Intracellular function in rehydrated lyophilized platelets

Selective targeting of bioengineered platelets to prostate cancer vasculature: new paradigm for therapeutic modalities

Androgen deprivation therapy (ADT) provides palliation for most patients with advanced prostate cancer (CaP); however, greater than 80% subsequently fail ADT. ADT has been indicated to induce an acute but transient destabilization of the prostate vasculature in animal models and humans. Human re-hydrated lyophilized platelets (hRL-P) were investigated as a prototype for therapeutic agents designed to target selectively the tumour-associated vasculature in CaP. The ability of hRL-P to bind the perturbed endothelial cells was tested using thrombin- and ADP-activated human umbilical vein endothelial cells (HUVEC), as well as primary xenografts of human prostate tissue undergoing acute vascular involution in response to ADT. hRL-P adhered to activated HUVEC in a dose-responsive manner. Systemically administered hRL-P, and hRL-P loaded with super-paramagnetic iron oxide (SPIO) nanoparticles, selectively targeted the ADT-damaged human microvasculature in primary xenografts of human prostate tissue. This study demonstrated that hRL-P pre-loaded with chemo-therapeutics or nanoparticles could provide a new paradigm for therapeutic modalities to prevent the rebound/increase in prostate vasculature after ADT, inhibiting the transition to castration-recurrent growth.

Use of fresh platelet concentrate or lyophilized platelets in thrombocytopenic dogs with clinical signs of hemorrhage: a preliminary trial in 37 dogs

OBJECTIVE

To examine the safety and feasibility of using lyophilized platelets (LYO) and fresh platelet concentrate (FRESH) in bleeding thrombocytopenic dogs.

DESIGN

Preliminary prospective randomized clinical trial.

SETTING

Two private referral centers and 3 university teaching hospitals.

ANIMALS

Thirty-seven dogs with a complaint of hemorrhage associated with thrombocytopenia (platelet count <70 × 10(9) /L [70,000/μL], a hematocrit >15%, and that had received neither vincristine nor platelet-containing transfusions within 72 h of enrollment were studied.

INTERVENTIONS

Animals were randomized to receive LYO or FRESH, dosed according to weight. Physical examination, complete blood counts, and coagulation testing (prothrombin time and activated partial thromboplastin time) were performed at enrollment. Physical examinations were also performed immediately post transfusion, and at 1 and 24 h after transfusion. Complete blood counts were repeated immediately post transfusion and at 24 h. Collected data included bleeding score (BLS), response to transfusion, adverse reactions, hospitalization time, need for additional transfusions, survival to discharge, and 28-d survival.

MEASUREMENTS AND MAIN RESULTS

Twenty-two dogs received LYO and 15 received FRESH. There was no difference between groups in age, weight, BLS, platelet count, white blood cell count, hematocrit, or presence of melena. There was no difference between groups in transfusion reaction rates, the need for additional transfusions, 24-h BLS, hospitalization time, survival to discharge, or 28-d survival.

CONCLUSIONS

Transfusion of LYO was feasible and associated with a low transfusion reaction rate in this limited study of thrombocytopenic canine patients presenting with mild-to-severe hemorrhage. LYO were easy to use and provided storage advantages over FRESH. Further study of this product, including examination of efficacy and platelet life span, is warranted.

Evaluation of lyophilized platelets as an infusible hemostatic agent in experimental non-compressible hemorrhage in swine

INTRODUCTION

Human lyophilized platelets hold promise as a novel hemostatic infusion agent for the control of traumatic hemorrhage. Rehydrated, lyophilized platelets (Stasix) were investigated as an infusible hemostatic agent in experimental non-compressible hemorrhage, using a porcine liver injury model.

METHODS

Yorkshire swine underwent a grade III liver injury and uncontrolled bleeding. After 15 min, animals were infused with Stasix (n = 10) or normal saline vehicle (n = 10). At 2 h, the liver was repaired, and the animals were monitored for another4 h. Resuscitation, including blood transfusion, was administered during the hospital phase. Laboratory data, including arterial blood gas, complete blood count, thromboelastography (TEG), and coagulation parameters, were collected. All animals underwent necropsy with complete histopathologic examination.

RESULTS

Overall survival in the Stasix group [8/10 (80%)] was significantly higher than in the control group [2/10 (20%)] (P = 0.023). Mean total blood loss index (g kg(-1)) was lower in Stasix-treated animals (22.2 +/- 3.5) than in control animals (34.7 +/- 3.4) (P = 0.019). Hemodynamic parameters were improved in the Stasix group, and a trend towards higher hemoglobin and lower lactate was observed. Coagulation and TEG parameters were not different between the groups. One surviving animal in the Stasix group had evidence of thrombi on necropsy.

CONCLUSIONS

This is the first reported study to evaluate rehydrated, lyophilized platelets as an infusible hemostatic agent for non-compressible hemorrhage. Stasix improved survival and reduced blood loss in a liver injury porcine model. However, evidence of thrombotic complications warrants further investigation prior to human use in the setting of traumatic hemorrhage.

The interaction of factor VIIa with rehydrated, lyophilized platelets

The experiments presented here were undertaken to determine if factor VIIa (rFVIIa, the Novo Nordisk product NovoSeven) will directly bind to rehydrated, lyophilized (RL) platelets for the formation of a catalytic surface with an enhanced ability to generate thrombin. The interaction between rFVIIa and the RL platelet surface was examined by measuring equilibrium and non-equilibrium binding of the coagulation factor to the cells and by following the effects of the surface modification on the kinetics of thrombin generation. The association of rFVIIa with RL platelets was rapid with saturation occurring within minutes. Disassociation was slow, with over half of the coagulation factor remaining bound after two hours. Densities of over one million molecules of rFVIIa per RL platelet were obtained when high concentrations of rFVIIa were incubated with RL platelets. Thrombin generation measurements showed that RL platelet-bound rFVIIa was catalytically active. Thus we can expect that RL platelets, which have been shown to effectively bind to sites of vascular injury, will localize rFVIIa to wounds for an increase in therapeutic index. These studies indicate that rFVIIa-RL platelets are worthy of preclinical and clinical development as an infusion agent for severe bleeding.

Correction of the Bleeding Time With Lyophilized Platelet Infusions in Dogs on Cardiopulmonary Bypass

Lyophilized canine platelets were infused in a single large bolus dose into splenectomized dogs after 2 hours' perfusion on cardiopulmonary bypass to test their possible efficacy in restoring hemostasis after compromise of platelet function. The vessel bleeding time (VBT) was monitored by venipuncture of the exposed jugular vein. During cardiopulmonary bypass, platelet counts fell quickly and the VBTs became prolonged over baseline. Infusion of lyophilized platelets reconstituted in normal saline occurred just before or immediately after weaning from the cardiopulmonary bypass pump. The results showed consistent and persistent lowering of the VBTs by the infused lyophilized platelets. Controls showed continuously prolonged VBTs. The weighted average VBT in infused subjects was significantly lower than the average in controls: 3 minutes 10 seconds versus 6 minutes 59 seconds, respectively ( t test, P = .01). These results in this setting indicate the possible effectiveness of similar human lyophilized platelet preparations in reducing postoperative bleeding in open heart surgery.

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.

A novel approach to improving recombinant factor VIIa activity with a preserved platelet preparation

Recombinant activated factor VII (NovoSeven, rFVIIa) is used to abrogate bleeding in haemophiliacs with inhibitors and is hypothesised to work by increasing activated factor X generation on the platelet surface. We hypothesised that rFVIIa activity could be increased by the co-addition of platelet procoagulant surface. This study characterised the ability of a rehydrated, lyophilised (RL) platelet preparation to increase rFVIIa activity in haemophilic conditions. RL platelets supported thrombin generation in the presence of factors VIII and IX but, in the absence of factors VIII and IX, thrombin generation was significantly reduced. RL platelets supported rFVIIa-mediated thrombin generation in a rFVIIa-concentration dependent manner. In a cell-based in vitro model of haemophilia, the presence of RL platelets increased the rFVIIa-dependent thrombin generation rate 2.8-fold compared with rFVIIa alone. Similarly, the addition of RL platelets plus rFVIIa to the in vitro model of haemophilia and to haemophilic platelet-rich plasma shortened the onset of clot formation and increased clot stability in a fibrinolytic environment versus rFVIIa alone. These results suggest that RL platelets can support rFVIIa-mediated thrombin generation, and that co-administration of RL platelets with rFVIIa may increase the efficacy of rFVIIa in some patients.

Primary and secondary hemostatic functionalities of rehydrated, lyophilized platelets

BACKGROUND

The rehydrated, lyophilized (RL) platelet (PLT) is being developed as a hemostatic infusion agent for the control of active bleeding. The key to the method for preparing RL PLTs is a mild aldehyde stabilization that allows for freezing and lyophilizing without cellular rupture. RL PLTs have been shown to be effective at rapidly controlling bleeding in animal models of cardiopulmonary bypass induced PLT dysfunction and washout thrombocytopenia, yet the rehydrated cells have proved to be safe with respect to induction of pathologic intravascular coagulation.

STUDY DESIGN AND METHODS

In vitro and in vivo studies were performed to better understand the differential effect of the RL PLT manufacturing method on primary and secondary hemostatic processes. The functionality of the von Willebrand factor (VWF) receptor (glycoprotein Ib) complex, the PAR receptors, integrin-mediated aggregation (inside-out signaling), and surface membrane prothrombin to thrombin conversion systems were investigated.

RESULTS

RL PLTs were found to retain native VWF-mediated adhesion and surface thrombin generation functions. In contrast, the coupling of thrombin receptors to integrin inside-out signaling was largely inhibited.

CONCLUSION

These results suggest that RL PLTs may stop bleeding by forming primary hemostatic plugs and providing a localized source of thrombin for secondary hemostatic processes, yet do not build up occlusive pathologic clots possibly because integrin functions for forming PLT-PLT aggregates are partially inhibited.

Paraformaldehyde induces elevation of intracellular calcium and phosphatidylserine externalization in platelets

Paraformaldehyde fixation of platelets stabilizes surface antigens while altering some that are associated with cellular activation. Present experiments show that the asymmetric distribution of phosphatidylserine in platelets was especially sensitive to paraformaldehyde treatment. It was found that this reagent induced a dose- and time-dependent translocation of phosphatidylserine to the membrane surface as measured by annexin V binding and flow cytometry. The percent phosphatidylserine-positive cells increased from about 5% to >90%. Chelation of extracellular Ca(2+) with EGTA partially blocked this translocation. Spectrofluorimetric analysis of fluo-3 loaded platelets indicates that paraformaldehyde caused a concomitant elevation of intracellular Ca(2+) concentrations, [Ca(2+)](i). ATP levels also declined in paraformaldehyde-treated cells, suggesting that the rise in [Ca(2+)](i) ensued in part from decreased activity of calcium pumps. Previous studies indicate that phosphatidylserine externalization arises from Ca(2+)-activated randomization of membrane phospholipids and decreased transport of phosphatidylserine from the outer to the inner leaflet of the plasma membrane. In light of present results, paraformaldehyde fixation is best avoided particularly in studies involving platelet apoptosis or activation.

Comparison of structural and hemostatic properties of the poly-N-acetyl glucosamine Syvek Patch with products containing chitosan

Polysaccharides are becoming increasingly developed as therapeutics and medical products, as the new field of Glycomics expands. Glycosaminoglycans that contain N-acetyl glucosamine constituents have been the focus of research leading to medical devices. A new hemostatic bandage, the Syvek Patch, has been introduced in the recent past for the control of bleeding at vascular access sites in interventional cardiology and radiology procedures. This product consists of poly-N-acetyl glucosamine (pGlcNAc) isolated in a unique fiber crystalline structural form from the large-scale culture and processing of a marine diatom. The Syvek pGlcNAc fiber material has chemical, physical, and biological properties that result in its favorable performance as a hemostat. Two new products, the Clo-Sur PAD and ChitoSeal, have recently become available also as patch hemostats. These two products both use chitosan, another N-acetyl glucosamine containing glycosaminoglycan, as their active ingredient. Structural, chemical, and biological comparisons of Syvek pGlcNAc and chitosan reveal a number of important differences. Syvek pGlcNAc fibers contain approximately 50 fully acetylated, high molecular weight pGlcNAc molecules in a crystalline, three-dimensional beta structure array, and are insoluble. Chitosan is a low molecular weight mixed amorphous cationic polymer with no regular structure as a solid, and is water-soluble taking on a random coil configuration when in solution. These structural dissimilarities result in differences in the hemostatic properties of the two materials. Syvek pGlcNAc is able to significantly reduce the in vitro fibrin clot formation time of platelet-rich plasma samples and has the ability to cause aggregation of red blood cells in vitro. Chitosan is no better than gauze or other controls in these in vitro assays. The Syvek Patch is able to control the bleeding and cause hemostasis in a coagulopathic swine spleen-bleeding animal model 100% of the time, whereas Clo-Sur PAD was completely unsuccessful (0%) and ChitoSeal (25%) was worse than a gauze pad control (50%) in the same model. Syvek pGlcNAc fibers have structural and chemical properties that provide a unique basis for their ability to interact with blood components to cause hemostasis. Chitosan does not have the same properties and capabilities.

Thrombus formation with rehydrated, lyophilized platelets

Stored human platelets are frequently used in hemorrhagic emergencies, but have limited immediate utility for controlling bleeding due to storage lesion and are frequently contaminated with microorganisms. The development of paraformaldehyde-treated, lyophilized and rehydrated (RL) platelets, which are sterile and have a prolonged shelf life (years), ameliorate the efficacy and sterility problems with stored platelets. RL platelets have been shown to have many native functions of fresh platelets in vitro and to mediate hemostasis in vivo in large animal models of hemorrhagic shock and cardiopulmonary bypass induced platelet dysfunction. To further evaluate the functional properties of this transfusion product, we studied the role of RL platelets in three aspects of thrombus formation and lysis. First, the interaction between RL platelets and fibrinogen was investigated. The surface density of unligated GPIIb-IIIa on RL and fresh platelets were, respectively 30000 and 70000 molecules per cell as detected with the monoclonal antibody 10E-5. Freezing, lyophilization and rehydration steps in the preparation of RL platelets resulted in the surface presentation of 120000 molecules of fibrinogen per cell from alpha granule sources. After ADP activation, RL platelets bound exogenous 125I-labeled fibrinogen in a dose-dependent manner with an affinity that is similar to that of fresh platelets and was inhibited by RGD peptides. 125I-Labeled fibrinogen binding to RL and fresh platelets, respectively, saturated at 14000 and 32000 molecules per cell. Scanning electron microscopic ultrastructural analysis showed that fibrin strands interacted with the surface of RL platelets in a normal manner. The second set of studies investigated the ability of RL platelets to catalyze and amplify the clot formation process in an activation-dependent manner. We showed that RL platelets undergo degranulation in fibrin in clots and functioned as thrombogenic surfaces for the generation of activated coagulation factors and fibrin generation. A final set of studies was performed to investigate fibrin of clots that contained RL platelets. RL platelet clots were lysed in the presence of tissue plasminogen activator with a similar time course as clots without platelets, and lysis occurred faster than when fresh platelets were included in the fibrin mass. The results of these three studies demonstrate that RL platelets are capable of mediating thrombus formation and do not inhibit lysis. Our results help explain how RL platelets restore hemostasis in vivo, and indicate that these cells might be a viable alternative to fresh stored platelets in transfusion medicine.

Splenic clearance mechanisms of rehydrated, lyophilized platelets

A variety of platelet substitutes (e.g., rehydrated, lyophilized (RL) platelets, thromboerythrocytes, plateletsomes, infusible platelet membranes, synthocytes, fibrinogen-coated microcapules) are potentially useful as hemostatic agents in transfusion medicine. However, as "foreign" particles, platelet substitutes interact to varying extents with elements of the reticulo-endothelial system for clearance, reducing hemostatic efficacy. Experiments were performed to better understand the interaction of RL platelets with elements of the innate and acquired immune systems. The infusion of heterologous RL platelets into rats resulted in rapid clearance from the free circulation with half-life values of minutes. The clearance of RL platelets was inhibited when macrophages were rendered apoptotic with gadolinium. Transmission EM analysis of splenic tissue after infusion of lyophilized cells, as well as in vitro mixing studies with splenic macrophages and RL platelets, indicated that macrophage-mediated phagocytosis mechanisms were operant in RL platelet clearance by the reticulo-endothelial system. Studies with IV IgG, as a competitive inhibitor of the macrophage Fc receptor, provides evidence that RL platelet destruction is in part mediated by platelet surface bound IgG. This hypothesis was further supported by the finding that RL platelets react with IgG class antibodies that are pre-existing in naïve animals. These studies provide a rational basis for prolonging the circulation time of RL platelets and other platelet substitutes.

Revisitation of the clinical indications for the transfusion of platelet concentrates

Platelet transfusion is indicated when the expected benefits of increasing the number of functional platelets in the patient's circulation outweigh the potential risks generated by exposing the patient to allogeneic, manipulated and stored blood products such as platelet concentrates. Although reassuring evidence has been collected indicating that current risks associated with blood transfusion are lower than those of several voluntary and involuntary human activities, balancing benefits and risks of platelet transfusion may not be easy in a proportion of patients and in a number of conditions. To facilitate this task, guidelines have been developed, with particular attention to cancer patients. As witnessed by the most recent guidelines, over the last few years there has been a progressive, although not absolute, consensus on: (i) the routine use of platelets as a tool to prevent hemorrhage in oncohematology (the so called 'prophylactic approach') as opposed to limiting platelet transfusion to actual bleeding episodes (the so-called 'therapeutic approach') and (ii) lowering the trigger for prophylactic platelet transfusion in stable oncohematology recipients from 20 x 109 to 10 x 109 platelets/L. This has been accompanied by a reduction of platelet use per oncohematology patient of about 20%, an important outcome in view of the progressive increase of platelet demand due to more aggressive therapy in cancer patients. In selected clinical conditions, specific triggers ranging from 30 x 10(9) to 100 x 10(9) platelets/L have been recommended, with higher values when surgical procedures are required for the patient's treatment. Indications and trigger values proposed in the guidelines must be considered within the context of careful clinical evaluation of each patient, with a clear appreciation of the power of discrimination of automated platelet counters at low counts, and of the quality and local availability of platelet products for emergency.