Impact of hormone-associated resistance to activated protein C on the thrombotic potential of oral contraceptives: a prospective observational study

Abstract

Introduction: The increased thrombotic risk of oral contraceptives (OC) has been attributed to various alterations of the hemostatic system, including acquired resistance to activated protein C (APC). To evaluate to what extent OC-associated APC resistance induces a prothrombotic state we monitored plasma levels of thrombin and molecular markers specific for thrombin formation in women starting OC use. Elevated plasma levels of thrombin have been reported to characterize situations of high thrombotic risk such as trauma-induced hypercoagulability, but have not yet been studied during OC use.

Patients and methods: Blood samples were collected prospectively from healthy women (n = 21) before and during three menstruation cycles after start of OC. APC resistance was evaluated using a thrombin generation-based assay. Plasma levels of thrombin and APC were directly measured using highly sensitive oligonucleotide-based enzyme capture assay (OECA) technology. Thrombin generation markers and other hemostasis parameters were measured additionally.

Results: All women developed APC resistance as indicated by an increased APC sensitivity ratio compared with baseline after start of OC (p = 0.0003). Simultaneously, plasma levels of thrombin, prothrombin fragment 1+2, and of thrombin-antithrombin complexes did not change, ruling out increased thrombin formation. APC plasma levels were also not influenced by OC use, giving further evidence that increased thrombin formation did not occur.

Conclusions: In the majority of OC users no enhanced thrombin formation occurs despite the development of APC resistance. It cannot be ruled out, however, that thrombin formation might occur to a greater extent in the presence of additional risk factors. If this were the case, endogenous thrombin levels might be a potential biomarker candidate to identify women at high thrombotic risk during OC treatment. Large-scale studies are required to assess the value of plasma levels of thrombin as predictors of OC-associated thrombotic risk.

Figure 1. Change of the APC sensitivity ratio during the study period.

Box-whisker plots show the ratio [ETP_+APC/ETP_−APC] at visits 1–4. The center horizontal solid line is drawn at the median. Top and bottom edges of the boxes are located at the sample 75^th and 25^th percentiles. Top and bottom of the whiskers are located at the 90^th and 10^th percentiles. Each outlier is shown. P values indicate the changes of visits 2–4 compared to visit 1.

Figure 2

A. Thrombin plasma levels, determined by OECA; Limit of quantification (LOQ, solid line), 0.039 ng/ml; Limit of detection (LOD, dotted line), 0.017 ng/ml. B. APC plasma levels, determined by OECA; LOQ (solid line), 0.116 ng/ml; LOD (dotted line), 0.022 ng/ml.

Figure 3. Changes of variables during the study period.

Columns show the median. Parameters with significant changes between visit 1 and visit 4 are listed. P values indicate the changes of visits 2–4 compared to visit 1. *indicates highly significant changes of p<0.005.