A liposome based platelet substitute, the plateletsome, with hemostatic efficacy

Advancement and Applications of Platelet-inspired Nanoparticles: A Paradigm for Cancer Targeting

Platelet-inspired nanoparticles have ignited the possibility of new opportunities for producing similar biological particulates, such as structural cellular and vesicular components, as well as various viral forms, to improve biocompatible features that could improve the nature of biocompatible elements and enhance therapeutic efficacy. The simplicity and more effortless adaptability of such biomimetic techniques uplift the delivery of the carriers laden with cellular structures, which has created varied opportunities and scope of merits like; prolongation in circulation and alleviating immunogenicity improvement of the site-specific active targeting. Platelet-inspired nanoparticles or medicines are the most recent nanotechnology-based drug targeting systems used mainly to treat blood-related disorders, tumors, and cancer. The present review encompasses the current approach of platelet-inspired nanoparticles or medicines that have boosted the scientific community from versatile fields to advance biomedical sciences. Surprisingly, this knowledge has streamlined to development of newer diagnostic methods, imaging techniques, and novel nanocarriers, which might further help in the treatment protocol of the various diseased conditions. The review primarily focuses on the novel advancements and recent patents in nanoscience and nanomedicine that could be streamlined in the future for the management of progressive cancers and tumor targeting. Rigorous technological advancements like biomimetic stem cells, pH-sensitive drug delivery of nanoparticles, DNA origami devices, virosomes, nano cells like exosomes mimicking nanovesicles, DNA nanorobots, microbots, etc., can be implemented effectively for target-specific drug delivery.

Intravenous Hemostats: Foundation, Targeting, and Controlled-Release

Uncontrollable blood loss is the greatest cause of mortality in prehospital patients and the main source of disability and death in hospital care. Compared with external hemostats, intravenous hemostats are more appropriate for preventing and treating uncontrolled bleeding in vivo and large bleeding on the body surface. This Review initially establishes intravenous hemostats' response basis, including the coagulation mechanism, fibrinolytic pathway, and protein corona. Second, the study of advancement of intravenous hemostat targeting was expanded from two perspectives, cellular hemostatic agents and synthetic hemostatic agents. Meanwhile, after discussing the progress of controlled-release intravenous hemostats with platelets as the stimuli, this Review offers insight into the possibility of controlled-release intravenous hemostats with microenvironment as the stimuli, combining the studies of controlled-release targeted thrombolysis.

Platelet-inspired nanomedicine in hemostasis thrombosis and thromboinflammation

Platelets are anucleate cell-fragments derived predominantly from megakaryocytes in the bone marrow and released in the blood circulation, with a normal count of 150 000-40 000 per μl and a lifespan of approximately 10 days in humans. A primary role of platelets is to aid in vascular injury site-specific clot formation to stanch bleeding, termed hemostasis. Platelets render hemostasis by a complex concert of mechanisms involving platelet adhesion, activation and aggregation, coagulation amplification, and clot retraction. Additionally, platelet secretome can influence coagulation kinetics and clot morphology. Therefore, platelet defects and dysfunctions result in bleeding complications. Current treatment for such complications involve prophylactic or emergency transfusion of platelets. However, platelet transfusion logistics constantly suffer from limited donor availability, challenges in portability and storage, high bacterial contamination risks, and very short shelf life (~5 days). To address these issues, an exciting area of research is focusing on the development of microparticle- and nanoparticle-based platelet surrogate technologies that can mimic various hemostatic mechanisms of platelets. On the other hand, aberrant occurrence of the platelet mechanisms lead to the pathological manifestation of thrombosis and thromboinflammation. The treatments for this are focused on inhibiting the mechanisms or resolving the formed clots. Here, platelet-inspired technologies can provide unique platforms for disease-targeted drug delivery to achieve high therapeutic efficacy while avoiding systemic side-effects. This review will provide brief mechanistic insight into the role of platelets in hemostasis, thrombosis and thromboinflammation, and present the current state-of-art in the design of platelet-inspired nanomedicine for applications in these areas.

Bioinspired artificial platelets: past, present and future

Platelets are anucleate blood cells produced from megakaryocytes predominantly in the bone marrow and released into blood circulation at a healthy count of 150,000-400,00 per μL and circulation lifespan of 7-9 days. Platelets are the first responders at the site of vascular injury and bleeding, and participate in clot formation via injury site-specific primary mechanisms of adhesion, activation and aggregation to form a platelet plug, as well as secondary mechanisms of augmenting coagulation via thrombin amplification and fibrin generation. Platelets also secrete various granule contents that enhance these mechanisms for clot growth and stability. The resultant clot seals the injury site to stanch bleeding, a process termed as hemostasis. Due to this critical role, a reduction in platelet count or dysregulation in platelet function is associated with bleeding risks and hemorrhagic complications. These scenarios are often treated by prophylactic or emergency transfusion of platelets. However, platelet transfusions face significant challenges due to limited donor availability, difficult portability and storage, high bacterial contamination risks, and very short shelf life (~5-7 days). These are currently being addressed by a robust volume of research involving reduced temperature storage and pathogen reduction processes on donor platelets to improve shelf-life and reduce contamination, as well as bioreactor-based approaches to generate donor-independent platelets from stem cells in vitro. In parallel, a complementary research field has emerged that involves the design of artificial platelets utilizing biosynthetic particle constructs that functionally emulate various hemostatic mechanisms of platelets. Here, we provide a comprehensive review of the history and the current state-of-the-art artificial platelet approaches, along with discussing the translational opportunities and challenges.

Platelet Transfusion-Insights from Current Practice to Future Development

Since the late sixties, therapeutic or prophylactic platelet transfusion has been used to relieve hemorrhagic complications of patients with, e.g., thrombocytopenia, platelet dysfunction, and injuries, and is an essential part of the supportive care in high dose chemotherapy. Current and upcoming advances will significantly affect present standards. We focus on specific issues, including the comparison of buffy-coat (BPC) and apheresis platelet concentrates (APC); plasma additive solutions (PAS); further measures for improvement of platelet storage quality; pathogen inactivation; and cold storage of platelets. The objective of this article is to give insights from current practice to future development on platelet transfusion, focusing on these selected issues, which have a potentially major impact on forthcoming guidelines.

Fibrinogen-Coated Albumin Nanospheres Prevent Thrombocytopenia-Related Bleeding

Thrombocytopenia (TCP) may cause severe and life-threatening bleeding. While this may be prevented by platelet transfusions, transfusions are associated with potential complications, do not always work (platelet refractory) and are not always available. There is an urgent need for a synthetic alternative. We evaluated the ability of fibrinogen-coated nanospheres (FCNs) to prevent TCP-related bleeding. FCNs are made of human albumin polymerized into a 100-nm sphere and coated with fibrinogen. We hypothesized that FCNs would bind to platelets through fibrinogen-GPIIb/IIIa interactions, contributing to hemostasis in the setting of TCP. We used two murine models to test these effects: in the first model, BALB/c mice received 7.25 Gy total-body irradiation (TBI); in the second model, lower dose TBI (7.0 Gy) was combined with an anti-platelet antibody (anti-CD41) to induce severe TCP. Deaths in both models were due to gastrointestinal or intracranial bleeding. Addition of antiplatelet antibody to 7.0 Gy TBI significantly worsened TCP and increased mortality compared to 7.0 Gy TBI alone. FCNs significantly improved survival compared to saline control in both models, suggesting it ameliorated TCP-related bleeding. Additionally, in a saphenous vein bleeding model of antibody-induced TCP, FCNs shortened bleeding times. There were no clinical or histological findings of thrombosis or laboratory findings of disseminated intravascular coagulation after FCN treatment. In support of safety, fluorescence microscopy suggests that FCNs bind to platelets only upon platelet activation with collagen, limiting activity to areas of endothelial damage. To our knowledge, this is the first biosynthetic agent to demonstrate a survival advantage in TCP-related bleeding.

Bioinspired artificial platelets for transfusion applications in traumatic hemorrhage

Among blood components, platelets (PLTs) present the toughest logistic challenges in transfusion due to limited availability, difficult portability and storage, high contamination risks, and very short shelf life (approx. 5 days). Robust research efforts are being directed to develop biologic PLTs in vitro as well as design biosynthetic and artificial PLT technologies that can potentially resolve these challenges to allow adequate availability and timely transfusion to improve survival in trauma.

Recent Advances in Hemostasis at the Nanoscale

Rapid and effective hemostatic materials have received wide attention not only in the battlefield but also in hospitals and clinics. Traditional hemostasis relies on materials with little designability which has many limitations. Nanohemostasis has been proposed since the use of peptides in hemostasis. Nanomaterials exhibit excellent adhesion, versatility, and designability compared to traditional materials, laying a good foundation for future hemostatic materials. This review first summarizes current hemostatic methods and materials, and then introduces several cutting-edge designs and applications of nanohemostatic materials such as polypeptide assembly, electrospinning of cyanoacrylate, and nanochitosan. Particularly, their advantages and working mechanisms are introduced. Finally, the challenges and prospects of nanohemostasis are discussed.

Novel platelet products under development for the treatment of thrombocytopenia or acute hemorrhage

Controlling hemorrhage has been a focus of survival since man recognized that the loss of blood led to death. Papyri from 1600 BCE describe methods for hemorrhage control including; direct pressure, ligature and the use of sutures. Multiple studies have demonstrated the survival advantage of early transfusion of whole blood or red cells and plasma. The added survival impact of early transfusion of platelets was recently reported in a substudy of the prospective Pragmatic, Randomized Optimal Platelet and Plasma Ratios (PROPPR) trial. Early transfusion of platelets demonstrated a statistically significant survival benefit at 24 h and 30 days post-injury. [1] Platelet availability is limited due to the short shelf life (5-7 days) and storage requirements (room temperature with constant agitation). Providing platelets or platelet derived products for prehospital treatment and to rural and some urban hospitals is an unmet medical need. The interest in novel and alternative platelet products has grown over the past decade and the status of novel platelet products is presented herein. Development, approval, and distribution of hemostatically effective approved platelet products for prehospital use and routine stockage in rural and urban centers could significantly increase survival rates in bleeding patients.

Biomimetic microgels with controllable deformability improve healing outcomes

Platelets mediate hemostasis by aggregating and binding to fibrin to promote clotting. Over time, platelets contract the fibrin network to induce clot retraction, which contributes to wound healing outcomes by increasing clot stability and improving blood flow to ischemic tissue. In this study, we describe the development of hollow platelet-like particles (PLPs) that mimic the native platelet function of clot retraction in a controlled manner and demonstrate that clot retraction-inducing PLPs promote healing in vivo. PLPs are created by coupling fibrin-binding antibodies to CoreShell (CS) or hollow N-isopropylacrylamide (NIPAm) microgels with varying degrees of shell crosslinking. We demonstrate that hollow microgels with loosely crosslinked shells display a high degree of deformability and mimic activated platelet morphology, while intact CS microgels and hollow microgels with increased crosslinking in the shell do not. When coupled to a fibrin-binding antibody to create PLPs, hollow particles with low degrees of shell crosslinking cause fibrin clot collapse in vitro, recapitulating the clot retraction function of platelets, while other particle types do not. Furthermore, hollow PLPs with low degrees of shell crosslinking improve some wound healing outcomes in vivo.

Liposome-chaperoned cell-free synthesis for the design of proteoliposomes: Implications for therapeutic delivery

Cell-free (CF) protein synthesis has emerged as a powerful technique platform for efficient protein production in vitro. Liposomes have been widely studied as therapeutic carriers due to their biocompatibility, biodegradability, low toxicity, flexible surface manipulation, easy preparation, and higher cargo encapsulation capability. However, rapid immune clearance, insufficient targeting capacity, and poor cytoplasmic delivery efficiency substantially restrict their clinical application. The incorporation of functional membrane proteins (MPs) or peptides allows the transfer of biological properties to liposomes and imparts them with improved circulation, increased targeting, and efficient intracellular delivery. Liposome-chaperoned CF synthesis enables production of proteoliposomes in one-step reaction, which not only substantially simplifies the production procedure but also keeps protein functionality intact. Building off these observations, proteoliposomes with integrated MPs represent an excellent candidate for therapeutic delivery. In this review, we describe recent advances in CF synthesis with emphasis on detailing key factors for improving CF expression efficiency. Furthermore, we provide insights into strategies for rational design of proteoliposomal nanodelivery systems via CF synthesis.

STATEMENT OF SIGNIFICANCE

Liposome-chaperoned CF synthesis has emerged as a powerful approach for the design of recombinant proteoliposomes in one-step reaction. The incorporation of bioactive MPs or peptides into liposomes via CF synthesis can facilitate the development of proteoliposomal nanodelivery systems with improved circulation, increased targeting, and enhanced cellular delivery capacity. Moreover, by adapting lessons learned from natural delivery vehicles, novel bio-inspired proteoliposomes with enhanced delivery properties could be produced in CF systems. In this review, we first give an overview of CF synthesis with focus on enhancing protein expression in liposome-chaperoned CF systems. Furthermore, we intend to provide insight into harnessing CF-synthesized proteoliposomes for efficient therapeutic delivery.

Recent Advances of Membrane-Cloaked Nanoplatforms for Biomedical Applications

In terms of the extremely small size and large specific surface area, nanomaterials often exhibit unusual physical and chemical properties, which have recently attracted considerable attention in bionanotechnology and nanomedicine. Currently, the extensive usage of nanotechnology in medicine holds great potential for precise diagnosis and effective therapeutics of various human diseases in clinical practice. However, a detailed understanding regarding how nanomedicine interacts with the intricate environment in complex living systems remains a pressing and challenging goal. Inspired by the diversified membrane structures and functions of natural prototypes, research activities on biomimetic and bioinspired membranes, especially for those cloaking nanosized platforms, have increased exponentially. By taking advantage of the flexible synthesis and multiple functionality of nanomaterials, a variety of unique nanostructures including inorganic nanocrystals and organic polymers have been widely devised to substantially integrate with intrinsic biomoieties such as lipids, glycans, and even cell and bacteria membrane components, which endow these abiotic nanomaterials with specific biological functionalities for the purpose of detailed investigation of the complicated interactions and activities of nanomedicine in living bodies, including their immune response activation, phagocytosis escape, and subsequent clearance from vascular system. In this review, we summarize the strategies established recently for the development of biomimetic membrane-cloaked nanoplatforms derived from inherent host cells (e.g., erythrocytes, leukocytes, platelets, and exosomes) and invasive pathogens (e.g., bacteria and viruses), mainly attributed to their versatile membrane properties in biological fluids. Meanwhile, the promising biomedical applications based on nanoplatforms inspired by diverse moieties, such as selective drug delivery in targeted sites and effective vaccine development for disease prevention, have also been outlined. Finally, the potential challenges and future prospects of the biomimetic membrane-cloaked nanoplatforms are also discussed.

Biomaterials and Advanced Technologies for Hemostatic Management of Bleeding

Bleeding complications arising from trauma, surgery, and as congenital, disease-associated, or drug-induced blood disorders can cause significant morbidities and mortalities in civilian and military populations. Therefore, stoppage of bleeding (hemostasis) is of paramount clinical significance in prophylactic, surgical, and emergency scenarios. For externally accessible injuries, a variety of natural and synthetic biomaterials have undergone robust research, leading to hemostatic technologies including glues, bandages, tamponades, tourniquets, dressings, and procoagulant powders. In contrast, treatment of internal noncompressible hemorrhage still heavily depends on transfusion of whole blood or blood's hemostatic components (platelets, fibrinogen, and coagulation factors). Transfusion of platelets poses significant challenges of limited availability, high cost, contamination risks, short shelf-life, low portability, performance variability, and immunological side effects, while use of fibrinogen or coagulation factors provides only partial mechanisms for hemostasis. With such considerations, significant interdisciplinary research endeavors have been focused on developing materials and technologies that can be manufactured conveniently, sterilized to minimize contamination and enhance shelf-life, and administered intravenously to mimic, leverage, and amplify physiological hemostatic mechanisms. Here, a comprehensive review regarding the various topical, intracavitary, and intravenous hemostatic technologies in terms of materials, mechanisms, and state-of-art is provided, and challenges and opportunities to help advancement of the field are discussed.

Bio-inspired nanomedicine strategies for artificial blood components

Blood is a fluid connective tissue where living cells are suspended in noncellular liquid matrix. The cellular components of blood render gas exchange (RBCs), immune surveillance (WBCs) and hemostatic responses (platelets), and the noncellular components (salts, proteins, etc.) provide nutrition to various tissues in the body. Dysfunction and deficiencies in these blood components can lead to significant tissue morbidity and mortality. Consequently, transfusion of whole blood or its components is a clinical mainstay in the management of trauma, surgery, myelosuppression, and congenital blood disorders. However, donor-derived blood products suffer from issues of shortage in supply, need for type matching, high risks of pathogenic contamination, limited portability and shelf-life, and a variety of side-effects. While robust research is being directed to resolve these issues, a parallel clinical interest has developed toward bioengineering of synthetic blood substitutes that can provide blood's functions while circumventing the above problems. Nanotechnology has provided exciting approaches to achieve this, using materials engineering strategies to create synthetic and semi-synthetic RBC substitutes for enabling oxygen transport, platelet substitutes for enabling hemostasis, and WBC substitutes for enabling cell-specific immune response. Some of these approaches have further extended the application of blood cell-inspired synthetic and semi-synthetic constructs for targeted drug delivery and nanomedicine. The current study provides a comprehensive review of the various nanotechnology approaches to design synthetic blood cells, along with a critical discussion of successes and challenges of the current state-of-art in this field. WIREs Nanomed Nanobiotechnol 2017, 9:e1464. doi: 10.1002/wnan.1464 For further resources related to this article, please visit the WIREs website.

Nanodiagnostics, nanopharmacology and nanotoxicology of platelet–vessel wall interactions

In physiological conditions, the interactions between blood platelets and endothelial cells play a major role in vascular reactivity and hemostasis. By contrast, increased platelet activation contributes to the pathogenesis of vascular pathology such as atherosclerosis, thrombosis, diabetes mellitus, hypertension and carcinogenesis. Nanomedicine, including nanodiagnostics and nanotherapeutics is poised to be used in the management of vascular diseases. However, the inherent risk and potential toxicity resultant from the use of nanosized (<100 nm) materials need to be carefully considered. This review, basing on a systematic search of literature provides state-of-the-art and focuses on new discoveries, as well as the potential benefits and threats in the field of nanodiagnostics, nanopharmacology and nanotoxicology of platelet–vessel wall interactions.

Synthetic Strategies for Engineering Intravenous Hemostats

While there are currently many well-established topical hemostatic agents for field administration, there are still limited tools to staunch bleeding at less accessible injury sites. Current clinical methods to restore hemostasis after large volume blood loss include platelet and clotting factor transfusion, which have respective drawbacks of short shelf life and risk of viral transmission. Therefore, synthetic hemostatic agents that can be delivered intravenously and encourage stable clot formation after localizing to sites of vascular injury are particularly appealing. In the past three decades, platelet substitutes have been prepared using drug delivery vehicles such as liposomes and PLGA nanoparticles that have been modified to mimic platelet properties. Additionally, structural considerations such as particle size, shape, and flexibility have been addressed in a number of reports. Since platelets are the first responders after vascular injury, platelet substitutes represent an important class of intravenous hemostats under development. More recently, materials affecting fibrin formation have been introduced to induce faster or more stable blood clot formation through fibrin cross-linking. Fibrin represents a major structural component in the final blood clot, and a fibrin-based hemostatic mechanism acting downstream of initial platelet plug formation may be a safer alternative to platelets to avoid undesired thrombotic activity. This Review explores intravenous hemostats under development and strategies to optimize their clotting activity.

Platelet-like nanoparticles: mimicking shape, flexibility, and surface biology of platelets to target vascular injuries

Targeted delivery of therapeutic and imaging agents in the vascular compartment represents a significant hurdle in using nanomedicine for treating hemorrhage, thrombosis, and atherosclerosis. While several types of nanoparticles have been developed to meet this goal, their utility is limited by poor circulation, limited margination, and minimal targeting. Platelets have an innate ability to marginate to the vascular wall and specifically interact with vascular injury sites. These platelet functions are mediated by their shape, flexibility, and complex surface interactions. Inspired by this, we report the design and evaluation of nanoparticles that exhibit platelet-like functions including vascular injury site-directed margination, site-specific adhesion, and amplification of injury site-specific aggregation. Our nanoparticles mimic four key attributes of platelets, (i) discoidal morphology, (ii) mechanical flexibility, (iii) biophysically and biochemically mediated aggregation, and (iv) heteromultivalent presentation of ligands that mediate adhesion to both von Willebrand Factor and collagen, as well as specific clustering to activated platelets. Platelet-like nanoparticles (PLNs) exhibit enhanced surface-binding compared to spherical and rigid discoidal counterparts and site-selective adhesive and platelet-aggregatory properties under physiological flow conditions in vitro. In vivo studies in a mouse model demonstrated that PLNs accumulate at the wound site and induce ∼65% reduction in bleeding time, effectively mimicking and improving the hemostatic functions of natural platelets. We show that both the biochemical and biophysical design parameters of PLNs are essential in mimicking platelets and their hemostatic functions. PLNs offer a nanoscale technology that integrates platelet-mimetic biophysical and biochemical properties for potential applications in injectable synthetic hemostats and vascularly targeted payload delivery.

Hemostatic strategies for traumatic and surgical bleeding

Wide interest in new hemostatic approaches has stemmed from unmet needs in the hospital and on the battlefield. Many current commercial hemostatic agents fail to fulfill the design requirements of safety, efficacy, cost, and storage. Academic focus has led to the improvement of existing strategies as well as new developments. This review will identify and discuss the three major classes of hemostatic approaches: biologically derived materials, synthetically derived materials, and intravenously administered hemostatic agents. The general class is first discussed, then specific approaches discussed in detail, including the hemostatic mechanisms and the advancement of the method. As hemostatic strategies evolve and synthetic-biologic interactions are more fully understood, current clinical methodologies will be replaced.

Pharmacokinetic study of adenosine diphosphate-encapsulated liposomes coated with fibrinogen γ-chain dodecapeptide as a synthetic platelet substitute in an anticancer drug-induced thrombocytopenia rat model

A fibrinogen γ-chain (dodecapeptide HHLGGAKQAGDV, H12)-coated, adenosine diphosphate (ADP)-encapsulated liposome [H12-(ADP)-liposome] was designed to achieve optimal performance as a homeostatic agent and expected as a synthetic platelet alternative. For the purpose of efficient function as platelet substitute, H12-(ADP)-liposomes should potentially have both acceptable pharmacokinetic and biodegradable properties under conditions of an adaptation disease including thrombocytopenia induced by anticancer drugs. The aim of this study was to characterize the pharmacokinetics of H12-(ADP)-liposomes in busulphan-induced thrombocytopenic rats using (14) C, (3) H double radiolabeled H12-(ADP)-liposomes, in which the encapsulated ADP and liposomal membrane (cholesterol) were labeled with (14) C and (3) H, respectively. After the administration of H12-(ADP)-liposomes, they were determined to be mainly distributed to the liver and spleen and disappeared from organs within 7 days after injection. The encapsulated ADP was mainly eliminated in the urine, whereas the outer membrane (cholesterol) was mainly eliminated in feces. The successive dispositions of the H12-(ADP)-liposomes were similar in both normal and thrombocytopenic rats. However, the kinetics of H12-(ADP)-liposomes in thrombocytopenic rats was more rapid, compared with the corresponding values for normal rats. These findings, which well reflect the clinical features of patients with anticancer drug-induced thrombocytopenia, provide useful information for the development of the H12-(ADP)-liposomes for future clinical use.

Pharmacokinetic study of the structural components of adenosine diphosphate-encapsulated liposomes coated with fibrinogen γ-chain dodecapeptide as a synthetic platelet substitute

Fibrinogen γ-chain (dodecapeptide HHLGGAKQAGDV, H12)-coated, ADP-encapsulated liposomes [H12-(ADP)-liposomes] were developed as a synthetic platelet alternative that specifically accumulates at bleeding sites as the result of interactions with activated platelets via glycoprotein IIb/IIIa and augments platelet aggregation by releasing ADP. The aim of this study is to characterize the pharmacokinetic properties of H12-(ADP)-liposomes and structural components in rats, and to predict the blood retention of H12-(ADP)-liposomes in humans. With use of H12-(ADP)-liposomes in which the encapsulated ADP and liposomal membrane cholesterol were radiolabeled with (14)C and (3)H, respectively, it was found that the time courses for the plasma concentration curves of (14)C and (3)H radioactivity showed that the H12-(ADP)-liposomes remained intact in the blood circulation for up to 24 hours after injection, and were mainly distributed to the liver and spleen. However, the (14)C and (3)H radioactivity of H12-(ADP)-liposomes disappeared from organs within 7 days after injection. The encapsulated ADP was metabolized to allantoin, which is the final metabolite of ADP in rodents, and was mainly eliminated in the urine, whereas the cholesterol was mainly eliminated in feces. In addition, the half-life of the H12-(ADP)-liposomes in humans was predicted to be approximately 96 hours from pharmacokinetic data obtained for mice, rats, and rabbits using an allometric equation. These results suggest that the H12-(ADP)-liposome has potential with proper pharmacokinetic and acceptable biodegradable properties as a synthetic platelet substitute.

Mimicking adhesive functionalities of blood platelets using ligand-decorated liposomes

Platelet transfusion is used for treating a variety of bleeding complications. Natural platelet-based transfusion products have very short storage life (3-7 days) and high risks of biological contamination and side effects. Consequently, there is significant clinical interest in synthetic platelet-mimetic constructs that can promote hemostasis, while allowing convenient large-scale production, easy portability, long storage life, and minimal biological risks. To this end, research efforts are being directed toward particles that can amplify aggregation of activated platelets or can mimic platelet's ability to undergo adhesion to various vascular matrix proteins. Here, we report on a synthetic construct design that combines the mimicry of platelet's shear-dependent adhesion to vWF and shear-independent adhesion to collagen under flow, on a single particle. For this, we have used 150-nm-diameter liposomes as model particles and have decorated their surface simultaneously with vWF-binding and collagen-binding recombinant protein fragments or synthetic peptide motifs. We demonstrate in vitro that these surface-modified liposomes are able to adhere onto vWF surfaces in a shear-dependent fashion and onto collagen surfaces in a shear-independent fashion under flow. Moreover, when the vWF-binding and the collagen-binding were integrated on a single liposomal platform, the resultant heteromultivalent liposomes showed significantly enhanced adhesion to a vWF/collagen mixed surface compared to liposomes bearing vWF-binding or collagen-binding ligands only, as long as the ligand motifs did not spatially interfere with each other. Altogether, our results establish the feasibility of efficiently mimicking platelet's dual adhesion mechanisms on synthetic particles.

Liposomes bearing fibrinogen could potentially interfere with platelet interaction and procoagulant activity

Background

The contribution of fibrinogen (FBN) to hemostasis acting on platelet aggregation and clot formation is well established. It has been suggested that FBN-coated liposomes could be useful in restoring hemostasis. In the present study, we evaluated the modifications induced by multilamellar raw liposomes (MLV) or fibrinogen-coated liposomes (MLV-FBN) on hemostatic parameters.

Materials and methods

Different experimental settings using whole blood or thrombocy-topenic blood were used. Thromboelastometry, aggregation studies, platelet function analyzer (PFA-100^®) tests and studies under flow conditions were applied to detect the effect of MLV-FBN on hemostatic parameters.

Results

The presence of MLV-FBN in whole blood modified its viscoelastic properties, prolonging clot formation time (CFT) (226.5 ± 26.1 mm versus 124.1 ± 9.4 mm; P < 0.01) but reducing clot firmness (45.4 ± 1.8 mm versus 35.5 ± 2.3 mm; P < 0.05). Under thrombocy-topenic conditions, FIBTEM analysis revealed that MLV-FBN shortened clotting time (CT) compared to MLV (153.3 ± 2.8 s versus 128.0 ± 4.6 s; P < 0.05). Addition of either liposome decreased fibrin formation on the subendothelium (MLV 8.1% ± 4.7% and MLV-FBN 0.8% ± 0.5% versus control 36.4% ± 6.7%; P < 0.01), whereas only MLV-FBN significantly reduced fibrin deposition in thrombocytopenic blood (14.4% ± 6.3% versus control 34.5% ± 5.2%; P < 0.05). MLV-FBN inhibited aggregation induced by arachidonic acid (52.1% ± 8.1% versus 88.0% ± 2.1% in control; P < 0.01) and ristocetin (40.3% ± 8.8% versus 94.3% ± 1.1%; P < 0.005), but it did not modify closure times in PFA-100^® studies. In perfusion experiments using whole blood, MLV and MLV-FBN decreased the covered surface (13.25% ± 2.4% and 9.85% ± 2.41%, respectively, versus control 22.0% ± 2.0%; P < 0.01) and the percentage of large aggregates (8.4% ± 2.3% and 3.3% ± 1.01%, respectively, versus control 14.6% ± 1.8%; P < 0.01).

Conclusion

Our results reveal that, in addition to the main contribution of fibrinogen to hemostasis, MLV-FBN inhibits platelet-mediated hemostasis and coagulation mechanisms.

Peptide-decorated liposomes promote arrest and aggregation of activated platelets under flow on vascular injury relevant protein surfaces in vitro

Platelet-mimetic synthetic hemostats are highly attractive in transfusion medicine. To this end, past research reports have described particles that either amplify platelet aggregation or mimic platelet adhesion. However, a construct design that effectively combines both functionalities has not been reported. Here we describe the design of a liposomal construct simultaneously surface-decorated with three peptides (a vWF-binding peptide (VBP), a collagen-binding peptide (CBP), and an active platelet clustering cyclic-RGD (cRGD) peptide), that can integrate platelet-mimetic dual hemostatic activities of adhesion and aggregation. We first demonstrate that surface-immobilized cRGD-liposomes are capable of aggregating activated platelets onto themselves. Subsequently, we demonstrate that hetero-multivalent liposomes bearing VBP, CBP, and cRGD, when introduced in flow with ≈ 20,000 activated platelets per microliter, are capable of adhering to vWF/collagen surfaces and promoting the recruitment/aggregation of platelets onto themselves. We envision that optimizing this construct can lead to a highly refined synthetic hemostat design for potential application in transfusion medicine.

Ultrastructural analysis of thrombin-induced interaction between human platelets and liposomes carrying fibrinogen γ-chain dodecapeptide as a synthetic platelet substitute

BACKGROUND

The dodecapeptide HHLGGAKQAGDV (H12) in the carboxy-terminus of the fibrinogen γ-chain is a specific binding site of the ligand for platelet GPIIb/IIIa complex. We have evaluated liposomes carrying fibrinogen γ-chain dodecapeptide as a synthetic platelet substitute.

OBJECTIVES

We examined the interaction between human platelets and H12-liposomes during thrombin-induced activation using flow cytometry and electron microscopy (EM).

METHODS AND RESULTS

After thrombin-activation, a remarkable time-dependent increase in binding of the H12-liposomes to platelets was found by flow cytometry. A large-sized swollen open canalicular system (OCS) was observed in the spheroidal platelets from 60 sec to 5 min after thrombin-activation, but intact H12-liposomes were not evident by conventional EM. Cryoultramicrotomy and immunogold staining with anti-H12 antibody were successful in identifying the liposomes; they appeared as small particles with a unit membrane around 0.2 to 0.4 μm in diameter, and gold labels representing H12 were distributed homogeneously on the surface. Abundant H12-liposomes were localized not only on the surface membrane but also in the lumen of the large-sized swollen OCS in the platelets at 60 sec after thrombin-activation. The formation of the large-sized swollen OCS was inhibited by pre-incubation with unbound H12, EDTA or anti-GPIIb/IIIa antibody. In thrombin-induced platelet aggregates we observed electron-transparent areas between adherent platelets, in which abundant H12-liposomes were distributed.

CONCLUSIONS

We demonstrate morphologically that H12-liposomes bind to thrombin-activated platelets and accumulate between adherent platelets like fibrinogen, leading to large-scale aggregation.

Current concepts in platelet transfusion

This is the era of component therapy. Therefore there is a need for rational use of platelet concentrate. Lot of knowledge has been added recently in the field of platelet specially about the platelet rich plasma and its application in clinical practice. The current review focuses on improvement in preparation of platelet rich plasma, the procedure to make the same more safe and its rational use. Furthermore newer aspects of platelet concentrate use in surgical practice and for regenerative medicine has also been discussed. It also covers some progress and hurdles in preparation of platelet substitutes.

Prolonged hemostatic ability of polyethylene glycol?modified polymerized albumin particles carrying fibrinogen ?-chain dodecapeptide

BACKGROUND

Second-generation platelet (PLT) substitutes for treatment of bleeding were studied and the focus was on a dodecapeptide, HHLGGAKQAGDV (H12), which is a fibrinogen gamma-chain carboxy-terminal sequence (gamma 400-411) and exists only in a fibrinogen domain.

STUDY DESIGN AND METHODS

H12 was conjugated to the surface of polymerized albumin particles (polyAlb) modified with polyethylene glycol (PEG) chains to produce biocompatible particles (H12-PEG-polyAlb) that had prolonged blood circulation t((1/2)) and were more stable in vitro and in vivo compared with H12-polyAlb (not modified with PEG). H12-PEG-polyAlb was administered intravenously into thrombocytopenic rats and the t((1/2)) of the particles and the tail bleeding time were measured to evaluate the prolongation in the hemostatic effect.

RESULTS

H12-PEG-polyAlb particles modified with PEG prolonged the t((1/2)) and maintained specific binding ability to activated PLTs. The particles dose dependently shortened the tail bleeding time of thrombocytopenic rats 6 hours after injection.

CONCLUSION

H12-PEG-polyAlb may be a suitable candidate for treatment of bleeding into thrombocytopenic patients as an alternative to PLT concentrate transfusion.

Hemostatic effects of polymerized albumin particles bearing rGPIa/IIa in thrombocytopenic mice

The recombinant fragment of the platelet membrane glycoprotein Ia/IIa (rGPIa/IIa) was conjugated to the polymerized albumin particles (polyAlb) with the average diameter of 180 nm. The intravenous administration of rGPIa/IIa-polyAlb to thrombocytopenic mice ([platelet] = 2.1+/-0.3 x 10(5) particles/ microL) with three doses of ca. 2.4 x 10(10), 7.2 x 10(10), and 2.4 x 10(11)particles/kg, respectively, significantly reduced their bleeding time to 426+/-71, 378+/-101, and 337+/-46 s, respectively, whereas that of the control groups (PBS) was 730+/-198 s. The injection of rGPIa/IIa-polyAlb (2.4 x 10(11)particles/kg) was approximately equal to the effect of the injection of the mouse platelets at a dose of 2.0 x 10(10) particles/kg. It was confirmed that rGPIa/IIa-polyAlb had a recognition ability against collagen and could contribute to the hemostasis in the thrombocytopenic mice as a platelet substitute.

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.

Reconstitution of adhesive properties of human platelets in liposomes carrying both recombinant glycoproteins Ia/IIa and Ib alpha under flow conditions: specific synergy of receptor-ligand interactions

Liposomes carrying both recombinant glycoprotein Ia/IIa (rGPIa/IIa) and Ib alpha (rGPIb alpha) (rGPIa/IIa-Ib alpha-liposomes) instantaneously and irreversibly adhered to the collagen surface in the presence of soluble von Willebrand factor (VWF) at high shear rates, in marked contrast with translocation of liposomes carrying rGPIb alpha alone on the VWF surface. In the absence of soluble VWF, the adhesion of rGPIa/IIa-Ib alpha-liposomes to the collagen surface decreased with increasing shear rates, similar to liposomes carrying rGPIa/IIa alone. While adhesion of liposomes with exofacial rGPIa/IIa and rGPIb alpha densities of 2.17 x 10(3) and 1.00 x 10(4) molecules per particle, respectively, was efficient at high shear rates, reduction in rGPIb alpha density to 5.27 x 10(3) molecules per particle resulted in decreased adhesion even in the presence of soluble VWF. A 50% reduction in the exofacial rGPIa/IIa density resulted in a marked decrease in the adhesive ability of the liposomes at all shear rates tested. The inhibitory effect of antibody against GPIb alpha (GUR83-35) on liposome adhesion was greater at higher shear rates. Further, the anti-GPIa antibody (Gi9) inhibited liposome adhesion more than GUR83-35 at all shear rates tested. These results suggest that the rGPIa/IIa-collagen interaction dominates the adhesion of rGPIa/IIa-Ib alpha-liposomes to the collagen surface at low shear rates, while the rGPIa/IIa-collagen and rGPIb alpha-VWF interaction complements each other, and they synergistically provide the needed functional integration required for liposome adhesion at high shear rates. This study thus has confirmed for the first time the proposed mechanisms of platelet adhesion to the collagen surface under flow conditions using the liposome system.

Platelets: preparations, transfusion, modifications, and substitutes

OBJECTIVE

To summarize current knowledge and recent progress pertaining to platelet concentrate preparations, modifications, and future prospects for platelet substitutes.

METHODS

Current publications identified through a search of an electronic literature database were evaluated and reviewed. Relevant data were abstracted into this article. Abstractions of the data were made depending on their relevance. This review starts with standard methods of platelet preparation and goes on to describe different modifications intended to optimize the product and increase its safety. The article concludes with a discussion of the use of hematopoietic growth factors and novel kinds of platelet components for future use.

CONCLUSIONS

Many modifications in the preparation of platelet transfusions have occurred in recent years. Platelets prepared by standard techniques contain significant numbers of donor leukocytes, which are responsible for several adverse effects. Awareness of this problem has lead to the development of effective means for their removal. Several methods to reduce the risk of viral and bacterial transmission through platelet transfusions are emerging. New technologies in the use of platelet substitutes have attempted to prolong the platelet storage potential and prevent the development of recipient alloimmunization. As the biological activities of growth factors become better understood, the clinical applications of novel recombinant products may redefine the concept of future platelet transfusions. It is important that research continues into the optimal methods for the preparation and use of platelet transfusions to provide maximal clinical benefits with minimal risk of complications.

Characterization of liposomes carrying von Willebrand factor-binding domain of platelet glycoprotein Ibalpha: a potential substitute for platelet transfusion

Platelet glycoprotein (GP) Ib/IX/V complex is a receptor for von Willebrand factor (vWf), which plays a crucial role in primary hemostasis by mediating platelet adhesion to injured blood vessels. We have expressed in CHO cells a fragment of GPIba that retained a vWf-binding function. The recombinant fragment (rGPIba) was incorporated into liposomes and evaluated their functions in vitro. rGPIba on the liposome surface was detectable by flow cytometric analysis. Addition of vWf and ristocetin caused specific agglutination of rGPIbalpha-liposomes, as evaluated by an aggregometer or a fluorescent microscopy. When ristocetin was added to platelet-rich plasma (PRP) pre-mixed with rhodamine-labeled rGPIbalpha-liposomes, platelets aggregated and rhodamine-fluorescence was strongly positive in the platelet thrombi, suggesting that heterologous aggregation (attachment of liposomes to platelets) occurred. Platelet aggregation in PRP at low platelet concentration (20-80 x 10(6)/ml) was enhanced by rGPIbalpha-liposomes in a dose-dependent manner. Thus, rGPIbalpha-liposomes may accumulate on vWf-exposed subendothelial tissues and enhance platelet function in vivo, supporting hemostasis in thrombocytopenic individuals.