Increased thromboxane production in women with a history of venous thromboembolic event: effect of heparins

Investigation of a Thromboxane A2 Receptor-Based Vaccine for Managing Thrombogenesis

BACKGROUND

Despite the well-established role for the thromboxane A2 receptor (TPR) in the development of thrombotic disorders, none of the antagonists developed to date has been approved for clinical use. To this end, we have previously shown that an antibody targeted against TPR's ligand-binding domain inhibits platelet activation and thrombus formation, without exerting any effects on hemostasis. Thus, the goal of the present studies is to design a novel TPR-based vaccine, demonstrate its ability to trigger an immune response, and characterize its antiplatelet and antithrombotic activity.

METHODS AND RESULTS

We used a mouse keyhole limpet hemocyanin/peptide-based vaccination approach rationalized over the TPR ligand-binding domain (ie, the C-terminus of the second extracellular loop). The biological activity of this vaccine was assessed in the context of platelets and thrombotic diseases, and using a host of in vitro and in vivo platelet function experiments. Our results revealed that the TPR C-terminus of the second extracellular loop vaccine, in mice: (1) triggered an immune response, which resulted in the development of a C-terminus of the second extracellular loop antibody; (2) did not affect expression of major platelet integrins (eg, glycoprotein IIb-IIIa); (3) selectively inhibited TPR-mediated platelet aggregation, platelet-leukocyte aggregation, integrin glycoprotein IIb-IIIa activation, as well as dense and α granule release; (4) significantly prolonged thrombus formation; and (5) did so without impairing physiological hemostasis.

CONCLUSIONS

Collectively, our findings shed light on TPR's structural biological features, and demonstrate that the C-terminus of the second extracellular loop domain may define a new therapeutic target and a TPR vaccine-based approach that should have therapeutic applications.

Postmenopausal hormone replacement therapy increases plasmatic thromboxane β2

OBJECTIVE

This study shows the effect of hormone replacement therapy (HRT), using oral estrogen exclusively or in combination with progestin, on platelet activation in healthy menopaused women.

BACKGROUND

Recent evidence from studies of postmenopausal HRT in healthy women demonstrated a short-time increased risk of coronary heart disease. Platelet activation, which generates vasoconstrictory thromboxane A(2) (TxA(2)), has been related to the risk of cardiovascular diseases.

METHODS

By means of a placebo-controlled study twenty-seven postmenopausal patients were continuously orally administered estrogen in combination with progestin or estrogen exclusively for an 8-week period. Platelet activation was evaluated by flow cytometric P-selectin expression and by enzyme immunoassay plasmatic TxA(2) (TxB(2)) concentrations.

RESULTS

P-selectin binding index changed from 6.3+/-3.6 to 7.0+/-3 in the placebo group (n=10); from 5.9+2.2 to 7.9+/-3.3 in the E+P group (n=8) and from 6.4+2.7 to 7.1+/-1.9 in the E group (n=9). Plasma concentrations of TxB(2) before and after intervention, changed from 1.2+1.2 to 1.5+1.4 (pg/well) in the placebo group; significantly (p=0.005) in the E+P group (n=8), from 0.9+0.3 to 6.1+6.5 (pg/well), and from 1.3+1.5 to 0.8+0.4 (pg/well) in the E group (n=8; mean+standard deviation, basal x therapy, p<0.05).

CONCLUSIONS

Healthy menopaused women who were administered estradiol in association with norethisterone continuously had an increase of plasmatic thromboxane, possibly determined by platelet activation, which indicates a higher short-term thrombotic risk. P-selectin expression analyses failed to demonstrate the impact of HRT on platelets.