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Overview of efficacy and safety of available topical thrombin products

Achieving rapid and effective hemostasis is a key component to any surgical procedure.^1,2 Typically, surgical ligation of larger vessels or electrocautery of smaller vessels is used to control excess vascular bleeding that may occur during surgery; however, these methods may not always be feasible or possible. Electrocautery can result in excessive postoperative drainage while surgical ligation is restricted to accessible vessels of an acceptable minimum size. As a result, during various surgical situations, topical thrombin may be administered to aid in the control of hemostasis. This paper will explore and analyze the 3 available formulations—bovine thrombin (Thrombin-JMI), human thrombin (Evithrom), and recombinant thrombin (Recothrom)—in terms of efficacy and safety.

Mechanism of action of thrombin
Thrombin plays a crucial role in coagulation; it is directly responsible for the generation of fibrin, a glycoprotein that forms the mesh-like insoluble matrix of a blood clot.^3,4 After vascular injury, thrombin acts as the gateway protease through the tissue factor pathway, or extrinsic pathway, to the final coagulation pathway. Within the final coagulation pathway, thrombin converts inactive fibrinogen into active fibrin, the key building block in the formation of a hemostatic clot. Additionally, thrombin activates antihemophilic factor (Factor VIII), proaccelerin (Factor V), and a coagulation inhibitor, protein C. As a result, thrombin plays a vital role in effective and timely clot formation.

Topical thrombin products
All topical thrombins are indicated as an aid to hemostasis whenever minor bleeding from capillaries or small venules is accessible and whenever control of bleeding by standard surgical techniques is ineffective or not possible.^5-7 Systemic injection of topical thrombin is contraindicated as this can result in the formation of a fatal thrombus in circulatory vessels resulting in death. Because of this risk, it is recommended that topical thrombin application strictly follow the manufacturers’ instructions for proper use.

The manufacturing processes used for the 3 topical thrombin products are summarized in the Table.^5-7 Thrombin-JMI is the only bovine-derived thrombin commercially available and is highly purified.⁵ Historically, other formulations of bovine thrombin (Thrombostat and Thrombogen) were also available to control hemostasis; however, these formulations contained increased levels of bovine factor V contaminants resulting in a cross-reactive immune response and subsequent coagulopathy.^8-10 Thrombin-JMI was recently improved to be highly purified through the implementation of a more rigorous viral filtration process.^5,9 As a result, in March 2008, the Food and Drug Administration (FDA) approved updated package labeling for Thrombin-JMI to reflect the enhanced purification process and its capacity to remove extraneous proteins.^5,11,12 Nonetheless, Thrombin-JMI is still composed of bovine proteins and carries the rare risk of antibody development with anaphylactic consequences, alterations in prothrombin time (PT) and partial thromboplastin time (PTT), or Factor V deficiency.^5,8,13 The package labeling for Thrombin-JMI includes a boxed warning regarding this risk.⁵ Patients should avoid this product if they are allergic to any bovine or bovine-derived products.

Topical human thrombin (Evithrom) is a human plasma-derived product; therefore, like other human plasma-derived products, it carries a small risk of transmitting blood-borne pathogens.⁶ However, human plasma used for the production of human thrombin is screened for blood-borne pathogens and the final thrombin product undergoes viral inactivation treatment (solvent-detergent and nanofiltration) to ensure product safety. Adverse events that may occur with topical human thrombin include elevations in international normalized ratio (INR), decreased lymphocyte and neutrophil levels, and increased PT and PTT times.

Topical recombinant thrombin (Recothrom) was developed to serve as an alternative to bovine and human topical thrombin.⁷ It is derived from recombinant DNA human protein sequences, not human or bovine plasma sources.^7,14 As a result, it is thought that topical recombinant thrombin avoids the antibody mediated response associated with topical bovine thrombin and the viral transmission risk associated with topical human thrombin. However, precautions must be taken if an individual has had a previous hypersensitivity reaction to snake or hamster proteins since these proteins are used in the formation of topical recombinant thrombin.⁷

Table. Manufacturing processes of the topical thrombin products.^5-7

Comparative efficacy of topical thrombins
Two recent studies have been published that have evaluated the efficacy of bovine-derived thrombin to recombinant- or human-derived. Chapman and colleagues found that the use of topical recombinant thrombin was non-inferior to bovine thrombin in achieving adequate surgical hemostasis.¹⁵ In this phase 3, double-blind, comparative study, 411 patients were randomized to treatment with either topical bovine thrombin (Thrombin-JMI) or topical recombinant thrombin (Recothrom) while undergoing either liver resection, spine, peripheral arterial bypass, or dialysis access surgery. Pertinent inclusion criteria were patients over the age of 18 and no known hypersensitivity to any component of topical bovine or recombinant thrombin. Pertinent exclusion criteria were known use of any study agent or use of any blood product in the last 24 hours. The primary endpoint was the incidence of hemostasis within the first 10 minutes after application of the blinded study drug to 1 of 4 prespecified bleeding sites. A total of 198 patients in the recombinant thrombin group and 203 patients in the bovine thrombin group completed the study. The primary endpoint of incidence of hemostasis in the first 10 minutes after application of study drug was found in 95.4% of patients in the recombinant group and in 95.1% of patients in the bovine group. The adjusted lower limit of the 95% confidence interval (CI) was -3.73%, within the prespecified non-inferiority margin of -15%, indicating non-inferiority of the recombinant thrombin to the bovine product.

Doria and colleagues found that the use of topical human thrombin was equally efficacious to topical bovine thrombin at achieving hemostasis at 3, 6, and 10 minutes.¹⁶ In this phase 3, multicenter, double-blind study, 305 patients were randomized to receive either topical human thrombin (Evithrom) or topical bovine thrombin (Thrombin-JMI) during predetermined cardiovascular, neurological, or general surgical procedures where adequate hemodynamic control could neither be established nor maintained through conventional methods. Patients included in the study were over the age of 18, scheduled for one of the predetermined surgical procedures, and had at least 1 mild to moderate bleeding site within the operative field unable to be managed by conventional methods. Pertinent exclusion criteria consisted of pathological hemostasis disorders, concurrent anticoagulant therapy within 5 days of surgery, and known antibodies to bovine thrombin preparations. The primary endpoint was successful hemostasis within 10 minutes of study drug application. Secondary endpoints were successful hemostasis at 3 and 6 minutes after study drug application and incidence of adverse events. After 10 minutes of application, hemostasis was reported in 97.4% (149/153) of patients who received topical human thrombin and 97.3% (148/152) of patients given topical bovine thrombin, indicating comparable efficacy (ratio human:bovine successes of 1, 95% CI, 0.96 to 1.05 [equivalence was based on a prespecified CI of 0.80 to 1.25]). Similar results were found between human and bovine topical thrombin for achieving hemostasis at 3 minutes (73.2% vs. 72.4%; ratio 1.01, CI 0.88-1.16) and at 6 minutes (94.8% vs. 92.8%; ratio 1.02, CI 0.96-1.09). No difference was seen in median blood loss between the 2 treatment groups, nor in the percentage of patients requiring blood transfusions (22% and 26% for human and bovine thrombins, respectively).

Comparative safety of topical thrombins
In addition to evaluating the efficacy of the topical thrombins, the 2 available comparative trials also assessed the safety of the topical thrombins. Chapman and colleagues concluded in the previously described phase 3 study that both topical recombinant and bovine thrombin maintained a similar safety profile as well.¹⁵ Safety endpoints included the incidence and severity of adverse events, clinical laboratory abnormalities, and the development of post-exposure antibodies. The frequency of adverse events, including bleeding (13% vs. 11%), hypersensitivity reactions (14% vs. 17%), nausea (35% vs. 28%), postoperative wound infections (9% vs. 10%), cardiac events (20% vs. 18%), and thromboembolic events (6% vs. 5%) were similar between recombinant and bovine thrombin within 30 days of application. However, antiproduct antibodies were detected (either seroconversion or a  1 unit change from baseline) in 1.5% of recombinant thrombin patients (3/198) versus 21.5% of patients (43/200) who received bovine thrombin. For bovine thrombin, 5% of patients had antiproduct antibodies detected at baseline (prior to surgery); 80% of these patients experienced an increase in antibody titers of 1 or more units. In contrast, none of the patients in the recombinant thrombin group with baseline antibodies (1.5%) experienced an increase in antibody titers. A post-hoc analysis in patients who developed antiproduct antibodies revealed that patients with newly formed bovine antibodies had an increased incidence of bleeding, thromboembolic events, hypersensitivity, and abnormal laboratory parameters (however, no statistical analyses were conducted).

Doria and colleagues also reported that human and bovine thrombins had similar safety profiles, with comparable rates and severity of adverse events.¹⁶ Among human and bovine thrombin treatment arms, nausea (41% vs. 32%), blood disorders (25.5% vs. 18.4%), and administration site disorders (34% vs. 31.6%) were the most common adverse events with similar event rates between treatment groups. The authors also found that patients who were treated with bovine thrombin were more likely to develop antibodies than those treated with human thrombin (12.70% vs. 3.28%, 95% CI, 0.09 to 0.71; p=0.01); however, antibody formation was not linked to any increase in adverse event incidence. Four different antibody assays were conducted: any antibody, anti-bovine thrombin, anti-bovine factor V/Va, anti-human thrombin, and anti-human factor V/Va. Anti-bovine thrombin antibodies were seen in 2.46% of patients given human thrombin and in 7.94% of patients in the bovine thrombin group (p=0.053). The incidence of anti-bovine factor V/Va antibodies was also higher with bovine thrombin exposure (9.52% vs. 1.64% for human thrombin; p=0.01). Two patients in the human thrombin group who seroconverted for anti-bovine antibodies had been previously exposed to bovine thrombin. No patients in the human thrombin group developed antibodies to human thrombin, compared with 2.38% of the bovine group (p=0.09). And no patients in either group developed antibodies to human factor V/Va.

Antibody development to bovine thrombin and risk of bleeding
The development of antibodies to bovine-derived topical thrombin has been well documented in the literature.^17,18 However, these data are primarily in the form of case reports, following exposure to older, less pure bovine thrombins. To quantify the effects of bovine thrombin on antibody development, Ortel and colleagues conducted a prospective study on patients undergoing cardiovascular surgeries.¹⁸ A total of 151 patients were evaluated before and after exposure to bovine thrombin (Thrombogen) for immune response and coagulation abnormalities. At 4 to 8 weeks after surgery, 90% of patients exposed to bovine thrombin developed IgG antibodies (regardless of the type of surgery), primarily to bovine factor V or factor Va (80% and 90%, respectively). Elevations were also seen in IgM antibodies, increased in about 50% of patients exposed. In addition, about half of patients had increases in antibodies to human coagulation proteins, including factor V, thrombin, or both, with no effect of dose or past surgical history; nearly all of these patients had antibodies to the corresponding bovine proteins. However, for adverse events (including hemorrhagic and nonhemorrhagic), only elevation in antibodies to 2 or more bovine proteins prior to surgery (at baseline) was found to increase the risk for an adverse event (p=0.0042). The authors concluded that bovine thrombin preparations are immunogenic and may predispose patients to increased postoperative adverse events with re-exposure.

In contrast to the findings by Ortel, a recent systematic review by Reynolds concluded that the risk of abnormal bleeding was not significantly different between procedures using topical bovine thrombin and procedures without topical thrombin.¹⁰ A total of 21 clinical trials were identified that addressed the risk of bleeding in surgery. In 16 of the trials (88,058 patients in 10 retrospective and 6 prospective designs), patients had no exposure to bovine thrombin. Five trials (with 1012 patients in 1 retrospective and 4 prospective designs) evaluated the risks of bleeding with topical bovine thrombin. Two of these studies identified the bovine thrombin product used as the currently available Thrombin-JMI. The authors reported bleeding rates with bovine thrombin to range between 0% to 11%, with most studies reporting rates between 2% and 3%. In comparison, for the 16 studies without exposure to bovine thrombin, bleeding rates ranged from 0.1% to 20.2%, with most reported rates between 2.6% to 4%. However, it is important to note that the authors did not conduct any statistical analyses on pooled data to determine if there were any significant differences in these bleeding rates.

To date, only 1 case report is available describing coagulation abnormalities following the use of the bovine thrombin Thrombin-JMI.⁸ In this report, a 76-year-old male underwent a five-vessel coronary artery bypass graft (CABG) procedure, during which a surgical sealant containing bovine thrombin was used. On postoperative day 7, elevations in both PT and PTT were seen, which remained high for the next 10 days. The maximum PT INR was 2.65, and the maximum PTT was 65 seconds. Both of these parameters returned to normal by postoperative day 65, and the patient did not experience any clinical bleeding events. Based on postoperative day 12 plasma samples, high levels of both anti-bovine factor Va and anti-human factor V antibodies (IgG) were found, along with a decrease in human factor V activity (9.2 units; normal range 79 to 133). These assays were later found to be normal (by postoperative day 149). Based on the patient’s surgical history (4 procedures), the patient was likely exposed to bovine thrombin during at least 1 prior surgery.

Summary
Both topical human thrombin and topical recombinant thrombin have been compared individually to topical bovine thrombin in similarly designed, randomized, clinical trials with similar efficacy and safety results observed. Based on these comparisons, the current evidence indicates that these 3 agents are comparable in their ability to achieve hemostasis when hemostasis cannot be achieved through conventional methods. The major concern with topical thrombin appears to be the risk of antibody formation and subsequent abnormalities in coagulation and clinical bleeding, especially with bovine-derived thrombin. Although the currently available bovine thrombin (Thrombin-JMI) has improved purity over older bovine thrombin products (with undetectable levels of bovine factor V contaminants), studies have still reported antibody formation following exposure to bovine thrombin among a small percentage of patients, and at higher rates than with either human or recombinant thrombins. While studies have suggested that postoperative antibody formation is not associated with increased risks of postoperative adverse events (including bleeding), the presence of anti-bovine thrombin antibodies prior to re-exposure may increase a patient’s risk of postoperative complications. For patients with a history of bovine thrombin exposure or with anti-bovine antibodies preoperatively, use of a human or recombinant topical thrombin product may be warranted.

References

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3. Haines ST, Zeolla M, Witt DM. Venous Thromboembolism. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, eds. Pharmacotherapy: A Pathophysiological Approach. 6^th ed. New York, NY: McGraw-Hill; 2005:373-413.
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5. Thrombin-JMI [package insert]. Bristol, TN: King Pharmaceuticals, Inc.; 2008.
6. Evithrom [package insert]. Kiryat Ono, Israel: Omrix biopharmaceuticals, Inc.; 2007.
7. Recothrom [package insert]. Seattle, WA: ZymoGenetics, Inc.; 2008.
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11. Up To Date Online Web site. http://www.Uptodate.com. Accessed Jan 16, 2009.
12. Micromedex Healthcare Series Web site. http://www.micromedex.com. Accessed Jan 16, 2009.
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14. Heffernan JK, Ponce RA, Zuckerman LA, et al. Preclinical safety of recombinant human thrombin. Regul Toxicol Pharmacol. 2007;47(1):48-58.
15. Chapman WC, Singla N, Genyk Y, et al. A phase 3, randomized, double-blind comparative study for the efficacy and safety of topical recombinant human thrombin and bovine thrombin in surgical hemostasis. J Am Coll Surg. 2007;205(2):256-265.
16. Doria C, Fischer CP, Wood CG, Li PM, Marra S, Hart J. Phase 3, randomized, double-blind study of plasma-derived human thrombin versus bovine thrombin in achieving hemostasis in patients undergoing surgery. Curr Med Res Opin. 2008;24(3):785-794.
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By Andrius Cepenas, PharmD candidate