Measurement of Thromboxane Biosynthesis in Health and Disease

Figure 1. Assessment of platelet thromboxane (TX)A2 biosynthesis ex vivo and in vivo. The left panel depicts the chain of enzymatic reactions triggered by thrombin generation during whole-blood clotting, and the resulting formation of very large amounts of TXB2 (the stable hydration product of TXA2) that can be easily measured in serum as a sensitive and specific index of platelet COX-1 activity. The right panel depicts the metabolic fate of TXA2in vivo and the calculated rate of its production in healthy subjects on the basis of TXB2 infusions and measurement of its major urinary metabolites, 11-dehydro-TXB2 and 2,3-dinor-TXB2. The latter represent a non-invasive index of platelet activation in vivo. Modified and redrawn from Patrono et al. (2013).

Figure 2. Urinary excretion rates of 11-dehydro-TXB2 in clinical settings at high cardiovascular risk. Mean (± standard deviation) or median (interquartile range) urinary excretion rates of 11-dehydro-TXB2 in clinical settings characterized by high cardiovascular risk. PAD, peripheral arterial disease; PCI, percutaneous coronary intervention; T2DM, type 2 diabetes mellitus; TIA, transient ischemic attack. Modified and redrawn from Patrono et al. (2013).

Figure 3. Long-term effects of low-dose (0.45 mg/kg per day) aspirin on platelet TXB2 and renal PGI2 synthesis. Serum TXB2 concentrations and urinary excretion of 6-keto-PGF1α were measured in three healthy subjects before, during, and after aspirin therapy. Mean values ± SEM are plotted as percentage of control measurements performed prior to aspirin administration. The arrows indicate duration of daily aspirin therapy. Redrawn from Patrignani et al. (1982).

Figure 4. Nonlinear relationship between inhibition of serum thromboxane (TX) B2 and urinary 11-dehydro-TXB2 excretion. In the lower left panel, individual percentage inhibition values are depicted from all on-treatment and post-treatment measurements performed in 48 healthy subjects randomized to receive aspirin 100 mg daily for 1 to 8 weeks. The nonlinear relationship between percent inhibition of serum TXB2 and urinary 11-dehydro-TXB2 showed that for 0 to 97% of COX-1 inhibition, TXA2 biosynthesis in vivo was linearly inhibited by <40% and that >97% suppression of serum TXB2 was necessary to maximally reduce TX metabolite (TXM) excretion. The upper right panel represents a detail of the left panel and is based on mathematical modeling of the experimental data. Modified and redrawn from Santilli et al. (2009), with permission from the publisher.

Figure 5. Serum thromboxane (TX)B2 levels at 24 h after aspirin intake (100 mg once daily) vs body mass index or body weight. Log-transformed serum TXB2 levels measured 24 h after a witnessed aspirin intake are represented in relation to body mass index (panel A) and body weight (panel B). Triangles represent individual patients receiving aspirin for primary or secondary cardiovascular prevention, n = 71; circles represent individual healthy subjects, n = 25. Solid lines and green areas: predicted serum TXB2 levels and 95% confidence intervals, respectively, as a function of increasing body mass index (panel A) and body weight (panel B) in the 96 study subjects. Horizontal dotted lines were drawn to indicate serum TXB2 levels of 3, 6, and 9 ng/mL, corresponding to 99%, 98%, and 97% inhibition of platelet COX-1 activity, respectively, measured in healthy subjects. The vertical dotted lines were drawn to indicate on the abscissa scales the highest calculated values of body mass index (panel A) and body weight (panel B) compatible with these different levels of platelet COX-1 inhibition. Modified and redrawn from Petrucci et al., 2019, with permission from the publisher.