C1s-IN-1 trihydrochloride (Compound A1) is the selective inhibitor for C1s protease with the Ki of 5.8 μM. C1s-IN-1 trihydrochloride inhibits cleavage of C2 by C1s with an IC50 of 85 μM, inhibits the activation of classic pathway with an IC50 of 22 μM. C1s-IN-1 trihydrochloride is the competitive inhibitor for thrombin with the Ki of 51.2 μM.