Development of an efficient strategy for the synthesis of the ETB receptor antagonist BQ-788 and some related analogues

Development, synthesis and evaluation of novel fluorescent Endothelin-B receptor probes

The endothelin (ET) signaling system is comprised of three endothelin peptides (ET-1, -2 and -3) and two corresponding endothelin-A and -B receptors (ET_AR and ET_BR), which belong to the G-protein coupled receptor (GPCR) superfamily. The endothelin axis, as this system is also referred to, contributes to the maintenance of vascular tone, functions as regulator of inflammation and proliferation and helps in balancing water homeostasis. In pathological settings, the ET axis is known to contribute to endothelial activation in cardiovascular diseases, to cell proliferation, chemoresistance and metastasis in cancer and to inflammation and fibrosis in renal disease. Antagonists of ET_AR and ET_BR, either subtype-specific compounds or substances with high affinity to both receptors, have been developed for more than 30 years. In the preclinical context, in vivo imaging of endothelin receptor expression has been utilized to assess ET-axis contribution to e.g. cancer or myocardial infarction. In this work, we present the development and synthesis of two novel ET_BR-specific fluorescent probes, based on the available antagonists BQ788 and IRL2500 and their preliminary evaluation in a breast cancer context.

Enzymatic pathways involved in the generation of endothelin-1(1-31) from exogenous big endothelin-1 in the rabbit aorta

We investigated whether blood vessels contribute to the production of ET-1(1-31) from exogenous big endothelin-1 (BigET-1) in the rabbit and assessed which enzymes are involved in this process. Vascular reactivity experiments, using standard muscle bath procedures, showed that BigET-1 induces contraction in endothelium-intact rabbit aortic rings. Preincubation of the rings with phosphoramidon, CGS35066 or thiorphan reduced BigET-1-induced contraction. Conversely, chymostatin did not affect BigET-1-induced contraction. Thiorphan and phosphoramidon, but not CGS35066 or chymostatin, reduced ET-1(1-31)-induced contraction. None of the enzymatic inhibitors affected the contraction afforded by ET-1.BQ123-, but not BQ788-, selective antagonists for ET(A) and ET(B) receptors, respectively, produced concentration-dependent rightward displacements of the ET-1(1-31) and ET-1 concentration-response curves. By the use of enzymatic assays, we found that the aorta, as well as the heart, lung, kidney and liver, possess a chymase-like activity. Enzyme immunoassays detected significant levels of Ir-ET-1(1-31) in bathing medium of aortas after the addition of BigET-1 (30 nM). Neither thiorphan nor chymostatin altered the levels of Ir-ET-1(1-31). Conversely, the levels of Ir-ET-1(1-31) were increased in the presence of phosphoramidon. This marked increase of the 31-amino-acid peptide was abolished when phosphoramidon and chymostatin were added simultaneously. The major new finding of the present work is that the rabbit aorta generates ET-1(1-31) from exogenously administered BigET-1. Additionally, by measuring the production of ET-1(1-31), we showed that a chymase-like enzyme is involved in this process when ECE and NEP are inhibited by phosphoramidon. Our results also suggest that ET-1(1-31) is an alternate intermediate in the production of ET-1 following BigET-1 administration. Finally, we showed that NEP is the predominant enzymatic pathway involved in the cleavage of ET-1(1-31) to a bioactive metabolite that will act on ET(A) receptors to induce contraction in the rabbit aorta.