Targeting biased signaling by PAR1: Function and molecular mechanism of parmodulins

The G protein-coupled receptor (GPCR) protease activated receptor 1 (PAR1) is a therapeutic target that was originally pursued with the aim of restricting platelet activation and the burden of cardiovascular disease. In clinical studies the use of orthosteric PAR1 inhibitors was associated with an increased risk of hemorrhage, including intracranial hemorrhage. As (i) PAR1 is expressed by various cell types, including endothelial cells, (ii) conveys in mice a physiological indispensable function for vascular development during embryogenesis, and (iii) is subject to biased signaling dependent on the activating proteases, orthosteric PAR1 inhibition may be associated with unwanted side effects. Alternatively, the protease activated protein C (aPC) and its variants can promote valuable anti-inflammatory signaling via PAR1. Most recently, small molecule biased allosteric modulators of PAR1 signaling, so called parmodulins, have been developed. Parmodulins inhibit coagulation and platelet activation, yet maintain cytoprotective effects typically provoked by PAR1 signaling upon activation by aPC. Here we review the discovery of parmodulins and their preclinical data, summarize the current knowledge about their mode of action, and compare the structural interaction of parmodulin - PAR1 with that of other intracellularly binding allosteric GPCR modulators. Thus, we highlight the pharmaceutical potential and challenges associated with the future development of parmodulins.