Development of Novel Therapeutics Targeting the Urokinase Plasminogen Activator Receptor (uPAR) and Their Translation Toward the Clinic

The urokinase plasminogen activator receptor (uPAR) mediates cell motility and tissue remodeling. Although uPAR may be expressed transiently in many tissues during development and wound healing, its constitutive expression appears to be associated with several pathological conditions, including cancer. uPAR expression has been demonstrated in most solid tumors and several hematologic malignancies including multiple myeloma and acute leukemias.Unlike many tumor antigens, uPAR is present not only in tumor cells but also in a number of tumor-associated cells including angiogenic endothelial cells and macrophages. The expression of uPAR has been shown to be fairly high in tumor compared to normal, quiescent tissues, which has led to uPAR being proposed as a therapeutic target, as well as a targeting agent, for the treatment of cancer. The majority of therapeutic approaches that have been investigated to date have focused on inhibiting the urokinase plasminogen activator (uPA)-uPAR interaction but these have not led to the development of a viable uPAR targeted clinical candidate. Genetic knockdown approaches e.g. siRNA, shRNA focused on decreasing uPAR expression have demonstrated robust antitumor activity in pre-clinical studies but have been hampered by the obstacles of stability and drug delivery that have limited the field of RNA nucleic acid based therapeutics. More recently, novel approaches that target interactions of uPAR that are downstream of uPA binding e.g. with integrins or that exploit observations describing the biology of uPAR such as mediating uPA internalization and signaling have generated novel uPAR targeted candidates that are now advancing towards clinic evaluation. This review will discuss some of the pitfalls that have delayed progress on uPAR-targeted interventions and will summarize recent progress in the development of uPAR-targeted therapeutics.