-1 h^-1 infusion; n = 5) decreased plasma PC activity by 15%, from 105% to 90%, to a new equilibrium within 60 min. APC levels increased from 7.5 ng mL^-1 to 86 ng mL^-1 by 40 min, then declined, but remained elevated at 34 ng mL^-1 at 240 min. A 22-fold higher dose WE (n = 5) decreased PC levels to 60% by 60 min without significant further depletion in 5 h. The APC level was 201 ng mL^-1 at 40 min and decreased to 20 ng mL^-1 within 120 min despite continued activator infusion. Co-infusion of WE and equimolar soluble thrombomodulin (n = 5) rapidly consumed about 80% of the PC pool with significant temporal increase in APC generation. In conclusion, low-grade PC activation by WE produced sustained, clinically relevant levels of circulating APC. Limited PC consumption in WE excess was consistent with the rapid depletion of cofactor activity before depletion of the PC zymogen. Reduced utilization of circulating PC might have similar mechanism in some patients.