Aprotinin. Boxes containing vials of aprotinin, a drug used during open heart surgery. The drug is an enzyme, obtained from the lungs of cows. It breaks down the proteins in the blood that prevent blood from clotting. Therefore, it reduces blood loss during surgery and decreases the need for blood transfusions. It is administered intravenously, either by injection, or mixed with saline in an intravenous drip.