Validation of a SCID mouse model for transfusion by concurrent comparison of circulation kinetics of human platelets, stored under various temperature conditions, between human volunteers and mice

Mechanisms of action of an investigational new freeze-dried platelet-derived hemostatic product

BACKGROUND

A safe and efficacious hemostatic product with a long shelf-life is needed to reduce mortality from hemorrhage due to trauma and improve surgical outcomes for persons with platelet deficiency or dysfunction. Thrombosomes, a trehalose-stabilized, leukoreduced, pooled blood group-O freeze-dried platelet-derived hemostatic (FPH) with a 3-year shelf-life, may satisfy this need.

OBJECTIVES

To characterize the mechanism of action of FPH.

METHODS

FPH's ability to adhere to collagen, aggregate with and without platelets, and form clots was evaluated in vitro. Nonobese diabetic-severe combined immunodeficiency mouse models were used to assess circulation persistence and hemostatic efficacy.

RESULTS

FPH displays the morphology and surface proteins of activated platelets. FPH adheres to collagen, aggregates, and promotes clots, producing an insoluble fibrin mesh. FPH is rapidly cleared from circulation, has hemostatic efficacy comparable to apheresis platelets in a murine tail-cut, and acts in a dose-dependent manner.

CONCLUSION

FPH is a first-in-class investigational treatment and shows strong potential as a hemostatic agent that is capable of binding exposed collagen, coaggregating with endogenous platelets, and promoting the coagulation cascade. These properties may be exploited to treat active platelet-related or diffuse vascular bleeding. FPH has the potential to fulfill a large unmet patient need as an acute hemostatic treatment in severe bleeding, such as surgery and trauma.

CPT2 K79 Acetylation Regulates Platelet Lifespan

CPT2 K79 acetylation caused by NAD⁺ exhaustion and Sirt3 dysfunction resulted in LCAC accumulation and platelet damage.

Blocking acylcarnitine generation with AMPK or CPT1 inhibitors, Sirt3 agonists, and antioxidants retarded platelet storage lesion.

The short life span of platelets is a major challenge to platelet transfusion services because of the lack of effective intervention. Here, we found that the accumulation of long-chain acylcarnitines (LCACs) is responsible for mitochondrial damage and platelet storage lesion. Further studies showed that the blockade of fatty acid oxidation and the activation of AMP-activated protein kinase (AMPK)/acetyl-CoA carboxylase/carnitine palmitoyltransferase 1 (CPT1) pathways that promote fatty acid metabolism are important reasons for the accumulation of LCACs. The excessive accumulation of LCACs can cause mitochondrial damage and a short life span of stored platelets. The mechanism study elucidated that NAD⁺ exhaustion and the subsequent decrease in sirtuin 3 (Sirt3) activity caused an increase in the level of CPT2 K79 acetylation, which is the primary cause of the blockade of fatty acid oxidation and the accumulation of LCACs. Blocking LCAC generation with the inhibitors of AMPK or CPT1, the agonists of Sirt3, and antioxidants tremendously retarded platelet storage lesion in vitro and prolonged the survival of stored platelets in vivo posttransfusion with single or combined use. In summary, we discovered that CPT2 acetylation attenuates fatty acid oxidation and exacerbates platelet storage lesion and may serve as a new target for improving platelet storage quality.

Current challenges in platelet transfusion

The supply of platelets for transfusion is a logistical challenge due to the physiology of platelets and current measures of transfusion performance dictating storage at 22°C and a short product shelf-life (<7 days). Demand for platelets has increased in recent years and changes in the demographics of the population may enhance this further. Many studies have been conducted to understand what the optimal dose and trigger for transfusion should be, mainly in hematology patients who are the largest cohort that receive platelets, mostly to prevent bleeding. Emerging data suggests that for bleeding patients, where immediate hemostasis is a key consideration, the current standard product may not be optimal. Alternative platelet preparation methods/storage options that may improve the hemostatic properties of platelets are under active development. In parallel with research into alternative platelet products that might enhance hemostasis, better measures for assessing bleeding risk and platelet efficacy are needed.

P38 mitogen activated protein kinase inhibitor improves platelet in vitro parameters and in vivo survival in a SCID mouse model of transfusion for platelets stored at cold or temperature cycled conditions for 14 days

Platelets for transfusion are stored at room temperature (20-24°C) up to 7 days but decline in biochemical and morphological parameters during storage and can support bacterial proliferation. This decline is reduced with p38MAPK inhibitor, VX-702. Storage of platelets in the cold (4-6°C) can reduce bacterial proliferation but platelets get activated and have reduced circulation when transfused. Thermocycling (cold storage with brief periodic warm ups) reduces some of the effects of cold storage. We evaluated in vitro properties and in vivo circulation in SCID mouse model of human platelet transfusion of platelets stored in cold or thermocycled for 14 days with and without VX-702. Apheresis platelet units (N = 15) were each aliquoted into five storage bags and stored under different conditions: room temperature; cold temperature; thermocycled temperature; cold temperature with VX-702; thermocycled temperature with VX-702. Platelet in vitro parameters were evaluated at 1, 7 and 14 days. On day 14, platelets were infused into SCID mice to assess their retention in circulation by flow cytometry. VX-702 reduced negative platelet parameters associated with cold and thermocycled storage such as an increase in expression of activation markers CD62, CD63 and of phosphatidylserine (marker of apoptosis measured by Annexin binding) and lowered the rise in lactate (marker of increase in anaerobic metabolism). However, VX-702 did not inhibit agonist-induced platelet aggregation indicating that it does not interfere with platelet hemostatic function. In vivo, VX-702 improved initial recovery and area under the curve in circulation of human platelets infused into a mouse model that has been previously validated against a human platelet infusion clinical trial. In conclusion, inhibition of p38MAPK during 14-days platelet storage in cold or thermocycling conditions improved in vitro platelet parameters and platelet circulation in the mouse model indicating that VX-702 may improve cell physiology and clinical performance of human platelets stored in cold conditions.

Generation and manipulation of human iPSC-derived platelets

The discovery of iPSCs has led to the ex vivo production of differentiated cells for regenerative medicine. In the case of transfusion products, the derivation of platelets from iPSCs is expected to complement our current blood-donor supplied transfusion system through donor-independent production with complete pathogen-free assurance. This derivation can also overcome alloimmune platelet transfusion refractoriness by resulting in autologous, HLA-homologous or HLA-deficient products. Several developments were necessary to produce a massive number of platelets required for a single transfusion. First, expandable megakaryocytes were established from iPSCs through transgene expression. Second, a turbulent-type bioreactor with improved platelet yield and quality was developed. Third, novel drugs that enabled efficient feeder cell-free conditions were developed. Fourth, the platelet-containing suspension was purified and resuspended in an appropriate buffer. Finally, the platelet product needed to be assured for competency and safety including non-tumorigenicity through in vitro and in vivo preclinical tests. Based on these advancements, a clinical trial has started. The generation of human iPSC-derived platelets could evolve transfusion medicine to the next stage and assure a ubiquitous, safe supply of platelet products. Further, considering the feasibility of gene manipulations in iPSCs, other platelet products may bring forth novel therapeutic measures.

Antioxidant prevents clearance of hemostatically competent platelets after long-term cold storage

BACKGROUND

Cold storage of platelets (PLTs) has the potential advantage of prolonging storage time while reducing posttransfusion infection given the decreased likelihood of bacterial outgrowth during storage and possibly beneficial effects in treating bleeding patients. However, cold storage reduces PLT survival through the induction of complex storage lesions, which are more accentuated when storage is prolonged.

STUDY DESIGN AND METHODS

Whole blood-derived PLT-rich plasma concentrates from seven PLT pools (n = 5 donors per pool). PLT additive solution was added (67%/33% plasma) and the product was split into 50-mL bags. Split units were stored in the presence or absence of 1 mM of N-acetylcysteine (NAC) under agitation for up to 14 days at room temperature or in the cold and were analyzed for PLT activation, fibrinogen-dependent spreading, microparticle formation, mitochondrial respiratory activity, reactive oxygen species (ROS) generation, as well as in vivo survival and bleeding time correction in immunodeficient mice.

RESULTS

Cold storage of PLTs for 7 days or longer induces significant PLT activation, cytoskeletal damage, impaired fibrinogen spreading, enhances mitochondrial metabolic decoupling and ROS generation, and increases macrophage-dependent phagocytosis and macrophage-independent clearance. Addition of NAC prevents PLT clearance and allows a correction of the prolonged bleeding time in thrombocytopenic, aspirin-treated, immunodeficient mice.

CONCLUSIONS

Long-term cold storage induces mitochondrial uncoupling and increased proton leak and ROS generation. The resulting ROS is a crucial contributor to the increased macrophage-dependent and -independent clearance of functional PLTs and can be prevented by the antioxidant NAC in a magnesium-containing additive solution.