Interference with the urokinase plasminogen activator system: a promising therapy concept for solid tumours

There is abundant evidence that the plasminogen activator (PA) system with its key components uPA (urokinase-type plasminogen activator), its cell surface receptor uPA-R (CD87) and its inhibitor PAI-1 plays a key role in tumour invasion and metastasis. Elevated levels of these factors in tumour tissue are associated with tumour aggressiveness and poor patient outcome. Animal models suggest that the PA system is not essential for fertility or survival under physiological conditions. Thus, it seems well suited as a therapeutic target for patients with solid malignant tumours. Novel therapy concepts targeting the uPA system are currently being explored. A variety of different synthetic uPA inhibitor classes have been developed over the last decades. First generation inhibitors displayed a low uPA inhibitory potency combined with broad specificity. More recently, structure based design, x-ray crystallographic screening or NMR based screening have revealed a large number of new, potent and selective uPA-inhibitors. A few modern compounds have shown promising results in preclinical testing and are now ready for Phase I clinical studies. Other therapeutic strategies such as antagonists of uPA/uPA-R interaction or gene therapeutic approaches to suppress the uPA-system are still being evaluated in in vitro and in vivo models. For clinical application, a combination therapy targeting more than one of the interacting proteolytic pathways may be required for effective antiproteolytic therapy. In addition, antiproteolytic agents may provide additive or synergistic treatment benefits if used in combination together with conventional therapeutics, in particular in those solid tumours for which potent conventional regimens already exist.