Chemotherapy for breast cancer decreases plasma protein C and protein S.

An increased incidence of thromboembolic events has been described in women receiving chemotherapy for breast cancer. The etiology of this enhanced thrombotic state has not been defined. We performed serial coagulation studies in 15 women during 1 monthly cycle of cyclophosphamide, methotrexate, and fluorouracil (CMF) chemotherapy for breast cancer; seven adjuvant and eight metastatic. Plasma protein C levels were measured by anticoagulant, amidolytic, and antigenic techniques. Antigen levels of both total and free plasma protein S were quantitated by immunoelectrophoresis. Plasma levels of protein C, an important vitamin K-dependent inhibitor of blood coagulation and a profibrinolytic agent, and protein S, a cofactor for protein C, decreased 1 and 2 weeks after initiation of chemotherapy compared with pretreatment values. Plasma levels of factor VII and fibrinogen also decreased. The changes in protein C and protein S may contribute to the enhanced thrombotic tendency described in this setting. Possible mechanisms for the decreases in plasma protein C, protein S, factor VII, and fibrinogen are discussed.