Blood, 1 July 2002, Vol. 100, No. 1, pp. 364-366
CORRESPONDENCE
To the editor:
Platelet-dependent action of high-dose factor VIIa
We read with interest the recent article by Butenas et
al,1 who conclude that the prohemostatic effect
of high-dose factor VIIa in hemophilic blood or a synthetic plasma
system is strictly dependent on tissue factor (TF). The authors note
that their proposed mechanism differs from our previously published
conclusion that high-dose factor VIIa can, in the absence of TF,
generate factor Xa on the surface of activated platelets at
concentrations sufficient to ameliorate hemophilia.2
Butenas et al propose that differences in the results from our 2 labs are due to differences in experimental design or protein preparations. We feel it is very likely that most of the differences in
results between our 2 labs are due to our use of tissue
factor-expressing cells as the initiating stimulus in our model system
rather than tissue factor protein reconstituted into phospholipid
vesicles. For example, Mann's group has previously reported that the
presence of zymogen factor VII in a synthetic plasma model competes
with factor VIIa for binding to TF on phospholipid
vesicles.3 By contrast, we see no effect of zymogen factor
VII on thrombin generation in a model system initiated by
cell-associated TF.4
Butenas et al suggest that high-dose factor VIIa therapy works in
hemophilia to directly enhance activation of factor X through the
factor VIIa/TF complex, resulting in an enhanced activation of
platelets. Our data indicates that, although the factor VIIa/TF complex
is essential to initiate coagulation, once platelets have been
activated, high-dose factor VIIa binds to the surface of activated
platelets and replaces the absent factor IXa/VIIIa complex to generate
factor Xa, which then boosts platelet-surface thrombin generation.2 Our view is consistent with the observation
that the tissue factor pathway is intact in hemophilic
patients5 and, indeed, is responsible for platelet
activation, accounting for the tendency of hemophilics to initially
stop bleeding as a normal platelet plug forms6,7 but then
suffer severe delayed rebleeding.
These disparate views of the mechanism of action of factor VIIa have
important implications for dosing. The TF-dependent effect described by
Butenas et al is saturated at levels below those now used
therapeutically. By contrast, the binding of factor VIIa to the
activated platelet surface is nowhere close to being saturated at
therapeutically relevant concentrations. Thus, our mechanism predicts that escalating the dose of factor VIIa should enhance its hemostatic effect, while the mechanism of Butenas et al suggests that escalating the dose would waste money without benefiting patients.
Taken together, we believe the data supports a platelet-dependent, TF-independent mechanism for high-dose factor VIIa in hemophilia.
Maureane Hoffman, Dougald M. Monroe, and Harold R. Roberts
Correspondence: Dougald M. Monroe, UNC Chapel Hill,
Hematology/Oncology, 932 Mary Ellen Jones Building, Chapel Hill, NC
27599-7035; e-mail: dmonroe{at}med.unc.edu.
References
1.
Butenas S, Brummel KE, Branda RF, Paradis SG, Mann KG.
Mechanism of factor VIIa-dependent coagulation in hemophilia blood.
Blood.
2002;99:923-930[Abstract/Free Full Text].
2.
Monroe DM, Hoffman M, Oliver JA, Roberts HR.
Platelet activity of high-dose factor VIIa is independent of tissue factor.
Br J Haematol.
1997;99:542-547[CrossRef][Medline]
[Order article via Infotrieve].
3.
van't Veer C, Golden NJ, Mann KG.
Inhibition of thrombin generation by the zymogen factor VII: implications for the treatment of hemophilia A by factor VIIa.
Blood.
2000;95:1330-1335[Abstract/Free Full Text].
4.
Hoffman M, Monroe DM.
The action of high-dose factor VIIa (FVIIa) in a cell-based model of hemostasis
Sem Hematol.
2001;38(suppl 12):6-9[Medline]
[Order article via Infotrieve].
5.
Bauer KA, Kass BL, ten Cate H, Bednarek MA, Hawiger JJ, Rosenberg RD.
Detection of factor X activation in humans.
Blood.
1989;74:2007-2015[Abstract/Free Full Text].
6.
Wester J, Sixma JJ, Geuze JJ, Heijnen HF.
Morphology of the hemostatic plug in human skin wounds: transformation of the plug.
Lab Invest.
1979;41:182-192[Medline]
[Order article via Infotrieve].
7.
Sixma J, van den Berg A.
The haemostatic plug in haemophilia A: a morphological study of haemostatic plug formation in bleeding time skin wounds of patients with severe haemophilia A.
Br J Haematol.
1984;58:741-753[Medline]
[Order article via Infotrieve].
Response:
Mechanism of factor VIIa-dependent coagulation in
hemophilia blood
There are 3 areas in which the experimental results and their
interpretations are in discord between our 2 groups: (1) whether factor
VII and factor VIIa compete for tissue factor, (2) whether tissue
factor is required for the function of factor VIIa in the condition of
hemophilia A or B, and (3) whether vigorous propagation of thrombin
generation by factor VIIa can occur on activated platelets in the
absence of tissue factor, factor VIII, and factor IX. We are unsure of
the basis for the discrepancies between the results. But we can offer
the following facts in order to rationalize some potential bases for
the differences:
First, the competition of factor VII and factor VIIa for tissue factor
(TF) can only be observed at low tissue factor concentrations relative to the factor VII and factor VIIa concentrations. As the
concentration of tissue factor is increased, the feedback activation of
factor VII by factor VIIa-TF, factor Xa-membrane, and thrombin
obliterates this phenomenon.1(Fig2)
Second, the tissue factor requirement essential to trigger the reaction
appears to be indisputable. The relative rate of factor VIIa to factor
VIIa-TF in factor X activation is approximately 0.0 001-fold,2 and the relative efficiency of factor
VIIIa-factor IXa compared to factor VIIa-TF in factor X activation is
approximately 50-100 to1.3 Our experimental data in
synthetic system and in whole blood4 are consistent with
these experimental ratios determined by other laboratories. During the
propagation phase of thrombin generation, the rate of factor X
activation by factor VIIa without tissue factor in the hemophilia A or
B situation would be depressed by approximately 1 000 000-fold
relative to that observed in normal blood in the presence of tissue factor.
Third, the data of our recent publication4 indicate that
factor VIIa at supraphysiologic concentrations has little effect on
thrombin generation either in congenital hemophilia A or "acquired" hemophilia B blood. This observation is valid for experiments conducted
in the absence of tissue factor and for those conducted at fixed TF
concentrations. A prolonged clotting time in "acquired" hemophilia
B blood (>40 min)4(Fig4A) and a lack of response to the
anti-TF antibody in normal blood (see comments to Figure
24) indicate that the blood used in our
experiments had no functional tissue factor. In contrast to the
observation by Monroe et al,5 in our experiments
thrombin generation in hemophilia blood is almost not affected by the
initial rate of platelet activation. Data
show4(Fig4C) that additions of 10 to 50 nM factor
VIIa to "acquired" hemophilia B blood in the absence of tissue
factor restores platelet activation to the levels observed in normal
blood. But thrombin generation,4(Fig4B) however, is almost
not affected by these factor VIIa additions. Similarly, while
supraphysiologic concentrations of factor VIIa in congenital hemophilia
A blood triggered with tissue factor lead to an increased rate of
platelet activation,4(Fig5D) robust thrombin generation is
not observed,4(Fig5A) even when platelets are completely
activated. These observations would seem to contradict the notion that
factor VIIa activity on platelets can provide for the rapid activation
of factor X in the absence of the factor VIIIa-factor IXa complex and
tissue factor.
The experimental systems used by the Roberts group and our group differ
in the source of tissue factor that is used to trigger the
reaction. It is conceivable that the tissue factor source or the
environment in which tissue factor is presented could be the cause of
some of the differences in our results.
Kenneth G. Mann and Saulius Butenas
Correspondence: Kenneth G. Mann, Department of Biochemistry,
University of Vermont, C-401 Given Building, 89 Beaumont Ave,
Burlington, VT 05405
References
1.
van 't Veer C, Golden NJ, Mann KG.
Inhibition of thrombin generation by the zymogen factor VII: implications for the treatment of hemophilia A by factor VIIa.
Blood.
2000;95:1330-1335[Abstract/Free Full Text].
2.
Komiyama Y, Pedersen AH, Kisiel W.
Proteolytic activation of human factor IX and X by recombinant human factor VIIa: effects of calcium, phospholipids, and tissue factor.
Biochem.
1990;29:9418-9425[CrossRef][Medline]
[Order article via Infotrieve].
3.
Rawala-Sheikh R, Ahmad SS, Ashby B, Walsh PN.
Kinetics of coagulation factor activation by platelet-bound factor IXa.
Biochem.
1990;29:2606-2611[CrossRef][Medline]
[Order article via Infotrieve].
4.
Butenas S, Brummel KE, Branda RF, Paradis SG, Mann KG.
Mechanism of factor VIIa-dependent coagulation in hemophilia blood.
Blood.
2002;99:923-930[Abstract/Free Full Text].
5.
Monroe DM, Hoffman M, Oliver JA, Roberts HR.
Platelet activity of high-dose factor VIIa is independent of tissue factor.
Br J Haematol.
1997;99:542-547[CrossRef][Medline]
[Order article via Infotrieve].
