technician placing a sample on assay tube

Hemophilia A is primarily treated with factor VIII (FVIII) replacement therapy. The reason for the continued use of this prophylactic treatment is that it is generally successful in reducing both morbidity and mortality in patients with the disease. 

Regling and colleagues wrote, “Prophylaxis with FVIII concentrates was, until recently, the gold standard of treatment for severe [hemophilia A] and has significantly improved the overall bone and joint health in these patients.”

The principle of replacement therapy is remarkably simple: replace FVIII to therapeutic levels and the risk of bleeding drops significantly. In most patients, this is indeed what happens. In addition, FVIII therapy is now highly individualized, prescribed according to a patient’s body weight and personal pharmacokinetic studies. 


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However, FVIII therapy has encountered 2 notable problems: 

  • Some patients with supposedly adequate FVIII trough levels still experience breakthrough bleeding. 
  • Patients with the same baseline FVIII activity level can experience vastly different bleeding phenotypes. 

The lack of a strict correlation between plasma FVIII activity and clinical expression of the disease is a problem, since it means plasma FVIII levels cannot be relied upon to accurately predict the risk of breakthrough bleeding. Any given dose of antihemophilic factor concentrate cannot guarantee a desired clinical response. 

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Why is this the case? A proposed theory is that this is caused by differences between individuals in their levels of pro- and anti-coagulant factors.

Studies have indicated that thrombin generation profiles correspond more closely to bleeding phenotype than FVIII activity levels do. Hence, some scientists have proposed that thrombin generation assays be used to monitor the interplay between pro- and anti-coagulant factors, since they measure the hemostatic balance as a whole. 

A Sharper Tracking Tool 

Salvagno and colleagues describe thrombin generation as “a key process that determines the extent of a hemostatic plug or a thrombotic process.” Thrombin plays an important role in the clotting of fibrinogen. Hence, the abnormal production of thrombin can lead to thrombotic or hemorrhagic disease. 

The thrombin generation test was first developed in the 1980s. Researchers then defined endogenous thrombin potential (ETP) as the plasma’s overall capacity to generate thrombin after coagulation has been induced. The ETP is a useful measurement of the coagulation capacity of an individual, since it is a reflection of the total enzymatic activity of the generated thrombin over the course of its lifespan. Since then, this parameter has been proposed as a sensitive indicator for every type of anticoagulation. 

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Verhagen and colleagues reviewed the medical literature, through September 22, 2021, on using thrombin generation assays to monitor the impact of different hemophilia A treatment modalities. They reported that 4 key parameters can be obtained via thrombin generation assays. These are: 

  • Lag time, which is the time needed for the thrombin generation signal to increase by two standard deviations from baseline
  • Time to thrombin peak
  • Thrombin peak height, which is the maximum amount of thrombin produced
  • Endogenous thrombin potential. 

Dividing the thrombin peak height by the time between the lag time and time to thrombin peak reveals the velocity of thrombin generation. Verhagen et al wrote, “Whereas the ETP only shows the total amount of thrombin formation, the velocity of TG reflects the amount of thrombin that is generated in the acceleration phase.” Studies have suggested that this parameter correlates more closely with FVIII activity levels than ETP as it represents the capacity of an individual to produce enough thrombin in a short period of time to stop bleeding.

Indicating a ‘More Realistic Hemostatic Response’

Another key finding of this review was that a strong correlation exists between FVIII activity levels and thrombin generation parameters, with the exception of lag time. Interpatient variability remains but can typically be explained by differences in individual pharmacokinetics and individual response of thrombin generation to FVIII replacement therapy. 

“[Thrombin generation assays] can solve the problem of the discrepant results between one-stage and chromogenic assays for monitoring recombinant and extended half-life FVIII products because it indicates the more realistic hemostatic response provided by these products compared with the assumed FVIII activity level provided by FVIII assays,” the research team concluded. 

This means that thrombin generation assays can monitor the efficacy of hemophilia A treatment in a way that the mere monitoring of plasma FVIII activity cannot. By using thrombin generation assays, physicians can lower the risk of FVIII overdosing, which has wide implications, such as the increased cost of treatment. 

Salvagno and colleagues wrote, “[Thrombin generation assays] could be a promising tool to establish the hemostatic potential of a patient at T=0 and then the response to factor substitution therapy.” 

The role of thrombin generation assays as a tool for monitoring hemophilia A therapies may extend beyond those that are currently available. Presently, much research is being devoted to refining gene therapy for public use. If gene therapy is approved for hemophilia A in the future, thrombin generation assays may likewise play an important role in monitoring its efficacy. 

References

Verhagen MJA, Valke LLFG, Schols SEM. Thrombin generation for monitoring hemostatic therapy in hemophilia A: a narrative reviewJ Thromb Haemost. 2022;10.1111/jth.15640. doi:10.1111/jth.15640

Salvagno GL, Berntorp E. Thrombin generation testing for monitoring hemophilia treatment: a clinical perspectiveSemin Thromb Hemost. 2010;36(7):780-790. doi:10.1055/s-0030-1265295

Regling K, Callaghan MU, Sidonio R Jr. Managing severe hemophilia A in children: pharmacotherapeutic optionsPediatric Health Med Ther. 2022;13:27-35. doi:10.2147/PHMT.S293246