The urokinase plasminogen activator receptor (uPAR) as a target for the diagnosis and therapy of cancer.

The identification and characterization of validated molecular targets for cancer drug and diagnostic development is rapidly changing the way that promising new anti-cancer compounds are developed and evaluated. A significant body of in vitro and in vivo data has established the urokinase plasminogen activator (uPA) system as a promising target for cancer drug development. The uPA system has been demonstrated to have pleiotropic activities in the development of tumors, and in tumor progression and angiogenesis. There are multiple ways to target this system, the most straightforward being the development of small molecule active site inhibitors of the serine protease, uPA. However, compounds of this type have not entered into clinical trials, and issues related to selectivity and specificity of this class of inhibitors have yet to be satisfactorily resolved. Recent evidence suggests that in addition to uPA, its specific cell surface receptor (uPAR) may also be a suitable target for the design and development of cancer therapeutic and diagnostic agents. uPAR is central to several pathways implicated in tumor progression and angiogenesis. The binding of the uPA zymogen (scuPA) to uPAR appears to be a pre-requisite for efficient cell-surface activation of scuPA to the active two-chain form (tcuPA) by plasmin, and simple ligand occupancy of uPAR by scuPA initiates various signaling pathways leading to alterations in cell motility and adhesion. One therapeutic rationale that is currently being investigated is the simple displacement of scuPA or tcuPA from suPAR, which may effectively inhibit both the proteolytic and signal-transducing cascades. In addition, other approaches to the modulation of the activity of this system that may also be useful include blocking the interaction of uPAR with integrins and extracellular matrix proteins as well as strategies to down-regulate the expression of uPA and uPAR in target cells. This review will summarize these approaches, and also describe the targeting of uPAR for diagnosis and imaging.