Osocimab is a fully human monoclonal immunoglobulin G1 antibody that binds adjacent to the active site of factor XIa, and prevents it from activating factor IX.
A related editorial suggests that based on the results of this dose-finding study, it is not possible to conclude that the risk of bleeding using osocimab, the monoclonal antibody to factor XI, is less than that of currently available anticoagulants. The rate of bleeding in the group receiving the 1.8-mg/kg antibody dose preoperatively signals that a high degree of factor XI inhibition is not devoid of bleeding risk.