Preclinical safety of recombinant human thrombin

Current status of hemostatic agents, their mechanism of action, and future directions

The bleeding problem might seem straightforward, but it involves a plethora of complex biochemical pathways and responses. Hemorrhage control remains one of the leading causes of “preventable deaths” worldwide. The past few decades have seen a wide range of biomaterials and their derivatives targeted to serve as hemostatic agents, but none can be deemed as an ideal solution. In this review, we have highlighted the current diversity in hemostatic agents and their modalities. We have enclosed a comprehensive outlook of the proposed solutions and their clinical performance so far. In addition to these, several promising compositions are still in their infancy or developmental phases. The inclusion of novel upcoming nanocomposites has further widened the potencies of existing formulations as well.

Results of a randomised controlled trial between an ORC collagen hemostatic agent and a carrier-bound fibrin sealant

INTRODUCTION

Despite ongoing optimisation of surgical techniques, hemostasis continues to be a fundamental challenge in many operations today. This randomised controlled trial compared the efficacy of a new hemostatic agent made of oxidised regenerated cellulose and collagen (ORC-Coll) with that of a conventional carrier-bound fibrin sealant (CBFS).

METHODS

Hemostasis was investigated representatively in the case of post-thyroidectomy bleeding from the resection surface. To demonstrate that ORC-Coll (mediCipio® A) has at least the same hemostatic efficacy as CBFS (Tachosil®), the volume of drainage fluid at the time of drain removal was used as the primary endpoint in a non-inferiority test with a significance level of 5%. The secondary endpoints included number and size of hemostatic agents required, adhesion of the hemostatic agent to the bleeding surface, intraoperative hemostatic effect, duration of drainage and adverse events during a six-month follow-up period.

RESULTS

A total of 150 patients (ORC-Coll: 75; CBFS: 75) were included. After operation, total volume of drainage fluid was 68.20±44.56mL in the ORC group and 68.21±40.20mL in the CBFS group. The non-inferiority of ORC-Coll compared to CBFS with regard to hemostatic efficacy was shown at a significance level of 5%. The results demonstrated effectiveness in achieving hemostasis without adverse events.

CONCLUSIONS

ORC-Coll is an effective hemostatic agent and barrier sealant without blood components, which ensures reliable prevention of intra- and postoperative bleeding. With use of the new technique, any risks associated with the use of human blood components are a priori eliminated.

Self-Propelled Dressings Containing Thrombin and Tranexamic Acid Improve Short-Term Survival in a Swine Model of Lethal Junctional Hemorrhage

Hemorrhage is the leading cause of preventable death in trauma, and hemorrhage from noncompressible junctional anatomic sites is particularly difficult to control. The current standard is QuikClot Combat Gauze packing, which requires 3 min of compression. We have created a novel dressing with calcium carbonate microparticles that can disperse and self-propel upstream against flowing blood. We loaded these microparticles with thrombin and tranexamic acid and tested their efficacy in a swine arterial bleeding model without wound compression. Anesthetized immature female swine received 5 mm femoral arteriotomies to induce severe junctional hemorrhage. Wounds were packed with kaolin-based QuikClot Combat Gauze (KG), propelled thrombin-microparticles with protonated tranexamic acid (PTG), or a non-propelling formulation of the same thrombin-microparticles with non-protonated tranexamic acid (NPTG). Wounds were not compressed after packing. Each animal then received one 15 mL/kg bolus of hydroxyethyl starch solution followed by Lactated Ringer as needed for hypotension (maximum: 100 mL/kg) for up to 3 h. Survival was improved with PTG (3-h survival: 8/8, 100%) compared with KG (3/8, 37.5%) and NPTG (2/8, 25%) (P <0.01). PTG animals maintained lower serum lactate and higher hemoglobin concentrations than NPTG (P <0.05) suggesting PTG decreased severity of subsequent hemorrhagic shock. However, total blood loss, Lactated Ringer infusion volumes, and mean arterial pressures of surviving animals were not different between groups (P >0.05). Thus, in this swine model of junctional arterial hemorrhage, gauze with self-propelled, prothrombotic microparticles improved survival and 2 indicators of hemorrhagic shock when applied without compression, suggesting this capability may enable better treatment of non-compressible junctional wounds.

Optimization of expression of untagged and histidine-tagged human recombinant thrombin precursors in Escherichia coli

The present study is focused on preparation of proper Escherichia coli expression system to ensure high yields of various modified precursors of human recombinant thrombin, a potential biopharmaceutical reagent. Two thrombin precursors, the smallest single-chain α-thrombin precursor prethrombin-2 and its shortened form prethrombin-2∆13, and their His-tagged forms were used. In order to determine the effect of the different lengths and amino acid compositions of affinity His-tag on the target protein expression level, a variety of the His-tag sequences were used. We found out that the protein expression efficiency was closely related to the codons used for encoding of amino acids of fusion histidine tag. Optimization of culture medium composition is another way to increase yield of the target protein. Suitable medium composition can ensure cell growth to high densities which is related to total yield of expressed protein. In this study, a new optimized complex medium for batch fermentation was developed. Addition of nutrients like a yeast extract and enzymatic casein hydrolysate to the defined medium components had a positive impact on protein expression, where relatively high expression level of the target protein from total amount of cellular proteins was achieved. Further, we have focused on trace element solution composition, and the optimized nickel and selenium concentrations were determined. Our results show that the composition of essential trace metal solution has a major impact not only on expression level, but it can also affect cell growth rate.

Autoactivation of thrombin precursors

Trypsin-like proteases are synthesized as inactive zymogens and convert to the mature form upon activation by specific enzymes, often assisted by cofactors. Central to this paradigm is that the zymogen does not convert spontaneously to the mature enzyme, which in turn does not feed back to activate its zymogen form. In the blood, the zymogens prothrombin and prethrombin-2 require the prothrombinase complex to be converted to the mature protease thrombin, which is unable to activate prothrombin or prethrombin-2. Here, we show that replacement of key residues within the activation domain causes these zymogens to spontaneously convert to thrombin. The conversion is started by the zymogen itself, which is capable of binding ligands at the active site, and is abrogated by inactivation of the catalytic residue Ser-195. The product of autoactivation is functionally and structurally equivalent to wild-type thrombin. Zymogen autoactivation is explained by conformational selection, a basic property of the trypsin fold uncovered by structural and rapid kinetics studies. Both the zymogen and protease undergo a pre-existing equilibrium between active and inactive forms. The equilibrium regulates catalytic activity in the protease and has the potential to unleash activity in the zymogen to produce autoactivation. A new strategy emerges for the facile production of enzymes through zymogen autoactivation that is broadly applicable to trypsin-like proteases of biotechnological and clinical interest.

Quantification of prothrombin in human plasma amplified by autocatalytic reaction

By site directed mutagenesis, we have produced recombinant mutants of human and mouse prethrombin-2 which are able to convert themselves autocatalytically into α-thrombin. We also have created a new method to amplify the signal of bioanalytical assays based on the autocatalytic activation of these mutated proenzymes. The activation of the mutants by active α-thrombin triggers an autocatalytic reaction which leads to more active thrombin resulting in the amplification of the readout signal. Addition of mutated mouse prethrombin-2 into the conventional assay for prothrombin level in human plasma, employing ecarin and the fluorogenic substrate, resulted in improvement of the detection limit by 2 orders of magnitude.

Cross-Reactivity of Various Thrombin Products With Anti-Rabbit Antibodies to Bovine, Human, and Recombinant Thrombin

JMI-thrombin is used as topical hemostatic agent. While earlier clinically available JMI were reported to produce immunologic responses upon repeated exposure, the improved JMI, Recothrom?, and Evithrom? are claimed to be less immunogenic. Recothrom, despite its reduced immunogenic nature, upon repeated administration may result in the generation of antibodies (Abs) and that may cross react with bovine and human thrombin. Therefore, groups of rabbits were challenged repeatedly with Recothrom, Evithrom, and JMI over a 9-month period. Pre-immune blood and antiserum were collected from each rabbit on different time point. To determine their relative cross reactivity, JMI, Recothrom, and Evithrom were evaluated by western blotting using the rabbit IgG fractions. The results suggest that anti-Recothrom Abs cross-react with Evithrom and JMI in a time dependent fashion. Anti-JMI Abs did not cross-react with Recothrom, and Evithrom. Also, anti-Evithrom did not show any cross-reactivity with Recothrom and JMI at any time.

Clinical use of topical thrombin as a surgical hemostat

When surgical ligation of bleeding fails, or is not possible, surgeons rely on a number of hemostatic aids, including thrombin. This review discusses the history, pharmacology and clinical application of thrombin as a surgical hemostat. The initial thrombin was bovine in origin, but its use has been complicated by the formation of antibodies that cross react with human coagulation factors. This has been associated with life threatening bleeding and in some circumstances anaphylaxis and death. Human thrombin, isolated from pooled plasma of donors, has been developed in an effort to minimize these risks, but its downside is the potential of transmitting blood-borne pathogens and limited availability. Recently a recombinant thrombin has been developed and approved for use by the FDA. It has the advantage of being minimally antigenic and devoid of the risk if viral transmission. Thrombin is often used in conjunction with other hemostatic aids, including absorbable agents (like gelfoam, collagen, and cellulose), and with fibrinogen in fibrin glues. The last part of this review will discuss these agents in detail, and review their clinical applications.

Cross-reactivity of rabbit anti-bovine prothrombin/thrombin IgGs with bovine factor V/Va-related antigens

The purpose of this study was to determine whether rabbit anti-bovine prothrombin/thrombin immunoglobulin Gs (IgGs) would cross-react with bovine factor V/Va-related antigens. Bovine prothrombin, crude thrombin, as well as 2 purified versions of thrombin, that is, thrombin 4A (the previous version of Thrombin-JMI marketed prior to 2008) and 4B (the currently marketed version of Thrombin-JMI), were administrated to individual groups of rabbits on days 0, 21, 42, 91, 123, and 151 using standard immunologic methods. Blood was drawn from each rabbit on days 30, 50, 105, 137, and 165 and the pooled antisera from individual groups were purified to obtain the IgGs using protein G affinity columns. By probing bovine factor V/Va samples, the possible cross-reactivity of each IgG collected at different time points (from day 30 to day 165) was explored using Western blotting techniques. The results indicated that rabbit anti-bovine prothrombin and crude thrombin IgGs could cross-react strongly with bovine factor V/Va in an immunization time-dependent manner. However, antibodies generated in thrombin 4A-treated rabbits presented much weaker cross-reactivity with bovine factor V/Va. Furthermore, no cross-reactivity with bovine factor V/Va-related antigens was observed when the anti-bovine thrombin 4B IgG collected at any time point was used. The results suggest that thrombin 4B preparation contains the least bovine factor V/Va contaminants among the bovine prothrombin/thrombin preparations studied and the amount of bovine factor V/Va contaminants in bovine thrombin 4B is too small to elicit the generation of antibodies against bovine factor V/Va in rabbits.

Thrombin use in surgery: an evidence-based review of its clinical use

When surgical ligation of bleeding fails, or is not possible, surgeons rely on a number of hemostatic aids, including thrombin. This review discusses the history, pharmacology and clinical application of thrombin as a surgical hemostat. The initial thrombin was bovine in origin, but its use has been complicated by the formation of antibodies that cross-react with human coagulation factors. This has been associated with life-threatening bleeding and in some circumstances anaphylaxis and death. Human thrombin, isolated from pooled plasma of donors, was developed in an effort to minimize these risks, but its downsides are its limited availability and the potential for transmitting blood-borne pathogens. Recently a recombinant thrombin has been developed, and approved for use by the FDA. It has the advantage of being minimally antigenic and devoid of the risk of viral transmission. Thrombin is often used in conjunction with other hemostatic aids, including absorbable agents such as Gelfoam, and with fibrinogen in fibrin glues. The last part of this review will discuss these agents in detail, and review their clinical applications.

Cross-reactivity of rabbit anti-bovine thrombin IgGs with human alpha-thrombin and a recombinant version of human thrombin (Recothrom)

It has been reported that patients exposed to topical bovine thrombin preparations may develop antibodies against bovine thrombin, factr V or various other proteins found in these preparations. Such antibodies can cross-react with human endogenous coagulation proteins and may lead to alterations in the coagulation laboratory parameters, hypersensitivity reactions, and severe bleeding or thrombosis. The underlying mechanisms for the coagulopathy are not fully understood yet. To better understand the cross-reactivity of anti-bovine thrombin antibodies with human corresponding coagulation proteins, bovine crude thrombin, and its purified versions, thrombin 4A (the old version of Thrombin-JMI before year 2008) and thrombin 4B (the current version of Thrombin-JMI on market), were used to generate relevant anti-bovine thrombin immunoglobulin G (IgGs) in rabbits. Using Western blotting, the cross-reactivity of each IgG with human alpha-thrombin and a recombinant version of human thrombin (Recothrom) was investigated. The results indicated that no cross-reactivity with either human alpha-thrombin or Recothrom was observed with both anti-bovine crude thrombin IgGs and thrombin 4B IgGs. However, anti-bovine thrombin 4A IgGs showed apparent cross-reactivity with human alpha-thrombin and Recothrom in a protein amount-dependent manner. Furthermore, the results revealed that the cross-reactivity of anti-bovine thrombin 4A IgGs with human alpha-thrombin and Recothrom was immunization time-dependent. The minimum concentration of 4A IgG required to exhibit cross-reactivity with human alpha-thrombin and Recothrom varied considerably among individual rabbits. These results indicate that rabbit anti-bovine thrombin IgGs can cross-react with human alpha-thrombin and Recothrom, suggesting that human antibodies against bovine thrombin may also cross-react with human recombinant thrombin. Thus, the patients who were previously exposed to bovine thrombin may also develop antibodies which can cross-react with human recombinant thrombin.

Topical recombinant thrombin at a concentration of 1000 IU/mL reliably shortens in vivo TTH and delivers durable hemostasis in the presence of heparin anticoagulation and clopidogrel platelet inhibition in a rabbit model of vascular bleeding

Background

This study was designed to evaluate the effect of recombinant human thrombin (rThrombin) concentration on time to hemostasis (TTH), clot durability, and clot strength in settings that replicate the heparinization and platelet inhibition often found in surgical populations.

Methods

A modified, anticoagulated rabbit arteriovenous shunt preparation was selected to model vascular anastomotic bleeding. Rabbits were treated with heparin or heparin + clopidogrel and TTH was measured after applying a range of topical rThrombin concentrations or placebo, in combination with absorbable gelatin sponge, USP. Treatments (placebo, rThrombin) were randomly assigned and the investigator was blinded to treatment. TTH was evaluated with the Kaplan-Meier method. After hemostasis was achieved, clot burst assessment was performed for heparin + clopidogrel treated animals. Clot viscoelastic strength and kinetics were measured in ex-vivo samples using thromboelastography (TEG) methods.

Results

TTH decreased with increasing concentrations of rThrombin in heparin-treated animals and was shorter after treatment with 1000 IU/mL rThrombin (73 seconds) than with 125 IU/mL rThrombin (78 seconds; p = 0.007). TTH also decreased with increasing concentrations of rThrombin in heparin + clopidogrel treated animals; again it was significantly shorter after treatment with 1000 IU/mL rThrombin (71 seconds) than with 125 IU/mL rThrombin (177 seconds; p < 0.001). Variability in TTH was significantly smaller after treatment with 1000 IU/mL rThrombin than after 125 IU/mL rThrombin, indicating greater reliability of clot formation (p < 0.001 for heparin or heparin + clopidogrel treatments). Clot durability was examined in heparin + clopidogrel treated animals. Clots formed in the presence of 1000 IU/mL rThrombin were significantly less likely to rupture during clot burst assessment than those formed in the presence of 125 IU/mL rThrombin (0% versus 79%, p < 0.001). In vitro clot strength and clot kinetics, as determined by TEG in heparin + clopidogrel samples, were positively associated with the amount of rThrombin activity added for clot initiation.

Conclusion

In an animal model designed to replicate the anti-coagulation regimens encountered in clinical settings, topical rThrombin at 1000 IU/mL more reliably controlled the pharmacological effects of heparin or heparin + clopidogrel on hemostasis than rThrombin at 125 IU/mL. Results from in vitro assessments confirmed a positive relationship between the amount of rThrombin activity and both the rate of clot formation and clot strength.

Adipose tissue mesenchymal stem cell expansion in animal serum-free medium supplemented with autologous human platelet lysate

BACKGROUND

Mesenchymal stem cells (MSCs) have been considered for human regenerative therapy applications, and safe culture and expansion protocols are needed especially in the context of interspecies contamination. Human platelet lysate (PL) has been proposed as animal serum substitute during in vitro MSC expansion. In this work, a simplified and efficient method to obtain autologous PL to replace animal serum in cell culture applications is described.

STUDY DESIGN AND METHODS

PL obtained by freezing and centrifugation procedures was tested as medium supplement for human adipose mesenchymal stem cell (hASC) culture. Differential proliferation, immunophenotypic changes, and differentiation under PL or fetal bovine serum (FBS) were assessed.

RESULTS

In contrast to 10% FBS supplementation, cell population doubling time was significantly lower when hASCs were cultured with the same concentration of PL (PL 22.9 +/- 1.5 hr vs. FBS 106.7 +/- 6.5 hr, t test, p < 0.05). Furthermore, hASCs maintained with 2.5% PL supplementation also showed satisfactory results. Immunophenotypic analysis revealed no differences between hASCs cultivated with PL or FBS supplementation and both cultures retained the potential to differentiate into adipose cells. These results demonstrate that autologous PL obtained from the same donor can be used as animal serum substitute in hASC culture.

CONCLUSIONS

Taken together, evidence is provided that platelets provided by a single donor are sufficient to obtain PL for hASC propagation for clinical-scale applications mitigating the potential untoward side effects associated with the use of animal-derived reagents.

Immunogenicity of biologically-derived therapeutics: assessment and interpretation of nonclinical safety studies

An evaluation of potential antibody formation to biologic therapeutics during the course of nonclinical safety studies and its impact on the toxicity profile is expected under current regulatory guidance and is accepted standard practice. However, approaches for incorporating this information in the interpretation of nonclinical safety studies are not clearly established. Described here are the immunological basis of anti-drug antibody formation to biopharmaceuticals (immunogenicity) in laboratory animals, and approaches for generating and interpreting immunogenicity data from nonclinical safety studies of biotechnology-derived therapeutics to support their progression to clinical evaluation. We subscribe that immunogenicity testing strategies should be adapted to the specific needs of each therapeutic development program, and data generated from such analyses should be integrated with available clinical and anatomic pathology, pharmacokinetic, and pharmacodynamic data to properly interpret nonclinical studies.

Thrombin, Topical (Recombinant)

Each month, subscribers to The Formulary Monograph Service receive 5 to 6 well-documented monographs on drugs that are newly released or are in late phase 3 trials. The monographs are targeted to your Pharmacy & Therapeutics Committee. Subscribers also receive monthly 1-page summary monographs on the agents that are useful for agendas and pharmacy/nursing inservices. A comprehensive target drug utilization evaluation (DUE) is also provided each month. With a subscription, the monographs are sent to you in print and are also available online. Monographs can be customized to meet the needs of your facility. Subscribers to the The Formulary Monograph Service also receive access to a pharmacy bulletin board, The Formulary Information Exchange (The F.I.X.). All topics pertinent to clinical and hospital pharmacy are discussed on The F.I.X.

Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. If you would like information about The Formulary Monograph Service or The F.I.X., call The Formulary at 800-322-4349. The July 2008 monograph topics are on subcutaneous methylnaltrexone bromide, certolizumab pegol, regadenoson injection, olopatadine hydrochloride nasal spray, and triamcinolone acetonide injectable suspension. The DUE is on subcutaneous methylnaltrexone bromide.

A comparison of recombinant thrombin to bovine thrombin as a hemostatic ancillary in patients undergoing peripheral arterial bypass and arteriovenous graft procedures

OBJECTIVES

Recombinant thrombin (rThrombin) is a potential hemostatic alternative to bovine and human plasma-derived thrombin. This report examines the clinical results for the vascular surgery subgroup of patients enrolled in a larger double-blind, randomized, multicenter trial, which evaluated the comparative safety and efficacy of rThrombin and bovine plasma-derived thrombin (bThrombin) when used as adjuncts to surgical hemostasis.

METHODS

Data from the 164 vascular patients who underwent either a peripheral arterial bypass (PAB) or arteriovenous graft (AV) procedure are included in this analysis. Time to hemostasis at proximal and distal anastomotic sites at 1.5-, 3-, 6-, and 10-minute intervals was determined by procedure (PAB or AV) and overall (PAB + AV). Baseline and day 29 immunologic sera were analyzed. The incidences of postoperative adverse events were compared between treatment groups. Categorical adverse events were evaluated in relation to thrombin product antibody formation.

RESULTS

Patients were randomized to either bThrombin (n = 82) or rThrombin (n = 82). Procedures included PAB (n = 88) and AV (n = 76). The bThrombin and rThrombin groups were well matched for demographics and baseline characteristics. A comparable incidence of anastomotic hemostasis was observed in both treatment groups at 10 minutes (94% bThrombin, 91% rThrombin). The incidence of hemostasis was lower at all time points for PAB procedures compared with AV procedures. In the PAB group, a significantly greater proportion of patients receiving rThrombin (55%) achieved hemostasis at 3 minutes compared with bThrombin (39%; P < .05). Adverse event profiles and laboratory findings were similar between groups. No patients in the rThrombin group developed anti-rThrombin product antibodies at day 29, whereas 27% of patients in the bThrombin group developed antibodies to bThrombin product (P < .0001).

CONCLUSIONS

rThrombin or bThrombin used as a hemostatic ancillary for anastomotic bleeding was equally effective at 10 minutes; however, rThrombin compared with bThrombin may provide a more rapid onset of hemostasis at 3 minutes in PAB procedures. Adverse events were similar between the two thrombins. In patients undergoing vascular surgery, both treatments were similarly well tolerated, although rThrombin demonstrated a superior immunogenicity profile.

Pharmacologic Interventions for the Management of Critical Bleeding

Hospital pharmacists are often consulted for their knowledge about coagulation and therapeutic interventions for the management of critical bleeding. Many pharmacotherapies are available for this purpose, both systemic and topical, and others are in development. These agents and their mechanisms of action are reviewed, and perspectives are provided regarding their use in various clinical settings. Also provided are associated precautions to promote safe use. Current controversies surrounding pharmacotherapeutic agents used to control serious bleeding (e.g., in various types of surgery, trauma, obstetrics, and intracranial hemorrhage) are also discussed.