The angiotensin AT-receptor agonist compound 21 is an antagonist for the thromboxane TP-receptor - Implications for preclinical studies and future clinical use.

Since the AT-receptor (ATR) agonist C21 has structural similarity to the AT-receptor antagonists Irbesartan and Losartan, which are antagonists not only at the ATR, but also at thromboxane TP-receptors, we tested the hypothesis that C21 has TP-receptor antagonistic properties as well. Isolated mouse mesenteric arteries from C57BL/6 J and ATR-knockout mice (ATR) were mounted in wire myographs, contracted with either phenylephrine or the thromboxane A (TXA) analogue U46619, and the relaxing effect of C21 (0.1 nM - 10 µM) was investigated. The effect of C21 on U46619-induced platelet aggregation was measured by an impedance aggregometer. Direct interaction of C21 with TP-receptors was determined by an β-arrestin biosensor assay. C21 caused significant, concentration-dependent relaxations in phenylephrine- and U46619-contracted mesenteric arteries from C57BL/6 J mice. The relaxing effect of C21 was absent in phenylephrine-contracted arteries from ATR mice, whereas it was unchanged in U46619-contracted arteries from ATR mice. C21 inhibited U46619-stimulated aggregation of human platelets, which was not inhibited by the ATR-antagonist PD123319. C21 reduced U46619-induced recruitment of β-arrestin to human thromboxane TP-receptors with a calculated K of 3.74 µM. We conclude that in addition to ATR-agonistic properties, C21 also acts as low-affinity TP-receptor antagonist, and that - depending on the constrictor - both mechanisms can be responsible for C21-induced vasorelaxation. Furthermore, by acting as a TP-receptor antagonist, C21 inhibits platelet aggregation. These findings are important for understanding potential off-target effects of C21 in the preclinical and clinical context and for the interpretation of C21-related myography data in assays with TXA-analogues as constrictor.