Although results of some phase III clinical trials of new oral anticoagulants 
are now known, it is important to understand the mechanisms of their actions. 
These new agents exert their anticoagulant effect via direct inhibition of a 
single Factor within the coagulation cascade (such as Factor Xa or thrombin). 
Rivaroxaban--the first oral, direct Factor Xa inhibitor--is a small-molecule 
oxazolidinone derivative that binds directly and reversibly to Factor Xa via the 
S1 and S4 pockets. Rivaroxaban competitively inhibits Factor Xa and is more than 
10,000-fold more selective for Factor Xa than other related serine proteases, 
and it does not require cofactors (such as antithrombin) to exert its 
anticoagulant effect. Unlike indirect Factor Xa inhibitors, rivaroxaban inhibits 
both free and clot-bound Factor Xa, as well as prothrombinase activity, thereby 
prolonging clotting times. Dabigatran etexilate is a direct thrombin inhibitor 
that inhibits both free and fibrin-bound thrombin. Although the mechanism of 
action differs between the direct Factor Xa and direct thrombin inhibitors, 
phase III studies of these new agents confirmed that both Factor Xa and thrombin 
are viable anticoagulation targets.