Lyophilized platelets: fifty years in the making

Starting with the work of Klein et al. in the early 1950s, there has been a concerted effort to apply the process of freeze-drying for the preservation of platelets in order to provide hemorrhagic patients with a stable infusible hemostatic agent to stop bleeding. The original attempts did not preserve platelet structural integrity and proved to be of little clinical benefit. However, it was known that fixation by various cross-linking agents rendered platelets able to withstand structurally intact the stresses of lyophilization but with (assumed) complete loss of functionality. Read and coworkers showed that fixed and freeze-dried platelets could respond to ristocetin-induced agglutination, and thus devised a widely accepted assay for von Willebrands factor that demonstrated that reconstituted platelets participated well in this in vitro model of an important interaction in primary hemostasis. This review chronicles the efforts of the authors to refine the fixation process so that the freeze-dried and reconstituted platelets retain fundamental hemostatic properties necessary to stop bleeding. The resultant product has demonstrated correction or reduction of the bleeding times in animal models with platelet deficits including the thrombocytopenic rabbit model of Blajchman and coworkers, a canine cardiopulmonary bypass model of open-heart surgery at East Carolina University (ECU), and a porcine trauma model at The University of North Carolina at Chapel Hill (UNC-CH) involving exsanguination and complete blood exchange with a hemoglobin-based oxygen carrier (HBOC). In addition, it has been shown that the fixation process kills viruses and bacteria spiked into the platelet suspension, indicating that the final material may indeed be the first truly sterile cellular transfusion product. The initial goal for clinical benefit is to prevent exsanguination and hypovolemic shock in combat casualties of armed services personnel, for whom platelet transfusions are most often unavailable. Commercial interests are being brought to bear by Entegrion Inc. (formerly known as Hemocellular Therapeutics Corporation) to transfer this technology to a scaleable manufacturing platform for the production of Stasix, a pharmaceutical preparation of fixed and freeze-dried platelets for intravenous or topical use in the arrest of active hemorrhage in a wide variety of patients with a platelet-related bleeding diathesis. It has taken fifty+ years from the first attempt at making a clinically useful freeze-dried platelet preparation to get to the rapidly-approaching clinical trials of Stasix; stabilization of the platelets has been the key to realizing this advance.