Targeting of antioxidant and anti-thrombotic drugs to endothelial cell adhesion molecules.
The endothelium represents an important therapeutic target for containment of oxidative stress, thrombosis and inflammation involved in a plethora of acute and chronic conditions including cardiovascular and pulmonary diseases and diabetes. However, rapid blood clearance and lack of affinity to the endothelium compromise delivery to target and restrict medical utility of antioxidant enzymes (e.g., catalase) and fibrinolytics. The use of "stealth" PEG-liposomes prolongs circulation, whereas conjugation with antibodies to endothelial determinants permits targeting. Constitutive endothelial cell adhesion molecules (CAM, such as ICAM-1 and PECAM-1, which are stably expressed and functionally involved in oxidative stress and thrombosis) are candidate determinants for targeting of antioxidants and fibrinolytics. CAM antibodies and compounds conjugated with anti-CAM bind to endothelial cells and accumulate in vascularized organs (preferentially, lungs). Pathological stimuli enhance ICAM-1 expression in endothelial cells and facilitate targeting, whereas PECAM-1 expression and targeting are stable. Endothelial cells internalize 100-300 nm diameter conjugates possessing multiple copies of anti-CAM, but not monomolecular antibodies or micron conjugates. This permits size-controlled sub-cellular targeting of antioxidants into the endothelial interior and fibrinolytics to the endothelial surface. Targeting catalase to PECAM-1 or ICAM-1 protects endothelial cells against injury by oxidants in culture and alleviates vascular oxidative stress in lungs in animals. Anti-CAM/catalase conjugates are active for a few hours prior to lysosomal degradation, which can be delayed by auxiliary drugs. Conjugation of fibrinolytics to monovalent anti-ICAM permits targeting and prolonged retention on the endothelial surface. Therefore, CAM targeting of antioxidants and fibrinolytics might help to contain oxidative and thrombotic stresses, with benefits of blocking CAM. Avenues for improvement and translation of this concept into the clinical domain are discussed.