The Rate of Prothrombin Conversion Is Increased in Antiphospholipid Syndrome and Is Insensitive to Thrombomodulin

Background: The antiphospholipid syndrome (APS) is characterized by the presence of antiphospholipid antibodies directed mainly against prothrombin and β₂-glycoprotein I. The syndrome is associated with an increased risk of thrombosis. The global hemostatic state in a patient can be tested by measuring thrombin generation (TG). Recently, we developed a method to study the main pro- and anticoagulant processes at the basis of TG, called the thrombin dynamics method.

Aim: In this study we investigated the dynamics of thrombin generation in healthy subjects and APS patients.

Materials and methods: Healthy subjects (n=129) and antiphospholipid syndrome (APS) patients (n=31) were included in the study. Sixty-eight percent of the APS patients were lupus anticoagulant positive, anti-cardiolipin antibodies were detected in 84% of the patients, and 52% presented with anti-β₂-glycoprotein I antibodies. Patients on anticoagulant therapy were excluded from the study. Thrombin generation was measured at 1 pM tissue factor (TF) and activated protein C (APC) system sensitivity was tested by measuring TG in the presence and absence of 20 nM thrombomodulin (TM).

Results: Thrombin generation was measured in platelet poor plasma at 1 pM tissue factor. The lag time and time-to-peak were significantly prolonged in APS patients compared to healthy subjects (lag time: 3.30 ± 0.59 vs. 6.69 ± 4.26 min, p<0.001; time-to-peak: 8.33 ± 1.29 vs. 10.76 ± 4.51 min, p<0.001). The peak height was significantly higher in APS patients (240 ± 84 vs. 214 ± 58 nM, p<0.05) and the velocity index was elevated in APS patients (134 ± 66 vs. 70 ± 32 nM/min, p<0.001) compared to healthy subjects. The ETP values were comparable between healthy subjects and APS patients (1260 ± 235 vs. 1176 ± 362 nM*min).

The pro- and anticoagulant processes underlying thrombin generation were studied separately. The total amount of prothrombin converted during thrombin generation (PCtot) did not differ between healthy subjects and patients (1234 vs. 1165 nM). However, the maximum rate of prothrombin conversion (PCmax) was significantly elevated in APS patients (291 vs 425 nM/min; p<0.001). The amount of thrombin-antithrombin (T-AT) complexes formed was comparable between patients and controls (1169 vs. 1098 nM), and the thrombin decay capacity (TDC) was comparable as well (0.675 vs. 0.674 min^-1). These results are in line with the finding that the plasma levels of the main thrombin inhibitors are unchanged in APS patients. Antithrombin levels are on average 2.31 ± 0.44 μM in healthy subjects and 2.36 ± 0.56 μM in APS patients, and the mean α₂-macroglobulin levels were 3.22 ± 0.77 μM in healthy subjects and 3.23 ± 1.11 μM in patients.

Thrombomodulin reduced the ETP by 45% in healthy subjects, but had significantly less effect in APS patients (10%). The addition of TM decreased total prothrombin conversion by 40% and the maximum prothrombin conversion rate by 50% in healthy subjects. In patients, TM only slightly reduced total prothrombin conversion (8%) and the maximum prothrombin conversion rate (7%).

Discussion: The thrombin generation results indicate a predisposition to thrombosis in APS patients, as the TG parameters peak height and the velocity index are increased. Examination of the underlying pro- and anticoagulant processes of prothrombin conversion and thrombin inactivation revealed that although the same amount of prothrombin is converted in patients, the maximum activity of the prothrombinase complex is higher, indicating that patients can generate thrombin faster. In addition, APS patients have a dysfunctional APC system, as prothrombin conversion and thrombin generation could be only slightly inhibited by the addition of thrombomodulin.