Aprotinin is a protease inhibitor that has been used in various medical applications due to its ability to inhibit several enzymes, including trypsin, chymotrypsin, plasmin, and kallikrein. Its primary applications include:

1. **Cardiac Surgery**: Aprotinin was widely used to reduce blood loss and the need for blood transfusions during cardiac surgeries, particularly in procedures involving cardiopulmonary bypass. It works by inhibiting fibrinolysis (the breakdown of blood clots) and reducing inflammatory responses.

2. **Orthopedic Surgery**: It has also been used in major orthopedic surgeries to minimize blood loss.

3. **Trauma and Hemorrhage**: Aprotinin has been investigated for use in trauma patients to control bleeding and reduce the need for blood transfusions.

4. **Pancreatitis**: It has been studied for the treatment of acute pancreatitis, as it inhibits trypsin and other proteases that can exacerbate pancreatic inflammation.

5. **Research**: In laboratory research, aprotinin is commonly used to inhibit proteolytic degradation of proteins during cell lysis and protein extraction procedures.

However, the use of aprotinin has been controversial. In 2007, concerns about its safety led to its temporary withdrawal from the market after studies suggested an increased risk of renal failure, myocardial infarction, and stroke in patients undergoing cardiac surgery. Subsequent re-evaluations and studies have led to its reintroduction in some countries, but its use remains carefully regulated and limited to specific cases where the benefits outweigh the risks.