Augmenting the protein C pathway with endothelial targeted biotherapeutics: Strategies to promote partnering of TM and EPCR

The design of targeted recombinant biotherapeutics is a rapidly growing area of translational biomedical research, with particular relevance to acute and life-threatening conditions, in which the available treatment options have narrow therapeutic indices. Although vascular immunotargeting typically has been thought of as a strategy for controlling and altering pharmacokinetics, in the context of biotherapeutic delivery, precise localization may be the primary goal, allowing optimal interaction of drug with endogenous partners. The protein C pathway has important protective roles in a variety of human illnesses, including sepsis and acute lung injury. We recently reported a strategy for augmenting this pathway by anchoring thrombomodulin (TM, CD141) to the endothelium via an affinity ligand to platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31). Endothelial PECAM-1, however, is believed to localize to a different portion of the cell membrane than the majority of endogenous TM and its key co-factor, the endothelial protein C receptor (EPCR, CD201). The current document includes new data indicating that recombinant TM anchored to endothelial PECAM-1 does not partner effectively with EPCR and describes the design, implementation, and validation of two strategies for more effectively replicating the enzymatic partnering of these two molecules. In both cases, proximity of these co-factors on the surface of the endothelial membrane appears to be the key variable and has significant implications, affecting not only functional activity in vitro but therapeutic efficacy in vivo. These findings underscore the complexity of targeting biotherapeutics to the plasmalemma, and suggest that precision on a nanometer scale is necessary for optimal biotherapeutic effect