A Robust Bioassay of the Human Bradykinin B ₂ Receptor That Extends Molecular and Cellular Studies: The Isolated Umbilical Vein

Abstract

Bradykinin (BK) has various physiological and pathological roles. Medicinal chemistry efforts targeted toward the widely expressed BK B ₂ receptor (B ₂R), a G-protein-coupled receptor, were primarily aimed at developing antagonists. The only B ₂R antagonist in clinical use is the peptide icatibant, approved to abort attacks of hereditary angioedema. However, the anti-inflammatory applications of B ₂R antagonists are potentially wider. Furthermore, the B ₂R antagonists notoriously exhibit species-specific pharmacological profiles. Classical smooth muscle contractility assays are exploited over a time scale of several hours and support determining potency, competitiveness, residual agonist activity, specificity, and reversibility of pharmacological agents. The contractility assay based on the isolated human umbilical vein, expressing B ₂R at physiological density, was introduced when investigating the first non-peptide B ₂R antagonist (WIN 64338). Small ligand molecules characterized using the assay include the exquisitely potent competitive antagonist, Pharvaris Compound 3 or the partial agonist Fujisawa Compound 47a. The umbilical vein assay is also useful to verify pharmacologic properties of special peptide B ₂R ligands, such as the carboxypeptidase-activated latent agonists and fluorescent probes. Furthermore, the proposed agonist effect of tissue kallikrein on the B ₂R has been disproved using the vein. This assay stands in between cellular and molecular pharmacology and in vivo studies.