COVER: Biased signals through GPCRs and their evaluation. Signals mediated by GPCRs is roughly divided into two pathways: the heterotrimeric G-protein-mediated pathway and the β-arrestin-mediated pathway. Each pathway transduces distinct intracellular signals comprised of different signaling molecules, which often lead to benefi cial or adverse effects of the drug. Biased agonists are agonists that preferentially activate one of these signaling pathways and is expected to reduce side effects by minimizing the signals leading to unwanted effects. Evaluation of the biased signal of a drug is mostly performed in cell-based assays using a reference lignad. By measuring the relative activity of a drug of interest to a reference ligand in individual assays and comparing that relative activity across different assay systems, one can evaluate which signals the drug preferentially activates. The pharmacological parameters Emax and EC50 are often used for the evaluation of bias, and the results of their calculations can be presented in various ways. Comprehensive analysis of drugs at multiple signals would contribute to the development of signal pathway-selective drugs with better safety profi les. Kise R. and Inoue, A. (2024) GPCR signaling bias: an emerging framework for opioid drug development. J. Biochem. 175, 367-376