Summary
We studied the inhibitory action of recombinant desulphatohirudin (CGP 39393) on thrombin
generation in whole plasma. Human plasma was activated either with thromboplastin
or factor IXa. Hirudin delayed thrombin generation, but it was unable to prevent the
explosive appearance of thrombin. The
dose-dependent prolongation of the lag phase of the intrinsic and extrinsic thrombin
generation curve was not the result of titration of thrombin activity by hirudin but
the result of a delayed formation of the prothrombin converting complex (prothrombinase).
In case of extrinsic activation, hirudin did not affect factor Xa generation, but
prolonged the lag phase of the factor Va generation curve, causing its appearance
when factor Xa generation was already in the decay phase. Because of its inhibitory
action on the thrombin-mediated activation of factor VIII, hirudin prolonged the lag
phase of the factor X converting complex that consists of factor IXa and factor VIIIa.
Our observations with hirudin are in keeping with the notion that inhibition
of the thrombin-mediated amplification reactions in blood coagulation is a very efficient
way to delay or inhibit completely thrombin generation. However, although hirudin
neutralizes stoichiometric amounts of thrombin, the interaction between in situ generated
thrombin and hirudin appears not to be fast enough to prevent trace amounts of thrombin
to activate factors VIII and V. Consequently, an explosive thrombin generation is
observed even when free hirudin is present.
