Displacement of Fibrin-bound Thrombin by r-Hirudin Precludes the Use of ¹³¹l-r-Hirudin for Detecting Pulmonary Emboli in the Rabbit

Summary

Pulmonary emboli are detectable by filling defects in the pulmonary vasculature upon pulmonary angiography. Emboli derived from venous thrombi are rich in fibrin to which thrombin remains bound. Hirudin, a specific thrombin inhibitor, binds to thrombin to yield a 1:1 stoichiometric complex. We examined whether ¹³¹I-recombinant hirudin (r-hirudin) could be used to detect pulmonary emboli in rabbits. Clots were formed by re-calcifying rabbit plasma in vitro, and then injected (0.034 ml) into a femoral vein to lodge in the lungs. ¹³¹I-r-hirudin (29 ± 4 μCi/kg) was injected intravenously but emboli could not be detected by gamma camera in real time. Post-mortem analysis of lung tissue showed that ¹³¹I-r-hirudin did not associate with emboli prepared with ¹²⁵I-fibrin. Because of these findings, we used different techniques to look at the binding of hirudin to plasma clots. Clots formed in vitro were incubated with ¹³¹I-r-hirudin in the presence of equimolar amounts of ¹²⁵I-albumin; specific binding of ¹³¹I-r-hirudin was not observed. Experiments with immobilized fibrin(ogen) showed that ¹²⁵I-r-hirudin did not bind to and remain with fibrin-bound ¹³¹I-thrombin but did lead to the inactivation and displacement of up to 70% of bound thrombin as r-hirudin-thrombin complex; residual thrombin bound to fibrin remained active. Thus, released r-hirudin-thrombin complex is probably cleared rapidly from the region of the embolus in vivo; radioiodinated r-hirudin may not, therefore, be useful as a marker for detecting emboli.

• References

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