An Expert Report on Human Thrombin (DB11571): A Plasma-Derived Topical Hemostatic Agent

I. Biochemical Profile and Pharmaceutical Formulation of Human Thrombin

1.1. Identification, Nomenclature, and Classification

Human thrombin is a highly purified, protein-based biotech therapeutic agent central to the process of hemostasis.[1] It is identified by the DrugBank Accession Number DB11571 and the Chemical Abstracts Service (CAS) Number 9002-04-4.[1] As a fundamental enzyme in the coagulation cascade, it is known by a variety of synonyms that reflect its biological function and origin, including Thrombin (human), Thrombin human plasma-derived, Trombina humana, Coagulation Factor IIa, fibrinogenase, and thrombofort.[1]

Biochemically, human thrombin is classified as a highly specific, endolytic serine protease belonging to the S1 (trypsin) family of enzymes.[1] It is encoded in the human genome by the F2 gene and is also categorized as a platelet activating factor due to its potent effects on platelet aggregation.[1] Its enzymatic activity is cataloged under the Enzyme Commission (EC) number 3.4.21.5.[4] This precise classification underscores its role as a targeted biological catalyst, distinct from small-molecule drugs, and places it within a well-understood family of proteolytic enzymes.

1.2. Source and Manufacturing: From Pooled Plasma to Purified Protein

The primary source material for therapeutic human thrombin is pooled human plasma, collected from carefully screened donors.[1] This origin is a critical feature that distinguishes it from two other classes of therapeutic thrombin: bovine-derived thrombin, which is purified from cow plasma, and recombinant thrombin (thrombin alfa), which is produced using recombinant DNA technology in a genetically modified Chinese Hamster Ovary (CHO) cell line.[1]

The manufacturing process is a multi-step biochemical purification designed to isolate and activate the enzyme while ensuring maximum safety. The process begins with cryo-poor plasma, from which the zymogen (inactive precursor) prothrombin is purified using chromatographic techniques.[1] Following purification, the prothrombin is converted into its active form, thrombin, through a controlled activation step involving calcium chloride.[1]

A paramount concern for any product derived from human plasma is the potential for transmitting blood-borne infectious agents. Consequently, the manufacturing process for human thrombin incorporates a series of rigorous, validated safety measures. These include comprehensive screening of plasma donors for prior exposure to certain viruses, direct testing of plasma units for the presence of current viral infections, and dedicated viral inactivation and removal steps integrated into the purification process.[7] For example, commercial preparations are sourced from plasma that has been certified negative for Hepatitis B surface antigen (HBsAg) and for antibodies to Human Immunodeficiency Virus (HIV) and Hepatitis C Virus (HCV).[12] This layered safety approach is fundamental to the product's regulatory approval and clinical acceptance. The use of a human source provides a protein that is biologically identical to the patient's own, which minimizes immunogenic potential, but it concurrently introduces the risk of human pathogens. This contrasts with recombinant alternatives, which eliminate the human pathogen risk but may introduce their own manufacturing-related impurities.[7]

1.3. Physicochemical Properties, Formulation, and Stability

Human thrombin is formulated for therapeutic use as a sterile, frozen solution, typically maintained at a physiological pH between 6.8 and 7.2.[1] Commercial products, such as Evithrom, are supplied in vials containing a standardized activity of 800-1200 International Units (IU) per milliliter.[11] Formulations may also include various excipients to ensure stability and proper function, such as sodium chloride, a citrate buffer, a cryoprotectant like polyethylene glycol (PEG)-8000, and a stabilizing protein like Bovine Serum Albumin (BSA).[12]

The protein itself has a molecular weight of approximately 37 kDa.[3] As a complex protein, it exists in multiple isoforms, which is reflected in its isoelectric point (pI) range of 6.35 to 7.6; the major isoform exhibits a pI of 7.31.[4] The stability of the final product is a critical consideration for clinical use. Once thawed or reconstituted, solutions like Evithrom are stable for a limited period, typically up to 8 hours at room temperature, and must be used within this timeframe.[10] Lyophilized preparations intended for research or other applications have different storage requirements, often necessitating temperatures of -20°C or -80°C for long-term storage, with defined stability periods after reconstitution.[12]

II. Comprehensive Pharmacological Review

2.1. Mechanism of Action: The Terminal Enzyme of the Coagulation Cascade

The therapeutic effect of human thrombin is derived from its function as the central executive enzyme in the final phase of blood coagulation. Its core mechanism of action is the proteolytic cleavage of soluble plasma fibrinogen to form insoluble fibrin monomers.[1] These monomers then spontaneously polymerize to form a fibrin mesh, the structural backbone of a blood clot. This process directly replicates the terminal step of the physiological coagulation cascade, providing a rapid and potent hemostatic effect at the site of application.[1]

A key pharmacological feature of topically applied thrombin is its ability to bypass the complex upstream enzymatic steps of the intrinsic and extrinsic coagulation pathways.[1] In normal hemostasis, a cascade of factor activations is required to generate endogenous thrombin. Topical application delivers the active enzyme directly to the bleeding site, where it acts immediately on the patient's available fibrinogen without the need for any intermediate physiological agents.[1] This direct action provides a "hemostatic shortcut," which is the fundamental rationale for its use as a fast-acting surgical hemostat.[8]

The action of thrombin extends beyond initial clot formation to include clot stabilization. Thrombin activates coagulation Factor XIII, converting it to its enzymatic form, Factor XIIIa.[1] Factor XIIIa, a transglutaminase, then catalyzes the formation of covalent bonds between adjacent fibrin strands, specifically between lysine and glutamine residues.[1] This cross-linking process transforms the soft, soluble fibrin gel into a mechanically strong and stable clot that is more resistant to fibrinolysis and can effectively seal bleeding tissues.[1] The potent, direct, and unregulated nature of this activation is the source of both the drug's primary efficacy and its most significant risk; if it were to enter the systemic circulation, it could trigger widespread, uncontrolled clotting.[10] This duality underscores the absolute importance of the "topical use only" directive.

2.2. Pharmacodynamics: Activation of Fibrinogen, Coagulation Factors, and Platelets

The pharmacodynamic effects of thrombin are multifaceted, influencing multiple components of the hemostatic system to create a robust and physiologically complete clot.

• Primary Target Interaction: The principal target of thrombin is fibrinogen, including its alpha, beta, and gamma chains, which it cleaves at specific Arg-Gly bonds to release fibrinopeptides A and B, initiating fibrin polymerization.[4]
• Amplification of Coagulation: Thrombin serves as a powerful catalyst in a positive feedback loop that amplifies its own production and the overall coagulation response. It activates key upstream cofactors, including Factor V to Va, Factor VIII to VIIIa, and Factor XI to XIa.[1] This ensures a rapid burst of coagulation activity localized to the site of injury.
• Platelet Activation: Beyond its role in the fibrin mesh formation, thrombin is one of the most potent physiological activators of platelets. It binds to and cleaves Protease-Activated Receptors (PARs), primarily PAR1 and PAR4, on the surface of platelets.[8] This triggers intracellular signaling cascades that lead to platelet shape change, granule release, and aggregation, forming the primary hemostatic plug that is then reinforced by the fibrin mesh. This dual action on both fibrin and platelets is critical for creating a stable, integrated clot.
• Other Physiological Effects: In its endogenous role, thrombin also induces vasoconstriction at the site of injury, which helps to reduce blood flow and facilitate hemostasis.[7]

The rate and extent of clot formation are directly dependent on the concentrations of both the exogenously applied thrombin and the endogenous fibrinogen available in the patient's plasma at the wound site.[1] In rare cases where a patient has a primary clotting defect due to the absence of fibrinogen (afibrinogenemia), topical thrombin will be ineffective.[1]

2.3. Pharmacokinetics and Biodistribution of a Topically Applied Biologic

The pharmacokinetic profile of human thrombin is defined by its intended local application and the body's natural mechanisms for controlling systemic coagulation.

• Absorption: As a topical agent applied directly to a surgical wound, systemic absorption of human thrombin is expected to be minimal and is generally considered clinically insignificant.[1] The large size of the protein and its rapid action at the application site limit its potential for entering the systemic circulation. Consequently, formal pharmacokinetic studies to assess parameters like bioavailability or peak plasma concentration were not conducted for its approval.[1] The primary pharmacokinetic concern is not therapeutic absorption but rather accidental intravascular administration, which is strictly contraindicated.[10]
• Metabolism and Clearance: In the event that any human thrombin reaches the systemic circulation, it is expected to be metabolized and cleared via the same pathways as endogenous thrombin.[1] The circulatory system has robust mechanisms to prevent free, active thrombin from circulating. It is rapidly inactivated by forming 1:1 stoichiometric complexes with a variety of endogenous plasma protease inhibitors (serpins).[1] The most important of these is Antithrombin III (ATIII), with other contributors including alpha-2-macroglobulin and heparin cofactor II.[1] This rapid inactivation is a critical physiological safeguard that prevents localized hemostatic events from propagating into systemic thrombosis.
• Elimination Pathways: Once inactivated, the thrombin-inhibitor complexes are cleared from the circulation through two primary routes. The first involves recognition and uptake by specific receptors in the liver, leading to lysosomal degradation.[1] The second pathway involves direct binding of thrombin to thrombomodulin on the surface of endothelial cells, which leads to internalization and degradation, and also initiates an anticoagulant signaling pathway via Protein C activation.[1] These efficient clearance mechanisms ensure that the half-life of any systemically available active thrombin is extremely short, effectively containing its potent procoagulant activity.

III. Clinical Efficacy and Surgical Applications

3.1. Approved Indications as an Adjunctive Hemostatic Agent

Human thrombin is approved for a specific and well-defined clinical niche as a topical hemostatic agent. Its primary indication is as an aid to hemostasis for controlling oozing blood and minor bleeding from capillaries and small venules that are surgically accessible.[1] A crucial element of this indication is that its use is reserved for situations where conventional methods of achieving hemostasis—such as suture, ligature, or cautery—are either ineffective or impractical.[1] This positions human thrombin not as a primary surgical tool, but as an essential adjunctive therapy for managing challenging, diffuse bleeding that cannot be controlled by mechanical means.

The product is approved for use either as a standalone solution or, more commonly, in conjunction with an absorbable carrier like a gelatin sponge.[10] This combination allows the thrombin solution to be held in direct contact with the bleeding surface, providing a matrix for clot formation. Human thrombin is also a critical active component in multi-component fibrin sealant kits, where it is packaged separately from a fibrinogen solution and mixed at the point of application to form a fibrin glue.[1] Its utility has been established across a broad spectrum of surgical procedures, including cardiovascular, neurological, general, orthopedic, and dental surgeries, where such bleeding is a common challenge.[7]

3.2. Dosage, Administration Protocols, and Application Techniques

The administration of human thrombin requires strict adherence to protocols designed to maximize local efficacy while preventing systemic exposure.

• Route of Administration: The product is exclusively for topical use. It must be applied only to the surface of bleeding tissue and is strictly contraindicated for any form of intravascular injection.[6]
• Site Preparation: Before application, the target surface should be cleared of pooling blood, typically by gentle sponging or suction. Wiping the surface is discouraged as it can dislodge early, fragile clots and exacerbate bleeding.[10]
• Application Methods:

1. Direct Application/Spray: The thrombin solution can be applied directly onto the bleeding area, often by flooding the surface using a sterile syringe with a small-gauge needle or a specialized spray applicator.[10] After application, the site should be left undisturbed to allow the clot to form and stabilize securely.
2. Application with Absorbable Gelatin Sponge: The thrombin solution is transferred to a sterile container. A piece of absorbable gelatin sponge, cut to the desired size, is then immersed in the solution. The sponge should be kneaded with moistened, gloved fingers to expel all trapped air and ensure complete saturation. The saturated sponge is then applied directly to the bleeding site and held in place with moderate pressure, typically using a gauze pledget, until hemostasis is achieved.[10]

• Dosage and Concentration: There is no standard dose for human thrombin. The volume and concentration required are dictated by the clinical situation, including the size of the wound and the rate of bleeding.[22] In clinical studies supporting the approval of Evithrom, volumes of up to 10 mL were used in conjunction with gelatin sponges.[11] Commercial preparations are supplied in standardized concentrations, such as 800-1200 IU/mL for Evithrom, to ensure predictable activity.[11]

3.3. Analysis of Pivotal Efficacy Trials and Supporting Clinical Evidence

The clinical efficacy of human thrombin was definitively established in a pivotal Phase III trial that compared it to the then-standard-of-care, bovine-derived thrombin. This study was strategically designed not to prove superiority in efficacy, which was expected to be similar, but to establish non-inferiority, thereby allowing the product's superior safety profile to become the key differentiator.

The study, published by Doria et al. in 2008, was a multicenter, prospective, randomized, double-blinded clinical trial that enrolled 305 subjects undergoing elective cardiovascular, neurologic, or general surgical procedures.[7] Patients were randomized to receive either plasma-derived human thrombin (Evithrom, n=153) or bovine thrombin (Thrombin-JMI, n=152), both applied topically with an absorbable gelatin sponge.[24]

• Primary Efficacy Outcome: The primary endpoint was the proportion of patients who achieved hemostasis within 10 minutes of product application. The results demonstrated perfect equivalence between the two groups: 97.4% of patients in the human thrombin arm achieved hemostasis, compared to 97.4% in the bovine thrombin arm (ratio, 1.00; 95% CI, 0.96-1.05).[7]
• Secondary Efficacy Outcomes: This equivalence was maintained at earlier time points as well. At 6 minutes post-application, 94.8% of the human thrombin group and 92.8% of the bovine thrombin group had achieved hemostasis. At 3 minutes, the rates were 73.2% and 72.4%, respectively.[24]
• Conclusion: The trial successfully demonstrated that plasma-derived human thrombin provides hemostatic efficacy that is clinically equivalent to that of bovine thrombin.[7] This critical finding provided assurance to the clinical community that adopting this newer agent for its safety benefits would not come at the cost of reduced performance in the operating room.

Further evidence for the efficacy of human thrombin comes from its use as a component in fibrin sealants like VeraSeal. In studies supporting VeraSeal's approval, the combination of human thrombin and human fibrinogen was shown to be highly effective at stopping bleeding within 4 minutes during various surgical procedures, including vascular, organ, and soft tissue surgeries.[20]

Study Feature	Doria et al., 2008 (Evithrom vs. Bovine)	Chapman et al., 2007 (Recothrom vs. Bovine)
Study Design	Randomized, double-blind, multicenter	Randomized, double-blind, multicenter
Patient Population (N)	305 (Cardiovascular, Neurologic, General Surgery) 25	411 (Spinal, Hepatic, Vascular Surgery) 26
Primary Efficacy Endpoint	Incidence of hemostasis within 10 minutes 25	Incidence of hemostasis within 10 minutes 26
Efficacy Result (% Hemostasis)	97.4% (Human) vs. 97.4% (Bovine) 25	95.4% (Recombinant) vs. 95.1% (Bovine) 7
Primary Safety Endpoint	Antibody Formation 25	Antibody Formation 26
Immunogenicity Result	3.3% (Human) vs. 12.7% (Bovine) 25	1.5% (Recombinant) vs. 21.5% (Bovine) 7
Key Conclusion	Equivalent Efficacy, Superior Immunogenicity 25	Equivalent Efficacy, Superior Immunogenicity 26

IV. Safety, Tolerability, and Risk Management

The safety profile of human thrombin is well-characterized, with a clear set of contraindications and warnings designed to mitigate its primary risks. Its main advantage over historical alternatives lies in its significantly reduced potential for immunogenicity.

4.1. Contraindications and High-Risk Patient Populations

The use of human thrombin is strictly prohibited in several clinical scenarios:

• Intravascular Application: It must never be injected directly into the circulatory system. Such an administration would trigger widespread intravascular coagulation, leading to extensive thrombosis, potential end-organ damage, and a high risk of fatality.[14]
• Severe Arterial Bleeding: It is not indicated for the treatment of severe or brisk arterial bleeding. The high pressure of arterial flow would wash away the topical agent before a stable clot could form, rendering it ineffective.[10]
• Known Hypersensitivity: It is contraindicated in individuals with a known history of anaphylactic or severe systemic reactions to human blood products, as this would predispose them to a severe allergic reaction to the thrombin product itself.[10]

4.2. Warnings and Precautions

The product labeling includes several important warnings for clinicians:

• Risk of Thrombosis: A primary warning reinforces the risk of thrombosis if the product is absorbed systemically. This reiterates the critical importance of ensuring strictly topical application.[10] Patients should be counseled on the signs and symptoms of thromboembolic events, such as unexplained leg tenderness or swelling, acute shortness of breath, or chest pain.[10]
• Risk of Infectious Agent Transmission: As with all products derived from human plasma, there is a remote but theoretical risk of transmitting infectious agents, including viruses and the agent for Creutzfeldt-Jakob disease (CJD). Although this risk is substantially reduced through rigorous donor screening and viral inactivation/removal steps during manufacturing, it cannot be completely eliminated. This is a standard class warning for all plasma-derived therapeutics.[7]
• Risk of Hypersensitivity Reactions: Anaphylactic or other severe hypersensitivity reactions may occur, although they are considered rare. In the pivotal clinical trials for Evithrom, no such events were reported. Standard protocols for managing anaphylaxis should be available whenever the product is used.[10]

4.3. Adverse Reaction Profile from Clinical Trials

The adverse event profile of human thrombin has been characterized in controlled clinical trials, which showed it to be generally well-tolerated and comparable to bovine thrombin in terms of overall event frequency.

• Overall Incidence: In the pivotal Phase III study, the overall incidence of adverse events was not statistically different between the human thrombin and bovine thrombin groups.[10] Most events were related to the underlying surgical procedure and the patient's medical condition rather than the study drug.
• Common Adverse Reactions: The most frequently reported adverse reactions (incidence ≥2%) in subjects treated with Evithrom were primarily transient laboratory abnormalities, including prolonged activated partial thromboplastin time (APTT), increased international normalized ratio (INR), decreased lymphocyte count, prolonged prothrombin time (PT), and increased neutrophil count.[10] These findings are consistent with localized consumption of clotting factors at the surgical site and are not indicative of a systemic effect. Other commonly reported events were procedural complications and pruritus (itching).[11]
• Serious Adverse Events (SAEs): SAEs were reported in 17% of patients in the human thrombin group compared to 11% in the bovine thrombin group. However, upon adjudication, these events were determined to be associated with post-surgical complications (e.g., wound infection) and were not considered causally related to the administration of human thrombin.[10]

4.4. Immunogenicity: A Detailed Assessment of Antibody Formation Potential

The most significant safety advantage of human thrombin over its bovine-derived predecessor is its markedly lower immunogenicity. The development of antibodies against bovine thrombin and contaminating bovine Factor V was a major clinical concern, as these antibodies could cross-react with human coagulation factors, leading to life-threatening bleeding or thrombotic complications.[7]

The pivotal trial for Evithrom was specifically designed to assess this risk by testing for the development of antibodies against four distinct antigens: human thrombin, human Factor V/Va, bovine thrombin, and bovine Factor V/Va.[10] The results provided a clear and compelling demonstration of human thrombin's superior immunogenic profile:

• In the bovine thrombin group, 12.7% of subjects developed antibodies to at least one of the four antigens tested.[7] Specifically, 7.9% developed antibodies against bovine thrombin and 9.5% developed antibodies against bovine Factor V/Va.[10]
• In stark contrast, only 3.3% of subjects treated with human thrombin developed antibodies to any of the antigens.[7] This difference was statistically significant (p=0.01).[7]
• Most importantly, zero patients in the human thrombin group developed detectable antibodies against human thrombin or human Factor V/Va.[10] This finding confirms that the homologous human protein does not trigger the production of potentially cross-reactive or neutralizing autoantibodies, thereby eliminating the specific immune-mediated coagulopathy risk associated with the bovine product. This superior immunogenicity profile is the core value proposition and the primary clinical justification for its use over bovine-derived alternatives.

V. Comparative Analysis: Positioning Within the Topical Thrombin Landscape

The development of human thrombin was a direct response to the limitations of earlier hemostatic agents. Its clinical value is best understood through a comparative analysis with both its predecessor, bovine-derived thrombin, and its technological successor, recombinant human thrombin. This evolution from animal-derived to plasma-derived to recombinant products represents a clear and deliberate progression toward enhancing safety while maintaining efficacy.

5.1. Human Plasma-Derived vs. Bovine-Derived Thrombin: Efficacy, Immunogenicity, and Regulatory Discrepancies

The comparison between human and bovine thrombin is a classic case study in pharmaceutical development, where a new agent with equivalent efficacy supplants an older one due to a superior safety profile.

• Efficacy: The two products are clinically equivalent in their ability to achieve surgical hemostasis. The pivotal Phase III trial by Doria et al. found identical success rates of 97.4% for achieving hemostasis within 10 minutes.[7] Clinicians can therefore expect the same level of performance from both agents.
• Immunogenicity: This is the critical point of differentiation. Human thrombin is demonstrably superior. It has a significantly lower rate of overall antibody formation (3.3% vs. 12.7%) and, crucially, does not induce the formation of antibodies against endogenous human coagulation factors.[7] Bovine thrombin carries a known risk of inducing antibodies that can cross-react with human Factor V, leading to a potentially fatal factor deficiency and uncontrolled bleeding, or with human thrombin, potentially causing thrombosis.[9]
• Regulatory Status: The difference in safety is directly reflected in their regulatory labeling by the U.S. FDA. Bovine thrombin products carry a Black Box Warning—the FDA's most stringent warning—highlighting the risk of severe bleeding and thrombosis complications related to antibody formation.[7] Human thrombin products do not carry this warning, signifying their recognized safety advantage.[7]
• Source-Related Risks: The two products present a trade-off in source-related risks. Bovine thrombin introduces the risk of immunogenicity from a xenogeneic (animal) protein. Human thrombin eliminates this risk but introduces the theoretical risk of transmitting human blood-borne pathogens, a risk that is managed through rigorous manufacturing controls.[7]

5.2. Human Plasma-Derived vs. Recombinant Human Thrombin: A Trade-off Analysis of Source, Purity, and Risk

The development of recombinant human thrombin (thrombin alfa, Recothrom) represents the next step in the safety evolution, aiming to eliminate the risks associated with human plasma.

• Source and Purity: Human plasma-derived thrombin is purified from large pools of donated human plasma.[1] Recombinant human thrombin is produced using DNA technology in a controlled bioreactor environment using a CHO cell line.[7] This recombinant process yields a highly purified product that is free of human plasma proteins and eliminates the risk of transmitting human viruses or prions.[7]
• Efficacy: Both products are structurally and functionally identical to native human thrombin and are considered to have equivalent efficacy. The pivotal trial for recombinant thrombin (Chapman et al., 2007) also demonstrated non-inferiority to bovine thrombin, with hemostasis rates of 95.4% vs. 95.1%.[7]
• Immunogenicity: Both human-sequence products have very low immunogenicity compared to the bovine product. In its pivotal trial, recombinant thrombin had an antibody formation rate of only 1.5%, compared to 21.5% for bovine thrombin in the same study.[7] This rate is even lower than the 3.3% observed for plasma-derived thrombin in its trial, though direct cross-trial comparisons should be made with caution.
• Differential Risks: The choice between plasma-derived and recombinant human thrombin involves a nuanced trade-off between two different types of low-probability risks. With the plasma-derived product, the risk is the theoretical possibility of an unknown or emerging blood-borne pathogen bypassing the safety measures.[10] With the recombinant product, the risk is a potential hypersensitivity reaction to trace amounts of non-human proteins from the manufacturing process, such as host cell (hamster) proteins or reagents like the snake venom enzymes used for activation.[7] For a hospital formulary committee, this means weighing a theoretical infectious risk against a theoretical allergic risk.

Feature	Bovine Thrombin (e.g., Thrombin-JMI)	Human Plasma-Derived Thrombin (e.g., Evithrom)	Recombinant Human Thrombin (e.g., Recothrom)
Source	Bovine Plasma 9	Pooled Human Plasma 1	Recombinant (CHO cells) 8
Primary Manufacturing Risk	Xenogeneic Immunogenicity 9	Human Pathogen Transmission (theoretical) 10	Host Cell Protein/Reagent Sensitivity 7
Hemostatic Efficacy (vs. Bovine)	Baseline	Equivalent 7	Equivalent 7
Immunogenicity (Antibody Rate)	High (~12-22%) 7	Low (~3.3%) 7	Very Low (~1.5%) 7
FDA Black Box Warning	Yes 7	No 7	No 7

VI. Drug Interactions and Use in Medically Complex Patients

6.1. Theoretical vs. Clinical Risk of Systemic Drug Interactions

The concept of drug-drug interactions for a topically applied biologic like human thrombin must be framed differently than for a systemically absorbed drug. Due to its local application and minimal entry into the circulation, human thrombin is not subject to classical pharmacokinetic interactions involving metabolic enzymes (e.g., cytochrome P450) or plasma protein binding displacement.[1] The product's prescribing information lists no known systemic drug interactions.[38]

The more clinically relevant issue is not a direct interaction, but rather the pharmacodynamic performance of topical thrombin in a patient whose systemic hemostatic environment has been deliberately altered by anticoagulant or antiplatelet medications. In this context, topical thrombin is often used specifically to overcome the effects of these systemic agents at a localized bleeding site.

6.2. Efficacy and Safety Considerations in Patients on Systemic Anticoagulant Therapy (Warfarin, Heparin)

A growing number of surgical patients are on chronic anticoagulant therapy for conditions like atrial fibrillation or venous thromboembolism, making intraoperative hemostasis a significant challenge.

• Use with Warfarin: Warfarin is a vitamin K antagonist that inhibits the hepatic synthesis of active coagulation factors II (prothrombin), VII, IX, and X.[40] It acts upstream by depleting the pool of functional clotting factor precursors. However, it does not inhibit the activity of thrombin itself or its ability to cleave fibrinogen. Because topical human thrombin is the active enzyme applied directly to the site of bleeding, it bypasses the synthesis steps inhibited by warfarin. Therefore, its mechanism of action remains intact, and it is expected to be effective in achieving local hemostasis in warfarinized patients.[44]
• Use with Heparin: Heparin exerts its anticoagulant effect by binding to and potentiating the activity of Antithrombin III, which in turn rapidly inactivates systemic thrombin (Factor IIa) and Factor Xa.[45] While systemic heparin would effectively neutralize any thrombin that enters the bloodstream, the clinical goal of topical application is to deliver a very high concentration of thrombin directly to the wound. This high local concentration is intended to overwhelm the capacity of any local inhibitors and rapidly catalyze fibrin formation before significant systemic neutralization can occur. Preclinical evidence supports this concept; a study in a rabbit vascular bleeding model demonstrated that topical recombinant human thrombin was able to achieve rapid and durable hemostasis even in animals systemically anticoagulated with heparin.[46]

6.3. Efficacy and Safety Considerations in Patients on Systemic Antiplatelet Therapy (Clopidogrel)

Patients with coronary artery disease or peripheral vascular disease are frequently on antiplatelet agents, which also increases the risk of surgical bleeding.

• Use with Clopidogrel: Clopidogrel is a prodrug that is metabolized to an active form that irreversibly inhibits the P2Y12 adenosine diphosphate (ADP) receptor on platelets, thereby blocking a key pathway of platelet activation and aggregation.[47]
• Overcoming Platelet Inhibition: While clopidogrel effectively blocks the ADP pathway, it does not affect all pathways of platelet activation. Thrombin is an independent and extremely potent platelet activator that functions through the PAR receptor pathway.[8] Therefore, even in a patient on clopidogrel, the high local concentration of topical thrombin can still directly activate platelets at the wound site, contributing to the formation of a primary hemostatic plug.
• Supporting Evidence: This mechanistic rationale is supported by both in vitro and in vivo data. An in vitro study using blood from donors taking clopidogrel confirmed that while ADP-induced platelet aggregation was significantly inhibited, thrombin-induced aggregation remained robust. The study concluded that thrombin-based hemostatic matrices are effective at initiating clotting even in the presence of clopidogrel.[49] Furthermore, the aforementioned rabbit model also demonstrated that topical thrombin was effective in animals treated with a combination of both heparin and clopidogrel, simulating a common and challenging clinical scenario.[46] This makes topical thrombin a critical tool for surgeons operating on this high-risk patient population.

VII. Global Regulatory and Approval Status

The regulatory history of human thrombin reflects its development as a safer alternative to bovine-derived products and shows some variation in marketing strategy between major global regions.

7.1. U.S. Food and Drug Administration (FDA) Regulatory History and Labeling

In the United States, human thrombin was approved as a standalone hemostatic agent, providing surgeons with flexibility in its application.

• Approval and Branding: The U.S. FDA approved Evithrom on August 27, 2007.[6] The Biologics License Application (BLA 125247) was submitted by Omrix Biopharmaceuticals, Ltd., which was later acquired by Johnson & Johnson.[6]
• Regulatory Significance: The approval of Evithrom was a significant milestone, as it was the first human thrombin approved as a standalone product and was explicitly marketed as a bovine-free alternative designed to mitigate the immunogenicity risks that had led to a black box warning on bovine thrombin products.[6]
• Approved Labeling: The FDA-approved label for Evithrom specifies its indication as an adjunctive hemostatic agent, contains the critical warning for "Topical Use Only," and includes the standard precaution for plasma-derived products regarding the theoretical risk of infectious agent transmission.[10] Its clean label, devoid of the black box warning for immune-mediated coagulopathy, is a key regulatory and clinical advantage.[7]
• Use in Specific Populations: The label supports its use in pediatric patients based on data from studies of Evicel (a fibrin sealant in which Evithrom is a component) and by extrapolation of safety and efficacy findings from adult populations.[11]

7.2. European Medicines Agency (EMA) Assessment and Authorised Combination Products

In the European Union, human thrombin has been primarily authorized as an integral component of two-part fibrin sealant kits, rather than as a standalone product. This reflects a different marketing and regulatory approach favoring integrated systems.

• VeraSeal: This fibrin sealant, containing human fibrinogen and human thrombin as its two active substances, was granted marketing authorization by the EMA on November 10, 2017.[20] The European Public Assessment Report (EPAR) for VeraSeal details its efficacy in stopping bleeding during surgery and outlines its safety profile, which is consistent with other fibrin sealants. The report emphasizes the risk of thromboembolic complications if the product is accidentally injected intravascularly.[20]
• Raplixa (Authorization Withdrawn): Raplixa, another combination product of human fibrinogen and human thrombin formulated as a powder, was granted EU marketing authorization on March 19, 2015.[52] However, the authorization was later withdrawn by the marketing authorization holder. This withdrawal was explicitly for commercial reasons and was not related to any new safety or efficacy concerns.[52] This event highlights the competitive and economic pressures within the biosurgicals market, where even an approved product may not achieve commercial viability.
• Non-Medical Use: It is important to note a separate, non-therapeutic application where bovine and porcine thrombin have been approved by the European Food Safety Authority (EFSA) for use as a food additive to bind pieces of meat together, a use colloquially known as "meat glue".[54] This is entirely distinct from its medical use.

The divergence in regulatory approvals—standalone in the U.S. versus combination kits in the E.U.—suggests that global market strategies for biosurgical products must be adapted to regional regulatory precedents and clinical practice preferences.

VIII. Expert Synthesis and Recommendations

8.1. Integrated Assessment of the Risk-Benefit Profile

The comprehensive body of evidence establishes that human plasma-derived thrombin possesses a highly favorable risk-benefit profile for its approved indication as an adjunctive topical hemostatic agent.

• Benefit: The clinical benefit is clear and unequivocal. It provides rapid and effective hemostasis in surgical situations where conventional methods fail. Pivotal clinical trials have demonstrated its efficacy to be equivalent to the long-standing standard of care, bovine-derived thrombin, ensuring no loss of performance for clinicians transitioning to the newer agent.[7]
• Risk: The risk profile represents a substantial improvement over bovine thrombin. The primary historical risk associated with topical thrombin therapy—the development of immunogenic antibodies leading to potentially fatal coagulopathies—is virtually eliminated by the use of a homologous human protein. Clinical trial data confirm that human thrombin does not elicit the production of antibodies against endogenous human clotting factors.[10] The residual risks are well-understood and manageable. The risk of thrombosis is mitigated by strict adherence to topical application, and the theoretical risk of pathogen transmission is minimized by modern, rigorous plasma sourcing and manufacturing standards common to all plasma-derived products.[7]

Therefore, the assessment concludes that human thrombin effectively decouples the high efficacy of topical thrombin from the most severe risks associated with its xenogeneic predecessor.

8.2. Recommendations for Clinical Practice and Formulary Placement

Based on the superior safety profile established in robust clinical trials, human-sequence thrombins (both plasma-derived and recombinant) should be considered the standard of care over bovine-derived thrombin for topical surgical hemostasis.

• Clinical Practice: Clinicians should preferentially use human-sequence thrombin products to avoid the risk of immune-mediated coagulopathy associated with bovine thrombin. While the incidence of this complication is low, its consequences can be catastrophic. Given the equivalent efficacy, there is no compelling clinical reason to expose patients to this avoidable risk.
• Formulary Placement: The placement of human thrombin on hospital and institutional formularies is strongly recommended. While the acquisition cost may be higher than that of bovine thrombin, a comprehensive pharmacoeconomic analysis must account for the potential downstream costs of managing a severe bleeding or thrombotic complication arising from bovine thrombin-induced antibodies. These costs—which include extended hospital stays, intensive care, additional procedures, and extensive blood product transfusions—would far outweigh the initial drug cost differential. The adoption of human thrombin represents a prudent investment in patient safety and risk mitigation.

8.3. Future Perspectives in Topical Hemostasis

The evolution of topical thrombin from bovine to human plasma-derived to recombinant products is emblematic of a broader trend in medicine toward safer, more precisely engineered biological therapies. The future of topical hemostasis will likely see continued innovation in several areas:

• Dominance of Recombinant Technology: Recombinant production platforms offer the ultimate control over purity and eliminate the reliance on biological source materials (animal or human), thereby removing the associated risks of immunogenicity and pathogen transmission. It is anticipated that recombinant products will increasingly become the preferred choice as manufacturing technologies mature and costs decrease.
• Novel Hemostatic Agents: Research continues into novel agents that target different aspects of the coagulation cascade, as well as the development of advanced biomaterials that provide a more effective scaffold for clot formation.
• Advanced Delivery Systems: Innovation in sprayers, applicators, and combination products (e.g., thrombin-infused patches or powders) will continue to improve the ease of use and precision of application in complex surgical environments.

In this evolving landscape, human thrombin stands as a pivotal product that successfully addressed a major safety concern and paved the way for even safer recombinant alternatives, solidifying the role of topical thrombin as an indispensable tool in the modern surgical armamentarium.

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