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(Circulation. 1996;94:2389-2395.)
© 1996 American Heart Association, Inc.

Articles

Distinct Effects of Recombinant Desulfatohirudin (Revasc) and Heparin on Plasma Levels of Fibrinopeptide A and Prothrombin Fragment F1.2 in Unstable Angina

A Multicenter Trial

A. Koneti Rao, MD; Ling Sun, MD; James H. Chesebro, MD; Valentin Fuster, MD; Robert A. Harrington, MD; Darryl Schwartz, MD; Paul Gallo, PhD; Deborah Matos, MS; Eric J. Topol, MD; for the Hirudin in Unstable Angina Trial*

the Sol Sherry Thrombosis Research Center, Department of Medicine, Temple University School of Medicine, Philadelphia, Pa; the Mayo Clinic, Rochester, Minn; Massachusetts General Hospital, Boston; Duke University Medical Center, Durham, NC; CIBA-Geigy, Summit, NJ; and the Cleveland (Ohio) Clinic.

	Abstract

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 
Background Thrombin plays an important role in the pathogenesis of acute coronary thrombosis. We studied the effects of a direct thrombin inhibitor, recombinant desulfatohirudin, and heparin on plasma levels (at 0, 4, 12, and 24 hours) of fibrinopeptide A (FPA), which reflects thrombin action, and prothrombin fragment F1.2, which reflects thrombin generation, in patients with unstable angina.

Methods and Results Patients were randomized to one of two doses of heparin (n=50) (target activated partial thromboplastin time, 65 to 90 seconds or 90 to 110 seconds) or one of four doses of r-hirudin (n=113) (0.05, 0.10, 0.20, or 0.30 mg·kg^-1·h^-1 by infusion). r-Hirudin induced a dose-dependent decline in plasma FPA. At 24 hours, FPA levels with 0.1- to 0.3-mg·kg^-1·h^-1 r-hirudin regimens were significantly lower than with 0.05 mg·kg^-1·h^-1 r-hirudin; levels with 0.1- to 0.2-mg·kg^-1·h^-1 r-hirudin regimens were lower than with both heparin regimens. Plasma F1.2 did not decline significantly during therapy with heparin or hirudin except at 0.3 mg·kg^-1·h^-1 hirudin. At 24 hours, they were higher with the 0.05-mg·kg^-1·h^-1 r-hirudin regimen than with other regimens. For comparable levels of thrombin generation (F1.2 levels), FPA levels were higher in heparin patients than in hirudin patients. For the same FPA values, the corresponding F1.2 values were higher in the hirudin group.

Conclusions Our findings provide evidence for distinct in vivo effects of the two agents and suggest that r-hirudin is a relatively more potent inhibitor of thrombin action but a less effective inhibitor of thrombin generation than heparin. The lower FPA levels in hirudin patients may reflect its ability to inactivate clot-bound thrombin. The relative clinical efficacies of the two agents need to be defined by clinical trials in progress.

Key Words: angina • coagulation • anticoagulants • hirudin • heparin

	Introduction

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 
Thrombin plays a pivotal role in the pathogenesis of acute coronary syndromes by virtue of its multifaceted actions, including fibrin deposition and platelet activation.^{1 2} In line with this, plasma levels of FPA, generated by thrombin proteolysis of fibrinogen, and F1.2, released during prothrombin conversion to thrombin, are elevated in patients with acute coronary syndromes.^{3 4 5} This has been generally considered to reflect ongoing thrombin action and generation. Because of the role of thrombin in thrombus formation, there has been an intense interest in evaluating the role of direct thrombin inhibitors in the management of patients with acute coronary syndromes based on the premise that more potent and direct inhibitors of thrombin may extend the currently available therapies. One such agent is hirudin, a naturally occurring anticoagulant derived from the leech Hirudo medicinalis. Hirudin is a 65-amino-acid single-chain polypeptide that is a highly potent, selective, and essentially irreversible inhibitor of thrombin.^{6 7 8 9} Hirudin and the widely used anticoagulant heparin differ in their mechanisms of action as antithrombotic agents. Whereas hirudin requires no cofactor for its anticoagulant action,^{8 9} heparin derives its anticoagulant action primarily through its ability to accelerate the neutralization of thrombin and other serine proteases (including factor Xa) by a plasma cofactor, antithrombin III.^{10 11} During blood coagulation, thrombin activates factor V, a critical cofactor in the generation of the prothrombinase that converts prothrombin to thrombin. It has been suggested^{12 13 14} that inhibition of these thrombin-dependent positive-feedback reactions is important to the expression of anticoagulant activity of heparin. Hirudin is an effective inhibitor of clot-bound thrombin (which still retains its enzymatic activity), whereas heparin inhibits predominantly soluble thrombin.¹⁵

The availability of recombinant desulfatohirudin (CGP 39,393, Revasc, CIBA-Geigy) has made it feasible to conduct adequate clinical trials to evaluate the potential of hirudin as an antithrombotic agent. Several experimental models of arterial thrombosis^{6 16 17 18 19} have shown recombinant hirudin to be an effective antithrombotic agent, and phase I studies have demonstrated the safety of this agent.^{19 20 21 22} Hirudin has been evaluated in patients with acute myocardial infarction as an adjunct to thrombolytic therapy^{23 24 25 26} and during coronary angioplasty.²⁷ We have evaluated recombinant hirudin in a multicenter, dose-escalating randomized phase II trial in patients with unstable angina pectoris and have reported the angiographic findings.²⁸ The present report describes our findings in these patients with respect to the effect of recombinant hirudin and heparin on plasma coagulation tests and markers of thrombin activity and generation as measured by plasma FPA and F1.2, respectively. The goal of these studies was to determine whether the two agents differed in their relative effects on thrombin generation and action. We demonstrate that the two antithrombotic agents have distinct effects on these mechanisms.

	Methods

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 
Details of the study design have been published previously.²⁸ At 10 participating sites ("Appendix"), patients with rest ischemic chest pain undergoing coronary angiography were screened to determine eligibility for inclusion provided that the following criteria were met: (1) at least 5 minutes of rest ischemic pain, presumed to be myocardial in origin, occurring within the past 48 hours; (2) minimal angiographic stenosis 60% in a major epicardial coronary artery or saphenous vein graft interpreted as having an intraluminal thrombus and believed to represent the culprit vessel for the patient's clinical presentation; and (3) transient or fixed ECG abnormalities suggestive of ischemia. Patients more than 48 hours after intravenous thrombolytic therapy for acute myocardial infarction were eligible, as were patients receiving intravenous heparin, provided that entry criteria were met. All patients gave informed consent to participate, and the protocol was approved by the Institutional Review Board at each site.

Study Drug Dosing and Assignment
Six groups of patients were enrolled in the trial, with escalating doses of recombinant hirudin.²⁸ For all groups, the planned treatment duration was 72 to 120 hours. In the first five groups, the randomization was planned to be 23 patients assigned to hirudin and 7 assigned to heparin (target aPTT, 65 to 90 seconds). In the last group, recruitment of 21 patients in both the high-dose heparin group (target aPTT, 90 to 110 seconds) and the hirudin group was planned. The trial progressed in an open-label, sequential manner through each dose group. The dose of hirudin (CIBA-Geigy) ranged from a bolus of 0.15 mg/kg and infusion of 0.05 mg·kg^-1·h^-1 to a bolus of 0.9 mg/kg and an infusion of 0.3 mg·kg^-1·h^-1. Twenty-three patients each were treated with the following hirudin doses: 0.15 mg·kg^-1·h^-1 as a bolus with 0.05 mg·kg^-1·h^-1 as an infusion; 0.3 mg/kg bolus and 0.1 mg·kg^-1·h^-1 infusion; and 0.6 mg/kg bolus with 0.2 mg·kg^-1·h^-1 infusion. Forty-four patients were administered hirudin 0.6 mg/kg bolus followed by 0.3 mg·kg^-1·h^-1 infusion. Only 3 patients were treated with a hirudin dose of 0.9 mg/kg bolus and 0.3 mg·kg^-1·h^-1 infusion; these patients are not included in this report. The various groups are referred to in this report according to the infusion rates. Porcine heparin was obtained from a single batch (Organon, Inc) and was supplied in the appropriate study drug kits. The dose of heparin was a 5000-U bolus and 1000 U/h adjusted to maintain the aPTT between 65 and 90 seconds (28 patients). Based on the prolongation in aPTT noted in patients receiving the highest hirudin dose, in the last group of 21 patients treated with heparin, the dose was raised to adjust the aPTT to 90 to 110 seconds (high dose). This was done to obtain a group of patients with each drug with comparable levels of anticoagulation and to examine whether the effects of hirudin and heparin were related to the intensity of in vitro anticoagulant effect.

Angiographic Studies
At baseline, complete coronary angiography was performed, with specific attention to interrogation of the culprit vessel, as previously described.²⁸ The angiograms were obtained after administration of 200 µg nitroglycerin IC with 7F or 8F catheters. Open-label heparin was given as a single 2500- to 5000-U bolus during the procedure and was withheld thereafter. In some patients, no heparin was administered before the angiography. The infusion of study drug was begun 1 to 2 hours after hemostasis following sheath removal. After 72 to 120 hours of study drug administration, a repeat angiogram was obtained by use of the same technique with intracoronary nitroglycerin. The cineangiograms were assessed at the Core Angiographic Laboratory (Cleveland Clinic Foundation) in a blinded fashion.

Hematological Studies
Patients had serial measurements of aPTT and PT at baseline and during study drug infusion. Serial blood samples were drawn into 0.1 volume of 3.8% sodium citrate, and the plasma was harvested by centrifugation and shipped on dry ice to a core laboratory established at the Sol Sherry Thrombosis Research Center, Temple University School of Medicine, Philadelphia, Pa. The plasma samples were stored at -80°C until the time of assay. The measurement of aPTTs was performed according to the manufacturer's recommendations with a KoaguLab 32-S (Ortho Diagnostic Systems) instrument and partial thromboplastin from Organon Teknika Corp (General Diagnostics, automated aPTT reagent). The PT was measured with the same instrument with a rabbit brain tissue thromboplastin reagent, Simplastin Excel (Organon Teknika Corp). In addition, aPTT was determined at the local sites by the CIBA-Corning 512 monitor (Biotrack, Inc). The values obtained by this method were used to adjust the dose of heparin, and the results have been reported.²⁸

Plasma levels of FPA and F1.2 levels were measured in samples obtained at baseline before initiation of drug infusion (but after the angiography); at 4, 12, and 24 hours; and just before discontinuation of the drug. The numbers of patients studied are shown in the figure legends. Blood samples (4.5 mL) for FPA levels were collected into tubes containing 0.5 mL of 3.8% sodium citrate and 100 µL of a cocktail containing 5000 KIU/mL Trasylol (Miles Inc) and 5000 U/mL heparin (Organon Inc).²⁹ FPA levels were measured with an ELISA (American Bioproducts Co). F1.2 levels were measured in citrated plasma collected in 0.1 vol of 3.8% sodium citrate with an ELISA (Behring Diagnostics). Plasma TAT complexes were measured in citrated plasma with a commercially available ELISA from Behring Diagnostics. TAT levels were measured in 7 to 12 patients in each group except for the highest hirudin dose (0.3 mg·kg^-1·h^-1), in which 17 to 19 patients were analyzed.

Data Analysis
Summary statistics were computed for the variables obtained in this study separately for each dose group. For variables measured on continuous scales, between-treatment comparisons were performed with Kruskal-Wallis tests when more than two groups were compared and Wilcoxon rank-sum tests for pairwise comparisons; the Wilcoxon signed-rank tests were performed for within-group comparisons. These tests were considered appropriate because the parameters tested did not follow normal distributions. For incidence data, Fisher's exact test was performed. All test results for pairwise tests are presented as two-tailed probability values.

For hypothesis tests involving between-treatment comparisons, the following "step-down" procedure³⁰ was used: an initial overall test addressing the equality of all dose groups was performed. If this test result was not significant at the P=.05 level, then no pairwise tests were performed, and it was concluded that the dose groups were not different. If the overall test result was significant, then pairwise tests were performed; no adjustment of significance criteria was needed, because the chance of making any false-positive statements was controlled at 5% by the performance of the overall test. The probability values shown in this report for comparisons between treatment groups are derived from this step-down procedure. In addition, for the aPTT data, a repeated-measures ANOVA was also performed with PROC. MIXED in SAS (SAS Institute); restricted maximum likelihood estimation was used, with an unstructured covariance matrix. Correlations were computed with Spearman's rank correlation coefficient. Linear regressions were performed to characterize the relationships between certain parameters; for some of these (FPA, F1.2), log transformations were applied before regression lines were fitted so that the data would more closely follow normal distribution assumptions.

	Results

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 
The baseline characteristics of the patients, the angiographic findings, and results of the aPTT measured at local sites with a CIBA-Corning 512 monitor have been reported previously.³⁰ Fig 1 shows measurements made at the centralized Core Laboratory; the aPTTs were more prolonged at the 0.2- to 0.3-mg·kg^-1·h^-1 infusion rates of hirudin than at lower levels and at the higher heparin dose than the lower dose. A greater prolongation of the PT was observed at higher hirudin doses, and there was a lack of adequate sensitivity of this test to monitor anticoagulation with either agent. To better assess the intensity of anticoagulation between various groups, the following analysis was performed. Because anticoagulation (aPTT) appeared to be fairly stable between 12 and 72 hours (Fig 1), a single median value representing each patient was computed, taking all aPTTs performed in that patient during this time period. Further analyses were carried out using the within-patient median aPTT values for each patient. The overall test of equality among treatment groups revealed that they were not the same (P<.001). The median aPTTs computed for each drug regimen by use of such within-patient median values were comparable for lower-dose heparin (68 seconds, n=27 patients), hirudin 0.05 mg·kg^-1·h^-1 (64 seconds, n=23), and hirudin 0.1 mg·kg^-1·h^-1 (65 seconds, n=23). The median aPTTs at the higher heparin dose (78 seconds, n=22) and the two higher hirudin doses (0.2 mg·kg^-1·h^-1, 78 seconds, n=23; and 0.3 mg·kg^-1·h^-1, 88 seconds, n=44) were longer (P=.03 to P<.001) than at lower doses of hirudin or heparin but were not different from each other. To formally test these hypotheses, regression lines were fit in a repeated-measures ANCOVA model. The overall slope was not different (P=.89), indicating that the average values were constant in the time range 12 to 72 hours. The overall test of equality among treatment groups revealed that they were not the same (P<.0001). Higher-dose heparin and the two higher hirudin doses were not significantly different from each other, but each had significantly longer values than the lower doses of hirudin or heparin (P=.012 to P<.001).

View larger version (28K): [in this window] [in a new window]   Figure 1. Effect of hirudin (HIR) and heparin (HEP) infusion on aPTT.

Plasma Levels of FPA, F1.2, and TAT
The baseline (0-hour) FPA levels were higher in the hirudin 0.05- and 0.3-mg·kg^-1·h^-1 groups compared with the others (Fig 1). The baseline samples were obtained after the initial angiography but just before initiation of the study drug (hirudin or heparin). In the initial study groups, not all patients received heparin during the angiography. This would provide an explanation for the differences noted in baseline value between groups. FPA levels declined significantly during therapy in all treatment groups (Fig 2) with the exception of the high-dose heparin, the latter possibly being due to a low baseline value. At 4 hours, plasma FPA levels were significantly lower than baseline values in these groups (P=.036 to P<.001). Plasma FPAs were not significantly different between the two heparin regimens at any time point. The overall test indicated that there were differences among the treatment groups (P<.001) in 24-hour FPA levels, the levels in the lowest hirudin dose (0.05 mg·kg^-1·h^-1) were higher than the three higher hirudin doses (P<.001 for all comparisons), and the levels in the two heparin groups were higher than those in hirudin groups of 0.1 and 0.2 mg·kg^-1·h^-1 infusion (P=.02 to P=.008). The FPA levels at 12 hours also showed a similar trend, with the levels at the lowest hirudin dose being higher than at higher doses. The 24-hour time was specifically chosen for comparison because it was expected to be relatively less influenced by the differences noted in the levels at the 0 time point, which are affected by heparin administration during the preceding coronary angiography and by differences in duration of the interval between the initial angiography-related heparin bolus and initiation of study drug. Moreover, they would reflect the effect of a 24-hour drug infusion. Given that the half-lives of FPA and F1.2 are 3 to 5 minutes and 90 minutes, respectively,^{31 32} it is unlikely that the baseline levels have a major impact on the 24-hour levels. The greater inhibition of FPA levels at 24 hours with 0.1- and 0.2-mg·kg^-1·h^-1 hirudin doses compared with both the heparin regimens (which induce comparable or greater aPTT prolongations than these hirudin groups, Fig 1) suggests that the differences between the two agents with respect to FPA levels may be independent of aPTT.

View larger version (23K): [in this window] [in a new window]   Figure 2. Effect of hirudin (HIR) and heparin (HEP) infusion on plasma FPA levels. Shown are median values for each treatment group (n=12 to 19 in each group except the hirudin 0.3-mg·kg-1·h-1 group, in which n=17 to 19 patients). FPA levels declined significantly during therapy in all groups with the exception of the highest hirudin (0.3-mg·kg-1·h-1) group. At 24 hours, FPA levels in the hirudin 0.05-mg·kg-1·h-1 group were higher (P<.02 to P=.008) than all other groups. FPA levels in the hirudin 0.1- and 0.2-mg·kg-1·h-1 groups were lower than the levels at hirudin 0.05 mg·kg-1·h-1 and both heparin regimens, which did not differ from each other.

As shown in Fig 3, plasma F1.2 levels at 0 hour were significantly (P=.02 to P=.005) higher in the hirudin 0.05- and 0.3-mg·kg^-1·h^-1 regimens compared with heparin regimens. This may be related, as indicated earlier for FPA levels, to differences in heparin anticoagulation during coronary angiography preceding the initiation of therapy. At 4 hours, plasma levels in the highest hirudin dose (0.3 mg·kg^-1·h^-1) were significantly lower than at 0 hour (P=.004). In the rest of the groups (both hirudin and heparin), no significant declines were observed during treatment. At 24 hours, the overall test indicated differences between the groups (P=.019); F1.2 levels were significantly higher (P=.015 to P=.004) in the 0.05-mg·kg^-1·h^-1 hirudin group compared with both of the heparin groups and with all other hirudin groups.

View larger version (21K): [in this window] [in a new window]   Figure 3. Plasma F1.2 levels during hirudin (HIR) and heparin (HEP) therapy. Shown are median values in each group. During therapy the levels did not decline significantly in any of the groups with the exception of the 0.3-mg·kg-1·h-1 hirudin regimen. At 24 hours, F1.2 levels were higher (P=.02 to P=.004) in the hirudin 0.05-mg·kg-1·h-1 group than in both heparin groups and the higher hirudin doses. Number of patients: heparin 65 to 90 seconds, 19 to 22; heparin 90 to 110 seconds, 11 to 14; hirudin 0.05 mg·kg-1·h-1, 15 to 17; 0.1 mg·kg-1·h-1, 9 to 12; 0.2 mg·kg-1·h-1, 11 to 15; and 0.3 mg·kg-1·h-1, 23 to 30.

Thrombin can complex with antithrombin or hirudin. TAT levels were measured in a smaller number of patients (n=7 to 12 per group; hirudin 0.3-mg·kg^-1·h^-1 group, n=17 to 19), and differences between groups were generally not significant. However, certain trends were suggested. TAT levels for the two heparin groups were comparable at most time points. Interestingly, the median TAT levels at 12 and 24 hours in the 0.05-mg·kg^-1·h^-1 hirudin group (14.2 and 17.4 µg/L, respectively) were moderately higher than in all other groups, both heparin and hirudin. This is compatible with the hypothesis that with increasing hirudin levels there is greater complex formation of thrombin with hirudin over AT-III, resulting in lower TAT levels.

Relationship Between Plasma FPA and F1.2 Levels
FPA levels reflect the thrombin proteolysis of fibrinogen, whereas F1.2 levels indicate thrombin generation from prothrombin by factor Xa in the presence of factor Va. To examine whether heparin and hirudin had differential effects on thrombin generation and thrombin-induced fibrinogen proteolysis, we examined the relationships between plasma levels of F1.2 and FPA. This analysis was performed on log-transformed data from patients in whom both F1.2 and FPA levels were available (4, 12, and 24 hours of therapy), and all patients treated with each drug were combined. With increasing plasma F1.2, there was an increase in FPA levels (Fig 4). There was a direct correlation between plasma F1.2 and FPA levels in both the hirudin (r=.458, P<.001) and heparin (r=.544, P<.001) groups. When log-transformed data for FPA were fitted as a function of F1.2, the intercepts were significantly different between hirudin and heparin (P<.001) but not the slopes, which were also not significantly different from zero. Transforming the results into the original units, the approximate relationships between FPA and F1.2 were heparin FPA=7.21xF1.2 and hirudin FPA=3.11xF1.2, indicating that for hirudin the rate of increase was less than half of that for heparin. This is in line with the median FPA and F1.2 ratios calculated from untransformed data: heparin, 6.14 (n=94); hirudin, 3.17 (n=209); P<.001. These findings indicate that for comparable levels of thrombin generation (F1.2 levels), FPA levels were higher in heparin patients. Moreover, for the same FPA values, the corresponding F1.2 value was higher for the hirudin group than for the heparin group. This suggests that for the same amount of FPA formed, there is a greater thrombin generation in hirudin patients than in heparin patients.

View larger version (20K): [in this window] [in a new window]   Figure 4. Relationship between plasma F1.2 and FPA levels during therapy with heparin (broken line) and hirudin (continuous line). The intercepts are significantly different between treatment groups (P<.001). Correlation coefficient for F1.2 vs FPA: heparin 0.54, P<.001; hirudin 0.46, P<.001.

Relationship Between aPTT and Plasma Levels of FPA and F1.2
When FPA levels (0 to 24 hours) were plotted as a function of aPTT (Fig 5A), the slopes and intercepts were not significantly different between the two treatments. The slope of a single regression line combining both treatments was significantly less than zero (P<.001); it was significantly different from zero for hirudin (P<.001) but not heparin. Thus, FPA levels decreased with increasing aPTT with a slightly but not significantly larger decrease for hirudin. The combined correlation coefficient was -0.211 (P<.001); for the hirudin and heparin groups, they were -0.239 (P<.001) and -0.140 (P=.089), respectively. In similar plots for F1.2 and aPTT data (Fig 5B), the slopes were not significantly different from zero or each other. However, the hirudin intercept was higher than that of the heparin group (P=.024), reflecting the higher F1.2 levels for hirudin over heparin patients associated with fixed aPTT levels. The correlation coefficients failed to indicate a relationship between aPTT and F1.2.

View larger version (49K): [in this window] [in a new window]   Figure 5. Relationship between aPTT and plasma FPA (top) and F1.2 (bottom) levels during therapy with hirudin (solid line, +) and heparin (broken line, ). Top, Slopes and intercepts of the regression lines for FPA are not significantly different between groups. The slope of the FPA regression line for hirudin is different from zero (P<.001). Correlation coefficients between FPA and aPTT were hirudin, r=-.239 (P<.001); heparin, r=-.140 (P=.089). Bottom, Intercepts of the F1.2 regression lines are significantly different from each other (P=.024). The slopes are not significantly different from each other or zero.

Relationship of Angiographic Findings to Plasma FPA and F1.2 Levels
As we previously reported,²⁸ patients treated with hirudin tended to show more improvement angiographically than those treated with heparin. Therefore, we examined the relationship between the change in average cross-sectional area of the culprit artery on repeat angiography and FPA levels at 24 hours. Patients were arbitrarily divided into two groups on the basis of 24-hour FPA levels of 5 ng/mL and >5 ng/mL, which represents approximately twice the upper limit of the range of values noted in control subjects. If all patients are taken together, those with FPA levels at 24 hours of 5 ng/mL had a significantly greater (P=.005) increase in cross-sectional area on repeat angiography relative to those with higher levels (Fig 6). A similar relationship was noted for hirudin patients (P=.01). In the heparin group, the difference was not statistically significant, possibly because of fewer patients. Interestingly, at comparable FPA levels, the change in average cross-sectional area tended to be greater in patients receiving hirudin. The average cross-sectional area on repeat angiography of the culprit artery tended to be higher in patients with 24-hour FPA levels 5 ng/mL, but this did not reach statistical significance (not shown). Further, the proportion of patients showing improvement in the repeat angiogram (defined as TIMI flow grade improvement) tended to be higher in patients with FPA levels 5 ng/mL than in those with higher levels (70% versus 53%, P=.08, Fisher's exact test). In a similar analysis, no significant relationship was observed between angiographic findings and F1.2 levels.

View larger version (24K): [in this window] [in a new window]   Figure 6. Relationship between 24-hour FPA levels and change in average cross-sectional area on repeat angiography.

	Discussion

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 
The findings of this multicenter dose-escalating study indicate that recombinant desulfatohirudin induces a dose-dependent prolongation in the aPTT. Our findings with respect to plasma levels of FPA and F1.2 and their relationships to each other and to aPTT provide new insights into the in vivo mechanisms of action of these two agents. FPA levels declined during therapy with both drugs, and at 24 hours of therapy, FPA levels were lower in the three higher-hirudin regimens (0.1 to 0.3 mg·kg^-1·h^-1 compared with heparin; Fig 2). These findings indicate that in vivo hirudin is a more potent inhibitor of thrombin-induced fibrinogen proteolysis than heparin at all but the lowest dose (0.05 mg·kg^-1·h^-1). Moreover, for the same magnitude of thrombin generated (F1.2 levels), FPA levels were lower in hirudin patients (Fig 4), as also reflected by the lower ratio of FPA to F1.2. Thrombin binds avidly to the clot and continues to induce fibrinogen proteolysis.^{9 15} Clot-bound thrombin is inhibited by hirudin but not heparin,¹⁵ although both of these agents are potent inhibitors of fluid-phase thrombin. We interpret the superior inhibition of FPA levels by hirudin in our studies as being a result of its additional effect on clot-bound thrombin. These findings indicate that even in vivo hirudin manifests ability to inhibit clot-bound thrombin, as previously suggested by in vitro observations.¹⁵

Our findings suggest that hirudin has a relatively smaller effect than heparin in inhibiting continued thrombin generation. When viewed in relation to equivalent FPA levels, F1.2 levels generated by prothrombin conversion to thrombin were clearly higher with hirudin infusion than with heparin (Fig 4). Moreover, at comparable levels of anticoagulation, F1.2 levels were higher with hirudin (Fig 5B). Hirudin does not inactivate factor Xa, which can effectively activate factors V and VIII when proteolytic action of thrombin is blocked.^{12 13} Factors Va and VIIIa play an important role in continued thrombin generation. In contrast, heparin is an effective inhibitor of factor Xa in the fluid phase, and inhibition of thrombin-dependent positive feedback reactions leading to prothrombin activation is important to the expression of its anticoagulant effects.^{12 13} Studies in purified systems have demonstrated that the association of factor Xa with platelet surface or phospholipid micelles protects the enzyme against inhibition by heparin-antithrombin complex.^{32 33 34 35 36} However, studies in plasma and whole blood indicate that the magnitude of enzyme (factor Xa) protection provided by the platelet surface is strongly correlated with the extent of prothrombin activation at the time of heparin addition³⁶ ; at a low level of prothrombin conversion, factor Xa is completely inhibited by heparin-antithrombin complex.³⁶ We propose that hirudin at the doses administered has a relatively smaller effect on the feedback-amplifying mechanisms leading to prothrombinase generation and thrombin formation. These observations confirm similar conclusions made from studies in vitro^{9 37} and in patients with stable coronary disease²² on effects of hirudin on thrombin generation and on the feedback mechanisms. Our findings attest to differences in the action in vivo of the two anticoagulants, one with a relatively superior ability to inhibit thrombin generation (heparin) and the other with a potent ability to inhibit even clot-bound thrombin (hirudin).

Patients receiving hirudin at the lowest dose (0.05 mg·kg^-1·h^-1) had higher levels of plasma FPA (Fig 2) and F1.2 (Fig 3) than patients treated with higher hirudin doses and heparin. This is consistent with previously reported inferior angiographic findings²⁸ at this dose. The lowest dose assessed appears inadequate for use of hirudin alone as an antithrombotic agent. At the next higher dose (0.3 mg/kg bolus, 0.1 mg·kg^-1·h^-1 infusion), plasma FPA levels declined during therapy, and at 24 hours, they were lower than corresponding levels in heparin groups, indicating a discernible thrombin-inhibitory effect. In the GUSTO IIB trial currently in progress, the hirudin infusion rate is the same, although the initial bolus is lower (0.1 mg/kg). In the presence of hirudin, thrombin binds hirudin preferentially over AT-III,³⁷ resulting in thrombin-hirudin complexes rather than TAT. In vitro³⁷ and in vivo²² studies show that with increasing hirudin concentration, TAT levels decline with an increase in hirudin-thrombin complexes. TAT levels were noted to be higher at the lowest hirudin dose compared with the highest dose, again supporting the above conclusion regarding inadequacy of the 0.05-mg·kg^-1·h^-1 infusion rate.

The inhibition of FPA levels appears to have a relationship to inhibition of continued thrombosis in the coronary artery. Despite the relatively small number of patients, the change in average cross-sectional area on repeat angiography (Fig 6) favored those with lower FPA levels at 24 hours. Not unexpectedly, such a relationship was not observed for F1.2 levels at 24 hours, because hirudin did not have a major inhibitory effect on F1.2 levels. It remains to be established whether recombinant hirudin, with its greater inhibitory effect on thrombin action than that of heparin, will translate to an enhanced clinical efficacy. In vivo studies in a pig model suggest that hirudin at comparable aPTTs is superior to heparin in preventing arterial thrombus formation after angioplasty.³⁸

In conclusion, our studies provide convincing evidence that standard unfractionated heparin and hirudin differ in their mechanisms of action in vivo with respect to inhibition of thrombin action versus thrombin generation. In this clinical study, hirudin was more effective in inhibiting thrombin action and fibrin formation but not thrombin generation, even at relatively high levels of anticoagulation. More importantly, this study provides evidence that the distinctions in the action of the two agents previously shown^{9 12 13 15} in in vitro studies operate in vivo as well, both with respect to the effect on clot-bound thrombin and on thrombin generation. These mechanistic differences appear to be unrelated to their effects on aPTT. Because of these demonstrated differences, in the appropriate clinical scenario there may be a role for both classes of thrombin inhibitors. Their relative clinical efficacies need to be defined by the large trials currently in progress.

	Selected Abbreviations and Acronyms

 

aPTT = activated partial thromboplastin time F1.2 = prothrombin fragment F1.2 FPA = fibrinopeptide A PT = prothrombin time TAT = thrombin-antithrombin

	Acknowledgments

 
This study was supported by CIBA-Geigy, Summit, NJ. The excellent secretarial assistance of Mary Merrick and JoAnn Hamilton is appreciated.

	Footnotes

 
Reprint requests to A. Koneti Rao, MD, Division of Hematology and Thromboembolic Diseases, Temple University School of Medicine, 3400 N Broad St, 300 OMS, Philadelphia, PA 19140.

Presented at the annual meeting of the American Federation for Clinical Research, Baltimore, Md, April 29-May 2, 1994, and the 67th Scientific Sessions of the American Heart Association, Dallas, Tex, November 14-17, 1994, and published in abstract form (Circulation. 1994;90[pt 2]:I-552).

*A full list of coinvestigators in the study group is provided in the "Appendix."

	Appendix

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 

	Coinvestigators in the Study Group

Top Abstract Introduction Methods Results Discussion Appendix Coinvestigators in the Study... References

 
Cleveland Clinic, Cleveland, Ohio. Eric J. Topol, MD; Stephen G. Ellis, MD; Patrick L. Whitlow, MD; Irving Franco, MD; Conrad Simpfendorfer, MD; Russell Raymond, DO; Michele Webb, RN.

Duke University Medical Center, Durham, NC. Robert Harrington, MD; Robert Califf, MD; Eric Berrios, RN; Kirby Quintero, RN.

Massachusetts General Hospital, Boston, Mass. Valentin Fuster, MD, PhD; Herman Gold, MD; Wendy Werner, RN.

Christ Hospital Medical Center, Cincinnati, Ohio. Dean J. Kereiakes, MD; Karen Ibanez, RN.

Methodist Hospital, Houston, Tex. Neal S. Kleiman, MD; Dale Rose, BS; Kathy Trainor, RN.

Hahnemann University Medical Center, Philadelphia, Pa. Marc Cohen, MD; Kathy Stoakes, RN, BSN.

Mayo Clinic, Rochester, Minn. James Chesebro, MD; Dawn Gaspar, RN.

Riverside Methodist Hospital, Columbus, Ohio. Anthony Chapekis, MD; Barry S. George, MD; Steven Yakabov, MD; Richard Candela, MD; Karla Rusk, RN.

Iowa Heart Center, Des Moines, Iowa. Phillip Bear, DO; Marc Polich, RN; Mark Tannenbaum, MD; Mary Beth Craig, RN, BSN; Dawn Stangl, RN.

Washington Cardiology Center, Washington, DC. Jeffrey J. Popma, MD; Leslie Sweet, RN.

Angiographic Core Laboratory, Cleveland Clinic, Cleveland, Ohio. Darrell Debowey, MS; Timothy D. Crowe, BS.

Coagulation Core Laboratory, Temple University Health Sciences Center, Philadelphia, Pa. A. Koneti Rao, MD; Ling Sun, MD.

Continuous ECG Recording Core Laboratory, Mayo Medical Services, Rochester, Minn. Mary Colman.

CIBA-Geigy, Summit, NJ. Marc Henis, MD; Malcolm MacNab, MD; Catherine Cabot, MD; Darryl Schwartz, MD; Paul Gallo, MD; Deborah Mattos, MS.

Received March 27, 1996; revision received June 4, 1996; accepted June 11, 1996.

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