Blood Clearance and Activity of Erythrocyte-Coupled Fibrinolytics

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Abstract
Conjugating tissue-type plasminogen activator (tPA) to red blood cells (RBCs) endows it with features useful for thromboprophylaxis. However, the optimal intensity and duration of thromboprophylaxis vary among clinical settings. To assess how the intrinsic properties of a plasminogen activator (PA) affect functions of the corresponding RBC/PA conjugate, we coupled equal amounts of tPA or Retavase (rPA; a variant with an extended circulation time, lower fibrin affinity, and greater susceptibility to PA inhibitors). Conjugation to RBC markedly prolonged the circulation of each PA in rats and mice, without detrimental effects on carrier RBC. The initial blood clearance of RBC/tPA was faster than RBC/rPA, yet it exerted greater fibrinolytic activity, in part due to greater resistance of tPA and RBC/tPA to plasma inhibitors versus rPA and RBC/rPA observed in vitro. Soluble and RBC-coupled tPA and rPA exerted the same amidolytic activity, yet RBC/tPA lysed fibrin clots more effectively than RBC/rPA, notwithstanding comparable fibrinolytic activity of their soluble counterparts. Conjugation to RBC suppressed rPA's ability to be activated by fibrin, whereas the fibrin activation of RBC-coupled tPA was not hindered. Therefore, the functional profile of RBC/PA is influenced by: pharmacokinetic features provided by carrier RBC (e.g., prolonged circulation), intrinsic PA features (e.g., clearance rate, resistance to inhibitors), and changes imposed by conjugation to RBC (e.g., loss of cofactor stimulation). These factors, different from those guiding the design of soluble PA for lysis of existing clots, can be exploited in the rational design of RBC/PA tailored for specific prophylactic indications.