Analysis of the structural and mechanical effects of procoagulant agents on neonatal fibrin networks following cardiopulmonary bypass

Effect of In Vivo Administration of Fibrinogen Concentrate Versus Cryoprecipitate on Ex Vivo Clot Degradation in Neonates Undergoing Cardiac Surgery

BACKGROUND

Neonates undergoing cardiac surgery require fibrinogen replacement to restore hemostasis after cardiopulmonary bypass (CPB). Cryoprecipitate is often the first-line treatment, but recent studies demonstrate that fibrinogen concentrate (RiaSTAP; CSL Behring) may be acceptable in this population. This investigator-initiated, randomized trial compares cryoprecipitate to fibrinogen concentrate in neonates undergoing cardiac surgery (ClinicalTrials.gov NCT03932240). The primary end point was the percent change in ex vivo clot degradation from baseline at 24 hours after surgery between groups. Secondary outcomes included intraoperative blood transfusions, coagulation factor levels, and adverse events.

METHODS

Neonates were randomized to receive cryoprecipitate (control group) or fibrinogen concentrate (study group) as part of a post-CPB transfusion algorithm. Blood samples were drawn at 4 time points: presurgery (T1), after treatment (T2), arrival to the intensive care unit (ICU) (T3), and 24 hours postsurgery (T4). Using the mixed-effect models, we analyzed the percent change in ex vivo clot degradation from a patient's presurgery baseline at each time point. Intraoperative blood product transfusions, coagulation factor levels, perioperative laboratory values, and adverse events were collected.

RESULTS

Thirty-six neonates were enrolled (intent to treat [ITT]). Thirteen patients in the control group and seventeen patients in the study group completed the study per protocol (PP). After normalizing to the patient's own baseline (T1), no significant differences were observed in clot degradation at T2 or T3. At T4, patients in the study group had greater degradation when compared to those in the control group (826.5%, 95% confidence interval [CI], 291.1-1361.9 vs -545.9%, 95% CI, -1081.3 to -10.4; P < .001). Study group patients received significantly less median post-CPB transfusions than control group patients (ITT, 27.2 mL/kg [19.0-36.9] vs 41.6 [29.2-52.4]; P = .043; PP 26.7 mL/kg [18.8-32.2] vs 41.2 mL/kg [29.0-51.4]; P < .001). No differences were observed in bleeding or thrombotic events.

CONCLUSIONS

Neonates who received fibrinogen concentrate, as compared to cryoprecipitate, have similar perioperative ex vivo clot degradation with faster degradation at 24 hours postsurgery, less post-CPB blood transfusions, and no increased bleeding or thrombotic complications. Our findings suggest that fibrinogen concentrate adequately restores hemostasis and reduces transfusions in neonates after CPB without increased bleeding or thrombosis risk.

Ultrasoft platelet-like particles stop bleeding in rodent and porcine models of trauma

Uncontrolled bleeding after trauma represents a substantial clinical problem. The current standard of care to treat bleeding after trauma is transfusion of blood products including platelets; however, donated platelets have a short shelf life, are in limited supply, and carry immunogenicity and contamination risks. Consequently, there is a critical need to develop hemostatic platelet alternatives. To this end, we developed synthetic platelet-like particles (PLPs), formulated by functionalizing highly deformable microgel particles composed of ultralow cross-linked poly (N-isopropylacrylamide) with fibrin-binding ligands. The fibrin-binding ligand was designed to target to wound sites, and the cross-linking of fibrin polymers was designed to enhance clot formation. The ultralow cross-linking of the microgels allows the particles to undergo large shape changes that mimic platelet shape change after activation; when coupled to fibrin-binding ligands, this shape change facilitates clot retraction, which in turn can enhance clot stability and contribute to healing. Given these features, we hypothesized that synthetic PLPs could enhance clotting in trauma models and promote healing after clotting. We first assessed PLP activity in vitro and found that PLPs selectively bound fibrin and enhanced clot formation. In murine and porcine models of traumatic injury, PLPs reduced bleeding and facilitated healing of injured tissue in both prophylactic and immediate treatment settings. We determined through biodistribution experiments that PLPs were renally cleared, possibly enabled by ultrasoft particle properties. The performance of synthetic PLPs in the preclinical studies shown here supports future translational investigation of these hemostatic therapeutics in a trauma setting.

Influence of Fibrinogen Concentrate on Neonatal Clot Structure When Administered Ex Vivo After Cardiopulmonary Bypass

BACKGROUND

Bleeding is a serious complication of cardiopulmonary bypass (CPB) in neonates. Blood product transfusions are often needed to adequately restore hemostasis, but are associated with significant risks. Thus, neonates would benefit from other effective, and safe, hemostatic therapies. The use of fibrinogen concentrate (FC; RiaSTAP, CSL Behring, Marburg, Germany) is growing in popularity, but has not been adequately studied in neonates. Here, we characterize structural and degradation effects on the neonatal fibrin network when FC is added ex vivo to plasma obtained after CPB.

METHODS

After approval by the institutional review board and parental consent, blood samples were collected from neonates undergoing cardiac surgery and centrifuged to yield platelet poor plasma. Clots were formed ex vivo from plasma obtained at several time points: (1) baseline, (2) immediately post-CPB, and (3) post-transfusion of cryoprecipitate. In addition, we utilized post-CPB plasma to construct the following conditions: (4) post-CPB +0.5 mg/mL FC, and (5) post-CPB +0.9 mg/mL FC. The resultant fibrin networks were imaged using confocal microscopy to analyze overall structure, fiber density, and alignment. Clots were also analyzed using a microfluidic degradation assay. Fibrinogen content was quantified for all plasma samples.

RESULTS

The addition of 0.5 or 0.9 mg/mL FC to post-CPB samples significantly enhanced the median fiber density when compared to untreated post-CPB samples (post-CPB = 0.44 [interquartile range {IQR}: 0.36-0.52], post-CPB +0.5 mg/mL FC = 0.69 [0.56-0.77], post-CPB +0.9 mg/mL FC = 0.87 [0.59-0.96]; P = .01 and P = .006, respectively). The addition of 0.9 mg/mL FC to post-CPB samples resulted in a greater fiber density than that observed after the in vivo transfusion of cryoprecipitate (post-transfusion = 0.54 [0.45-0.77], post-CPB +0.9 mg/mL FC = 0.87 [0.59-0.96]; P = .002). Median fiber alignment did not differ significantly between post-CPB samples and samples treated with FC. Degradation rates were not statistically significant from baseline values with either 0.5 or 0.9 mg/mL FC. In addition, we found a significant correlation between the difference in the baseline and post-CPB fibrinogen concentration with patient age ( P = .033) after controlling for weight.

CONCLUSIONS

Our results show that clots formed ex vivo with clinically relevant doses of FC (0.9 mg/mL) display similar structural and degradation characteristics compared to the in vivo transfusion of cryoprecipitate. These findings suggest that FC is effective in restoring structural fibrin clot properties after CPB. Future studies after the administration of FC in vivo are needed to validate this hypothesis.

Activated 4-Factor Prothrombin Complex Concentrate as a Hemostatic Adjunct for Neonatal Cardiac Surgery: A Propensity Score-Matched Cohort Study

BACKGROUND

Prothrombin complex concentrates are an emerging "off-label" therapy to augment hemostasis after cardiopulmonary bypass (CPB), but data supporting their use for neonatal cardiac surgery are limited.

METHODS

We retrospectively reviewed neonates undergoing open heart surgery with first-time sternotomy between May 2014 and December 2018 from a hospital electronic health record database. Neonates who received activated 4-factor prothrombin complex concentrate (a4FPCC) after CPB were propensity score matched (PSM) to neonates who did not receive a4FPCC (control group). The primary efficacy outcome was total volume (mL/kg) of blood products transfused after CPB, including the first 24 hours on the cardiovascular intensive care unit (CVICU). The primary safety outcome was the incidence of 7- and 30-day postoperative thromboembolism. Secondary outcomes included 24 hours postoperative chest tube output, time to extubation, duration of CVICU stay, duration of hospital stay, 30-day mortality, and incidence of acute kidney injury on postoperative day 3. We used linear regression modeling on PSM data for the primary efficacy outcome. For the primary safety outcome, we tested for differences using McNemar test on PSM data. For secondary outcomes, we used linear regression, Fisher exact test, or survival analyses as appropriate, with false discovery rate-adjusted P values.

RESULTS

A total of 165 neonates were included in the final data analysis: 86 in the control group and 79 in the a4FPCC group. After PSM, there were 43 patients in the control group and 43 in the a4FPCC group. We found a statistically significant difference in mean total blood products transfused for the a4FPCC group (47.5 mL/kg) compared with the control group (63.7 mL/kg) for PSM patients (adjusted difference, 15.3; 95% CI, 29.4-1.3; P = .032). We did not find a statistically significant difference in 7- or 30-day thromboembolic rate, postoperative chest tube output, time to extubation, incidence of postoperative acute kidney injury (AKI), or 30-day mortality between the groups. The a4FPCC group had a significantly longer length of intensive care unit stay (32.9 vs 13.3 days; adjusted P = .049) and hospital stay (44.6 vs 24.1 days; adjusted P = .049) compared with the control group.

CONCLUSIONS

We found that the use of a4FPCC as a hemostatic adjunct for post-CPB bleeding in neonatal cardiac surgery was associated with a decrease in mean total blood products transfused after CPB without an increased rate of 7- or 30-day postoperative thromboembolism. Our findings suggest that a4FPCCs can be considered as part of a hemostasis pathway for refractory bleeding in neonatal cardiac surgery.

COVID-19 patient fibrinogen produces dense clots with altered polymerization kinetics, partially explained by increased sialic acid

BACKGROUND

Thrombogenicity is a known complication of COVID-19, resulting from SARS-CoV-2 infection, with significant effects on morbidity and mortality.

OBJECTIVE

We aimed to better understand the effects of COVID-19 on fibrinogen and the resulting effects on clot structure, formation, and degradation.

METHODS

Fibrinogen isolated from COVID-19 patients and uninfected subjects was used to form uniformly concentrated clots (2 mg/ml), which were characterized using confocal microscopy, scanning electron microscopy, atomic force microscopy, and endogenous and exogenous fibrinolysis assays. Neuraminidase digestion and subsequent NANA assay were used to quantify sialic acid residue presence; clots made from the desialylated fibrinogen were then assayed similarly to the original fibrinogen clots.

RESULTS

Clots made from purified fibrinogen from COVID-19 patients were shown to be significantly stiffer and denser than clots made using fibrinogen from noninfected subjects. Endogenous and exogenous fibrinolysis assays demonstrated that clot polymerization and degradation dynamics were different for purified fibrinogen from COVID-19 patients compared with fibrinogen from noninfected subjects. Quantification of sialic acid residues via the NANA assay demonstrated that SARS-CoV-2-positive fibrinogen samples contained significantly more sialic acid. Desialylation via neuraminidase digestion resolved differences in clot density. Desialylation did not normalize differences in polymerization, but did affect rate of exogenous fibrinolysis.

DISCUSSION

These differences noted in purified SARS-CoV-2-positive clots demonstrate that structural differences in fibrinogen, and not just differences in gross fibrinogen concentration, contribute to clinical differences in thrombotic features associated with COVID-19. These structural differences are at least in part mediated by differential sialylation.

Differential sialic acid content in adult and neonatal fibrinogen mediates differences in clot polymerization dynamics

Increased sialic acid in neonatal fibrinogen influences fibrin knob-hole interactions during polymerization.

Neonatal fibrin polymerization involves more B knob– and fewer A knob–mediated interactions compared with adults.

Neonates possess a molecular variant of fibrinogen, known as fetal fibrinogen, characterized by increased sialic acid, a greater negative charge, and decreased activity compared with adults. Despite these differences, adult fibrinogen is used for the treatment of bleeding in neonates, with mixed efficacy. To determine safe and efficacious bleeding protocols for neonates, more information on neonatal fibrin clot formation and the influence of sialic acid on these processes is needed. Here, we examine the influence of sialic acid on neonatal fibrin polymerization. We hypothesized that the increased sialic acid content of neonatal fibrinogen promotes fibrin B:b knob-hole interactions and consequently influences the structure and function of the neonatal fibrin matrix. We explored this hypothesis through analysis of structural properties and knob:hole polymerization dynamics of normal and desialylated neonatal fibrin networks and compared them with those formed with adult fibrinogen. We then characterized normal neonatal fibrin knob:hole interactions by forming neonatal and adult clots with either thrombin or snake-venom thrombin-like enzymes that preferentially cleave fibrinopeptide A or B. Sialic acid content of neonatal fibrinogen was determined to be a key determinant of resulting clot properties. Experiments analyzing knob:hole dynamics indicated that typical neonatal fibrin clots are formed with the release of more fibrinopeptide B and less fibrinopeptide A than adults. After the removal of sialic acid, fibrinopeptide release was roughly equivalent between adults and neonates, indicating the influence of sialic acid on fibrin neonatal fibrin polymerization mechanisms. These results could inform future studies developing neonatal-specific treatments of bleeding.

Synthetic platelet microgels containing fibrin knob B mimetic motifs enhance clotting responses

Native platelets are crucial players in wound healing. Key to their role is the ability of their surface receptor GPIIb/IIIa to bind fibrin at injury sites, thereby promoting clotting. When platelet activity is impaired as a result of traumatic injury or certain diseases, uncontrolled bleeding can result. To aid clotting and tissue repair in cases of poor platelet activity, our lab has previously developed synthetic platelet-like particles capable of promoting clotting and improving wound healing responses. These are constructed by functionalizing highly deformable hydrogel microparticles (microgels) with fibrin-binding ligands including a fibrin-specific whole antibody or a single-domain variable fragment. To improve the translational potential of these clotting materials, we explored the use of fibrin-binding peptides as cost-effective, robust, high-specificity alternatives to antibodies. Herein, we present the development and characterization of soft microgels decorated with the peptide AHRPYAAK that mimics fibrin knob 'B' and targets fibrin hole 'b'. These "Fibrin-Affine Microgels with Clotting Yield" (FAMCY) were found to significantly increase clot density in vitro and decrease bleeding in a rodent trauma model in vivo. These results indicate that FAMCYs are capable of recapitulating the platelet-mimetic properties of previous designs while utilizing a less costly, more translational design.

Neonatal coagulopathies: A review of established and emerging treatments

Despite the relative frequency of both bleeding and clotting disorders among patients treated in the neonatal intensive care unit, few clear guidelines exist for treatment of neonatal coagulopathies. The study and treatment of neonatal coagulopathies are complicated by the distinct hemostatic balance and clotting components present during this developmental stage as well as the relative scarcity of studies specific to this age group. This mini-review examines the current understanding of neonatal hemostatic balance and treatment of neonatal coagulopathies, with particular emphasis on emerging treatment methods and areas in need of further investigative efforts.

Neonatal Fibrin Scaffolds Promote Enhanced Cell Adhesion, Migration, and Wound Healing In Vivo Compared to Adult Fibrin Scaffolds

INTRODUCTION

Fibrin scaffolds are often utilized to treat chronic wounds. The monomer fibrinogen used to create such scaffolds is typically derived from adult human or porcine plasma. However, our previous studies have identified extensive differences in fibrin network properties between adults and neonates, including higher fiber alignment in neonatal networks. Wound healing outcomes have been linked to fibrin matrix structure, including fiber alignment, which can affect the binding and migration of cells. We hypothesized that fibrin scaffolds derived from neonatal fibrin would enhance wound healing outcomes compared to adult fibrin scaffolds.

METHODS

Fibrin scaffolds were formed from purified adult or neonatal fibrinogen and thrombin then structural analysis was conducted via confocal microscopy. Human neonatal dermal fibroblast attachment, migration, and morphology on fibrin scaffolds were assessed. A murine full thickness injury model was used to compare healing in vivo in the presence of neonatal fibrin, adult fibrin, or saline.

RESULTS

Distinct fibrin architectures were observed between adult and neonatal scaffolds. Significantly higher fibroblast attachment and migration was observed on neonatal scaffolds compared to adults. Cell morphology on neonatal scaffolds exhibited higher spreading compared to adult scaffolds. In vivo significantly smaller wound areas and greater epidermal thickness were observed when wounds were treated with neonatal fibrin compared to adult fibrin or a saline control.

CONCLUSIONS

Distinctions in neonatal and adult fibrin scaffold properties influence cellular behavior and wound healing. These studies indicate that fibrin scaffolds sourced from neonatal plasma could improve healing outcomes compared to scaffolds sourced from adult plasma.

Comparison of Neonatal and Adult Fibrin Clot Properties between Porcine and Human Plasma

BACKGROUND

Recent studies suggest that adult-specific treatment options for fibrinogen replacement during bleeding may be less effective in neonates. This is likely due to structural and functional differences found in the fibrin network between adults and neonates. In this investigation, the authors performed a comparative laboratory-based study between immature and adult human and porcine plasma samples in order to determine if piglets are an appropriate animal model of neonatal coagulopathy.

METHODS

Adult and neonatal human and porcine plasma samples were collected from the Children's Hospital of Atlanta and North Carolina State University College of Veterinary Medicine, respectively. Clots were formed for analysis and fibrinogen concentration was quantified. Structure was examined through confocal microscopy and cryogenic scanning electron microscopy. Function was assessed through atomic force microscopy nanoindentation and clotting and fibrinolysis assays. Lastly, novel hemostatic therapies were applied to neonatal porcine samples to simulate treatment.

RESULTS

All sample groups had similar plasma fibrinogen concentrations. Neonatal porcine and human plasma clots were less branched with lower fiber densities than the dense and highly branched networks seen in adult human and porcine clots. Neonatal porcine and human clots had faster degradation rates and lower clot stiffness values than adult clots (stiffness [mmHg] mean ± SD: neonatal human, 12.15 ± 1.35 mmHg vs. adult human, 32.25 ± 7.13 mmHg; P = 0.016; neonatal pig, 10.5 ± 8.25 mmHg vs. adult pigs, 32.55 ± 7.20 mmHg; P = 0.015). The addition of hemostatic therapies to neonatal porcine samples enhanced clot formation.

CONCLUSIONS

The authors identified similar age-related patterns in structure, mechanical, and degradation properties between adults and neonates in porcine and human samples. These findings suggest that piglets are an appropriate preclinical model of neonatal coagulopathy. The authors also show the feasibility of in vitro model application through analysis of novel hemostatic therapies as applied to dilute neonatal porcine plasma.

Ultrasound enhanced synthetic platelet therapy for augmented wound repair

Native platelets perform a number of functions within the wound healing process, including interacting with fibrin fibers at the wound site to bring about retraction after clot formation. Clot retraction improves clot stability and enhances the function of the fibrin network as a provisional matrix to support cellular infiltration of the wound site, thus facilitating tissue repair and remodeling after hemostasis. In cases of traumatic injury or disease, platelets can become depleted and this process disrupted. To that end, our lab has developed synthetic platelet-like particles (PLPs) that recapitulate the clot retraction abilities of native platelets through a Brownian-wrench driven mechanism that drives fibrin network densification and clot retraction over time, however, this Brownian-motion driven process occurs on a longer time scale than native active actin/myosin-driven platelet-mediated clot retraction. We hypothesized that a combinatorial therapy comprised of ultrasound stimulation of PLP motion within fibrin clots would facilitate a faster induction of clot retraction on a more platelet-mimetic time scale and at a lower dosage than required for PLPs acting alone. We found that application of ultrasound in combination with a subtherapeutic dosage of PLPs resulted in increased clot density and stiffness, improved fibroblast migration in vitro and increased epidermal thickness and angiogenesis in vivo, indicating that this combination therapy has potential to facilitate multiphase pro-healing outcomes. Additionally, while these particular studies focus on the role of ultrasound in enhancing specific interactions between fibrin-binding synthetic PLPs embedded within fibrin networks, these studies have wide applicability in understanding the role of ultrasound stimulation in enhancing multi-scale colloidal interactions within fibrillar matrices.

Nanosilver composite pNIPAm microgels for the development of antimicrobial platelet-like particles

Platelets crucially facilitate wound healing but can become depleted in traumatic injury or chronic wounds. Previously, our group developed injectable platelet-like particles (PLPs) comprised of highly deformable, ultralow crosslinked pNIPAm microgels (ULCs) coupled to fibrin binding antibodies to treat post-trauma bleeding. PLP fibrin-binding facilitates homing to sites of injury, promotes clot formation, and, due to high particle deformability, induces clot retraction. Clot retraction augments healing by increasing clot stability, enhancing clot stiffness, and promoting cell migration into the wound bed. Because post-traumatic healing is often complicated by infection, the objective of these studies was to develop antimicrobial nanosilver microgel composite PLPs to augment hemostasis, fight infection, and promote healing post-trauma. A key goal was to maintain particle deformability following silver incorporation to preserve PLP-mediated clot retraction. Clot retraction, antimicrobial activity, hemostasis after trauma, and healing after injury were evaluated via confocal microscopy, colony-forming unit assays, a murine liver trauma model, and a murine full-thickness injury model in the absence or presence of infection, respectively. We found that nanosilver incorporation does not affect base PLP performance while bestowing significant antimicrobial activity and enhancing infected wound healing outcomes. Therefore, Ag-PLPs have great promise for treating hemorrhage and improving healing following trauma.

Clot Structure and Implications for Bleeding and Thrombosis

The formation of a fibrin clot matrix plays a critical role in promoting hemostasis and wound healing. Fibrin dynamics can become disadvantageous in the formation of aberrant thrombus development. Structural characteristics of clots, such as fiber diameter, clot density, stiffness, or permeability, can determine overall clot integrity and functional characteristics that have implications on coagulation and fibrinolysis. This review examines known factors that contribute to changes in clot structure and the presence of structural clot changes in various disease states. These insights provide valuable information in forming therapeutic strategies for disease states where alterations in clot structure are observed. Additionally, the implications of structural changes in clot networks on bleeding and thrombus development in terms of disease states and clinical outcomes are also examined in this review.