DISPLACEMENT OF FIBRIN-BOUND THROMBIN BY R-HIRUDIN PRECLUDES THE USE OF I-131 R-HIRUDIN FOR DETECTING PULMONARY EMBOLI IN THE RABBIT

Pulmonary emboli are detectable by filling defects in the pulmonary va sculature upon pulmonary angiography. Emboli derived from Venous throm bi are rich in fibrin to which thrombin remains bound. Hirudin, a spec ific thrombin inhibitor, binds to thrombin to yield a 1:1 stoichiometr ic complex. We examined whether I-131-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. I-131-r-hirudin (29 +/- 4 mu Ci/kg) was injected intravenously but emboli could not be detected by gamma camera in real time. Post-morzem analysis of lung tissue show ed that I-131-r-hirudin did not associate with emboli prepared with I- 125-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 I-131-r-hirudin in the presence of equimolar amou nts of I-125-albumin; specific binding of I-131-r-hirudin was not obse rved. Experiments with immobilized fibrin(ogen) showed that I-125-r-hi rudin did not bind to and remain with fibrin-bound I-131-thrombin but did lead to the inactivation and displacement of up to 70% of bound th rombin as r-hirudin-thrombin complex; residual thrombin bound to fibri n remained active. Thus, released r-hirudin-thrombin complex is probab ly cleared rapidly from the region of the embolus in vivo; radioiodina ted r-hirudin may not, therefore, be useful as a marker for detecting emboli.