Antiphospholipid antibodies after surgical exposure to topical bovine thrombin

Thrombin and Thrombin-Incorporated Biomaterials for Disease Treatments

Thrombin, a coagulation-inducing protease, has long been used in the hemostatic field. During the past decades, many other therapeutic uses of thrombin have been developed. For instance, burn treatment, pseudoaneurysm therapy, wound management, and tumor vascular infarction (or tumor vasculature blockade therapy) can all utilize the unique and powerful function of thrombin. Based on their therapeutic effects, many thrombin-associated products have been certificated by the Food and Drug Administration, including bovine thrombin, human thrombin, recombinant thrombin, fibrin glue, etc. Besides, several thrombin-based drugs are currently undergoing clinical trials. In this article, the therapeutic uses of thrombin (from the initial hemostasis to the latest cancer therapy), the commercially available drugs associated with thrombin, and the pros and cons of thrombin-based therapeutics (e.g., adverse immune responses related to bovine thrombin, thromboinflammation, and vasculogenic "rebounds") are summarized. Further, the current challenges and possible future research directions of thrombin-incorporated biomaterials and therapies are discussed. It is hoped that this review may provide a valuable reference for researchers in this field and help them to design safer and more effective thrombin-based drugs for fighting against various intractable diseases.

Pathophysiology of the Antiphospholipid Antibody Syndrome

The antiphospholipid syndrome is characterized by antibodies directed against phospholipid-binding proteins and phospholipids attached to cell membrane receptors, mitochondria, oxidized lipoproteins, and activated complement components. When antibodies bind to these complex antigens, cells are activated and the coagulation and complement cascades are triggered, culminating in thrombotic events and pregnancy morbidity that further define the syndrome. The phospholipid-binding proteins most often involved are annexins II and V, β2-glycoprotein I, prothrombin, and cardiolipin. A distinguishing feature of the antiphospholipid syndrome is the "lupus anticoagulant". This is not a single entity but rather a family of antibodies directed against complex antigens consisting of β2-glycoprotein I and/or prothrombin bound to an anionic phospholipid. Although these antibodies prolong in vitro clotting times by competing with clotting factors for phospholipid binding sites, they are not associated with clinical bleeding. Rather, they are thrombogenic because they augment thrombin production in vivo by concentrating prothrombin on phospholipid surfaces. Other antiphospholipid antibodies decrease the clot-inhibitory properties of the endothelium and enhance platelet adherence and aggregation. Some are atherogenic because they increase lipid peroxidation by reducing paraoxonase activity, and others impair fetal nutrition by diminishing placental antithrombotic and fibrinolytic activity. This plethora of destructive autoantibodies is currently managed with immunomodulatory agents, but new approaches to treatment might include vaccines against specific autoantigens, blocking the antibodies generated by exposure to cytoplasmic DNA, and selective targeting of aberrant B-cells to reduce or eliminate autoantibody production.

Platelet-rich preparations to improve healing. Part II: platelet activation and enrichment, leukocyte inclusion, and other selection criteria

Multiple platelet-rich preparations have been reported to improve wound and bone healing, such as platelet-rich plasma (PRP) and platelet rich fibrin (PRF). The different methods employed during their preparation are important, as they influence the quality of the product applied to a wound or surgical site. Besides the general protocol for preparing the platelet-rich product (discussed in Part 1 of this review), multiple choices need to be considered during its preparation. For example, activation of the platelets is required for the release and enmeshment of growth factors, but the method of activation may influence the resulting matrix, growth factor availability, and healing. Additionally, some methods enrich leukocytes as well as platelets, but others are designed to be leukocyte-poor. Leukocytes have many important roles in healing and their inclusion in PRP results in increased platelet concentrations. Platelet and growth factor enrichment reported for the different types of platelet-rich preparations are also compared. Generally, TGF-β1 and PDGF levels were higher in preparations that contain leukocytes compared to leukocyte-poor PRP. However, platelet concentration may be the most reliable criterion for comparing different preparations. These and other criteria are described to help guide dental and medical professionals, in large and small practices, in selecting the best procedures for their patients. The healing benefits of platelet-rich preparations along with the low risk and availability of simple preparation procedures should encourage more clinicians to incorporate platelet-rich products in their practice to accelerate healing, reduce adverse events, and improve patient outcomes.

Estimating the pathogen safety of manufactured human plasma products: application to fibrin sealants and to thrombin

BACKGROUND

Plasma fractionators have implemented many improvements over the past decade directed toward reducing the likelihood of pathogen transmission by purified blood products, yet little has been published attempting to assess the overall impact of these improvements on the probability of safety of the final product.

STUDY DESIGN AND METHODS

Safety margins for human immunodeficiency virus (HIV), hepatitis C virus (HCV), hepatitis B virus (HBV), hepatitis A virus (HAV), parvovirus B19, and variant form of Creutzfeldt-Jakob disease (vCJD) were calculated for the two fibrin sealants licensed in the United States and for thrombin. These products were selected because their use in a clinical setting is, in most cases, optional, and both were relatively recently approved for marketing by the US Food and Drug Administration (FDA). Moreover, thrombin and fibrinogen both undergo two dedicated virus inactivation steps and/or removal steps in accord with the recommendations of regulatory agencies worldwide. Safety margins were determined by comparing the potential maximum viral loads in contaminated units to viral clearance factors, ultimately leading to the calculation of the residual risk per vial.

RESULTS

The residual risk of pathogen transmission per vial was calculated to be less than 1 in 10(-15) for HIV, HCV, HBV, and HAV for both fibrinogen and thrombin. Owing to the greater quantities that can be present and its greater thermal stability, the calculated risk for parvovirus transmission was 1 in 500,000 vials for fibrinogen and less than 1 in 10(7) per vial for thrombin. Assuming that vCJD is found to be present in plasma donations, its risk of transmission by these purified and processed plasma derivatives would appear to be very low.

CONCLUSIONS

The pathogen safety initiatives implemented by plasma fractionators over the past 10 to 20 years have resulted in products with excellent pathogen safety profiles. Of the agents examined, parvovirus continues to have the lowest calculated margin of safety. Despite this, parvovirus transmissions should be rare. Manufacturers are encouraged to continue exploring processes to further enlarge parvovirus safety margins and to continue exploring ways of eliminating prions.

Topical bovine thrombin and adverse events: a review of the literature

OBJECTIVE

To review published evidence suggesting a link between topical bovine thrombin (TBT) and important adverse events (AEs).

RESEARCH DESIGN AND METHODS

English language articles and abstracts were obtained from MEDLINE using combinations of text and MeSH terms for thrombin, bovine thrombin and their trade names. References from summary articles were also retrieved. Published case reports, review articles, and retrospective, prospective or observational studies involving either immunogenicity or AEs were selected for further assessment. Retrieved articles were evaluated separately as AE case reports, quantitative studies of antibodies, or quantitative studies of AEs.

MAIN OUTCOME MEASURES

Presence of case causal information, temporal pattern of case report publication, reproducibility of aggregate data findings, and study design features.

RESULTS

The major limitations of reviewed publications were insufficient information regarding TBT and other exposures, and designs in which linkage between laboratory immune phenomena and AEs could not be evaluated. While immunogenicity studies did support an increased risk for post-TBT antibodies, there was no consistent evidence that this led to an increased AE risk or severity. Common evidentiary deficiencies included case reports from high incidence environments, studies of combination or mixture products, biased study designs, lack of patient-level exposure data, inadequate control groups and insufficient sample sizes. The best designed study (a randomized, controlled comparison of TBT to a recombinant bovine product) documented post-TBT antibody production, but no important efficacy or AE differences. An examination of publication dates for case reports showed a peak between 1992 and 1994 followed by a substantial drop. Since 1997 the number of published AE case reports has continued to decline.

CONCLUSIONS

TBT increases the risk for antibody elevations in patients. A careful review of published evidence does not show that either TBT itself or any associated elevations in anti-bovine antibodies are risk factors for clinically important AEs.

Preclinical safety of recombinant human thrombin

Recombinant human thrombin (rhThrombin) is being developed as an alternative to thrombin products purified from pooled human or bovine plasma, which are currently marketed for topical hemostasis. Preclinical studies of rhThrombin were conducted prior to its evaluation as a topical adjunct to surgical hemostasis in clinical trials. No overt clinical pathology or signs were observed in cynomolgus monkeys following implantation of a gelatin sponge containing either rhThrombin or bovine thrombin to a surgical liver wound, and similar gross and microscopic wound healing characteristics were observed over an eight-week recovery period with either compound. Repeated subcutaneous injections of rhThrombin or bovine thrombin to cynomolgus monkeys produced no treatment-related effects. Whereas no monkeys demonstrated anti-rhThrombin antibody seroconversion, specific anti-bovine antibodies were detected in all tested monkeys exposed to bovine thrombin. Addition of rhThrombin or bovine thrombin to mouse fibroblast cells resulted in expected detachment and shape change. Topical application of rhThrombin to rabbits did not cause irritation to the eye, normal skin, or abraded skin. These studies showed that topical, subcutaneous, or implanted rhThrombin was minimally immunogenic, safe, and well tolerated in nonclinical models, and supported the clinical evaluation of rhThrombin in a variety of surgical settings.

Hypercoagulable states and antithrombotic strategies in recurrent vascular access site thrombosis

Vascular access site thrombosis is a major cause of morbidity in patients receiving hemodialysis. The role of hypercoagulable states in recurrent vascular access site thrombosis remains poorly understood. Data are limited regarding systemic anticoagulation to improve access graft patency, because of concern about hemorrhagic complications. We determined the prevalence of hypercoagulable states and clinical outcome (thrombotic and hemorrhagic) after initiation of antithrombotic therapy in a series of patients with recurrent vascular access site thrombosis. We evaluated 31 patients who had sustained 119 thrombotic events that resulted in vascular access graft failure during the year before evaluation. Sixty-eight percent of patients tested had elevated concentrations of antibody to anticardiolipin or topical bovine thrombin, and 18% of patients tested had heparin-induced antibodies. More than 90% of patients had elevated factor VIII concentration, 62% had elevated fibrinogen concentrations, and 42% had elevated C-reactive protein concentrations. Twenty-nine patients were given antithrombotic therapy: 13 with warfarin sodium, 12 with unfractionated heparin (UFH), and 11 with low molecular weight heparin (LMWH). Seven patients received more than one antithrombotic agent, sequentially. Nineteen patients have had no thrombotic events since beginning antithrombotic therapy (10 with warfarin, 3 with UFH, 6 with LMWH). Mean follow-up was 8.6 months (median, 7 months). Eight patients sustained 10 bleeding complications (5 with warfarin, 3 with UFH, and 2 with LMWH). In conclusion, hypercoagulable states are common in patients with recurrent vascular access site thrombosis. Antithrombotic therapy may increase vascular access graft patency, but is associated with significant risk for hemorrhage. Prospective studies are needed to evaluate the role and safety of antithrombotic agents in improving vascular access graft patency.