Nafamostat is a potent human diamine oxidase inhibitor possibly augmenting hypersensitivity reactions during nafamostat administration .

Nafamostat is an approved short acting serine protease. However, its administration is also associated with anaphylactic reactions. One mechanism to augment hypersensitivity reactions could be inhibition of diamine oxidase (DAO). The chemical structure of nafamostat is related to the potent DAO inhibitors pentamidine and diminazene. Therefore we tested whether nafamostat is a human DAO inhibitor. Using different activity assays nafamostat reversibly inhibited recombinant human DAO with an IC50 of 300 to 400 nM using 200 µM substrate concentrations. The Ki of nafamostat for the inhibition of putrescine and histamine deamination is 27 nM and 138 nM respectively. For both substrates nafamostat is a mixed mode inhibitor with p-values <0.01 compared to other inhibition types. Using 80% to 90% EDTA plasma the IC50 of nafamostat inhibition was approximately 360 nM using 20 µM cadaverine. In 90% EDTA plasma the IC50 concentrations were 2-3 µM using 0.9 µM and 0.18 µM histamine as substrate. In silico modeling showed a high overlap compared to published diminazene crystallography data, with a preferred orientation of the guanidine group towards topaquinone. In conclusion, nafamostat is a potent human DAO inhibitor and might increase severity of anaphylactic reaction by interfering with DAO‑mediated extracellular histamine degradation. Significance Statement Treatment with the short-acting anticoagulant nafamostat during hemodialysis, leukocytapheresis, extracorporeal membrane oxygenator procedures and disseminated intravascular coagulation is associated with severe anaphylaxis in humans. Histamine is a central mediator in anaphylaxis. Potent inhibition of the only extracellular histamine-degrading enzyme diamine oxidase could augment anaphylaxis reactions during nafamostat treatment.